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Palaeogeography, Palaeoclimatology, Palaeoecology
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Composition, structure and evolution of a lacustrine carbonate margin dominated bymicrobialites: Case study from the Green River formation (Eocene; Wyoming, USA)
Claire Seard a,⁎, Gilbert Camoin a, Jean-Marie Rouchy b, Aurélien Virgone c
a Aix-Marseille Université, CNRS, CEREGE UM34, 13545 Aix en Provence, Franceb Muséum National d'Histoire Naturelle, Paris, Francec TOTAL, Pau, France
Article history:Received 18 July 2012Received in revised form 6 April 2013Accepted 17 April 2013Available online 28 April 2013
Keywords:Green River FormationGosiute LakeEoceneLacustrine carbonatesInsect-microbial build-upsStromatolite
During mid Eocene times, the northwestern margin of the Gosiute Lake (Wyoming) has been characterisedby the extensive development of a microbialite dominated carbonate sequence corresponding to the LaneyMember of the Green River Formation, thus providing a potential analogue for petroleum carbonate reser-voirs developed in similar marginal lacustrine settings. A detailed sedimentological study carried out onthe Little Mesa plateau near LaBarge (SW Wyoming) allows the reconstruction of: 1) lateral and verticalfacies variations of the carbonate sequence across the margin, 2) development patterns of complex microbialbioconstructions, and 3) the evolution of the margin through time. The carbonate sequence, up to 18 m-thick,starts with the deposition of sandy limestones which fill channels in the underlying siliciclastic unit of theCathedral Bluff Formation. The increasing microbial contribution in the upper part of the sequence resultsin the development of large insect-microbial build-ups which represent up to 80% of the total volume ofthe carbonate deposits and which aggregated to form reef-like geometries up to several tens of metres indiameter, including bioherms, spur and groove alignments and reef-flat structures. The initiation and devel-opment of the lacustrine carbonate platform was controlled both by the general lake expansion associated tothe Eocene Climatic Optimum and by local drainage diversions responsible for a new water inflow in thebasin from the northern volcanic provinces. Short-term lake level changes driven by high-frequency climatefluctuations which are superimposed to this general transgressive trend may explain the discontinuous andpolygenic growth of the individual insect-microbial build-ups. The basin infilling by prograding alluvialvolcaniclastic deposits originated from the northwestern volcanic province caused the rapid turn off of themarginal lacustrine carbonate factory during mid Eocene time.
Lacustrine sources and reservoirs are important in many areas ofcurrent and future exploration opportunities: Africa, South America,Southeast Asia and China (Bohacs et al., 2000). The recent discoveryof vast hydrocarbon reservoirs in Cretaceous marginal lacustrinedeposits dominated by composite algal/microbial features in theSouth Atlantic has reopened the interest of the scientific communityand oil companies for the study of those deposits. Carbonate depositsincluding composite algal/microbial build-ups that are potentialanalogues to these reservoirs have been previously reported inmany lacustrine systems ranging in age from late Cretaceous toRecent times (Great Salt Lake: Carozzi, 1962; Halley, 1977; Pedoneand Folk, 1996; Tanganyika Lake: Cohen and Thouin, 1987;Casanova and Hillaire-Marcel, 1992; Tiercelin et al., 1992; Cohen etal., 1997; Bolivia: Rouchy et al., 1996; Camoin et al., 1997; Argentina:
opole de l'Arbois BP 80, 13545.
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Palma, 2000; USA: Link et al., 1978; Benson, 1994; France: Casanovaand Nury, 1989; Bertrand-Sarfati et al., 1994). In these examples,the algal/microbial build-ups have been usually described in detailsbut their relationships with coeval lacustrine deposits and the mor-phology and evolution of the margin have received little attentionso that the architecture of the lacustrine margins and the relative con-tribution of the various processes controlling its development in timeand space are still obscured.
The Eocene Green River Formation (Hayden, 1869) of Wyoming,Colorado, and Utah was selected for this study as one of the mostvaluable potential analogue to the South Atlantic carbonates becausethe Green River Formation represents one of the best-documentedancient lacustrine system and has long been considered as a casestudy to understand lacustrine depositional systems (Bradley, 1929;Eugster and Surdam, 1973; Carroll and Bohacs, 1999). The open lacus-trine facies contain the richest and thickest oil shale deposits reportedso far and have been extensively documented in the literature. In con-trast, the carbonate margins of the Green River lacustrine systemshave received less attention (e.g. Roehler, 1973, 1993; Leggitt andCushman, 2001; Leggitt and Loewen, 2002; Leggitt et al., 2007).
The Green River marginal lacustrine carbonates include a wide vari-ety of facies that include algal–microbial build-ups, tufas, coquinas,skeletal and oolitic grainstones, shales, sandstones and organic-rich fa-cies that have been the subject of an extensive sedimentological, min-eralogical and petrophysical study to reconstruct the architecture ofthe lacustrine margins and to unravel their evolution in time andspace. Furthermore, the large size of the Green River Basin affords theopportunity to study these lacustrine systems at an exploration scale.
The present paper focuses on the northern carbonate margin ofthe lake which crops out in southwest Wyoming, near the town ofLaBarge (Little Mesa; Fig. 1). This margin developed during theLaney Member of the Green River Formation, when the GosiuteLake was expanding over the floodplains and mudflats of the Cathe-dral Bluffs Tongue of the Wasatch Formation. The main feature ofthese margins corresponds to the extensive development of distinc-tive insect-microbial build-ups which contribute to the constructionof reef-like morphologies. The study site, characterised by wellexposed continuous outcrops, allows the study of lateral and verticalfacies variations across the margin as well as the morphology and thestructure of the carbonate margin. With the association of extensivecomposite microbial constructions, various carbonate sediments andtravertine deposits, this margin provides a valuable potential ana-logue for the Cretaceous pre-salt carbonate systems from the SouthAtlantic basin. The objectives of this paper are: (1) to document theorganisation and composition of the microbial-dominated lacustrine
GreenRiverBasin
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Fig. 1. Location map of the major Eocene basins in which the Green River Formation wasdeposited: Green River,Washakie, Uinta, Piceance Creek and Fossil basins (modified fromLeggitt and Cushman, 2001). These basins are separated by chains of anticlinal uplifts.The Gosiute Lake occupied the Green River andWashakie basins; its maximal extent dur-ing the Laney member is represented. The studied site Little Mesa (red star) is situatedNorth of the town of LaBarge.
margin, (2) to determine the development patterns of insect-microbial build-ups, and (3) to unravel the evolution of the lacustrinemargin through time.
2. Geological setting
The Green River Formation was deposited in a series of lacustrinebasins that occupy a broken foreland province to the east of the Sevierfold and thrust belt: Greater Green River Basin (Green River/Bridger,Great Divide, Washakie and Sand Wash Basins), Uinta and PiceanceCreek Basins (Fig. 1). These basins are separated by chains ofLaramide-style Precambrian basement-cored uplifts that were vari-ably active from the Maastrichtian through the Eocene (Beck et al.,1988; Dickinson et al., 1988; DeCelles, 2004). The initiation of thelacustrine deposition in these basins coincided with the late stage ofLaramide uplift (Dickinson et al., 1988; Carroll et al., 2006). The lacus-trine depocenters developed shortly after the eastward retreat of thelate Cretaceous seaway and formed in structural depressionsbordered by Laramide ranges where subsidence rates exceeded sedi-mentation rates.
The Greater Green River Basin and the Uinta–Piceance CreekBasin, which are separated by the east–west-trending Uinta uplift(Bradley, 1964; Johnson, 1985; Roehler, 1992a), correspond respec-tively to the lakes Gosiute (King, 1878) and Uinta (Bradley, 1931).Each of these lakes varied greatly in chemistry and areal extentduring the deposition of the Green River Formation. These systemsexpanded and contracted in response to tectonic and climaticvariations, resulting in the interfingering of lacustrine and fluvialsediments along the basin margins (Carroll and Bohacs, 1999). Thechronology of the Green River Formation deposits is rather wellconstrained by lithostratigraphy, mammalian biostratigraphy (e.g.Wood et al., 1941; Lillegraven, 1993; Robinson et al., 2004; Smith etal., 2004), magnetostratigraphy (e.g., Tauxe et al., 1994; Clyde et al.,2001) and 40Ar/39Ar geochronological studies of tuff beds (Wing etal., 1991; Smith et al., 2003, 2004, 2006, 2008). These stratigraphicdata have demonstrated that the Green River Formation was deposit-ed during the warmest period of the Cenozoic corresponding to theend of the Early Eocene Climatic Optimum (ca. 53–49 Ma).
In the Gosiute Lake, the Green River Formation rests on the WasatchFormation and is overlain by the Bridger and Washakie Formations(Roehler, 1992a,b; McCarroll et al., 1996; Evanoff et al., 1998). The sedi-mentary succession is divided into four major members: the LumanTongue and the Tipton, Wilkins Peak, and Laney Members (Roehler,1992b). These units document the spatial and chemical evolution ofthe lake through time, involving the successive development of anoverfilled freshwater lake (Luman and Niland tongues), a balanced-fillsaline lake (Tipton), an underfilled hypersaline playa-lake (WilkinsPeak), a balanced-fill saline lake (lower Laney), and eventually anoverfilled freshwater lake (upper Laney) (Roehler, 1993; Carroll andBohacs, 1999; Bohacs et al., 2000; Rhodes et al., 2002; Smith et al., 2003).
The current study concerns the marginal lacustrine systems thatcorrespond to the LaClede Bed of the Laney member. This memberthat ranges from 0 to 630 m in thickness in the basin centre(Bradley, 1964), records a major lacustrine expansion in all sub-basins of the Greater Green River Basin with spill over into down-stream Lake Uinta during highstands (Smith et al., 2008). At its max-imum areal extent during the LaClede Bed, the lake covered a surfaceof about 40,000 km2, i.e. more than 75% of the whole Green RiverBasin. Its depositional axis paralleled the north flank of the UintaMountains with the maximum depocenter located in the Washakiesub-basin (Roehler, 1993). The deposition of the Lower Laclede Bedstarted slightly after 49.62 ± 0.17 Ma, which is the age of the SixthTuff bed, an ash layer intercalated in the uppermost part of theWilkins Peak Member (Smith et al., 2008). This formation ischaracterised by m-scale cyclic sedimentary successions recording re-peated expansion and desiccation of Gosiute Lake (Surdam and
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Stanley, 1979). These cycles commonly include shallow-water stro-matolite and ostracod, ooid, or pisoid grainstone facies that gradeinto deeper-water organic-rich laminated micrites (oil shales) andthen to mud-cracked dolomicrite exhibiting casts of evaporite min-erals such as nahcolite and trona (Roehler, 1973; Rhodes, 2002). Adistinct marker horizon named the Buff Marker Bed (Roehler, 1973,1992a, 1993) that represents a period of intense desiccation(Rhodes et al., 2007) is intercalated between these cycles (Carroll etal., 2008; Doebbert et al., 2010). The transition from the lower tothe upper part of the LaClede Bed occurred slightly after 48.94 ±0.18 Ma, the age of the Analcite Tuff located beneath this boundary(Smith et al., 2008), and represents the permanent filling of GosiuteLake to a spill point located to the southeast (Carroll and Bohacs,1999; Rhodes, 2002) indicating a permanent increase in average netwater supply to the lake. The upper LaClede bed corresponds toprofundal facies that comprise laminated to massive organic-richcalcitic mudstone; lake expansion–contraction cycles, mud cracks,and evaporite minerals are absent (Doebbert et al., 2010); it thengrades laterally and upward into dolomitic siltstone and volcaniclasticsiltstone and sandstone facies of the Sand Butte Bed.
3. Results
In the Little Mesa area, the LaClede Bed corresponds to marginalcarbonate deposits which form a tabular unit cropping out over
Fig. 2. Lithological and paleobiological compositions of the studied sections. The colours repwith large reworked elements, (F3) marly limestones, (F4) ooid and ostracod rich limestonestone and sandstone. The different units (U1, Unit 1; U2, Unit 2; and U3, Unit 3) are correlW110 12–16°) displays the location of studied sites (red points). The extension of the Laneysites.
several square km to form large undeformed and unfaulted plateaus(Fig. 2). They onlap the floodplain and mudflat deposits of the Cathe-dral Bluffs Tongue of the Wasatch Formation corresponding tovaricoloured mudstone and brown sandstone (Roehler, 1989, 1991).The carbonate marginal lacustrine sequence, 10 to 18 m-thick, islocally overlain by dm to m-thick travertine deposits and fluvialsiltstone and sandstone. In this paper, the term travertine is used todesignate continental carbonate deposits with laminated and/orshrub-like fabrics that are the result of the precipitation of calciumcarbonate from ambient temperature surface waters, either by bioticor abiotic processes. They lack in situ macrophyte remains. 23 stan-dard stratigraphic sections and numerous panoramas were studiedacross the study site (Fig. 2) to characterise these sequences.
3.1. Organisation and composition of the microbial build-ups andassociated deposits
The marginal carbonate sequence is characterised by the largedevelopment of insect-microbial build-ups that occur throughoutthe carbonate sequence, although their abundance increases sharplyupwards as they form most of the plateaus in the studied area(highlighted in yellow in Fig. 2). Those build-ups (Facies 1, F1,Table 1) are associated with other carbonate deposits includingooid-and-ostracod-rich limestones or sandy limestones, floatstonerich in stromatolite fragments, and tufas that occur between the
resent the various facies types: (F1) insect-microbial build-ups, (F2) sandy limestoness, (F5) floatstone with stromatolite fragments, (F6) tufas, (F7) travertines and (F8) silt-ated between the sections. The aerial photograph of the Little Mesa area (N42 21–23°;member is outlined in yellow. The kilometric values indicate the distance between the
Table 1Facies characteristics: thickness, lateral continuity and morphology of the beds and their lithological and biological compositions.
F1. Build-ups (Figs. 4 and 6) Bioherms up to 9 m-thickand 40 m-large
km-scale, they formmost of the plateaus
Reef-like geometries: bioherms,spur and groove alignments andreef flat-like structures
Nucleus of aggregates of insect cases (Trichoptera) thatform erect columnar or domal-shape constructions,up to 1.5 m-thick. Outer stromatolitic crusts around theaggregates from 1 to 60 cm-thick, with a preferentiallateral growth. Stromatolite accretions include flat, wavy,small columnar, branching or bulbous growth forms.
F2. Sandy limestones(Figs. 4, 6 and 7)
0.4 to 1.5 m Up to 250 m Channel features with basalerosional surfaces and nearly flatupper surfaces
Floatstone with mm to dm elements includingstromatolitic accretions, stromatolite desiccation chipsor gravels, fragments of insect cases aggregates, lithoclasts(fragments of marlstone, mudstone or ooid and/orostracod-rich limestones), megaoncoids, phytoclasts,bone fragments, ooids, ostracods and detrital grains.
F3. Marly limestones Up to 3 m Up to 100 m Lenticular beds Grey fine-grained marly limestones including locallyburrows and pedogenetic features.
F4. Ooid and ostracodrich limestones
8 to 80 cm 1 and 50 m Tabular beds; fill the depressionbetween adjacent build-ups
Wackestone to grainstone with well sorted ooids,ostracod shells and mm to cm sized stromatoliticaccretions, stromatolite desiccation chips or gravels andfragments of aggregates of insect cases. Localcross-bedded stratifications.
F5. Stromatolite fragmentsfloatstone (Fig. 7)
1 to 2 m 10 to 100 m (sites 7, 9and 23)
Massive beds with channelmorphologies
Floatstone with well-rounded mm to cm-sized fragmentsof stromatolites and lithoclasts in a marly matrix. Nograded bedding.
F6. Tufas (Fig. 7) 50 to 80 cm 100 m (site 23) Tabular bed Bushy tufas with digitate branches of 1 to 3 cm-largeand 5 to 20 cm-long. Branches are composed of crudelylaminated micrite.
F7. Travertines(Figs. 8 and 9)
0.2 to 2 m Up to 10 m (sites 6and 12 to 19)
Small terraces Centimetric horizontal layers of shrubs with a downwardarborescent growth alternating with dense micritic layers.
build-ups. In this paper, the term tufa corresponds to continental car-bonate deposits whose precipitation is associated to the occurrence ofsubaqueous vegetation (macrophytes) (Pedley, 2009).
3.1.1. Internal structure of the build-upsThe size of individual insect-microbial build-ups ranges from 0.5
to 2 m in thickness and from 0.5 to 3 m in width. Their most commonmorphologies consist of hemispherical domes, but columns, smalltowers, and cones also occur (Figs. 3A and 4B and C).
The nucleus of the build-ups is comprised of abundant insect cases(Fig. 3A, C and F) which represent the major volumetric component ofthe whole build-ups (up to 90% of the volume), thus exerting a directcontrol on their shape and size. The insect cases are 1 cm long,2.5 mm in diameter, and are aggregated to form erect columnar ordomal-shape constructions up to 1.5 m-thick. These aggregates arecomposed of the stacking of several generations of insect cases ar-ranged perpendicular to the bedding. The cases are in growth positionor slightly reworked and are locally agglutinated around vegetalstems which may have acted as substrates for their settling. Theinsects have been interpreted as caddisfly (Insecta: Trichoptera;Leggitt and Cushman, 2001; Leggitt and Loewen, 2002) pupae be-cause they are similar in length and diameter (Donsimoni, 1975;Leggitt et al., 2007); however, fossil caddisfly pupae have not beenfound within the cases (Leggitt and Cushman, 2001). In thin section,the insect cases appear as circles, ellipses and subparallel rows corre-sponding to a single layer of small particles bound together (Fig. 5Aand B). These particles, up to 400 μm-long and 200 μm-wide, includeostracod shells, coated grains, peloids and quartz grains and arecemented together by external and internal layers of microsparitecalcite cement. The intergranular porosity of the tubes is eitheropen or rimmed by an isopachous layer of prismatic calcite cementand then filled by detrital particles. The cases may be closely packed,separated by grainstones bearing ostracod shells, coated grains andpeloids (Fig. 5A), or directly encrusted by stromatolites (Fig. 5E).
The stromatolites generally encrust the core of insect cases andtherefore represent the final stage of the build-up development;however, alternations between insect cases and centimetre-thick
stromatolite layers are observed locally. Desiccation cracks are com-monly reported in internal laminae (Fig. 3E) throughout the sequenceand also affect the top of the build-ups. The stromatolite crusts are 1to 60 cm-thick and display a concentric growth around the nucleus;their thickness is maximal on the sides of the nucleus (Fig. 3C) andminimal at its top. These crusts correspond to the succession ofseveral generations of stromatolite accretions typified by variousmesostructures including flat or wavy laminations which form alayer of up to 2 cm-thick at the centre, and stacked generations ofsmall columnar, branching or bulbous growth forms that constitutethe thickest part of the encrustation (2 to 50 cm-thick; Fig. 3C andF). These small columnar growths, up to 4 cm-long and 1 cm-large,are generally digitate and coalescent and composed of convex andparallel laminae. A final thin crust, up to 3 cm-thick, with flat orwavy laminations may cover the whole build-up and give a smooth,pustular or mamillated aspect to the outer surfaces of the domes.The outer encrustation locally displays a cerebroid pattern related tothe merging of the last generation of stromatolite columns. Thecontinuity of the outer stromatolite crust indicates that the heightand shape of the domes existed prior to burial by sediments.
At a microscopic scale, the flat, wavy and convex laminations of thestromatolites generally correspond to alternations of light-colouredmicrite or microsparite and 200 to 400 μm-thick dark-brown micritelayers without clearly defined boundaries (Fig. 5D).
Many columnar, branching and bulbous accretions include smalllight coloured spheres of 100–200 μm in diameter filled by sparry cal-cite that locally form the main component of the laminae (Fig. 5C, Dand E). Given the lack of preserved organic structures (i.e. no larvaor pupa, no filaments), the nature of these spheres cannot be deter-mined. They could be interpreted as casts of individual cells of unicel-lular green algae, fungal spores, insect eggs or bacterial concretions(Bradley, 1929; Leggitt and Cushman, 2001). The scarcity of detritalparticles in the laminae suggests that these stromatolites wereformed mostly by in-situ precipitation of carbonate by microbialcommunities rather than sediment trapping and binding.
Stromatolites also form isolated accretions and small domes, up to20 cm-thick, without any core of insect cases (Fig. 3E) in sandy
Fig. 3. Photographs of A) a insect-microbial build-up with a core of insect cases (i) coated by digitated columnar stromatolite accretions; B) a columnar megaoncoid with a centralcore outlined by bark impressions filled with sediments (p) coated by mamillated stromatolite accretions; the central core was initially a phytoclast; C) CT-scan of the sample Ashowing its internal structure with a core of insect cases (i) laterally coated by digitated columnar stromatolite accretions with convex laminations; D) CT-scan of the sample Bshowing its internal structure with a core of sediments (p) coated by digitated columnar stromatolite accretions with convex laminations; E) Superposition of several generations(1, 2, 3) of branching and digitated stromatolite accretions; the second one is affected by desiccation; and F) Digitated columnar stromatolite accretions coating insect cases aggre-gates (i).
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limestones and ooid-and-ostracod-rich limestones. They display avertical evolution which is similar to that of the insect-microbialbuild-ups, with flat or wavy laminations up to 2 cm-thick at thebase overlain by digitate columnar and branching forms with convexlaminations from 1 to 10 cm in thickness. Small domes, mostly found
in the south-west sections, are composed of bushy accretions withthin digitate branches (2–4 mm-large) that form a pustular morphol-ogy at their top. These domes also include vertical stem traces of1.5–2 cm in diameter at their top, tentatively related to the presenceof reeds.
Fig. 4. Photographs fromMesa 5 site: A) Panoramic view of the Mesa 5 outcrop showing Unit 1 with channelled beds of ostracod and ooid-rich limestones with reworked elements andUnit 2 with abundant insect-microbial build-ups. B and C) Tilted and in-situ dm-sized insect-microbial domes fromUnit 2; domes are separated by ooid-rich limestones. D) Lithoclast (li)coated by insect larval cases (in) and then by stromatolites (st) to form a megaoncoid (Unit 1). E) Phytoclasts (p) locally coated by stromatolites (Unit 1). F) Stromatolite chips (c) (Unit1). G) Columnar build-up with a core of insect larval cases coated by stromatolites (Unit 1).
3.1.2. Distribution of lacustrine carbonate depositsThe build-ups display variable abundance, size and distribution
throughout the lacustrine sequence. The sequence can be dividedinto two major units based on the abundance of the build-ups(Figs. 4, 6 and 7), from base to top: (1) the Unit 1, up to 9 m-thick,is dominated by channel infillings with isolated build-ups formingless than 20% of the total volume. (2) The Unit 2, up to 18 m-thick,is characterised by the greater development of stromatolitic accre-tions which form larger coalescing build-ups which represent up to80% of the total volume and display locally reef-like geometries. Thethickest development of this unit (18 m-thick) is reported in the east-ern part of the margin. Evidences of short-term episodes of subaerialexposure (e.g. desiccation cracks) occur throughout the sequence.
In Unit 1, the individual build-ups, up to 80 cm-thick, are isolatedand irregularly spaced, from 2 to 10 m, on top of channel infillingsconsisting of alternations of beige to light brown massive sandy lime-stones and fine-grained grey marly limestones (Figs. 2, 4, 6 and 7;Table 1). These channel deposits which are well developed inthe western parts of the plateaus display a NW–SE orientation,i.e. perpendicular to the shoreline.
The sandy limestones (F2) appear as lenticular massive bodies 0.4to 1.5 m-thick filling a complex network of intersected channel fea-tures. They are composed of mm to dm reworked elements in awackestone to packstone matrix with ooids, ostracods and variableamount of fine to medium siliciclastic grains (Fig. 5F), includingmainly sub-angular quartz grains, and scarce potassium feldspars
and micas. The ooids ( 0.2 to 1 mm) have spheroidal, ovoid and ellip-soidal shapes and display a concentric fabric around a nucleus ofrounded micrite aggregate, ostracod shell or detrital grain. The ostra-cod shells ( 0.4 to 0.8 mm) are thin, smooth and composed of pris-matic calcite. The reworked elements are angular and poorly sorted;they include stromatolite chips (Fig. 4F) or gravels, fragments of in-sect cases aggregates, litho- and phytoclasts (Fig. 4E), megaoncoids(Fig. 4D and G) and bone fragments. The megaoncoids, from 10 cmto 2 m in size, have cylindrical (Fig. 4G), ovoid or conical shapesand are composed of a core corresponding to a phytoclast (vegetalstems and tree branches or trunks) or a lithoclast coated by insectcases (Fig. 4D), or directly by stromatolites. The megaoncoid corecan be empty and outlined by bark impressions (altered phytoclast)or more commonly filled with sandy limestones (Fig. 3B and D).Phytoclasts are more abundant in the southern sites than in thenorthern ones. Siliciclastic grains are usually scarce in the matrixwhere they represent less than 5% of the grains.
The marly limestones interbeds (F3) are better developed in thenorthern sites where they form 1–3 m-thick continuous beds, whileonly up to 0.4 m-thick lenticular beds are reported in the southernsites (Fig. 2). They exhibit connected vertical and horizontal burrows,2 to 3 cm in diameter. In the lowermost part of the unit, these marlylimestones may include one or two thin levels, less than 10 cm inthickness, displaying pedogenetic features as root traces and spots ofoxidation. More regular and thinner beds of ooid and ostracod-richlimestones occur laterally to these channel deposits. They locally
Fig. 5. Photomicrographs of insect-microbial build-ups: A) Insect cases (circles) separated by grainstones with ostracod shells, coated grains and peloids; B) Detailed view of a casecomposed of a single layer of small particles including small ostracod shells, peloids and terrigeneous grains bind together. These particles are cemented by external and internallayers of microsparite calcite cement; C) Branching stromatolite accretions comprising abundant small light coloured spheres; D) Columnar stromatolite accretion with alternationsof light-coloured micrite and dark-brown micrite layers without clearly defined boundaries including small light coloured spheres; E) Stromatolite accretions including small lightcoloured spheres (st) coating insect cases (i). These accretions are generally coated by a 100 to 400 μm-thick continuous layer of brown micrite. The thin section was stained withAlizarin; and F) Thin section picture of sandy limestones (F2) corresponding to an ostracod-rich wackestone to packstone including quartz grains; the thin section was stained withAlizarin.
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form a thin (10 cm-thick) but continuous (over hundreds of metres)decarbonated yellowish bed, composed mainly of potassium feldsparsand nodules of iron oxides, that is interpreted as a weathered horizon.
Unit 2 is characterised by the development of abundant build-ups,the sizes of which significantly increase upwards but also eastwardfrom 20 cm at site 1 to 1.5 m at site 22. In this unit, the individualbuild-ups commonly coalesce to form larger composite build-ups of0.7 to 2 m in thickness and 1 to 8 m in width. These compositebuild-ups are irregularly spaced from 0.2 to 10 m or may coalesceto form reef-like geometries including bioherms, spur and groovealignments and reef flat-like structures (Fig. 6).
The bioherms are 4 to 9 m thick and 4 to 40 m wide; they form arelief displaying a hemispherical shape that results from the vertical
piling-up and lateral coalescence of successive generations of individ-ual and composite build-ups of various thicknesses and morphologies(Figs. 4A and 6A and B). The bioherms that form topographic highsare better developed over channel deposits and separated from eachother by depressions filled by ooid and ostracod-rich limestones bear-ing isolated insect-microbial build-ups, defining a spur and groovemorphology (Fig. 6A). These alignments are irregularly spaced, from40 to 100 m, and display the same NW–SE trending than the channelbodies, but with a morphological inversion. Erosion features affectingthese structures are scarce, suggesting that they correspond to themorphology of the initial sedimentary slopes.
The upper part of the sequence is characterised by the develop-ment of flat sedimentary bodies composed of laterally coalescent
Fig. 6. Photographs from Mesa 8 to Mesa 9 sites (see Fig. 2 for the details of the log from Mesa 8). A) panoramic view of the Mesa 8 to 9 outcrops showing Units 1 and 2; Unit 2,composed of insect-microbial build-ups, form reef-like geometries including spur and groove alignments; B) Bioherms from Unit 2 formed by the vertical and lateral coalescence ofsuccessive generations of composite build-ups; and C) Sub-circular upper surface of a dome from a reef flat-like structure at the top of Unit 2.
large composite domes that are 3 to 6 m-large and 3 to 8 m-long witheroded, flat and sub-circular to ovoid upper surfaces (Fig. 6C). Theyextend over large areas, up to 14,000 m2 at the top of the easternpart of the plateaus, with a NW–SE long axis that appears perpendic-ular to the shoreline and corresponds to the maximal development ofbuild-ups.
In this unit, the build-ups are generally associated with beige towhite ooid and ostracod rich limestones (F4) and, more occasionally,to floatstone made up of stromatolite fragments (F5) (Fig. 7A,Table 1). Ooid-and-ostracod-rich limestones form 8 to 80 cm-thicktabular beds at the base, between and above insect-microbialbuild-ups and fill the depression between the adjacent build-ups.The lateral continuity of these beds ranges from 1 to 50 m, dependingon the spacing of the build-ups. They correspond to wackestone/grainstone that is predominantly composed of well sorted ooidsand, to a less extent, ostracod shells with an intergranular porositygenerally filled by brownmicrite. They may contain randomly distrib-uted larger (mm to cm-sized) elements including stromatolitic accre-tions, stromatolite desiccation chips or gravels and fragments ofaggregates of insect cases. A single bed of bushy colonies interpretedas tufas (F6, Table 1) occurs at the top of Unit 2 in the western part ofthe study area (Mesa 23; Figs. 2 and 7). The well organised and regu-lar shape of these bushy accretions (Fig. 7) typifies macrophytes inlife position which originally provided a framework for later microbi-al colonisation and carbonate precipitation.
The thickness and composition changes of Units 1 and 2 are clearlyrelated to their position on the margin (Fig. 2). These large scale lat-eral variations allow the differentiation of the proximal and distalparts of the margin. The southern sites, characterised by abundantphytoclast deposits and the occurrence of desiccation features andpedogenetic levels, probably correspond to the internal part of themargin. In contrast, the northern sites, typified by a higher develop-ment of fine-grained marly limestones and a lower accumulation of
phytoclasts, seemingly characterise deeper environments in a moredistal position. An intense fracturation affects the whole carbonate se-quence at the northern sites. The fractures, with no lateral displace-ment, are mainly present where units 1 and 2 are stacked, thusforming a 10–15 m-thick sequence; they form a connected networkwith horizontal, vertical and oblique joints and affect both thebuild-ups and the marly limestones. The abundance of fractures inthis area, close to the edge of the lake margin, could be related tosteeper sedimentary slopes which could have destabilised the sedi-ment pile. An apparent transition to more open facies towards theNorth-East is seemingly associated to the irregularities of the margin,as the paleogeographic maps of the Gosiute Lake indicate a generalEast/South-East opening of the margin (e.g. Chetel and Carroll, 2010).
3.2. Non-lacustrine deposits
The end of the lacustrine margin development (Unit 3) ischaracterised by the formation of travertines (F7) and then by thedeposition of continental siltstone to sandstone (F8). The travertinedeposits occur only in the most elevated parts of the northern marginsites where they form outcrops and small terraces of 0.2 to 2 m-thickand several metres in lateral extent at the edge of the last generationof insect-microbial domes (Figs. 8A and 9A). The travertines generallyconsist of alternating several cm-thick horizontal layers of delicatedendritic accretions and dense micrite (Fig. 8B). The dendritic accre-tions are similar to shrub morphologies described in travertinedeposits (Chafetz and Folk, 1984; Chafetz and Guidry, 1999), andform crusts of up to 6 cm-thick that nucleate on the micrite layerand display downward arborescent growth forms. Travertines alsodisplay laminated globular to columnar fabrics (Fig. 8C) exhibitingsimilarities with speleothem deposits.
Travertine deposits also occur in karstic cavities within the lastmetres of the lacustrine deposits where they form discontinuous 2
Fig. 7. A) Panoramic view of Mesa 23 outcrops showing Units 1, 2 and 3 (see Fig. 2 for the details of the log from Mesa 23). Unit 1 is composed by alternating beige to light brownmassive sandy limestones and grey fine-grained marly limestones (in orange) with isolated build-ups. The top of the Unit 1 is marked by the development of a continuous level oftufas (in purple). Unit 2 comprised abundant insect-microbial build-ups (in yellow). The reworking of the bioherm edges produce massive channelled beds of stromatolite frag-ments floatstone (in red). Unit 3 is composed of siltstone and sandstone. B) 30 cm-thick bushy tufa colonies that coalesce laterally to form a continuous bed; occurrence ofcm-thick micritic horizontal layers within the colonies; C) Slab of a tufa colony showing branches (b) with no visible structure between micritic deposits (m); D) Detailed viewof digitate branches of a colony; and E) Thin section picture displaying the branches (b) composed of crudely laminated micrite. Peloidal micrite (m) fill the spaces between thebranches.
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to 6 cm-thick carbonate crusts with globular to dendritic fabrics. Theynucleate at the top of the cavities and grow downward perpendicularto the substrate formed by insect-microbial build-ups (Fig. 9B and C).The thickness of these crusts is subordinate to the shape and size ofthe cavities. These cavities are also the place of an intense precipita-tion of silica with the occurrence of several mm to cm-thick silica ce-ments that postdate the formation of the carbonate crusts (Fig. 8D).
In thin section, the finely laminated crusts consist of thin and reg-ular concentric bands of brown and light-coloured fibrous calcite(Fig. 8E and G). Some long crystals are subperpendicular to the lami-nation and intersect it. XRD measurements indicate that they aremainly composed of low-magnesian calcite and, to a less extent,dolomite. The dendritic forms correspond to the superposition of sev-eral layers up to 500 μm-thick of acicular aragonite pendant cementsseparated by thin layers of brown micrite (Fig. 9C and D). XRD mea-surements demonstrate that they are mainly composed of aragoniteand, in a lower proportion, of low-magnesian calcite.
Carbonate crusts, up to 15 cm-thick, also occur in open fracturesthat affect the northern part of the margin where their growth isseemingly coeval to the fracture development. They are thicker inthe horizontal joints than in the vertical and oblique ones due totheir prevalent downward growth. They consist of finely laminatedcalcite that grew centripetally in cm-wide fissures. The laminationsare parallel and display various colours (grey, beige, white andbrown) (Fig. 10). These crusts generally evolve from the edges ofthe cavity to its centre with successive cm-thick flat or wavy lamina-tions to globular and stalactitic growth forms (Fig. 10A). They alsodisplay dendritic growth forms (Fig. 10B). In thin section, these pla-nar to globular laminae are composed of thin micrometric, paralleland isopachous layers of fibrous calcite cements separated by darkbrown micrite lines. Some levels display stromatolitic growth forms(e.g. columnar; Fig. 10D and E) and then could correspond to cavityor fissure-dwelling stromatolites (“endostromatolites”; Monty,1982). XRD measurements indicate that they are mainly composed
Fig. 8. Photographs of travertine facies: A) Panoramic view of Mesa 13 outcrops showing Units 1, 2 and 3. Travertines (Unit 3) form small terraces at the edge of the last generationof insect-microbial build-ups (Unit 2); B) Detailed view of the travertines deposits from picture A composed by the superposition of centimetre-thick horizontal layers of dendriticaccretions; C) Finely laminated calcitic layers with globular growth forms displaying a downward growth development; D) Thick grey silica cements that developed in theremaining pore spaces; E) Thin section (cross-polarised light) of a travertine showing thin concentric bands of brown and light-coloured calcite with a radial extinction; F) Thinsection (cross-polarised light) of a travertine displaying fibrous calcite with fanlike morphologies; the porosity is filled by silica cement; and G) Thin section (plan-polarisedlight) of a travertine showing thin and regular concentric bands of brown and light-coloured calcite.
of low-magnesian calcite and, to a less extent, quartz. A widespreadsilicification affects these joints, as demonstrated by the occurrenceof several mm to cm-thick silica cement layers that develop in theremaining pore spaces (Fig. 10B) and by the silicification of somelaminae (Fig. 10C). The authigenic quartz precipitated in the openpores or in some laminae as overgrowths of xenomorphic quartzcrystals forming equant or drusy cements.
Brown and grey fine-grained sandstone and siltstone form thetop of the sequence at most of studied sites although they are gen-erally lacking at the top of travertine deposits. They consist of 1 to5 m-thick superposition of thin platy horizontal beds overlying thelast generation of build-ups. They display planar and ripple cross
laminations but do not display any grading nor pedogenetic alter-ation. The sandstone and siltstone are mainly composed of wellsorted fine sand to silt particles of quartz. XRD measurementsindicate that they are mainly composed of quartz (62%) and in aminor proportion of potassium feldspars (sanidine, 19%), calcite(14.5%), dolomite (3%) and muscovite (1.5%). Their compositionis similar to that of the siliciclastic fraction of the lacustrinesandy limestones from Unit 1 indicating the persistence of thesame external source during the relevant time span. The sandstoneand thin-bedded interlayered siltstone are interpreted as havingbeen deposited in a lower floodplain environment close to thelake margin.
Fig. 9. Photographs of travertine facies: A) Travertines deposits composed by the superposition of layers of dendritic accretions that occur around the last generation of build-ups;B) Centimetre-thick carbonate crust (c) formed by dendritic accretions with a downward growth (shrub-like morphology) that nucleate at the top of insect (i)–microbial (st) domecavities; C) Thin section (plan-polarised light) of light-coloured aragonite pendant cements (c) that developed in the insect (i)–microbial dome cavities; D) Thin section(plan-polarised light) of a dendritic accretion composed by the superposition of several layers of acicular aragonite pendant cements alternating with thin micritic layers.
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4. Discussion
4.1. Insect-microbial build-ups
The particularity of the Little Mesa build-ups is the major contri-bution of the caddisfly cases in their edification. The aggregates of in-sect cases that form the nucleus of the constructions play a prominentrole in their initiation and development as (1) they produce a hard
and stable substrate for the benthic microbial mat development andthen controlled the location and distribution of the build-ups;(2) their shapes influenced the morphology of the build-up; and(3) they enhanced the vertical development of the build-ups as theyform the main volume of the construction. On the other hand, thebenthic microbial mats enhanced the calcification of the caddisflycases and contribute to their stabilisation and preservation. Similarconstructions principally made by insect cases were described in the
Fig. 10. Photographs of the carbonate crusts occurring in the fissures that affect the northern part of the margin. A) Slab of finely laminated calcite with flat laminations to globularand stalactitic growth forms that grew orthogonally toward the cavity centre. B) Detailed views of calcitic accretions with globular and dendritic growth forms; translucent silicacements (s) occur in the remaining pore spaces; C) Thin section (cross-polarised light) of the sample B displaying a concretion that evolves from thin concentric laminae to den-dritic shrub-like crystals composed of radiating branches; some laminae are affected by silicification; D) Slab of a lithoclast (li) encrusted by columnar stromatolites (st) and then bymore regular wavy laminae of calcite (c); and E) Thin section (plan-polarised light) of the columnar stromatolites from picture D.
Early Cretaceous Lake Jahyeri (Korea; Paik, 2005), in the Tipton ShaleMember of the Eocene Green River Formation (Wyoming; Leggitt etal., 2007), in the Oligocene and Miocene lakes from the Limagne gra-ben (Massif Central, France; Hugueney et al., 1990; Bertrand-Sarfatiet al., 1994; Wattine et al., 2003; Wattinne, 2004) and in the modernLake Dziani (Mayotte; GC pers. obs.).
On the contrary to the common stromatolite build-ups that coulddevelop up to 40 m water-depth (e.g. Lake Tanganyika stromatolites;Cohen et al., 1997), the studied build-upswere constrained in very shal-lowmarginal settings, with water depths probably lower than 5 m. It istypified by: (1) the abundant development of caddisfly larvae andpupae that lived in wave-washed littoral areas (Leggitt et al., 2007),probably no more than 50 cm deep, and (2) the morphology of thebuild-ups that generally have a flat top and a preferentially lateral stro-matolitic development around the insect case aggregates. Moreover,frequent temporary periods of subaerial exposure are indicated by theoccurrence of desiccation cracks and chip levels within the build-ups.
The analogy with the modern caddisfly species allows the recon-struction of the build-up development. Trichopterans attach their lar-val and pupal cases to stable substrates once a year in very shallowlittoral areas (Leggitt and Cushman, 2001). As previously observedby Leggitt et al. (2007), the growth of the insect columns in littoralareas of the Gosiute Lake dominated by soft carbonate mud andsand (ooid and ostracod-rich limestones), was predominantlygoverned by the competition for suitable solid substrates (e.g. lithi-fied sands, former build-ups). This results in local gregarious concen-trations of large numbers of closely-spaced vertically-aligned pupalcases which, during lake level rise, led to the construction of erectcolumns or domes formed by stacking of multiple generations ofcases. During lake level highstand, microbial mats developed overthe pre-existing relief and stable substrates corresponding to theaggregates of cases and the prevalent lateral growth of stromatolitesformed an external layer around the aggregates. Lake level fluctua-tions then controlled the development of those build-ups.
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The build-up growth rates are difficult to constrain accurately, buttheir estimate can be helpful to unravel the short term lake levelfluctuations. According to Leggitt and Cushman (2001), one layer ofcases about 1 cm of average thickness would correspond to anannual deposit, so that the edification of a 1 m-thick column couldbe very fast (~100 yr). The estimation of growth rates of Holocenelacustrine stromatolites based on 14C and U series measurementsshows a great variability. The most common ones that have beenreported range from 2.5 to 5 cm/1000 yr for stromatolites from thePavilion Lake, a slightly alkaline freshwater lake (Laval et al., 2000;Brady et al., 2009) and from 5 to 8 cm/1000 yr in stromatolitesfrom the Tanganyika Lake, a middle alkaline open lake (Casanovaand Hillaire-Marcel, 1992). Stromatolite growth rates ranging from6.6 to 39 cm/1000 yr were reported recently from the Walker Lake,a closed basin alkaline lake (Petryshyn et al., 2012). Exceptionallyfast growth rates reaching 3.5 mm yr−1 and up to 3 cm yr−1 haveeven been reported in alkaline lakes respectively Manito Lake (Lastet al., 2010) and Big Soda Lake (Rosen et al., 2004). Considering themost common values, a single insect-microbial build-up composedof a 50 cm-thick stromatolite crust should have been formed in sever-al millennia (6000 to 20,000 yr). Indeed, the installation of well-developed stromatolites requires relatively stable high-stands as itwas especially reported in Quaternary counterparts from lakesTanganyika (East Africa, Casanova and Hillaire-Marcel, 1992) andMagadi/Natron (Kenya, Tanzania; Hillaire-Marcel and Casanova,1987). Taking into account the average values proposed for the mod-ern stromatolites of Pavilion and Tanganyika lakes (i.e. 5 cm/1000 yr)and that the most common stromatolite thicknesses average 50 cm, itis suggested that the duration for the formation of a single build-upgeneration should be ~10,000 yr; this duration corresponds thereforeto that of a transgressive parasequence. In contrast, the duration ofthe associated regressive parasequence is seemingly shorter, as afew yellowish pedogenetically altered levels may have recorded pe-riods of subaerial exposure that were long enough to induce the de-velopment of pedogenetic processes, but too short to produce asignificant erosional surface. Indeed, no indication of long term andlarge scale subaerial exposure, such as karstification or massive ero-sion processes, has been reported. Individual build-ups then do notexhibit a continuous growth but correspond to polygenic structuresthat resulted from successive growth and break phases related tohigh frequency fluctuations of the lake level.
4.2. Controls on the evolution of the lacustrine carbonate margin
Large scale changes affecting the basins during the deposition of theGreen River Formation were triggered by both tectonics and climatechanges (e.g. Roehler, 1992a; Smith et al., 2008). In particular, episodesof crustal deformation and volcanism created drainage diversions andreintegrations which played a key role in determining the hydrologicalbalance of the Green River lacustrine system (Smith et al., 2008). Thesedimentological analysis of the Little Mesa sequence coupled withstratigraphical and chronological data obtained on wells and otherGreen River Basin outcrops (e.g. Roehler, 1973, 1992a,b, 1993; Smithet al., 2003, 2008; Chetel and Carroll, 2010) allows the reconstructionof the architecture of the carbonate margin and its evolution at differ-ent time scales.
4.2.1. Long-term evolution
4.2.1.1. General transgressive sequence. The general evolution of the la-custrine carbonate margin can be constrained through the chronolog-ical frame defined on the Green River Formation (Smith et al., 2003,2004, 2006, 2008). The Little Mesa sedimentary succession corre-sponds to a single sequence of carbonate margin development(LaClede bed of the Laney Member) over the floodplain and mudflatdeposits of the Wasatch Formation. This sequence was related to a
major phase of expansion of the Gosiute Lake which overflowed occa-sionally into the downstream Lake Uinta during periods of lakehighstands (Smith et al., 2008). This transgression was associatedwith the opening of the Gosiute Lake, which caused a change fromthe deposition of evaporite facies in the tectonically isolated GreenRiver Basin during the Wilkins Peak Member (Pietras and Carroll,2006) to the deposition of fluctuating profundal facies in the GreenRiver and Uinta basins during the Laney Member (Smith et al.,2008). The permanence of evaporitic conditions in the PiceanceCreek Basin indicates that this shift from underfilled to balanced-filllake type in the Green River Basin was not driven by a regional cli-matic cooling and/or humidity increase. Indeed, during the periodca. 51–45 Ma, the deposition of evaporites occurred nearly continu-ously in the various Green River basins (Smith et al., 2008) and coin-cided with the end of the Early Eocene Climatic optimum (53–49 Ma;Zachos et al., 2001). During the deposition of the upper Wilkins PeakMember (ca. 50 Ma; Smith et al., 2008) and the lower Laney Member(ca. 49.3 Ma; Smith et al., 2008) that predates the development of thecarbonate margin, the climate was subtropical with mean annualtemperatures of 19.6 ± 2.1 °C and mean annual precipitation of75.8 + 33.2/−23.2 cm (Wilf, 2000). The relevant paleohydrologicalchanges were induced by increasing tectonic activity in the ChallisVolcanic field and the Absaroka Volcanic Province located North ofthe Green River Basin (see review in Doebbert et al., 2010) causingthe capture of an extrabasinal Northern drainage by the GosiuteLake ca. 49.5 Ma (Smith et al., 2008; Chetel and Carroll, 2010). Thismodification from an East to West drainage to a North-West toSouth-East drainage pattern between 49.5 and 48.9 Ma induced anew water inflow and resulted in the progressive filling of the basinfrom the North-West corner by volcaniclastic sediments that finallycaused the end of the development of the carbonate margins of theGosiute Lake (Chetel and Carroll, 2010).
The general lacustrine expansion induced the development ofwide carbonate margins including laterally extensive shallow-waterareas characterised by the growth of stromatolites and the coevaldeposition of associated oolitic and skeletal grainstone beds thatcompose the major facies of the Little Mesa sequence. Theseshallow-water depositional areas developed only in the LowerLaClede Bed, indicating that the Little Mesa sequence was depositedduring a period that started slightly after 49.62 ± 0.17 Ma (i.e. theage of the Sixth Tuff bed) and ended slightly after 48.94 ± 0.18 Ma(i.e. the age of the Analcite Tuff; Smith et al., 2008; Doebbert et al.,2010). In contrast, the sedimentation in the centre of the basinconsisted of profundal facies during the upper LaClede Bed, withoutany evidence of shallow-water facies or subaerial exposure(Doebbert et al., 2010). A more accurate timing regarding the deposi-tion of the Little Mesa sequence can be assessed based on the strati-graphic evolution of the Laney Member (Chetel and Carroll, 2010;Doebbert et al., 2010) as the studied sequence records a transgressionreaching the north-western part of the Green River Basin, thus corre-sponding to the configuration of the Gosiute Lake at ca. 49.5 Ma. Theend of the Little Mesa carbonate sequence which is characterised bythe deposition of travertines and fluvial siltstone/sandstone couldbe coeval to the complete desiccation of the Gosiute Lake during theBuff Marker Bed, at ca. 49.3 Ma. This would imply that the studied la-custrine marginal sequence could have been deposited in 200 ka orless (49.5–49.3 Ma time window).
4.2.1.2. Development stages of the carbonate lacustrine margin4.2.1.2.1. First stage: onset and development of the margin. During
the first stage of the margin evolution (Fig. 11 t1 and t2), correspond-ing to Unit 1, the drainage system was active with the deposition ofsandy limestones in subaqueous depressions. These limestones in-clude abundant carbonate grains (ostracods and ooids) and largerpoorly-sorted elements (lithoclasts, stromatolite and insect casesfragments) (Fig. 4) that were reworked from the carbonate lacustrine
Fig. 11. Large scale evolution of the lacustrine margin: (t1) and (t2) Development ofsubaqueous channels filled by sandy limestones during the lacustrine transgression;isolated build-ups developed on the elevated parts and on the edges of the channels.(t3) The cessation of the drainage systems and the decreasing lake-level rise inducedthe progradation of the build-ups over the channels. (t4) The following lake levelrise led to the vertical stacking of several generations of build-ups and the develop-ment of large bioherms. (t5) The stabilisation of the lake level induced the formationof a large reef-flat structure. The edges of the bioherms were reworked locally, produc-ing massive channelled beds of stromatolite fragments floatstone.
margin by rivers entering the lake. The occurrence of such depositsindicates that a carbonate composed of insect-microbial build-upsassociated to ooid-rich limestones was already present laterally andupstream from these channels. These drainage processes did nottransport large amounts of external particles as siliciclastic grainsform generally less than 5% of the matrix, thus indicating that theerosion did not affect significantly the surrounding basement highs;furthermore, no coarse siliciclastic deposits have been reported inthe study area. However, the occurrence of large phytoclasts impliesthat the nearby coastal areas of the lake were covered by an extensivevegetation, in agreement with the climatic reconstructions involvinghumid subtropical conditions (Wilf, 2000).
The sandy limestone beds display an aggradational pattern andthe associated build-ups developed on the elevated parts and on theedges of the channels, where they remained scattered and neverformed large bioherms. The decreasing activity of the drainage
system and its subsequent cessation due to the complete infill ofthe channels induced a major change in the depositional evolutionof the carbonate margin. This could reflect the interfering effects ofthe general lacustrine transgression and the capture or deviation ofthe drainage system, possibly related to the modification of thebasin drainage system associated to the capture of a Northern drain-age that occurred between 49.5 and 48.9 Ma (Chetel and Carroll,2010).
4.2.1.2.2. Second stage: extensive development of the insect-microbialbioconstructions. Although water depth did not change significantlybetween Units 1 and 2, the modification of the depositional systemassociated with the cessation of the reworking of the marginal car-bonate deposits promoted the development of the bioconstructionsthat characterise Unit 2. The isolated build-ups located upstreamand on the channel edges prograded laterally and eventually coveredthe former channels (Fig. 11 t3). This progradation may indicate a pe-riod of reduced accommodation space probably associated to a slowerrise in lake level coeval to the cessation of the local river discharge.
The following lake level rise led to the vertical stacking of severalgenerations of build-ups and the development of large bioherms overthe channelled deposits (Figs. 6 and 11 t4), thus inducing an inversionof the initial topography. The formation of the large reef-flat structurewith flat-top build-ups ending the sequence was produced by thelateral growth of the build-ups during the lake level high-stand. Thelateral occurrence of a single bed of tufas may be interpreted asrepresenting algal tufts in growth position that formed a dense algalmeadow covering the lake bottom. Their development occurredduring this lake level high-stand because the colonisation of thelake floor by a stable benthic flora requires a short interval of lakelevel stabilisation (Benson, 1994). Similar development of porousbranching tufas during lake level highstand has been reported inmodern deposits from Pyramid Lake (Benson, 1994).
After this episode, the edges of the bioherms were locallyreworked to form massive channelled beds composed of stromatolitefragments (Fig. 11 t5). Larger gravity mechanisms occurred at theedges of the lacustrine margin that were characterised by steepersedimentary slopes which may have induced the destabilisation ofthe accumulated sediment pile and the subsequent formation of net-works of fractures in the poorly consolidated sediments on the north-ern part of the margin. Fluid expulsions caused by sediment overloadmay have also been associated with the opening of the fractures.
4.2.1.2.3. End of the lacustrine episode. The carbonate marginal se-quence ended by the development of travertine deposits and by thedeposition of alluvial siltstone and sandstone. The travertines formedsmall terraces on the edge of the last generation of build-ups, thus in-dicating that the top of the margin was subaerially exposed in thenorthern part of the area. The intercalation of small and thin stromat-olite colonies within these levels suggests the local occurrence ofcoeval small ponds. The travertine deposits are not related to faultsor spring vents, thus indicating that they did not correspond tohot-water travertine deposits. Their formation was rather related tolocal water flow and seepage through the insect-microbial construc-tions during the subaerial exposure of the top of the lacustrine margin.The spring waters that favoured the development of these meteogenictravertine deposits and of structures referred-to endostromatolites inopen fractures should have been enriched in Ca during their circulationthrough the carbonate build-ups although we cannot preclude the Capartly derive from the carbonate mudflats that surrounded the lake(Surdam and Stanley, 1979). The occurrence of thick silica cements inthe remaining pore spaces of the travertine deposits indicates a changein fluid composition of the groundwaters which became saturated in Sidue to the weathering of the alluvial siliciclastic deposits.
The final thin-bedded siltstone and sandstone facies is interpretedto have been deposited in a low-energy lower floodplain environ-ment. The lack of root structure and pedogenetic developmentsuggest that this environment was poorly drained and could have
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therefore corresponded to swamp or interdistributary ponds (Cheteland Carroll, 2010). Their late deposition throughout the studiedarea is related to the progradation of the alluvial facies over the la-custrine sequence, therefore indicating a continuous decrease inlake level. This period of subaerial exposure could have been causedby the complete desiccation of the Gosiute Lake typified by the BuffMarker Bed in theWashakie Basin (Doebbert et al., 2010). The sourceof the siliciclastic material could correspond to pre-Cretaceous base-ment rocks from the Sevier fold-and-thrust belt located on the west-ern side of the Green River Basin. The Laney sandstone strata of anearby site (Fontanelle Reservoir) has been interpreted as resultingfrom the erosion of the Sevier fold-and-thrust belt (Chetel andCarroll, 2010).
4.2.2. Short-term evolutionThe short-term evolution of the carbonate build-ups may be
reconstructed from their internal structure and organisation, andfrom the lateral and vertical facies distribution. The two lacustrine se-quences are characterised by short scale facies variations that indicaterapid environmental changes related to high frequency fluctuationsof the lake level superimposed on the general transgressive trend.In Unit 1, these facies variations correspond to repetitive successionsof m-thick beds of massive coarse sandy limestones and dm-thickbeds of fine-grained marly limestones that could reflect respectivelyshallow high energy, and deeper and quieter depositional environ-ments. In Unit 2, typical facies variations are defined by the successivedevelopment of insect cases and stromatolites characterising adeepening-upward sequence. The lake-level fluctuations associatedwith these dm to m-scale facies alternations were of small amplitude,probably less than 1 to 2 m, but have greatly impacted the arealextent of the marginal carbonate deposits as the Gosiute Lake wascharacterised by a very smooth topography. Such fluctuations inlake level were principally driven by climate changes. The tectonicactivity was low during the studied period (Doebbert et al., 2010),as the compression associated with the motion of the eastern Sevierthrusts ended ca. 50 Ma (DeCelles, 2004) and the Laramide WindRiver Thrust fault was inactive by ca. 49–50 Ma (Steidtmann andMiddleton, 1991). Climatically driven high frequency lake level varia-tions are common in balanced-fill lake basins due to modification ofthe balance between on-lake precipitations, lake evaporation andcatchment runoff (Bohacs et al., 2000).
The study of the structure and composition of the build-ups indi-cating high frequency changes in their development allows to betterdefine the short-term lake-level fluctuations. The duration of a trans-gressive cycle deduced from the modern stromatolites growth ratesand estimated to ~10,000 yr (see Section 4.1) can be compared tothe duration of expansion–contraction lacustrine cycles recorded inthe underlying Wilkins Peak Member during an underfilled basinphase of Gosiute Lake (Carroll and Bohacs, 1999; Pietras and Carroll,2006). These cycles consist of up to four successive facies associa-tions: littoral, profundal–sublittoral, palustrine and salt pan describedin Pietras and Carroll (2006). These cycles were first interpreted as19–23 ka precessional cycles (e.g. Roehler, 1993; Fischer et al.,2004) but 40Ar/39Ar geochronology of tuff beds (Smith et al., 2003,2004, 2006) provided the opportunity to better constrain the averageduration of each cycle, ranging from 10,000 yr to less than 1000 yr,thus too short to relate it to a precessional forcing (Pietras andCarroll, 2006). The duration of such lacustrine expansion and contrac-tion cycles is in good agreement with the results that we obtained onbuild-up development patterns. Although the origin of the WilkinsPeak cycles is not clear, they could be linked either (1) to changesin shortwave radiations that impact lake evaporation (Morrill et al.,2001), or (2) to autogenic cycles related to geomorphic instability ofthe surrounding landscape modulating the flux of water andsediment to downstream lake (Hasbargen and Paola, 2000; Pietrasand Carroll, 2006). Because the study site is protected from outer
sedimentary inputs, we conclude that a climate forcing with periodi-cal changes of the precipitation/evaporation ratio may better ac-count for the interpretation of such sub-Milankovitch highfrequency sedimentary cyclicity, even if the periodicity is less wellapparent in the build-ups than in the profundal deposits due to com-plex growth features.
Lake-level fluctuations on a shorter time scale are also recorded inthe build-ups. Erosional surfaces and desiccation features recordedrespectively in columns of insect cases and in stromatolitic crusts(Fig. 3E) record short periods of subaerial exposure as a consequenceof annual to centennial lake-level fluctuations. The Great Salt Lake(Utah) provides a valuable recent counterpart to better understandthe impact of such high frequency lake level changes on the growthof composite microbial build-ups. Between 1860 and 2005, this lakerecorded 30 to 40 yr long cycles of 3 to 6 m in amplitude and annualcycles of around 60 cm in amplitude more recently (from USGSwebsite and authors unpublished observations). More important fluc-tuations have been reported in Lake Malawi whose water level variesannually between 1 and 3 m (Lyons et al., 2011). These annual to cen-tennial fluctuations are governed by very short-term variations inmean annual precipitation. In a shallow margin with gentle slopessuch as those of the Great Salt Lake or the Gosiute Lake, even smallfluctuations in the water surface elevation may induce extensivechanges in the surface of the lake. For example, at its 1963 historicallowest level, the extent of the Great Salt Lake was reduced to 50% ofits average surface. The marginal areas and the carbonate depositionare greatly impacted by these very high frequency fluctuations thatinduced periodical desiccation and erosion of the build-ups separatedby periods of renewed growth at least in the more marginal settings.However, these short-lived episodes of subaerial exposure didnot induce the formation of well-expressed erosional morphologiesbut promoted the development of parasequences composed ofinterlayered oolitic deposits and mud-cracked stromatolitic layers,sometimes truncated and reworked as desiccation chips.
5. Conclusions
The sedimentological study of the Gosiute Lake carbonate marginsfrom the mid Eocene Green River Fm (Lower LaClede Bed) allows thereconstruction of the development of a microbial-dominated marginin a transgressive setting.
(1) The most prominent feature of this carbonate margin lies in thelack of siliciclastic components and the deposition of a carbonatesequence characterised by the extensive growth of microbialbuild-ups intimately associated to aggregates of insect larvalcases that lead to the formation of large reef-like morphologies.The insect-microbial build-ups are associated with ooid andostracod-rich limestones, stromatolite fragment floatstone andtufas. These build-ups are specific as they are mainly composedof abundant insect cases that constitute the nucleus of the struc-ture and control the volume and the shape of the final construc-tions. Stromatolites form an external crust and represent the laststage of build-up development. These build-ups do not display acontinuous growth but correspond to polygenic structuresresulting from successive growth and break phases that can beused to unravel short-term environmental changes. The lateraland vertical coalescences of these constructions lead to the for-mation of extensive carbonate bodies with reef-like geometriesincluding large flat-top composite mounds and spurs and grovesstructures.
(2) The large-scale evolution of the margin was controlled both bythe lacustrine expansion associated to the opening of the lakesystem and new water inflow in the basin from the North volca-nic provinces and by local drainage diversions. The development
of the carbonate margin over the alluvial deposits of theWasatchFm occurred in three main stages:
(a) During its initiation, the sedimentary systemwas dominated bychannelled detrital carbonates associated to a local drainagesystem. These deposits were mainly reworked from the up-stream carbonate margin by the river currents entering thelake, whereas build-ups were scattered and never formedlarge bioherms.
(b) The transition between the first and second stage coincidedwith the interruption of the drainage system activity thatpromoted the development of abundant bioconstructions overthe former channels. The abundance of the build-ups increasedupwards and they coalesced to form reef-like geometries in-cluding bioherms, spur and groove alignments and reef-flatstructures.
(c) The end of the lacustrine episode was due to the subaerial ex-posure of the margin leading to the development of travertinedeposits and the progradation of alluvial siltstone and sand-stone.
(3) The short-scale facies variations and the build-up growthwere controlled by rapid environmental changes includingclimatically-driven high-frequency lake level fluctuationssuperimposed to a general transgressive trend. These decimetreto metre-scale fluctuations greatly impacted these shallow mar-gins exhibiting a smooth topography and gentle slopes by in-ducing periodical subaerial exposure and drowning events.Several lake-level fluctuation frequencies controlled the growthof the build-ups: (a) environmental changes on millennial topluri-millenial time scales controlled the establishment of a gen-eration of build-ups and its subsequent expansion in time andspace, while (b) environmental changes on annual to decadaltime scales controlled the internal growth of a build-up thatwas periodically interrupted by short-lived episodes of subaerialexposure.
This study emphasises the heterogeneity of marginal lacustrinecarbonate sequences that are very sensitive to environmentalchanges on various time scales.
This study shows an example of a lacustrine margin characterisedby the extensive development of a true carbonate platform dominat-ed by composite microbial-insect build-ups with no significantsiliciclastic contribution, during an episode of lacustrine expansion.A deeper knowledge of the mechanisms involved in the evolution ofthe lacustrine margins and their sedimentary architecture could beobtained by comparing different types of margins such as those thatformed in the other Green River sub-basins and in basins of differentages.
Acknowledgements
This work has been made possible thanks to the funding fromTOTAL. The authors would like to thank Philippe Lapointe, CécilePabian and Emmanuelle Poli for their fruitful discussions on theproject.
References
Beck, R.A., Vondra, C.F., Filkins, J.E., Olander, J.D., 1988. Syntectonic sedimentation andLaramide basement thrusting, Cordilleran foreland: timing of deformation. In:Schmidt, C.J., Perry Jr., W.J. (Eds.), Interaction of the Rocky Mountain Forelandand the Cordilleran Thrust Belt: Geol. Soc. Am. Mem, 171, pp. 465–487.
Benson, L.V., 1994. Carbonate deposition, Pyramid Lake subbasin, Nevada: 1. Sequence offormation and elevational distribution of carbonate deposits (tufas). Palaeogeography,Palaeoclimatology, Palaeoecology 109, 55–87.
Bertrand-Sarfati, J., Freytet, P., Plaziat, J.C., 1994. Microstructures in Tertiary nonmarinestromatolites (France). Comparison with Proterozoic. In: Bertrand-Sarfati, J.,Monty, C. (Eds.), Phanerozoic Stromatolites II. Kluwer Academic Publishing,Dordrecht, The Netherlands, pp. 155–191.
Bohacs, K.M., Carroll, A.R., Neal, J.E., Mankiewicz, P.J., 2000. Lake-basin type, sourcepotential, and hydrocarbon character: an integrated sequence stratigraphic–geochemical framework. In: Gierlowski-Kordesch, E.H., Kelts, K.R. (Eds.), LakeBasins Through Space and Time: Am. Assoc. Petrol. Geol., Studies in Geology, 46,pp. 3–34.
Bradley, W.H., 1929. Algal reefs and oolites of the Green River Formation. GeologicalSurvey Professional Paper 154, 203–223.
Bradley, W.H., 1931. Origin and microfossils of the oil shale of the Green River Formationof Colorado and Utah. Geological Survey Professional Paper 168, 1–58.
Bradley, W.H., 1964. Geology of the Green River and associated Eocene rocks in south-western Wyoming and adjacent parts of Colorado and Utah. Geological SurveyProfessional Paper 496-A, 1–86.
Brady, A., Slater, G.F., Laval, B., Lim, D., 2009. Constraining carbon sources and growthrates of freshwater microbialites in Pavilion Lake using 14C analysis. Geobiology7, 544–555.
Camoin, G., Casanova, J., Rouchy, J.M., Blanc-Valleron, M.M., Deconinck, J.F., 1997. Envi-ronmental controls on perennial and ephemeral carbonate lakes: the centralpalaeo-Andean Basin of Bolivia during Late Cretaceous to early Tertiary times.Sedimentary Geology 113, 1–26.
Carozzi, A.V., 1962. Observations on algal biostromes in the Great Salt Lake, Utah. Journalof Geology 70, 246–252.
Carroll, A.R., Bohacs, K.M., 1999. Stratigraphic correlation of ancient lakes: balancingtectonic and climatic controls. Geology 27, 99–102.
Carroll, A.R., Doebbert, A.C., Booth, A.L., Chamberlain, C.P., Rhodes-Carson, M.K., Smith,M.E., Johnson, C.M., Beard, B.L., 2008. Capture of high altitude precipitation by alow altitude Eocene lake, western U.S. Geology 36, 791–794. http://dx.doi.org/10.1130/G24783A.1.
Casanova, J., Hillaire-Marcel, C., 1992. Late Holocene hydrological history of LakeTanganyika, East Africa, from isotopic data on fossil stromatolites. Palaeogeography,Palaeoclimatology, Palaeoecology 91, 35–48.
Casanova, J., Nury, D., 1989. Biosédimentologie des stromatolites fluvio-lacustres dufossé oligocène de Marseille. Bulletin de la Société Géologique de France 8,1173–1184.
Chafetz, H.S., Folk, R.L., 1984. Travertines: depositional morphology and the bacteriallyconstructed constituents. Journal of Sedimentary Petrology 54, 289–316.
Chafetz, H.S., Guidry, S.A., 1999. Bacterial shrubs, crystal shrubs, and ray-crystal shrubs:bacterial vs. abiotic precipitation. Sedimentary Geology 126, 57–74.
Chetel, L.M., Carroll, A.R., 2010. Terminal infill of Eocene Lake Gosiute, Wyoming, USA.Journal of Sedimentary Research 80 (6), 492–514.
Clyde, W.C., Sheldon, N.D., Koch, P.L., Gunnell, G.F., Bartels, W.S., 2001. Linking theWasatchian/Bridgerian boundary to the Cenozoic global climate optimum; newmagnetostratigraphic and isotopic results from South Pass,Wyoming. Palaeogeography,Palaeoclimatology, Palaeoecology 167, 175–199.
Cohen, A., Thouin, C., 1987. Nearshore carbonate deposits in Lake Tanganyika. Geology15, 414–418.
Cohen, A.S., Talbot, M.R., Awramik, S.M., Dettman, D.L., Abell, P., 1997. Lake level andpaleoenvironmental history of Lake Tanganyika, Africa, as inferred from lateHolocene and modern stromatolites. Geological Society of America Bulletin 109,444–460.
DeCelles, P.G., 2004. Late Jurassic to Eocene evolution of the cordilleran thrust belt andforeland basin system, western U.S.A. American Journal of Science 304, 105–168.http://dx.doi.org/10.2475/ajs.304.2.105.
Dickinson, W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., Lundin, E.R., McKittrick, M.A.,Olivares, M.D., 1988. Paleogeographic and paleotectonic setting of Laramidesedimentary basins in the central Rocky Mountain region. Geological Society ofAmerica Bulletin 100, 1023–1039.
Doebbert, A.C., Carroll, A.R., Mulch, A., Chetel, L.M., Chamberlain, C.P., 2010. Geomorphiccontrols on lacustrine isotopic compositions: evidence from the Laney Member, GreenRiver Formation (Wyoming). Geological Society of America Bulletin 122, 236–252.
Donsimoni, M., 1975. Etude des calcaires concrétionnés lacustres de l'Oligocènesupérieur et de l'Aquitanien du bassin de Limagne (Massif Central, France). Thèse3e cycle (unpublished).Université de Paris VI, Paris.
Eugster, H.P., Surdam, R.C., 1973. Depositional environments of the Green River Formation ofWyoming—a preliminary report. Geological Society of America Bulletin 84, 1115–1120.
Evanoff, E., Brand, L.R., Murphey, P.C., 1998. Bridger Formation (middle Eocene) ofsouthwest Wyoming: widespread marker units and subdivisions of Bridger Bthrough D. Dakoterra 5, 115–122.
Fischer, A.G., Grippo, A., Teerman, S., 2004. Eocene Green River Formation: orbital cyclesversus argon–argon chronology. Geological Society of America Abstracts withPrograms 36 (5), 99.
Halley, R.B., 1977. Ooid fabric and fracture in the Great Salt Lake and the geologicrecord. Journal of Sedimentary Petrology 47, 1099–1120.
Hasbargen, L.E., Paola, C., 2000. Landscape instability in an experimental drainagebasin. Geology 28, 1067–1070.
Hayden, F.V., 1869. Preliminary field report (third annual) of the United States. Geolog-ical Survey of Colorado and NewMexico.United States Government Printing Office,Washington (155 pp.).
Hillaire-Marcel, C., Casanova, J., 1987. Isotope hydrology and paleohydrology of the Magadi(Kenya)–Natron (Tanzania) basin during the late Quaternary. Palaeogeography,Palaeoclimatology, Palaeoecology 58, 155–181.
Hugueney, M., Tachet, H., Escuillé, F., 1990. Caddisfly pupae from the Miocene indusiallimestone of Saint-Gérand-Le-Puy, France. Palaeontology 33, 495–502.
Johnson, R.C., 1985. Early Cenozoic history of the Uinta and Piceance Creek basins, Utahand Colorado, with special reference to the development of Eocene Lake Uinta. In:
144 C. Seard et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 381–382 (2013) 128–144
Flores, R.M., Kaplan, S.S. (Eds.), Cenozoic Paleogeography of the West-CentralUnited States, Rocky Mountain Section. Society of Economic Petrologists andMineralogists, Denver, pp. 247–276.
King, C., 1878. Systematic geology. U.S. Geological Exploration of the Fortieth ParallelReport, 1 (803 pp.).
Last, F.M., Last, W.M., Halden, N.M., 2010. Carbonate microbialites and hardgroundsfrom Manito Lake, an alkaline, hypersaline lake in the northern Great Plains ofCanada. Sedimentary Geology 225, 34–49.
Laval, B., Cady, S.L., Pollack, J.C., McKay, C.P., Bird, J.S., Grotzinger, J.P., Ford, D.C., Bohm,H.R., 2000. Modern freshwater microbialite analogues for ancient dendritic reefstructures. Nature 407 (6804), 626–629.
Leggitt, V.L., Cushman Jr., R.A., 2001. Complex caddisfly-dominated bioherms from theEocene Green River Formation. Sedimentary Geology 145, 377–396.
Leggitt, V.L., Loewen, M.A., 2002. Eocene Green River Formation “Oocardium tufa”reinterpreted as complex arrays of calcified caddisfly (Insecta: Trichoptera) larvalcases. Sedimentary Geology 148, 139–146.
Leggitt, V.L., Biaggi, R.E., Buchheim, H.P., 2007. Palaeoenvironments associated withcaddisfly dominated microbial-carbonate mounds from the Tipton Shale Memberof the Green River Formation: Eocene Lake Gosiute. Sedimentology 54, 661–699.
Lillegraven, J.A., 1993. Correlation of Paleogene strata across Wyoming— a users' guide.In: Snoke, A.W., et al. (Ed.), Geology of Wyoming: Geological Survey of WyomingMemoir, 5, pp. 414–477.
Link, M.H., Osborne, R.H., Awramik, S.M., 1978. Lacustrine stromatolites and associatedsediments of the Pliocene Ridge Route Formation, Ridge basin, California. Journal ofSedimentary Petrology 48, 143–158.
Lyons, R.P., Scholtz, C.A., Buoniconti, M.R., Martin, M.R., 2011. Late Quaternary stratigraphicanalysis of the Lake Malawi Rift, East Africa: an integration of drill-core and seismic-reflection data. Palaeogeography, Palaeoclimatology, Palaeoecology 303, 20–37.
McCarroll, S.M., Flynn, J.J., Turnbull, W.D., 1996. Biostratigraphy and magnetostratigraphyof the Bridgerian–Uintan Washakie Formation, Washakie Basin, Wyoming. In:Prothero, D.R., Emry, R.J. (Eds.), The Terrestrial Eocene–Oligocene Transition inNorth America. Cambridge University Press, Cambridge, UK, pp. 25–39.
Monty, C.L.V., 1982. Cavity or fissure dwelling stromatolites (endostromatolites) fromBelgian Devonian mud mounds (extended abstract). Annales de la SociétéGéologique de Belgique 105, 343–344.
Morrill, C., Small, E.E., Sloan, L.C., 2001. Modeling orbital forcing of lake level change:(Eocene), North America. Global and Planetary Change 29, 57–76.
Paik, I.S., 2005. The oldest record of microbial-caddisfly bioherms from the Early Creta-ceous Jinju Formation, Korea: occurrence and palaeoenvironmental implications.Palaeogeography, Palaeoclimatology, Palaeoecology 218, 301–315.
Palma, R.M., 2000. Lacustrine facies in the Upper Cretaceous Balbuena subgroup (SaltaGroup): Andina basin, Argentina. In: Gierlowski-Kordesch, E.H., Kelts, K.R. (Eds.),Lake Basins Through Space and Time: AAPG Studies in Geology, 46, pp. 323–328.
Pedley, M., 2009. Tufas and travertines of the Mediterranean region: a testing groundfor freshwater carbonate concepts and developments. Sedimentology 56, 221–246.
Pedone, V.A., Folk, R.L., 1996. Formation of aragonite cement by nannobacteria in theGreat Salt Lake Utah. Geology 24, 763–765.
Petryshyn, V.A., Corsetti, F.A., Berelson, W.M., Beaumont, W., Lund, S.P., 2012. Stromat-olite lamination frequency, Walker Lake, Nevada: implications for stromatolites asbiosignatures. Geology 40, 499–502. http://dx.doi.org/10.1130/G32675.1.
Pietras, J.T., Carroll, A.R., 2006. High-resolution stratigraphy of an underfilled lakebasin: Wilkins Peak Member, Eocene Green River Formation, Wyoming, U.S.A.Journal of Sedimentary Research 76, 1197–1214.
Rhodes, M.K., 2002. Lacustrine stratigraphy and strontium isotope geochemistry of theLaney member, Green River Formation, southwestern Wyoming. Ph.D. thesisUniversity of Wisconsin, Madison (367 pp.).
Rhodes, M.K., Carroll, A.R., Pietras, J.T., Beard, B.L., Johnson, C.M., 2002. Strontiumisotope record of paleohydrology and continental weathering, Eocene GreenRiver Formation, Wyoming. Geology 30, 167–170.
Rhodes, M.K., Malone, D.H., Carroll, A.R., 2007. Sudden desiccation of Lake Gosiute at49 Ma: downstream record of Heart Mountain faulting? Mountain Geologist 44, 1–10.
Robinson, P., Gunnell, G.F., Walsh, S.L., Clyde, W.C., Storer, J.E., Stucky, R.K., Froehlich, D.J.,Ferrusquia-Villafranca, I., McKenna, M.C., 2004. Wasatchian through Duchesneanbiochronology. In: Woodburne, M.O. (Ed.), Late Cretaceous and Cenozoic Mammalsof North America. Columbia University Press, New York, pp. 106–155.
Roehler, H.W., 1973. Stratigraphic divisions and geologic history of the Laney memberof the Green River Formation in the Washakie Basin in southwestern Wyoming.U.S. Geological Survey Bulletin 1372-E (28 pp.).
Roehler, H.W., 1989. Correlation of surface sections of the intertongued EoceneWasatch and Green River Formations along the western margins of the GreaterGreen River Basin in southwest Wyoming. US Geol. Surv. Misc. Field Studies MapMF-2103.
Roehler, H.W., 1991. Revised stratigraphic nomenclature for the Wasatch and GreenRiver Formations of Eocene Age, Wyoming, Utah, and Colorado. United StatesGeological Survey Professional Paper 1506-B (38 pp.).
Roehler, H.W., 1992a. Correlation, composition, areal distribution, and thickness of Eo-cene stratigraphic units, Greater Green River Basin, Wyoming, Utah, and Colorado.United States Geological Survey Professional Paper 1506-E (49 pp.).
Roehler, H.W., 1992b. Description and correlation of Eocene rocks in stratigraphic ref-erence sections for the Green River and Washakie basins, southwest Wyoming.United States Geological Survey Professional Paper 1506-D (83 pp.).
Roehler, H.W., 1993. Eocene climates, depositional environments, and geography,greater Green River basin, Wyoming, Utah, and Colorado. United States GeologicalSurvey Professional Paper 1506-F (74 pp.).
Rosen, M.R., Arehart, G.B., Lico, M.S., 2004. Exceptionnally fast growth rate of b100-yr-oldtufa, Big Soda Lake, Nevada: implications for using tufa as a paleoclimate proxy.Geology 32, 409–412.
Rouchy, J.M., Servant, M., Fournier, M., Causse, C., 1996. Extensive carbonate algalbioherms in upper Pleistocene saline lakes of the central Altiplano of Bolivia.Sedimentology 43, 973–993.
Smith, M.E., Singer, B.S., Carroll, A.R., 2003. 40Ar/39Ar geochronology of the EoceneGreen River Formation, Wyoming. Geological Society of America Bulletin 115,549–565.
Smith, M.E., Singer, B., Carroll, A.R., 2004. 40Ar/39Ar geochronology of the Eocene GreenRiver Formation, Wyoming. Reply. Geological Society of America Bulletin 116,253–256.
Smith, M.E., Singer, B., Carroll, A., Fournelle, J.H., 2006. High-resolution calibration ofEocene strata: 40Ar/39Ar geochronology of biotite in the Green River Formation.Geology 34, 393–396. http://dx.doi.org/10.1130/G22265.1.
Smith, M.E., Carroll, A.R., Singer, B.S., 2008. Synoptic reconstruction of a major ancientlake system: Eocene Green River Formation, western United States. GeologicalSociety of America Bulletin 120, 54–84.
Steidtmann, J.R., Middleton, L.T., 1991. Fault chronology and uplift history of the south-ern Wind River Range, Wyoming: implications for Laramide and post-Laramidedeformation in the Rocky Mountain foreland. Geological Society of America Bulle-tin 103, 472–485.
Surdam, R.C., Stanley, K.O., 1979. Lacustrine sedimentation during the culminatingphase of Eocene Lake Gosiute, Wyoming (Green River Formation). GeologicalSociety of America Bulletin 90, 93–110.
Tauxe, L., Gee, J., Gallet, Y., Pick, T., Bown, T., 1994. Magnetostratigraphy of theWillwood Formation, Bighorn Basin, Wyoming: new constraints on the locationof Paleocene/Eocene boundary. Earth and Planetary Science Letters 125, 159–172.
Tiercelin, J.J., Soreghan, M., Cohen, A.S., Lezzar, K.E., Bouroullec, J.L., 1992. Sedimenta-tion in large rift lakes: example from the Middle Pleistocene–Modern deposits ofthe Tanganyika Trough, East African Rift System. Bulletin des Centres deRecherches Exploration–Production Elf-Aquitaine 16 (1), 83–111.
Wattinne, A., 2004. Evolution d'un environnement carbonaté lacustre à bioconstructionsen Limagne bourbonnaise Oligo-miocène (massif central, France). Thèse de 3ecycle.Muséum National d'Histoire Naturelle, Paris (195 pp.).
Wattinne, A., Vennin, E., De Wever, P., 2003. Evolution d'un environnement carbonatélacustre à stromatolithes, par l'approche paléo-écologique (carrière de Montaigu-le-Blin, bassin des Limagnes, Allier, France). Bulletin de la Société Géologique deFrance 174, 49–66.
Wilf, P., 2000. Late Paleocene–early Eocene climate changes in southwesternWyoming: paleobotanical analysis. Geological Society of America Bulletin 112,292–307.
Wing, S.L., Bown, T.M., Obradovich, J.D., 1991. Early Eocene biotic and climatic changein interior western North America. Geology 19, 1189–1192.
Wood, H.E., Chaney, R.W., Clark, J., Colbert, E.H., Jepson, G.L., Reedside, J.B., Stock, C.,1941. Nomenclature and correlation of the North American continental Tertiary.Geological Society of America Bulletin 52, 1–48.
Zachos, J.C., Pagani, M., Sloan, L.C., Thomas, E., Billups, K., 2001. Trends, rhythms, andaberrations in global climate 65 Ma to present. Science 292, 686–693. http://dx.doi.org/10.1126/science.1059412.
