The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition
Eileen D. GROGANBiology Department, St. Joseph’s University, Philadelphia Pa 19131 (USA)
Research Associate, The Academy of Natural Sciences in Philadelphia (USA)[email protected]
Richard LUNDResearch Associate, Section of Vertebrate Fossils,
Carnegie Museum of Natural History (USA)
Grogan E. D. & Lund R. 2002. — The geological and biological environment of the BearGulch Limestone (Mississippian of Montana, USA) and a model for its deposition.Geodiversitas 24 (2) : 295-315.
ABSTRACTThe Bear Gulch Limestone (Heath Formation, Big Snowy Group, FergusCounty, Montana, USA) is a Serpukhovian (upper Mississippian, NamurianE2b) Konservat lagerstätte, deposited in the Central Montana Trough, atabout 12° North latitude. It contains fossils from a productive Paleozoicmarine bay including a diverse biota of fishes, invertebrates, and algae. Wedescribe several new biofacies: an Arborispongia-productid, a filamentous algaland a shallow facies. The previously named central basin facies and upper-most zone are redefined. We address the issue of fossil preservation, superblydetailed for some of the fish and soft-bodied invertebrates, which cannot beaccounted for by persistent anoxic bottom conditions. Select features of thefossils implicate environmental conditions causing simultaneous asphyxiationand burial of organisms. The organic-rich sediments throughout the centralbasin facies are rhythmically alternating microturbidites. Our analyses suggestthat these microturbidites were principally generated during summer mon-soonal storms by carrying sheetwash-eroded and/or resuspended sedimentsover a pycnocline. The cascading organic-charged sediments of the detachedturbidity flows would absorb oxygen as they descended, thereby suffocatingand burying animals situated below the pycnocline. Seasonal climatic vari-ability would have provided the cycling between vertically mixed to density-stratified water column. These dynamics are likely to have promoted the highbiodiversity of the bay, would have produced the rhythmic repetition ofmicroturbidites that characterize the Bear Gulch Limestone, and provide acompelling explanation for the detailed preservation of its fossils.
KEY WORDSBear Gulch Limestone,
deposition, paleoecology, paleoclimate,
microturbidite.
INTRODUCTION
The Bear Gulch Limestone of Central Montana(USA) was first described as a part of the TylerFormation (Pennsylvanian), barren of fossils andcorrelated with subsurface limestones farther tothe east (reviewed by Nelson 1993). The fossilif-erous Bear Gulch deposit was discovered throughthe activities of local ranchers who found fishremains while recovering ornamental buildingstone. This discovery initiated quarrying opera-tions in 1968 by William Melton (Montana StateUniversity), one of the coauthors (R. L.), andtheir field parties and led to a stratigraphic studyof the exposures by Horner (1985) and a sedi-mentologic study by Williams (1981, 1983).
These latter works placed the Bear GulchLimestone within the Mississippian HeathFormation, rather than the Tyler Formation.They also demonstrated that there is no directhorizontal (layer-cake) correlation between theBear Gulch Limestone and other limestone unitsto the east (the fundamental premise for desig-nating all of these limestones as the Bear Gulchmember of the Tyler Formation). Our continu-ing field explorations (e.g., Lund et al. 1993;Feldman et al. 1994) have substantiated Horner’sand William’s interpretation and so, their strati-graphic designations are adhered to here. Wehave expanded our database of geologic, sedi-mentary, and faunal and floral information andnow provide a more in depth examination of the
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RÉSUMÉL’environnement géologique et biologique du Calcaire de Bear Gulch(Mississipien du Montana, USA) et son modèle de dépôt.Le Calcaire de Bear Gulch (Heath Formation, Big Snowy Group, FergusCounty, Montana, USA) est un Konservat lagerstätte déposé dans le bassindu Montana Central, à environ 12° de latitude Nord. Il contient des fossilesd’une baie marine comportant un biote varié de poissons, d’invertébrés etd’algues. Nous décrivons plusieurs nouveaux biofaciès : à Arborispongia-productidés, à algues filamenteuses et un faciès peu profond. Le faciès précé-demment nommé central du bassin et la zone supérieure sont redéfinis. Nousabordons la question de la conservation des fossiles, dont le détail est parfoistrès fin chez certains poissons et invertébrés à corps mou, et qui ne peut êtreexpliquée par des conditions anoxiques persistantes sur le fond. Chezquelques fossiles, l’observation de certains caractères impliquent des condi-tions environnementales provoquant simultanément l’asphyxie et l’ensevelis-sement des organismes. Les sédiments organiques dans tout le faciès centraldu bassin sont des microturbidites qui alternent de manière rythmique. Nosanalyses suggèrent que ces microturbidites se sont formées principalementpendant les tempêtes des moussons d’été par transport de sédiments lessivéset/ou remis en suspension au dessus d’un pycnocline. Le déferlement de sédi-ments, chargés en éléments organiques provenant des flux de turbidites misesen suspension, devait absorber l’oxygène au cours de leur descente, de ce faitsuffoquant et ensevelissant les animaux situés au dessous du pycnocline. Lavariation saisonnière climatique a dû provoquer les alternances entre mélangeet stratification par densité dans la colonne d’eau. C’est cette dynamique quiexplique la forte biodiversité de la baie, la répétition rythmique des microtur-bitites qui caractérise le Calcaire de Bear Gulch. Elle fournit aussi une explica-tion incontournable à la conservation de détails des fossiles.
MOTS CLÉSCalcaire de Bear Gulch,
dépôt, paléoécologie,
paléoclimat, microturbidite.
environmental conditions influencing the deposi-tion of the Bear Gulch Limestone. Over the 33 years of study, the Bear GulchLimestone has revealed a large assemblage of ver-tebrate and invertebrate fossils. Vertebrate faunaldiversity is very high (Lund & Poplin 1999).These fossils are preserved along a spectrum thatranges from scattered scales and disarticulatedskeletal elements to entire bodies including skinpigments and pigmented outlines of venousblood vessels and internal organs (Grogan &Lund 1997). Fine traces of soft tissues are alsopreserved in some invertebrates. Their faunalcomposition is unusual in that most of theshelled forms characteristic of the lateMississippian (Lutz-Garihan 1985) are conspicu-ously rare or absent. Preservational considera-tions are not a viable causative factor in thisabsence. In fact, this rock unit has been classifiedas a plattenkalke (Williams 1981, 1983) and aKonservat lagerstätte (Briggs & Gall 1990; Lundet al. 1993; Feldman et al. 1994) based on thediversity and the extraordinary preservation ofsoft-bodied animals and traces of vertebrate inter-nal organs.Classical explanations for the preservation of suchdiverse and high quality fossil faunas invoke per-sistent anoxic, dysoxic, or hypersaline bottomwaters or sediments (Barthel et al. 1990). Yet, wefind the Bear Gulch deposit reveals a bottom-living fish fauna (including a probable burrowingcomponent) that is ubiquitous, they are found inall lithologies and throughout the basin. Theydemonstrate the benthic environment was inhab-itable, that despite the very fine nature of themud (Lund et al. 1993) this environment musthave been aerobic. Our collective analyses of field and laboratoryobservations indicate seasonal climatic variationsthat would promote high faunal diversity andquality preservation and may account for aerationof fine bottom muds. Data are compiled to pres-ent a reconstruction of the probable prevailingpaleoecologic, paleoclimatic and paleocirculatoryconditions in this region during the Serpu-khovian; Namurian E2b. This paper discusseshow these conditions could explain the remark-
able features of the Bear Gulch bay; its faunaldiversity and quality of fossil preservation.
MATERIALS
Information for this study has been derived fromfossil quarrying operations conducted from 1968through 2000, and from the works of Williams(1981, 1983), Horner (1985), and Feldman et al.(1994). Quarrying has resulted in over 5000 fishfrom 85 sites within and around the outcrop areaof the Bear Gulch lens, and innumerable inverte-brate and algal specimens. Locality and catalogdata are on file at Carnegie Museum of NaturalHistory, Pittsburgh, Pennsylvania, USA. Museumcollection designations are as follows: CarnegieMuseum of Natural History (CM), RoyalOntario Museum, Toronto, Ontario (ROM),
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297GEODIVERSITAS • 2002 • 24 (2)
Surenough Beds
Bear Gulch Beds
Becket Beds
Gypsum
CameronCreekFm.
Surenough Beds
Bear Gulch Beds
Becket Beds
Gypsum
blackshale
Heath Fm.
Otter Fm.
Kibbey Fm.
green shales
red shales
Namurian
Mississippian
Big Snowy Group
Westphalian
Pennsylvanian
Amsden Group
Visean
MadisonGroup
??
AlaskaBenchFm.
sandstone
FIG. 1. — Schematic diagram of stratigraphy, modified fromWilliams (1983).
University of Montana Geological Museum,Missoula, Montana, USA (MV).
GEOGRAPHIC AND GEOLOGIC SETTING
OF THE BEAR GULCH LIMESTONE
Regional stratigraphyThe Bear Gulch Limestone is one of a series oflimestone lenses within the Heath Formation ofMontana and North Dakota that have collective-ly been named the Bear Gulch LimestoneMember (Williams 1983; Horner 1985) or upperHeath Formation (Feldman et al. 1994) (Fig. 1).The Heath Formation along with the underlyingOtter and Kibbey Formations are called the BigSnowy Group, are all Namurian E2b in age(Lund et al. 1993), and correlate with theSerpukhovian (Riley 2000; Menning et al. 2000).The Kibbey Formation is a basal transgressivesandstone that lies unconformably upon karsttopography at the top of the massive middleMississippian Madison Group limestones. TheOtter Formation contains littoral-zone litholo-gies. The Heath Formation consists of shallowmarine and brackish water shales, linear channelsandstones, littoral to fresh-brackish water shales,limestone lenses representing pockets of less tur-bid or deeper water, and supralittoral gypsumlayers. The upper boundary of the Heath Formationin the outcrop area marks the Mississippian-
Pennsylvanian boundary. Sands within theHeath Formation and erosional features at itsupper boundary indicate uplift progressing froma southerly direction and a possible source of flu-viatile influx from the south. Unconformablyoverlying the Heath Formation is the CameronCreek Formation, consisting of soil zones, fresh-water pond deposits, and some littoral zonedeposits. Above this is the Alaska BenchLimestone (Amsden group), a shallow transgres-sive marine unit. The entire upper HeathFormation as well as the Alaska Bench Limestonethins to zero at an apparent structural high atForest Grove, Mt.Stratigraphic studies show that the Chesterianwas a period of brief epicontinental sea transgres-sion upon the emergent surface at the top of theMadison Group in this region of western NorthAmerica (Fig. 2) (see paleomap reconstructions atwww.scotese.com and www.ucmp.berkeley.edu/geology). It resulted in the deposition of theBig Snowy Group in the Williston basin and thenarrow Central Montana Trough.The Central Montana Trough (Central Montanalineament) was an intermittently active geologicalfeature extending from the Williston basin in theeast, across Montana to the Montana-Idaho bor-der. Surface geological mapping and oil well logdata show a 160 km series of in-line limestonelenses within the Heath Formation that succes-sively overlap one another from east to west, anddemonstrate that the trough was subsiding inter-mittently (east to west) during the deposition ofthe Heath Formation (Williams 1981). The BearGulch Limestone is one of these lenses formedduring this period of tectonic instability.
Bear Gulch stratigraphyThe fossiliferous exposures of the Bear Gulch lensare visible in outcrop over about 85 km2. Theymeasure about 14 km east-west by 10 km north-south at its widest extent along the eastern expo-sure line (Fig. 3). A maximum sedimentaryaccumulation of about 30 m is exposed near itsnortheastern margin. The western edge of anoth-er limestone lens, the Becket, lies underneath theeastern edge of the Bear Gulch Limestone and
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298 GEODIVERSITAS • 2002 • 24 (2)
10°
EQ
NN
BGLM
CMT
WB
0 km 1000
FIG. 2. — Namurian paleogeography of North America, redrawnfrom Witzke (1990). Emergent lands shaded; present politicalboundaries outl ined. Abbreviations: BGL , Bear GulchLimestone; CMT, Central Montana Trough; EQ, Namurian equa-tor; M, miogeosyncline; NN, Namurian North; WB, Willistonbasin; 10o, Namurian 10o North latitude.
extends eastward into the subsurface. Oil well logdata show at least five other lenses of similarrocks below and to the east of the Becket lime-stone (Williams 1981). Another small lens withaccompanying fossils and linear sandstones, theSurenough beds, is found above the northwesternmargin of the Bear Gulch Limestone and repre-sents the last small basin episode (Horner 1985).The tectonic activity indicated by the HeathFormation chain of basins and evidence of south-ern uplift supports Williams’ (1981, 1983) pro-posed origin of the bay (Fig. 4). Yet, the commonpenecontemporaneous slumps that occurthroughout the Bear Gulch Limestone also sug-gest that a series of small seismic events may alsobe implicated in the history of this deposit. Thus,Feldman et al. (1994) advanced the scenario of anumber of seismic events leading to the gradual
or episodic subsidence and filling-in of a shallowbay. This view of the Paleozoic bay has beenexpanded through our continued studies. Fieldobservations (1995-1997) have included the dis-covery of complex large-scale channeling in theeast wall of southern Rose Canyon (see Fig. 3)which correlates with graded, fish-bearing beds atthe top of the section in the west wall of thecanyon. Similar observations (1998-2000) weremade at other exposures in the more northernreaches of this canyon and Bear Canyon. Weinterpret these data to indicate that towards thelatter stages of Bear Gulch formation and prior tothe development of the Surenough Creek lens,the center of deposition of the Bear GulchLimestone apparently shifted abruptly (paleo-)westward and was most likely due to seismicactivity (Fig. 3; Upper Bear Gulch bay facies).
FIG. 3. — Conformation of the Bear Gulch lens, Upper Heath, Fergus County, Montana. Boundaries of the facies are approximate.The grid is in km, Universal Transverse Mercator, zone 12, from the U.S. Geological Survey Becket and Forest Grove 7 1/2’ (topo-graphic) quadrangle maps. Abbreviation: NN, Namurian North, from Witzke 1990.
A GEOLOGIC/SEDIMENTARY VIEW OF THE
BEAR GULCH BASIN
SedimentsThe Bear Gulch is considered to be a lithographiclimestone or plattenkalke (Feldman et al. 1994).The sediments are composed of very fine silts,lime silts, shell fragments, organic debris, andclays; silicates comprise up to 50% by volume(Williams 1981). Sedimentation in the centralbasin and nearby facies was in the form of rhyth-mically alternating sets of dark, thick, hard, finegrained, massive to graded beds and sets oflighter-colored fine laminar beds (Fig. 5); oneunit of alternating light and dark beds mayapproach 1 m in thickness. Bed sets can be tracedfor at least 1 km along outcrops. There is no sig-nificant pyrite present in the sediment except for
local occurrences, such as among tightly packedArborispongia Rigby, 1985 accumulations in onepart of the eastern area. Occasional thin zones ofsmall, oxidized iron-mineral nodules in theuppermost beds may also have been weatheredfrom pyrite. Bedded cherts are common in thedense sets of the upper beds. Calcite and arago-nite were mobilized away from shells in the baycenter but not at the western margin. The thick,dense beds are sufficiently rich in decomposedplant matter and organic chemicals that a distinc-tive (oil-like) odor emanates from freshly brokensurfaces. There are no obvious traces of primaryevaporites within the Bear Gulch, although abun-dant star-shaped gypsiferous nodules disrupt theuppermost laminated beds near the northeasternmargin.
Basin dimensionsData from measured stratigraphic sections, thestrike and dip of abundant dewatering micro-faults, occasional channels and foreset beds, andthe orientation of the penecontemporaneousslumps and rare current-aligned features havebeen considered together to generate a picture ofthe Bear Gulch basin, its shape and flow regime(Fig. 3). Where the uppermost layers of the Bear GulchLimestone can be found, they are characterizedby littoral and supralittoral lithologies of stroma-tolites, mudcracks, disrupted beds, chert bedsand nodules, dolomitic nodules, and local terres-trial plant material. These are overlain in placesby a 1-3 m thick conglomerate consisting of BearGulch clasts, and above this, by a fresh to brack-ish water marl in many places. To the west, theBear Gulch Limestone thins to zero immediatelysouth of Forest Grove, Mt., and is replaced by azone of freshwater fish- and plant-bearing clays,shales, and a bed of dolomitic silts bearing thedisrupted carcasses of large chondrichthyans,osteichthyans, and abundant acanthodians. Shortlywest of this area, the lower beds of the HeathFormation contain zones with a marine upperMississippian shelly fauna and rare vertebrateteeth and spines that have not been found withinthe Bear Gulch. Along its northern margin the
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300 GEODIVERSITAS • 2002 • 24 (2)
sea level
infill of original basin
W EA
original basin fill
sea level
new basin infill
W EB
W E
Surenough BedsBear Gulch Beds
Becket Beds
C
exposed land
FIG. 4. — Illustration of basin formation, modified from Williams(1983); A, infilling of original basin to the east; B, subsidence ofCentral Montana trough (indicated by arrow) leading to drop inoriginal basin and formation of a second basin to the west. Thenew basin is subject to infilling; C, three successive beds result-ing from repeated events of subsidence and basin infilling.Abbreviations: E, east; W, west.
Bear Gulch rapidly grades into thin, dirty tan,poorly consolidated marine silts and shales. A fewof the upper layers along the northern quarrysites contain considerable quantities of charcoalfragments, supporting the interpretation thatthese outcrops were very near shore and thatcharcoal blew in from on-shore fires to the north(similar conditions have been reported for theDevonian fish-bearing deposits of Miguasha,Quebec; H.-P. Schultze pers. comm. to R. L.).The exposed eastern margin is characterized byseries of complex structures. High-energy facies-fossils and associated bars flank foreset beds of asmall prograding delta that indicate a principaloutlet (the basin mouth) at the northeastern cor-ner (Williams 1981). Less than 1 km to thesouthwest, the thickest part of the sequence isfound. Both slope and current indicators demon-strate that on the west side of the bar the basinfloor declines to the north. Channel and slopedirections converge upon the bay opening fromthe southwest, west, and west-northwest.Dewatering faults further delineate a principalbasin axis that extended, roughly northwest tosoutheast, for a minimum of 7 km. (The basinaxis can be seen in section in the north wall ofAtherton Gulch). Dense brachiopod, sponge,and annelid worm zones, and indications of aseparate (presumably tidal) inlet are found alongthe southernmost aspect of this eastern margin.Articulated crinoid evidence in this area suggeststhat open water (i.e. stabile marine) conditionsexisted in close proximity to the inlet. Theremainder of the eastern Bear Gulch margin passesinto the subsurface and many sedimentologicalfeatures suggest there is no significant subsurfaceextension. Basally, the Bear Gulch Limestone grades intoirregularly bedded, often peloidal dark gray toblack shales.
Facies: lithographic and biologicThe initial sedimentary depositional model ofWilliams (1981, 1983) defined four lithofacies;marly (upper beds), marginal, basin-slope, andbasin facies. Subsequent field investigations haveprovided extensive biological and ecological data
for inclusion in the model and so, require itsrefinement. We introduce three newly definedbiofacies: 1) an Arborispongia-productid facies; 2)a filamentous algal facies; and 3) a shallow facies.Williams’ marginal facies, central basin facies andthe uppermost zones are redefined. The locationand extent of these facies are illustrated (Fig. 3).The central basin facies is characterized by thehighest diversity of fish (Lund & Poplin 1999),by mobile and nektonic invertebrates, very limitedamounts of algae (principally filamentous or nar-row-fronded), and the nearly total absence of ses-sile benthic invertebrates. Fossil preservation inthe central basin ranges from poor to superb. Arhythmic sedimentological unit in the centralbasin of the Bear Gulch Limestone consists of a
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301GEODIVERSITAS • 2002 • 24 (2)
FIG. 5. — Outcrop of central basin facies, showing rhythmicalternation of dense and laminar beds. Abbreviations: W, wetseason deposits; D, dry season deposits.
dark gray, dense, poorly fissile to massive subunitand a light tan to yellow, laminar, fissile subunit.Organizationally, one complete sedimentologicalunit is comprised of a dense basal zone and aseries of laminar beds (Fig. 5). The dense subunitconsists of a thin basal zone, a zone of massivenon-bedded to very irregularly bedded sediment,an upper zone containing several graded laminae,and not infrequently, a thin heavily bioturbatedor unbedded uppermost zone (Fig. 6). The denselayer is succeeded by thin laminar beds of well-sorted sediments that vary from light gray belowthrough tan to light yellow, and then to gray-brown at the base of the next dense bed. Organicdrapes blanket individual graded beds, and verylittle scouring of underlying laminae is found.Williams (1983) speculated the massive bedsmight reflect periods in which turbidity currentsflowed frequently, the laminae resulting fromseasonal currents in a quieter period. We nowextend her interpretation and propose that mostof the dense subunits in the central basin faciesare somewhat reminiscent of a Bouma sequence(as described by Kennett 1982), a deep-seapattern of turbidite sedimentation.An Arborispongia-productid facies occurs to thenorth of the central basin facies and along theeastern margin, as well as locally in the upper-most beds. It is characterized by dense stands ofan arborescent sponge (Arborispongia) (Fig. 7E)frequently serving as the substrate for spiny artic-
ulate brachiopods, bivalve molluscs, and conu-lariids (Babcock & Feldman 1986). Many algalforms are evident. Of these, both calcareous andnon-calcareous dasyclad algae are common, aswell as an alga closely resembling the modernValonia Agardh, 1823 (Fig. 7E, F, H). Diverseshrimp and worms, an unbranched species ofSphenothallus Hill, 1978 (Hill pers. comm. toR. L.), crinoids, branching bryozoans, anddiverse fishes are also part of this habitat (Figs 7;8). Sediments are predominantly light tan tolight gray and laminar bedded, with some micro-turbidites. Preservation ranges from poor toexcellent.A filamentous algal facies lies south of the centralbasin facies. It is characterized by abundant algalfilaments (Fig. 7H) accompanied by very smallspiny productid brachiopods in all lithologies. Abranched (colonial) species of Sphenothallus Hill,1978 is also relatively common (Fig. 7G).Laminar to irregularly bedded sediments are tanto dark brown with the dense limestones beingdark gray to black and very rich in organic mate-rial; lower in calcium carbonate and higher in ter-riginous clastics than the rocks of the centralbasin facies. Graded microturbidites are less com-mon than in the central basin facies, these arelight gray in color and less rich in organic materi-al. Preservation ranges from disturbed and dis-persed skeletal elements to good, but specimensare predominantly less well preserved. The marginal facies is characterized by black,irregularly bedded to occasionally laminar, bio-turbated rocks in which original plant cuticle andthe calcium carbonate of shells often remain.Sediments of the marginal facies are lower in cal-cium carbonate and particularly lower in micrite,and high in peloids and organic materials(Williams 1981). Vertebrates and shrimps arerarely preserved intact and when intact are usual-ly disturbed; gastropods, which are very rare inother facies, are regularly present here. The acan-thodian Acanthodes lundi Zedik, 1980, consid-ered an indicator of brackish water conditions, iscommon in the western marginal beds. Largespiny productid brachiopods associated with con-centrations of filamentous algae are most com-
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G
RAINY
B
M
D D
D
D
FIG. 6. — Rainy season component of central basin facies sedi-mentary unit. Polished section of beds from the central basinfacies. Arrow indicates up; scale in cm. Abbreviations: D, dewa-tering fault; B, bioturbated zone; G, graded, laminar beds;M, massive non-graded bed.
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303GEODIVERSITAS • 2002 • 24 (2)
B
A E
D
F
G
C
H
FIG. 7. — Invertebrate and algal fossils; A, Aenigmocaris Schram, 1979; B, lingula; C, segmented worm; D, Lepidasterella Welch,1984; E, Arborispongia Rigby, 1985; F, sea-lettuce-like algae; G, Sphenothallus Hall, 1978; H, fine filamentous algae. All housed(currently uncatalogued) at the Royal Ontario Museum. Scale bars: 1 cm.
mon in the northern marginal facies and theArborispongia facies.The upper zone, or shallow facies of the BearGulch Limestone is characterized by whitish, tanto yellow beds, local relief surfaces with layerscontaining algal laminae and stromatolites, chertlaminae and nodules, and star-shaped gypsiferousnodules in the softer laminated beds. Calciumcarbonate may not be totally absent from skeletalmaterials. Cherts can imply fluviatile silica insolution being precipitated upon contact withmarine water, or shallow water solution and rede-position of biogenic silica from sponge spiculesand diatoms (Williams 1981). AbundantArborispongia communities, conulariids, largespiny productid and other articulate brachiopods,bivalved molluscs, stony bryozoans, somecrinoids, and a branching species of Sphenothalluscharacterize the biota. Larval and juvenile fish aswell as small shrimp are common. The upperfacies is similar to the Arborispongia facies in fau-nal content and ecological significance but it isnot localized to the eastern margin. The shallowfacies caps the Bear Gulch lens across its much ofits area of outcrop. It represents the final phase ofshallowing and filling of the bay, starting fromwider access to higher energy marine water andprogressing to shoal conditions. A limited variety of terrestrial leaves has beenfound in the uppermost zone, immediately belowa conglomerate composed of Bear GulchLimestone fragments and a superposed thickmarl zone or marly facies. Ostracod shells havebeen found associated with the marl (Williams1981). The conglomerate and marl are trueintraformational deposits, manifesting the changein environment from that of the marine BearGulch lens to that of the terrestrial CameronCreek Formation.
Sedimentological trends across the faciesA few gross, basin-wide sedimentological trends(first noted by Williams 1983, and furtherexpanded by observations of subsequent field sea-sons) provide evidence of a dynamic link betweenthe defined facies. There is a strong gradient fromdarker sediments with higher organic content
marginally to those with very low organic contentin the central basin facies. There is a stability gra-dient of skeletal calcium carbonate; it rangesfrom being present (but altered) in the southernand western marginal facies to being totallyabsent in the central basin. Silts decrease relativeto carbonates from the marginal to the centralbasin facies, as do peloids. Iron and manganesemineralization of vertebrate skeletal tissues ishighest in the southeast, and decreases north-ward. As noted above, there also is a significant trend inthe distribution, nature, and sequence of micro-turbidites. In transects from the marginal to thecentral basin facies the rhythmically varyinglithological units become divisible into more con-spicuous subunits. Classical graded microtur-bidite laminae are concentrated in the centralbasin axis. The proportion of these beds to thelower, darker, more organic-rich, massively bed-ded rocks increases into the basin axis where theyconstitute the middle and lower portions of eachcentral basin facies sedimentary unit. These datawere especially significant to reconstructing thepaleocirculation of the basin and the depositionof its sediments. They also provided insight intothe means by which excellent fossil preservationcould be achieved.
A BIOLOGICAL VIEW OF THE BEAR GULCH BASIN
The fauna of the Bear Gulch Limestone essential-ly contains marine fossils; fresh-water macro-fossils are extremely rare (Zidek 1980; Horner &Lund 1985). All biotic evidence points to a high-ly productive Paleozoic bay and a complexecosystem. Plant and fungal spores and acritarchs are foundin the shallow water and marginal facies (Stuckepers. comm.). Algal filaments and large quantitiesof unidentified plant debris are observed in virtu-ally every layer. Yet, there is some preferentiallocalization among the different types of algae(apparently reflective of the hydrodynamic andnutrient regimes). Calcareous and non-calcareousdasyclad algae, and other undescribed attachedalgae, are most common in the Arborispongiafacies to the north of the axis, while the abundant
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filamentous algae are concentrated south of theaxis. Worms, crustaceans, and cephalopods wereabundant (Schram & Horner 1978; Schram1979a, b; Factor & Feldmann 1985; Landman &Davis 1988). The fish fauna was diverse and thesize distribution of recovered fish fossils is com-parable to that of a small, shallow, modern bay(reviewed in Lund 1990). Preservation of the bio-facies reveals life assemblages in place and, likethe algae, demonstrates habitat selection for mostof the sponge, invertebrate and vertebrate species(Lund et al. 1993; Lund & Poplin 1999). Thisincludes fine scale habitat selection between eco-morphologically similar or “sibling” species ofactinopterygians (Staropoli 1993) and species- orlife stage-based patchiness among select residentfish (Lund 1990; Lund et al. 1993). A large com-ponent of migratory, vagrant, and opportunistic,fish species (Lund 1990) is also indicated.Collectively, these features are consistent withthat of a productive modern marine or estuarinebay, particularly one with seasonal characteristics.Unlike today’s marine or estuarine bays, however,there is no appreciable evidence that any sessilebenthic invertebrate actually lived on the sedi-ment in the central basin facies. Small spiny pro-ductid brachiopods are commonly found in thefilamentous algal facies, the large ones accompa-ny filamentous algae and Arborispongia. Diverseother sessile invertebrates are often foundattached to Arborispongia but have also beenfound attached to cephalopod shells and, in oneinstance, to a submerged log which was preservedin the central basin facies. Some contribute toflotsam assemblages (McRoberts & Stanley1989) analogous to the modern SargassumAgardh, 1820 assemblages but based on filamen-tous or fronded algae rather than seaweed. We find it also significant that several inverte-brate faunal elements typical of the upperMississippian are conspicuously absent or haveextremely localized occurrences. These benthicforms include corals, attached echinoderms,foraminifera, ostracods, and stony bryozoans. Yetthe central basin and adjacent environments werenot necessarily inhospitable, for bottom- andburrow-dwelling fish have been routinely identi-
fied. These data suggest to us that unfavorableturbidity conditions may have been responsiblefor such a selective distribution.
PRESERVATION OF THE FAUNA
We have noted a correlation between animal sizeand the quality of preservation for the fishes ofthe Bear Gulch. The well-preserved fish arestrongly skewed toward the smaller sizes (up to150 mm). Well-preserved intermediate-sized fish(200 mm to upwards of 1.5 m) are rare and,although tooth, spine, and scale/denticle evi-dence demonstrates they did exist, there are nointact fish above 1.5 m. Several factors mayaccount for this correlation. Small microhabitatdwellers would be more susceptible to quick andcomplete burial while the larger and the migrantfishes may have selectively escaped killing events.In addition, the larger individuals that died in ornear the bay are likely to have bloated and floatedfor an appreciable amount of time. This wouldpermit their body parts to have been distributedover wide areas as decomposition, water trans-port, and scavenging occurred. In fact, ample evidence of decomposition, preda-tion, and scavenging does exist among the fossilremains. Yet, given the density/distribution ofvarious fossils, we find it striking that other poten-tial prey items such as worms and other soft-bodied invertebrates are often preserved asflawlessly as the small to intermediate-sized fish. Itis even more intriguing to find exquisite preserva-tion of fish heads or other incomplete fish remainsthat were obviously subject to some initial preda-tion but for which the process was halted or inter-rupted. (It is virtually inconceivable to us that anyorganism’s remains, surrounded by diverse andabundant live shrimp, cephalopods, and fish,could lie undisturbed on the bottom of a bay).There are also instances of excellently preservedfish and shrimp extending through several lami-nae of the Bear Gulch Limestone or through up to10 mm of rock. Yet, it is established that duringthe burial and fossilization processes the upperbody surface commonly collapses down onto thelower surface for remains found in laminar layers(Elder 1985; Elder & Smith 1988). We can only
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305GEODIVERSITAS • 2002 • 24 (2)
envision that some episodic and dynamic burialprocess might account for these occurrences.The central basin facies is a focal point in address-ing this quandary of preservation. As stated earli-er, the quality of preservation in the central basinfacies can often be particularly extraordinary, asexemplified by the preservation of features of softtissues (Grogan 1993; Grogan & Lund 1995,1997) (Fig. 8). Skin outlines, skin pigmentationpatterns and imprints of internal organs arerecorded. The blood pigments from highly vascu-larized tissues such as livers, spleens, and gonadsare preserved either as black colored areas or, inthe case of livers, are occasionally evidenced asbituminous layers of measurable thickness. Liverpigment preservation seems to vary taxonomical-ly and according to ecomorphology; a coelacanth(Allenypterus Lund & Lund, 1985), select chon-drichthyans (code name El Weirdo; Echino-chimaera meltoni Lund, 1977; Debeerius ellefseniGrogan & Lund, 2000) and a paleoniscoid
(Paratarrasius hibbardi Lund & Melton, 1982)typically provide the best examples. Splenic tissueis frequently indicated in the chondrenchelyid,Harpagofututor volsellorhinus Lund, 1982 and inthe petalodont Netsepoye hawsi Lund, 1989.Gonadal imprints have been observed in the lam-prey, Hardistiella montanensis Janvier & Lund,1983, and permit identification of reproductivelymature Harpagofututor (Grogan & Lund, 1997)females. A bilobed internal organ and the pre-sumed gut of the enigmatic protochordateTyphloessus Melton & Scott, 1973 (Scott 1973;Morris 1985, 1990) also preserve very well. Insurvey, these organisms are inferred to have dif-ferent life history patterns (Lund 1990) andreflect organisms ranging from a benthic tosponge reef to migratory habitus.The preservation of the fishes’ venous circulation(rather than the entire vascular or arterial plan) isparticularly intriguing, as these capacious, thin-walled structures would normally be most likely
to decompose on a time-scale of hours ratherthan days after death. The fine preservation ofvenous elements, as evidenced in the detailedstructures of the gills, suggests mortality due toasphyxiation and rapid burial of the fish (Grogan& Lund 1995, 1997). It is also indicated by theobservation of distended gills and raised opercu-lums in paraselachian specimens, features thatare diagnostic for asphyxiation in recent fish.Similarly, certain actinopterygian taxa and thechondrichthyans Falcatus falcatus Lund, 1985and Damocles serratus Lund, 1986 are frequentlyfound curled up, like watch springs. This obser-vation cannot be simply attributed to postmortemrigor mortis. Such behavior has been observed forlive specimens of the Recent catshark Scyliorhinuscapensis Smith, 1838 (Compagno et al. 1989)and is correlated with the stressful physiologicalconditions that accompany asphyxiation (Groganpers. obs.).Field experience with and common knowledge ofrecent forms also demonstrates that these symp-toms of asphyxiation are not elicited by salinitychanges alone. Furthermore, other possible causesof asphyxiation may also be ruled out. For exam-ple, we do not find massive kill horizons as mightbe induced by a significant rise in water tempera-ture. The oxidation of algal debris and organicmatter could logically reduce the level of dis-solved oxygen and promote an anoxic environ-ment. Yet, the preservation of blood pigments(which requires the oxidation of hemoglobin orother oxygen sensitive molecule) would suggestthat the bottom waters were not consistentlyanoxic. The finding of benthic inhabitantsthroughout the basin would also negate the ideaof a persistently anoxic bottom. Cumulatively, the biological data suggest thatdeath by asphyxiation and burial were intricatelylinked. Coincident death and rapid burial is alsoimplicated in the preservation of many inverte-brates. The starfish are preserved in relaxed atti-tudes. Yet, like the worms, it is virtuallyimpossible to kill starfish by simply coveringthem with sediment (Welch 1984) and burialalone does not solve the preservation quandary. Arise in water temperature would not kill them but
would merely put the starfish to sleep (by invok-ing reduced metabolic activity). Toxic algalblooms would however kill with no other trace ofdisturbance but would not ensure burial of theorganisms without either predation or post-mortem degradation. Although there are notraces of significant fluviatile input, if a suddenfreshwater influx were involved it would princi-pally float upon the denser marine waters and so,would have essentially no effect upon benthicorganisms. Marked reductions in salinity alonecannot account for the preservational conditionseither. Some factor or combination of factors hadto both kill and bury the organisms quickly and,in the process hinder or prevent predation andmicrobial decay.In summary, the biological data indicate thatthere was selective, high quality preservation oforganisms, that the benthic habitat was not pref-erentially anoxic, and that there appear to havebeen repeated events that caused both death byasphyxiation and rapid burial. The evidence ofthese conditions is best recorded in the fossils ofthe basin facies, the sediments of which alsoreflect rhythmic changes (in terms of theirnature, color, and thickness). To identify factorswhich may explain the above, we considered thedynamics of the physical environment (indica-tions of water circulation, climate, sedimenta-tion) and how they relate to the biologicalenvironment.
CLIMATOLOGY
GLOBAL AND REGIONAL PALEOGEOGRAPHY
Reconstruction of the paleoclimatic conditionsand paleocirculation of the Bear Gulch bay is onlyfeasible through comparison to modern studies ofworld climate and physical, chemical and biologi-cal oceanography. What follows is a model for theBear Gulch based on these data and on fundamen-tal tenets of physical oceanography.Like the climate and circulation of today’s landand water masses, those of the Paleozoic BearGulch bay environment would have been definedby the latitudinal position of the bay, its connection
Environment and deposition of Bear Gulch (USA)
307GEODIVERSITAS • 2002 • 24 (2)
to the shallow epicontinental sea, the config-uration of the continental landmasses, and theinfluence of a falling Namurian sea level. Witzke(1990) and Scotese & McKerrow (1990) recon-struct the Visean-Namurian North Americancontinent as rotated approximately 35-40° clock-wise relative to its present orientation, with thetectonic plate moving north. Geomagnetic dataplace Central Montana at about 10-12° north ofthe equator in upper Mississippian time and onor near the border between an arid climatic beltto the North and a tropical climatic belt to theSouth (Hidore & Oliver 1993; www.scotese.com;www.ucmp.berkeley.edu/geology) (Fig. 9).
ATMOSPHERIC AND CIRCULATORY CONDITIONS
The latitude at which the Bear Gulch bay existed320 million years ago is most likely to have beenimpacted by shifts in the planetary scaleIntertropical Convergence Zone (ITCZ) (Figs 9;10) and characterized by a monsoonal-climatic
regime of rainy and dry seasons (Figs 11; 12).According to Blanchard (1997; webspinners.com/dlblanc/climate/climmods.html), a twoHadley cell atmospheric circulation plan pre-vailed and the general climate conditions wouldbe classified as a wet cycle, with global rainfallexpected to be in excess of 500 cm per year, near-ly uniform temperatures across the surface of theearth, and winds ranging from a calm, steady 2-3miles per hour (3-5 KPH) to storm conditionsthat are not expected to exceed 5 miles per hour(8 KPH).
Calm, dry/winter seasonWhen the ITCZ was positioned closest to thepaleo-equator the 10-12° latitudes would mostlikely to have been dominated by the dry windsof a winter season (Figs 9; 11A). Because this dryseason would have represented an energeticallyquiet period, the influence of atmospheric or cli-matic conditions on bay circulation would have
Grogan E. D. & Lund R.
308 GEODIVERSITAS • 2002 • 24 (2)
Tropical
Arid
Arid
Warm temperate
Cool temperate
Warm temperate
Cool temperate
OceanEpicontinental and shelf waters Emergent land
*BG
FIG. 9. — Paleogeographic and paleoclimate map of the Late Early Carboniferous (modified from C. R. Scotese, PALEOMAP Project,www.scotese.com). Arrow indicates position of the Bear Gulch deposit (BG).
been limited to evaporative and tidal influences.The absence of significant river or freshwaterchannel input suggests bay circulation was princi-
pally driven by winds. Light easterly trade windswould have pushed epicontinental (Willistonbasin) sea-water into the bay mouth and generatedanticlock wise flow in cross section as they blewtransverse to the long axis of the Bear Gulch bay(Fig. 11A). Surface waters would have built uptowards the paleo-western margin and edge ofthe central bay channel or trough. Geostrophiceffects would force the flow of this water down-ward and across the bottom, to upwell at thepaleo-eastern margin of the trough. The entranceof daily tidal and easterly wind-driven epiconti-nental seawaters would have afforded turnover orrefreshment of these waters. Minimal rainfall andevaporative heating would have promoted somehorizontal stratification of the water (particularlyin the shallows of the western margin) and theextension of a bottom layer of seawater intothe higher reaches of the central bay (Fig. 12A).Vertical mixing of water types would haveoccurred at density-different interfaces andwould have been promoted by tidal forces.
Energy pulsed, wet/summer seasonProgression from the winter season into springand summer would have been defined by anorthward shift in the ITCZ. As the ITCZincreasingly encroached upon the latitude of theBear Gulch it would have introduced anincreased probability of strong winds and precipi-tation in spring and into summer. Winds would have become westerly to south-westerly and basin circulation would switch to aclockwise flow, with surface water downwellingnear the eastern margin and bottom waterupwelling on the western margin of the centralbay trough (Fig. 11A). Under spring to earlysummer conditions the influx of terriginous sedi-ments would have been restricted to wind-driventransport and are likely to have accumulated inthe shallow western bay margin. Reversal ofgeostrophic flow may have contributed to sedimen-tation in the channel and along the western slope.The relatively stronger westerly to southwesterlywinds would have effectively increased the impactof geostrophic flow, inducing resuspension of anysediment which had built up along the eastern
Environment and deposition of Bear Gulch (USA)
309GEODIVERSITAS • 2002 • 24 (2)
N
S
L
H
H
H
H
L
L
Polar easterlies
Westerlies
Northeast trades
Southeast trades
Westerlies
Polar easterlies
Hadleycell
N
S
L
H
H
L
L
Polar easterlies
Northeast trades
Polar easterlies
Hadleycell
ITCZ
ITCZ
A
B
Southeast trades
FIG. 10. — Atmospheric circulation patterns; A, six cell patternprevailing today; B, two cell pattern predicted (Blanchard 1997)for the Late Paleozoic. The air flow pattern of the tropical Hadleycell follows the same general plan in each scenario. Cool, dry eastto northeasterly winds would have been generated as tropicalHadley cell air subsided (at about 30° for the 6 cell model, towardsthe poles in the 2 cell model). These winds would have undergoneadiabatic warming as surface flow continued toward the equatorand contributed to desertification of the land under the central andmore northern span of the Hadley cell. Closer to the equatorial re-gion, however, increased evaporation and cloud cover wouldcause the easterlies to become more moist. With the seasonal mi-gration of the ITCZ northward the lands previously subjected toarid conditions would become subject to warmer temperaturesand increased rainfall. Arrows indicate direction of winds and at-mospheric circulation. Abbreviations: H, high pressure; ITCZ, in-tertropical convergence zone; L, low pressure; N, North; S, South.
channel margins during the winter and springseasons. Refreshment of the bay with epicontinental sea-water would have been extremely restricted inthis period compared to calm and dry (winter)period. This is because wind-driven waters wouldhave principally entered from the southeast chan-nel rather than the bay mouth proper andbecause wind direction would have counteractedthe impact of daily tidal flow by reducing theforcing influence of the epicontinental sea.Therefore, salinity stratification would havebecome more oblique to nearly vertically orientedas evaporative loss continued and as the bottomlayer of seawater was deterred from extendinginto the uppermost spans of the bay (Fig. 12B).Compared to the mouth of the bay, the shallowuppermost regions would also be most likely tohave become susceptible to hypersaline condi-tions at all levels of the water column as seasonaltemperatures increased. Only the most tempera-ture resilient or burrowing components of thefauna (those normally protected by pore waters)
are expected to have inhabited the uppermostand shallowest extent of the bay during suchconditions. As summer progressed, disturbances of these con-ditions would have occurred with the onset ofmonsoonal winds and occasional torrential rain-fall. Episodes of heavy rains would likely havegenerated an increased outflow of surface waters.They would also have provided some moderationof salinity differences in the uppermost section ofthe water column throughout the bay. In con-trast, the shallow waters of the western marginsand bay head would have experienced an extremereduction in salinity at all depths.Rapid rainfall would have introduced massiveamounts of sediment into the bay from the vege-tatively depauperate surrounding land as theycontributed to mixing of marginal and headwa-ters. Downstream to these waters the deeper,warm, and hypersaline waters of the channelwould persist and permit the sediment rich, rain-fall-induced headwaters to flow over this layerand down the bay. This sediment-laden flow
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310 GEODIVERSITAS • 2002 • 24 (2)
Wet, energetic summerArid, calm winter
NNN
1
2
Prevailing seasonal winds
Advection currents
Surface water flow
Bottom water flow
Turbidity flow direction
1, 2Cross-sectional transects
A
B Rain-induced flow
FIG. 11. — Reconstructed Namurian wind and flow conditions at two transects across the Bear Gulch bay; A, wind and geostrophicflow conditions at the specified cross-sections (1, 2) during arid, calm “winter” and wet, energetic “summer” seasons respectively.Relative magnitude of wind and flow indicated by arrow size; B, monsoonal-rain induced turbidity flow at transect 2. Shading reflectsdifferent densities of water masses. Abbreviation: NN, Namurian North.
would travel along the pycnocline (Fig. 11B)until its momentum matched that of the sur-rounding water and initiated its deposition.The magnitude of seasonal shifts would be drivenby the extent of ITCZ migration. So, during theestimated 1000 years that the bay existed it is alsolikely that conditions intermediate to those pre-dicted in the scenario above also occurred. In theevent that the annual flux was more moderateand the center of the ITCZ became localized ator around the latitude of the Bear Gulch the shiftin climate would have been less dramatic. Just asin the horse latitudes of today, stable low velocitywinds would have continued to flow from east towest during the summer and only occasionalchanges in atmospheric circulation would causeunstable weather in the form of squalls, wind-storms and rainstorms.
JUSTIFICATION OF THE MODEL
Although any reconstruction of the paleoclimateand paleocirculation is potentially flawed by thelack of detailed, first hand evidence for the pre-vailing air and water conditions, we are confidentof this model because it agrees with the prepon-derance of the geological, biological, and preser-vational data.The microturbidites are key to our analysis. Themicroturbidites reported in most facies couldhave been generated by the resuspension ofunconsolidated shoreward sediments by strongonshore winds, storms, and/or earthquakesbefore being transported to the area of deposition.Yet, we find these sources cannot adequatelyexplain the deposition of the central basin facies.There are no current indicators in this facies;there is no preferred orientation of any fossils.Furthermore, little to no scouring of underlyingmicroturbidite laminae or their organic drapes isidentified in this facies. Thus, the sedimentaryevidence agrees with deposition resulting fromsediment-charged water being carried over a pyc-nocline, and having cascaded vertically to thebottom. Such a phenomenon, identified as a cas-cading or detached turbidity flow (Pierce 1976),is exactly the type of sedimentary pattern weexpect during the wet monsoonal period.
Hypersaline bottom waters or a layer of fresh(rainwater runoff) water over denser saline waterare known to produce the density-stratified watercolumn necessary for the formation of a turbiditycurrent of very fine particles and particle-organicaggregates (Pierce 1976). Our climatic model forthe Bear Gulch environment accounts for hyper-salinity and freshwater influx. In the transition tothe wet season, all waters, but especially the shal-lowest, would be susceptible to higher salinities asthe winds restricted tidal flow and warmer tem-peratures promoted increased evaporation. Theoblique to nearly vertical stratification of relative-ly hypersaline waters would permit a rainwater-induced turbidity flow to extend down the bay(Figs 11; 12). Torrential monsoonal rains wouldproduce a hyposaline upper water layer withinwhich resuspended sediment and algal bloom-derived organics would flow, as sheetwash, outacross the bay and over more saline layers. Somehorizontal density stratification could haveoccurred during the dry season (because of thearid conditions) but under easterly to northeast-erly winds, bay water would be routinely mixedwith the epicontinental seawater forced into thebay. This tidal forcing would have ensured morevertical mixing and greater homogeneitythroughout the bay waters.The rhythmicity of the Bear Gulch lithologysuggests that microturbidite deposition occurredregularly. The seasonal component of the modelaccounts for this periodicity and explains thesubunit sequences noted for the central basinfacies sedimentary unit (Figs 5; 6). The densebasal zone (whose pattern is reminiscent of theBouma sequence divisions A, B, and D; Kennett1982) would correspond to turbidity flowdeposits during a wet season. The series of lami-nar beds would correspond to deposition duringthe arid season (approximating Bouma divisionE). The predicted pattern of winter and summercirculation is also in accord with all current-aligned and channel features and the wind-blown charcoal along the northeastern edge.Furthermore, inter- and intra-seasonal variationin the amount of wind forcing and rains (asdetermined by the extent of the ITCZ shift, for
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311GEODIVERSITAS • 2002 • 24 (2)
example) could easily account for the statisticallyinconsistent thicknesses reported (Feldman et al.1994) for several repeating rhythmic subunits.Monsoon-induced resuspension of accumulatedmarginal sediments that contain high amounts ofplant debris, algal mucus and organic compoundsis consistent with the alternating (varve-like)organic-rich and organic-poor parts of each sedi-mentary unit and the quantities of organic sedi-mentary components needed to produceasphyxia. Bottom water layers would quicklybecome lethal as resuspended sediments andorganics cascaded downward from the upperwater column and absorbed the dissolved oxygenof the lower water column (Pierce 1976; Kennett1982; Feldman et al. 1994). Within minutes theresultant combination of higher water tempera-ture and oxygen depletion would generate lethalasphyxiating conditions for any living thingtrapped in the lower water column. The descend-ing sediment would immediately bury anytrapped and/or killed organism. This scenariothereby accounts for the higher frequency ofmore detailed fossil preservation in the centralbasin facies compared to other facies. The condi-tions leading up to the microturbidite event may
also explain why the fauna of the shallow water isnot entrained into the central basin depositionalevent. The predicted hypersalinity of the shallowmarginal waters would preclude the possibility ofwashing occasional shallow water fauna/flora intothe central basin because the biota of this areawas probably restricted to burrowing compo-nents at best.The predicted seasonal shifts in wind, rain, andsedimentary influx and their consequences onhydrodynamics sufficiently agree with other bio-logical observations such as the localization of theArborispongia communities in the more hydrody-namically active region (north of the centralbasin) and filamentous algal community in theless active waters (south of the central basin). Asreviewed under our biological observations, theregularity of high turbidity conditions, the highrate of fine sediment deposition, and the seasonalcutoff of (epicontinental sea-derived) replenishingbottom water inflow could very well have prevent-ed the establishment or long term survival of a ses-sile benthic biota in the central basin sediments. It should also be mentioned that episodic disrup-tion of ecological conditions is strongly implicatedin the maintenance of any high diversity ecosystem.
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312 GEODIVERSITAS • 2002 • 24 (2)
NNN
Salinity / density isobars
N
E
W
SLongitudinal transect through bay
bay head bay mouth
Arid, calm season
Wet, energetic season
Epicontinentalseawater wedge
Epicontinentalseawater wedge
Monsoonal rain-induced turbidity flow
Mixing of water types
Most saline water
A
B
More saline water
FIG. 12. — Reconstruction of bay waters and salinity regimes along a transect; A, general conditions favored during a calm, arid sea-son; B, prevailing conditions favored at the early stage of an energetic, wet season and relative position of subsequent rain-inducedturbidity flow. Salinity moderation of upper flow waters during turbidity flow not illustrated. Abbreviation: NN, Namurian North.
CONCLUSION
Stratigraphic data present the Bear Gulch Lime-stone as a small, narrow, shallow lens of lime-stone deposited in the narrow Central MontanaTrough and surrounded by littoral and supralit-toral sediments. Paleogeographical reconstruc-tions locate the Bear Gulch Limestone at alatitude between 10° and 12° north of theNamurian equator and position the marine bayas having been oriented in a northwest to south-east direction. It is characterized by up to fivefacies; the Arborispongia-productid, filamentousalgal, shallow, marginal, and central basin facies.The paleoclimate of this region would mostprobably have oscillated between semi-arid toarid and tropical conditions as the change of sea-sonal atmospheric circulation patterns shiftedfrom a dry to monsoonal phase and vice versa. The sedimentologic data present classical indica-tors of arid to semi-arid climatic conditions. Thedeposits of gypsum in the adjacent Heath Shalefacies and of gypsum and chert nodules in shal-low facies corroborate the interpretation of theregional paleoclimate. Gypsum in the shallowfacies and along the paleo-eastern margin wouldreflect high rates of evaporation during the drytimes and the terminal stages of filling in of thebay. The rhythmically recurring sedimentary lay-ers, especially those of the central basin faciessedimentary unit, show alternation of darkorganic-rich and light-colored laminar organic-poor subunits which, on a very large scale,resemble varve and sapropel deposits.Hydrologic and climatic analyses in concert withall sedimentologic data strongly suggest thatmicroturbidites form the bulk of the dark organ-ic-rich subunits and that these were principallygenerated during summer monsoonal storms.Torrential seasonal rains would have generatedthese microturbidites from sheetwash-erodedand resuspended, organic rich marginal sedi-ments and carried them in a hyposaline upperwater layer towards the central basin and baymouth. Repeating events of cascading turbiditydeposition during the monsoonal season accountfor microturbidite layers found atop scour-less
underlayers and the lack of any evidence forshorward transport of fauna.The paleontologic data from the Bear GulchLimestone present a high-diversity marine verte-brate record. The invertebrate record is of low tomoderate diversity, with highly localized occur-rences and conspicuous absences among the moretypical invertebrates of the upper Mississippian.There are virtually no terrestrial macrofossils orindicators of fluviatile input, in spite of the nar-rowness of the Central Montana Trough and theproximity of the shores of the Bear Gulch bay.Had there been significant rainfall throughoutthe year, the shallow marginal and terrestrialenvironments would have provided the ideallocations to generate fluviatile or terrestrial fos-sils. Persistent anoxia or hypoxia of the bottomwaters or sediments is inconsistent with the ubi-quitous benthic and burrowing fishes.The preservation of the fishes and soft-bodiedinvertebrates of the Bear Gulch Limestonerequires that death and burial were essentiallysimultaneous. Paleopathologic examination indi-cates asphyxia as the cause of death for severalfish. The zone of best preservation is limited tothe central basin, and all lines of evidence pointto the deposition of organic-charged, cascadingmicroturbidite flows as the agents of both deathand immediate burial.Seasonal variability in African Sahel-like climaticand wind conditions would have provided theessential atmospheric, hydrologic, sedimentary,and biologic conditions necessary to produceboth the repetitive microturbidites and thedetailed fossil preservation that characterize theBear Gulch Limestone.
AcknowledgementsThe impetus for this paper was provided by thepersistent question asked by four generations of aparticular ranching family, “How did the fish getbetween those rocks?”. We are deeply grateful tothis family for their friendship, hospitality, andcuriosity and respect their request for anonymity.This analysis would not have been possible with-out the fundamental works of L. A. Williams and
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313GEODIVERSITAS • 2002 • 24 (2)
J. Horner. We are indebted to H. Feldmanfor his many insightful discussions and thankL. J. V. Compagno, C. Lavett Smith, C. Bartho-lomew, H.-P. Schultze, and K. Porter for theirviews and comments on various aspects of thisproblem. We are also thankful to C. Scotese forproviding us with his paleomap image and per-mission to include it in this paper and toC. Poplin for her French translation of our writ-ing. It is unfortunately impossible to list themany volunteers who have contributed theirtime, effort, and resources to the excavations inthe Bear Gulch Limestone during the years 1968-2000. Particular thanks, however, go toR. Bugosh, C. DeBell, B. Hopkins, B. Kleinhaus,M. Moore, and B. Snyder for their years offriendship and unflagging devotion to the BearGulch field expeditions. We also thank thereviewers (D. J. Bottjer and J. Hagadorn) fortheir time and contributions.
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Submitted on 7 May 2001;accepted on 26 November 2001.
