16 RESEARCH REPORTS

Copyright Q 2002, SEPM (Society for Sedimentary Geology) 0883-1351/01/0017-016/$3.00

Ichnology, Sedimentology and Paleontology ofEocene Calcareous Paleosols From a

Palustrine Sequence, Argentina

RICARDO N. MELCHORCONICET & Universidad Nacional de La Pampa, Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina

JORGE F. GENISECONICET & Departamento de Icnologıa, Museo Paleontologico ‘‘Egidio Feruglio’’, Av. Fontana 140, 9100 Trelew,

Chubut, Argentina

SERGIO E. MIQUELCONICET & Museo Argentino de Ciencias Naturales ‘‘Bernardino Rivadavia’’, Av. Angel Gallardo 470, 1405 Buenos

Aires, Argentina

PALAIOS, 2002, V. 17, p. 16–35

Integrated analysis of the ichnology, sedimentology, geo-chemistry, and fossil content of three trace fossil-bearingcalcareous paleosols from the early Eocene Gran SalitralFormation is used to reconstruct the detailed paleoenviron-mental and paleoecological setting of this insect-dominatedichnofossil association. This continental sequence, locatedin southwestern La Pampa province (Argentina), is com-posed of palustrine marls and minor lacustrine mudstonesand sandstones arranged in shallowing-upward cycles.The ichnologic association is the first comprehensively de-scribed from a palustrine sequence and could be used for fu-ture comparisons with other similar assemblages. The ich-nofauna is dominated by bee cells, Celliforma germanica,C. roselli, and Rosellichnus isp.; Teisseirei barattinia, aninsect trace fossil that is redescribed and recorded for thefirst time outside its type locality; Taenidium barretti; andSkolithos linearis. The ichnologic association also includesovoid structures, ornamented burrow fillings, and planttrace fossils (rootlets, rhizoliths, and a tree / shrub stump).Abundant freshwater (Pomacea sp.) and terrestrial gastro-pods (Plagiodontes spp., Bostryx sp., and Bulimulus sp.)are used to characterize the paleoenvironmental setting.The weakly developed paleosols are interpreted as Incepti-sols, developed in a low-gradient lake margin periodicallyexposed to pedogenesis with low (probably saline) vegeta-tion and sparse shrubs. They developed under a semi-aridand warm climate (mean annual temperature higher than208C). Short saline / alkaline conditions were prevalentduring lake lowstands, accompanied by incipient karst de-velopment. The association of trace fossils is attributed pro-visionally to the Coprinisphaera ichnofacies, although itsdistinctive features and recurrence in time and space maydeserve recognition as a separate ichnofacies.

INTRODUCTION

Calcareous paleosols bearing insect trace fossils havebeen recorded from several localities in the world. Fren-guelli (1930) was the first to mention a fossil insect nest oc-

curring in a calcareous paleosol, later assigned to the Pa-leocene (Martınez et al., 1997). Other records were report-ed by Retallack (1984) from the Oligocene of the USA andby Thackray (1994) from the Miocene of Kenya. These pa-leosols share a similar paleontological content: terrestrialsnails, hackberry endocarps, fossil bee cells, and other un-determined invertebrate and plant traces.

This paper documents early Eocene calcareous paleo-sols occurring in a palustrine sequence, that bear a similarassociation of continental snails, fossil bee cells, and otherinsect trace fossils (Fig. 1A, B). Palustrine sequences com-monly are bioturbated (e.g., Platt and Wright, 1992), butno detailed description of their trace fossil content isknown, to date, except for the recent report of Edwards etal. (1998). Ichnofossils and gastropods are analyzed in theframework of a detailed description of the trace-bearingpaleosols, including field logging, micromorphology, x-raydiffractometry, major oxides ratios, and stable isotope geo-chemistry. These analyses have permitted detailed pa-leoenvironmental reconstruction and calibration of the en-vironmental and ecological parameters of this community.In this context, this particular insect-dominated ichnolog-ic assemblage may be used as a template for comparisonwith future ichnological/paleoecological studies of semiar-id wetland environments. Furthermore, considering its re-currence in time and space (Paleocene of Uruguay, Oligo-cene of USA, Miocene of Kenya, and Eocene of Argentina),this association in calcareous paleosols is considered a po-tential continental ichnofacies, as previously suggested byGenise et al. (2000).

GEOLOGICAL SETTING

The surface geology of the area bordering the large de-pression of Gran Salitral (meaning great salt lake, Fig. 1C)is composed of Permo-Triassic to Recent units (Linares etal., 1980; Melchor and Casadıo, 2000). Fault blocks of vol-canic rocks assigned to the Choiyoi volcanism (ChoiqueMahuida Formation) are overlain by early Danian shal-low-marine carbonates (Roca Formation), early Eoceneshallow-lacustrine deposits with paleosols (Gran SalitralFormation), and Eocene shallow- to deep-lacustrine sedi-

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 17

FIGURE 1—Location map and lithologic section of the Gran Salitral Formation. (A, B) Position of La Pampa province in South America andlocation of the study area. (C) Geologic map of the Gran Salitral depression area (modified from Melchor and Casadıo, 2000). Black star(arrowed) indicates the study locality (378 329 490 S; 678 049 140 W). (D) Composite lithologic log of the Gran Salitral and El Sauzal formationsat the study locality. The interval represented in Figure 2 is bracketed.

mentary rocks (El Fresco Formation). This succession iscapped unconformably by Pliocene (?) sandstones and cal-crete included in the El Sauzal Formation (Fig. 1D), whichare overlain by modern deposits.

The Gran Salitral Formation is exposed in a ca. 40-m-high mesa-like feature on the southeast and east marginsof the Gran Salitral depression (Fig. 1D). This unit wasproposed by Melchor and Casadıo (2000) to include conti-nental lithic arenites, siltstones, mudstones, and lime-stones of probable Miocene age that crop out in the west-ernmost area of the geologic map 3766-III ‘‘La Reforma’’(southwest La Pampa province). Further mapping and ad-ditional geochronologic information (Melchor, unpub-lished) indicate that this unit is restricted to outcrops onthe margin of the Gran Salitral depression and is early Eo-cene in age (Fig. 1C).

METHODS

Thin sections for micromorphology were prepared thin-ner than usual (;20 mm) and the descriptive terminologyof Bullock et al. (1985) was followed. To estimate modalcomposition of the paleosols, a total of 500 points per thinsection were counted. Rock color was measured dry with aRock Color Chart (Goddard et al., 1980).

A total of 33 stable isotopes analyses (C and O) on 7 bulkcarbonate samples, including one to three replicates toevaluate sample heterogeneity, were performed at theUniversity of Michigan Stable Isotope Laboratory. A pol-ished chip of sample 259A was drilled in spar-filled fenes-trae and micritic matrix to compare its isotopic composi-tion. Powdered carbonate samples weighting a minimumof 10 milligrams were placed in stainless steel boats andheated at 380 8C in vacuo for one hour to remove volatiles.Samples then were placed in individual borosilicate reac-tion vessels and reacted at 76 6 2 8C with 3 drops of an-hydrous phosphoric acid for 8 minutes in a Finnigan Kiel

preparation device coupled directly to the inlet of a Finni-gan MAT 251 triple collector isotope-ratio mass spectrom-eter. Isotopic enrichments are corrected for acid fraction-ation and 17O contribution by calibration to a best-fit re-gression line defined by two NBS standards, NBS-18 andNBS 19. Data are reported in ‰ notation relative to VPDB(Vienna Pee Dee Belemnite). At least six powdered car-bonate standards were reacted and analyzed daily, brack-eting the sample suite at the beginning, middle, and end ofthe day’s run. Measured precision is maintained at betterthan 0.1 ‰ for both carbon-and-oxygen isotope composi-tions.

Major element and some trace elements of bulk marland tuff samples were analyzed at Activation LaboratoriesInc. (Canada) by Inductively Coupled Plasma mass spec-trometry. FeO was analyzed by titration and CO2 by infra-red. Detection limits were 0.01%, except for FeO (0.1%)and CO2 (0.05%). Due to the low-Mg content of calcite (asrevealed by x-ray diffractometry), the total carbonate con-tent in Figure 2 was estimated by considering all CO2 con-tent (Appendix 1) as coming from CaCO3. The guidelinesof Retallack (1990a) were followed in the calculation andinterpretation of molecular weathering ratios. The chemi-cal index of alteration (CIA) was calculated as (Nesbittand Young, 1982; Fedo et al., 1995):

CIA 5 Al O /[Al O 1K O1Na O1CaO*]2 3 2 3 2 2

where CaO* 5 mol CaO2mol CO22(0.5 x mol CO2)2(10/3x mol P2O5).

The mineralogy of seven samples was identified from x-ray diffractometer patterns recorded on whole-rock pow-der, for the angular range 28–608 2u, with Cu Ka radiation,at 18 2u / minute, 35 Kv and 15 mA, on a Rigaku DenkiDMAX-IIIC diffractometer from the Universidad Nacionaldel Sur, Bahıa Blanca, Argentina.

Institutional abbreviations used in this paper are as fol-lows: MFLR: Museo ‘‘Francisco Lucas Roselli’’ (Nueva Pal-

18 MELCHOR ET AL.

FIGURE 2—Paleosol details including lithology, sedimentary structures, trace fossils, body fossils, sample position, dry color, micromorphology,modal composition, calcium carbonate content, molecular weathering ratios, and chemical index of alteration (CIA). 1, 2, 3 indicate paleosols1, 2, and 3, respectively. (1) Molecular weathering ratio formulae: Leaching 5 Ba/Sr, hydrolysis 5 (CaO 1 MgO 1 K2O 1 Na2O) / Al2O3,

salinization 5 Na2O/K2O, oxidation 5 (Fe2O3 1 FeO 1 MnO) / Al2O3, hydration 5 SiO2 / (Fe2O3 1 Al2O3), clay formation 5 Al2O3 / SiO2.

→

FIGURE 3—Selected paleosol macrofeatures. (A) Skolithos linearis in paleosol 1 (hammer 5 35 cm long). (B) Medium-sized rhizolith (seearrows) and Skolithos burrow (Sk) from top of paleosol 1 (scale in cm). (C) Dissolution pit filled with intraformational breccia near the top ofpaleosol 2. Notice abrupt truncation on top. (D) Tubular cavities, snails (s), and pseudo-microkarst (p), in the upper part of paleosol 3 (pen 513 cm). (E) Polished slab of sample 259A from top of paleosol 3 showing incipient glaebule (g), fine tubular cavities (t), and spar filled fenestrae(f). (F) Close-up of the top of the section showing lenticular bedding and the uppermost tuff bed (t). Hammer head is 15 cm long.

mira, Uruguay); MACN-LI: Museo Argentino de CienciasNaturales ‘‘Bernardino Rivadavia,’’ Laboratorio de Icnolo-gıa (Buenos Aires, Argentina); GHUNLPam: Catedra deGeologıa Historica, Universidad Nacional de La Pampa(Santa Rosa, La Pampa, Argentina).

RESULTS

Sedimentology

Dominant lithologies of the Gran Salitral Formation aregrayish orange pink (5YR7/2) to pale-red (10R6/2) marlswith common pedogenic modification (as described below),limestones, red laminated mudstones, fine-grained sand-stones, and vitric tuffs (Fig. 1D). The succession at the lo-cality studied begins with 6.6 m of pale orange (10YR7/2)to grayish orange pink (5YR7/2) limestones interbeddedwith sandstones and minor siltstones (Fig. 1D). The over-lying 9-m-thick section includes pedogenically modifiedtuffaceous marl, massive limestones, and discrete vitrictuff beds (Figs. 1D, 2); these are arranged in three shal-lowing-upward cycles (Fig. 2). The lower half of this inter-val is mostly structureless or exhibits poorly defined bed-ding. The upper half, is characterized by laterally contin-uous 0.1–0.5-m-thick beds with undulating contacts (Fig.3F). Carbonate content ranges from 11 to 36 % in volumeand is represented mostly by micrite and minor sparry cal-cite (Fig. 2). The succession is capped by 1.5-m-to-4 m-thick laminated pale red (10R6/2) to grayish red (5R4/2)lacustrine mudstone with minor interbedded tuff, lime-stone, and sandstone.

Paleosol Description

Three conspicuous paleosols were recognized in themeasured section of the Gran Salitral Formation (identi-fied by numbers 1–3 in Fig. 2), mostly based on their fieldfeatures. These paleosols are weakly developed (scale ofRetallack, 1990a, p. 265), poorly horizonated, and show anupward trend toward slightly more developed soil fea-tures. Special attention was paid to this interval becauseof the occurrence of abundant insect trace fossils and gas-tropods, which are accompanied by other invertebrate ich-nofossils, rhizoliths and root marks, and scarce vertebratebones (probably a notoungulate; Tonni, pers. comm.,2000). Figure 2 summarizes macroscopic and micromor-phologic attributes, as well as lithologic, petrographic, geo-chemical, ichnologic, and paleontologic data on these pa-leosols.

Macrofeatures

All paleosols mostly are developed on tuffaceous marl.Paleosol 1 (1.20 m thick) is composed of grayish pink (5R7/2) massive marl with common, distinct, fine pale yellow(10Y8/2) mottles (descriptive terminology after Retallack,1990a) and is devoid of insect trace fossils and gastropodsremains. A slight reddening is apparent toward the top ofthe soil profile, which is truncated erosively by the overly-ing bed. Its salient macroscopic feature is the abundanceof vertical burrows (Skolithos linearis), which obliteratesthe original sedimentary fabric (Fig. 3A). This paleosol

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 19

20 MELCHOR ET AL.

contains a few laterally spreading rhizoliths of mediumsize in its upper part (Fig. 3B).

Paleosol 2 (2.8 m thick) exhibits well-defined stratifica-tion in the lower half and is massive in the rest of the pro-file. Its color is very pale-orange (10YR8/2) with pale-yel-low (10Y8/2) mottles that change to grayish orange pink(5YR7/2) in the upper half-meter (Fig. 2). Locally, paleo-karst pits were observed in this and the overlying paleosol;some start in the upper third of paleosol 3 and penetratedown into the upper part of paleosol 2. These pits are ver-tical, cylindrical to funnel-shaped dissolution cavities(about 0.4 m wide and 0.5 m to ca. 3 m long) filled withsandstone-supported intraformational breccia (Figs. 2,3C). Pseudo-microkarst (Freytet and Plaziat, 1982) wasrecognized in the upper half-meter of paleosols 2 and 3, asa horizontal to vertical, intricate network of tubular cavi-ties (0.5–1 cm diameter) filled with calcareous sandstone.The most common ichnofossils in this paleosol are abun-dant bee cells, which are distributed throughout the soilprofile, even in the lowermost 0.15 m, where evidence forpedogenesis is scarce. This ichnofauna is associated withterrestrial gastropods (Plagiodontes spp.) and scarce no-toungulate remains.

Bedding in paleosol 3 (2.10 m thick) is well-defined andbecomes thinner towards the top. The topmost meter iscomposed of lenticular laminae interbedded with millime-ter-thick gypsum laminae; this is capped by a 0.35-m-thickash-fall tuff (Fig. 2). The tuff bed is highly indurated andits color is slightly redder (pale red) than the underlyingbeds (grayish orange pink), which display nodular weath-ering. Planar and curved voids (terminology after Freytetand Plaziat, 1982), incipient glaebules (Fig. 3E), and inter-connected tubular vugs (diameter less than 1 cm; see Fig.3D) were identified in the upper 3 cm of this paleosol.Plant traces occur as common fine tubules (Fig. 3E) and asingle tree or shrub stump cast is present.

Microfeatures

Main micromorphological attributes and compositionaldata are summarized in Figure 2. In paleosol 1, there arealso thin clay coatings on skeletal grains and patchy de-velopment of angular blocky micropeds limited by carbon-ate veins (Fig. 4A).

In paleosols 2 and 3, there is an upward increase in theproportion of volcanic grains (Fig. 2). Subspherical and su-bangular, scattered calcareous ooids and oomolds are pre-sent in the entire profile of paleosol 2 and part of paleosol3, although their abundance is less than 1% (Fig. 4B). Ooidmaximum diameter ranges between 0.48 mm and 0.15mm, averaging 0.28 mm. The best-preserved ooids exhibita faint radial structure and commonly a micritized outerrim (Fig. 4B). Mostly micrite and clay minerals, in addi-tion to minor opaque minerals, compose the groundmassof paleosol 2. Alveolar structure and sparite-filled fenes-trae (Esteban and Klappa, 1983) are very rare at 1.8 mfrom the top of paleosol 2 an upward increase in abun-dance. Microlaminated clay fillings, geopetal fabrics, claycoatings (grain cutans), and probable fecal pellets were de-tected only in the uppermost meter of the profile (Fig. 2).The uppermost 30 cm of paleosol 2 shows a clotted-peloi-dal structure (Fig. 4D) in the sense of Armenteros and Dal-ey (1998). Abundant sparitic cementation in localized are-

as transformed the rock into a diagenetic peloidal grain-stone (Fig. 4E). Peloids (40 mm 2 170 mm in diameter) aremore or less differentiated from the homogeneous micriticmatrix and usually are bound by a drusy sparite mosaic orveins. Probable silica nodules also were identified in theupper part of this paleosol.

In paleosol 3, groundmass composition, b-fabric, andclotted-peloidal structure are similar to that at the top ofpaleosol 2. The abundance of alveolar structure and claycoatings, along with geopetal fillings and fenestrae, aredistinct micromorphological features of this paleosol (Fig.5B, C, D). Fenestral cavities (up to 2 mm in diameter) arecemented by calcite crystal silt (below) and drusy sparitemosaic (above); a marginal, laminated micritic coating islocally present (Fig. 5D). Septarian and circumgranularcracks, pendant cement, and probable phytoliths alsowere identified sporadically. Clay coatings are more abun-dant in the uppermost part of paleosol 3 (reaching about1% in volume). They coat terrigenous grains, peloids, andcavities; they are microlaminated and commonly showcrescent morphology and cracks (Fig. 5C). In the middlepart of paleosol 3, sparse silica nodules, up to 2.5 mm in di-ameter, were observed in thin section. They contain spher-ulites (about 1 mm in diameter) consisting of quartzine(length-slow chalcedony) and coarse granular calcite crys-tals (Fig. 5E).

Whole-Rock Chemistry

Major element data are tabulated in Appendix 1. All themajor oxides except P2O5 and SiO2 increase upward in pa-leosol 2. For paleosol 3, this vertical trend is different (es-sentially reflecting differences between the topmost tuffand the underlying tuffaceous marl); all oxides increaseexcept for P2O5, Na2O, CaO, and MnO, which are depletednear the upper part of the soil. The chemical data in Ap-pendix 1 were used to estimate six molecular ratios (fol-lowing Retallack, 1990a) that are displayed in Figure 2.The chemical index of alteration (CIA; Nesbitt and Young,1982) ranges between 42 and 64, with the higher valuescorresponding to the upper part of the paleosols (Fig. 2).Gain and losses during soil formation were estimated us-ing a sample of the base of paleosol 2 as possible parentmaterial (sample 257A), and assuming TiO2 as a stableconstituent. This sample was chosen because of its verylow CIA and preservation of primary sedimentary struc-tures. Results of this procedure are plotted in Figure 6.

Stable Isotope Data

Analyses of C and O stable isotopes (n 5 33) correspondto whole-rock powder, carbonate matrix, and sparry cal-cite from fenestrae (the two last components only for sam-ple 259A). Results are tabulated in Appendix 2 and plottedin Figure 7. The carbonate phase analyzed is low-Mg cal-cite as inferred from x-ray diffractometry data.

Most of the data lie on an approximately linear trend inthe negative d13C, negative d18O quadrant, with spreads ofd13C from 24‰ to 27‰ and of d18O from 23‰ to 26‰(Fig. 7A). Whole rock values for paleosols 2 and 3 showlimited variation (d13C 5 25.65‰ to 24.52‰, d18O 523.51‰ to 23.03‰), although there is a general upwardtrend to lighter values, especially for d13C (Fig. 7B). The

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 21

FIGURE 4—Photomicrographs of paleosols 1 and 2. (A) Localized development of blocky micropeds from paleosol 1 (sample 256A). (B) Complete(above) and subangular broken ooid (below) with planar nucleus (skeletal remain; sample 257C). (C) Laminated clay filling (sample 257D). (D) Clotted-peloidal fabric with abundant peloids and crystallitic b-fabric (c). Note that siliciclastic grains are included in some peloids (sample 257C). (E) Diageneticgrainstone composed of peloids and abundant drusy sparite (sample 257D). Photographs with ordinary light except for C (crossed nicols).

components analyzed separately in sample 259A displaygreater heterogeneity; the more enriched values approachthe isotopic composition of the micritic matrix carbonate,and the more depleted values are closer to the compositionof the sparry calcite of fenestrae (Fig. 7A). Intermediatevalues likely reflect different contributions of each com-ponent (matrix and spar).

X-ray Mineralogy

The dominant minerals identified by X-ray diffractom-etry in the seven samples analyzed are low-magnesiumcalcite, plagioclase feldspar, zeolites (in particular mor-denite, phillipsite, and clinoptilolite), and quartz (Appen-dix 3). MgCO3 content of calcite is usually 1–2%, although

22 MELCHOR ET AL.

FIGURE 5—Photomicrographs of paleosol 3. (A) Microscopic view of the uppermost tuff bed within paleosol 3 showing abundant glass shards,micrite, and probable phytoliths, in addition to drusy calcite cementation and localized development of peloids (arrow; Sample 259A). (B)Fenestral and alveolar structures from sample 258A. pl 5 plagioclase, q 5 quartz. (C) Cavity with a complex, geopetal filling including micriteplus silt-sized siliciclastic grains (margin), microlaminated clays with quasi radial cracks (arrow), and drusy calcite (center; sample 259A). (D)Geopetal filling of cavities. Microlaminated micrite and clay with pendant geometry (m), crystal silt (c), and calcite spar (s). (E) Nodule composedby quartzine (qz) and calcite (c). Sample 258A. Photographs with ordinary light except for E (crossed nicols).

in a single sample it reaches about 3–4%. Other mineralsrecognized in minor amounts are K-feldspar and probablesepiolite. The presence of zeolites is important in the se-quence (about 20% to 30% whole rock).

Trace Fossil AssemblageThe ichnologic assemblage is dominated by insect trace

fossils (especially bee cells and chambers of uncertain ori-

gin), meniscate burrows, ornamented burrow fillings,ovoid structures, and plant ichnofossils (Figs. 2, 3A, B, E,8–12). Bee cells are so abundant in paleosol 2 that, whenbroken with a hammer, almost every fist-sized piece ofrock bears at least one cell.

Three ichnotaxa of bee cells have been distinguished:Celliforma germanica, Celliforma roselli, and Rosellichnusisp. The Celliforma specimens include isolated cells and

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 23

FIGURE 6—Estimated elemental gains and losses during soil formation.The plotted ratio measures the degree of departure from the estimatedconcentration of each element (see text for explanation). Changes inpaleosol bulk density were not measured. Data from Appendix 1.

FIGURE 7—Carbon and oxygen isotopic data (note that symbols usedin A are not equivalent to those used in B). (A) Cross-plot of d13C andd18O (PDB) for the different phases analyzed by stable isotope geo-chemistry (see text for discussion). (B) Vertical distribution of d13C andd18O (PDB). Open symbols: whole-rock data; solid symbols: materialdrilled on polished slab (sample 259A). See Figure 2 for additionalreferences. Data from Appendix 2.

cells arranged in rows, while Rosellichnus is characterizedby clusters of cells (cf. Genise, 2001). Celliforma germanica(Figs. 8A, B, C, 9A) includes elongated, tear-shaped cellshaving rounded bottoms and flat tops, which are precededby a constriction or neck. In some specimens, the neck maybe masked by a ring of matrix (i.e., Frenguelli, 1938a; fig.3). The length of the cells ranges from 14 mm to 20 mmand the maximum diameter from 4 mm to 7 mm (mean 56 mm, n 5 35). Their orientation in the paleosols is sub-vertical to subhorizontal. Cells may occur as isolated trac-es or grouped in sloping rows of two to four cells (Fig. 9A).Two tiered rows of cells were observed. In one case, thebottom of a single cell was in contact with the top of thethird cell of a row of three. In the other case, a row of twocells was separated by a short distance from a row of four.Rows of cells attached to one side of a tunnel are commonin the nests of many digging bees. On the other hand, cellclusters composed of cells arranged simultaneously inrows and in end-to-end series are unknown from modernnests, suggesting that this tiered arrangement resulted,more likely, from the close construction of end-to-end se-ries of cells at the distal extremes of independent, lateraltunnels excavated in close proximity. Cells having necksand arranged in an end-to-end design are common in nestsof species of the Agapostemonini (Roberts, 1969) in whichcells may be so densely grouped that they may look like ti-ered series as those described herein (e.g., Abrams andEickwort, 1980). Specimens described by Schutze (1907)and Frenguelli (1938a), and some of those described byFrenguelli (1930, 1938b) and Martınez et al. (1997; Celli-forma isp. A), belong also to C. germanica and occur in cal-careous paleosols.

Celliforma roselli (Figs. 8D, E, 9B) comprises cylindricalto barrel-shaped, stout cells having rounded bottoms andflat or conical tops and lacking any other distinctive fea-ture. The length of the collected cells ranges from 20 to 39mm, but most of these specimens were broken, precludingthe proper estimate of the mean value. The largest mea-sured specimen is 39 mm (GHUNLPam 12474; Fig. 8E)and the second largest (34.5 mm) known is that collectedby Frenguelli (1938b, plate VII, fig. 9). The length of the

remaining three complete specimens ranges from 29 to 31mm. The maximum diameter of the longest cell(GHUNLPam 12474) was 18 mm. The maximum diame-ter of the remaining cells (including those described byFrenguelli) ranges from 11 to 15 mm (mean 5 13.28 mm,n 5 16). Disposition of the cells in the paleosol was obliqueto subvertical. Cells occur mostly as isolated traces but agroup arranged in a row of six cells was also collected (Fig.9B). Diameter of these cells is 11 mm and, in some of them,

24 MELCHOR ET AL.

FIGURE 8—Selected specimens of Celliforma germanica (A, B, C)and Celliforma roselli (D, E). (A) MACN-LI 1314, X 1.6. (B) GHUNLP-am 12465, X 0.95. (C) GHUNLPam 12460, X 0.9. (D) GHUNLPam12464–3, X 1.0. (E) GHUNLPam 12474, X 0.8.

FIGURE 10—Rosellichnus isp. (A) Oblique field photograph of cluster(scale divisions on bottom 5 1 cm). (B) View of cluster from aboveshowing overall geometry and discrete wall indicated by arrow (X 1.1).(C) View of the rounded ends of cells (X 1.2). Notice the presence ofmatrix around cells. Specimen MACN-LI 1316.

FIGURE 9—Field photographs of groups of Celliforma cells from pa-leosol 2. (A) Groups of two and three cells of C. germanica in slopingrows. Scale divisions 5 1 cm. (B) Close-up of row of six cells of C.roselli (individual cells arrowed). Only five cells are shown.

it was possible to recognize remains of a thin, discretewall. Some of the fossil cells mentioned by Frenguelli(1930, 1938b) and by Martınez et al. (1997; Celliforma isp.B) also belong to C. roselli and occur in calcareous paleo-sols.

Rosellichnus isp. (Fig. 10) is composed by cells disposedin two rows of three and two cells, respectively. Cell di-ameter is 7 mm and cell length ranges from 21 to 25 mm.However, cell length may be overestimated because of thepresence of a matrix hood at the tops. Isolated cells are inall aspects identical to Celliforma germanica, suggestingthat at least some of them or those arranged in rows prob-ably represent the first instars of the cluster construction.Thus, there is a high probability that the same trace mak-er constructed at least part of Celliforma germanica and

Rosellichnus isp. In contrast with the described ichnospe-cies, R. arabicus and R. patagonicus, of uncertain origin,Rosellichnus isp. closely resemble in morphology, size, andthin cell walls the augochlorine clusters.

Teisseirei barattinia (Fig. 12A, B, C, D, E, I), another in-sect ichnotaxon (although of uncertain origin), which isabundant and restricted to the top of paleosol 3. Teisseireibarattinia is redescribed herein on the basis of abundantmaterial collected in La Pampa (this study), Colon (alsofrom Argentina), and many localities in Uruguay (Appen-dix 4). Other trace fossils of probable insect affinity are‘‘ovoid structures’’ (Fig. 12G, H), that frequently have beeninterpreted as wasp cocoons (e.g., Thackray, 1994; Bownet al., 1997). They display an ovoid outline, having round-ed extremes (length: 14–17 mm, width: 8 mm).

Meniscate burrows occur associated stratigraphicallywith T. barattinia (Fig. 2) and are assigned to Taenidiumbarretti (Fig. 11). This ichnotaxon includes slightly curved,unwalled, subhorizontal burrows of uniform diameter (8–9 mm) with thin, deeply arcuate menisci.

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 25

FIGURE 11—Taenidium barretti from top of paleosol 3 (GHUNLPam12486). Three subhorizontal specimens are shown (arrowed) alongwith a transverse section of probable empty bee cells (b).

FIGURE 12—Teisseirei barattinia (A-E, I) and other trace fossils (F-H). All photographs of Teisseirei barattinia taken with the concave sideup except for E. See complete description in Appendix 4. (A) Largestcollected specimen, GHUNLPam 12462–1, X 0.8. (B) Medium-sizedspecimen, GHUNLPam 12473–5, X 0.8. (C) Laterally curved speci-men of T. barattinia, GHUNLPam 12478–5, X 0.7. (D) T. barattiniawith antechamber, MACN-LI 1309, X 0.7. (E) Lateral view of T. bar-attinia, GHUNLPam 12462–2. (F) Ornamented burrow filling with scalysurficial texture, GHUNLPam 12487, X 0.9. (G, H) Ovoid structures,GHUNLPam 12475–1/2, X 1.05. (I) Detail of the surface ornamenta-tion in Teisseirei barattinia (MACN-LI 1308), X 1.6.

Two additional types of burrow were recognized: Skoli-thos linearis (Fig. 3A) and ornamented burrow fillings(Fig. 12F). Skolithos linearis comprises dominantly verti-cal, straight to slightly curved burrows with subcircularcross-section (burrow diameter ranges from 7.5 mm to11.0 mm), smooth boundaries, and no wall. They are veryabundant in paleosol 1 (Figs. 2, 3). Ornamented burrowfillings are internally massive and display a distinct scalysurface morphology, that is similar to the scratches ob-served in the best preserved specimens of Teisseirei (com-pare Fig. 12F and 12I).

Three different types of plant ichnofossils were identi-fied: a single tree / shrub stump cast in paleosol 3 (about0.15 m in diameter), medium sized rhizoliths (2 cm in di-ameter) with laterally spreading root molds on top of pa-leosol 1 (Fig. 3B), and common fine rootlets (Fig. 3E) main-ly in the upper part of paleosols 2 and 3 (see also Fig. 2).Rootlets are open or spar-filled, tubular cavities, up to 2mm in diameter, which are abundant in the upper part ofpaleosols 2 and 3 (Figs. 2, 3E).

Gastropod Association

The collected material, largely preserved as internalmolds, comprises both freshwater and terrestrial gastro-pods whose stratigraphic distribution is indicated in Fig-ure 2. In the upper part of paleosol 3, terrestrial snails laywith their largest axis subhorizontal and with the aper-ture facing down, which is similar to the usual positionduring life. Terrestrial gastropods found in situ are re-stricted to the upper part of paleosol 3 (associated withfreshwater gastropods) and in the lower part of paleosol 2.

The gastropod association includes numerous speci-mens of the freshwater snail Pomacea sp. (Fig. 13A) andthree taxa of terrestrial snails, including Plagiodontes spp.(Figs. 13C, D) and scarce remains of Bostryx sp. (Fig. 13B)and Bulimulus sp. (Fig. 13E). A detailed taxonomic treat-

ment of this fauna will be the focus of an additional contri-bution (Miquel, in prep.). In Argentina, the species of theAmpullariidae (which includes Pomacea sp.) inhabit sub-tropical to temperate areas up to 388 S. The Ampullariidaehave branchia and lungs, preferring quiet and clear bodiesof water, rivers, as well as temporary and permanentponds. They also can withstand periods of desiccation andcan live in waters of various salinities (Castellanos andFernandez, 1976). The modern species of Plagiodontes in-habit principally semiarid areas of northern and centralArgentina, reaching the Tandilia-Ventania mountain sys-tems (; 388 S) in Buenos Aires province (Parodiz, 1939;Fernandez, 1973). Similarly, the modern species of Buli-mulus that closely resemble the fossil material is found inthe semiarid Monte region of Argentina (Cabrera and Wil-link, 1973).

26 MELCHOR ET AL.

FIGURE 13—Freshwater and terrestrial gastropods. (A) Pomacea sp.,GHUNLPam 12467–5, X 0.7. (B) Bostryx sp., GHUNLPam 12463–2,X 0.7. (C) Plagiodontes sp., GHUNLPam 12470–10, X 0.8. (D) Pla-giodontes sp. MACN-LI 1330, X 0.65. (E) Bulimulus sp., MACN-LI1332, X 1.5.

TABLE 1—Key sedimentologic, ichnologic and paleoecologic featuresof the Gran Salitral palustrine sequence.

Lithology and paleoenvironmental settingTuffaceous marl with vadose textures and minor interbedded mud-stones deposited in a shallow-lacustrine low-gradient margin. Thelake experienced repeated hydrological restrictions.

Palustrine featuresShallow water facies, shallowing upward cycles with freshwatergastropods capped by paleosols and terrestrial gastropods, rhizo-liths, grainification, pseudomicrokarst, clotted-peloidal microstruc-ture, paleokarst pits.

Lake salinityLonger freshwater periods during lake highstands (abundantfreshwater gastropods, waterlain structures) punctuated by mod-erately saline/alkaline periods (evaporite nodules, gypsum lami-nae, radial aragonitic ooids) during lake lowstands. Lake marginswere flooded about one month each year.

Paleosol characteristicsPoorly horizonated, weakly developed calcareous paleosols (Incep-tisols), commonly with well-drained profile. Mildly saline/alkalinesoil conditions favored zeolite authigenesis.

Ichnologic associationBee cells (Celliforma germanica, C. roselli, Roselichnus isp.), otherinsect trace fossils (Teisseirei barattinia, ovoid structures), menis-cate burrows (Taenidium barretti), dense concentration of Skolithoslinearis burrows, unidentified burrow fillings, medium-sized rhi-zoliths, and tree or shrub stump.

Invertebrate trace makersInsect dominated trace fossil association. Three or more species ofsweat bees (tribes Augochlorini and Agapostemonini), other un-identified insects (producers of Teisseirei and ovoid structures), ar-thopods and/or annelids (Skolithos, Taenidium).

Gastropod associationFreshwater (Pomacea sp.) and terrestrial snails (Plagiodontes spp.,Bostryx sp., Bulimulus sp.), the latter commonly restricted to pa-leosol tops.

VegetationSparse low vegetation, probably saline shrubs and sage (featuresof plant ichnofossils, absence of dung-beetle trace fossils).

ClimateSemiarid (as suggested by sparse plant ichnofossils, abundant hy-menopterous nests, gastropod association, paleokarst in calcareouspaleosols, and scarce evaporites) and warm climate (mean annualtemperature higher than 208C).

DISCUSSION

Table 1 summarizes essential descriptive and interpre-tative characteristics on the ichnology, sedimentology,andpaleontology of the studied Eocene continental succession.Each of these are discussed below.

Paleoenvironmental Setting

The sequence is interpreted as a palustrine carbonatedeposit (mostly pedogenically modified lacustrine marl-stones) in the sense of Freytet (1973, 1984), Freytet andPlaziat (1982), and Platt (1989). Palustrine facies are typ-ical of low-gradient, shallow freshwater environments inwarm climates with reduced clastic supply (Platt andWright, 1992). Palustrine carbonates have received in-creased attention recently and detailed descriptions werepublished by Esteban and Klappa (1983), Alonso-Zarza etal. (1992), Platt and Wright (1992), and Armenteros andDaley (1998).

The main palustrine features found in the Gran Salitralmarls include: shallow water lithofacies; shallowing-up-ward cycles containing water-lain structures capped bypaleosols; an association of freshwater and terrestrial gas-tropods, rhizoliths, root tubules; ‘‘grainification’’ (Alonso-Zarza et al., 1992); ‘‘pseudo-microkarst’’ (Freytet and Pla-ziat, 1982); and clotted-peloidal microstructures (Armen-teros and Daley, 1998). The Gran Salitral palustrine faciesare interpreted as having been deposited in a low-gradientlake-margin setting (cf. Freytet and Plaziat, 1982; Platt,1989; Platt and Wright, 1991) subjected to repeated desic-cation and flooding in response to water-level fluctuations.These palustrine deposits are closer to the semi-arid typedistinguished by Platt and Wright (1992), because mostcycles commence with horizons preserving freshwater fau-na but cycle tops contain minor evaporites, terrestrial andfreshwater gastropods, and evidence of desiccation. In ad-dition to sedimentary features, a subaqueous origin for theparent material of paleosols 2 and 3 is reinforced by thepresence of the freshwater gastropod genus Pomacea. Theoccurrence of Pomacea sp. and terrestrial snails (Plagio-

dontes spp.) on top of paleosol 3 also attest to the inferredsubaqueous origin and later subaerial exposure of thesesediments. Water-level changes exposed large littoral ar-eas to pedogenesis, vadose diagenesis, and to colonizationby burrowing animals (mostly insects) and sparse plants.

For the exposure surfaces described in this study, a firstapproximation to the mean number of days per year thatthe ground surface would have been covered with water,can be made by comparison with the exposure index forfreshwater palustrine environments of Platt and Wright(1992). It is likely that paleosols 2 and 3 were emergent formost of the year, being covered with water for about 30days per year (cf. fig. 8 of Platt and Wright, 1992). Proba-ble hydromorphic features (mottling, laterally spreadingrhizoliths) and absence of bee cells in paleosol 1 might in-dicate a longer flooding period over the course of its devel-opment.

Clotted-peloidal microstructure usually is interpretedas the result of repeated wetting and drying of lacustrinecarbonate mud and likely formed via the following se-

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 27

quence: carbonate mud → nodulization (pelletization) →curved fissuring → coating of nodule (peloid) surfaces→isolation of nodules (e.g., Freytet and Plaziat, 1982; Platt,1989; Armenteros and Daley, 1998). Pseudo-microkarststructures, described and interpreted in detail by Freytetand Plaziat (1982) and Alonso-Zarza et al. (1992), are re-lated to palustrine facies. They are considered as a pedo-genic to very early diagenetic feature related to root pene-tration prior to lithification with minor participation ofdissolution processes, distinguishing it from true karst.Other processes in pseudo-microkarst formation are des-iccation, reworking, and microorganism activity (Alonso-Zarza et al., 1992). Paleokarst pits associated with thetops of paleosols 2 and 3 indicate a significant hiatus be-tween successive lake-flooding events. Karst processesprobably accompanied soil formation and were related tolocalized dissolution of the carbonate substrate, possiblyenhanced by concentration of rainwater by tree stem-flow,in a similar way as proposed by Vanstone (1998).

Paleosol Interpretation

All described paleosols are similar to Inceptisols (SoilSurvey Staff, 1999). They are distinguished from Entisolsbased on their finer grain size, composition of the parentmaterial, and presence of a cambic horizon (denoted bylimited accumulation of carbonate and clays in the upper-most preserved horizons). Other typical features for Incep-tisols that could be identified or inferred include the abun-dance of easily weatherable volcanic material, develop-ment in young geomorphic surfaces, preference for depres-sions, and reduced rainfall (Foss et al., 1983; Fanning andFanning, 1989; Buol et al., 1990). Limited salinization andabsence of a calcic, petrocalcic, or gypsic horizon distin-guishes these soils from Aridisols (cf. Fanning and Fan-ning, 1989). Furthermore, the relative abundance of vol-caniclastic particles and zeolites is not considered signifi-cant enough to compare these paleosols with Andisols; thecontent of volcanic glass commonly is reduced, whereasAndisols typically support abundant vegetation and devel-op in areas of considerable rainfall (Soil Survey Staff,1999).

Paleosol parent material was a carbonate-cemented,moderately zeolitized, secondary volcaniclastic depositthat have been reworked in a shallow lake. Carbonate par-ticipation was derived, in part, from clastic bedload (andprobably eolian input) and from pedogenesis/early diagen-esis. Molecular weathering ratios (Fig. 2) and the calcula-tions of geochemical changes during soil formation (Fig. 6)indicate that these paleosols suffered little transformationduring pedogenesis.

The uppermost part of paleosol 1 exhibits a darker color,rhizoliths, moderate leaching, and clay formation. Therest of the profile is compared to a C horizon with about30% CaCO3, and lacking any conspicuous segregation ofcarbonate. The total destruction of the sedimentary fabricin this interval was due chiefly to the activity of burrowingorganisms that excavated vertical dwelling burrows (Sko-lithos linearis).

A cambic horizon (Bw) is identified within the uppermeter of paleosol 2, especially based on the presence ofmassive structure and slightly darker color than the un-derlying soil profile. Due to the profusion of bee cells found

in this horizon, bees likely played an important role inmodification of the depositional fabric. Alveolar and fenes-tral structures were recorded as deep as 1.8 m below thetop of paleosol 2, suggesting a probable maximum depth ofroot penetration. Very low CIA values (ranging from 41 to47, see Fig. 2) for the underlying C horizon and the pres-ence of relict layering and scarce microscopic pedofeaturesare indicative of minor weathering and pedogenic modifi-cation. The upward decrease in the amount of P2O5 in theprofiles for paleosols 2 and 3 is interpreted as recycling byplants and downward movement of percolating waters(Fenton, 1983).

In paleosol 3, an upper cambic horizon also can be dis-cerned (Bw) on the basis of its color, structure (massivewith abundant voids and rootlets), high induration, evi-dence for translocation of clays (clay coatings), and deple-tion of carbonates (Fig. 2). The lower horizon is labeledtentatively as Ck due to the sharp increase (11 to 29%) incalcium carbonate content compared with the overlyingBw horizon (Fig. 2). Paleosol 3 also is weakly developed,but the presence of a clear cambic horizon reflects a slight-ly more developed state. Paleokarst pits in paleosols 2 and3 may suggest a slightly higher elevation during their de-velopment than in paleosol 1 (probably less than 3–5 mabove lake level; see Platt and Wright, 1992).

The isotopic values for carbonates (Appendix 2) fall inthe known range for calcretes (e.g., Talma and Netteberg,1983; Rossinsky and Swart, 1993) and are close to avail-able isotopic values for palustrine carbonates (Platt, 1989;Wright and Alonso-Zarza, 1992, Armenteros et al., 1992;Armenteros and Daley, 1998; Alonso-Zarza and Calvo,2000; Tanner, 2000). A trend towards lighter isotopic val-ues upsection is evident in Figure 7B, which parallels aconcurrent higher degree of soil development inferredfrom macroscopic and micromorphological features. Inparticular, there is a slight decrease in d13C values and asubtle increase in d18O indicative of paleoexposure surfac-es near the tops of paleosols 2 and 3 (Allan and Mathews,1982; Goldstein, 1991). This is consistent with the valuesfor the chemical index of alteration (CIA), which denotesan increase in chemical weathering toward the top of pa-leosols 2 and 3 (Fig. 2).

The light d13C values obtained are interpreted as reflect-ing modification due to influence of fractionated meteoricgroundwater and isotopically light, soil-derived CO2 in anear-surface, vadose diagenetic setting (Allan and Mat-thews, 1982; James and Choquette, 1990; Goldstein, 1991)or pedogenic setting (Beier, 1987; Cerling et al., 1988;Platt, 1989; Wright and Alonso-Zarza, 1992; Armenterosand Daley, 1998). The lightest d13C value (ca. 27.1 ‰) like-ly represents the closest approximation to the carbon iso-topic composition of the soil-gas charged diagenetic fluids(cf. Cerling, 1984).

Lake and Soil Salinity

In the present study, the morphology of calcareous la-custrine ooids provides clues about lake salinity. The highpercentage of syndepositionally broken ooids (Fig. 4B),faint radial microstructure in some ooids combined withsmall diameters (mean 0.28 mm), and evidence for ooiddissolution (oomolds) suggest that Gran Salitral ooidswere originally radial aragonitic ooids associated with hy-

28 MELCHOR ET AL.

persaline shorelines similar to those of the Great SaltLake (e.g., Halley, 1977) and the Late Triassic MerciaMudstone Group at Clevedon, U.K. (Milroy and Wright,2000). The presence of scarce silica nodules composed ofquartzine are considered as a by-product of replacement offormer gypsum or anhydrite nodules (Milliken, 1979;Hess, 1990). Sample 257D from the top of paleosol 2 con-tains about 3–4% MgCO3, which might indicate a high-Mgcalcite precursor. This mineral is most frequent in salineor brackish-water lakes (Eugster and Kelts, 1983). Thisevidence, along with the occurrence of abundant remainsof freshwater gastropods (Pomacea sp.) throughout thesection, would imply that the lake experienced short (hy-per)saline periods (during lake draw down) within longerfreshwater intervals. Despite evidence for moderate sali-nization, the stable isotopic data do not indicate fraction-ation associated with evaporative processes (Fig. 7A).

There is additional geochemical and mineralogical evi-dence that suggests partially saline and/or alkaline condi-tions during lake lowstands and soil development. Thesoda/potash ratio, which is close to 2 in the succession, in-dicates that pedogenesis was salt-influenced (Retallack,1997). The increase in the ratio of bases to alumina (aproxy for the degree of hydrolysis) is suggestive of alkalin-ity or poor development of the soil (Retallack, 1990a). Theincrease in the ratio of silica to sesquioxides is related to ascarcity of hydrated minerals (Fig. 2).

Sedimentary zeolites are considered authigenic miner-als, formed under saline and alkaline soil or lake settings(e.g., Hay, 1981; Gude and Sheppard, 1986; Retallack,1990a; Renaut, 1993). The presence of abundant volcanicglass and plagioclase favors zeolite authigenesis, althoughthese features are not considered an essential factor (Re-naut, 1993). Following Hall (1998), it is considered that re-working of pyroclastic detritus in a lacustrine environ-ment and subsequent pedogenesis in an alkaline soil en-vironment probably is responsible for zeolite authigenesisin the Gran Salitral Formation.

Ichnology

The insect trace fossil association found in the calcare-ous paleosols of the Gran Salitral Formation is similar tothat recorded from the Mercedes Formation of Uruguay(Martınez et al., 1997). Specimens of Celliforma are simi-lar not only in general aspect but also in the arrangementof cells in rows. Moreover, the material mentioned and il-lustrated by Frenguelli (1930, 1938a,) from the MercedesFormation and the material described herein includes oneof the largest and one of the smallest specimens of Cellifor-ma known. These ichnological similarities are also accom-panied by the preservation of continental snails and ovoidstructures in paleosols from both deposits. In contrast, thehackberry endocarps found in Uruguay were not recordedin La Pampa, although many kilometers of outcrops arestill unexamined. It is also noteworthy that the Teisseireibarattinia recognized in the current study is absent in theMercedes Formation. This ichnotaxon is recorded for thefirst time outside the typical Chacoparanense region (i.e.,Uruguay and the neighboring Entre Rıos province in Ar-gentina). This expands its regional importance althoughits biochron seems to be restricted to the Late Cretaceous-Oligocene interval (cf. Genise et al., 2000).

Calcareous paleosols recorded from United States andKenya (Retallack, 1984; Thackray, 1994), apart fromshells of continental snails and hackberry endocarps, con-tain similar fossil bee nests, which, in turn, are differentfrom the clustered type described herein. This fact couldreflect the predominance of different types of nest archi-tecture in the Halictinae (the possible trace makers for allmentioned cases), inhabiting the northern and southernHemispheres. In North America and Africa, the nests arecomposed of cells directly attached to main tunnels, a com-mon design for nests of the cosmopolitan tribe Halictini. Incontrast, the clustering of cells, like that of Rosellichnusisp., is more common in the neotropical Augochlorini(Eickwort and Sakagami, 1979). Similar bee traces en-countered in two different South American paleosols sug-gest the existence of bees (halictines?) that preferred cal-careous, alkaline soils to nest, an observation apparentlynot recorded in the literature of modern bees. Sakagamiand Michener (1962) noted that, for the Halictinae, chem-ical and pedological properties of soils seemed to be of littleimportance in the process of selecting a nesting site. Cane(1992) analyzed soil texture and moisture where bees nest,pointing out that there were no studies for other soil prop-erties. Rozen (1964) recorded the only bee, Svastra obliqua(Anthophorinae), known to nest in salt flats. In terms ofbee diversity, it is possible to identify at least three speciesin the paleosols of La Pampa, each one constructing a dif-ferent nesting trace. Nonetheless, this number may behigher if the dispersion of cell sizes included in both ich-nospecies of Celliforma is considered. At least in Cellifor-ma roselli, the two largest specimens probably belong toone species of bee, while the smaller specimens (30 mm inlength and 15 mm in diameter) seem to belong to anotherspecies. The trace maker of Rosellichnus isp. could havebeen an Augochlorini (as suggested by cell morphology,size, and thin walls), whereas that of C. germanica mayhave been another species of sweat bee, possibly from theAgapostemonini (cf. Roberts, 1969; Abrams and Eickwort,1980); both taxa are well represented in the Neotropicalregion at present.

The remaining invertebrate trace fossils convey little in-formation about paleoenvironmental conditions and tracemakers. Skolithos has been reported from a wide variety ofenvironments, both marine and continental (e.g., Fillionand Pickerill, 1990). Dense groupings of Skolithos in con-tinental settings (like that within paleosol 1) have beenrecognized in fluvial channel facies and point bars (Fitz-gerald and Barrett, 1986; Woolfe, 1990; Buatois and Man-gano, 1996). This ichnotaxon is interpreted as a dwellingburrow probably produced by an arthropod (cf. Ratcliffeand Fagerstrom, 1980; Fitzgerald and Barrett, 1986). Inthe present study, periodic fluctuation of the water table isconsidered to have been a critical factor for the occurrenceof dense groupings of Skolithos (cf. Fitzgerald and Barrett,1986). Taenidium barretti is recorded only from continen-tal facies (Keighley and Pickerill, 1994). Different organ-isms have been suggested as producers of Taenidium(Buatois and Mangano, 1996), although the environmen-tal setting and diameter of the described specimens prob-ably restrict the potential trace makers to annelids (earth-worms?) and arthropods.

The association of fossil bee cells in paleosols, like thatdescribed herein for paleosols 2 and 3, is taken as indica-

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 29

TABLE 2—Comparison of fundamental features for four known ichnological associations of insect trace fossils in calcareous paleosols.

Unit(location)

Mercedes(Uruguay)

Gran Salitral(Argentina)

Brule(South Dakota)

Higewi(Kenya)

Soil horizons Cca? Bw-C A1-A2-A3-B2irt-B3ir-Cca A-A/C-CParent material Quartz sandstone Reworked intermediate py-

roclastic materialReworked volcanic ash fall

and alluviumReworked carbonatitic vol-

canic ashSoil development Moderate Weak Strong WeakSoil type Inceptisol Petrocalcic PalustalfAge Paleocene Early Eocene Early Late Oligocene Early–Middle MioceneInsect trace fossils Celliforma germanica, Cel-

liforma roselli(?)Celliforma germanica, Cel-

liforma roselli, Rosel-lichnus isp., Teisseireibarattinia, ovoid struc-tures

Celliforma ficoides, Pallich-nus dakotensis

Celliforma habari, cocoons

Associated ichnofos-sils

Rhizoliths Root traces, Skolithos li-nearis, Taenidium bar-retti, unnamed burrowfillings

Various root traces Root traces, burrows

Bee tribe Augochlorini Augochlorini, Agaposte-monini

Halictini Halictini

Associated fossils Hackberry endocarps, ter-restrial snails

Freshwater and terrestrialsnails, notoungulatebones, herbaceous re-mains

Hackberry endocarps, ter-restrial snail, vertebratebones

Hackberry endocarps, ter-restrial snails, grassleaves

Vegetation Trees (Celtis) Saltbush and sage (lowvegetation)

Gallery woodland 1 savan-na (Celtis)

Savanna 1 dry forest

Paleoclimate Warm, arid Warm, semiarid Seasonal dry or cool, sub-humid, sutropical towarm temperate

Humid–subhumid

Source Martınez et al. (1997), Ve-roslavsky et al. (1997)

This paper Retallack (1984) Thackray (1989, 1994)

tive of open herbaceous paleoenvironments, consideringthat soil-nesting bees commonly prefer well-drainedground, that is exposed to solar radiation (Genise andBown, 1994; Genise et al., 2000). In addition, the other in-sect trace fossil occurring at the Gran Salitral Formation,Teisseirei barattinia, is associated with bee and dung-bee-tle trace fossils in the type locality (Genise and Laza,1998), suggesting a similar environment. The absence ofdung-beetle trace fossils might indicate sparse plant cov-erage, such as that found within xeromorphic shrublands,which is consistent with the gastropod association, salini-ty and alkalinity indicators, and lack of extensive rooting.

A comparison with the other three known occurrences ofthe association of bee nests and continental gastropods incalcareous paleosols from Uruguay, Kenya, and the USA(Table 2), allows for the identification of some shared fea-tures. These include: (1) parent material commonly com-posed of reworked pyroclastic material; (2) colonization bysweat bees, which were important agents in the modifica-tion of the sedimentary fabric; (3) moderate alkalinityand,in some cases, also salinity; (4) association with low vege-tation; and (5) development under warm, semiarid to sub-humid climate.

A similar occurrence of palustrine facies, probable in-sect ichnofossils, and terrestrial and freshwater snailsalso is known from the late Eocene Bembridge Limestoneof England (Edwards et al., 1998). In that deposit, thetrace fossil association (whose taxonomy is still unre-solved) is restricted to ovoid-cylindrical casts of chamberspossibly constructed by insects for pupation or by otheranimals for aestivation or hibernation (Edwards et al.,1998). The trace fossil association described in the current

paper also differs significantly from that typical of conti-nental evaporite sequences described by Rodrıguez-Aran-da and Calvo (1998), especially with regard to the absenceof hymenopterous nests in the latter.

Genise et al. (2000) suggested that associations of differ-ent types of bee cells, probable wasp cocoons (ovoid struc-tures), and dung-beetle pupal chambers in calcareous pa-leosols be considered as a potentially separate ichnofacies.The new data presented herein add another occurrence ofthis potential ichnofacies, as well as the inclusion of Teis-seirei. Additionally, this report broadens the range of en-vironmental settings in which the association is found toinclude palustrine facies. At present, the association de-scribed herein can be provisionally included in the Coprin-isphaera ichnofacies (Genise et al., 2000). Comparisonswith other continental and lacustrine ichnofacies are notconsidered necessary due to the large differences with thedescribed ichnologic association (cf. Buatois et al., 1998).

Gastropod Fauna

The ability of the extant Ampullariidae (which includesPomacea) to withstand periods of desiccation and slightchanges in salinity, and its general distribution in temper-ate to subtropical areas of Argentina, are in agreementwith the general paleoenvironmental setting as deducedfrom sedimentological criteria. The association of the landsnails Plagiodontes spp., Bostryx sp., and Bulimulus sp. in-dicates an environment similar to that found at present inthe central-western provinces of Argentina, related to theSierras Pampeanas, and particularly the Monte biogeo-graphic region of Argentina (Cabrera and Willink, 1973).

30 MELCHOR ET AL.

The later is characterized by a subtropical to temperateclimate dominated by xeric vegetation, similar to that ofthe xeromorphic shrublands (Mueller-Dombois and Ellen-berg, 1980).

Vegetation

Insight about the vegetation that supported the Eocenepaleosols come from micro- and macrofeatures and geo-chemical data. Alveolar and fenestral structures (0.2 to 2mm in diameter) are interpreted as rootlet tubules filledby sparry calcite and occasionally by microlaminated clay,which were formed very early during diagenesis (cf.Mount and Cohen, 1984; Wright et al., 1988; Alonso-Zarzaet al., 1992). Paleosol 3 supported sparse herbaceousplants (as suggested by relatively abundant, fine alveolarstructure and root traces in its cambic horizon, Fig. 3E), aswell as rare medium-sized trees or shrubs. Hence, lakemudflats probably were covered by sparse, low vegetationlike saltbush and sage.

The Cerling model for carbon isotopic composition of soilcarbonates (e.g., Cerling, 1984; Cerling and Quade, 1993)is not applicable to the Gran Salitral palustrine paleosolsbecause its matrix was not leached of calcium carbonate(T. Cerling, pers. commun., 2001). The complications im-posed by uncertainties about isotopic composition, CO2

concentration of the atmosphere, and the discriminationby plants impedes the estimation of the proportion of ped-ogenic carbonates in palustrine samples following themethodology of Magaritz and Amiel (1980).

Paleoclimate

The presence of abundant hymenopterous nests andsparse vegetation reinforce xeric conditions as determinedfrom other geologic data. The association of incipient sur-face karst with relatively thick calcareous paleosols, asthose described in this paper, is linked to carbonate parentmaterial that has been exposed to meteoric diagenesis un-der a semiarid and warm climate (James and Choquette,1990). Precipitation of evaporites (former evaporite nod-ules replaced by length-slow chalcedony and gypsum lam-inae) also is indicative of semiarid conditions, at least inpart of the sequence. The low values of the chemical indexof alteration (CIA; from 42 to 64) are equivalent to fresh ormildly weathered intermediate igneous rocks (cf. Nesbittand Young, 1982) and also suggest that the availability ofwater was restricted during weathering. Furthermore, thedevelopment of clotted-peloidal structure requires varia-tion in the availability of water during the year and henceis related to seasonal climate (e.g., Freytet and Plaziat,1982; Platt, 1989; Armenteros and Daley, 1998).

The d18O of pedogenic carbonate is in equilibrium withpalustrine/soil water, whose isotopic composition is relat-ed to meteoric water, although there are various compli-cating factors that are not understood completely (Cerlingand Quade, 1993). Some authors (e.g., Suchecki et al.,1988; Quade et al., 1995; Yemane and Kelts, 1996) haveused absolute d18O values of pedogenic carbonate to esti-mate paleotemperature, although this practice is prelimi-nary (Cerling and Quade, 1993). Mean annual tempera-ture for paleosol 3 was estimated using the equation ofHays and Grossmann (1991) for inland recharge, and as-

suming d18O for Paleogene seawater of 21 ‰ SMOW(Shackleton and Kennett, 1975). The equation for coastalrecharge of Hays and Grossman (1991) is more suitable,given the early Eocene paleogeographic setting for thestudy area (Franchi et al., 1984). Among the solutions forthat equation, which are 08C and 268C, the lower temper-ature estimate is rejected because it conflicts with the sed-imentological and faunal evidence that supports a temper-ate-warm climate. The approximation of 268C correspondsto average soil temperature at about 1 m depth where car-bonate forms, which is 2–38C above the mean annual airtemperature (cf. Quade et al., 1995). An air temperatureestimate of 238C 2 248C agrees with warm climatic condi-tions characteristic of palustrine carbonates, as well aswith the general view that the early Eocene was a time ofglobal warmth and reduced latitudinal temperature gra-dients compared with the present day (e.g., Zachos et al.;1994 Greenwood and Wing, 1995; Sloan and Rea, 1995).

CONCLUSIONS

Early Eocene calcareous paleosols from La Pampa prov-ince (Argentina) developed on lake-margin, carbonate-dominated deposits subject to repeated flooding and des-iccation. Short (annual?) and long-term water-level fluc-tuations under warm, seasonal climate favored the devel-opment of distinctive palustrine features. However,paleosols are not fully developed because of prevailingsemiarid conditions. The succession contains a moderatelydiverse insect ichnofauna (Celliforma, Rosellichnus, Teis-seirei, and probably ovoid structures), few plant ichnofos-sils, and other animal trace fossils of uncertain origin. Re-markable features of the Gran Salitral ichnofauna are theabundance of bee nests; apparent preference for calcare-ous, alkaline and mildly saline soils in a playa-like setting;and association with low and sparse vegetation. The doc-umentation of this insect-dominated ichnologic associa-tion from calcareous paleosols strengthens the cause fordistinguishing a new continental ichnofacies (cf. Genise etal., 2000) with particular environmental implications, al-though further studies are needed.

ACKNOWLEDGMENTS

Preliminary observations on the ichnology and sedi-mentology of the Gran Salitral Formation were made aspart of mapping work for the Servicio Nacional de Geolo-gıa y Minerıa of Argentina under contract with the Univ-ersidad Nacional de La Pampa. Nestor Sandoval kindlyconducted part of the photographic artwork for this arti-cle. Lora Wingate (Stable Isotopes Laboratory, Universityof Michigan, Ann Arbor) patiently analyzed the C and Oisotopic composition of samples for this study. Thure Cer-ling is thanked for his comments on the stable isotopicdata.

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EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 33

APPENDIX 1Major oxides data.

Sample ID

256A % 257A % 257B % 257C % 257D % 258A % 259A %

SiO2

Al2O3

Fe2O3

FeOMnO

42.819.772.45

20.100.051

43.708.562.30

20.100.047

41.498.562.13

20.100.038

45.136.561.81

20.100.025

51.105.811.66

20.100.025

54.465.421.50

20.100.036

67.096.871.83

20.100.026

MgOCaONa2OK2OTiO2

1.2921.011.751.440.315

1.1520.731.701.290.295

1.0723.391.571.200.279

0.9221.331.370.960.233

0.8618.971.320.960.202

0.7618.410.940.760.180

1.138.290.790.990.204

P2O5

LOITotalCO2

0.4719.50

100.7614.40ppm

0.4019.6199.6813.90ppm

0.3220.81

100.7614.80ppm

0.2220.0398.4916.10ppm

0.2619.21

100.2814.80ppm

0.4818.04

100.8912.70ppm

0.0813.19

100.394.70

ppmBaSrYScZrBeV

605787227

7921120

4841271

196

7721125

1961042

166

6921127

140791134

6021160

337793134

6521129

321945204

7421112

66838

6365

2176

the Triassic—Jurassic Hartford and Fundy rift basins: Journal ofSedimentary Petrology, v. 58, p. 801–811.

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YEMANE, K., and KELTS, K., 1996, Isotope geochemistry of UpperPermian early diagenetic calcite concretions: Implications for LatePermian waters and surface temperatures in continental Gond-wana: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 125,p. 51–73.

ZACHOS, J.C., STOTT, L.D., and LOHMANN, K. C., 1994, Evolution ofearly Cenozoic marine temperatures: Paleoceanography, v. 9, p.353–387.

ACCEPTED JULY 5, 2001

34 MELCHOR ET AL.

APPENDIX 2Carbon and oxygen isotopic data from low-Mg calcite.

SampleID Lithology

d13C(‰ VPDB)

d18O(‰ VPDB)

256A256A256A256A257A

marlmarlmarlmarlmarl

24.2524.2624.3524.2724.59

25.5425.4625.6925.8123.15

257A257B257B257B257C

marlmarlmarlmarlmarl

24.5224.8424.7524.7425.34

23.1623.0323.0723.0723.47

257C257D257D257D257A

marlmarlmarlmarlmarl

25.2425.3925.4525.3925.37

23.4823.4923.4023.5123.30

258A259A259A259A*259A*

marltufftuff50% matrix 50% sparmostly sparry calcite

25.6525.3825.4025.7126.76

23.2523.5223.4923.3824.89

259A*259A*259A*259A*259A*

matrix85% matrix85% matrix95% spar95% spar

25.5925.3725.4625.7426.01

23.2522.8223.0123.5823.84

259A*259A*259A*259A*

80% matrix80% matrix95% spar85% matrix

26.0825.7127.1325.66

23.6823.3525.5123.05

259A*259A*259A*259A*

85% matrix90% matrix95% spar95% spar

25.6025.3526.6526.60

23.0922.7825.1124.92

* Material obtained by drilling on polished slab.

APPENDIX 3Mineralogy from x-ray diffractrometry.

SampleID Lithology

Dominantmineral Secondary minerals

256A247A257B257C257D258A259A

limestonemarllimestonemarlmarlmarltuff

cal–plagcalcal–plagcal–plagcalcalq–cal

q–phi–zeo?phi–q–plag–kcli–zeo–qq–mor–cli?mor–plagmor–zeo–smplag–mor–cli–phi?

cal: low-magnesium calcite, plag: plagioclase, k: potassium feld-spar, q: quartz, zeo: unidentified zeolites; mor: mordenite; phi: phil-lipsite; cli: clinoptilolite; sm: smectite.

EOCENE PALEOSOL INSECT ICHNOFOSSILS, ARGENTINA 35

APPENDIX 4Redescription of Teisseirei.

Teisseirei barattinia Roselli 1938Figs. 12A, B, C, D, E, I

Teisseirei barattinia. Roselli, 1938, p. 82.Tesseireichnus barattinia. Roselli, 1976, p. 167 (junior synonym).Teisserichnus barattinia. Roselli, 1987, p. 24 (lapsus).Teisseirichnus barratini. Retallack, 1990b, p. 219 (junior synonym).Teisseirei barattinia. Genise and Laza, 1998, p. 213.Holotype: MFLR 645. A complete specimen, broken in two pieces, illustrated in Roselli (187, fig. 5, p. 45). Paratypes: MFLR 645a and645b. Complete specimens, illustrated in Roselli (1987, figs. 5a and 5b, p. 45).

Examined material; The types and 230 specimens collected in Nueva Palmira, Las Flores, Palmitas, Carmelo, Soriano, Paraje Mollesand Paysandu, Uruguay (MACN-LI 825 to 876 and 890 to 926; MFLR 800 to 941). Twelve specimens collected in Colon (from the LateCretaceous-Early Tertiary Puerto Unzue Formation), Entre Rıos, Argentina (MACN-LI 877 to 889) and 57 specimens collected in GranSalitral, La Pampa, Argentina (GHUNLPam 12462-1/2; 12471-1/8; 12473-1/7; 12478-1/10 and MACN-LI 1266 to 1280, 1296 to 1309).

Type locality: Palacio Member of the Asencio Formation (Late Cretaceous-Early Tertiary), Nueva Palmira, Uruguay.Emended diagnosis: Depressed chambers, slightly arched downwards (Figs. 12A, 12E), that may show an anterior small, rounded,antechamber (Fig. 13D). The inner surface displays, in the best preserved specimens, a distinct lining bearing small elliptical scratchesoriented mostly longitudinally. Some specimens are surrounded by a thick wall of uncertain (concretionary?) origin.

Remarks: The depressed cross section along with the internal microrelief distinguishes this ichnotaxon from any other attributed toinsects.

Description: These traces may be commonly found in the field showing three preservational variants. As empty or passively filledchambers found in situ in paleosols; as isolated, detached, ‘‘clasts’’ composed of the chamber fillings; or, as empty or filled chamberssurrounded by a thick wall. The original of this wall is still uncertain, the trace maker may construct it, or it may be of diageneticorigin. Specimens from the Gran Salitral Formation occur mostly as detached chamber fillings with the exception of one specimen thatwas found in situ, oriented horizontally in the paleosol. Only one of the collected specimens preserved the antechamber. The length ofthe chambers ranges from 30 mm to 52 mm (mean 5 40.20 mm, n 5 50); the width ranges from 13 mm to 20 mm (mean 5 16.98 mm,n 5 50) and the height ranges from 9 mm to 15 mm (mean 5 11.63 mm, n 5 50). The average proportion height: width: length in 50specimens is 1 : 1.5 : 3.5, which gives the trace its depressed aspect. The length is the parameter that shows more dispersion, the heightand width being more conservative, a fact that suggests that the trace makers showed a great variation in body length but preservingsimilar width and height. The antechamber displays a less depressed aspect than the chamber, measuring 15 mm in length, 12 mm inwidth, and 11 mm in height. Three specimens (GHUNLPam 12473-3; 12462-1 and MACN-LI 1309) show remains of the microreliefcomposed of small, elliptical, parallel scratches (about 2 mm long), preserved as positive epirelief (Fig. 12I).

The Eocene specimens from La Pampa are indistinguishable from those of the type locality in shape, size, microrelief, and position inpaleosol, leaving no doubts about their assignment to T. barattinia.