Beraldi-Campesi (2006) -- Sedimentology n' Paleoecology of Los Ahuehuetes (Eocene-Oligocene)

1 (2006) 227254www.elsevier.com/locate/sedgeoSedimentary Geology 19Sedimentology and paleoecology of an EoceneOligocenealluviallacustrine arid system, Southern Mexico

Hugo Beraldi-Campesi a,, Sergio R.S. Cevallos-Ferriz a,1, Elena Centeno-Garca a,2,Concepcin Arenas-Abad b,3, Luis Pedro Fernndez c,4

a Institute of Geology, UNAM, Ciudad Universitaria, Coyoacn, 04510, DF Mxicob Area of Stratigraphy, Department of Earth Sciences, University of Zaragoza, E-50009 Zaragoza, Spain

c Area of Stratigraphy, Department of Geology, University of Oviedo, C/ Jess Arias de Velasco s/n, E-33005 Oviedo, Spain

Received 18 May 2005; received in revised form 24 January 2006; accepted 23 March 2006Abstract

A depositional model of the EoceneOligocene Coatzingo Formation in Tepexi de Rodrguez (Puebla, Mexico) is proposed,based on facies analysis of one of the best-preserved sections, the Axamilpa Section. The sedimentary evolution is interpreted asthe retrogradation of an alluvial system, followed by the progressive expansion of an alkaline lake system, with deltaic, palustrine,and evaporitic environments. The analysis suggests a change towards more arid conditions with time. Fossils from this region, suchas fossil tracks of artiodactyls, aquatic birds and cat-like mammals, suggest that these animals traversed the area, ostracodspopulated the lake waters, and plants grew on incipient soils and riparian environments many times throughout the history of thebasin. The inferred habitat for some fossil plants coincides with the sedimentological interpretation of an arid to semiarid climatefor that epoch. This combined sedimentologicalpaleontological study of the Axamilpa Section provides an environmental contextin which fossils can be placed and brings into attention important biotic episodes, like bird and camelid migrations or the origin ofendemic but extinct plants in this area. 2006 Elsevier B.V. All rights reserved.Keywords: TepexiCoatzingo; EoceneOligocene; Sedimentary evolution; Fossil tracks; Fossil plants; Riparian ambients Corresponding author. Present address: School of Life Sciences,LSE 418, Arizona State University, Tempe, AZ 85287-4601, USA.Tel.: +1 480 727 7762; fax: +1 480 965 7599.

E-mail addresses: [email protected] (H. Beraldi-Campesi),[email protected] (S.R.S. Cevallos-Ferriz),[email protected] (E. Centeno-Garca), [email protected](C. Arenas-Abad), [email protected] (L.P. Fernndez).1 Tel.: +52 55 5622 4312.2 Tel.: +52 55 5622 4310.3 Tel.: +34 976 762 129.4 Fax: +34 98 510 3103.

0037-0738/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2006.03.0181. Introduction

Extensive magmatism and tectonic activity havecontributed to the complex geological history ofcentral Mexico during the Cenozoic. Paleogene inlandbasins there are poorly known, mainly because of theextensive cover by younger rocks and imprecisecorrelations (e.g. Ferrari et al., 1999; Morn-Zentenoet al., 1999). Paleontological studies have providedreferences for dating rocks and have contributed to amore comprehensive understanding of the biotic

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228 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254communities, but yet such studies do not alwaysprovide an environmental context for fossils and theyrarely involve sedimentological studies in theirpaleoenvironmental reconstructions. Therefore, Paleo-gene basins with well-preserved outcrops and fossilsFig. 1. (A) Map showing the study area and fossil localities from the TepexiTepexi de Rodriguez area. Pz=Paleozoic; Mz=Mesozoic; K=CretaceouQ=Quaternary.can be extremely valuable components in facilitatingreconstruction studies.

The EoceneOligocene Coatzingo Formation, lo-cated in the CentralSouth area of Puebla, Mexico(Fig. 1A), is known for its plant and animal fossili-Coatzingo basin. (B) Local geology (solid line-box in map A) of thes; E=Eocene; EO=EoceneOligocene; P=PliocenePleistocene;

229H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254ferous localities (e.g. Buitrn and Malpica-Cruz,1987), particularly artiodactyl (camel-like and cervid)fossil tracks, together with those of cat-like mammals,proboscideans, birds, reptiles and the skeleton of aflamingo (Cabral-Perdomo, 1996). Palynomorphassemblages described from several localities in theFormation (Carranza-Sierra, 2001; Carranza-Sierra andMartnez-Hernndez, 2002; Martnez-Hernndez andRamrez-Arriaga, 1999) have served as tools fordating, biostratigraphical correlations and floral des-criptions. An ever-growing collection of fossil plantsfrom the Ahuehuetes locality, one of the most well-preserved Oligocene records in Mexico, has been usedto estimate ages, paleotemperatures and to improvethe understanding of biogeographical patterns ofNorth Americas' flora (Calvillo-Canadell and Ceval-los-Ferriz, 2002; Magalln-Puebla and Cevallos-Fer-riz, 1994a; Velasco de Len and Cevallos-Ferriz,2000; Ramrez-Garduo and Cevallos-Ferriz, 2002).Other fossils from the Formation include fossil wood(currently being studied), stromatolites, and ostracods.Outcrops of the Coatzingo Fm in the Pie de Vaca andnearby localities have been interpreted as lacustrine inorigin (Buitrn and Malpica-Cruz, 1987; Pantoja-Aloret al., 1988) based on field observations, but nosedimentological studies that describe the type ofpaleolake, its evolution or the overall paleoenviron-mental conditions that existed have previously beenundertaken.

In this paper, we provide a paleoenvironmentalreconstruction of part of the Coatzingo Formation,based on facies analysis of one of its most completesections, termed here the Axamilpa Section (AS). Anoverall integration of the sedimentological and paleon-tological data from the region is taken into considerationto further discuss relevant biotic events, such asinterchanges and endemisms, that may have occurredin this area, some of them with profound evolutionaryimplications.

2. Local geology

The geology of the Tepexi de Rodriguez area (Fig.1B) has not been studied completely. In general, aCenozoic succession is underlain by Cretaceous rocks,e.g. the Tlayua Fm (Kashiyama et al., 2003; Pantoja-Alor, 1990) and the Rosario Fm, and by a Paleozoicbasement, the metamorphic Tecomate Formation,which represents the northernmost part of the AcatlnComplex (Ortega-Gutierrz et al., 1999). The lowerrocks of the Cenozoic succession correspond to awhite, calcareous, clast-supported conglomerate thatcrops out in many areas of the TepexiCoatzingobasin, which has been informally linked with theEocene Balsas Conglomerate (Martnez-Hernndezand Ramrez-Arriaga, 1999; Pantoja-Alor et al.,1988). It crops out West of the AS (although theircontact is not clearly visible), and unconformablyunderlies the eastern part of the Ahuehuetes section,being then at least part of the local basement of theCoatzingo Fm. Finally, the Cenozoic succession iscovered with the Plio-Pleistocene Agua de LunaFormation (Pantoja-Alor et al., 1988) in the SE partof the Tepexi area (Fig. 2).

2.1. The Coatzingo Formation

The Coatzingo Formation, previously named Pie deVaca Fm (Pantoja-Alor et al., 1988), correspond toPaleogene rocks within the TepexiCoatzingo basin.That Formation has been locally divided into two units:the lower Pie de Vaca Unit, composed mainly oflimestones and cherty and sandy limestones, and theupper Ahuehuetes Unit, composed mainly of tuff andtuffaceous sandstones (Silva-Romo and Gonzlez-Torres, in: Calvillo-Canadell and Cevallos-Ferriz,2002; Fig. 2), both interpreted as fluvial to lacustrinelow-energy environments.

The geographical limit of the Coatzingo Fm is atpresent uncertain; however, several localities (e.g.Chigmecatitln, Zaragoza, and Cuayuca) have beencorrelated biostratigraphically (Martnez-Hernndez andRamrez-Arriaga, 1999), extending its area further to thewest and south of the study site (Fig. 1A).

2.3. The Axamilpa Section

The Axamilpa Section (AS) is located along theAxamilpa River, close to the Ahuehuetes locality, 3 kmN of the town of Tepexi de Rodrguez, in the centralsouthern part of Puebla (975548W, 183642N;1680 masl). This section, which represents the Piede Vaca Unit (Fig. 2), is thought to underlie theAhuehuetes Unit, although they are geographicallyseparated.

Initially, the Pie de Vaca Unit was interpreted to be ofMiocene to PliocenePleistocene age (Buitrn andMalpica-Cruz, 1987; Cabral-Perdomo, 1995; Rodrguezde la Rosa et al., 2005), based upon fossil mammaltracks from the Pie de Vaca locality. Later on, extensivepalynologic and paleobotanic studies have pointed to anEoceneOligocene age for both the Pie de Vaca andAhuehuetes Units (Calvillo-Canadell and Cevallos-Ferriz, 2002; Carranza-Sierra and Martnez-Hernndez,

Fig. 2. General stratigraphy of the regional lithologies within the TepexiCoatzingo basin and the position of the Coatzingo Formation Units. Thelocalities Axamilpa Section, Ahuehuetes, and Pie de Vaca (right) are shown in a stratigraphical context.

230 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 2272542003; Martnez-Hernndez and Ramrez-Arriaga,1999), interpretation also supported by regional geo-logical studies (Silva-Romo, 1998 in Calvillo-Canadelland Cevallos-Ferriz, 2002).

3. Stratigraphy and facies of the Axamilpa Section

Well-exposed horizontal strata of the AS crop outmainly in a vertical cliff (Fig. 3A) that shows noevidence of major deformation. Three stratigraphicsections were measured at sites A, B, and C (Fig. 3B),then correlated and combined in a single log (Fig. 4,Log 1). A second section (Log 2) was measuredat site D.

Three lithological groups are recognized in the AS: a)detrital rocks (conglomerates, coarse sandstones andminor limestone, from the base of the section to20 m),b) detrital and chemical rocks (sandstones, minorlimestones and marls, from 20 to 36 m), whichcommonly appear interbedded, and c) rocks mainly ofchemical origin (limestones and evaporites, marls andscarce terrigenous, from 36 m to the top). Log 2contains only groups b) and c). Facies descriptions andinterpretations are summarized in Table 1. The verticaldistribution of facies and their associations is shown inFig. 5. Microscopic features of the facies are shown inFig. 6.Conglomeratic faciesMatrix-supported conglomerates (Cgm, Fig. 7A)This facies comprises polymictic, matrix-supportedconglomerates, which form irregular beds up to 2 mthick. The matrix is a sandsilt mixture with a highpercentage of carbonate. Clasts are angular to sub-rounded, 3 to 7 cm in diameter, and composed of schists,white or reddish limestone, claystone and quartz. Someparts are replaced and cemented with hematite.Interpretation:These conglomerates represent cohesive debris flowdeposits. Matrix strength and buoyancy preventedlarge clasts to sink, keeping them floating andfavoring a disorganized fabric. These conglomeratesalways form on slopes, and are typical of theproximal or mid parts of alluvial fans (e.g. Colombo,1992; Reading, 1986).

Clast-supported conglomerates (Cgc, Fig. 7BC)These consist of poorly sorted clast-supportedconglomerates. Clasts are angular to subrounded,up to 14 cm long, consisting mainly of schist,although some quartz and limestone clasts are alsopresent. Pore spaces between clasts are commonlyfilled by calcite cements. This facies forms tabular,dm- to m-thick, strata with erosive bases. Bedsdisplay planar and trough cross-stratification, with

Fig. 3. (A) Panoramic view, facing NE, of the Axamilpa Section and surroundings. (B) Topographic map of the Axamilpa Section showing the sites where logs were measured. Log 1 was measured insites A, B, and C; Log 2 was measured in site D (logs shown in Fig. 5). 231

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232 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254normal or inverse grading. Some beds pass laterallyinto coarse sandstone. Clasts are regularly imbricat-ed, showing N40 paleocurrent directions.Interpretation:This kind of deposits would form in braided channelsand bars in middle parts of alluvial systems (Miall,1996). Some coarsening-upward examples of thisfacies may record progradation of minor deltaic-typebars, given their close relationship to calcareouslacustrine facies, or merely reflect migration of thecoarser head of longitudinal bars on the finer-graineddowncurrent tail.Sandstone faciesCross-stratified sandstones (Sc, Fig. 7D)These consist of very coarse to fine-grained sand-stones, arranged in dm to m thick tabular or lenticularstrata, which display normal or inverse grading andcross stratification. These beds can contain conglom-eratic lenses, and may pass laterally into fine-grainedconglomerates. Fluid escape structures are locallypresent.Interpretation:This facies is common in fluvial systems ofmedium tolow energy (Miall, 1996), where gravel deposits mayaccumulate at the base of the channels (Davis, 1983).It is interpreted to have formed from channeled andunconfined flows, in the middle-to-distal parts ofalluvial fans. Fluid escape structures may be evidenceof contemporary seismic movements or other factorsthat led to sediment disturbance.

Horizontal-laminated and cross-stratified sandstones(Shc, Fig. 7E)This facies is formed by fine to medium (rarelycoarse) sandstones arranged in cm- to dm-thick beds,which are amalgamated or, more rarely, separated bypale-green marl layers, containing clay, quartz andlimestone grains. Sandstone beds, which may displayload structures and marl intraclasts in their base,show internal horizontal lamination, sometimes withcross and flaser stratification and ripple lamination.Sandstones contain large amounts of schist fragmentsand also scattered angular quartz and other rockfragments (930 mm in diameter). Calcareouscement and hematized zones are common in thisfacies. The marls are porous and poorly indurated,and may contain algal lumps, peloids, ostracodremains, oolites, bioturbation galleries. Brecciationand nodulation, as well as gypsum crystals andchalcedony, may also be present. The abundance ofoolites and ostracods in this facies varies along thesection.Interpretation:This facies may represent deposits laid down onthe floodplains of a distal alluvial-fan setting oreven in the delta plain of a lacustrine delta system(cf. Miall, 1992, 1996; Rust, 1980). In suchsettings, unconfined floods can deposit sand sheetswith a variety of structures (e.g. parallel, flaser, andripple lamination) that reflect variable energyconditions. The common normal grading in thesandstones reflects the waning behavior of theseflows, although turbulent episodes capable oferoding the substratum could have occurred, asimplied by the many marly rip-up clasts. Duringquiet periods or the weakest flows, alternated clayand carbonate accumulation would account for theformation of marls among the clay-rich layers. Thepresence of oolites, which are common in largesaline and calcareous lakes fringes (e.g. Kowa-lewska and Cohen, 1998; Swirydezuk et al., 1979),indicates currents or oscillation produced by windand swell during periods of absence of terrigenous.The bioturbation suggests the presence of intersti-tial organisms and, therefore, the existence of awater layer. During drought periods, the substratewould be exposed to desiccation and oxidation,resulting in brecciation and the precipitation ofhematite.

Sandstone and marl alternations (Scs, Fig. 7FG)This facies is composed of sandstone and marlalternations, cm- to dm-thick beds, which typicallyform overall coarsening-upward units. The separa-tion of both lithologies is not abrupt but continuous.Marl layer thickness decrease toward the top, whilethickness of sandstones increase. Loading structuresare observable at the base of the beds, and ripple- andlow-angle stratification are present in the uppersandstones.Interpretation:The coarsening upward trend and the thinner bedstowards the top are common indicators of deltaicprogradation (e.g. Bhattacharya and Giosan, 2003).Given the thin nature of this facies and itsstratigraphical position between rocks of chemicalorigin, it is interpreted as the progradation of smalldeltaic lobes in distal flat areas.

Massive or graded sandstones (Sm, Fig. 7H)This facies comprises mostly fine, but also mediumgrain-size sandstones, mainly formed of schist,quartz and carbonate grains extensively cementedwith calcite and locally with hematite. Sorting is

Fig. 4. Stratigraphic logs from the Axamilpa Section.

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Table 1General description and interpretation of the Axamilpa Section facies (see text for details)

Facies Description Interpretation

Conglomeratic Cgm Polymictic, matrix-supported conglomerates. Debris-flow deposits in proximal or middle parts of alluvialfans.

Cgc Clast-supported conglomerates with erosive bases.Planar cross-stratification, normal or inverse grading.

Braided channel and bar deposition in middle to distal parts ofalluvial systems.

Sandstone Sc Coarse- to fine-grained sandstones in tabular or lenticularstrata. Cross-stratification and conglomeratic lenses.

Channelled and unconfined flows in the middle to distal parts ofalluvial fans.

Shc Fine- to medium-grained sandstones with horizontal andcross-stratification, and cm-thick interbedded marllayers.

Deposits on floodplains of distal alluvial-fan or lacustrine deltasystems.

Scs Sandstone and marl alternations. Net coarsening-upwardintervals. Load structures, ripple and low-angle crossstratification.

Progradation of small deltaic lobes in flat marginal lacustrineareas.

Sm Fine to medium massive sandstone; inverse or normallygraded, internal horizontal lamination and rarefluid-escape structures.

Suspended deposition (flash floods) in flat alluvial or deltaicsettings, near lake margins.

Marl Mm Marl layers, massive or with horizontal lamination. Chertbands and gypsum nodules, brecciation and bioturbationare present.

Periodic or continuous settling of carbonate muds in offshore,low-energy lacustrine areas, with reduced terrigenous input.Subaerial exposure events.

Mh Alternation of calcareous marls and clay-rich layers.Load structures, fluid escape structures, desiccationcracks, ripples, peloids and bioturbation galleries arefound. Fossil plants found at one level.

Palustrine setting with soil formation and carbonate deposition.

Calcareous Lm Limestones in massive or stratified layers containingdesiccation cracks, load structures, vertebrate tracks,rhizocretions, pores, microgranular and nodular gypsum,chert bands and nodules, chalcedony, brecciation,bioturbation, microbial-like lamination, nodulation,ostracod remains, oolites, bacterial or algalmicroorganisms, and peloids.

Carbonate deposition in shallow lacustrine to palustrine settings,where herbaceous vegetation became established.

Lo Massive oolitic limestones with gypsum nodules andchert bands, vertebrate ichnofossils, peloids, andostracods. Coated grains and pisoids, bioturbation andalgal lumps are present.

Margins of saline carbonate lakes with agitated shallow waters.

Ll Varve-like laminated limestones with flaser stratification,silicified oncolites, chert and gypsum nodules.

Rhythmic carbonate deposition in relatively deep, low-energyoffshore lacustrine areas.

Ls Stromatolites capped with gypsum. Marginal shallow and low-energy lacustrine environments, withprolonged evaporation and depletion of water.

Evaporitic Gy Gypsum and calcite laminae. Nodular gypsum. Halitealso present.

Intense and prolonged lake evaporation events.

237H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254good, although the basal part of some beds containsscattered grains up to 8 mm. Beds are massive orinversely or normally graded, although, in somecases, they show horizontal lamination and scarcefluid escape structures.Interpretation:This facies suggest direct sedimentation of sandfrom waning, turbulent sediment-laden flows withno (or little) traction at the bed, perhaps with aneolian contribution. The differences in gradingwould reflect sedimentation from high to lowdensity fluids (e.g. Lowe, 1979). The presence offluid escape structures also indicates liquefiedflows that resulted from rapid flow decelerationand quick deposition. These probably arose fromflow expansion at the mouth of confined conduits(channel mouths?), in a wide and flattened alluvialor deltaic setting. The flows loaded with sandwere perhaps generated by sudden floods duringheavy rainstorms (e.g. Marzo, 1992; Mutti et al.,1996). Neighboring carbonate strata suggest depo-sition in a distal floodplain to marginal lacustrinesetting.Marl faciesMassive or laminated marls (Mm, Fig. 7I)These marls form tabular layers, cm- to dm-thick,and are either massive or have horizontal lamination.Chert bands and microcrystalline gypsum nodules up

238 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254to 1 cm are present. Some marls show brecciation andbioturbation.Interpretation:These marls represent periodic or continuoussettling of carbonate mud in offshore, low-energylacustrine areas with little terrigenous input. Gyp-sum nodules may have formed during periods ofevaporation.

Marls and claystones (Mh, Fig. 7JK)Alternation of cm- to dm-thick layers of calcareousmarls with cm-thick greenish clay-rich vermiculitelayers. Thin, mm-thick laminae of carbonate-cemented sandstone may be present in the marllevels. Ripples may be present locally, as well aspeloids and bioturbation. Fossil plants were foundat one level. Load structures, fluid escape struc-tures, and desiccation cracks are very common (Fig.7K). The vermiculite forms layers 13 cm thickthat rhythmically interrupt the marls.Interpretation:Vermiculite can be found in soils (McCarthy andPlint, 2003; Schroeder et al., 1997) and in paleosols(Davies and Gibling, 2003), indicating that soilsmay have formed in a palustrine setting wherecarbonate deposition occurred during rainy seasonswith high lake levels. The poor preservation of theplant fragments implies an early diageneticdegradation.Limestone faciesMassive limestones (Lm, Fig. 7L)This facies consists of white-yellowish, beige or greenmudstones that form cm- to dm-thick tabular layers.Desiccation cracks, load structures, vertebrate fossiltracks, rhizocretions, andpores (1 to3mmindiameter)are present. Replaced and fragmentary ostracodvalves, complete and broken oolites, and peloidsoccur but are uncommon. Fungal or algal filaments arealso present. Some beds contain microgranular andmicrocrystalline nodular gypsum, radial calcite crys-tals, bands andnodulesof chert, andchalcedony.Somehorizons are brecciated, bioturbated, and showmicro-bial-like lamination (with bacterial or algal remainspreserved,althoughrarely).Thesurfacesof theserocksare commonly replaced by hematite.Interpretation:This facies formed from carbonate deposition inshallow lacustrine to palustrine areas that underwentperiodic subaerial exposure. The presence of rhizo-cretions indicates that herbaceous vegetation coveredparts of the surface, possibly grass-like plantsadapted to basic conditions.Oolitic limestones (Lo, Fig. 8A)Beige oolitic grainstones to packstones representthis facies. They are typically partially silicified,with large amounts of oolites, peloids, ostracods,coated grains and pisoids, as well as algal-likeaggregates. In a few places, they show partial dolo-mitization, bioturbation and hematite replacements.Some are porous and contain microcrystallinegypsum nodules, mm to cm in diameter, and chertbands, mm to cm thick. Vertebrate fossil tracks wereobserved at one level.Interpretation:These deposits are typical of those that form alongthe edges of saline carbonate lakes (e.g. Kowalewskaand Cohen, 1998; Swirydezuk et al., 1979). Ashallow or fringing lacustrine environment withagitated water that promoted the formation of oolitesis inferred for this facies.

Laminated limestones (Ll, Fig. 8BD)This facies is represented by cm- to dm-thick tabularstrata of mudstones. Laterally they have mm-thickvarve-like horizontal and wavy laminae and nobioturbation is observable. Locally they show flaserstratification and contain scarce silicified oncolites,chert and gypsum nodules, 0.5 mm to 10 cm indiameter. Porosity is conspicuous with pores up to4 mm in diameter. In thin section, small anhedralgypsum crystals, microgranular chert, clays andquartz grains are seen.Interpretation:These deposits may have formed in relatively deepand low-energy lacustrine offshore areas by carbon-ate deposition and variable terrigenous inputs. Thevarve-like appearance suggests that the carbonateversus terrigenous sedimentation was influenced byseasonal factors. This may imply also changes in thelake level, and thus the lake area. The scarcity ofoncolites suggests that they were formed in shallowerzones and transported to deeper zones. Likewise,scattered terrigenous suggest eolian inputs duringhigh level stages.

Stromatolitic limestones (Ls, Fig. 8EF)Stromatolites appear associated with facies Ll andLm, in strata varying from 60 to 70 cm in thickness.Two types of stromatolites were found at twodifferent levels: a) large, columnar and domalstromatolites, 30 to 40 cm high, which formabundant, continuous laterally coalescent bioherms;and b) small, hemispherical stromatolites, 10 to12 cm high, which occur alone or in clusters of two or

Fig. 5. Facies distribution in the Axamilpa Section. See text for explanation of facies associations.

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Fig. 6. Microscopic features in thin section. (A) Pellets within a spar matrix. (B) Nucleated oolites with chalcedony crystals (C). (C) Large gypsumcrystal (G) within a micritic matrix. (D) Radial growing of diagenetic calcite. (E) Rhizoidal pores (P, black) and nodules (N, dark grey). (F) Rhizoidpore with inner concentric layers of calcite. (G) Brecciated zone with intraclasts (I). (H) Bioturbated microbial-like lamination. (I) Partiallydolomitized limestone clast, with a miliolid foraminifer (arrow). (J) Echinoderm fragment (E) along with quartz grains (Q) and pelloidal limestonefragments (P).

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244 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254

Fig(C)Fin2occ

Fig. 8. (A) General view of the Facies Lm and Lo next to a rupestrian painting. (B) Facies Ll containing gypsum nodules (arrows). (C) Close-up of thefacies Ll showing compacted mm lamina. (D) Sequence of the facies Mm, Lm, Lo, Lm, and Ll (25 m in log 1). (E) Large stromatolites (facies Ls).(F) Small isolated stromatolites (facies Ls). (G) Transversal cut of a small stromatolite showing the outside and its internal lamination. (H) Partiallydissolved and collapsed stratum of gypsum (facies Gy) with calcite laminae. (I) Intricate growth of gypsum crystals.

245H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254three domes. The stromatolitic lamination shows thecharacteristic dark/light-lamina alternation (Reid etal., 2000; Seong-Joo et al., 2000), with the lighterlaminae being thicker and dominant. Partial silicifi-cation is present toward the centre in both types. Bothstromatolitic intervals are noteworthy capped bygypsum in honeycomb-like frameworks. The matrix. 7. (A) Close-up of the facies Cgm, showing poor sorted floating clasts. (B) EFacies association A in log 1 (12 m) showing two cycles (corks). Hammere lamination of facies Shc. (F) Facies Scs lamination. (G) Load structures of5 m (scale=1 m). (J) Facies Mh showing marl (M, light) and claystone (C, darkur. (L) General view of Lm facies in site D (11 m; hammer encircled between the stromatolites is massive or laminatedcarbonate.Interpretation:These facies formed in low energy, shallow andmarginal lake environments. Their continuous andsmooth lamination, and the regular morphology ofthe structures suggest that relatively stable conditionsrosive contact between facies Cgc (above) and facies Cgm (below).encircled is scale. (D) Cross-stratification of the facies Sc. (E)the facies Scs. (H) Facies Sm. (I) Facies Mm (arrows) in Log 1 at). (K) Flame structures of the facies Mh where thin sandstone layersis scale).

246 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254prevailed during their formation. The extensivegypsum on their upper surfaces suggest alkalineconditions with periods of prolonged evaporation.The limited presence of stromatolites in the ASsuggests that favorable conditions for their growthwere not frequent.Evaporitic faciesGypsum (Gy, Fig. 8GH)This facies consists of dm-thick, tabular to veryirregular beds of dissolved and collapsed gypsum.Thin and broken carbonate layers and halite casts arealso present. Microgranular, microcrystalline andmacrocrystalline textures are observable, althoughmacrocrystalline gypsum is more common in irreg-ular beds.Interpretation:This facies represents increasing solute concentra-tion in the lake and sediment pore water due tolowering of the water table and intenseevaporation.

3.1. Silicification processes

Many of the facies above described contain chertnodules and bands and chalcedony. They are clearlydiagenetic features and are found replacing some of theprimary components of the deposits (e.g. oncolites,stromatolites, limestones). The source of silica, as inmany silicified deposits elsewhere, can be debatable.Diatoms have not been detected and thus they canlikely be ruled out as a major source of silica.Although microbial mats have also been claimed as asource of silica in lacustrine deposits (Bustillo et al.,2003), where silicification takes place in littoral andeulittoral sequences of lacustrine carbonates, thepaucity of microbial deposits in the AS argues againstmicrobial mats being a major source for silica. Thus, itis more likely that the extensive magmatic activity thatprevailed in this region during the Paleogene (Martinyet al., 2000; Morn-Zenteno et al., 1999) would haveaccounted for the silica input, as it has been seen inother basins (e.g. Dunagan and Turner, 2004; Eugster,1980; Pirajno and Grey, 2002). In some cases,hydrothermal pulses may have loaded the pores ofthe preexistent rocks with the silica that would laterprecipitate as chert. In other cases, silicification mayhave occurred during early diagenesis by the mixing ofmeteoric waters with the evaporite pore waters,causing dissolution and collapse of the evaporitelayers, calcite cementation and silicification, probablyas a complex and recurrent sequence of processes(Arenas et al., 1999).3.2. Facies associations

The facies described above are associated in verticalsequences that represent the superposition of differentsubenvironments through time. Facies that are inferredto be genetically linked have been grouped, and theirvertical associations (Fig. 6) can be interpreted as asequence of events that describe the sedimentaryevolution of the section.

A association: CgcSc, SmShc, Lo, MmLmThis fining upward sequence begins with erosive or

planar contacts. It comprises mainly clastic facies withthicknesses from 30 to 250 cm, followed by massivelimestones that are up to 35 cm thick.

This association occurs cyclically and suggestsgravel and sand deposition in braided bar and channelsystems of alluvial fans that reached lacustrine areas, inwhich sheet-like flows deposited part of the detritalsediments as facies Shc. Finer sediment reached distalzones of the fan. Decreasing terrigenous inputs alloweddeposition of massive limestone facies in shallow oreven fringing lacustrine conditions. The sequencerepresents the retreat of an alluvial system and theestablishment of lacustrine and palustrine environments.

Features of the alluvial-fan conglomerates, such asthe clast size (814 cm), the local debris-flow events,and their close relationship with lacustrine facies,indicate alluvial fans of small size, mostly with activefluvial-dominated sectors in relatively steep areas, withtheir apices close to the palustrine and lacustrine areasthat existed down stream. Paleocurrents inferred fromthe conglomerate clast imbrication suggest that thesource of alluvial sediments was located to the SWof thestudy area.

B association: ScsShc, SmScCgcThis comprises a clastic facies with thickness from

50 to 250 cm and coarsening upward evolution,beginning at the base with sandstone facies, eithermassive or interbedded with marls, overlain by coarsersandstones that grade into conglomerates towards thetop. Marls are evidence of a water body with carbonatedeposition. This association records the progradationof the alluvial system by channeled flows that entereda shallow lake area, giving rise to small deltaicsystems.

C association: (Shc), Sm, ScMmLm, Lo, LlThis consists of clastic and carbonate facies,

varying from 4 to 70 cm thick, with planar contactsand a fining-upward pattern. It begins with fine detritalinputs that induced a subsequent rise of the water table,and caused offshore marl deposition (facies Mm).Later, shallower lacustrine conditions gave place to

247H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254carbonate deposition in marginal areas with or withoutagitation (facies Lo and Lm, respectively). Facies Llcould represent a slightly deeper lake setting withepisodic carbonate deposition, and would mark theestablishment of lacustrine, probably shallow, carbon-ate depositional conditions after an initial deepeningevent. This association reflects the end of the alluvialdeltaic dominance, followed by deepeningshallowinglacustrine events.

D association: Mm, MhLl, LmLo, LsGyThis is a complex association formed of cm- to dm-

thick claystone, marl, calcareous and evaporitic facies.Commonly the strata are tabular, except for thegypsum, which is found in deformed, collapsed andpartially dissolved strata. It begins with deposition inlow-energy offshore areas as a consequence of a lakedeepening and expansion. It continues with carbonateprecipitation in either relatively deep (facies Ll) orshallower waters (facies Lo, Lm) in which palustrineconditions were present. Two of the most commonsequences of this shallowing process are D1:MmLlLm; and D2: MhLl, Lm. Facies Lmand Ll can be found alternating through time, whichindicates cyclic water level changes attributed todeepeningshallowing events, for example, D3: Mm,MhLmLlLm (Fig. 6).

Facies Lo and Ls would also imply shallow, butprobably more saline, carbonate lacustrine conditionsin marginal areas affected by waves (Lo) or in calmareas (Ls), followed by high rates of evaporation anddesiccation that formed facies Gy in very shallowponds, and also caused brecciation, nodulation, andgypsum nodules to form in earlier carbonate facies.Two of the associations that reflect that evolution are:MmLl, LsGy, and MhLm (Ll)LoGy.

The sequence represents an overall shallowing thatimplies the change from lacustrinepalustrine to evap-oritic conditions, due to supersaturation of the lake byincreased evaporation that caused sulfate deposition.Rises in the lake water level due to freshwater inputs,may have caused dilution of the lake waters, a relativedeepening and the beginning of a new sequence.

3.3. Fossil record of the Axamilpa Section

Stratigraphic appearance of the Axamilpa Sectionfossils is indicated in Fig. 4. Representative specimensare shown in Fig. 9.

Vertebrate fossil tracks: (Fig. 9AB) All the tracksobserved in the AS were found on top of exposedbedding planes at various levels of the section.Although footprints are not always well marked,tracks of at least 4 steps can be easily distinguished. Insome strata, the tracks are better defined than inothers, suggesting different consistency and watersaturation of the substrate at the time of the imprint.They are usually found displaying alternated right andleft steps. Foot lengths vary from 10 to 18 cm. Theform of the digits is typical of ungulates (artiodac-tyls). Fossil tracks with similar characteristics havebeen described from other localities and related to theCamelidae group (Cabral-Perdomo, 1995).Bioturbation galleries: (Fig. 9C) Galleries areobserved in thin laminae, in several levels of thesection. The traces are usually oriented in a samedirection, although they may appear randomlyarranged. Sometimes ostracods are found in thesame sediments, and it is possible that they contributedto the bioturbation, given the benthic and excavatorhabits of some species (Henderson, 1990). However,there are many other possible burrowing candidates.Leaves: (Fig. 9DI) At least six different types ofleaves were recognized in the facies Mh. They aremoderately well preserved carbonaceous imprints,both fragmental and complete specimens. Recog-nized genera include Pseudosmodingium and Pista-cia from the Anacardiaceae and Cedrelospermumfrom the Ulmaceae. The remaining material has notbeen identified due to the absence of diagnosticcharacters, although some plant remains resemblelegumes and aquatic plants in morphology.Oncolites: (Fig. 9JK) Scarce ellipsoidal oncolites, 3to 5 cm in diameter, were found in limestones (FaciesLl). Their concentric laminationmay be obliterated bysilicification, which is most intense toward the center,whereas the external surface remains unsilicified.Rhizocretions: (Fig. 9L) In some strata, relicts ofroots (rhizocretions) were observed. These are small(up to 10 mm in length), oxidized (inferred by thecolor), and mostly appearing as vertical traces.Ostracods: (Fig. 9MN) These are present in severalhorizons of the succession, always in low-energylacustrine marls and limestones. The shape and sizeof the valves, between 400 and 550 m in length, isconstant along the stratigraphic section, suggestingstability in diversity of species. They do not displayornamentation, being quite smooth, which is com-mon in non-marine ostracods (Henderson, 1990), asassumed for those of the AS. Ostracods are presenteither as conjoined or disarticulated valves, completeor fragmented. They are abundant regionally. Coqui-nas with silicified ostracods have been observed atthe Zaragoza locality (Fig. 1) and some 3 km E from

248 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254

Fig. 10. Sedimentary evolution proposed for the Axamilpa Section (see text for explanation). (A) Alluvialfluvial stage. (B) Transitional stage. (C)Lacustrine stage. (D) Evaporitic stage.

249H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254it, suggesting that one or more lakes existed at thattime. In that case, the similarity between faunas mayindicate a relationship between the water bodies.Microorganisms: (Fig. 9OP) Permineralized micro-fossils were observed in thin sections of chertsamples. They appear in groups of four cells, wrappedby a mucilage-like sheath. The diameter of each cellvaries from 5 to 7 m. The diameter of the groupsvaries from 10 to 15 m and they are arranged insmall colonies (6 to 20 individuals per colony).Stromatolites: (see Stromatolitic limestones, FaciesLs).

4. Evolution of the sedimentary system

The Axamilpa Section clearly shows a fining-upwardtrend from alluvial conglomerates and sandstones tolacustrine carbonates and evaporites. This evolutionreflects an overall retrogradation of an alluvialfluvialsystem, followed by expansion of a carbonate deposi-Fig. 9. (A) Track of vertebrate ichnites. (B) Shape of an ichnite delimitedCedrelospermum sp. (E) Carbonaceous impression of an unidentified leaflPseudosmodingium sp. (H) Fossil impression of a legume. (I) Fossil leaf of a ppartially silicified oncolite (white calcite coat). (L) Rhizoid impressions (scinternal remains. (N) Transversal cut of an ostracod with non-symmetric valvtional lacustrine system that eventually evolved to amore shallower and alkaline, intermittent lacustrinesettings. In the studied succession this can be conceivedas four main depositional stages: 1) alluvialfluvial (Fig.10A), characterized mainly by detrital facies (conglom-erates, sandstones and mudstones, 020 m); 2) transi-tional (Fig. 10B), characterized bymarls, sandstones andlimestones (alternating distal alluvial and carbonatelacustrine facies, 2036 m); 3) lacustrine (Fig. 10C),characterized by carbonates; and finally 4) evaporitic(Fig. 10D, 3655 m).

The alluvialfluvial stage, characterized by braidedchannel and bar deposits with rare debris flows,experienced a general retrogradation through time(associations A and B), that gave rise to the expansionof palustrine and lake environments northeastward,overlapping the alluvial domain, as indicated by lime-stones and marls that cap the retrograde cycles. Thetransitional stage records the influence of both thealluvial and the lacustrine depositional environments,by a draw. (C) Parallel bioturbation galleries. (D) Fossil leaves ofet. (F) Poorly preserved fossil leaf of Pistacia sp. (G) Fossil leaf ofossible aquatic plant. (J) Oncolites in the field. (K) Transversal cut of aale=1 cm). (M) Transversal view of an ostracod showing substitutedes. (O) Permineralized microorganisms. (P) Individual microbial cells.

250 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254with alluvial inputs in the form of lacustrine deltaic lobesand distal-fan sheet-like deposits (associations B and C).

Although a timeframe for these events is difficult toassess, the frequency of flow-generated deposits appearsto decrease starting from the transitional stage, favoringthe development of palustrine conditions around alacustrine water body, as inferred from desiccationcracks, rhizocretions, nodulation, brecciation, andfenestral porosity present in facies Lm.

Finally, with the cessation of the alluvial influence,carbonate lacustrine environments became extensive,and facies Lm, Ll, Lo, Ls, Mm, and Mh developed(sequence D). The common, but discontinuous, appear-ance of ostracods along the Axamilpa Section supportsthe existence of intermittent water bodies, whoseintermittency appears to be more frequent as theenvironment became dryer. This would imply aprogressive lowering of the lake level, giving rise toephemeral and shallow saline lakes. While subjected tointense evaporation, sulfates (e.g. gypsum) would haveconcentrated in the lake and formed early diageneticfeatures (e.g., gypsum nodules) in the exposed mudflats. During early diagenesis, the influence of meteoricwaters may have caused the dissolution and collapse ofthe evaporitic layers. The depth, salinity, and size of thewater bodies must have changed substantially over time,as shown by the different lithofacies, fossil content, andsedimentary features of the AS rocks. The decreasingdepth, increasing salinity, and ever-smaller water bodiesthat represent the lacustrine and evaporitic stages, seemto reflect a change towards more arid conditions.

Some modern hypersaline terminal lakes and playasshow sedimentary processes similar to those inferred forthe Axamilpa Section and could serve as models tobetter understand environmental factors. Extensiveplains are characteristic of these environments, withhigh slopes or mountains relatively close to the lakeenvironment. These lakes have very shallow and evenintermittent waters, scarce rainfalls and high evapora-tion, carbonate and evaporitic deposits. For example,Great Salt Lake, Utah, is a shallow calcareous-evaporiticlake (10 m) influenced along its margins by alluvialsystems. Oolites, stromatolites, ostracods and migratorybirds are common, and the vegetation is low and open(Kowalewska and Cohen, 1998). The Salton Sea insouthern California (Arnal, 1961; Gilmore and Castle,1983) is a saline lake with scarce peripheral vegetation.It has hydrothermal activity associated with rifting(Harmon, 1966) and combines fluvial and lacustrinesedimentary processes (Arnal, 1961).

As in modern analogs, regional events that weretaking place simultaneously in central Mexico duringthe Paleogene, such as extrusive and intrusive magma-tism, extensive faulting and block displacements (e.g.Martiny et al., 2000; Morn-Zenteno et al., 1999), musthave determined sedimentation and biota distribution inthe Coatzingo Fm to a great extent, along with localfactors, such as rain frequency and intensity, changingtopography and drainage patterns.

The fining-upward evolution of the sequence,inferred as the retrogradation of an alluvial system, isprobably the result of the basin extension. Hydrothermalpulses, deduced from the occurrence of magnesite andcherts in the AS and in nearby localities, could representthe existence of magmatic activity in the subsurface.Both ancient and present-day hydrothermalism process-es have been reported for this area (Carballido-Snchezand Delgado-Argote, 1989; Jimnez-Surez et al.,2001). Similarly, the cyclical deposition of some facies(e.g., associations A, B, D) was probably related eitherto tectonic and faulting episodes, or to climaticvariations, maybe related to the global climatic changesthat occurred in that epoch (Frakes et al., 1992; Wolfe,1994).

5. Paleoecology

The fossil record known from the region correlateswell with the sedimentological data, thus lending supportto paleoecological inferences. For example, the geologicsetting indicates the existence of shallow lacustrineenvironments, which is reinforced by the presence of aflamingo skeleton in the Pie de Vaca locality (Cabral-Perdomo, 1996). This suggests that environmentalconditions (shallow and saline waters) similar to thoseknown from where these birds live today (Mascitti andBonaventura, 2002) prevailed in this area of the Coat-zingo Formation. Flamingos (Phoenicopteridae) havebeen described from many Eocene and Oligocene strata(Martin, 1983; Olson, 1985). Because extant flamingosare gregarious and migratory (Nager et al., 1996), it ispossible that the discovery of this single flamingoskeleton represents a larger population. This notion mayalso be supported by the fact that the fossil record ofaquatic birds in Tepexi de Rodrguez is composedmainly of ichnofossils of their tracks (Cabral-Perdomo,1996). Further work is needed to correlate precisely allthe localities where avian ichnofossils are present, buttheir presence in the Coatzingo Formation contributes tothe understanding of the past habitats and ecologicalrequirements of these birds, as well as their temporal andspatial distribution during the EoceneOligocene.

Fossil plants from the Axamilpa Section, such asCedrelospermum, Pistacia and Pseudosmodingium, are

251H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254also found in the Ahuehuetes Unit. The Pseudosmo-dingium species, which today exists in the Tepexi deRodrguez area, is thought to be endemic to this regionand, along with Cedrelospermum, is considered apioneer in harsh environments (Ramrez and Cevallos-Ferriz, 2002). Cedrelospermum has been referred tosubhumid climates in communities of desert scrub(Magalln-Puebla and Cevallos-Ferris, 1994b; Velascode Len and Cevallos-Ferriz, 2000), which suggestssimilar climatic conditions prevailed in this regionduring the Oligocene. Furthermore, thermometric infer-ences from the Ahuehuetes locality fossil plants,estimate annual average temperatures of 1018 C(Velasco de Len, 1999), and deciduous scrub orchaparral-like communities are thought to have estab-lished there in temperate to semiarid areas (i.e. whereannual precipitation was less than annual evaporation,and where there were long dry periods) (Ramrez andCevallos-Ferriz, 2002).

Plant-bearing strata in the Ahuehuetes Unit arecomposed of volcanic ash, and although the AxamilpaSection lacks volcanic components, the major volca-nism that occurred in this epoch (Martiny et al., 2000;Morn-Zenteno et al., 1999; Silva-Romo et al., 2000)might have caused environmental disruption in thesurrounding areas, which in turn probably influencedthe origin and distribution of plants in this region.Pistacia fossils from the Ahuehuetes locality have closerelatives that are found today only in restricted points ofAsia and one Oligocene locality in Germany, suggestinga long history of exchange between the North Americanand Eurasian floras (Ramrez and Cevallos-Ferriz,2002). Given the stratigraphical position of the twolocalities (Fig. 2), it is possible that some floral elementsfound in the Ahuehuetes locality evolved earlier in theEocene and lived around the dry and saline environ-ments represented in the Axamilpa Section.

The presence of riparian-related plants from theAhuehuetes Unit, such as Salix, Populus and someAnacardiaceae (Ramrez-Garduo, 1999), which arealso represented in the AS, may indicate that near-riverenvironments became established more than once in thebasin. In the Axamilpa Section, however, the presenceof Graminidites sp. (Carranza-Sierra, personal commu-nication, 2003) and fossil roots, together with theevidence for high salinity and aridity, would indicate agrass-like vegetation relatively far from the waterwayswhere the riparian communities became established, asin modern saline-lake environments (e.g. Kowalewskaand Cohen, 1998; Soria et al., 2000). This fact agreeswith previous assumptions of grass-abundant biomes forthis area (Martnez-Hernndez and Ramrez-Arriaga,2003), and may narrow the existence or riparianenvironments to only a few places. Pollen assemblagesalso suggest that the mountain ranges that surroundedthe basin were populated mostly by conifers (Martnez-Hernndez and Ramrez-Arriaga, 1999), contrastinglowland xeric and upland mesic vegetations.

The distinction between riparian and non-riparianplants may have implications for their distribution andtaphonomy. Perhaps near-river plants had a higherpotential for fossilization due to their susceptibility ofbeing quickly buried after flooding events (Stromberg etal., 1991), so riparian settings would be suitable placesfor taphonomical processes. Moreover, higher plantdiversity in arid lands is found around the streams (Ali etal., 2000; Stromberg et al., 1991), so diversity of fossilplants in the Coatzingo Formation may be an indicativeof riparian environments.

The fact that riparian ecosystems may have existed inthis region would have profound paleobiologicalimplications. Riparian zones favor plant dispersal andmixing of seeds (Stromberg et al., 1991), and highnumber of endemisms can be found there (Stohlgren etal., 2005), especially in desert springs. These systemsmay also serve as dispersal ways for plant and animalspecies (Baker, 1986). Furthermore, resources such asdrinking water, shade, food, and shelter, would havebeen an attraction for animals and may have helped ascorridors for their dispersal within arid zones. In thatsense, these environments could have had an influenceon the biodiversity and distribution of plants andanimals, and serve as spots for endemisms.

The presence of this resource rich environment isfurther supported by the abundance of fossil tracksrelated to camel-like animals at various stratigraphiclevels of the AS and their extensive occurrence in theTepexi de Rodrguez region (Fig. 1A). The tracksindicate that these animals traversed the area regularly;furthermore, they are perhaps an indication of theirdispersion process. Camelids originated in NorthAmerica during the PaleoceneEocene in North Amer-ica (Stearn and Carrol, 1989), and 2 events ofdispersion, one to Eurasia and Africa, and one toSouth America, have been proposed for the Mioceneand PliocenePleistocene respectively (van der Made etal., 2002); however, the fossil record of this group isincomplete and the starting points of these migrationsare unknown. Their association with desiccation cracksand rhizocretions in the AS suggests animal movementacross emergent areas, although resources from streamswould have been their main attraction. Perhaps theTepexiCoatzingo basin served as a migratory path forthese animals, as well as a place to drink, feed or breed.

252 H. Beraldi-Campesi et al. / Sedimentary Geology 191 (2006) 227254Finally, the recurrence of some of the taxa (e.g. fossiltracks, ostracods, plants) in various levels of theCoatzingo Fm indicate their permanence over time,and suggests that either the environmental conditionsnecessary for their development remained stable or thatthey adapted to the changing environments.

6. Conclusions

While a wide variety of habitats exist today in thisregion of the globe, where life forms have existed andevolved, little is known about these diverse scenariosduring the Cenozoic. Interpreting past life and theplaces where they lived opens the opportunity tounderstand particular biological or geological phenom-ena, and to compare them at different moments, whichoffers a more dynamic understanding of the naturalprocesses.

The depositional model of the Axamilpa Sectionprovides a conception of the nature and evolution ofa dry paleoenvironment over time, and gives organ-isms, or communities, a context in which they wereable to evolve. Providing an environmental contextfor fossils allows a better understanding of theirecology and distribution over time and opens theopportunity for discussions with a timeline vision.Moreover, fossils and paleoenvironments from theAxamilpa Section reinforce the importance of under-standing the reciprocal biotic and abiotic interactionsand influences.

While arid and semiarid areas in Mexico are widelydistributed and assumed to be of a more recent origin,the new evidence provided by the well-preservedsediments of the Axamilpa Section expands our viewon how arid environments may have looked like atthat time, their depositional evolution, and theinfluence of physical factors on the local biota andthe environment.

This study may encourage more detailed studies inthe Coatzingo Formation and other Cenozoic basins inMexico that can provide data to further precisetimeframes of sediment deposition, reconstruct paleoen-vironments, and to understand the evolution, biogeog-raphy and adaptation of organisms in tropical NorthAmerica.

Acknowledgments

We thank Dr. Ana Luisa Carreo, M.Sc. ClaudiaCarranza-Sierra, Ing. Ciro Daz, Dr. Jerjes Pantoja Alor,Ing. Diego Aparicio from the Institute of Geology,UNAM. Sr. Flix Aranguti and his family from theMuseum of Paleontology (Tepexi de Rodrguez); Dr.Gonzalo Pardo from the University of Zaragoza, Spain;Dr. Jos Carlos Garca Ramos from the University ofOviedo, Spain; the staff of the Biological Sciences andEarth Sciences Postgraduate Department of UNAM; Dr.Scott Bates, Dr. Robin Renaut, Dr. Blas Valero Garcs,and Dr. Bruce Sellwood for their comments. Thisresearch was supported by Project DGAPA (IN 208500)to Sergio R.S. Cevallos Ferriz.

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Sedimentology and paleoecology of an EoceneOligocene alluviallacustrine arid system, Souther.....IntroductionLocal geologyThe Coatzingo FormationThe Axamilpa Section

Stratigraphy and facies of the Axamilpa SectionSilicification processesFacies associationsFossil record of the Axamilpa Section

Evolution of the sedimentary systemPaleoecologyConclusionsAcknowledgmentsReferences