Geographic setting

The study area (Figure 1) lies between 17°25’-17°30’ North Latitude and 97°29’-97°36’ West Longitude, and 2200-2500 m a.s.l. It comprises a ~90 km² surface of rugged relief; the main currents that drain it are the ríos Teposcolula, Yodonda and Mixteco/Negro, all draining to the north. Most of the area is high ground, covered by forest; the western fourth is relatively lower, less forested and partly used as plow fields; it also includes a belt of badlands (in the central-west part). The area is well populated (^{INEGI, 1984}), and includes the Santiago Yolomécatl town, and the villages, from north to south: San Miguel Tiza, Ixtapa, Tecolotitlán, Santa Catarina, Santo Domingo Ticú, and San José de Gracia. The federal highway 125 traverses the area joining Tiza, Ixtapa and Yolomécatl, continuing further southwesternly to Tlaxiaco. The villages are largely connected by unpaved roads.

Figure 1 Index map of Oaxaca showing the location of the Yolomécatl area, northwestern Oaxaca, Mexico (pointed by the arrow), as well as the villages, rivers and sites mentioned in the text. 

Geologic setting

The following synopsis largely draws from ^{Ferrusquía-Villafranca et al. (2016b)}. In this area, rock units of late Jurassic to Quaternary ages are exposed (Figures 2-3). The Late Jurassic San Juan Limestone barely reaches the area: it is a sequence of dark gray to black, well indurated, partly dolomitized, largely pelmicritic limestone set in medium to thick strata. The Albian-early Coniacian Teposcolula Limestone unconformably overlies San Juan Limestone; it consists of biomicritic and pelmicritic limestone set in thick strata, which are strongly faulted and folded into N-S trending anticlines and synclines forming a massif/range bound to the west by the left-lateral Tamazulapam fault. A small Teposcolula Limestone tectonic scale detached from the massif, and thrusted over the Nduayaco Group (see below), is also present (Figure 2).

Figure 2 a) Geologic map and b) Structural section of the Yolomécatl area, northwestern Oaxaca, Mexico. Notice that the Yolomécatl Formation is preserved in the namesake tectonic basin. 

Figure 3 Generalized lithostratigraphic column of the Yolomécatl area, northwestern Oaxaca, Mexico. 

The Cenozoic sequence includes the late middle Eocene (pre-40.3 Ma) Nduayaco Group [in the sense of the ^{NACSN (2005)}, Art. 28, Remark (a)], which is a volcanic succession that extensively crops out in the area, forming partly peneplained hills and small cuestas; it mainly consists of basaltic andesite and other intermediate to mafic flows, autobreccias, a few avalanche deposits, and subordinate amounts of ash flow tuff sheets; it is in fault contact with the Teposcolula Limestone, and unconformably underlies the Yolomécatl Formation, with which is also in fault contact. The latter formation lies in a small (~40 km² surface), fault-bound depression here designated Yolomécatl tectonic basin, which is genetically related to the activity of the left lateral, strike-slip Tamazulapam fault, making the basin analogous to the much larger Ridge Basin (^{Crowell, 2003a}, ^2003b). This formation is a pinkish red, continental sequence ~650 m thick; however, we could measure only ~250 m of continuosly exposed thickness [see Figures 4 and 5, stratotype and principal reference section], because the overall uniformity of this sequence, local faulting, lack of bed markers, and of suitable outcrops, prevented us from objectively tracing any given bed or stratigraphic level along the outcrop area of this formation, which forms a badlands belt.

Figure 4 Schematic representation of the Yolomécatl Formation stratotype, northwestern Oaxaca, Mexico, slightly modified to include the fluvial (^{Miall, 2006}) and lacustrine lithofacies (this work) codes. The latter was generated following ^{Miall’s (2006)} criteria. The small, punctuated areas indicate isolated outcrops within the given interval. 

Figure 5 Schematic representation of the Yolomécatl Formation principal reference section, northwesterm Oaxaca, Mexico, slightly modified to include the fluvial (^{Miall, 2006}) and lacustrine lithofacies (this work) codes. The latter was generated following ^{Miall’s (2006)} criteria. The small, punctuated areas indicate isolated outcrops within the given interval. 

The Yolomécatl Formation chiefly consists of friable to moderately indurated, largely fine-grained quartzitic-volcanic arenite, fine-grained sandstone, clayey siltstone of similar composition, and silty smectite claystone set in medium to very thick (up to 5 m) strata (Tables 1, 2, and Figures 4 and 5). Interbeds include: 1) Sparse, thin to medium thick, pale brown tuff sheets of intermediate to felsic composition; one of these yielded a ⁴⁰Ar-³⁹Ar age of 40.3±1.0 Ma datum (^{Ferrusquía-Villafranca et al., 2016b}), that places this unit and its biota in the early late Eocene. 2) Clast- to matrix-supported, boulder bearing, pebble to cobble volcanic conglomerate set in medium to thick strata, forming tongues, lenses and continuous beds, mostly in the eastern part of the Yolomécatl Formation outcrop area; their clast composition is the same as that of the Nduayaco Group flows, a fact that indicates provenance from such group. 3) Medium to thick beds of milky white to brown chalcedony or opal, an enigmatic rock formed by amorphous silica, interpreted as a heavily altered tuff. 4) Up to 2 m thick paleosols are present at various levels, they usually carry fungal palynomorphs (see the Supplementary Material). The lower contact of this unit does not crop out; the upper contact is unconformable with the Chilapa Formation and the Nicananduta Group; it also bears the southernmost Paleogene vertebrate fauna of North America.

The Ticú dome is a small, oval, hypabyssal, dome-like rock body that lies in and around Santo Domingo Ticú. The rock is heavily altered to pinkish powdery matrix with rock fragments/remnants irregularly positioned; the less altered ones show an intermediate (dacitic) composition, which suggests some affinity with the early Oligocene, felsic Nundichi Group (a lithostratigraphic unit that crops outside but close to this area). If so, the Ticú dome would have been emplaced after the Yolomécatl deposition and before the Nundichi emplacement. Some large, altered blocks of the San Juan Limestone crop out over the dome, indicating that such unit was the host rock body, or at least part of it.

The late Oligocene 27.7 ± 0.7 Ma old Nicananduta Group (⁴⁰Ar-³⁹Ar method, ^{Ferrusquía-Villafranca et al., 2016b}) is a thick volcanic succession of lava flows and autobreccias of (largely) andesitic composition and varied texture that makes up steep-slope sierras; outside but close to the area, it unconformably overlies the early Oligocene Nundichi Group which in turn overlies the Yolomécatl Formation. In the northwestern part of the area, a small part of the Nicananduta Group unconformably rests on the Yolomécatl Formation. This group also seems to intertongue the late Oligocene Chilapa Formation, a tuffaceous, stratiform rock body intercalated with lacustrine limestone strata, thus resembling the Chilapa Formation. This unit was described in the northernly adjacent Tamazulapam-Teposcolula-Yanhuitlán area (^{Ferrusquía-Villafranca, 1976}), and as such it is cautiously regarded here; then, the Nicananduta Group and the Chilapa Formation are partly contemporaneous.

Quaternary alluvial deposits in the area lie in the valley of río Teposcolula (northeast of Yolomécatl), and in the western plain of río Mixteco; soils and plow fields are found in these sediments. Regolith mantles developed over all kinds of units, the easier ones to observe are those of unforested areas. We did not discriminate them though, to avoid clouding/crowding the map.

Cartographic material

Map used in this study were scale 1:50000 and included: (a) Carta Topográfica Yolomécatl E14D35 (^{INEGI, 1984}) and (b) Geologic map of the Yolomécatl-Tlaxiaco area (^{Ferrusquía-Villafranca et al., 2016b}). The software Google Earth Pro for cartographic analyses at various scales was also used.

Methodology

The geologic work was done following standard procedures applied to discriminating and describing fluvial and lake deposits (^{Talbot and Allen, 1996, p. 83-92}; ^{Miall, 2006, p. 162-352}). Field work was conducted during 2014, 2015 (two weeks each), and 2016 (four weeks); it consisted of walking along and describing the outcrops of all roads and trails that showed the Yolomécatl Formation (Figures 1 and 2). This work included to take detailed photographs of the outcrops, tracing profiles, investigating binding surfaces, traversing the outcrops to recognize and describe lithofacies, recording of structural data, and make preliminary integrations to detect/interprete lake and fluvial sedimentary patterns. Positioning was done with a GPS equipment, and measurements with a 50 m steel tape. Cartographic plotting was done directly on topographic or geologic maps. The sedimentary petrographic nomenclature is largely that of ^{Folk (1974)} and ^{Boggs (2003)}; the pyroclastic terminology is that of ^{Schmid (1981)}, ^{Fisher and Schmincke (1984}; ^{Branney and Kokelaar (2002)}, ^{Sulpizio et al. (2014)}.

Fluvial facies recognition and description follows ^{Miall (2006)}. Depositional systems attributes were described largely after ^{Collinson (1996)}, ^{Miall (2006}, ²⁰¹⁰⁾, and ^{Talbot and Allen (1996)}. The fluvial architectural elements and lithofacies used in this study are: CH, Channel; LA, Lateral accretion; GB, Gravel bar and bed form; and DA, downstream accretion elements, as well as these lithofacies: Gmm, Gmg, Gcm (alluvial and related deposits); Gm, Gh, Gp, Gt, Sp (channel deposits); Sr, Sl, Sh, and Fl (floodplain deposits) FF (overbank fines deposit) (acronyms explanation in Figures 4 and 5). Although ^{Tha et al. (2015)} proposed a lithofacies code for lacustrine clastic facies, based on ^{Miall (2006)} fluvial lithofacies code, they used the same acronym (e.g. Fl, Lsm and so on), for both fluvial and lacustrine lithofacies, which leads to confusion. To avoid it, we replaced the F for an L (lacustrine), and keep the lower case letters (which refer to the lithofacies main textural attributes), as shown here: Ll (lacustrine, laminated fine-grained sandstone, siltstone and mudstone: Lsm (lacustrine siltstone and mudstone), and Lm (mud and clay): offshore lithofacies; Ll and Lsm (as above): playa lake and mud flats deposits.

Bed thickness was described following ^{Ingram (1954)}. The palynologic analysis was performed using standard techniques (^{Batten, 1999}). Depositional systems modeling was done integrating all pertinent information, i.e. recognition and discrimination of depositional systems and their lithofacies, location, description and photographic illustration of specific outcrops, plotting spatial association of similar systems and/or lithofacies on a topographic base map, and filling of gaps assuming continuation of nearest features/data.

Initial remarks

^{Ferrusquía-Villafranca et al. (2016b)} showed that the Yolomécatl Formation records lake, and fluvial sedimentation, as well as intermittent felsic pyroclastic deposits preserved in a small, triangle-shaped tectonic basin bound by the Teposcolula fault in the north, the Ticú fault in the east, and the río Mixteco-río Negro fault in the west and south (Figure 2). The contrasting lithology on both sides of these faults is an evidence of their presence. Additionally, the Teposcolula fault plane is visible in the Yolomécatl-Nicananduta road, some 1.6 km west of Yolomécatl, as well as in La Presita area, where the dip of Yolomécatl strata is high. It should be noted that these three faults are synsedimentary, but the Ticú fault was more active, thus the sedimentary fill is thicker in the east.

This tectonic basin lies close to the regional, N-S trending, left-lateral, strike-slip Tamazulapam fault, and thus, very probably could be genetically related to the dynamics of this fault. It is noteworthy that the east-ward continuation of the Teposcolula and río Mixteco-río Negro faults place them in contact with the Tamazulapam fault (Figure 6), thus defining a larger graben (so far unnamed), where the Yolomécatl tectonic basin is just one of three major blocks. This rather complex structural array associated to the dynamics of a major strike-slip fault (Tamazulapam), is scale aside, also similar to that reported by ^{Crowell (2003a}, ^2003b) for the Ridge Basin, and ^{Harding et al. (1985, p. 68}, fig. 19, particularly in the bottom part) for the Andaman Sea, northwestern Pacific Ocean, which are areas of concentrated transtensional/transpressional stress along curved zones of major strike-slip faults. Due to the objectives of this paper, the structure of blocks A and C need no further description.

Figure 6 Structural setting of the Yolomécatl tectonic basin, Block (B) with the adjacent Eastern (A) and Northern (C) Blocks, as well as their spatial relationship with the Tamazulapam Fault. The main structural features of the Yolomécatl Formation are also depicted. 

Structurally, the best part of the Yolomécatl Formation shows a moderate, largely east-southeastward dipping (10-15°); however, in the southern portion, the dips are usually higher (up to 27°), and strike north and northeast; this anomalous dipping is related to the presence of NW-SE striking faults (of normal and lateral type, Figure 6). The presence of fibrous gypsum-fillings in sigmoid tensional fractures arranged in echelon, evidences transpression (Figure 7). Alternatively, it could be possible that the río Mixteco-río Negro fault had another (younger) pulse of activity, so that the anomalous dip values reflect dragging near the fault plane. Actually, both hypotheses complement each other, producing a more plausible solution to the anomalous dipping mentioned above.

Figure 7 Picture showing the presence of fibrous gypsum-fillings in sigmoid tensional fractures (indicated by the opposing arrows). By contrast, in the upper part a fibrous gypsum pseudobed fills a nearly horizontal fracture. 

The rocks that make up the Yolomécatl Formation are distributed in three geographic zones or belts that show a broad concentric pattern (Figure 8) around a N-S elongate, irregularly shaped, relatively large core, located in the east, which consists of deeply intertongued conglomerate (more abundant) and fine-grained stratal stacks, as well as tuff sheets (relatively scarce); the core is fairly rugged and set at higher altitude than that of the surrounding belts; its margins include fine-grained sediments. The inner belt consists of clay, silt and fine-grained sandstone set in thin to thick strata, showing vaguely horizontal laminations (i.e. laminar to very thin beds) that form a red, thick sequence that laps around the coarse-grained core, defining a distinctive geomorphology consisting of a semicircular outcrop area, expressed as a zone of badlands; several conglomeratic and pyroclastic strata are intercalated in the sequence; vertebrate fossils were recovered from this zone. The outer belt includes an irregular succession of tuff sheets (more abundant) and fine-grained strata; this fact indicates that volcanic eruptions were frequent but not continuous, and that during the eruption-free lapses, fine clastic sedimentation took place, thus producing interbedding of tuff and sedimentary strata. The outcrops extend northwestwards, finally meeting the volcanic Nicananduta Group.

Figure 8 Schematic space distribution of the depositional belts of the Yolomécatl Formation. Notice that the vertical dimension, in the sections, is exaggerated three times to better show sedimentary features. 

Fluvial system

The fluvial system largely forms the core mentioned above (Figure 8), where several kinds of deposits are found. Such deposits include the following ones:

Gmm, Gmg, Gcm Lithofacies: Alluvial fans and related deposits

These deposits partly include the CH architectural element (Figure 9); they are mainly located in the core, and consist of clast- or matrix-supported gravel (sensu lato) deposits, which usually show massive bedding, or less commonly, subtle thick bedding. The interstices between larger clasts are filled with unsorted sandstone, siltstone and clay; a few tuff sheets were observed on the Yolomécatl-Ticú road (Figure 2). The irregular shape of the core strongly suggests that its outgoing projections may correspond to alluvial fan lobes, which in turn disclose some lateral accretion; one of such lobes is partly traversed by the Yolomécatl-Ticú road. Along this road, there are some structureless, matrix-supported gravel deposits that could be of debris flow type.

Figure 9 Fluvial system of the Yolomécatl Formation: Alluvial fans and related deposits. a) Shows part of a terminal alluvial fan lobe conglomerate strata. b) Shows an outcrop of a debris flow deposit. Notice the lack of stratification. 

Gm, Gh, Gp, Gt, St, Sp Lithofacies: Channel lag deposits

These deposits include the CH architectural elements (Figure 10), and are made up of clast-supported conglomerate (largely pebble to cobble clasts) set in medium to thick (and even massive) strata; the small ones are typically lensoid, with concave bottoms, interpreted as subsidiary channels, others form blankets a few centimeters thick and probably record avulsion episodes. Typically, these small forms seem to be absent in the core, but are present in the inner belt at various stratigraphic levels; such belt, as mentioned above, largely records clastic lake deposition. The presence of conglomerate deposits in this belt, indicates that the lake bottom was intermittently subaerially exposed, and partly traversed by small channels; if so, the St lithofacies would also be present. The larger deposits could correspond to major channels, bars of diverse types, or subterminal fan facies, however the paucity of outcrops prevented making out their precise shapes. Only in one outcrop, ‘La Cascadita’ (Figures 2 and 11), sited on the west, there is a conglomerate body intercalated with tuff sheets and fine-grained epiclastic strata; its geographic position suggests that the conglomerate source area was located to the west, where the Nduayaco Group also crops out.

Figure 10 Fluvial System of the Yolomécatl Formation: Channel lag deposits. a) and b) show conglomerate lenses formed by channel lag conglomerate deposits filling scours on fine-grained lacustrine strata. 

Figure 11 La Cascadita outcrop. It shows tuff sheets (featureless zones) interbedded with conglomerate strata (Gp) of different clast size; the upper ones are finer-grained than the lower ones. 

Sr, Sl, Sh, Fl Lithofacies: Floodplain deposits

These deposits include the FF architectural element and partly make up the inner belt (Figure 12). The presence of fine gravel sensu stricto and sandstone rock bodies in the area is rare, a fact unexpected in fluvial deposits; none occurs in the ‘core,’ whereas a few were observed in the inner belt, whose constitution (medium to fine-grained sandstone or siltstone) makes them difficult to tell them apart from the true, fine-grained clastic lake deposist that largely form the inner belt. The main criterion to tell them apart is the occurrence of low angle laminar/tabular cross-bedding in the floodplain deposits, which contrasts with the vaguely thin to laminar bedding of the lake deposits. Further, this kind of deposits (floodplain) largely consist of friable to slightly indurated silty claystone, clayey siltstone and fine-grained sandstone that form tabular bodies usually < 1.0 m thick.

Figure 12 Fluvial System of the Yolomécatl Formation: Floodplain deposits. Notice that loose conglomerate fall and vegetation interrupt the continuity of the floodplain exposure. 

The presence of structureless zones in the fine-grained sequence, largely formed by dark clays allows us to interpret them as paleosols (Figure 13) developed in exposed flood plain deposits (and/or in lake bottoms). Paleosols were observed at least in three sites, El Llano, Rancho Ara, and CECYT (a high school). In the latter, Calamospora (a soil fungus) and Salvinia (an aquatic fern) are abundant (in different stratigraphic levels), which indicates that the site/area was aerially exposed and soil could develope, which later became flooded, so that the aquatic fern could thrive. Variable lake levels could foster a similar effect.

Figure 13 Paleosols. a) Paleosols developed in an exposed lake bottom. b) Close up of the lower right inset square in a), which shows numerous rootlets. The bottom left square in b) depicts one rootlet. 

This fine-grained, friable sequence formed badlands, thus showing resemblance to those cropping out in the Dakota Badlands (^{Retallack, 1983}). Fungal and plant palynomorphs recovered from these zones, bolster this interpretation, as does the finding of terrestrial mammals, which also disclose the availability there of lands not covered by water. Further, the palynomorphs recovered include an admixture of tropical and temperate plant taxa (in Table S1 of the Supplementary Material). This assemblage may shed light on the origins of the Cordilleran Flora (^{Axelrod and Raven, 1985}), and on the development of montane forests (^{Wijnga, 1995}).

Lacustrine system

The inner ‘belt’ (Figure 8) is a conspicuous red, badlands zone (devoided of a vegetation cover), formed by a clastic sequence at least 650 m thick (although in the principal reference section, the nearly continuously exposed thickness is only 250 m); it is mainly constituted by fine-grained sediments set in thin to thick strata or stratal packages vaguely showing laminar to very thin beds devoided of cross-stratification, thus indicating clastic deposition in (shallow) lakes. Local interbedding of thin conglomerate layers and/or lenses (already described above) indicates subaerial exposure of the lake bottom. The sequence also includes tuff sheets (> 0.60 m thick) which are more resistant to erosion than the adjacent fine-grained beds, thus forming within this sequence ridges at an early stage of erosion, or outstanding cuestas delimited by cliffs, where the overlying friable, fine-grained beds have been eroded away, and the underlying strata could not recede beyond the cuesta exposed limit.

Ll, Lm, Lsm: Lacustrine clastic (offshore) facies

This facies (Figure 14) consists of fairly abundant smectite claystone (determined by X-Ray difractometry, ^{Ferrusquía-Villafranca et al., 2016b}), clayey siltstone and fine-grained, subarkosic (quartz-rich) sandstone with a minor phyllarenitic composition, where the quartz grains have sharp edges, and most show undulatory extinction, a distinct feature of metamorphic origin. The feldspar grains (8-10%) are weathered (cloudy appearance) and less angular than those of quartz; there is an abundant clayey and slightly calcite-cemented matrix. The source-area of fine-grained sediments probably involved the Nduayaco Group, thus accounting for the volcanic (feldspar) component, whereas that of the metamorphic component (i.e. the undulatory extinction quartz grains) probably includes the undifferentiated, middle Jurassic Zorrillo/Taberna formations, or directly the late Paleozoic Acatlán Complex, which are the nearest geologic units that bear undulatory-extinction quartz crystals.

Figure 14 Lacustrine clastic offshore facies. a) and b) depict fine-grained sequences. 

The fine-grained clastics are set in thin to laminar beds forming strata up to ~3 meters thick, where a vague lamination is observed; this condition does not allow to detect varves, or to recognize cycles. The large thickness of the inner belt (~650 m) indicates either a very deep lake basin or a shallow one that subsided pari passu with sedimentation. The presence of thin channel lag and floodplain deposits in this largely fine-grained sequence, favors the second alternative.

Ll, Lsm: Lacustrine shore and mudflat deposits

Theoretically, this kind of deposits could be present, however a deep intertonguing that occurs between the conglomerate and fine-grained beds/facies (Figure 15) indicates that the lake shore (probably narrow) was very dynamic (and so was the lake level), repeatedly advancing inland or receding toward the lake, thus largely covering the lake shore deposits; however, we found three sites where fine-grained sandstone and siltstone (mudstone) vaguely set in thin to medium strata crop out, and bearing numerous reed plant remains, indicative of this kind of deposits (^{Talbot and Allen, 1996}). Additionally, the presence of these marginal/lake shore vegetation taxa: Typha sp., Saggitaria sp. and Cyperus sp. (see the Supplementary Material), strongly support this interpretation. The scarcity of suitable outcrops prevented us from detecting and describing other such deposits.

Figure 15 Intertonguing of coarse (conglomerate) and fine-grained (lake shore) strata that illustrates advance and retreat of the lake shore. 

Environmental significance of the palynoflora

The palynoflora recovered from the Yolomécatl Formation/sequence (see the Supplementary Material) consists of a moderately diverse array of lacustrine, marginal and terrestrial, largely tropical taxa: among the first are green algae like Zygnemataceae, the Tetrasporina Group, and abundant spores of the floating fern Salvinia; marginal (shore) aquatic plants include Typha, Saggittaria, Cyperus and Cupressaceae (cypresses); terrestrial plants comprise the arboreal taxa Acacia, Mimosa, Bombax, Anacardiaceae (mango family), Moraceae (fig family) and Arecaceae (palm family), known to have a tropical distribution since the Cretaceous. This flora indicates that the bearing geologic unit was deposited in diverse places: lake, lake shore and terrestrial (fluvial) sedimentary environments, under tropical climatic conditions, thus providing additional support to the environmental discrimination described above.

Pryroclastic sheets and associated lake deposits

Pyroclastic sheets, although do not conform a depositional system per se, are considered here, because they did contribute to generate the Yolomécatl Formation, aided to date it and to understand its depositional history. Such sheets largely occur in the outer belt (Figure 8), together with lacustrine strata similar to those of the inner belt (i.e. red claystone, siltstone and fine-grained sandstone strata), which tend to be thicker than the sheets.

Pyroclastic sheets

The ash flow tuff sheets (Figure 16) are usually light ochre (varying from whitish to light brown), and consist of glass shards and pumice. They exhibit a variable degree of welding (from unwelded, partly welded to fully welded); some show cross-bedding and dune structures, both are diagnostic of pyroclastic flow emplacement (^{Branney y Kokelaar, 2002}; ^{Sulpizio et al., 2014}). Some sheets are strongly altered by silicification (chertification). It should be noted that the tuff sheets show no evidence of previous movilization or reworking, thus indicating that they are primary deposits. However, given that tuffs of low grade welding erode rapidly, we might expect somewhere deposits formed by this process.

Figure 16 Pyroclastic System: Pyroclastic sheets (exposed as ridges or cuestas). a) Shows tuff sheets (marked by yellow arrows) intercalated in an offshore lake sequence. b) Depicts the badlands geomorphology, here dominated by small cuestas bound by cliffs developed in places where the friable beds overlying tuff sheets (marked by yellow arrows) have been largely eroded away, and the underlying ones could not recede beyond the tuff sheets margins. 

Both, tuffs sheets and epiclastic beds are set in strata of variable thicknesses, i.e. from laminar to thin to medium or thick strata. The superior toughnees/hardness of the tuffs makes them to stand out from the lacustrine beds, forming cliff-bound cuestas where the friable overlaying beds have been eroded away, and the underlying ones could not recede beyond the tuff sheet edge. In the outer belt, tuff sheets seem to be volumetrically more abundant than the epiclastic deposits. The alternation of tuff sheets and fine strata indicates that volcanic eruptions were frequent, but not continuous, and that during the eruption-free lapses, fine clastic sedimentation continued, until interrupted by another pyroclastic emplacement. This belt’s crop out area extends about 3 km northwest of Yolomécatl, until contacting the Nicananduta Group. This fact suggests that the source-area of the tuffs sheets probably laid northwest of the Yolomécatl Basin.

Concluding remarks on the depositional systems of the Yolomécatl Formation

It is quite possible that the Yolomécatl basin, as well as its sedimentary fill, experienced instability because it lies close to the regional, left-lateral strike-slip Tamazulapam fault, and its bounding faults (Teposcolula in the north, río Mixteco/río Negro in the west and south, and the Ticú in the east). It is likely that this tectonic activity might have caused variable lake levels and mobile lake shores.

Furthermore, the presence of paleosols at different stratigraphic levels indicates that floodplains and/or lake shores and even lake bottom deposits became episodically exposed to subaerial conditions, which induced development of soils (now they are paleosols), and of a vegetation cover, thus providing food and shelter to vertebrates that lived there.

The dynamics of the lake shore advancing toward the land or receding toward the lake accounts for the intertonguing of conglomerate and fine-grained strata (Figure 16). Perhaps another factor was the formation of piedmont deposits by gravity sliding of clasts on steep slopes. The lake or lakes received the suspended load from incoming rivers, which laid down in laminar to very thin beds formed by fine-grained sediments, and also received dust from the atmosphere. The pyroclastics indicate synsedimentary volcanism from a source probably located northwest of the Yolomécatl tectonic basin; the dominant small size of these pyroclastics may indicate that the volcano that produced them was not close to this basin. On the other hand, the mechanism of pyroclastic sheets chertification is not well understood; however, the presence of convolute confining surfaces in strata of chertified tuff, discloses the existence of a plastic, gel-like phase during the process of chertification.