versión On-line ISSN 2007-2902
Rev. mex. cienc. geol vol.24 no.1 México abr. 2007
Revista Mexicana de Ciencias Geológicas
Stratigraphy of the Balsas Group in the Amacuzac area, southern Mexico: relationship with Eocene volcanism and deformation of the TilzapotlaTaxco sector
Estratigrafía del Grupo Balsas en el área de Amacuzac, sur de México: relaciones con el volcanismo y deformación eocénicos del sector Tilzapotla-Taxco
Dante J. MoránZenteno1, Ahiram MonterRamírez3, Elena CentenoGarcía2, Leticia A. AlbaAldave2, and Jesus Solé2
1 Departamento de Geoquímica, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., Mexico. firstname.lastname@example.org
2 Departamento de Geoquímica, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., Mexico.
3 División de Ciencias de la Tierra, Facultad de Ingeniería, Universidad Nacional Autónoma de México, 04510 Mexico D.F., Mexico.
Manuscript received: April 28, 2006
Corrected manuscript received: November 28, 2006
Manuscript accepted: November 29, 2006
The ZapalcoAmacuzac area in the Morelos and Guerrero states, southern Mexico, constitutes one of the classic sectors where Paleogene continental beds of the Balsas Group were originally described. The exposed succession has an apparent thickness of 2,500 m and it is constituted by two distinct sedimentary units separated by a slightly angular unconformity and by two intercalated ignimbrites (35.1 ± 0.4 and 38.5 ± 0.7 Ma, sanidine, KAr). The basal beds of the lower sedimentary unit (Tepetlapa Formation) are constituted by matrixsupported conglomerates that grade upwards to sandstone with a poorly defined planar parallel stratification and some intercalated andesitic and mafic lava flows. One of the andesites yielded a whole rock KAr date of 44.5 ± 0.7 Ma. Intercalations of conglomerate beds increase again toward the top of this unit. The upper sedimentary unit (Huajintlán Formation) is constituted by a ~300 m thick succession of matrixsupported conglomerate with some lenses of conglomeratic sandstone. This unit unconformably underlies the Tilzapotla ignimbrite whose best age estimate is 34.3 Ma (sanidine ArAr). The beds of the two sedimentary formations dip east to northeast with higher dips for the lower unit (5030°). Recognized facies in both units are indicative of an alluvial fan environment with more proximal facies for the Huajintlán Formation.
The studied succession is located immediately northwest of a large elliptical (50x30 km) dome structure that resulted from the tumescence stage, in the early evolution of the Tilzapotla caldera, previous to the collapse. The northwestern termination of the dome merges and interferes with the NNEtrending Laramide fold and thrust front of the Iguala Cacahuamilpa highs, near Taxco. In the study area, the dome contours display a local inflection defined by a change in the dip direction of the Morelos and basal beds of the Tepetlapa Formation from north to east. The slight angular unconformity between the Tepetlapa Formation and the overlaying units indicates a first stage of eastward tilting related to an uplift episode in the Taxco area, probably originated by the emplacement of silicic magmas (ca. 38 Ma). The proximal facies of the overlaying Huajintlán Formation and the age (35.1 ± 0.4 Ma) of the Piedras Altas Tuff , indicate that this formation resulted from the rejuvenation of the source area triggered by the tumescence and the initiation of the volcanic activity in the nearby Tilzapotla caldera area. This is confirmed by the higher dips (50°) of the Tepetlapa Formation near the dome structure and the decreasing dips toward the northeast. The age of one andesite lava flow (44.5 Ma) intercalated in the lower unit is indicative of an Eocene age for the beginning of the Balsas Group accumulation in this area.
Key words: terrestrial sedimentation, Southern Mexico, Balsas Group, Tilzapotla caldera, Eocene.
El área de AmacuzacZacapalco, estados de Morelos y Guerrero, constituye una de las áreas clásicas en donde el grupo Balsas fue descrito originalmente. La sucesión expuesta en esta zona tiene un espesor aparente de 2,500 m y está formada por dos unidades separadas por una ligera discordancia angular y por dos ignimbritas intercaladas (35.1 ± 0.4 y 38.5 ± 0.7 Ma, KAr en sanidino). La unidad inferior (Formación Tepetlapa) presenta en su base conglomerados matrizsoportados que gradúan hacia la parte media y superior a capas de arenisca con estratificación imperfecta e intercalaciones de lentes de conglomerado matrizsoportado con algunos derrames intercalados de andesita y basalto. Para una de las andesitas se obtuvo una edad de KAr en roca entera de 44.5 ± 0.7 Ma. La unidad superior (Formación Huajintlán) presenta una granulometría notablemente más gruesa representada por conglomerados matrizsoportados con algunos lentes de arenisca conglomerática. Esta unidad subyace en discordancia a la ignimbrita Tilzapotla, para la cual se ha obtenido una edad ArAr en sanidino de 34.3 ± 0.09 Ma. Las capas de ambas unidades tienen buzamientos al oriente y nororiente con echados mayores para la unidad inferior (5030°). Las facies observadas para ambas unidades revelan que se trata de depósitos de abanico aluvial con facies proximales para la Formación Huajintlán.
La sucesión estudiada se localiza inmediatamente al noroeste de una gran estructura dómicaelíptica (50x30 km) que se originó en la etapa de tumefacción, durante la evolución temprana de la caldera Tilzapotla, previa al colapso. La terminación noroccidental del domo converge e interfiere con el cinturón de pliegues y cabalgaduras que forma los altos del sector IgualaCacahuamilpa cerca de Taxco. En el área de estudio, el contorno del domo presenta una inflexión local definida por un cambio en la dirección del echado de las capas de la formación Morelos y la base de la Formación Tepetlapa de norte a este. La ligera discordancia angular entre la Formación Tepetlapa y las unidades subyacente indican una primera etapa de inclinación hacia el noreste probablemente originada por el levantamiento en el área de Taxco que acompaño el emplazamiento de magmas silícicos (ca. 38 Ma). Las facies proximales de la Formación Huanjintlán y la edad (35.1 ± 0.4 Ma) de la Toba Piedras Altas indican que esta formación resultó de un rejuvenecimiento del área fuente provocado por la tumefacción en el área más cercana de la caldera Tilzapotla. Esto se confirma por los echados mayores de la Formación Tepetlapa (50°) en las capas próximas a la estructura dómica. La edad de una de las andesitas intercaladas (44.5 Ma) en la unidad inferior es indicativa de una edad eocénica para el inicio de la acumulación del Grupo Balsas en esta área.
Palabras clave: sedimentación continental, Sur de México, Grupo Balsas, caldera Tilzapotla, Eoceno.
The Balsas Group was originally described by Fries (1960) as a succession of terrestrial deposits made up of conglomerate, sandstone, siltstone and lacustrine limestone as well as pyroclastic and lava flows. Fries (1960) included within the Balsas Group successions that crop out in five different areas distributed within the Morelos and northern Guerrero states. Subsequently, de Cserna and Fries (1981) and de Cserna (1982) applied the name of Balsas Formation to successions equivalent to the Balsas Group distributed in northwestern Guerrero State, southern Estado de México and western Michoacán State. However, in neither case was this unit formally defined.
In general, the main criterion used to allocate terrestrial rocks as part of the Balsas Group in central Guerrero and Morelos states has been its stratigraphic position. All conglomeratic units thatunconformably overlie deformed Cretaceous marine beds and underlie EoceneEarly Oligocene volcanic rocks (i.e., "TilzapotlaRhyolite" in its broad sense) (Fries, 1960) have been considered part of the Balsas Group. Therefore, a diverse range of Tertiary terrestrial continental sedimentary successions widespread from western Michoacán State to western Guerrero State have been described as part of the Balsas Group (i.e., Fries, 1960; 1966; de Cserna and Fries, 1981, de Cserna, 1982). Given the scarcity of paleontological and geochronological data, inferences on the age range and regional chronological variations of the Balsas Group are still vague. However, in a few localities, more precise inferences based on geochronological data have been made. For example, near Tetelcingo (Figure 1), by the Balsas river, OrtegaGutiérrez (1980) and Cerca et al. (in press) reported intermediate and silicic volcanic rocks, with KAr and ArAr dates ranging from 60 to 68 Ma, that underlie terrestrial rocks assigned to the Balsas Group. Cerca et al. (in press) provided geochronological data and stratigraphic observations that indicate that the lower beds of the Balsas Group are intercalated with Paleocene volcanic rocks. These authors also reported and compiled geochronologic data from late Eoceneearly Oligocene volcanic rocks that overlie the Balsas Group in a broad area around the high Balsas river. MolinaGarza and OrtegaRivera (2006) obtained an ArAr age of 33.1 Ma for an ignimbrite intercalated in a succession attributed to the Balsas Group, southeast from the Tilzapotla caldera, between Tuzantlán and the Balsas river. Based on a magnetostratigraphic analysis, the authors suggested that the whole succession belongs to the Eocene.
In the Taxco area, AlanizÁlvarez et al. (2002) recognized different sedimentary units (previously included within the Balsas Group by Fries), separated by andesites and ignimbrites ranging in age from 33 to 38 Ma. These authors also described a "red conglomerate" previously mentioned by Edwards (1955) and Fries (1960) in the AcamixtlaTaxco sector. The succession of reddish conglomerate underlies the earliest late Eocene volcanic rocks in the area and do not display any folding effects.
De Cserna (1983) suggested that parts of the Balsas Group in southern Estado de México and westernmost Michoacán may be Late Cretaceous in age. This is coincident with inferences by Campa and Ramírez (1979) who interpreted continental beds as Late Cretaceous in age (Cutzamala Formation), based on the occurrence of marine limestone intercalations and the folding deformation. More recently, Benammi et al. (2005) reported in the Tiquicheo region, eastern Michoacán, the finding of dinosaur rests, confirming that continental deposits resembling those of the Balsas Group in this region are, at least partially, Late Cretaceous in age.
Available uptodate data suggest that not all the terrestrial successions that were deposited previously to the widespread late Eocene to early Oligocene volcanism belong to the Balsas Group in the original meaning given by Fries (1960). Therefore more detailed stratigraphic and geochronologic studies are required to differentiate the units that have been wrongfully considered as one single depositional event, under the name of Balsas Group.
There are also different interpretations on the origin of the basinfills that have been grouped under the Balsas Group. According to Cerca et al. (in press), Balsas Group rocks in the Balsas river region accumulated in intermontane basins originated as a consequence of the Late Cretaceous Laramide deformation. In contrast, AlanizÁlvarez et al. (2002) interpreted that Balsas Group units (Chontalcuatlán and Atzala formations) in the Taxco area and to the north were accumulated in a pullapart basin associated with NW and Ntrending strikeslip faults. Preliminary evidence indicates that several independent sedimentary basins in Guerrero and Morelos states, with similar depositional environments, developed at different times, but all were covered by late Eoceneearly Oligocene volcanic rocks (Cerca et al., in press) (Figure 1).
Beyond the problem of applying the same lithostratigraphic denomination to a diversity of terrestrial deposits distributed in several basins, there are uncertainties regarding the age range, sedimentary environment and tectonic conditions during their accumulation. In order to set some constraints on the stratigraphy, depositional environments and age of what is described as the Balsas Group by Fries (1960), we have undertaken detailed studies in some selected areas of Morelos and northern Guerrero States. In this paper we present the results of a stratigraphic and geochronological study of the Amacuzac Zacapalco area, which is located southwest of Cuernavaca city, and is traversed by the highway and roads to Taxco and Iguala (Figure 2). The AmacuzacZacapalco area contains extensive exposures of the Balsas Group rocks that allowed us to carry out detailed field observations.
The Balsas Group in the AmacuzacZacapalco area unconformably overlies deformed beds of the Morelos and Mezcala formations. The Morelos Formation consists of over 1,000 m of platform limestone successions, while the Mezcala Formation is constituted by alternated marine beds of sandstone, siltstone and shale. The Balsas Group is made up of conglomerate, sandstone and siltstone, including intercalations of silicic ignimbrites and andesitic lava flows (Fries 1960, MonterRamirez, 2004). On the basis of differences in the dominant lithologies and the occurrence of a minor angular unconformity, the Balsas Group is divided in this study into two sedimentary and two pyroclastic units. The basal unit, defined here as the Tepetlapa Formation, is dominated by sandstone beds with subordinated intercalations of siltstone and matrixsupported conglomerate. The Tepetlapa Formation is unconformably overlain by the two pyroclastic units, which from base to top are the Teacalco Tuff and the Piedras Altas Tuff. To the south, these volcanic units pinch out entirely and leave in direct contact the Tepetlapa and Huajintlán formations. The uppermost unit, also sedimentary, is named here as the Huajintlán Formation. This formation is made up of coarse matrixsupported conglomerate, unconformably covered by the late Eocene (34 Ma) Tilzapotla ignimbrite (MoranZenteno et al., 2004). These units are also unconformably covered by post Eocene andesite flows (Zacapalco Andesite) and by Pliocene volcanic debrisflow deposits (Cuernavaca Formation) (Figures 3, 4a, 4b) (Fries, 1960; Morán Zenteno et al., 2004).
The Tepetlapa Formation is here defined as a succession of intercalated siltstone, sandstone and matrixsupported conglomerate with an apparent thickness of 2000 m. It shows lateral and vertical variations in grain size as well as in bed thickness. The best exposures of the Tepetlapa Formation are located along highway 90, near Santa Fe de Tepetlapa village (Km. 1922, from 99° 24'30" to 99°46'00" longitude) (Figure 4a), forming a succession that we consider here as the type section. The contact with the underlying Mezcala and Morelos formations is an angular unconformity, but it was locally overprinted by NW trending left lateral faults, which can be observed in outcrops at the highway cuts near Zacapalco.
The succession was divided into three members (Figure 3). The basal member is 150 m thick, and is made up of matrixsupported, poorly sorted conglomerate, interbedded with pebbly sandstone and poorly sorted medium to finegrained sandstone. Conglomerate and sandstone beds range in thickness from 1 to 5 m, and show remarkable lateral variations from sheetlike to lenticular geometries, laterally extending for several tens of meters. The conglomerate is mainly composed of subangular fragments of sandstone and limestone resembling the dominant lithologies of the Mezcala and Morelos formations respectively. According to the facies classification of Miall (1996), this member is formed by matrixsupported pebbly conglomerate with weak grading (Gmm), massive sandstone (coarse to fine) (Sm), and horizontally laminated fine to coarse sandstone (Sh). Gmm facies is dominant in the lower 70 m of this member, and Sm and Sh facies are dominant in the upper 80 m. A 20 cm thick bed of felsic ashfall tuff was identified about 50 meters up section from the base.
The second member is 1200 mthick and is formed by coarse, poorly sorted sandstone, interbedded with siltstone, to finegrained sandstone. Coarse sandstone beds are 10 cm to 2 mthick. The base of the sandstone shows irregular, concaveup, erosional bounding surfaces, and upper boundaries are mostly flat. It is well sorted, but contains some isolated pebbles up to 5 cm in diameter. The coarse sandstone is a litharenite, formed mainly by limestone and finegrained sandstone grains, showing subordinate amounts of volcanic grains, mostly andesitic in composition. Siltstone and finegrained sandstone intervals are massive and vary in thickness from 2 to 7 m. They have some thin intercalations of matrixsupported pebbly conglomerate, lenticular in cross section. Conglomerate is constituted by subrounded and subangular clasts, made up of limestone and sandstone. The recognized facies, in order of abundance, are Sm, Fm (mud, silt) and Gmm.
The upper member is represented by a 700 mthick succession of massive to poorly stratified lithic sandstone interbedded with conglomerate. Conglomerate beds range in thickness from 40 cm to 3 0 m. Sandstone is poorly sorted and is formed by limestone and sandstone/siltstone grains in different proportions, with subordinated quartz, feldspar, biotite and hornblende grains, as well as some volcanic lithics. Conglomerate is mostly made up of limestone clasts with some sandstone and mafic volcanic clasts. Massive to weakly laminated sand (Sm) is the dominant facies, with Gmm bedforms defining sometimes fourthorder erosive surfaces over sand intervals. It also contains some beds of tuff and greenish epiclastic sandstone. This member is exposed in the southern section along the road to Zacapalco and seems to have been removed by erosion to the north.
At least three interbedded andesitic and basaltic lava flows and one dike were found within the upper member. Andesitic lava flows display plagioclase and hornblende phenocrysts in a groundmass of plagioclase laths and orthopyroxene, defining an intergranular to intersertal texture. Basaltic lava flows contain iddingsitized olivine phenocrysts, glomerocrysts of clinopyroxene in an intergranular groundmass with plagioclase, clinopyroxene and othopyroxene. The mafic dike was found 3 km east of Texcaltitla town, and is basaltic in composition with abundant (40%) olivine phenocrysts. One andesitic lava flow intercalated in the upper part of this succession yielded a whole rock KAr age of 44.5 ± 0.7 Ma (Table 1).
The Tepetlapa Formation is distributed from the TepetlapaZacapalco sector to the northnorthwest, defining a continuous band of E and NE dipping beds (Figures 4a and 4b). Bed dips range from 50° for lowest beds exposed east the highlands of the ZacapalcoChapulistlán to 30° in the eastern zone of the outcrop area. There are some minor NNWtrending, SWdipping normal faults exposed in the easternmost outcrops with up to 1 m of displacement. The small displacements of these rather scarce faults can not account for the tilting of the studied successions.
The Tepetlapa Formation is Eocene in age, as suggested by the KAr ages obtained from the upper member. It is unconformably overlain by the Teacalco Tuff in the northcentral part of the study area, and by the Huajintlán Formation in the southern part of the area. Facies associations are indicative of distal alluvialfan depositional environment and its coarseningupward sedimentation suggests progradationof the fan. Given the general geometry of the fan body, the total measured stratigraphic thickness of 2,000 m is greater than the real thickness in a vertical section.
We define the Teacalco Tuff as a vitric crystalline massive welded tuff (Figure 3) whose type area is located on the road cuts near Teacalco village (from 99°27'00" to 99°28'50" longitude) (Figure 4a). It has avariable thickness, with a maximum of 60 m in the northern exposure area, vanishing to the south where the Huajintlán Formation directly overlies the Tepetlapa Formation. The tuff is constituted by quartz, abundant sanidine, biotite and plagioclase in a groundmass of glass with relicts of shards displaying variable degrees of devitrification. It also includes fragments of fiammelike pumice and lithics of siltstone. This member has a characteristic pink color when weathered. KAr dating from concentrates of sanidine and biotite obtained from a sample collected from the middle part of the section yielded ages of 38.5 ± 0.7 and 38.5 ± 1.0 Ma respectively, indicating an Eocene (Bartonian) age.
The Teacalco Tuff overlies the Tepetlapa Formation with a slight angular unconformity, and also overlies the Mezcala and Morelos formations in the northern part of the area. It dips about 15° to the northeast and is distributed from northwest of Teacalco to the southern flank of the Piedras Altas hills (Figure 4a).
Piedras Altas Tuff
The Piedras Altas Tuff is here defined as a slightly to nonwelded ignimbrite that comprises at least two different ashflow units, with a total thickness of 120 m. The type area is located at the western flank of the Piedras Altas hills, 2 km southeast of Teacalco Village (18°36'102 latitude, 99°26'45"longitude). The lower flow is made up of slightly welded pink ignimbrite, containing pumice fragments up to 5 cm in size, quartz, plagioclase, biotite and scarce sanidine in a pumicitic matrix. The upper flow is constituted by a white, nonwelded ignimbrite richer in pumice, with fragments up to 10 cm in size. It contains some red lithic volcanic clasts, as well as crystals of quartz, plagioclase and scarce biotite. A concentrate of sanidine from a sample collected at the base of the lower flow yielded a KAr age of 35.2 ± 1 Ma (Priabonian) (Table 1).
The Piedras Altas Tuff unconformably overlies the Teacalco ignimbrite and the upper contact with the Huajintlán Formation is a disconformity. The best exposures of this ignimbrite are distributed along the highway between Teacalco and Huajintlán and at the western flank of Piedras Altas hills (Figure 4a). There are some isolated bodies of ignimbrite attributed to the Piedras Altas Tuff, located north of Zacapalco and in the La Escondida locality, at km 22.7 of highway 90 to Taxco (99°26'00", 18°32'30").
The Huajintlán Formation is defined as an up to 300 m thick succession of poorly sorted conglomerate dominated by limestone fragments (> 90%) with minor content of sandstone and volcanic fragments (Figure 3). Limestone clasts range from subangular to subrounded and reach up to 50 cm in diameter. The matrix is made up of sandstone, formed by limestone grains with subordinated amount of siltstone, volcanic lithics, as well as quartz and plagioclase. According to the facies classification of Miall (Miall, 1996), the Huajintlán Formation is dominated by Gmm (massive matrixsupported gravel) and, in lower extent, by Gmg (matrixsupported conglomerate with some grading) and Gh (crudely bedded gravel). The succession displays fourth and fifth order surfaces bounding minor and intermediate paleochannels as well as minor lenses of coarse and pebbly horizontally stratified sand.
The type section is in Huajintlán village and some of the best exposures are also located along highway 95 and along the Amacuzac River near Huajintlán (from 99°25'00" to 99°26'00" longitude). There are also good exposures along the highway between Casahuatlán and Zacapalco (from km 17 to km 19; 99°24'30" to 99°25'00" longitude).
In the northern part of the study area, the Huajintlán Formation unconformably overlies the Piedras Altas Tuff, whereas in the southern part it directly lies over the Tepetlapa Formation with a slight angular unconformity (10°) (Figures 4a and 4b). The Huajintlán Formation is unconformably overlain by the 3435 Ma Tilzapotla ignimbrite. Its exposure area define a NNWtrending belt extending from the northwest of Huajintlan to the northeast of Tepetlapa, with beds dipping from 20 to 10° to the northeast. In the Huajintlán locality the unit is cut by a 10 meters thick, northtrending andesitic dike.
The Tilzapotla ignimbrite unconformably overlies the Huajintlán Formation. It was formed during a major volcanic episode related to the collapse of the Tilzapotla caldera (MoránZenteno et al., 2004), and is the most widespread pyroclastic unit in the region. It displays contrasting thickness depending on its intracaldera or extracaldera character. In the Amacuzac area, the Tilzapotla ignimbrite is represented by proximal extracaldera facies; it has a maximum thickness of 50 m and gradually thins toward the north. It is constituted by a massive crystalrich ignimbrite with characteristic euhedral biotite phenocrysts, rounded quartz, broken plagioclase, as well as minor sanidine in a microcryptocrystalline groundmass. Some intervals contain altered, up to 10 cm pumice fragments with biotite phenocrysts. A biotite concentrate from this area yielded a KAr date of 34.3 ± 1.5 Ma, which is undistinguishable from other ages obtained for the same unit in different areas around and within the caldera (MoránZenteno et al., 2004).
This unit is constituted by an up to 20 mthick succession of andesitic lava flows, overlying in angular un conformity the tilted beds of the Tepetlapa Formation and Teacalco Tuff. It is distributed immediately northwest of the inferred northwestern segment of the Tilzapotla caldera ring. In many aspects, it is similar to the andesitic rocks distributed within and near the caldera ring (i.e., Buenavista Formation) (3332 Ma) that were interpreted by MoránZenteno et al. (2004) as volcanic rocks related to the postcollapse caldera resurgence. Lavas have a porphyritic texture with plagioclase phenocrysts in a groundmass of othopyroxene, plagioclase laths and brown glass, forming an intersertal to intergranular texture. They also include plagioclase xenocrysts displaying desequilibrium features (sieve texture), as well as amphiboles with opacite rims.
TECTONIC SETTING AND VOLCANOTECTONIC EVOLUTION OF THE TILZAPOTLA CALDERA
The Balsasrelated beds of the AmacuzacZacapalco area are distributed northeast from the intersection of two structural highs: the NNEtrending Laramide thrust and fold belt of the IgualaCacahuamilpa sector and the NWtrending elliptical dome associated with the Tilzapotla caldera (Figures 2, 5).
The Laramide sector of the IgualaCacahuamilpa region is formed by a tectonic stack of overturned folds and thrust sheets with a general east vergence (Figure 2) (de Cserna y Fries, 1981; RiveraCarranza et al., 1998). This forms a range dominated by extensive exposures of the Morelos Formation that thrust over the Mezcala Formation and the Morelos Formation itself. Deeper erosion of the Mezcala Formation east of the IgualaCacahuamilpa front produced an elongated topographic low, approximately 20 km wide, partly filled with the Balsasrelated sediments in Paleogene time. The NNEtrending topographic low was broken up by the Eoceneearly Oligocene Tilzapotla volcanic edifice and associated NWtrending elliptical dome structure. The dome is 50 km long, and extends from Quetzalapa to the Taxco area where it merges with the IgualaCacahuamilpa highs (Figures 2, 5). The structural dome interfered with the older Laramide NNEtrending structures. This is indicated by the attitude of Late Cretaceous fold axes, which plunge to the north and to the south at north and south flanks of the structure respectively (MoránZenteno et al., 2004). Based on stratigraphic and structural observations around the Tilzapotla caldera, MoránZenteno et al. (2004) considered this dome as a structure developed in the early stages of the caldera evolution, and a result of the tumescence associated with the overpressure in the magma chamber. The elongated shape of the dome is partly controlled by the tectonic lineaments at its northeast and southwest flanks (Figure 2). The deformation and volcanic events related to the caldera evolution, as well as their chronological relationships, have been previously documented by MoránZenteno et al. (2004). The Tilzapotla ignimbrite (3435 Ma) is the major volcanic product of caldera collapse. The studied area, where the Balsasrelated rocks are exposed, is located at the northwestern margin of the dome structure near the northwestern limit of the caldera ring. In this area, the dome margin displays a local inflection in the form of a curvature, convex to the northeast, defined by a change in the dip direction of the Morelos Formation and basal Tepetlapa beds from north to east (Figures 4a, 5).
One remarkable feature in the studied area is the relatively high northeast and east dip of the Balsasrelated beds, without recognizable shortening at outcrop scale (Figures 4a, 4b, 6). Dip angles become lower eastwards, being up to 50° in the basal beds exposed in the southwestern sector contiguous to the Tilzapotla dome structure, but less than 10° near the top, in the northeastern sector. The basal unconformity with the middle Cretaceous Morelos Formation is tilted as well, and rocks of the Morelos Formation form a steep topographic slope (Figure 4b). The decrease in angle of the Balsas Group toward the east and upsection was previously interpreted by Fries (1960) as the result of synsedimentary tilting produced by faulting or large amplitude folding. However, our data suggest that the wide folding and change in dips might be related to the evolution of the Tilzapotla caldera, as it is discussed below.
DISCUSSION AND CONCLUSIONS
The angular unconformity separating the basal Tepetlapa beds and the underlying folded beds of Mezcala and Morelos formations (Figure 4b) indicate a time interval between the cessation of the Laramide deformation and the accumulation of the terrestrial deposits. The alluvial fan that originated the Tepetlapa Formation probably originated from an outlet of the topographic front formed by the thrust anticline. This is also suggested by the general eastward decrease in the clast size of the Tepetlapa Formation. The location and origin of the eastern limit of the sedimentary basin is unknown because it is covered by Pliocene epiclastic and alluvial deposits that unconformably overlie the Balsas Group and the Mezcala Formation.
The sedimentary facies recognized in the Tepetlapa Formation are indicative of an alluvial fan environment dominated by debris flow and sheet flow deposits. The lower and middle members show distal fan and/or flood plain facies (Figure 7a). The upper member shows a coarseningupward tendency, which might be related to the rejuvenation of the landscape. This probably was due to a first uplift stage related to the emplacement of silicic magmas in the Taxco volcanic center located at the northwestern side of the Tilzapotla dome structure. This is also suggested by an increment in the percentage of volcanic clasts in conglomerate beds of the same upper member. The slight angular unconformity between the Tepetlapa formation and the overlaying units records a break in the sedimentation and is probably related to the eastward tilting produced by this uplift episode. Regional geocronology of silicic magmatism suggest that the doming process began near the Taxco volcanic center at around 38 Ma and was progressing toward the SE until 34 Ma, when the collapse of the Tilzapotla caldera occurred.
There is no evidence of normal or lateral faulting at the piedmont that could have produced a slope break between the accumulation area and highlands to the west. Therefore we suggest that the deposition of the Tepetlapa Formation occurred at the flank of an intermontane basin.
The age range of the Tepetlapa Formation is less well constrained than the ages of the overlying units. The age obtained for an andesitic flow comes from the upper member (44.5 ± 0.7 Ma), and indicates a minimum Eocene (Lutetian) age for the sedimentation. Therefore the Tepetlapa Formation was not related to the evolution of the Tilzapotla caldera, but to a previous stage in the evolution of the sedimentary basin.
Remarkably coarser textures of the Huajintlán Formation indicate a significant change in the depositional environments. The relatively high abundance of sandstone and siltstone lithic fragments in the Tepetlapa beds, resembling the lithology of the Upper Cretaceous Mezcala Formation, is not found in the Huaj intlán Formation. In contrast, the Huaj intlán Formation is dominated by limestone clasts derived from the Morelos Formation. This change in composition of the lithic components is indicative of a change in the dominant lithology exposed in the source area. In addition to the change in clast composition, there is a major change in the facies and depositional environments in the Huajintlán Formation, compared to those observed in the Tepetlapa Formation. The coarse matrixsupported conglomerate texture and the sedimentary features of rocks of the Huajintlán Formation are typical of proximal alluvial fan deposits (Figure 7d).
The change in clast composition and depositional environments recorded in the Huajintlán Formation suggests an uplift of the source area, which occurred in a more proximal position to the studied area. The best candidate for the rejuvenation of the landscape and the change in the sedimentary system observed in the area is the late Eocene doming produced by magma emplacement in the early stages of the evolution of the Tilzapotla volcanic center (Figures 5 and 7d). This is supported by the age of the ignimbrites underlying (Piedras Altas) and overlying (Tilzapotla) the Huajintlán Formation. The best age estimate for the caldera collapse is the 34.29 ± 0.09 Ma ArAr date obtained for the Tilzapotla ignimbrite (MoránZenteno et al., 2004) at the eastern margin of the caldera. These results, compared to the KAr date of 35.2 ± 0.4 Ma of the Piedras Altas Ignimbrite, left a time interval of at least 0.5 Ma for the volcanic center evolution since the first precollapse ignimbrite emplacement, which is a reasonable time for this kind and size of volcanic center (Smith and Bailey, 1968).
The northeastward and eastward relative high dip of the westernmost Tepetlapa beds at the dome border (50°) and their strike following the inflection in the contours of this structure indicate a tilting episode related to the tumescence in the Tilzapotla caldera (Figure 4a). The relatively slight angular relationship with the syn and post doming Huajintlán Formation farther to the northeast, indicates that the tilting effect produced by the tumescence decreased in a short distance from the dome boundary.
The KAr ages obtained from intercalated volcanic rocks (35.2, 38.5 and 44 Ma) in the studied succession (Figure 3, Table 1) indicate that the main episodes of deposition occurred during the Eocene. The age estimates for the Balsas Group in the study area are in part similar to those obtained in other areas within the GuerreroMorelos region. In the nearby Taxco area, AlanizÁlvarez et al. (2002) documented Eocene volcanic rocks intercalated with continental deposits, similar to those of the study area. There are no indications that the basal beds of the studied succession could be contemporary with the Paleocene beds exposed along the Balsas river in central Guerrero State (Cerca, 2004). Even assuming low rates of sedimentation for the beds underlying the oldest dated andesite (44 Ma) in the studied area, it could hardly be interpreted that the basal beds reach a Paleocene age, unless significant interruptions of sedimentation of the Tepetlapa Formation occurred, which is not evident in the studied sections. This indicates that the evolution of the Paleogene continental basins began diachronously in the region.
There are no signs of accumulation in a coeval, faultbounded, subsidence basin as it has been documented in the neighboring Taxco region. Inclination of Balsas beds in the area are clearly related to doming processes and no listric faults seem to have been involved in the tilting process. Besides the evidence of tilting related to the doming process in the Tilzapotla caldera area, the structural attitude does not support listric or lateral faulting associated with the main slop break, separating the accumulation area from the highlands to the west. There is no evidence of the prolongation in this area of the tectonic lineament that defines the northeastern boundary of the Tilzapotla caldera (Figures 2, 4a).
Sedimentary and volcanic facies identified in this work suggest that five successive paleoenvironmental settings characterized the sedimentation of the Balsas Group throughout time in the Amacuzac region (Figure 7ae): 1) distal alluvial fan or floodplain sedimentation of the lower and middle member of the Tepetlapa Formation; 2) progradation of the fan lobe at the time of the andesitic volcanism, recorded in the upper member of the Tepetlapa Formation; 3) tilting of the Tepetlapa Formation related to the first stage of uplift originated by doming near the Taxco area and emplacement of the Teacalco Tuff; 4) initial stages of volcanic activity in the Tilzapotla volcanic center (Piedras Altas Tuff) and development of a new fan lobe (Huajintlán Formation), originated by the precaldera tumescence; 5) postBalsas volcanism related to the caldera formation (Tilzapotla ignimbrite) and later initiation of intermediate magmatism (Zacapalco Andesite).
Regarding the concept of using the name Balsas Group, it should be considered that this name was proposed but not formally defined as a lithostratigraphic unit by Fries (1960). In addition, the name has been used for relatively diverse deposits accumulated in different basins and with different lithostratigraphic rank (i.e., Balsas Formation, de Cserna, 1982). We think that, in order to avoid further confusions, it should remain as an informal group, as originally described by Fries, until lateral relationships among sedimentary units distributed in these basins can be documented in more detail.
We are grateful to Luca Ferrari and Susana Alaniz for valuable suggestions to the manuscript. We also acknowledge Barbara Martiny, Armando Alcalá y Erick Ramírez for assistance during fieldwork. Patricia Girón provided support in sample analyses and dating. Financial support came from the Dirección General de Asuntos del Personal Académico, UN AM (grant PAPIIT IN1048022) and resources from the Instituto de Geología, UNAM.
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