LITHOSTRATIGRAPHY, STRUCTURE AND REGIONAL TECTONIC SIGNIFICANCE OF THE CENOZOIC BASIN FILL OF THE OAXACA FAULT ZONE AT LA CAÑADA [TECOMAVACA-CUICATLÁN AREA], NORTHERN OAXACA, SOUTHEASTERN MEXICO

La Cañada is part of the Oaxaca Fault Zone, a major regional NNW-SSE, ~250 km long tectonic feature, geomorphicly expressed as a narrow depression in Puebla-Oaxaca; in the latter, between 17°45'- 18°00'N Lat., 96°51'- 97°08', and ~550-1600 mamsl, it is bound by an Albian-Cenomanian silty-micritic sequence, and to the west by a high grade, ?Early Mesozoic metamorphic complex. A thick Tertiary continental sequence of complex architecture underlies La Cañada. Early Paleogene coalesced alluvial fans produced Unit A, a 200 m thick, calclithitic cobble conglomerate. Later, volcanic activity and fluvial sedimentation formed the discordantly overlying Unit B, a ~500 m thick felsic tuff and tuffaceous siltstone unit, which bears Middle Miocene mammals. Unit C, a ~600 m thick, red and green, lacustrine, gypsiferous, phyllarenitic rock body unconformably overlies Unit B in the north, and partly intertongues Unit D, a ~600 m thick, coarse, mainly phyllarenitic breccia and conglomerate that recorded intermittent, post-Middle Miocene pie de mont deposition. Locally, small, isolated, cobble, calclithitic conglomerate bodies 30 m thick (Unit E), unconformably overlie previous units. Quaternary alluvium, colluvium and soil unconformably overlie earlier units.

Structurally, the Tertiary sequence has a general ~20° dip, is fault-bound to the east and west, and is affected by two normal fault systems [N25°W-N25°E and N65°E-S65°W]. The sequence became preserved in a narrow half graben that makes up La Cañada, which was triggered by a large, eastern, intermittently active, moderately angled normal fault, with a ~1 000 m total throw, which is largely responsible for the half graben sedimentary fill architecture. This structure may correspond to a major Middle Paleozoic/Early Mesozoic fault zone, whose genesis and development have been modeled on little objective evidence. If so Cenozoic rejuvenation has occurred, but notice that north of La Cañada, in Puebla the half graben becomes a wider graben, filled by an Early-Middle Tertiary sequence, and to the south, it corresponds with the Y-shaped Valle de Oaxaca Graben, largely filled by a Middle Miocene sequence. Regionally then, the Cenozoic evolution and significance of this major basin generator feature in Southeastern Mexico, is complex and not yet well understood.