TRIASSIC FLUVIAL FACIES IN MIXTECA TERRANE, SOUTHERN MEXICO

In southern Mexico, to the north of Oaxaca, we found a fluvial sedimentary succession of Triassic age known as La Mora formation. The unit consists of lithic and feldspathic greywackes, generally fine to medium grained, with parallel and cross lamination, flaser structures, shale intraclasts. The siltstones are laminated in thin layers. The sandstone and siltstone have metamorphic clasts, quartz and feldspar. The beds are usually thin, however the succession includes some medium and sporadic thick layers. The facies association to propose a probable floodplain model with small drainage channels. La Mora formation presents a discordant contact over Acatlán Complex and appears covered with a lag of its own debris.

Detrital zircons from La Mora formation have been dated by U-Pb SHRIMP and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) to obtain a maximum depositional age of Carnian with peaks of 252, 235 and 227 Ma

La Mora formation presents a compressive strain manifests itself in tight minor folds and some closed-verging to the west and northwest and inclined hinges to the north and northeast. Deformation does not affect the rest of continental clastic Mesozoic succession Huajuapan Basin. According to the current position of minor structures, it is moderately inclined folds and gently dipping.