SEDIMENTARY PROVENENCE OF THE GRUPO CONSUELO IN THE TLAXIACO ANTICLINORIUM, AT ROSARIO NUEVO, OAXACA, MEXICO

Since 1882, the Jurassic sequence, from the Tlaxiaco Anticlinorium, has been geologically studied. In 2004, it was divided by Jiménez-Rentería in the Lower Liassic Consuelo (Rosario and Conglomerado Prieto formations) and the Upper Liassic-Middle Jurassic Tecocoyunca (Cualac, Zorrillo, Taberna, Simón, Otatera and Yucuñuti formations) groups, considering the Conglomerado Prieto as a lateral change of the Rosario Fm. facies. At the beginning, the Consuelo Group was considered as continental, because it contains vegetal macrofossils and coal layers; nevertheless, he reported marine palynomorphs in some levels of this Group, suggesting a marine influence.

Miall Method (1985), was used to describe, and to interpret the sedimentary environments of the Consuelo Group. Petrographic analysis permitted to establish changes in mineralogical components related to estuarine and shallow marine environments. This Group was divided in petrozonas: A, the lower one, has a fluvial predominance and a little marine influence; B, is dominantly estuarine and displays little fluvial, shallow marine and coastal influences; C, is a fluvial one but a lateral alluvial fans system was more active. These data indicated transgressive and regressive fine grained units (Rosario Fm), deposited in an anastomosed river valley, bordered by an alluvial fans system (Conglomerado Prieto Fm), which was gradually invaded, from NW, by the “Portal del Balsas” epicontinental sea, forming an estuary.

Tectonic-sedimentary data permitted to know that the Lower Liassic Consuelo Group was deposited in a SE-NW halfgraben, during an incipient “rifting” stage, above Triassic andesitic extrusions (Diquiyú Unit) which provided detritus to the basin from the E-SE. A metamorphic debris source, was located at the S and E of the basin, identified as a recycled Permian orogen, raised by continental collision. Metamorphic quartz, coming from a source, lifted by a hot spot, placed at the present central area of the Gulf of Mexico, became gradually more abundant upward in the Consuelo Group; it is the main constituent of the Upper Liassic Cualac Fm.