DEPOSITIONAL PROCESSES,CHRONOLOGY AND STRATIGRAPHY, TLALOC IGNIMBRITE APRON, BASIN OF MEXICO

In the Tlacuatonco and Chapingo valleys, east of Texcoco, vertical and lateral facies changes present a complex ignimbrite architecture over an altitude of 120m, between 2305-2425m (e.g. pits at Santa Maria Nativitas, San Dieguito, Tlaminca, La Joya, Agregados Como and around Huexotla). The youngest sediments include thick vertical aggradational sequences deposited from pyroclastic flows, block and ash flows, lahars and rhyolitic ash fall. The origin of stratified sand sequences, pumice gravel bedforms, thin basal sand sheets and deformation structures formed during late stages of sedimentation are considered in detail. The morphology of this sediment suite is a series of overlapping fans which forms an ignimbrite apron marking the eastern fringes of the Basin of Mexico. The fans probably debouched into a higher Lake Texcoco at around 2250m (e.g. evidence from San Vicente Chicoloapan).

Older pyroclastic flow sequences (37.5m) and fluvial channel fills eroded into the pyroclastic flows (asymmetric channels, cobble gravels/horizontally stratified sands (20m+) are located beneath palaeosol and lacustrine units. The latter two units marked a period of relative stability in the valleys between 33,500 and 41,000 BP. The older units are at least 42,320 BP.

Similar sequences and morphologies are located to the south, east of Coatepec and in a series of cañadas towards the Puebla motorway. For example at Mina El Quarenta a palaeosol between 39,000 to over 43,000 BP is underlain by multiple pyroclastic flow units over 42m thick.