Gilberto Silva-romo

Based on the recognition of a major angular unconformity, magmatic and detrital zircons dating, as well as facies analysis of pre-Oxfordian clastic succession in the Ayuquila basin, we propose to subdivide the succession into two formations. We documented that the succession accumulated in braided fluvial systems, draining mainly southwards. During Bajocian-Bathonian time, at the beginning of sedimentation, the climate conditions were semiarid and became wet during Callovian time, probably because of the shifting of southern Mexico towards a more equatorial setting. We formally propose the Ayuquila Formation to name the succession below the unconformity, which consists of monomictic conglomerate of metamorphic or sandstone clasts, and lithic and feldspathic greywacke. Ayuquila Formation bears fossil flora reported in the region and includes some lacustrine beds; these accumulated during Bajocian- Bathonian time. We propose as type locality of the Ayuquila Formation, the succession e...

ABSTRACT The La Mora Formation is the oldest Mesozoic floodplain succession in the Mixteca Terrane of Southern Mexico. The presence of Amazonian detrital zircons in the La Mora Formation and in the overlying volcanic Diquiyú Unit indicates a major fluvial system that drained the Mixteca Terrane. The La Mora Paleo-River crossed the Oaxaquia microcontinent and the Mixteca Terrane prior to the breakup of Pangea, during Late Triassic–Early Jurassic time, when the Acatlán–Oaxaquia block was part of the northwestern portion of the Amazonian craton. Detrital zircons in the La Mora Formation have ages between 3307 ± 31 and 210 ± 12 Ma, which suggest that they originated in Amazonia and Southern Mexico: 33.2% of the detrital zircons have ages that are found only in Amazonian sources; whereas 66.7% of the zircons may be associated with either the Amazonian craton, the Andean Basement, or Southern Mexico. We propose that the La Mora fluvial system drained the Amazon basin in a westward direction, with its mouth in central Pangea, and that it most likely fed the Tolimán submarine fan. The inferred location of the Acatlán–Oaxaquia tectonic block at the time of the La Mora fluvial system implies that the basement of Southern Mexico experienced a right lateral displacement of at least 2300 km with respect to South America during the Mesozoic.

... This paper describes the structural features located in the eastern part of the Sierra Madre del Sur, grouped by ages. ... Ramos et al., 2000; Sanchez-Rojas et al., 2000; Martinez-Amador et al., 2001) and from specific studies on major structures (Riller et al., 1992; Tolson et al ...

Revista Mexicana de Ciencias Geológicas, volumen 15, número 2, 1998. p. 115 131. 115 Universidad Nacional Autónoma de México, Instituto de Geología; y Sociedad Geológica Mexicana, México, DF GEOLOGY OF XITLE VOLCANO IN SOUTHERN MEXICO CITY—A ...

ABSTRACT We propose formally the Chivillas Formation, as a key litostratigraphic unit to decrypt the sedimentary record of Cuicateca basin. Its type locality is along Barranca Las Salinas creek northeastern ward Tehuacán, Pue., where the rocks are virtually without deformation. Chivillas Formation consists of basaltic flows frequently with pillow structure and siliciclastic deposits of turbidite currents and debris flows, on a very thick volcanosedimentary succession cut by dikes of similar composition. Estimate a total thickness of more than 4,875 m. Chivillas Formation is assigned at Barremian based on the maximum age of detrital zircon within sandstones, this age is consistent with previously reported fossil content. We recognized 16 facies into five classes: conglomeratic, sandy, fine, calcareous and igneous, which are grouped as eight facies associations: A) Medium-grained turbidites, B) Coarse-grained turbidites, C) Fine-grained turbidites, D) Structureless sandstones; E) Debrites clast supported, F) Debrites matrix supported; G) Synsedimentary disharmonic folds and H) Lavas and pillow lavas. From the recognized facies associations, we interpreted an environment of submarine fan deposit. The volcanic rocks of Chivillas Formation are alkaline mafic to intermediate, geochemical affinity MORB or OIB, with common lead isotopic values similar to values of enriched mantle, the isotopic signature Pb/Pb of lavas suggests a source from lower crust. The sandstones studied are graywackes composed mainly of monocrystalline quartz, mylonitic polycrystalline quartz, mosaic polycrystalline quartz, perthitic microcline, and some plagioclase; very abundant siltstone and limestone lithics, and fewer volcanic and metamorphic lithics, similar to granite or granulite gneiss; the clasts are angular to subrounded. The clasts into conglomerates are of limestone, sandstone (cross-stratified) and gneiss. These lithologies are typical of the Sierra de Juárez mylonitic Complex and Oaxaca Complex, and its sedimentary cover. Only few volcanic lithic in sandstones and some levels with pyroclastic falls were observed. The sandstones have their source on continental blocks, specifically from inner craton or uplifted basement, so we can assume that came from Oaxaquia. The presence of Pan-African-Brazilian zircons indicates a source from Gondwana; probably, they were transported by complex river systems within an advanced step on the rifting process of Gulf of Mexico.