Rogelio Monreal

Stratigraphy and structure of the Lower Cretaceous of Lampazos, Sonora, (northwest Mexico) and its relationship to the Gulf Coast succession

The stratigraphy and structure of the Lower Cretaceous (Aptian-Albian) rocks exposed in the area of Lampazos, Sonora, are redefined and correlated to well-known stratigraphic successions of Chihuahua and northeast Mexico (Coahuila and Nuevo Leon). Originally, seven stratigraphic units were proposed for the Cretaceous rocks exposed in the Lampazos area, but in this article only five units (El Aliso, Agua Salada, Lampazos, Espinazo del Diablo, and Los Picachos formations) have been used, and we propose herein to discontinue usage of the other two (Nogal and La Mesa formations). This Aptian-Albian succession of east-central Sonora yields facies and fossil content remarkably similar to coeval sequences in the Gulf of Mexico realm (Chihuahua, Coahuila, and Nuevo Leon). These sections contain abundant microfossils (benthic and planktonic foraminifera, colomiellids, nannoconids, and radiolarians) that permit detailed biostratigraphic determinations. A complete Aptian-lower Albian section (base of K-6 through top of K-15 of the biochronologic scheme of Longoria, 1984) was identified. Abrupt lithic changes through the Aptian-Albian section of Lampazos revealed three major paleoceanographic events having an overall regional marine transgression, including an early Aptian (K-6 to K-7) event, a late Aptian (K-10 to K-11) event, and an early Albian (K-13 to K-14) maximum flooding phase. We used physical and biochronologic data to tie the aforementioned events to geochemically constrained Aptian-Albian oceanic anoxic events (OAEs) in Santa Rosa Canyon of Nuevo Leon. The close correlation between the oceanic events in east Sonora (northwest Mexico) and central Nuevo Leon (northeast Mexico) invokes the closer paleogeographic development of the two regions and undoubtedly links the area of Lampazos to a rapid encroachment of the paleo-Gulf of Mexico through north-central Mexico (the Mexican Sea) into northwestern Mexico (Chihuahua trough and Sonoran basin). The early Aptian (biozones K-6 to K-7) transgression has its maximum extension during the early Albian (biozones K-13 to K-14), culminating with the drowning of the carbonate platforms. The Lower Cretaceous rocks exposed in the Lampazos area are complexly folded and faulted. The more conspicuous structures are isoclinal and box-shaped folds and thrust faults, mostly oriented north-south and northwest-southeast, having vergences to both the northeast and southwest. A very conspicuous feature is the opposite vergence of thrust faults, either converging or diverging. Two dextral strike-slip faults (northeast-southwest and east-west) separate the area into three large blocks that have different patterns of folds and thrust faults. Also, it is evident that the area was affected by at least two episodes of normal faulting. Furthermore, the structural style of deformation present in the Lampazos succession is remarkably similar to the style of deformation of coeval rocks in northeastern Mexico, especially to the structure of the Chihuahua tectonic belt. The Lampazos succession is paleogeographically and tectonically related to the Chihuahua tectonic belt, and it is considered to be the westernmost extension of the ancestral Gulf of Mexico. The fact that the Lampazos succession can be lithocorrelated to known sequences in northeastern Chihuahua (Lucero, Ahumada, and Loma Plata formations) and northeast Mexico (Cupido, San Angel, La Pena, and Tamaulipas formations), which are know to be source rocks in exploration wells in Nuevo Leon and Coahuila, makes the Lampazos area of Sonora potentially attractive as an exploration target.