--> Nomenclature Proposal and Provenance Analysis of the Concepción del Oro Formation (former Formación Caracol): Implications for the Tectonic Evolution of Southern North America in the Late Cretaceous

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Nomenclature Proposal and Provenance Analysis of the Concepción del Oro Formation (former Formación Caracol): Implications for the Tectonic Evolution of Southern North America in the Late Cretaceous

Abstract

Abstract

The Upper Cretaceous clastic successions outcropping around Concepcion del Oro is redefined in this work and formally named as Concepción del Oro Formation. This unit is key to understanding the geological evolution of the Mesozoic Basin of Central Mexico. Its type locality is in Tierras Blancas II Canyon, northwest of Concepción del Oro, Zacatecas. The Concepción del Oro Formation is composed of the Tierras Blancas and Rancho Viejo members. The Tierras Blancas Member consists of an aggradational succession composed of rhythmically interbedded shale and sandstone; using Innoceramus labiatus impressions indicate a late Turonian to early Coniacian age. The Rancho Viejo Member is characterized by an aggraddational and retrogradational succession of shale and sandstone; ammonite impressions (Texanites genus) indicate a late Coniacian to early Campanian age. Modal and geochemical whole rock analysis suggest arc sources for the Concepción del Oro Formation. Following a pattern of erosion and contribution sources change from felsic (MTB) to mafic (MRV), a contribution of low to high grade metamorphic rocks was determined; sedimentary and plutonic sources were subordinate. The detrital modes indicate an evolution from recycled orogen (MTB: Q43F17L40) to transitional arc (MRV: Q35F14L51), with moderate chemical weathering conditions. Stratigraphic, petrographic and geochemical characteristics recognized for the Concepcion del Oro Formation were used to define two stages of foreland system development in central Mexico. Stage 1 (early Turonian-early Coniacian): West-facing subduction and beginning of the foreland-basin system, related to Arperos basin closure and accretion-amalgamation of Guanajuato and Zihuatanejo/Guerrero terrains, as early as Aptian to late Cenomanian. Tectonically, this process could have been controlled by westward oblique subduction with development of a “proto” type foreland system with foredeep type depozones. Stage 2: East facing subduction, end of the foreland system in the Concepción del Oro area, late Coniacian to early Campanian. The late stage represents eastward subduction of Farallon plate which controlled double vergent thrust wedges, where a retro type foreland system was developed. Within this system the former foredeep deposits documented in the Tierras Blancas Member migrated as piggy-back deposits within a wedge-top depozone, while Rancho Viejo Member successions were deposited within a foredeep depozone.