Nomenclarure 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

The Upper Cretaceous clastic successions cropping out around Concepción del Oro, traditionally recognized as Caracol Formation, are 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 nor­mal graded succession composed of rhythmically interbedded shale and sandstone; Innoceramus labiatus impressions indicate a late Turonian to early Coniacian age. The Rancho Viejo Member is characterized by both, a normal and inverse graded succession of shale and sandstone; ammonite impressions (Texanites genus) indicate a late Coniacian to early Campanian age. A total thickness of 1200 m was estimated includ­ing both members of Concepción del Oro Formation. The stratigraphic and sedimentological characteristics described in this study are different from the original characteristics of the Caracol Formation; therefore, the name, Concepción del Oro Formation, is proposed for locally use in the vicinity of Concepción del Oro, Zacatecas.

Modal and geochemical whole rock analysis suggest arc sources for the Concepción del Oro Formation, changing from felsic (MTB) to mafic (MRV), and a contribution of low- to high-grade metamorphic rocks; 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: During early Turonian-early Coniacian occurred a west-facing subduction, and the foreland-basin system began to form, 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: During late Coniacian to early Campanian, east facing subduction, and ending of the foreland system in the Concepción del Oro area. The late stage represents eastward subduction of the 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.