Geologic framework of the Sierra Mojada mining district, Coahuila, Mexico : an integrative study of a Mesozoic platform-basin margin

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Title: Geologic framework of the Sierra Mojada mining district, Coahuila, Mexico : an integrative study of a Mesozoic platform-basin margin
Author: Gryger, Sean Michael
Abstract: The geology of the Sierra Mojada silver -lead -zinc mining district gives new insights into the stratigraphic evolution of the Coahuila Block and the Coahuila Folded Belt and the history of deformation along the basement -rooted San Marcos Fault Zone . Sierra Mojada provides the opportunity for substantial data collection relevant to the interaction of regional tectono -stratigraphic elements in a generally data -poor region of northeastern Mexico . Active mineral exploration has produced an extensive database of closely spaced drill core . Expansive underground workings facilitate subsurface geologic mapping . Sierra Mojada is situated at the northwestern edge of two tectono -stratigraphic provinces , the Coahuila Block , to the south , and the Coahuila Folded Belt , to the north . The San Marcos Fault , a west -northwest -trending regional structure extends through Sierra Mojada and is the informal boundary between these two provinces . Sierra Mojada is situated on uplifted and deformed late Paleozoic Ouachita siliciclastic strata intruded by Triassic diorites . This basement is diagnostic of the Coahuila Block . Basement rocks are overlain by an immature conglomerate that is interpreted to be the updip equivalent of the Jurassic La Casita Formation . The stratigraphy of Sierra Mojada principally consists of a continuous succession of Barremian through Albian carbonates unconformably overlying the basal conglomerate . The Barremian -Aptian Cupido Formation locally records deepening conditions from a clastic -influenced evaporitic interior to high energy , open water conditions . The shale and lime mudstone of the La Pena Formation were deposited during a Gulf -wide transgression that signals the end of the Aptian . The Sierra Mojada region of the Coahuila Block was inundated throughout the Aptian and was affected by the late Aptian transgression . The Albian Aurora Formation constitutes the bulk of the Cretaceous section . Sierra Mojada exposes the Aurora shelf rim , progressing from platform margin to shelf rim and platform interior facies . The structural features of Sierra Mojada affect the entire Cretaceous section . The high angle San Marcos Fault was reactivated with reverse motion during the Paleogene as a result of Laramide shortening . This juxtaposed basement and Jurassic conglomerate against the Cretaceous carbonates consistent with offset observed along the southern trace of the San Marcos Fault . A local colluvial unit suggests a lag in Laramide deformation . The carbonate strata and colluvial unit were overridden by a low angle , northeast -dipping thrust fault that placed a Neocomian through Aptian sequence atop the autochthonous Aptian -Albian carbonates . The allochthonous San Marcos Formation suggests regional -scale tectonic transport of this immature fluvial conglomerate from a downdip depozone within the Sabinas Basin . Kinematic indicators are consistent with the southwest -northeast axis for maximum compression established for Paleogene shortening throughout the Coahuila Folded Belt . The thrust fault bisects the principal ore zone within the Lower Aurora and upper La Pena Formations . This relation constrains the minimum age of ore emplacement to the Paleogene and suggests mineralization was genetically tied to the late stages of the Laramide Orogeny .
URI: http : / /hdl .handle .net /2152 /ETD -UT -2010 -12 -2637
Date: 2011-02-16

Citation

Geologic framework of the Sierra Mojada mining district, Coahuila, Mexico : an integrative study of a Mesozoic platform-basin margin. Master's thesis, University of Texas at Austin. Available electronically from http : / /hdl .handle .net /2152 /ETD -UT -2010 -12 -2637 .

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