MISSING ARC-VOLCANIC INPUT TO UPPER CRETACEOUS FORELAND BASIN DEPOSITS OF THE MEXCALA FORMATION, SOUTHERN MEXICO

Abstract

During the Jurassic through early Tertiary, the western margin of North America was a convergent plate boundary with the Farallon plate subducting below the North American Plate (Sevier-Laramide orogeny). Convergence is well evidenced in the western United States by arc volcanism, the fold and thrust belt, and foreland basin development (Western Interior Seaway). Infilling of the foreland from the fold and thrust belt to the west included numerous volcanic ash beds. In contrast to the western U.S. record, evidence for arc volcanism and the proximity of the arc to southern Mexico is not well understood due to limited stratigraphic, biostratigraphic, structural studies and due to overprinting by postLaramide deformation. One method used to address the proximity of arc volcanism to the Mexican craton is petrographic provenance studies of Cretaceous foreland basin sandstones. This study focuses on part of the Upper Cretaceous (Cenomanian-Campanian) turbiditic sandstones of the Mexcala Formation in southern Mexico (Guerrero state). Initial field observations from turbidite paleoflow indicators suggest northwest to westward transport directions. Preliminary petrographic results indicate abundant sandstone grains from plutonic and metamorphic sources (deformed quartz, white mica, biotite, epidote). However, little or no volcanically derived grains (pyroxenes, amphiboles, alkali plagioclase, volcanic lithics) have been observed. This apparent absence of volcanic input may be interpreted as the result of three possible scenarios: 1) the volcanic arc was too far removed from the foreland basin study area; 2) some physical barrier prevented detrital volcanic influx (i.e., fold and thrust belt or fault blocking); or 3) an absence of volcanic activity/erosion during the studied further petrographic m-thick Mexcala in coeval foreland the constrain tectonic models involving volcanic arc development adjacent to the