Biostratigraphy and Lithostratigraphy of the Middle Jurassic Tecocoyunca Group, Mexico

Abstract

<p>The Middle Jurassic Tecocoyunca Group of northeastern Guerrero, Mexico is situated on the allochthonous Mixteca tectostratiqraphic terrane. This group represents an overall transgressive trend with a few minor fluctuations of base level. Foreshore, shoreface, barrier island, washover, and laqoonal facies are identified within five coarsening-upward sequence. Abundant hummocky cross stratification, low-angle inclined stratification, and swash cross stratification indicate dominance of wave processes. These nearshore sequences are overlain by offshore marine shales, which are thought to represent a major global eustatic sea-level rise in the latest Bathonian, continuing into the Early Callovian. During the transgression, deep portions of the basin developed anoxic bottom conditions, resulting in the deposition of bituminous black shales. The combined effects of basinal subsidence and eustatic sea-level rise resulted in a rapid rate of transgression which exceeded that of sedimentation; ca. 110 cm/Ka. </p> <p> Five ammonite associations date the Tecocoyunca Group as Upper Bathonian (Retrocostatum Zone) to Lower Callovian (Calloviense Zone). Biogeoqraphic affinity of the ammonite fauna is mostly Andean with signicant west-Tethyan/mediterranean elements and a few endemic species. The ammonite faunas show: 1) a rapid faunal replacement, 2) shell morphology trends, and 3 ) varying degrees of endemism/cosmopolitanism, all in relation to sea level variation. </p> <p>Biostratiqraphy and lithostratigraphy of the Tecocoyunca Group suggest that: 1) the Mixteca terrane had a paleoposition, during the Middle Jurassic, near the Pacific opening of the Hispanic Corridor (proto-AtlanticJ, 2) the Hispanic corridor provided marine connections between the eastern Pacific Ocean and the western Tethys Sea, and 3) preponderance near shore sediments suggests close proximity to the Andes of South America. </p>