Tectonic evolution of phosphorite-bearing deep water remnants of the San Pedro del Gallo terrane in northeastern Mexico

Cross, Jr., E. K., Shell Oil, New Orleans, LA, (formerly The University of Texas at Dallas)

Abstract

New geochemical and sedimentological data obtained from out crop localities examined during the present study is combined with data from previous studies to propose a tectonic evolution model for selected San Pedro del Gallo terrane remnants (SPGTR) occurring southwest of the Walper megashear in northeastern Mexico. Four "remnants" of the San Pedro del Gallo terrane (SPGT), (lower Oxfordian to upper Hauterivian) are examined, concentrating on the outer bathyal to upper abyssal La Caja Formation (lower Kimmeridgian to upper Tithonian). The study area extends from the San Carlos Mountains (Cruillas remnant), Tamaulipas, Mexico, to the San Pedro del Gallo remnant located in the Mesa Central physiographic province, Durango, Mexico. The La Caja Formation was deposited within a backarc basin that underwent structural subsidence, tectonic transport (strike-slip), and dissemination from higher paleolatitudes (~ 40⁰ N of the Oxfordian; Upper Jurassic paleoequator; San Pedro del Gallo remnant) to lower paleolatitudes (~ 22⁰ N of the Berriasian; Lower Cretaceous paleoequator). Contemporaneous with tectonic transport, is evidence of a dramatic deepening event that initiated during lower Kimmeridgian time, and continued throughout the Lower Cretaceous for all SPGTR examined. During the late Tithonian, a worldwide phosphorogenic episode is recorded from deposits occurring along the western margin of continental masses. In Mexico, a study of the upper Tithonian phosphorite interval (Unit D; Mazapil and Iturbide remnants) has revealed the presence of five phosphatic zones (MPL-1 to MPL-5) from the Mazapil remnant, and three phosphatic zones from the Iturbide remnant (IPL-3 to IPL-5). In addition, three "phosphatic realms" (PR-1, PR-2, and PR-3) occurring in the upper portion of the La Caja Formation within the Mazapil, (Zacatecas) and Iturbide, (Nuevo Leon), remnants of the SPG terrane have been assigned. The San Pedro del Gallo remnant is completely lacking in bedded phosphatic limestones within the "Capas de San Pedro" or lower chert member (= Unit D from the Mazapil and Iturbide remnants) of the La Caja Formation. In addition to the PR-zones, the sequence of phosphatic limestone deposition within Unit D from the Mazapil and Iturbide remnants displays geochemical signatures resulting in the identification of three phases or pulses of phosphatic limestone turbidite activity. The paleogeographic configuration of continental masses and corresponding paleocurrent configurations, and at what point these currents encounter the zone of photosynthesis (abiotic), along with the effects of nutrient-rich upwelling waters (biotic), are proposed as primary limiting factors to the initiation and subsequent termination of bedded phosphorite formation within the San Pedro del Gallo basin during late Tithonian time. The petrologic phase of the current investigation has confirmed scanning electron microprobe results that indicate mafic and heavy minerals characteristic of andesitic island arc volcanism have been concentrated within the neritic and bathyal PR-zones from the Mazapil and Iturbide remnants, but are lacking within the deep-water phosphorites (PR-3). Furthermore, tectonic transport of SPGTR in northeastern Mexico extends eastward into the Gulf Coastal Plane as evidenced by the presence of La Caja sediments and the overlying Chapulhuacan Limestone within the San Carlos Mountains, Tamaulipas, Mexico.