Determination of paleogeography and depositional environment within phosphorite bearing remnants of the San Pedro del Gallo terrane in northeastern Mexico

Geochemical and sedimentological data obtained from outcrop localities examined during the present study are combined with data from previous studies to explain the origin of bedded phosphatic limestones 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.; 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 SPGT have been assigned. The SPGTR 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 in the Northern Hemisphere and corresponding paleocurrent configurations, and the depth at which these currents encounter the zone of photosynthesis, along with the effects of nutrient-rich upwelling waters, 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 Plain as evidenced by the presence of La Caja sediments and the overlying Chapulhuacan Limestone within the San Carlos Mountains, Tamaulipas, Mexico.