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 investigations in an attempt to explain the origin of bedded phosphatic limestones for selected San Pedro del Gallo terrane remnants occurring in northcentral and northeastern Mexico. Four “remnants” of the San Pedro del Gallo terrane (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 (SPGTR) located in the Mesa Central physiographic province, Durango, Mexico.

The upper Tithonian phosphorite interval contains five distinct phosphatic zones within the Mazapil remnant, and three phosphatic zones within the Iturbide remnant. In addition, three “phosphatic realms” occurring in the upper portion of the La Caja Formation within the Mazapil (Zacatecas) and Iturbide (Nuevo Leon) remnants of the SPGT have been identified. By contrast, the SPGTR appears to be completely lacking in upper Tithonian bedded phosphatic limestones. In addition to the PR-zones, the sequence of phosphatic limestone deposition from the Mazapil and Iturbide remnants displays geochemical signatures resulting in the identification of three distinct phosphatic limestone turbidite events.

Paleogeography, paleocurrent configurations, and 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 apparently lacking within the deep-water phosphorites. Furthermore, tectonic and structural transport of SPGTR in northeastern Mexico appears to have extended eastward into the Gulf Coastal Plain as evidenced by the presence of La Caja equivalent sediments within the San Carlos Mountains, Tamaulipas, Mexico.