Petroleum Systems of Deep-Water Basins - Gulf Coast Section SEPM

Determination of Paleogeography and
Depositional Environment Within
Phosphate Bearing Remnants of the
San Pedro del Gallo Terrane in Northeastern Mexico
Edgar K. Cross, Jr.,
Shell Exploration and Production Company
One Shell Square, Poydras St.
New Orleans, Louisiana
Abstract
Geochemical and sedimentological data obtained from outcrop localities examined during the present study are
presented 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. Three “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). 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 and Iturbide remnants of the SPGT have been assigned. 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.
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