Petroleum Systems of Deep-Water Basins - Gulf Coast Section SEPM
Petroleum Systems of Deep-Water Basins - Gulf Coast Section SEPM
Petroleum Systems of Deep-Water Basins - Gulf Coast Section SEPM
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Determination <strong>of</strong> Paleogeography and<br />
Depositional Environment Within<br />
Phosphate Bearing Remnants <strong>of</strong> the<br />
San Pedro del Gallo Terrane in Northeastern Mexico<br />
Edgar K. Cross, Jr.,<br />
Shell Exploration and Production Company<br />
One Shell Square, Poydras St.<br />
New Orleans, Louisiana<br />
Abstract<br />
Geochemical and sedimentological data obtained from outcrop localities examined during the present study are<br />
presented to explain the origin <strong>of</strong> bedded phosphatic limestones for selected San Pedro del Gallo terrane remnants<br />
(SPGTR) occurring southwest <strong>of</strong> the Walper Megashear in northeastern Mexico. Three “remnants” <strong>of</strong> the San Pedro<br />
del Gallo terrane (SPGT) (lower Oxfordian to upper Hauterivian) are examined, concentrating on the outer bathyal to<br />
upper abyssal La Caja Formation (lower Kimmeridgian to upper Tithonian). During the late Tithonian, a worldwide<br />
phosphorogenic episode is recorded from deposits occurring along the western margin <strong>of</strong> continental masses. In Mexico,<br />
a study <strong>of</strong> the upper Tithonian phosphorite interval (Unit D; Mazapil and Iturbide remnants) has revealed the<br />
presence <strong>of</strong> five phosphatic zones (MPL-1 to MPL-5) from the Mazapil remnant, and three phosphatic zones from the<br />
Iturbide remnant (IPL-3 to IPL-5). In addition, three “phosphatic realms” (PR-1, PR-2, and PR-3) occurring in the<br />
upper portion <strong>of</strong> the La Caja Formation within the Mazapil and Iturbide remnants <strong>of</strong> the SPGT have been assigned. In<br />
addition to the PR-zones, the sequence <strong>of</strong> phosphatic limestone deposition within Unit D from the Mazapil and Iturbide<br />
remnants displays geochemical signatures resulting in the identification <strong>of</strong> three phases or pulses <strong>of</strong> phosphatic<br />
limestone turbidite activity. The paleogeographic configuration <strong>of</strong> continental masses in the Northern Hemisphere<br />
and corresponding paleocurrent configurations, and the depth at which these currents encounter the zone <strong>of</strong> photosynthesis,<br />
along with the effects <strong>of</strong> nutrient-rich upwelling waters, are proposed as primary limiting factors to the<br />
initiation and subsequent termination <strong>of</strong> bedded phosphorite formation within the San Pedro del Gallo basin.<br />
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