--> Abstract: Improved Time Constraints Through Isotope Stratigraphy, Lower Cretaceous Carbonates, Northeastern Mexico, by C. Lehmann, I. P. Montanez, D. A. Osleger, J. L. Banner, and W. V. Sliter; #90937 (1998).

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Abstract: Improved Time Constraints Through Isotope Stratigraphy, Lower Cretaceous Carbonates, Northeastern Mexico

LEHMANN, CHRISTOPH, Univ.of California, Riverside; ISABEL P. MONTANEZ and DAVID. A. OSLEGER, Univ. of California, Davis; JAY L. BANNER, Univ. of Texas, Austin; WILLIAM V. SLITER, USGS, Menlo Park; and Annie Arnaud-Vanneau, University of Grenoble, France.

The Cupido and Coahuila platforms of northeastern Mexico are part of the extensive carbonate platform system that rimmed the ancestral Gulf of Mexico during the Early Cretaceous. The stratigraphic distribution of strontium and carbon isotope data, coupled with new biostratigraphic data, substantially change the time ranges of several formations, force a significant revision of the longstanding stratigraphic framework, and offer a revised chronology of platform evolution in the study area.

Sr isotope values of primary carbonate components were calibrated with the recently defined Early Cretaceous seawater secular Sr isotope curve (Bralower et al., 1997) and integrated with new biostratigraphic data. This improved chronostratigraphy, coupled with the recognition of previously documented carbon isotope excursions, indicate that shallow-marine carbonates previously mapped as the Albian “Aurora” Formation in the Sierra de Parras actually correspond to the Barremian to Aptian Cupido Formation. This revised stratigraphy restricts the Aurora Formation proper to Albian shallow-water carbonates overlying the Coahuila basement block to the northwest. Basal skeletal limestones of the Coahuilla block, originally thought to be late Aptian to early Albian, are now shown to be early to mid-Aptian in age. The new chemostratigraphic and biostratigraphic data also constrain the Aptian-Albian boundary to within the lowermost Acatita evaporites. Based on the occurrence of diagnostic microfossils and the stratigraphic distribution of d13C and 87Sr/86Sr values, the basal-most Acatita evaporites are coeval with the late Aptian La Pea shales, which blanket the Cupido platform surrounding the Coahuila block. The La Pe§a flooding event on the Coahuila block is preserved as a thin condensed and reworked interval within the transition from carbonates to evaporites in the lower Acatita Formation. This flooding event likely is associated with a global sea level rise and oceanic anoxia.

The new time contraints offered by the integration of biostratigraphic and chemostratigraphic data force three critical refinements of the paleogeographic arrangement of Lower Cretaceous carbonate platforms in northeastern Mexico. (1) The Cupido rudist-reef margin in the vicinity of Monterrey doglegs westward through the Sierra de Parras as a semi-continuous grainstone shoal. (2) The initial flooding of the Coahuila basement block occurred significantly earlier than previously believed. (3) Shallow-water carbonates (Aurora Formation) of the Albian Coahuila platform are restricted to the northwest, contrary to their previously inferred widespread distribution.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah