Evidence for Milankovitch Climate Forcing on High-Frequency Cycles and Early Dolomite Distribution from the Cupido Formation, Sierra Madre Oriental, NE Mexico

Altobi, Younis K.¹, Tina R. Foster², Robert K. Goldhammer¹ (1) The Jackson School of Gesosciences, Austin, TX (2) BP America Inc, Houston, TX

High-resolution sequence stratigraphy coupled with the ability to predict the distribution of diagenetic processes such as dolomitization lead to greatly improved reservoir characterization. The Cupido Formation (700-1200 m thick) is a Barremian to Aptian low-angle shallow water carbonate bank that rimmed parts of the ancestral Gulf of Mexico and is equivalent to the Texas Gulf Sligo Formation.

Detailed field, petrographic, and statistical analyses were conducted on outcrops exposing the Cupido platform. Our depositional model for the Cupido platform reveals high-energy shoals rimming the shelf with a rudist bank forming downslope. Meter-scale high-frequency cycles (HFCs) stack into at least 12 depositional/high-frequency sequences (HFSs) within the Cupido carbonates. These HFSs stack into 2 full and 2 partial composite sequences that correlate across the platform and coeval, nearby Texas, platforms. Peritidal and subtidal HFCs were identified based on their facies proportion and position across the platform to determine their driving mechanism. Five detailed measured sections were examined using spectral analysis techniques to determine the presence/absence of periodic components. Identified significant peaks representing Milankovitch precession and eccentricity periodicities (20 ky, 40 ky, and 100 ky) support a dominant allocyclic origin for Cupido cycles.

The study also demonstrates a link between sequence stratigraphy and early dolomite distribution. Two different textures of early dolomite with different stable-isotope signatures are recognized within individual Cupido cycles. Coarse-grained replacement dolomite occupies the transgressive cycle base whereas fine-grained dolomite inhabits the regressive cap. Interplay of sabkha and reflux processes are interpreted as potential dolomitization mechanisms. These systematic variations are a result of Milankovitch-controlled eustatic changes.