Milankovitch orbital forcing control on shallow-water carbonate cyclicity and early dolomitization insights from the Lower Cretaceous Cupido platform, NE Mexico /

by 1977- Altobi, Younis Khamis

The Lower Cretaceous (Barremian to Aptian) Cupido platform of northeastern Mexico forms part of an extensive carbonate platform system that surrounded the ancestral Gulf of Mexico. Detailed stratigraphic sections combined with comparative sedimentology analysis provide evidence for a revised depositional model for the Cupido platform. A stable grainstone shoal developed behind a discontinuous rudist coralline algal bank on the margin facing the ancestral Gulf of Mexico to the east. A thick succession of cyclic peritidal and subtidal deposits accumulated on a broad shelf lagoon (~ 100 km) on the lee side of this margin. The Cupido Formation exhibits meter-scale high-frequency cyclicity which stacks into lower-order depositional and composite sequences that correlate across the platform. High-frequency cycles stack into depositional sequences (43-86 m thick) which in turn stack into lower-order composite sequences (145- 168 m thick). A total of 12 depositional sequences (A-L) are interpreted within the Cupido Formation. Depositional sequences (A-F) compose the LCu and stack into partial composite sequence I (highstand part only) and composite sequences II. Overlying this, depositional sequences (G-L) comprise the transgressive Cupidito member which stack into composite sequences III and 62 partial composite sequence IV (transgressive part only). The 2nd-order sequence boundary is represented by a laterally extensive thick collapse-breccia caused by the dissolution of the underlying bedded evaporites, breakage, and mixing of overlying layered lithofacies. Two-dimensional correlation of the measured stratigraphic sections revealed two additional depositional sequences landwards of the Cupido platform margin deposited simultaneously with the La Peña Shale deposition as it drowned the shoal complex in the early stages of the 2nd-order transgression. These retrogradational geometries on the Cupido platform are supported by landward thickening in the transgressive portion of the equivalent subsurface Sligo Formation. Near the platform margin, the cyclicity is disturbed by the presence of large-scale syndepositional growth faults. Movement along these faults resulted in thicker depositional sequences in the outer shelf due to the dramatic increase of accommodation space available for sedimentation. The complete depositional model and high-resolution sequence stratigraphic framework for the Cupido Formation, presented herein, reveal subtle complexities of facies models and their controls, which were not in previous studies. Results serve as practical analogues for the subsurface equivalent Sligo Formation of Texas and can be applied to similar reservoirs for better understanding of facies distribution and models, and their lateral variations. 63