Milankovitch orbital forcing control on shallow-water carbonate cyclicity and early dolomitization : insights from the lower Cretaceous Cupido platform, NE Mexico
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High-resolution sequence stratigraphy coupled with understanding of diagenetic processes within carbonate strata improves our understanding of platform evolution controls, facies models, and reservoir quality. The Cupido platform (Barremian to Aptian) exhibits well developed cyclicity and provides unparalleled outcrops to investigate platform morphology, depositional settings, sequence stratigraphic development, origin of high-frequency cycles, and possible climatic controls on early dolomitization across the shelf. High-frequency cycles stack into twelve depositional sequences (A-L) that correlate across the platform, which, in turn, build into the highstand part of composite sequence I, two complete composite sequences (II and III), and the transgressive based portion of composite sequence IV. Cupido platform interior cycles show evidence for an allocyclic origin, including (a) symmetric and asymmetric peritidal and subtidal cycles, (b) lateral continuity across facies tracts, and (c) incomplete subtidal cycles. Spectral analyses of cycle thickness reveal a strong Milankovitch signal when sedimentation rate is assumed constant within cycles and cycles are tuned to orbital precession. High spectral density peaks are present for long and short eccentricity (E) period calculated for the Early Cretaceous times: E1 (264-392 kyr), E2 (89-98 kyr), and E3 (107-126 kyr). Obliquity component with peaks at period (38-59 kyr) are also present. Strong correlation of Milankovitch signals across the platform suggests that the lateral continuity of high-frequency cycles was caused by shelf-wide sea-level fluctuations. Early dolomitization of the highstand lower Cupido deposits (LCu) possibly occurred in modified seawater within subtidal and supratidal zones, both at highfrequency cycle-scale and depositional sequence-scale. Dolomitization patterns appear to be linked to Milankovitch-induced 4th-order sea-level events. The transgressive portions of depositional sequences are more pervasively or preferentially dolomitized compared to the highstand portions. Thickness and periodicity clusters of completely dolomitized cycles are equivalent to thickness and duration of depositional sequences. The results strengthen arguments for the role of Milankovitch-induced sea-level fluctuations in the development of greenhouse carbonate platforms and its control on early dolomite distribution in specific settings.