Abstract

The Lower Cretaceous Cupido (Barremian-Aptian) and Coahuila (Albian) carbonate platforms of northeastern Mexico exhibit thick successions of meter-scale cycles deposited in three unique paleoenvironmental settings. (1) The Cupido shelf lagoon is composed of peritidal carbonate cycles deposited in the protected lee of a reef-rimmed to barrier-shoal margin. (2) The restricted Coahuila ramp interior consists of cyclic alternations of subtidal evaporites and peritidal carbonates. (3) The deep-water parts of both the Cupido and Coahuila platforms are composed of foraminiferal wackestones and lime mudstones interspersed with firmgrounds and hardgrounds in a "cyclic" arrangement. Vertical successions of meter-scale evaporitic cycles and peritidal cycles exhibit systematic stacking patterns that build into intermediate-scale high-frequency sequences (tens to hundreds of meters thick), and large-scale composite sequences (hundreds of meters thick) that can be correlated across the Cupido and Coahuila platforms. These large-scale stacking patterns are interpreted to reflect longterm accommodation events and, when combined with the scale-independent architecture of all genetic units, permit the inference that all three meter-scale cycle types on the Cupido and Coahuila platforms are also governed by relative sea-level change. The composition, thickness, and number of meter-scale cycles within individual high-frequency sequences can be highly variable across the Cupido and Coahuila platforms, however, even though the overall upward-shallowing patterns are evident. The lateral complexity of cycle architecture and distribution is interpreted to be a natural response to fluctuations in regional climate interacting with autogenic processes such as variations in carbonate production and dispersal, intensity and frequency of tropical storms and monsoons, thermohaline circulation patterns, and ambient ocean chemistry and temperature. These interacting processes created laterally variable physiographic and oceanographic conditions across the Cupido and Coahuila platforms, complicating the sedimentary record generated by the composite sea-level signal. Well-documented evidence from Barremian-Cenomanian pelagic cycles throughout the Tethyan seaway strongly indicates that Milankovitch-driven climatic changes operated during the Early Cretaceous. Contemporaneous shallow-marine cyclicity in several locations suggests that these climatic changes may have had globally widespread effects. In an effort to link the shallow-water and deep-water realms, we propose a model whereby Milankovitch-driven global climatic changes generated low-amplitude, high-frequency eustatic fluctuations through some combination of thermal expansion and contraction of ocean water, waxing and waning of small ice caps and alpine glaciers, and changes in the storage capacity of aquifers and lakes to produce meter-scale cycles across Lower Cretaceous shallow-marine platforms.

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