CONTROLS ON CYCLOSTRATIGRAPHY OF LOWER CRETACEOUS CARBONATES AND EVAPORITES, CUPIDO AND COAHUILA PLATFORMS, NORTHEASTERN MEXICO

The Lower Cretaceous Cupido (Barremian-Aptian) and Coahuila (Albian) c arbonate platforms of northeastern Mexico exhibit thick successions of meter-scale cycles deposited in three unique paleoenvironmental setti ngs. (1) The Cupido shelf lagoon is composed of peritidal carbonate cy cles deposited in the protected lee of a reef-rimmed to barrier-shoal margin. (2) The restricted Coahuila ramp interior consists of cyclic a lternations of subtidal evaporites and peritidal carbonates. (3) The d eep-water parts of both the Cupido and Coahuila platforms are composed of foraminiferal wackestones and lime mudstones interspersed with fir mgrounds and hardgrounds in a ''cyclic'' arrangement. Vertical success ions of meter-scale evaporitic cycles and peritidal cycles exhibit sys tematic stacking patterns that build into intermediate-scale high-freq uency sequences (tens to hundreds of meters thick), and large-scale co mposite sequences (hundreds of meters thick) that can be correlated ac ross the Cupido and Coahuila platforms. These large-scale stacking pat terns are interpreted to reflect long-term accommodation events and, w hen combined with the scale-independent architecture of all genetic un its, permit the inference that all three meter-scale cycle types on th e Cupido and Coahuila platforms are also governed by relative sea-leve l change. The composition, thickness, and number of meter-scale cycles within individual high-frequency sequences can be highly variable acr oss the Cupido and Coahuila platforms, however, even though the overal l upward-shallowing patterns are evident. The lateral complexity of cy cle architecture and distribution is interpreted to be a natural respo nse to fluctuations in regional climate interacting with autogenic pro cesses such as variations in carbonate production and dispersal, inten sity and frequency of tropical storms and monsoons, thermohaline circu lation patterns, and ambient ocean chemistry and temperature, These in teracting processes created laterally variable physiographic and ocean ographic conditions across the Cupido and Coahuila platforms, complica ting the sedimentary record generated by the composite sea-level signa l. Well-documented evidence from Barremian-Cenomanian pelagic cycles t hroughout the Tethyan seaway strongly indicates that Milankovitch-driv en climatic changes operated during the Early Cretaceous, Contemporane ous shallow-marine cyclicity in several locations suggests that these climatic changes may have had globally widespread effects. In an effor t to link the shallow-water and deep-water realms, we propose a model whereby Milankovitch-driven global climatic changes generated low-ampl itude, high-frequency eustatic fluctuations through some combination o f thermal expansion and contraction of ocean water, waxing and waning of small ice caps and alpine glaciers, and changes in the storage capa city of aquifers and lakes to produce meter-scale cycles across Lower Cretaceous shallow-marine platforms.