Depositional Controls and Sequence Stratigraphy of Lacustrine to Marine Transgressive Deposits in a Rift Basin, Lower Cretaceous Bluff Mesa, Indio Mountains, West Texas

Successful hydrocarbon exploration in former rift basins of the South Atlantic pre-salt has generated interest in understanding depositional, diagenetic, and stratigraphic controls on pre-salt deposits. However, most studies to date have focused on attributes and controls on presalt lacustrine carbonate reservoir systems and little work has been done on the overlying marine sealing facies. Currently our standard sequence stratigraphic model of marine transgression of rift systems involves a single pulse of marine flooding over fluvially-incised valleys resulting in backstepping of fluvial and estuarine siliciclastic facies within the erosionally confined zone of an incised valley. However, the pre-salt systems of the South Atlantic involve deep and broad alkaline lakes containing microbial carbonate facies that were deposited on rift structurally generated geomorphic surfaces. Using an outcrop analogue from the Indio Mountains of West Texas, this study aims to provide a depositional and stratigraphic model for marine transgression of lacustrine rift basin sediments that are similar in age, tectonic regime, and climatic setting to the pre-salt sealing facies of the South Atlantic. The Lower Cretaceous Bluff Mesa Formation was deposited on the eastern margin of the Chihuahuan Trough failed rift and is exposed within multiple Laramide-age thrust panels in the Indio Mountains of West Texas. The mixed carbonate-siliciclastic system thins from 360 to 220 meters across the study area and contains five non-marine lithofacies and seven marine lithofacies. The depositional facies stack into five fourth-order sequences that record the transition from fluvio-lacustrine to shallow marine deposition. Sequences 1–2 are characterized by fluvio-lacustrine sandstones during lowstand systems tracts (LST) and lacustrine siltstones or thin marine wackestones during highstand systems tracts (HST). Sequences 3–5 are composed of exclusively marine facies characterized by shoreface to shelfal sandstones during transgressive systems tracts (TST) and thick ooid grainstones and fossiliferous packstones deposited during HST. Sequence 2 records a significant rise in base level and the onset of marine deposition with LST fluvial sandstones overlain by thick marine carbonates during HST. The presence of thin marine limestones in sequences 1and 2 suggest that periodic marine incursion occurred during HST, but the basin was still primarily an enclosed rift lake. This idea is supported by analysis of carbonate associated sulfate from septarian nodules of lacustrine and marine origin. The observed succession of mixed terrestrial-marine sequences suggests transgression of rift basins involves multiple pulses of marine incursion