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Forcing mechanisms on Late Cretaceous carbonate sedimentation: the Austin Chalk Group of central Texas


Despite the widespread occurrence of Upper Cretaceous chalk deposits in shallow and deep marine settings around the world, their paleoenvironmental significance still needs to be constrained to accurately reconstruct the dynamics of past oceans that may, in turn, constrain reservoir heterogeneities. We aim to elucidate forcing mechanisms that control chalk deposition by investigating facies in the Austin Chalk Group preserved in central Texas, to capture the interplay of key environmental parameters affecting these unique carbonate deposits. We hypothesize that local mechanisms controlled regional scale depositional patterns, whereas global phenomena contributed to the development of phosphatic hardground surfaces. In particular we: (i) establish a high-resolution stratigraphic framework to evaluate the timing of sea level changes; (ii) document the influence of environmental parameters on carbonate facies by monitoring geochemical proxies; and (iii) determine the sequence stratigraphic and environmental significance of phosphatic hardgrounds within the Austin Chalk Group through their detailed diagenetic study.

First, the integration of outcrop and subsurface data from central Texas provides an integrated stratigraphic framework that reveals a migration of depocenters during the deposition of the Austin Chalk Group. This is inferred to reflect movements of the basement highs (e.g. San Marcos Arch) and sags that modulate eustatic sea level changes, and lead to the deposition of coarse upper Austin Chalk facies in the San Antonio area, whereas coeval sedimentation was deeper and finer grained in west Texas.

Second, geochemical proxies indicate that muddy facies of the lower Austin Chalk Group were deposited under oligotrophic (low nutrient input) conditions with moderate terrigenous contribution from land, whereas coarser, oyster-rich facies of the upper Austin Chalk Group represent an adaptation of carbonate producing ecosystems to mesotrophic waters where brief suboxic periods developed.

Third, at least three hardground surfaces in the upper Austin Chalk Group have a high amount of glauconitic grains which maturity is assessed by their potassium concentration. These hardground surfaces developed on top of oyster-rich beds, and record periods of strong winnowing of the seafloor, favoring potentially enhanced porosity and permeability in the coarse oyster-rich facies.