Depositional Environments of Texas Cretaceous Carbonate Platforms Inferred from the Statistical Analysis of Geochemical Data




Walsh, Kenneth

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From the Late Jurassic to the Late Cretaceous, much of modern-day Texas was submerged beneath an epeiric sea. Eustatic sea-level rise increased accommodation space in many epicontinental seas, and the hot-arid to warm-tropical climate permitted extensive carbonate precipitation. The Late Cretaceous is known for its significant global chalk deposits: For instance, the Austin Chalk Group deposits stretch across Texas from the southwest to the northeast. Geochemical analysis of carbonate deposits allows researchers to reconstruct the prevalent paleoenvironmental conditions at the time of deposition.

A Turonian chalk section from South Texas, deposited on the northern edge of the ancient Maverick Basin, is analyzed for its geochemical composition to rebuild the paleoclimatic and paleoceanographic conditions in the northern margin of the Gulf of Mexico (GoM). X-ray fluorescence spectroscopy is applied to 84 samples collected in a 19 m section of exposed chalk and the elemental abundances calculated.

The resultant geochemical dataset is evaluated using classic proxies to constrain the paleoenvironmental conditions at the time of deposition. The South Texas chalk was deposited in a reducing, organic-matter-rich, open distally steepening ramp. However, throughout the section, the accommodation space vacillated with varying detrital input reflecting continual eustatic sea-level changes.

Multivariate statistical analysis techniques were applied to a selection of datasets: Another dataset of Austin Chalk Group deposits from Central Texas and two Glen Rose Formation (Albian) datasets from Central and South Texas. The Principal Component Analysis (PCA) of multiple datasets provides correlations among the datasets and elucidated distinct geochemical trends within the data. A second multivariate technique, agglomerative cluster analysis, was not as informative, and as a standalone analysis technique would not be useful. However, it confirms interrelationships between element data when used in conjunction with PCA.

This study discusses the paleoenvironmental conditions that prevailed in the South Texas region during the deposition of the Late Cretaceous Austin Chalk Group. The study also utilized multivariate techniques in conjunction with geochemical data to carry out a regional evaluation in larger scale paleoenvironmental trends amid Early and Late Cretaceous deposits, and in geographically disparate locations.

This study achieved two primary goals, the geochemical analysis of samples from a South Texas Austin Chalk Group deposit, and multivariate statistical analysis to determine paleoenvironmental signals from the South Texas dataset, and other datasets from the literature to compare forcing mechanisms on carbonate production during the Early and Late Cretaceous.


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Ancient Carbonates, Austin Chalk Group, Cretaceous, Maverick Basin, Paleoenvironament, Principal Component Analysis