Early-Cenomanian Terrestrial Facies and Paleoclimate Records of the Lower Tuscaloosa Formation, Southwestern Mississippi


David F. Ufnar

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Geography and Geology


Biological, Environmental, and Earth Sciences


Sphaerosiderite oxygen isotope values from the Early Cenomanian Lower Tuscaloosa Formation, or LTF, extend a mid-Cretaceous meteoric delta O-18 latitudinal gradient 9 degrees south to25 degrees N paleolatitude, and contribute to a better understanding of the hydrologic cycle in North America during a "greenhouse" period in Earth history. Sphaerosiderites have been used to reconstruct paleolatitudinal trends in meteoric O-18 values throughout western North America, and the O-18 values have been used in a stable isotope mass balance model to produce quantitative estimates of mid-Cretaceous precipitation rates and latent heat flux values. The LTF meteoric O-18 values help constrain the critical subtropical values on the coastal plain bordering Tethys and the southeastern coast of the Western Interior Seaway. The LTF, an important subsurface petroleum unit (2800-4000 m depths) in southern Mississippi, contains amalgamated pedogenicallymodified, sphaerosiderite-bearing mudstones and sandstones. Cored intervals of the Early-Cenomanian Stringer Sandstone Member of the Lower Tuscaloosa Formation (LTF) of Southwestern Misississippi are characterized by 6 lithofacies units. The descriptive lithofacies units include: (1) thickly-bedded sandstones; (2) thinly bedded sandstones; (3) carbonaceous mudstones and siltstones; (4) red-mottled rnudstones; (5) purple and yellow mottled mudstones; and (6) flaser and lenticular bedded carbonaceous muddy sandstones. The sphaerosiderites are most commonly observed in gleyed domains of the red-mottled mudstones. These mudstones were clays, and silty clay loam soils, and are characterized by prominent red mottles in a gleyed matrix with an abundance of translocated clays, pedogenic slickensides, carbonaceous debris, and root traces. The sphaerosiderites yield delta O-18 vs. delta C-18 values, meteoric sphaerosiderite lines (MSLs), that may be used as a proxy for paleoprecipitation delta O-18 values. The MSL (delta O-18) values range from -3.31% to - 5.22% (VPDB). Estimated meteoric water values for the LTF have an average value of -4.69 % VSMOW.

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