ABSTRACT: Sphaerosiderite oxygen isotope values from the Early Cenomanian Lower Tuscaloosa Formation, or LTF, extend a mid-Cretaceous meteoric 18O latitudinal gradient 9° south to 25° N paleolatitude, and contribute to a better understanding of the hydrologic cycle in North America during a “greenhouseâ€ period in Earth history. Sphaerosiderites have been used to reconstruct paleolatitudinal trends in meteoric 18O values throughout western North America, and the 18O 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 18O 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 mudstones; (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 18O vs. 18C values, meteoric sphaerosiderite lines (MSLs), that may be used as a proxy for paleoprecipitation 18O values. TheMSL ( 18O) values range from -3.31â€° to - 5.22â€° (VPDB). Estimated meteoric water values for the LTF have an average value of -4.69 â€° VSMOW.