LITHOSTRATIGRAPHY AND PALEOGEOMORPHOLOGY OF THE UPPER MIOCENE OGALLALA FORMATION IN THE TEXAS PANHANDLE

The Ogallala Formation (Upper Miocene) of the Texas Panhandle preserves a paleoenvironmental archive that directly precedes the expansion of C₄ grasses. The Coffee Ranch local fauna (Late Hemphillian) contains the first evidence of C₄ herbivory in North America and has been used for paleoecological and paleoclimatic studies. Here we present detailed interpretations of the lithostratigraphy and stable isotope geochemistry of the Coffee Ranch and neighboring Mendota Ranch sections, in order to reconstruct paleogeomorphologic variability within the Ogallala Formation. The Coffee Ranch Ash (6.6 Ma) is used as a datum to correlate roughly 20 m of strata across the c. 7 km wide study area. Paleoenvironments interpreted from facies analysis include fluvial channels, floodplain paleosols, pedogenically modified aeolian sandstones, backswamp depressions and shallow lacustrine deposits. Several facies successions are traceable across the entire study area, preserving penecontemporaneous, aggrading landscapes with discernable paleogeomorphic and paleocatenary (soil landscape) relationships. Preliminary results from stable isotope analysis are in general agreement with previous studies. The δ¹³C of organic matter in paleo-Vertisols and aeolian paleosols varies between -24.0‰ to -23.3‰ VPDB, consistent with 11-16% C₄ biomass. Likewise, the δ¹³C of organic matter preserved in lacustrine mudstones ranges from -24.2‰ to -23.7‰ VPDB, resulting from the delivery of mixed C₃ and C₄ biomass to lacustrine basins from the surrounding landscapes. Further analysis of organic matter and pedogenic carbonate δ¹³C will be targeted at specific landscape positions in order to resolve vegetative partitioning during the nascent stages of the C₄ grassland biome.