EVIDENCE OF LATE PALEOCENE-EARLY EOCENE TERRESTRIAL WEATHERING IN PALEOSOLS OF THE WILLISTON BASIN OF WESTERN NORTH DAKOTA

Paleosol characteristics preserve signatures of climatic inputs and can be used as stratigraphic indicators of significant environmental shifts in the rock record, especially if other common stratigraphic markers are obscured or absent. During the late Paleocene (56 Ma), a significant carbon isotope excursion (CIE) and shift in geochemical composition of marine sediments occurred due to a significant release of methane hydrates in the ocean, followed by a period of drastic global warming. Many terrestrially influenced sediment plumes of late Paleocene/early Eocene (P/E) age marine sequences around the world have marked increases in kaolinite relative to other clay minerals and the hypothesis has been that this abundance is due to increased weathering of soils at the time, followed by erosion into the sea. In modern environments, pedogenic kaolinite forms in wet, yet well-drained warm subtropical to tropical settings. Well-preserved P/E paleosols are relatively uncommon in the rock record, but within the P/E age Golden Valley (GV) Formation of the Williston Basin (WiB) of North Dakota, regionally similar paleosol sequences are present in mesa-like erosional remnants and preserve a record of climate changes following the CIE. The WiB received a continual fine-textured sediment influx from surrounding uplands during the Cretaceous into the early Paleogene without tectonic disturbance. These paleosols (the Mah-Dah-Hay Geosol) developed across the P/E boundary in wetlands with dense vegetation that experienced seasonal drying and fluctuating seasonal water tables during that time. The Mah-Dah-Hey Geosol consists of massive kaolinite-rich horizons with orthogonal crack patterns filled with goethitic interstitial fillings overlying pronounced Fe- and Si-enriched pans, all presumably evolved from weathering of pre-existing smectitic/kaolinitic deposits during the early Eocene warming event. These paleosols are virtually devoid of organic matter or fossil evidence. Up-section, the clay mineral assemblages (illitic species) suggest a return to wetter conditions. In the absence of diagnostic paleobotanic or paleontologic evidence, the morphology and pedogenic mineral assemblages of the Mah-Dah-Hey Geosol in the WiB is an excellent indicator of P/E warming and enhanced terrestrial weathering.