EVIDENCE OF PALEOCENE-EOCENE PALEOENVIRONMENTAL CHANGE IN THE PALEOSOL RECORD OF THE WILLISTON BASIN OF WESTERN NORTH DAKOTA

Stacked paleosol successions found in well-preserved outcrops of the Paleocene/Eocene (P/E) age Golden Valley Formation (GVf) of the Williston Basin indicate that the basin experienced shifts in both weathering regime and regional hydrologic conditions during the early Eocene global warming event. Multiproxy evidence from paleosols in the Bear Den Member (BDm) of the GVf suggest that the initial late Paleocene setting was a low-relief, low-energy braided stream system, heavily burdened with clay-rich sediments derived from both external sources (prevalently kaolinite and illite) and in situ mineral alteration (smectites and hydroxyl-interlayered 2:1 clays). This low energy system was disrupted by a drop in regional base level during the early Eocene, combined with warmer and wetter conditions, promoting enhanced leaching and formation of kaolinite-rich paleosols (the Maah-Daah-Hay Geosol) with pronounced coloration and features (paleofragipans, Fe pans and concentrations, low organic matter) indicative of warm climate, primarily wet with seasonal drying periods early, then continuously wet later. Following this interval of pronounced weathering, paleosols at the top of the BDm indicate that leaching environments persist, but at lower intensity than earlier episodes. Only one unlithified section of this uppermost paleosol has been identified in the Williston Basin sections. This paleosol has an upper horizon of ashy material highly enriched in quartz and zircon and nearly devoid of weatherable phases, overlying horizons with slightly higher Fe and clay contents indicative of pedogenic leaching. Other occurrences of this paleosol usually contain a silcrete with geochemical compositions nearly identical to the ashy bed of the unlithified section. Above this transitional paleosol, the Camels Butte Member (CBm) of the GVf (early to mid Eocene) is a very sandy unit with pronounced sedimentary structures indicative of a highly active erosive environment. The muscovite-dominated mineralogy of the CBm sediments indicates that sediment source changed during this time, increasingly shed from western sources as early uplift of the Rockies initiated. The Williston Basin P/E paleosols provide an excellent insight into shifting pedogenic conditions driven by changes in climate and hydrology over time.