Integrated geochronological and chemostratigraphic study of middle Miocene strata (Ogallala Group) at the eastern margin of the North American Great Plains

Abstract

Global and regional shifts in climate and environmental conditions during the Miocene gradually gave rise to the grassland biomes that now dominate the modern North American Great Plains. Strata comprising the Ogallala Group provide critical information for understanding these transitions. Geologic mapping and scientific drilling at the eastern edge of the Ogallala Group in northeastern Nebraska, reveal a basal, pedogenically modified siltstone-dominated interval that was hitherto barely known and never firmly placed in a regional stratigraphic context. Herein, we distinguish this basal siltstone unit of the Ogallala Group in the eastern Niobrara River Valley from the overlying sand-dominated strata of the Valentine Formation on the basis of lithologic characteristics, trends in organic-carbon δ¹³C chemostratigraphic profiles, and U-Pb dating of detrital zircons. This siltstone unit bears some similarities to the middle Miocene Fort Randall Formation in its type area, which lies ∼75 km to the northwest in the Bijou Hills of South Dakota. Organic δ¹³C chemostratigraphic profiles generated from outcrops and cores of the basal siltstone in Nebraska indicate that the study area consistently had C₃-dominated paleofloras while it was deposited, presumably in the middle Miocene. The youngest detrital zircons from the siltstone-dominated unit were probably deposited directly onto ancient landscapes by supervolcanic airfall events originating some 1500 km to the west near the intersection of the boundaries of present Oregon, Nevada, and Idaho. These youngest zircons yield a maximum depositional age of ∼15 Ma. This age is in general agreement with the Barstovian (Ba2) mammal biostratigraphic age of the Fort Randall Formation in South Dakota. It is also significantly older than the ∼13 Ma youngest single grain zircon we recovered from sands of the overlying Valentine Formation. Thus, we propose that our integrated geochronologic and chemostratigraphic approach can date Late Cenozoic strata with constraints on the order of 1 m.y., portending broad applicability of our methods in reducing the uncertainties in dating terrigenous sediments in continental basins.