Sequence Stratigraphy and Paleoclimate Analysis of Paleosols in Incised Valley Fills of the Cretaceous Blackhawk Formation, Book Cliffs, Utah

Several exceptionally well-preserved paleosols occur within the incised valley fill (IVF) of the Campanian Blackhawk Formation of the Book Cliffs, Utah. Uncertainties exist as to what climatic conditions prevailed between highstand and lowstand events, as well as the time available for paleosol development in each systems tract. Paleosols are better developed during lowstands (within incised valley fills) than during highstands, suggesting either different climatic conditions or duration of pedogenesis. The driving forces behind lowstand events in the Cretaceous are also poorly understood, i.e., are they initiated by a tectonic, eustatic, or climatic driving force. Analysis of the paleoclimatic signature of the paleosols in a sequence stratigraphic framework improves our understanding of incised valley fills, and will potentially help resolve whether the incised valleys are controlled, at least partially, by climatic events.

The Desert Member of the Blackhawk Formation in Tuscher Canyon contains at least five paleosols in succession at the top of the IVF, some of which are truncated by the Castlegate Sequence Boundary. The paleosol succession consists of stacked one to one-half meter thick Histosols and Inceptisols with well developed zonation and extensive rooting in certain horizons. The paleosols are part of the overbank deposits and overlie thin one to two meters thick channel-fill sandstone and siltstone, some with lateral accretion deposits. The channel-fills with the Inceptisols and the paleosols overlain by coal (Histosols) indicate a complete preservation of channel-fill deposits. The paleosols within the IVF, the underlying highstand deposit, and the basal portion of the overlying Castlegate Sandstone were sampled at ten to fifty centimeter intervals for isotopic analysis and for TOC δ¹³C. Changes in the levels of δ¹³C and TOC denote changes in the paleoclimate, specifically wetness, as well as atmospheric pCO_¬2 levels.

Comparison of data to δ¹³C global reference curves better constrain where the stratum and the associated sequence stratigraphic framework occur temporally. Understanding the link between paleoclimatic signals and the IVF allows for a better understanding of how climate affects valley fill and the duration of the incised valley.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California