SEQUENCE STRATIGRAPHY AND CHEMOSTRATIGRAPHY OF AN INCISED VALLEY FILL WITHIN THE CRETACEOUS BLACKHAWK FORMATION, BOOK CLIFFS, UTAH

Abstract

A better constraint on the timing and depositional development of the incised valley fill (IVF) of the Desert Member of the upper Cretaceous Blackhawk Formation was needed to help understand the development of incised valley fills. Paleosol analysis and chemostratigraphy were used to refine the previously published sequence stratigraphic interpretation of this valley fill. This study is a proof of concept project that tested the ability to use δ13Corg chemostratigraphy with sequence stratigraphy to refine the stages of an IVF at the parasequence level. The incised valley fill within the Desert Member of the Blackhawk Formation in Tuscher Canyon, Utah was measured and described for the following reasons: paleosol facies interpretations at the detailed parasequence-scale sequence stratigraphy, and δ13Corg chemostratigraphy. From field observations, three moderately developed paleo-Histosols, three weakly developed paleo-Inceptisols, and four very weakly developed paleo-Entisols were described within the study area. Based on the δ13Corg profiles from the measured sections, eight chemostratigraphic events were characterized. Using the analyses from the facies and paleosols, five parasequence boundaries were selected within the IVF of the Desert Member. The IVF was divided into nine stages of fill using the parasequence stratigraphy and chemostratigraphy. Stages of valley fill are defined by changes in base level. A minimum-average-age was determined for five stages (numbers 3, 4, 5, 6, and 8) based on paleosol maturity within each stage. A total minimum-average-age for pedogenesis within the IVF is 32,000 years. The paleoclimate of the Desert Member is determined from the paleosol facies to have been humid. Chemostratigraphy, sequence stratigraphy, and detailed paleosol interpretations are used to help refine the depositional history of an IVF. A greater understanding of the depositional history of the IVF allows for greater resolution of lateral and vertical variability of facies within the valley fill.