A Chemostratigraphic and Detrital Zircon Geochronological Analysis of Upper Cretaceous Strata: Applications for Dating and Correlating Strata

Abstract

Rates of sedimentation, length of hiatus, temporal relationships and absolute age of strata are all difficult to determine in clastic non-marine and shallow marine environments due to their erosive and rapidly fluctuating depositional nature. High energy environmental conditions, erosion, and diagenesis make preservation of common chronological markers unlikely. Lack of chronological information results in the interpretation of sedimentation rates, length of hiatus/erosion, and genetic relationships based on chronological information obtained from strata bracketing the section of interest. Temporal relationships between strata and sequence stratigraphic interpretation can also be based on lithostratigraphic correlation to other dated sections. However, this can be highly unreliable when the lithological data is discontinuous, complex, or appearing homogeneous (e.g. shales or amalgamated sandstones). To provide chronostratigraphic control, this study demonstrates the utility of using high resolution organic carbon (δ13C OM) stable isotope chemostratigraphy to better understand the timing of fluvial and shallow marine siliciclastic successions from a 200m thick section of Mid-Campanian Mesaverde Group strata. Because recognizable changes in the carbon cycle can occur in the 10’s to 100’s of the years, organic carbon chemostratigraphy has the potential to precisely date individual strata at a resolution greater than is achievable with 87Sr/86Sr techniques. Constraining the organic carbon isotopic data using detrital zircon geochronology allows better understanding of the relative and absolute timing of major periods of deposition, as well as the temporal relationships between strata. The studied rock units include: The Grassy highstand sequence set and Desert lowstand sequence set of the Desert Member of the Blackhawk Formation, the Castlegate Sandstone, and the Buck Tongue of the Mancos Shale. Detrital zircon (U/Pb) radiometric and chemostratigraphic studies conducted east of Green River, Utah, reveal that the top of the Grassy highstand sequence set was deposited sometime between 76.9–77.2 ± 0.1 Ma. The Desert lowstand sequence set was deposited sometime between 76.2–76.9 ± 0.4 Ma. The lower Castlegate Sandstone was deposited sometime between 75.8–76.2 ± 0.4 Ma. The Buck Tongue of the Mancos Shale was deposited sometime between 75.2–75.8 ± 0.4 Ma. The Sego Sandstone was deposited at 73.3 ± 1.3 Ma. Duration of unconformities during lowstand incisional events is estimated to be 100–200 kyr based on global eustatic curves for the Campanian.