WESTERN INTERIOR SEAWAY KIOWA-SKULL CREEK DELTAIC SHORELINE: INVESTIGATING THE ROLE OF SEA LEVEL CHANGE AND AUTOGENIC PROCESSES

The influence of autogenic versus allogenic signals on modern fluvial-deltaic sediment routing systems (SRS) has been well studied, however few studies exist for ancient systems. This study focuses on the mid to late Cretaceous upper Dakota group along the modern Colorado Front Range, which was the approximate center of a >800 km north-south oriented deltaic shoreline on the western side of the Western Interior Seaway during the Kiowa-Skull Creek cyclothem occurring in the Albian-Cenomanian. The fluvial channel-belt sandstones of the upper Dakota are referred to as the Horsetooth member of the Muddy sandstone, and the deltaic component is the Ft. Collins member. The Horsetooth sandstones are incised into the delta plain strata, forming an erosional surface that traditionally has been interpreted as unconformable and a sequence boundary in response to relative sea-level fall (allogenic). However, a previous study by Nazworth (2019) yielded younger maximum depositional ages (MDAs) for the Horsetooth, at ~98-100 Ma, than previously interpreted biostratigraphic ages, suggesting that fluvial incision was instead a result of normal autogenic processes. To better understand the role of autogenic versus allogenic processes in the stratigraphic record of the Western Interior Seaway, this study implemented detrital zircon U-Pb geochronology on the Ft. Collins member. Nine sandstone samples were collected from Ft. Collins sandstones at three paleoshoreline locations along strike, and two samples from an outcrop of fluvial deposits in a delta plain setting, located 80 km updip and to the west. An additional Horsetooth sandstone sample was taken at one of the along-strike locations, where previous studies had not dated the Horsetooth at that location. LA-ICP-MS dating was performed with a sample size of n=300. MDAs of 98-103.1 Ma were calculated, with all sample MDAs statistically indistinct from each other and indistinct from MDAs of the Horsetooth member as well. MDS plot differentiates two distinct DZ U-Pb age distributions, indicating two distributary channels in the system. Results support the view that the surface between the Horsetooth and Ft. Collins member of the Muddy sandstone is a composite scour surface driven by autogenic processes in response to normal shoreline regression.