Evolution of foreland basin fluvial systems in the mid-Cretaceous of Utah, USA (upper Cedar Mountain and Naturita formations)

Abstract An analysis of the sedimentary fill of a foreland basin can provide information on the relative effect of tectonics, base-level fluctuations and climate during basin development. This study analyzes fluvial strata from the upper Cedar Mountain and Naturita formations of the mid-Cretaceous foreland basin of Utah (USA) to determine the effects of spatial location within the foreland basin on fluvial planform and architecture. The study results document consistent planform and architecture across the distal foredeep and forebulge depozones during flooding of the Western Interior Seaway suggesting that fluvial planform and architecture did not change as a function of position within foreland basin depozone or as a result of base-level rise. Outcrop data, virtual outcrop models and satellite imagery were used to perform facies and architectural analysis and to collect dimensional and geometric data. Fluvial channel deposits consist of coarse-grained, laterally accreting bars deposited within narrow, low to moderate sinuosity channels, that were vertically stacked and encased in floodplain fines regardless of geographic location or stratigraphic position and may represent distal deposits of a distributive fluvial system. Bar elements increased in thickness over time indicating increasing channel depths and enhanced discharge. Palaeohydraulic calculations indicate an increase in discharge from between 315 and 1023 m3/s to between 1896 and 5270 m3/s, likely due to enhanced precipitation and/or drainage capture in the catchment and basin. Calculated drainage areas for the system increased from 104 to 105 km2 scale over time. The uppermost fluvial deposits were in close proximity (≤60 km) to contemporaneous shoreface systems as the Western Interior Seaway expanded; yet backwater effects are not observed suggesting that rising base level had no significant effect on fluvial planform or architecture. These results show that fluvial planform and architecture remained constant despite rising base-level, changing climate, increased discharge and position in different foreland basin depozones.