ARCHITECTURAL BIMODALITY IN FLUVIAL STRATIGRAPHY OF THE MORRISON FORMATION, USA

One of the most recognizable characteristics of fluvial stratigraphy is the relative proportion of preserved floodplain muds and channel-belt sand bodies. Stacking densities of sand bodies have been used as evidence of changes in environmental factors such as basin subsidence rate, relative base level change, and climate change. To explore potential origins of rapid changes in stacking density in fluvial stratigraphy, we investigated the Morrison Formation, Upper Jurassic of eastern Utah and western Colorado, U.S.A. It contains a rapid transition from the Salt Wash Member, 63% net sand, to the overlying Brushy Basin Member, 10% net sand. Nine sedimentary characteristics of the preserved transport system were measured along a 300-mile transect parallel to the paleo transport direction. Of all the characteristics, only the composition and abundance of floodplain clay minerals changed significantly across the boundary between the two members. We interpret the sharp change in stacking density to be related to a rapid increase in smectitic clay as a function of increased volcanic ash influx. The increased ash load boosted rates of floodplain accumulation. In this interpretation conservation of mass necessitates that more than half of the floodplain of the Brushy Basin Member would have to be composed of volcanic ash in order to explain the observed change in stacking pattern. The required volume flux rate could be provided by a Jurassic volcano equivalent to the modern Mt. St. Helens. This increase in ash deposition would also reduce the frequency of avulsion. Together, reduced avulsion and increased floodplain deposition rate could generate the observed change in channel sand stacking density. This example highlights the potential pitfalls of assuming that changes in stacking density observed in alluvial sequences are mainly controlled by environmental factors such as base-level, subsidence, or climate.