Application of paleoslope reconstruction method to the Jurassic Morrison Formation in southeast Utah
Abstract
In modern alluvial rivers the channel slope, flow depth, and the grain size of the bed interact in a systematic way which enables us to recreate the long-term evolution of ancient fluvial landscapes. In the Morrison Formation in southeast Utah the stratigraphy changes dramatically from the sandy, channel dominated Salt Wash Member to the overlying muddy, flood-plain dominated Brushy Basin Member; however, whether the change in stratigraphy resulted from a change in the behavior of the river systems between the two members remains an open question. In order to assess whether there was a noticeable change in the hydraulics of the rivers coincident with the change in stratigraphic style, we looked at changes in paleoflow depth from well-preserved bar forms, the median grain size of the channel bodies, and finally the reconstructed paleoslopes within and between the two members. Measurements of flow depth and optically measured grain-size distributions show no difference between the two members over a 200 km downstream transect in the study area. Instead the fluvial transport system remained unchanged throughout deposition of both members. Paleoslope reconstruction using an empirical relationship between bankfull flow depth, median grain size of the bed material, and slope of modern alluvial rivers indicate the average slope of the Morrison Formation in this region was around 1.1 x10-3 for the entire duration of deposition (~155-148 Ma). The evolution of the Morrison landscape does not appear to have featured any large-scale change of depositional slope for ~7 Myr and over 200 km downstream. This indicates that whatever controlled the stratigraphic change was not the result from a dramatic change in boundary conditions within the system: there is no evidence of surface tilting or a change in grain size related to tectonism, likewise there is not an apparent change in depth related to a change in precipitation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP53B0830T
- Keywords:
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- 1825 HYDROLOGY Geomorphology: fluvial;
- 1856 HYDROLOGY River channels;
- 1861 HYDROLOGY Sedimentation