Abstract: Controls on Sandbody Architecture in a Braided Fluvial System: The Salt Wash Member of the Morrison Formation, Southern Utah

John W. Robinson, Peter J. McCabe

The 160 m thick Salt Wash Member of the Morrison Formation of southern Utah consists of sandstones, interpreted as braided river deposits, and mudrocks, interpreted as overbank deposits. Sandstones form >80% of the lower third of the member but <70% of the upper third. Sandstone storys,

which were deposited by the infill or lateral migration of a single channel, are bounded below by major erosion surfaces. Some grade up into mudrocks but most are cut into by younger storeys. Storeys increase in average thickness from 5.8 m near the base of the Saltwash Member to 10.7 m near the top. Multistory sandstones consist of two or more storys that are cut into one another. They are separated by mudrock units that are between 1-15 m thick and 10-1500 m wide. These amalgamated sandstone bodies probably represent belts within which the braided rivers were stabilized for long periods of time. Near the base of the Saltwash Member, multistory sandstones average 20 m in thickness, and increase to about 33 m in thickness near the top.

The change in architecture, with an increase in mudrocks, and thicker sandstone storys and multistorys higher in the section is interpreted to be primarily a function of faster rates of creation of accommodation space through time, presumably because of higher rates of subsidence. However, an increase in vegetation in the overbank areas, suggested by the paleosols, may also have helped stabilize the channels during deposition of the upper part of the Saltwash Member.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994