SANDSTONE-BODY AND SHALE-BODY DIMENSIONS IN A BRAIDED FLUVIAL SYSTEM - SALT WASH SANDSTONE MEMBER (MORRISON FORMATION), GARFIELD COUNTY, UTAH

Excellent three-dimensional exposures of the Upper Jurassic Salt Wash Sandstone Member of the Morrison Formation in the Henry Mountains area of southern Utah allow measurement of the thickness and width of fluv ial sandstone and shale bodies from extensive photomosaics. The Salt W ash Sandstone Member is composed of fluvial channel fill, abandoned ch annel fill, and overbank/floodplain strata that were deposited on a br oad alluvial plain of low-sinuosity, sandy, braided streams flowing no rtheast. A hierarchy of sandstone and shale bodies in the Salt Wash Sa ndstone Member includes, in ascending order, trough cross-bedding, fin ing-upward units/mudstone intraclast conglomerates, single-story sands tone bodies/basal conglomerate, abandoned channel fill, multistory san dstone bodies, and overbank/flood-plain heterolithic strata. Trough cr oss-beds have an average width:thickness ratio (W:T) of 8.5:1 in the l ower interval of the Salt Wash Sandstone Member and 10.4:1 in the uppe r interval. Fining-up ward units are 0.5-3.0 m thick and 3-11 m wide. Single-story sandstone bodies in the upper interval are wider and thic ker than their counterparts in the lower interval, based on average W: T, linear regression analysis, and cumulative relative frequency graph s. Multistory sandstone bodies are composed of two to eight stories, r ange up to 30 m thick and over 1500 m wide (W:T > 50:1), and are also larger in the upper interval. Heterolithic units between sandstone bod ies include abandoned channel fill (W:T = 33:1) and overbank/flood-pla in deposits (W:T = 70:1). Understanding W:T ratios from the component parts of an ancient, sandy, braided stream deposit can be applied in s everal ways to similar strata in other basins; for example, to (I) det ermine the width of a unit when only the thickness is known, (2) creat e correlation guidelines and maximum correlation lengths, (3) aid in i nterpreting the controls on fluvial architecture, and (4) place additi onal constraints on input variables to stratigraphic and fluid-flow mo deling. The usefulness of these types of data demonstrates the need to develop more data sets from other depositional environments.