The Large-Scale Architecture of The Fluvial Westwater Canyon Member, Morrison Formation (Upper Jurassic), San Juan Basin, New Mexico
E. Jun Cowan, 1991. "The Large-Scale Architecture of The Fluvial Westwater Canyon Member, Morrison Formation (Upper Jurassic), San Juan Basin, New Mexico", The Three-Dimensional Facies Architecture of Terrigenous Clastic Sediments and its Implications for Hydrocarbon Discovery and Recovery, Andrew D. Miall, Noel Tyler
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The Westwater Canyon Member of the Morrison Formation (Upper Jurassic) has previously been interpreted as consisting of fluvial “channel systems” tens of kilometers wide and tens of meters thick. Reinvestigation of the member indicates that these “channel systems” actually represent post-depositional aquifer conduits, defined instead by differing sandstone colors, rather than primary depositional features. The member is composed of amalgamated, individual fining-up sandstone sheets each about 5-10 m thick. The absolute widths of these sheet sandstone bodies are at least 1 km and possibly exceed several kilometers. The width.thickness range of the sandstone sheets are well within the typical values of sandstone body dimensions reported from other fluvial sandstones, and are interpreted to represent aggradational channel-belts. Sandstone bodies thicker than about 12 m are the result of amalgamation of these individual unit sandstone bodies, and do not represent individual channel belts as interpreted previously.
Internally, the sheets contain abundant concave-up troughs typically 30 m wide and 5 m thick, filled both laterally and vertically with inclined parallel- to low-angle cross-stratified sandstone, in places exhibiting parting lineation. The laterally-limited extent of these large troughs and nature of their internal fills suggest that they represent short-lived scour fills rather than confined elongated channels. The concave-up erosional base, a negative feature, was most likely formed due to large-scale flow separation within a wider and shallower channel. Physical conditions similar to stream-flow convergence at channel confluences may be responsible for their formation. The abundant preservation of these troughs in the Westwater Canyon Member is consistent with the expected poor preservation of positive barforms in a sweeping, sandy-braided channel belt.
Review of the literature indicates that inferred channelbelt sandstone bodies mostly fall within the thickness range of 1 to 12 m, irrespective of their interpreted fluvial style. Post-depositional large-scale reservoir conduits are also expected to fall within this range for sandy fluvial systems-. Deviations from this range are due to amalgamation of the sandstone bodies or increased grain-size heterogeneity, resulting in an increase and decrease, respectively, of the conduit size.