Variations in Fluvial Style in Westwater Canyon Member, Morrison Formation (Jurassic), Southern San Juan Basin, Colorado Plateau

Andrew D. Miall, C. E. Turner-Peterson

The large-scale architecture of fluvial strata within the Westwater Canyon Member of the Morrison Formation consists mainly of a series of tabular sheets of sandstone 5-15 m thick and hundreds of meters wide separated by thin fine-grained units. These sandstone sheets are commonly flat bedded; however, lateral accretion surfaces and channels 10-20 m deep and up to at least 250 m wide are also present.

Where studied in detail, the sheets comprise a complex of elements and bounding surfaces unlike any previously described from ancient fluvial deposits. Lateral accretion deposits, typical indicators of moderate to high-sinuosity channels, coexist in the same outcrop with downstream-accreted foreset macroform deposits now thought to be typical of the sand flats of low-sinuosity, multiple-channel rivers. Broad deep channels with gently to steeply dipping margins have been mapped by carefully tracing major bounding surfaces in several of the outcrops. The locally thick accumulations of flat-laminated to low-angle cross-bedded lithofacies suggest high-energy flow, probably transitional to upper flow regime conditions. Such a depositional style is most typical of ephemeral rivers or those eriodically undergoing major seasonal or more erratic stage fluctuations.

Fining-upward sequences are rare in the project area, contrary to earlier descriptions. Analogies with the depositional architecture of the large Indian rivers, such as the Ganga and Brahmaputra, still seem reasonable, although convincing evidence now exists for aridity and for major stage fluctuations, which differs from those modern rivers and previous interpretations.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.