CHAN, MARJORIE A., University of Utah, Salt Lake City, UT

ABSTRACT: Tidal Deposits of the Precambrian Big Cottonwood Formation, Big Cottonwood Canyon, Central Utah

Spectacular outcrops of Precambrian quartzites and shales (4880 m) of the Big Cottonwood Formation are exposed in the lower gorge of Big Cottonwood Canyon, Utah. Despite the age and tectonic overprints, abundant, well-preserved sedimentary structures suggest deposition in tide-dominated settings of an estuary, tidal embayment, tidal flat, and/or tidal delta. Quartzites contain reactivation surfaces and compound cross-bed sets indicative of subtidal dunes and sand waves, with modification of the dunes by flow reversals. Shales can be divided into two facies. The purple shale facies contains ripples and mud polygons, suggesting tidal flat deposition with periodic desiccation. The black shale facies contains three orders/scales of prominent tidal rhythmites and cyclicities, previously un ocumented for the Big Cottonwood Formation. Rhythmites (couplets of interlaminated sandstone and mudstone), variable couplet thicknesses, and organized vertical cycles suggest a tidal origin with neap-spring tide fluctuations. The variety of structures and preservation of rhythmite cycles imply mostly subtidal conditions. Other Precambrian shale deposits (similar to the black shale facies) have been examined for the petroleum source rock potential. Understanding the origins of these shale facies can greatly contribute to understanding the distribution and geometries of the potential resources.

These tidal rhythmites examples are approximately middle to late Proterozoic (850 million to 1.0 billion yr old), and thus may represent the oldest tidal rhythmites known. Counts of tidal laminae can have enormous implication for understanding the Earth-moon relationships and lunar rates in the geologic past.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.