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Siliceous Cementation of Mesozoic Strata in the Four Corners Area: Evidence for Lacustrine Carbonates in the Jurassic, Morrison Fm. and Silcretes in the Cretaceous, Burro Canyon Fm.

Kim Miskell-Gerhardt
Consulting Geologist

Turner and Fishman (1991)* postulated ephemeral alkaline lacustrine environments in the Jurassic, Brushy Basin member of the Morrison Formation of SW Colorado based on the alteration of volcanic ash layers in a concentrically zoned succession ranging from smectite to zeolite to authigenic feldspar from margin to center. The current study of the upper Brushy Basin member differs by examining chert layers with clear microbial carbonate precursors, rather than volcanic ash. These cherts have laminated (stromatolitic) textures and minor (<5 cm) mounded morphologies. Rip-up clasts on bedding surfaces indicate storm-induced destruction and transport of algal mats. Individual deposits are limited in extent but such beds are found throughout Canyons of the Ancients National Monument north to Lake McPhee. This distribution suggests small, scattered carbonate lakes in an arid environment during the late Jurassic. Associated sediments consist of green, illitic and smectitic shales and minor silica-cemented sandstones. There are no associated organically-enriched mudstones, eliminating any source potential. Regionally the cherts sit about 40' below a heavily silica-cemented sandstone in the Early Cretaceous, Burro Canyon formation. Silicification along this upper horizon ranges from softer to harder quartzite, with minor alteration to chalcedony. Soil development on top of silicified beds is indicated by root penetration and indistinct pedogenic structures. Blocks of very angular quartzite carried as bedload in overlying fluvial conglomerates indicates early (intra-Cretaceous) cementation. These quartzites are interpreted as silcretes, or siliceous paleosols, formed during a widespread hiatus very late in the early Cretaceous. They occur throughout the study area noted above, as well as in north-draining washes of the Uncompahgre Plateau and in the Blanding region of SE Utah. If this unit is present in the subsurface it would negatively impact reservoir quality in conventional Burro Canyon / Dakota oil and gas plays. Both the Brushy Basin cherts and the Burro Canyon Quartzites were first noted by archaeologists looking for flakable stone quarry sites, without realizing the geologic significance. * Turner, C.E. and N.S. Fishman, 1991. Jurassic Lake T'oo'dichi': A large alkaline, saline lake, Morrison Formation, eastern Colorado Plateau. GSA Bull. V. 103, pp538-558.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013