Sedimentology and petrology of the Brushy Basin member, Morrison Formation (Late Jurassic), western Colorado
The 85-m Brushy Basin Member of the Morrison Formation in western Colorado comprises a 30-m lower division mostly made of red-brown mudstone and a 55-m upper division characterized by variegated smectitic mudstones. River-channel sandstones embedded in the floodplain mudstones tend to have cut down to resistant caliche paleosols. Also present are thin beds of lacustrine micritic limestone and altered silicic ashfalls. These strata were studied at three locations, from west to east: Trail Through Time (TT), Fruita Paleontological Resource Area (FP), and Echo Canyon (EC).
Morrison strata developed in a back-bulge depozone of a foreland basin system, with the debris eroded from thrust slices to the west. The Morrison strata differ from many foreland basins because the Morrison-age forebulge deposits west of the preserved basin were eroded due to plateau uplift of eastern Nevada and western Utah.
Petrographic modal analyses of 87 sandstones date a major pulse of the Nevadan Orogeny at the transition from the Salt Wash Member to the Brushy Basin Member in late Kimmeridgian time. Brushy Basin sandstones show significant increases upsection in plagioclase, total lithic fragments, and especially volcanic fragments. Abundant volcanic ash accumulated on the floodplains. The source areas for the Morrison Formation were the Ellko and Mogollon Highlands with feldspathic litharenite sands characterized by relatively high proportions of lithic and volcanic fragments and plagioclase.
Floodplain red-brown mudstones are highly smectitic with abundant smectite altered from silicic volcanic ash, in both the lower and the upper divisions.
Depositional facies and architectural element analysis show that the rivers were low gradient, mainly anastomosing, with perennial flow and seasonal peaks in discharge.
Dinosaur bone accumulations are found in some floodplain ponds. Isolated bones are present in anastomosing sandstones at the Trail Through Time. At Fruita Paleontological Resource Area, major acummulations of bones were rapidly buried in the deep pools at three bends in a meandering river.