SEDIMENTOLOGY OF THE BRUSHY BASIN MEMBER, MORRISON FORMATION (LATE JURASSIC), WESTERN COLORADO

Floodplain red mudstones and variegated smectitic mudstones dominate the 85m-thick Brushy Basin strata; minor river-channel sandstones are incised into resistant caliche paleosols. Channel sandstones occur in twelve zones at Fruita Paleontological Resource Area (FP) and six at Trail Through Time (TT). Fluvial architectural element analysis shows that most rivers were anastomosing with "flashy" discharge and some meandering. At FP, most channel sandstones have a straight map pattern and contain relatively few dinosaur bones. However, the second channel sandstone is sinuous and each of three bends has crevasse-splay deposits, levee complexes, and bone accumulations. The rivers are low-gradient, meandering to anastomosing, with perennial flow and seasonal peaks in discharge. There is no evidence for a playa-lake system at the three sections. The variegated mudstones contain numerous calcareous and non-calcareous paleosols with maturity stages I, II, and III of Retallack. Caliche paleosols are present at TT and FP, and to a lesser extent at Echo Canyon; they imply semiaridity and seasonal precipitation. Noncalcareous paleosols are incipient soils, whereas the caliche horizons are more mature and perhaps formed under drier conditions. Early gypsum crystals and local barite cement in the fluvial sandstones point to times of semiaridity. The sandstones have oversize pores, poikilitic calcite cement, and minus-cement porosities of 40%, indicating calcite cementation occurred before significant compaction. Floodplain red mudstones are highly smectitic, reflecting vigorous volcanic activity west of the study area, with silicic ashes carried eastward by the prevailing winds.