DEPOSITIONAL SYSTEM OF THE UPPER JURASSIC BRUSHY BASIN MEMBER OF THE MORRISON FORMATION, FOUR CORNERS REGION, USA

The idea that Upper Jurassic Brushy Basin Member of the Morrison Formation in the Four Corners region was deposited in a large, alkaline, saline lake (Lake T'oo'dichi') is well entrenched in the literature. Lake T'oo'dichi'has been envisioned as a closed lake basin 500 km N-S and 300 km E-W that was the locus of formation of authigenic zeolites (analcime, clinoptilolite and albite) that supposedly form a concentric zonation that corresponds to lake paleogeography. We question the existence of Lake T'oo'dichi' because: (1) sandstone bodies representing meanderbelt channels are found in the Brushy Basin across much of the Four Corners region; many have scoured bases, conglomeratic lags, lateral accretion forests and bioturbated tops indicative of a meandering fluvial system, not a lake; (2) unequivocal profundal lacustrine facies are absent on outcrop; (3) no data have been presented to document a concentric zonation of zeolites that would define a lake basin; (4) syndepositional formation of albite and analcime in the Brushy Basin Member is unlikely because it would require water temperatures (85-100^oC) and extreme thermal stratification of the water column for which there is no evidence;(5) Brushy Basin Member lithofacies extend well beyond the Four Corners--virtually all of the Morrison Formation off of the Colorado Plateau is Brushy Basin Member--which would extend Lake T'oo'dichi' to Montana and Oklahoma; the lithofacies that supposedly characterize Lake T'oo'dichi' extend well beyond its hypothesized limits; and (6) taphonomic and paleoecological studies of Brushy Basin Member fossil plants and vertebrate fossils do not support the presence of a large lake. Instead of deposition in Lake T'oo'dichi', all data indicate Brushy Basin Member deposition took place in a vast, low-gradient floodbasin characterized by a mosaic of meandering channels, small lakes or playas and stable, pedogenically-modified interfluves. Carbonate sediments were deposited during base-level highstands in ponds and marshes of limited aerial extent and subsequently subjected to pedogenic modification. Syndepositional authigenesis of ash-fall tuffs formed low-temperature zeolites, but post-depositional diagenesis at elevated temperatures is likely related to hydrothermal activity caused by Tertiary volcanism.