Abstract

A large alkaline, saline lake, Lake T'oo'dichi', occupied the entire eastern part of the Colorado Plateau region during deposition of the Brushy Basin Member of the Upper Jurassic Morrison Formation. The lake extended from near the site of Albuquerque, New Mexico, to near the site of Grand Junction, Colorado, and occupied a region that encompassed the San Juan and ancestral Paradox basins, making it the largest ancient alkaline, saline lake known.

The lake was typically shallow and frequently evaporated to dryness, although the water table was probably never far beneath the surface. Intermittent streams carried detritus from source areas to the west and southwest far out into the lake basin, a reflection of a low depositional gradient and the ephemeral nature of the lake. Prevailing westerly winds carried silicic volcanic ash to the lake basin from an arc region to the west and southwest. A lateral hydrogeochemical gradient, characterized by increasing salinity and alkalinity from the margin of the lake to the center, developed in pore waters of the lacustrine sediments and resulted in alteration of the ash to a variety of authigenic minerals that define concentric zones within the lake basin. The basinward progression of diagenetic mineral zones is smectite → clinoptilolite → analcime ± potassium feldspar → albite. A concomitant lateral zonation of mixed-layer illite/smectite developed, from highly smectitic in the outermost zones to highly illitic in the central zone.

Authigenic albite and illitic mixed-layer clay, minerals previously thought to require elevated temperatures for formation, crystallized during early diagenesis at syndepositional temperatures in the lacus trine sediments of Lake T'oo'dichi'. This study presents evidence for a syndepositional origin for both of these minerals. These findings have important implications in terms of limiting the use of these minerals as geothermometers.

Recognition of alkaline, saline-lake deposits in the Morrison Formation significantly alters interpretations of depositional environments of this formation, and it also has important implications for paleoclimatic interpretation. Late Jurassic climate was apparently much more and than had previously been thought. In fact, sedimentologic evidence suggests that the lake basin was typically dry for extended periods and enjoyed only brief wet intervals. This conclusion has important consequences for environmental interpretation of the habitat that was favorable for large herbivorous dinosaurs, which thrived in the Late Jurassic.

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