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Depositional Model for Carbonate-Evaporite Cyclicity: Middle Pennsylvanian of Paradox Basin

Alan C. Kendall

The Paradox basin is a classic area for the study of relations between carbonates and evaporites. Previous depositional models assume carbonates and evaporites are coeval, implying that the evaporites were deep-water deposits. Stratigraphic relationships are, however, complicated by previously unrecognized salt dissolution. Restoration of the missing salts indicates that evaporites entirely postdate marine carbonates in each cycle.

Anhydrites and silty dolomites that succeed halites are reinterpreted as shallow hypersaline to subaerial deposits. These playa-like sediments are abruptly overlain by organic-rich shales that represent anoxic and the deepest-water deposits in the sequence.

Paradox basin salts and succeeding playa deposits formed in a deep but desiccated basin. Sea level rises drowned the formerly exposed basin rims, causing sudden complete floodings of the basin and the abrupt contacts between playa deposits and deep-water shales.

The shale-carbonate-evaporite sequences that form lower parts of cycles resulted from sea level falls. These ultimately exposed basin rims, isolating the basin, and allowed evaporative drawdown and the deposition of basin-central evaporites. In contrast, the halite-anhydrite-silty dolomite sequences of the upper parts of cycles arose when sediment aggradation caused expansion of the evaporite depositional area onto basin flanks. There brine reflux became more significant. This reduced residence times of brines in the basin so that, progressively, salinities decreased and only less-saline sediments were able to persist in the playa environment.

Cycles end (or begin) when renewed sea level rises drowned the basin-central playas.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.