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Abstract

Continuous core from the Marsing No. 16 well in the southwestern portion of the Uinta Basin, Utah, contains both siliciclastic and carbonate marginal lacustrine and nearshore open lacustrine lithologies. Overall, a long-term stratigraphic record of lake evolution, characterized by an expanding lake, is recorded upward from deltaic through open lacustrine deposits. Coincident with this overall trend of lake expansion is an apparent reduction in supply of siliciclastics to the depositional site and a marked delta retreat.

Two contrasting suites of related environments that alternate in concert with changes in climate are postulated to explain the lithofacies recognized in the core. A "wet-climate model," distinguished by evidence for high lake levels combined with high fluvial discharge, is envisioned with a siliciclastic strand plain, lacking in dolomite, and containing channels that served as throughgoing delivery systems for clastics spewing out into the open lake. A "dry-climate model," with evidence for low lake levels combined with low fluvial discharge, consists of a dolomitic mud flat in which clastics are almost entirely lacking and practically the only limestone present occurs as shell-rich and coaly "lags" that represent brief lake-water incursions.

Superimposed on a long-term, wet-dry-wet trend, are a succession of finer-scale depositional cycles that are best expressed in the dry part of the trend, in the marginal lacustrine sediment, as limestone-dolomite couplets, but may also be expressed in deltaic and open lacustrine deposits. The typical cycle begins with interspersed shelly limestone and coal deposited with the rise in lake level. This is succeeded by burrowed pelecypod- and ostracod-rich limestone, and is locally capped by strand plain-deposited, laminated, ostracod grainstone, or exposed mudflat-deposited dolomite. The counterpart in the deltaic environment is a coarsening-upward cycle, beginning with interspersed shelly limestone and coal, which is succeeded by burrowed and rippled siltstone and mudstone, and eventually root-mottled siltstone or massive sandstone.

The black shale facies contains both oil-prone Type I and gas-prone Type III organic matter, with Type I volumetrically the more important. The open lacustrine limestones have the highest source potential, are oil-prone and immature, with total organic carbon averaging 3.1 percent. The vitrinitic coals within the basal portions of the lacustrine cycles are gas-prone, whereas the marginal lacustrine dolomites and clastics are lean.

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