Comparative petrology of Tertiary sandstones of southern Piceance Creek Basin, Colorado
Lacustrine and fluvial sandstones of the Wasatch and Green River Formations are described as they outcrop at several localities along the southern margin of the Piceance Creek basin. The description combined with petrographic analyses of thin-section samples provide a basis for determining the provenance of the sand, as well as crude comparison of compositional and textural features that resulted from differences in depositional processes. The sandstones in marginal lacustrine settings in the Wasatch and Green River Formations are proven reservoirs for significant hydrocarbon reserves in the Uinta and Piceance Creek basins. (JMT)
Depositional Environments of Oil Shale in the Green River Formation, Douglas Creek Arch, Colorado and Utah
Abstract Douglas Creek arch in western Colorado and eastern Utah separates the Uinta and Piceance Creek basins. During deposition of the lower part of the Eocene Green River Formation, the arch provided a physical barrier dividing ancient Lake Uinta into two segments. As the lake expanded, the arch became a shoal area where marginal-lacustrine and mudflat sediment was deposited. Near the end of Green River deposition, a major lacustrine transgression submerged the arch and deep-water open-lacustrine deposition prevailed. As a whole, rocks of the Green River Formation on Douglas Creek arch provide a comprehensive view of the initiation, evolution, and expansion of Lake Uinta in the Uinta and Piceance Creek basins. Oil shale on Douglas Creek arch is present in two stratigraphic intervals separated by approximately 250 m of sandstone, siltstone, marlstone and lacustrine limestone (micrite, boundstone, grainstone). The lower sequence, in the Garden Gulch Member, represents an early lacustrine phase of Lake Uinta and consists of approximately 29 m of interbedded clay-rich oil shale, gastropod grainstone and packstone, ostracode grainstone, algal boundstone and calcareous silty claystone. Fish fossils, plant debris, and pelecypods are locally present. Detrital clay minerals (smectite, chlorite and illite) are common and calcite abundance is greater than dolomite. The second oil-shale interval is located in the upper 70 m of the Parachute Creek Member. The Mahogany ledge crops out in the lower 15 m of this sequence. Oil shale in the Parachute Creek Member is a kerogenous dolomicrite (dolomite greater than calcite). Analcime, quartz, K-feldspar, and albite are also common. Illite is the dominant clay mineral, Fossils include insects, insect larvae, and plant debris. Marlstone and volcanic tuff are interstratified with oil shale. The two oil-shale types have different mineral contents, fauna and flora, stratification sequences, and sedimentary structures. Clay-rich oil shale in the Garden Gulch Member was deposited in a shallow, open-lacustrine environment characterized by fresh to slightly saline water. Oil shale in the Parachute Creek Member, in contrast, is the product of deposition in a deep, chemically-stratified, saline, alkaline, open-lacustrine environment.
Flume experiments on the production of stratification and cross-stratification
ABSTRACT Experiments are being conducted in flumes designed to simulate conditions of deposition (1) through stream current action, and (2) by slumping into a standing body of water. Previous experiments, made in a wind tunnel with dry sand, formed deposits (1) by wind currents and (2) by avalanching down steep slopes. Stratification, or cross-stratification, was developed under each of the four environments of deposition. The type of deposit formed depends upon such factors as speed of current, depth of water, size of grain, degree of sorting. Significant observations from the experiments include: (1) Foreset beds develop without bottomset beds where sorting is good and clay minerals are absent or sparse. (2) Topset beds commonly are eliminated in delta-front deposits through lowering of the water level. (3) Wind-deposited sand on the lee sides of dunes forms strata with dips that consistently are several degrees greater than those of foreset slopes in water-deposited sand of corresponding grain size and texture. (4) Channel profiles tend to be U-shaped where formed by streams flowing down the channel bottoms, semicircular where formed by currents in a standing body of water that fills the channels. (5) Cross-stratification that appears to be upside down, consisting of festoons that are convex upward, can be formed by coale cing lobes at the front of an advancing delta; strata in channel-fills, including "festoon" types, vary from those that curve in conformity with the channel to those that are nearly flat-lying as a result of differences in position of water level at or above the channel rims during deposition.