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.
The Inca Occupation of the South Coast of Peru
like the others, however; it forks into several nearly parallel ravines of which the largest are Huayuri, Rio Grande-Palpa, Ingenio (called Collao in ancient times), Nasca proper (the ancient Caxamarca), Taruga, and Poroma. Due to peculiarities of the local topography, the irrigated areas in Ica and the ravines of Nasca are located inland, at the foot of the mountains, rather than at the mouths of the rivers as is more usual in Peruvian coastal valleys. No sharp natural or cultural boundary separates the valley of Chincha from the valleys of Cafiete (ancient Huarco and Runahuana), Asia, and Mala, the next valleys going up the coast to the northwest. At the other end of the area we are considering, however, there is a sharp break; beyond Yauca the Andean foothills reach out to the sea, natural conditions are quite different, and there are important differences in the archaeological picture as well. The historical sources which mention the south coast are not particularly helpful, except in so far as they provide us with a few dates. To begin with, no chronicler gives an explicit account of the political organization of the south coast at the time it came into contact with the Incas. It can, perhaps, be inferred from Cabello Balboa's statements about common policy toward the Inca invaders that there was some sort of political connection between the valleys of Mala, Cafiete, and Chincha, but the connection need have been no more than a temporary military alliance. Cabello makes no mention at all of Pisco. The valleys of Ica, Nasca, and 125