Abstract In the Uinta Mountain area, the Gartra Formation (Middle? Triassic) unconformably overlies the Moenkopi Formation on the east and the Ankareh Formation on the west, and interfingers with the overlying purple unit of the Popo Agie Formation (Late Triassic). Based on lithology, sedimentary structures, and weathering characteristics, the Gartra is informally divided into three subunits. The lower subunit is characteristically conglomeratic, poorly sorted, massive, and poorly bedded sandstone. The middle subunit is characterized by finer-grained sandstone and by dominant planar and trough cross-stratification. The upper subunit is finer grained than either of the subjacent subunits, consisting of claystone, siltstone, and very fine to medium sandstone. Horizontal and small-scale cross-stratification are characteristic of the upper subunit. The Gartra probably was deposited by a series of west-northwest flowing streams on a broad alluvial plain. Detritus was derived from plutonic, sedimentary, and gneiss-schist terranes on the east and southeast (Ancestral Rockies and Uncompahgre uplift). Gradually decreasing velocity and turbulence of stream currents were responsible for the fining-upward sequences.
In June 1954 Nevada became the twenty-ninth oil-producing state in the United States (Picard 1955). Interestingly, production was from volcanic rocks from the open-hole interval 6,450 to 6,730 ft (1,966 to 2,051 m) in the Oligocene Garrett Ranch volcanics, an unexpected reservoir in the kind of rocks rarely productive anywhere in the world. The pour-point (65-80° F) and gravity (26-29° API) of the crude were high, similar to oils found in the Eocene Green River Formation of the Uinta Basin, northeast Utah. Cumulative production in the field through September 1978 was 3.3 million barrels of oil. An early estimate of ultimate primary reserves was four million barrels of oil (Bortz and Murray, 1979). The trap is a faulted truncated wedge of Oligocene and Cretaceous-Eocene rocks with a top seal of impermeable valley fill, a bottom seal of Paleozoic rocks, and an east-side seal formed by a basin boundary fault and impermeable Paleozoic rocks. The new field in Railroad Valley of east-central Nevada, finally totaling fourteen producing wells, was called Eagle Springs after the locality and the name of the discovery well drilled by the Shell Oil Company. Twenty-two years after the Eagle Springs discovery a larger oil field, Trap Spring, was discovered by Northwest Exploration Company less than ten miles west of Eagle Springs, in Tertiary ash-flow tuffs. Two hundred dry holes had been drilled in Nevada between the two discoveries. In 1982, six years after the Trap Spring discovery, Amoco Production Company drilled the first well outside of Railroad Valley at Blackburn field on the east side of Pine Valley in Eureka County. Blackburn, a structural trap above a Tertiary low-angle extensional fault, produces from Devonian reservoirs. In 1983, Northwest Production brought in the Grant Canyon field about 10 mi (6 km) south of Eagle Springs. The oil reservoir of Devonian carbonates there is entrapped in a ‘buried-hill’. The discovery in 2004 of the Covenant field in Central Utah, because of similarities to large oil fields in the thrust belt of Wyoming and Utah and some resemblance to the Nevada fields of the Great Basin, ignited a frenzy of leasing which still goes on when land is available. Located along the thrust-belt (hingeline), Covenant produces oil from the Jurassic Navajo Sandstone that apparently originated in the Paleozoic.
Detailed stratigraphic studies of the upper Chugwater Group (Triassic) in Wyoming permit well-defined lithologic correlation of units. Such correlation is necessary to reconstruct the paleogeography of the Triassic Wyoming shelf. The lower part of the Jelm Formation north of the type section in southeastern Wyoming correlates with the upper Crow Mountain Formation of central Wyoming; a tongue of the Jelm overlies the Crow Mountain in central and western Wyoming. The Ankareh Formation of western Wyoming is correlative with the Jelm tongue and with the Popo Agie Formation. The Alcova Limestone Member of the Crow Mountain is a tongue that extends southeastward from the upper Thaynes Formation in western Wyoming for approximately 250 mi. During deposition of the Crow Mountain, much of the Wyoming shelf was flooded by a shallow sea. Streams emptying into the sea in southeastern Wyoming deposited the basal Jelm and delivered sediment into the basin. A general regression followed and fluvial and fluvial-lacustrine sediment of the Jelm and Popo Agie was deposited over the marine Crow Mountain. Marine conditions prevailed longest in western Wyoming. Accumulation conditions were rather uniform on the Wyoming shelf during the Triassic, as indicated by the wide extent of thin stratigraphic units. After deposition of the Popo Agie, the Wyoming shelf was gently folded. Truncation before deposition of the Nugget Sandstone produced a regional unconformity. The folding and truncation that occurred between deposition of the Popo Agie and deposition of the Nugget suggest that the Nugget is Jurassic, rather than Late Triassic (?). The only major erosional break in the Wyoming Triassic is at the top of the Crow Mountain. Throughout central Wyoming, a disconformity separates the Crow Mountain and Jelm Formations. The writers suggest that at least part of Middle Triassic time is represented by this disconformity and the Jelm.
A correlation chart of the Mesozoic rocks of Utah has been prepared by a special committee of the Intermountain Association of Petroleum Geologists. For the purposes of illustrating the stratigraphic divisions and correlations of the Mesozoic, the state has been divided into twenty different physiographic and stratigraphic provinces each represented by a column in the chart. The work represents a summary of approximately one hundred published papers as well as considerable unpublished material particularly on subsurface correlations. The lower boundary of the Mesozoic in much of central and western Utah is marked by an erosional unconformity or disconformity accompanied by a change in sedimentation from marine carbonates of the Permian to Triassic redbeds. Over much of eastern and southeastern Utah the boundary falls within a redbed sequence with few fossils so that a satisfactory division is difficult to make. The Triassic-Jurassic boundary is drawn at the top of the Wingate sandstone (formerly considered to be questionably Jurassic) in accordance with recent paleontologic evidence gathered by the United States Geological Survey. The Jurassic-Cretaceous boundary is drawn at the base of the Cedar Mountain, Burro Canyon, and Kelvin formations in Utah. No consistent criteria can be laid down for separation of the Me ozoic and Cenozoic in Utah. This boundary has been located by various workers on the basis of one or more of the following criteria: (1) change from marine Cretaceous to continental Tertiary, (2) orogenic activity, or (3) biologic (floral or faunal) changes. The paper includes a bibliography and several index maps which supplement the chart.
Abstract Throughout the 19th century lacustrine rocks were widely recognized in the Tertiary of the United States and in the Devonian of Europe. This interpretation prevailed until Davis (1900) and Barrell (1916) gathered evidence that these deposits mainly formed in fluvial environments. The demise of lacustrine interpretations was rapid. For several decades geologists gave little attention to lacustrine rocks. The pioneering work of W. H. Bradley, beginning in the mid 1920s, initiated much of the current interest in lake deposits. There are few general lists of lacustrine criteria. The most comprehensive one is that of Feth (1964) which is based on an extensive bibliography of ancient lake deposits in the western United States. On the basis of many aspects of lacustrine rocks, Picard and High (1972a) presented a general summary of lacustrine criteria. From the criteria available— summarized here—environmental reconstructions can be made with considerable confidence if sufficient facts are obtained. With improved standards for recognition of lacustrine rocks, lacustrine studies are enjoying a minor renaissance. This has led to studies of formations in which lacustrine subenvironments are distinguished. The number of subenvironments recognized approaches the variety seen in marine marginal basins: mud-flat, beach shoreface, bar, lagoon, shoal, delta, and deep lake. The means to interpret the history of the lake basin are available now that the subenvironments and their patterns of succession have been recognized.
The Parachute Creek Member of the Green River Formation in the southern Piceance Creek basin Colorado and the eastern Uinta basin Utah displays four major lacustrine depositional facies sandstone bottom stromatolite marlstone and oil shale top In a vertical sequence these facies record the transgressive history of Lake Uinta during its most expansive stage The sandstone facies and the stromatolite facies represent deposition in a marginal lacustrine environment while the marlstone facies and oil shale facies were deposited in an open lacustrine setting Sulfur isotope values lp4S were determined for iron sulfide minerals pyrite marcasite and pyrrhotite from all facies except the marlstone facies Values were also determined for pyrite from theUinta Formation in theeastern Piceance Creek basin The sulfide minerals demonstrate a total range of about 72 permil and show strongenrichment in 34S when compared with sulfides from marine rocks Maximum 34S enrichment was found in sulfides from oil shale and marlstone beds of the oil shale facies averaging about 35 permil CD Sulfides from thestromatolite facies and the sandstone facies show less enrichment in 34S and average about 17 permil The Parachute Creek Member shows progressive and uniform upward enrichment in 34S which culminated during deposition of the rich oil shale beds of the Mahogany interval of the
Trichichnus, a thread-like burrow possibly the work of sipunculan worms, is widespread in Bouma E (turbidite claystone) and H (hemipelagic claystone) layers and some thin turbidite sandstone beds in basin-plain and outer fan-lobe deposits of the Marnoso-arenacea Formation. Trichichnus closely resembles that described elsewhere in chalk and marlstone, although the burrows are smaller (mean diameter=0.13 mm) and burrow curvature is tighter. Trichichnus fills are darker than the host claystone because of high organic content and disseminated pyrite. Any distinctive internal structure of the burrow wall or fill was destroyed by pyrite growth. Trichichnus is most abundant in Bouma E layers, where burrow spacing ranges from 0.4 to 50 cm and burrows extend to depths of 160 cm. The Trichichnus maker commonly burrowed vertically down through E and D layers (clay and silt) and then horizontally on top of C layers (sand). The burrows also vertically penetrate Bouma C sandstone layers up to 6 cm thick, but they are rare in beds thicker than 3 cm. They are less abundant or not well displayed in H layers where Chondrites predominates. Trichichnus overprints Chondrites. Following deposition of the Contessa megaturbidite, which exceeds 9 m in thickness, the sediment surface was repopulated chieflymore » by Trichichnus and Chondrites producers. Trichichnus extends 160 cm below the top of the Contessa, whereas Chondrites extend only to 64 cm. This suggests a greater tolerance of Trichichnus makers to low in situ oxygen levels. The near absence of laminations in the E division of the Contessa megabed may be the result of Trichichnus burrows that were not pyritized and which lack textural identity.« less
