Abstract The Nugget Sandstone, a Jurassic eolianite, has yielded few fossils. In addition, there are few studies of trace fossils in eolianites anywhere. The trace fossils discussed here, which are found immediately north of Peoa, Utah, are therefore significant as a reflection of life in the Early Jurassic Nugget sand sea. The trace fossils in the Nugget Sandstone are mostly meniscate burrows similar to those of other eolianites, including some that are roughly parallel with bedding and others that lack a preferred orientation. Most are assignable to the ichnogenera Taenidium and Entradichnus, and some may belong in the ichnogenus Beaconites. Tracks of a small vertebrate are exposed in one outcrop, and are predominantly directed upslope. These are assigned to the ichnogenus Brasilichnium. A few invertebrate (scorpionid?) tracks were found in float. The positions of the trace fossils indicate that the burrowed intervals formed on the middle or upper lee slopes of sand dunes. The depositional history of the thicker burrowed interval is reconstructed from an analysis of the bedding types, granulometry, and depositional slopes within that interval.
The thermal resistance (or Bullard) method is used to judge the utility of petroleum well bottom‐hole temperature data in determining surface heat flow and subsurface temperature patterns in a sedimentary basin. Thermal resistance, defined as the quotient of a depth parameter Δz and thermal conductivity k, governs subsurface temperatures as follows: [Formula: see text] where [Formula: see text] is the temperature at depth z=B, [Formula: see text] is the surface temperature, [Formula: see text] is surface heat flow, and the thermal resistance (Δz/k) is summed for all rock units between the surface and depth B. In practice, bottom‐hole and surface temperatures are combined with a measured or estimated thermal conductivity profile to determine the surface heat flow [Formula: see text] which, in turn, is used for all consequent subsurface temperature computations. The method has been applied to the Tertiary Uinta Basin, northeastern Utah, a basin of intermediate geologic complexity—simple structure but complex facies relationships—where considerable well data are available. Bottom‐hole temperatures were obtained for 97 selected wells where multiple well logs permitted correction of temperatures for drilling effects. Thermal conductivity values, determined for 852 samples from 5 representative wells varying in depth from 670 to 5180 m, together with available geologic data were used to produce conductivity maps for each formation. These maps show intraformational variations across the basin that are associated with lateral facies changes. Formation thicknesses needed for the thermal resistance summation were obtained by utilizing approximately 2000 wells in the WEXPRO Petroleum Information file. Computations were facilitated by describing all formation contacts as fourth‐order polynomial surfaces. Average geothermal gradient and heat flow for the Uinta Basin are [Formula: see text] and [Formula: see text], respectively. Heat flow appears to decrease systematically from 65 to [Formula: see text] from the Duchesne River northward toward the south flank of the Uinta Mountains. This decrease may be the result of refraction of heat into the highly conductive quartzose Precambrian Uinta Mountain Group. More likely, however, it is related to groundwater recharge in late Paleozoic and Mesozoic sandstone and limestone beds that flank the south side of the Uintas. Heat flow values determined for the southeast portion of the basin show some scatter about a mean value of [Formula: see text] but no systematic variation.
In northeastern Utah, the Entrada Sandstone contains two lithologic facies: a lower pale yellowish-orange, fine- to medium-grained sandstone (sandstone facies) , and an upper moderate reddish-orange, very fine-grained silty sandstone (silty sandstone facies). The silty sandstone facies is present only in the western part of the area where it interfingers with the sandstone facies. Differences in lithology and sedimentary structures provide a means for separating the sandstone facies into informal lower and upper units. The lower unit thickens eastward and is dominantly fine grained. Sedimentary structures include: small- to large-scale trough cross-stratification, horizontal stratification, small- to medium-scale tabular- and wedge-planar cross-stratification, large-scale tabular-planar cross-stratification, and multiple parallel-truncation bedding planes. The upper unit of the sandstone facies is dominantly very fink- to fine-grained sandstone. Locally glauconite and limy sandstone are abundant. The upper unit thins eastward and primarily is composed of horizontally stratified or structureless sandstone; small- to medium-scale tubular- and wedge-planar and trough cross-stratification are rare. The silty sandstone facies appears structureless. Disturbed and horizontal stratification are locally present where interfingering occurs with the underlying sandstone facies. Dominant paleocurrent directions, based on 275 measurements, are west, south, and southwest. Nine of 11 locations show a unimodal distribution; seventy-nine percent of these measurements are between 150 and 299; eighty-three percent are between 180 and 359. The remaining two locations show bimodal distributions; formed in subaqueous environments differ significantly from all other types of crossstratification. The mean azimuth for these troughs is 326, compared with 232 for all other types of cross-stratification. The lower unit of the sandstone facies probably was deposited in an arid or semiarid, inland-dune environment in the eastern part of the area, and as coastal dunes and shallow-water marine deposits (upper shoreface) bordering the Preuss seaway on the west. The upper unit may represent a reworking of the Entrada during a minor transgression of the Preuss sea. The silty sandstone facies is probably a shallow-water marine deposit. Eolian beds in the lower unit of the sandstone facies exhibit an average porosity of 15.9 percent and offer the best potential as reservoir rocks. Where eolian beds are close to favorable source rocks, low porosities in the overlying upper unit (9 percent) and the silty sandstone facies (1.9 percent) may provide stratigraphic traps for the accumulation of oil and gas. southeast-northwest and southwest-northeast. Azimuths of trough cross-stratification if:
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.
