The sedimentary deposits north of the western Snake River Plain host Idaho’s first and only producing oil and gas field. They consist of the mid-Miocene Payette Formation, the mid-late Miocene Chalk Hills Formation, and the Pliocene to early Pleistocene Glenns Ferry Formation. Using new geochronology, palynomorph biostratigraphy, and geologic mapping, we connect up-dip surface features to subsurface petroleum play elements. The Payette Formation is potentially the source of the hydrocarbons and acts as one of the reservoirs in the basin. Here we redefine the Payette Formation as 900 m of mudstone with lesser amounts of sandstone overlying and interbedded with the Columbia River Basalt Group and Weiser volcanics. Index palynomorphs, including Liquidambar and Pterocarya, present in Idaho during and immediately after the mid-Miocene climatic optimum, and new U/Pb dates of 16.39 and 15.88 Ma, help establish the thickness and extent of the formation. For the first time, these biostratigraphic markers have been defined for the oil and gas wells. The Chalk Hills Formation is a tuffaceous siltstone, claystone, and sandstone that is ~300 to 520 m thick. U/Pb dates are 9.00, 9.04, and 7.78 Ma. The Chalk Hills Formation acts both as a reservoir and the sealing mudstone facies. The overlying siltstone to fine conglomerate of the Glenns Ferry Formation acts as the overburden and sealing facies to the petroleum system in the subsurface but was important to the formations burial and hydrocarbon maturation. Both the Chalk Hills and Glenns Ferry Formation were deposited within ancient Lake Idaho during an overall increase in aridity and cooling after the mid-Miocene climatic optimum.
ABSTRACT The first reported Permian (Kungurian to Roadian) palynomorphs are described from Colorado, recovered from bedded gypsum and rare organic-rich shale intercalated in the red siltstone-dominated Lykins and State Bridge Formations. Surprisingly, these units generally lack the taeniate, saccate pollen that typifies most Permian continental rocks elsewhere, yet they contain abundant terrestrially derived palynomacerals, a low-diversity suite of sphaeromorph and acanthomorph acritarchs, and extremely rare non-taeniate, bissacate pollen grains. Acritarchs, known from one stratigraphic interval, are well-preserved and interpreted to represent autochthonous deposition during a marine incursion into the depocenter. This interpretation is consistent with their occurrence in a gray mudstone that is mantled by a mollusk-dominated coquina that bears conodonts, palaeoniscoid scales, and actinopterygian teeth. In contrast, most studied samples are dominated by wood fragments, charcoal, cuticles, and unidentified phytoclasts—all interpreted to represent dispersed plant cuticle and wood of continental origin. Fossils occur in black paper shale, gray fetid calcareous siltstone, and rhythmically bedded gypsum that is closely associated with thin limestone. Palynofacies analyses suggest that non-acritarch palynomacerals were deposited in dysoxic to anoxic waters that received minimal suspended terrigenous input. When combined with sedimentologic information, these non-acritarch fossils are hypothesized to have been deposited in shallow epicontinental lake-like settings that were periodically alkaline, hypersaline, and/or emergent.
ABSTRACT Clastic dikes that occur within the terrestrial, Oligocene White River Group strata at localities throughout the Great Plains typically display internal mud to fine sand layers that are subparallel to the walls. Shrink-swell weathering usually obscures details of the internal layer geometry of the dikes. Recent work in the Slim Buttes area documents internal layer cross-cutting relationships that indicate tens or more of recurrent opening and injection events for thicker individual dikes. Evidence of significant dike-wall modification also exists. Source beds were unobserved despite adequate outcrops. Dikes are enclosed within the Oligocene Brule Formation. Some are truncated at or near the contact with the overlying Miocene Arikaree Group strata, constraining formation timing, whereas others have upper and lower tips within the Brule Formation. Dike strikes test as random in distribution. These dike attributes are consistent with repeated fracture opening and tip propagation from diagenetically driven shrinkage that induced episodic fluid flow which mobilized host-rock sediment (crack-fill instead of crack-seal). Sediment fill is proposed to have come from dike-wall erosion in branching tip regions during propagation events. In general, clastic dikes are polygenetic, and the diagenetically driven, recurrent formation mode evident in the White River Group examples can be considered in addition to standard injection models associated with overpressurized source beds or Neptunian infill.
ABSTRACT We present the first description of Jurassic vertebrate fossils from Texas. The vertebrate specimens were collected from the Upper Jurassic Malone Formation in the Malone Mountains of western Texas. The specimens are fragmentary and not particularly diagnostic, but probably represent elements of plesiosaurians. One specimen is similar to the caudal vertebra of a pliosaurid plesiosaurian, whereas another may be a partial propodial of a small plesiosaurian. Additional bone fragments are not identifiable at this time. These vertebrates were discovered along with abundant plant and invertebrate fossils. Previous studies of the invertebrate fossils indicate a Kimmeridgian to Tithonian age for the Malone Formation, which is consistent with a single grain age of 151±2 Ma from detrital zircon U–Pb geochronology obtained in this study. The Malone Formation was deposited in shallow marine to marginal marine environments along the northern edge of the Chihuahua trough. It is correlative with the La Casita and La Caja Formations of northern Mexico, where similar marine vertebrates have been reported. The Malone Formation is also correlative with the Morrison Formation to the north.
ABSTRACT Facies of the Permian Lyons Sandstone are described and interpreted based on analyses of 23 cores from Larimer and Weld counties, Colorado. Here, the Lyons Sandstone consists of very fine- to medium-grained sandstone with minor silt and mudstone interbeds. The unit has five recurrent siliciclastic facies that can be grouped into two facies associations (FA). FA1 consists of (1) high-angle, cross-laminated sandstone (Facies 1; interpreted as eolian dune remnants); (2) low-angle, cross-laminated and horizontally laminated sandstone (Facies 2; interdune); and (3) chaotically bedded to folded sandstone (Facies 3; lower dune flanks). FA2, in contrast, is mainly (4) wavy- to irregularly laminated silty sandstone (Facies 4; wet to damp interdune); and (5) massive to wavy-laminated silt-rich mudstone (Facies 5; ponded water areas between dunes) with minor amounts of high-angle, cross-laminated sandstone (Facies 1) and low-angle, cross-laminated and horizontally laminated sandstone (Facies 2). FA1 is hypothesized to have been produced in an eolian system akin to those that might exist in the dune-dominated portion of an erg, whereas FA2 was deposited in the intermittently wet portion of this eolian system, perhaps along erg margins or in flat dune-adjacent settings that were impacted by the water table. Isopach data suggests that the study area is on the fringe of a larger Lyons system that spans > 100,000 km2, and was deposited close to the Ancestral Rockies—a paleogeography consistent with deposition in erg to erg-margin paleoenvironments. Detrital zircon populations from nearby Colorado Front Range outcrops and from 12 correlative eolian units are dominated by small, well-rounded Paleoproterozoic and Mesoproterozoic grain populations that are remarkably similar between units, signaling a well-mixed system that also received an influx of distally sourced sediment from the Appalachian orogen. Detrital zircon-based maximum depositional ages of the Lyons Sandstone and its equivalents are internally consistent with deposition of the unit during the latest Artinskian to Kungurian.
