Eagle Basin Research Articles

Provenance of Devonian–Carboniferous strata of Colorado: The influence of the Cambrian and the Proterozoic

ABSTRACTWe report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of > 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), > 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (< 0.1%; n = 2,927 grains).

Grain‐Size Control on Detrital Zircon Cycloprovenance in the Late Paleozoic Paradox and Eagle Basins, USA

AbstractDetrital zircon U‐Pb data are routinely used to assess provenance in sedimentary systems. However, recent studies have suggested that zircon provenance interpretation may be biased by grain‐size‐dependent selective transport and deposition. In this study, we present zircon grain‐size and U‐Pb data from 11 sandstone and 11 mudstone samples from the late Paleozoic Eagle and Paradox basins to test whether apparent provenance changes between sandstone and mudstone samples can be explained by changes in zircon grain‐size alone. The Eagle and Paradox basins formed in association with the Ancestral Rocky Mountains (ARM). These basins provide an excellent system to test grain‐size/provenance relationships because they are filled with cyclic stratigraphy that records, via grain size and facies changes, flooding/exposure driven by interglacial/glacial cycles. These cycles are interpreted to have substantially altered sediment transport pathways and provenance of ARM‐related sedimentary rocks. Data presented here suggest that although zircon grain size does correlate to sample grain size in most samples, age spectra appear to be independent of grain size. As such, differences in age spectra between sandstone and mudstone samples appear to be the result of changes in relative inputs of two endmember sources: (1) grains eroded from adjacent basement‐cored ARM uplifts and (2) far‐traveled sediment sourced from outside the ARM system. The data presented in this study also provide a successful proof‐of‐concept test for a manual method of determining zircon grain size. This method is simple and straightforward and can be applied to any sample analyzed using the Chromium off‐line targeting software.

Provenance of Pennsylvanian–Permian sedimentary rocks associated with the Ancestral Rocky Mountains orogeny in southwestern Laurentia: Implications for continental-scale Laurentian sediment transport systems

AbstractThe Ancestral Rocky Mountains system consists of a series of basement-cored uplifts and associated sedimentary basins that formed in southwestern Laurentia during Early Pennsylvanian–middle Permian time. This system was originally recognized by aprons of coarse, arkosic sandstone and conglomerate within the Paradox, Eagle, and Denver Basins, which surround the Front Range and Uncompahgre basement uplifts. However, substantial portions of Ancestral Rocky Mountain–adjacent basins are filled with carbonate or fine-grained quartzose material that is distinct from proximal arkosic rocks, and detrital zircon data from basins adjacent to the Ancestral Rocky Mountains have been interpreted to indicate that a substantial proportion of their clastic sediment was sourced from the Appalachian and/or Arctic orogenic belts and transported over long distances across Laurentia into Ancestral Rocky Mountain basins. In this study, we present new U-Pb detrital zircon data from 72 samples from strata within the Denver Basin, Eagle Basin, Paradox Basin, northern Arizona shelf, Pedregosa Basin, and Keeler–Lone Pine Basin spanning ∼50 m.y. and compare these to published data from 241 samples from across Laurentia. Traditional visual comparison and inverse modeling methods map sediment transport pathways within the Ancestral Rocky Mountains system and indicate that proximal basins were filled with detritus eroded from nearby basement uplifts, whereas distal portions of these basins were filled with a mix of local sediment and sediment derived from marginal Laurentian sources including the Arctic Ellesmerian orogen and possibly the northern Appalachian orogen. This sediment was transported to southwestern Laurentia via a ca. 2,000-km-long longshore and aeolian system analogous to the modern Namibian coast. Deformation of the Ancestral Rocky Mountains slowed in Permian time, reducing basinal accommodation and allowing marginal clastic sources to overwhelm the system.

Non-dinosaurian dinosauromorphs from the Chinle Formation (Upper Triassic) of the Eagle Basin, northern Colorado: Dromomeron romeri (Lagerpetidae) and a new taxon, Kwanasaurus williamparkeri (Silesauridae).

The “red siltstone” member of the Upper Triassic Chinle Formation in the Eagle Basin of Colorado contains a diverse assemblage of dinosauromorphs falling outside of Dinosauria. This assemblage is the northernmost known occurrence of non-dinosaurian dinosauromorphs in North America, and probably falls within the Revueltian land vertebrate estimated biochronozone (215–207 Ma, middle to late Norian). Lagerpetids are represented by proximal femora and a humerus referable to Dromomeron romeri. Silesaurids (non-dinosaurian dinosauriforms) are the most commonly recovered dinosauromorph elements, consisting of dentaries, maxillae, isolated teeth, humeri, illia, femora, and possibly a scapula and tibiae. These elements represent a new silesaurid, Kwanasaurus williamparkeri, gen. et sp. nov., which possesses several autapomorphies: a short, very robust maxilla with a broad ascending process, a massive ventromedial process, a complex articular surface for the lacrimal and jugal, and 12 teeth; 14 dentary teeth; an ilium with an elongate and blade-like preacetabular process and concave acetabular margin; a femur with an extremely thin medial distal condyle and a depression on the distal end anterior to the crista tibiofibularis. The recognition of K. williamparkeri further demonstrates the predominantly Late Triassic diversity and widespread geographic distribution across Pangea of the sister clade to Asilisaurus, here named Sulcimentisauria. Silesaurid dentition suggests a variety of dietary specializations from faunivory and omnivory in the Middle Triassic and early Late Triassic (Carnian), to herbivory in the Late Triassic (Carnian and Norian), with the latter specialization possibly coinciding with the radiation of Sulcimentisauria across Pangea. The extremely robust maxilla and folidont teeth of K. williamparkei may represent a strong herbivorous dietary specialization among silesaurids.

A new aetosaur from the Upper Triassic Chinle Formation of the Eagle Basin, Colorado, USA

Abstract A small aetosaur skull and skeleton and referred material from the Chinle Formation, Eagle Basin of Colorado, USA, is described as a new taxon, Stenomyti huangae gen. et sp. nov, distinguished from other aetosaurs by the following autapomorphies: three premaxillary teeth; four palpebrals; pronounced midline ridge on frontals and parietals; paired ridges flanking midline ridge on parietal and frontal; exclusion of quadratojugal from ventral margin of skull by contact between jugal and quadrate; exclusion of postorbital from infratemporal fenestra; infratemporal fenestra a horizontally oriented oval that embays the posterior edge of the jugal; retroarticular process longer than distance between articular glenoid and posterior edge of external mandibular fenestra; oval to irregularly shaped ventral osteoderms that do not contact each other. Paramedian and lateral osteoderms of S . huangae are nearly identical to those of Aetosaurus ferratus , and other shared cranial characters suggest that these taxa are closely related and lie outside the clade Typothoracisinae + Desmatosuchinae. This discovery indicates that other reports of Aetosaurus across Laurasia based on osteoderms should be reassessed. Similar confusion with the osteoderms of other non-typothoracisine/desmatosuchine aetosaurs such as Aetosauroides , Stagonolepis and Calyptosuchus suggests that osteoderms are not always reliable taxonomic indicators.

Paleoclimatic inferences from paleopedology and magnetism of the Permian Maroon Formation loessite, Colorado, USA

The Maroon Formation in the eastern Eagle basin (Colorado) consists of >700 m of lithified loess with >200 paleosols interpreted as Protosols and Argillisols on the basis of field, petrographic and geochemical data. Additionally, magnetic susceptibility aids assessment of the intensity of pedogenesis. Bulk magnetic susceptibility (χ b ) through the section repeatedly fluctuates between low values (average 3.51 × 10 −8 ± 1.59 × 10 −8 m 3 /kg) in parent loessite and higher values (average 5.70 × 10 −8 ± 2.70 × 10 −8 m 3 /kg) in paleosols. Moreover, magnetic susceptibility positively correlates with abundance of clay-sized material as well as Al 2 O 3 and K 2 O and effectively distinguishes Protosols and Argillisols. Low- temperature demagnetization indicates the presence of ultra-fine-grained magnetite. The integration of geochemical, petrographic, and rock magnetic data suggest that changes in magnetic susceptibility reflect pedogenesis and relate primarily to climate- and time-dependent pedogenic production and concentration of ultra-fine- grained magnetite. The Maroon Formation loess and associated soils accumulated in an overall arid system, documented in part by formation of incipiently formed paleosols developed into Argillisols by eolian clay and carbonate additions rather than by in situ clay formation. However, the paleosols showing bulk magnetic susceptibility values of greater than 200 χ b document a high- frequency (10 4 −10 5 yr) fluctuation between arid times of loess accumulation and slightly wetter times of reduced silt influx and resultant pedogenesis. This fluctuation likely reflects glacial-interglacial climate shifts that operated in low-latitude Pangea during icehouse conditions. These results suggest that climate-related magnetic susceptibility variations within loess successions can be preserved and useful in very ancient (pre– Pliocene–Pleistocene) sequences.

Genetic Sequences of the Lower Minturn Formation (Middle Pennsylvanian), Southeastern Eagle Basin, Colorado: ABSTRACT

Genetic Sequences of the Lower Minturn Formation (Middle Pennsylvanian), Southeastern Eagle Basin, Colorado: ABSTRACT

Sedimentology and Petroleum Occurrence, Schoolhouse Member, Maroon Formation (Lower Permian), Northwestern Colorado (1)

The Lower Permian Schoolhouse Member of the Maroon Formation (formerly considered the Schoolhouse Tongue of the Weber Sandstone) forms a partly exhumed petroleum reservoir in the Eagle basin of northwestern Colorado. The Schoolhouse consists mainly of yellowish gray to gray, low-angle to parallel bedded, very fine to fine-grained sandstone of eolian sand-sheet origin; interbedded fluvial deposits are present in most sections. The sand-sheet deposits of the Schoolhouse Member are sedimentologically and petrologically similar to those in the underlying red beds of the main body of the Maroon Formation, and the Schoolhouse is considered the uppermost sand sheet in the Maroon depositional sequence. The bleached and oil-stained Schoolhouse Member is distinguished from the unde lying Maroon red beds on the basis of its diagenetic history, which is related to regional hydrocarbon migration and development of secondary porosity. Geological and geochemical data suggest that Schoolhouse Member oils have upper Paleozoic sources, including the intrabasinal Belden Formation. Late Paleozoic faults have served as local conduits for vertical petroleum migration. Large-scale (>200 km) lateral migration from sources in the Permian Phosphoria Formation is also possible but less likely. Belden oil was generated and migrated before about 75 Ma. Subsequently, the Schoolhouse Member reservoir was uplifted, then partly exhumed on the monoclinal flank of the Laramide (latest Cretaceous-Paleogene) White River uplift. Based on this study, exploration models for Maroon Formation and Weber Sandstone reservoirs in northwestern Colorado should be expanded to more fully consider Belden source rocks and the controls of late Paleoz ic structures on hydrocarbon migration and trapping. Stratigraphic units of diagenetic origin comparable to the Schoolhouse Member are likely in other basin provinces, and their proper interpretation is critical for reconstructing the histories of associated petroleum systems.

Oil-Impregnated Outcrops and Their Relationship to Petroleum Generation in Late Paleozoic Eagle Basin, Northwestern Colorado: ABSTRACT

Oil-impregnated outcrops of Mississippian and Pennsylvanian rocks are present in widely separated areas of the Eagle basin. Along the northeast margin of the fault-bounded Eagle basin, oil-impregnated arkosic sandstones of the lower Minturn Formation (Desmoinesian) are interbedded with black, organic-rich shales that provided a local source for the oil. Algal mounds in the lower Minturn Formation in this area contain oil residue. Oil residue is also present in the Mississippian Leadville formation in several parts of the basin, both in paleokarst-related vugs and intergranular porosity in oolite grainstones. The Leadville formation was sourced by overlying organic-rich shales of the Belden Formation (Morrowan-Atokan). Under Pennsylvanian-Permian arkosic sandstones near the southwestern margin of the basin are not adjacent to potential source rocks. The Schoolhouse Tongue of the Weber Sandstone is heavily stained in many areas over its 50 mi of outcrop exposure. Numerous arkosic sandstones in the underlying Maroon Formation are also sained. Faults bounding intrabasinal horst blocks were conduits for upward oil migration, and the horsts may have provided the trap. Oil is also present in overlying Triassic and Jurassic sands in this area, supporting the theory of vertical migration up faults. Oil residue is also present in the Maroon Formationmore » 40 mi to the southeast in the Crystal River Valley. Secondary porosity from dissolution of calcite cement preceded oil emplacement. Beaching of normally red-brown sandstones is ubiquitous with the oil residue and probably represents reduction and partial removal of iron by acidic, reducing fluids generated from the Belden Shale.« less

Significance of Loessite in the Maroon Formation (Middle Pennsylvanian to Lower Permian), Eagle Basin, Northwest Colorado

ABSTRACT Quaternary loess deposits are widespread on the earth's surface, yet pre-Quaternary loess deposits have rarely been reported. The Maroon Formation (Middle Pennsylvanian to Lower Permian) of the Eagle Basin, northwest Colorado, includes a siltstone-dominated facies interpreted as loessite (lithified loess) along its downwind basin margin. The section of inferred loessite in the Maroon Formation is locally at least 490 m thick and consists in large part of structureless and nearly structureless beds of homogeneous sandy siltstone. Bed contacts are generally planar to undulatory and are either horizontal or are characterized by gentle relief. Loessite beds are separated by common claystone drapes and weakly developed paleosols, and by rare pond deposits, channel deposits, and eolian-ripp e-laminated deposits. The loess interpretation is based on 1) the homogeneity and dominance of the sandy silt grain-size; 2) the relative lack of primary sedimentary structures; 3) the gentle character of most bedding contacts and the common mantling of irregular depositional topography; 4) the inferred paleogeographic setting; and 5) the absence of suitable alternative interpretations. The loessite grades laterally into mixed fluvial-eolian deposits of the Maroon Formation in the main part of Eagle Basin, which served as the loessite sediment source. Deposition of the Maroon Formation was probably strongly affected by cyclic climatic changes synchronous with fluctuations in late Paleozoic continental ice sheets. The paleogeography and paleoclimatology of the Maroon Formation depositional system are not unique, suggesting that there are probably many other ancient loessites that have gone unrecognized.

Estimation of Paleogeothermal Gradients and Their Relationship to Timing of Petroleum Generation, Eagle Basin, Northwestern Colorado: ABSTRACT

Lopatin time-temperature index (TTI) modeling of three locations in the Eagle basin, northwestern Colorado, where vitrinite reflectance (R/sub m/) profiles were obtained, shows that paleogeothermal gradients and the timing of oil generation in the Belden Formation (Pennsylvanian) varied due to differing thickness of the Pennsylvanian section across the basin. At the Gilman location, where the Pennsylvanian section is thickest (7,900 ft or 2,408 m), two paleogeothermal gradient models were generated that match the average 3.70% R/sub m/ and the corresponding TTI value between 40,000 and 50,000. The first model assumes a constant geothermal gradient of 2.4/sup 0/F/100 ft (43.8/sup 0/C/km), which places the oil window between 270 and 230 Ma. The second model assumes a changing paleogeothermal gradient of 2.80/sup 0/F/100 ft (51/sup 0/C/km), from 320 to 265 Ma and 2.20/sup 0/F/100 ft (40.2/sup 0/C/km) from 265 Ma to present, which places the oil window between 275 and 250 Ma. For the Glenwood location, where the Pennsylvanian section is 4,960 ft (1,512 m), a constant paleogeothermal gradient of 1.80/sup 0/F/100 ft (32.9/sup 0/C/km) works the best in correlating the 2.50% R/sub m/ with the corresponding TTI value of approximately 2,700. Using this gradient, the oil window falls between 175 andmore » 75 Ma.« less

Surface Thermal Maturity Map of Uinta, Piceance, and Eagle Basins Area, Utah and Colorado: ABSTRACT

Knowledge of thermal maturity is essential for basin analysis studies because heat and heat flow play major roles in mineral formation, hydrocarbon generation and destruction, ground-water flow, and petroleum reservoir quality. The authors studied the thermal maturity of several hundred outcrop samples of coal and carbonaceous shale from Pennsylvanian through Quaternary rocks in the area encompassing the Uinta, Piceance, and Eagle basins of Utah and Colorado. All samples were analyzed for mean random vitrinite reflectance (R/sub m/); thus, thermal maturity is defined by R/sub m/ for this study. Their data show that R/sub m/ generally increases with age of the rock, reflecting increasing burial depth and residence time at maximum temperature. Some coals show local anomalously high maturities because of proximity to Tertiary intrusives. Rocks of the same age show a general decrease in maturity from south to north in all three basins, which probably results from a decrease in geothermal gradients from south to north. In the Uinta basin, R/sub m/ values range from about 0.40 to 0.70% around the margins of the basin to 0.45% in the Altamont-Bluebell oil field near the basin center. In the Piceance basin, R/sub m/ values range from about 0.50 to 0.70% aroundmore » the margins to about 0.25 to 0.35% in the center. The Eagle basin shows the widest range of R/sub m/ values, from 0.30 to 0.50% around the margins to values in the 4.0% range near the center. Some vitrinite anomalies in the study area are best explained by sampling and analytical problems, such as choosing recycled vitrinite grains from organically lean shales or collecting weathered, oxidized samples.« less

Origin and evolution of weber and tensleep formation waters in the greater green river and Uinta-Piceance basins, Northern rocky mountain area, U.S.A.

Laramide structures exercise strong control on fluid flow and differences in pore-water chemistry between the Weber Sandstone in the Uinta-Piceance Basin and the Weber in the adjacent Uinta Mountain and White River uplifts. Similar relationships between structure and variations in pore-water chemistry exist for the Permo-Pennsylvanian Weber and Tensleep sandstones in the Greater Green River Basin and the Weber in the Uinta Mountain Uplift. Stable-isotope and chemical data indicate that Weber pore water in the Uinta Mountain and White River uplifts and Tensleep pore water along the eastern margin of the Greater Green River Basin are young, fresh to saline, Ca SO 4 and Na Cl, meteoric waters which probably entered these Laramide structures subsequent to Mio-Pliocene epeirogenic uplift of the area. In contrast Weber pore waters in the Uinta-Piceance and central Greater Green River basins are Na Ca Cl and Na Cl SO 4 HCO 3 brines, respectively. These brines are separated from the meteoric waters in the uplifts by major Laramide thrust faults which mark the flanks of the uplifted blocks. Although the origin and evolution of the Na Cl SO 4 HCO 3 brine in the Greater Green River Basin are uncertain, stable-isotope and chemical data indicate that the Na Ca Cl brine in the Uinta-Piceance Basin originated as evaporated seawater during precipitation of the Pennsylvanian Eagle Valley Evaporites in the Eagle Basin which lies immediately east of the Uinta-Piceance Basin. This brine migrated out of the Eagle Basin, which is now part of the White River Uplift, prior to the Laramide orogeny and diagenetically altered the Weber Sandstone. The presence of ancient brines in the Weber Sandstone in both the Uinta-Piceance and Greater Green River basins indicates that there is no hydraulic continuity between the Weber Sandstone in the uplifts and the basins. Their presence also suggests that fluid flow in the deeper strata is stagnant and independent of fluid flow in the uplifts. Although these brines are now restricted to the post-Laramide basins, they probably were much more widespread throughout the Utah and Wyoming thrust belt and its foreland prior to Laramide tectonism. Much of the diagenetic alteration of the Weber Sandstone probably took place in the presence of these brines.

Variations in Vitrinite Reflectance and Clay-Mineral Composition Within Vertical Sections of Belden Shale, Eagle Basin, Colorado--Problems in Determining Thermal Maturity: ABSTRACT

A major problem in defining true thermal maturity of shales by mean random vitrinite reflectance (R/sub m/) and clay-mineral geothermometry is determining the correct population of organic matter and/or interstratified illite/smectite (I/S) clay, respectively. Shales commonly contain a variety of both organic matter and I/S clay; the types of both the organic matter and the I/S clays vary widely, as do the interpreted values, reflecting the heterogeneous sources. The organic-rich, carbonate-bearing Belden Shale (Early Pennsylvanian) contains three distinct populations of organic matter. The vitrinite with the lowest reflectance is weathered-looking or porous, and is generally in the 0.90-1.30% R/sub m/ range. The next higher population of vitrinite R/sub m/ values is consistently in the 2.0-3.0% range. The highest R/sub m/ population is fusinite, measuring in the 4.0-5.0% range. Other indicators of thermal maturity (T/sub max/ and time-temperature index reconstructions, which show that these rocks have been subjected to relatively high temperatures, are in best agreement with the intermediate R/sub m/ values 2.0-3.0%). Although the ordered and highly illitic I/S is in good agreement with the other thermal indicators, randomly interstratified I/S typically does not exist in sedimentary rocks with R/sub m/ > 0.80%. Possible explanations for the problems stated aremore » (1) that weathering of precursor organic matter and minerals can produce lower values in R/sub m/ and highly expandable I/S, respectively; (2) suppression effects reported for vitrinite in other organic-rich marine shales and for the smectite-to-illite reaction in other calcium-bearing rocks; and (3) very late diagenesis at near-surface conditions producing highly expandable I/S.« less

Thermal Maturation of Eagle Basin, Northwestern Colorado: ABSTRACT

Thermal Maturation of Eagle Basin, Northwestern Colorado: ABSTRACT

Fluvial Transitions and Paleogeography in Upper Part of Maroon Formation (Pennsylvanian and Permian), Northwestern Colorado: ABSTRACT

Six detailed lateral profiles (as much as 7 m thick and 50 m long) of fluvial channel sandstone bodies were measured at different locations in the northern part of the Eagle basin, northwestern Colorado. Contrasts in the grain size, internal geometry, and architecture of the sandstone bodies reveal significant facies changes across the basin. Proximal bodies that formed closest to the basin-margin ancestral Uncompahgre and Front Range uplifts consists of massive cross-bedded and flat-bedded coarse-grained to conglomeratic sandstone deposited mainly channel scour and fill and by vertical and lateral accretion on sand-gravel bars in low-sinuosity rivers. Medial sandstone bodies consist mostly of cross-bedded and flat-bedded fine to coarse-grained sandstone that was also deposited in low-sinuosity rivers on sand flats and in sandy bars. Bankfull discharge in proximal and medial channels was probably about 400 to 1000 m/sup 3//sec. The most distal sandstone body is composed of flat- and ripple-laminated, very fine to fine-grained sandstone that probably was deposited in a distributary area, in part by sheet floods. Overall facies changes indicate down-system decreases in competence and depth of flow. Paleocurrent data indicate that Maroon rivers draining the ancestral Uncompahgre uplift flowed northeastward across most of the basin before merging withmore » rivers draining the ancestral Front Range uplift. The depositional axis of the basin was therefore probably strongly skewed to the northeast during late Maroon time.« less

Pennsylvanian-Permian Block Faulting in Subsurface of Piceance Basin, Colorado: ABSTRACT

Pennsylvanian-Permian age block faulting has been identified on two regional seismic lines in the Piceance basin. At the south end of a north-south line along the west side of the basin, the Mesozoic section unconformably overlies the Precambrian. Five miles (8 km) to the north, reflectors of the Mississippian Madison Limestone appear in a series of fault blocks downthrown to the north. These faults generally do not displace overlying Mesozoic rocks. In the vicinity of Douglas Creek field, this block faulting created a large horst block on which Madison Limestone is faulted step-wise up the flanks of the structure. The overlying Pennsylvanian Maroon Formation is 50% thinner over the crest of this structure than it is 10 mi (16 km) to the north. Similar features can be see on a second seismic line running east-northeast through the central portion of the basin between DeBeque field and the Grand Hogback. Coming off the ancestral Uncompahgre highland, Madison reflectors appear near the southwest end of the line in a series of fault blocks downdropped to the northeast. Near Rulison field, a large Pennsylvanian-age horst block is present that may have Madison Limestone stripped from its crest. Near the Grand Hogback, Madison Limestone is faulted up to the east to form a Pennsylvanian-age basement high between the Piceance basin and the Eagle basin to the east. End_of_Article - Last_Page 869------------

Turbidite Fans in Upper Cretaceous Pierre Shale, Eagle Basin, Colorado: A New Reservoir Facies: ABSTRACT

Turbidites have been recognized increasingly in the Cretaceous Interior seaway. Most are described as thin, economically unimportant sandstone beds isolated in prodelta mudstone. This study documents the occurrence of southerly prograding, sand-dominated turbidite fans of sufficient size to be considered economically viable hydrocarbon reservoirs. Two fans intercalate with the Upper Cretaceous Pierre Shale and form cliffs over more than 10 mi (16 km) of continuous outcrop in the Eagle basin, north of Walcott, Colorado. Both units exhibit progradational sequences typical of turbidite fans. A common vertical succession of sedimentary structures consists of starved ripples, flat-bottomed ripple beds, thin flat beds grading into ripples of climbing ripples, and amalgamated flat beds. Massive to graded beds are rare and occur only in the upper part of each sandstone body. Associated sedimentary features include parting lineation, grooves, prod marks, mud chips, contorted bedding, and flute casts. Broad, low-relief channels occur at the top of the lower, more well-developed sequence. The sedimentary structures described correlate well with accepted models for turbidite-fan sedimentation. The upward-thickening and coarsening character of the sandstone bodies, the abundance of incomplete Bouma sequences, and the presence of broad low-relief channels are typical of the outer-fan lithofacies associations of E. Mutti and F. Ricci-Lucchi. Alternative interpretations of these laterally continuous, progradational sandstone bodies might include deposition in a shoreface or offshore bar environment. Hummocky cross-stratification and large-scale cross-stratified bed forms are not common in the sequence, as would be expected in a shoreface or marine-bar environment. Turbidite-fan deposits similar to those studied could be economically significant because of their extreme lateral continuity, updip seals, intercalation with hydrocarbon source rock,and possible overpressuring. In addition, the apparently distal nature of these sandstone bodies suggests the possibility of thicker, better developed turbiditic sandstone bodies to the north. The presence of submarine fans within the Cretaceous Western Interior seaway may increase significantly the hydrocarbon potential of previously unexplored, shaly portions of the basin. End_of_Article - Last_Page 498------------

Petrology and Paleoenvironments, Robinson Member, Minturn Formation (Desmoinesian), Eagle Basin, Colorado

The Robinson Member is one of eight carbonate members of the 6,000-ft (1,824 m), predominantly sandstone, Minturn Formation (Desmoinesian) in the study area west of the Gore Range in central Colorado. The Robinson Member consists of three limestone units (lower, middle, and upper) interstratified with marine and nonmarine sandstone, siltstone, and conglomerate. Total thickness of the Robinson Member ranges from 200 to 400 ft (60-120 m). Several fusulinid biozones can be recognized in the Robinson Member. They are parallel with and equivalent to the lower, middle, and upper limestones. Fusulina pristina and Fusulina curta do not appear to range above the lower Robinson. Fusulina rockymontana and Wedekindellina coloradoensis are present throughout the Robinson. Fusulina truncatulina has been found only in the upper Robinson and Fusulina plattensis is limited to the upper Robinson and overlying Elk Ridge Member. The areal distribution and the thickness of four time-transgressive facies which make up each of the Robinson limestone units imply that the late Paleozoic Eagle basin was an asymmetric depositional basin with a relatively deep marginal trough along the east side. The basin was bounded on the east by the Ancestral Front Range highland and on the southwest by an Ancestral Uncompahgre highland. From east to west the limestone units contain (1) an oolite facies composed of superficial oolites, pseudo-oolites, and oncolites in microspar or micrite; it is interpreted to be a shallow-water, high-energy deposit on the east edge of the marginal trough; (2) a tubular Foraminifera facies composed mainly of tubular Foraminifera micrite; (3) a phylloid algae facies of biomicrite containing abundant Archaeolithophyllum, Eugonophyllum, fusulinids, and Komia; and interpreted to be a deeper water deposit near the middle of the trough; (4) a stromatolite facies of stromatolites, laminated micrite, and vuggy ostracod-bearing intramicrudite, which indicates quiet-water to supratidal deposition near the western margin of the trough. Small Solenopora bioherms are present locally in this facies The stromatolite facies grades westward into and is interbedded with siltstones and evaporites of the Eagle Valley Evaporite near the center of the Eagle basin. The overall patterns of the limestone facies and sandstone beds indicate intervals of rapid nondepositional marine transgression, followed by relatively slow regression in which a series of limestone facies was deposited. A typical sequence of lithofacies in the Robinson limestones from bottom to top is: argillaceous micrite, phylloid algae biomicrite, tubular Foraminifera biomicrite, and oosparite and/or oomicrite. Bioherms as thick as 80 ft (24 m) are in the phylloid algae and stromatolite facies. Nonphylloid algae appear to be the major components of these bioherms. However, recrystallization and dolomitization make recognition of the alga types difficult. Diagenesis of the limestones involves recrystallization, silicification, dolomitization, and intrastratal solution.

Pre-Pennsylvanian Paleotectonic Framework of Ancestral Rockies of Colorado: ABSTRACT

The tectonic framework that controlled the locations of the late Paleozoic Ancestral Rockies was well established in pre-Pennsylvanian time. The Uncompahgre uplift in southwestern Colorado arose along tectonic lineaments that originated in the late Precambrian. Recurrent movements along the fault system continued through early and middle Paleozoic time and the resulting submarine topography controlled the sedimentation of reservoir facies in the adjacent Paradox basin. The Front Range and Wet Mountains uplifts of central Colorado were apparently low and inactive prior to Late Devonian time; at least, evidence is lacking that they were source areas during the early Paleozoic. The relation between basement tectonic trends and the location of the Permo-Pennsylvanian uplifts is obscure. The first demonstrable uplift of the Front Range element occurred in Late Devonian time when coarse clastic material was shed into the Parting sea from the west flank of the uplift and a satellite structure in the northern Sawatch Range. Uplift of these source areas recurred in Early Mississippian time and produced limestone conglomerates in the basal Leadville Formation. The Wet Mountains lay dormant until Early Mississippian time, and a broad lowland occupied the general region in the Late Devonian. Local deposits of limestone conglomerates in the Leadville Formation attest to the Early Mississippian time of uplift. Continued detailed petrologic and paleotectonic studies in the 1970s will lead to new petroleum discoveries in the related Paradox and Eagle basins by establishing favorable reservoir facies trends, paleohydrodynamic patterns, and times and paths of petroleum migration. End_of_Article - Last_Page 834------------