Absaroka Thrust Research Articles

Conditions during syntectonic vein formation in the footwall of the Absaroka Thrust Fault, Idaho–Wyoming–Utah fold and thrust belt

The Twin Creek Limestone in the footwall of the Absaroka thrust sheet contains three sets of bed-normal syntectonic calcite veins. Vein formation occurred during Cretaceous motion along the Absaroka thrust fault as indicated by (1) crosscutting relationships among these vein sets, (2) a previously dated solution cleavage, and (3) calcite twin analysis. Fluid inclusions in the veins and overburden estimates constrain inclusion entrapment temperatures to be between 175 °C and 328 °C. Results from stable oxygen isotopes indicate that the host and vein fluid compositions were in near isotopic equilibrium. Applying both reasonable geothermal gradients and constraints on overburden temperature yields fluid pressures during vein precipitation that are near hydrostatic. All data taken together suggest both that vein formation within the Twin Creek Formation occurred in a relatively closed system, and that the veins filled near hydrostatic fluid pressure. Because the veins fill precursory cracks, vein filling might not reflect the maximum fluid pressure that existed during the complete vein forming process.

Three-dimensional structural model of the Painter and East Painter reservoir structures, Wyoming fold and thrust belt

The Painter and East Painter reservoir structures are located in the hanging wall of the Absaroka thrust in the Wyoming fold-thrust belt. Balanced cross sections and interpreted logs and dipmeter data from more than 50 wells have been integrated to develop a three-dimensional (3-D) structural model of the Painter reservoir structures. The structures are interpreted as a pair of asymmetric faulted detachment folds formed along the hanging-wall ramp in Triassic–Jurassic units in the Absaroka thrust sheet. The Painter reservoir structure verges to the southeast and has a gentle backlimb and a steeply dipping to overturned forelimb, whereas the East Painter structure displays steep dips on both limbs. The front limbs of both structures contain forelimb thrust faults with small displacements. A tight syncline separates the structures and contains several out-of-syncline thrusts in the Jurassic Twin Creek limestones. Cross sections through the structures are restored using line-length balancing for the Nugget formation and area balancing of the Ankareh-Thaynes-Woodside formations and the Twin Creek Formation. The structures are interpreted to have initiated as a pair of detachment folds cored by moderately ductile limestones and shales of Triassic age. Increasing shortening resulted in tightening of the structures, the development of out-of-syncline thrusts, and the propagation of thrust faults on the steep forelimbs. The tight geometry of the East Painter structure resulted from frictional resistance to fault slip along the hanging-wall ramp in the Triassic–Jurassic units. The balanced 3-D structural model ensures consistency of the geometry of all interpreted horizons and faults and can be used to construct improved structural maps of reservoir units in the Painter reservoir structures.

Linkage of Sevier thrusting episodes and Late Cretaceous foreland basin megasequences across southern Wyoming (USA)

ABSTRACTDeposition and subsidence analysis, coupled with previous structural studies of the Sevier thrust belt, provide a means of reconstructing the detailed kinematic history of depositional response to episodic thrusting in the Cordilleran foreland basin of southern Wyoming, western interior USA. The Upper Cretaceous basin fill is divided into five megasequences bounded by unconformities. The Sevier thrust belt in northern Utah and southwestern Wyoming deformed in an eastward progression of episodic thrusting. Three major episodes of displacement on the Willard‐Meade, Crawford and ‘early’ Absaroka thrusts occurred from Aptian to early Campanian, and the thrust wedge gradually became supercritically tapered. The Frontier Formation conglomerate, Echo Canyon and Weber Canyon Conglomerates and Little Muddy Creek Conglomerate were deposited in response to these major thrusting events. Corresponding to these proximal conglomerates within the thrust belt, Megasequences 1, 2 and 3 were developed in the distal foreland of southern Wyoming. Two‐dimensional (2‐D) subsidence analyses show that the basin was divided into foredeep, forebulge and backbulge depozones. Foredeep subsidence in Megasequences 1, 2 and 3, resulting from Willard‐Meade, Crawford and ‘early’ Absaroka thrust loading, were confined to a narrow zone in the western part of the basin. Subsidence in the broad region east of the forebulge was dominantly controlled by sediment loading and inferred dynamic subsidence. Individual subsidence curves are characterized by three stages from rapid to slow. Controlled by relationships between accommodation and sediment supply, the basin was filled with retrogradational shales during periods of rapid subsidence, followed by progradational coarse clastic wedges during periods of slow subsidence. During middle Campanian time (ca. 78.5–73.4 Ma), the thrust wedge was stalled because of wedge‐top erosion and became subcritical, and the foredeep zone eroded and rebounded because of isostasy. The eroded sediments were transported far from the thrust belt, and constitute Megasequence 4 that was mostly composed of fluvial and coastal plain depositional systems. Subsidence rates were very slow, because of post‐thrusting rebound, and the resulting 2‐D subsidence was lenticular in an east–west direction. During late Campanian to early Maastrichtian time, widespread deposits of coarse sediment (the Hams Fork Conglomerate) aggraded the top of the thrust wedge after it stalled, prior to initiation of ‘late’ Absaroka thrusting. Meanwhile Megasequence 5 was deposited in the Wyoming foreland under the influence of both the intraforeland Wind River basement uplift and the Sevier thrust belt.

Fold–thrust styles in the Absaroka thrust sheet, Caribou National Forest area, Idaho–Wyoming thrust belt

The Bear Creek, Big Elk and Black Mountain anticlines are macroscopic structures located in the hanging wall of the Absaroka thrust within the Idaho–Wyoming fold–thrust belt. Structural mapping of the area was conducted by draping existing geologic maps and digital orthophotos over digital elevation models (DEM) to obtain a three-dimensional perspective of the area. The map was further refined by reconnaissance field mapping of selected outcrops. Construction of balanced cross-sections suggests that these structures detach at three stratigraphic levels, within the Cambrian Gros Ventre Formation, Devonian Darby Formation, and Triassic Dinwoody Formation. The folds in the upper stratigraphic package consist of symmetric to asymmetric detachment folds, which show variable vergence along trend. The structures in the lower stratigraphic packages primarily evolved as low-amplitude detachment folds, which were subsequently displaced over fault ramps to form fault–bend folds or duplexes. The duplex geometries in the Cambro–Ordovician units under the Bear Creek and Poker Peak anticlines vary from independent anticlines in the south to completely overlapping stacks in the north. The macroscopic structural patterns of folds proposed in this study will be useful in interpreting subsurface structures in other parts of the Idaho–Wyoming fold–thrust belt.

Mechanical stratigraphy and fault–fold interaction, Absaroka thrust sheet, Salt River Range, Wyoming

A distinct contrast in deformation style from tightly folded and faulted Cambrian through Ordovician strata to broadly folded and faulted Mississippian and younger strata is displayed in the hanging wall of the Absaroka thrust, Salt River Range, Wyoming. Based on style, the Paleozoic section can be divided into two main structural lithic units. Each unit is characterized by a relatively weak, ductile, anisotropic lower section and a relatively strong, brittle, more isotropic upper section. Where the two units were stacked within the thrust sheet, inverted fault–propagation folds formed in the center of each unit, and the overall transition upward from close- to wide-spaced folds and imbricate faults developed in the multilayer. Where the upper unit was isolated, deformation was dominated by imbricate faulting with little associated folding, and inverted fault–propagation folds did not form. Observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response. It is note-worthy that the boundary between the two structural lithic unts, in part defined by the roof thrust of the Stewart passive roof duplex, occurs within a dolostone rather than in overlying shale of the Darby Formation. I propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions. Cross-cutting relations suggest that the Absaroka sheet was cut by large imbricate faults during late-stage movement over a major dip ramp. A high deviatoric and low mean compressive stress state just beyond the upper ramp–flat corner promoted break-back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation.

A comparison of fluvial megafans in the Cordilleran (Upper Cretaceous) and modern Himalayan foreland basin systems

The Campanian–Maastrichtian Hams Fork Conglomerate Member of the Evanston Formation in northeastern Utah and southwestern Wyoming consists of a widespread (>10 000 km 2 ) boulder to pebble, quartzitic conglomerate that was deposited by east-southeastward–flowing, gravelly braided rivers on top of the frontal part of the Sevier fold-thrust belt and in the adjacent foredeep of the Cordilleran foreland basin. In northeastern Utah the conglomerate was deposited in a lobate fan-shaped body, up to 122 m thick, that trends southeastward away from its principal source terrane in the southern end of the Willard thrust sheet. The Willard sheet contains thick Proterozoic quartzite units that produced highly durable clasts capable of surviving long-distance fluvial transport. Although the main source of sediment for the Hams Fork Conglomerate was the Willard sheet, the active front of the thrust belt lay 40–50 km to the east along the Absaroka thrust system. Displacement along the Absaroka system uplifted and topographically rejuvenated the Willard sheet, and antecedent drainages carried detritus from hinterland source terranes into the proximal foreland basin. Although topographic ridges associated with fault-propagation anticlines along frontal thrusts locally influenced transport directions, they provided relatively little sediment to the Hams Fork Conglomerate. Lithofacies, paleocurrent, and isopach data indicate that the Hams Fork Conglomerate was deposited in fluvial megafans and stream-dominated alluvial fans, similar in scale and processes to megafans and alluvial fans in southern Nepal and northern India that are forming along the proximal side of the Himalayan foreland basin system. The Himalayan fluvial megafans have areas of 10 3 –10 4 km 2 , slopes of 0.05°–0.18°, and are deposited by large transverse rivers that are antecedent to frontal Himalayan structures and topography. The main fluvial channels on the upper parts of the megafans are anastomosed and braided at bankfull stage but commonly have braided thalwegs at low-flow stage. Downstream, these channels become predominantly braided and meandering and ultimately merge with the axial Ganges trunk river system. Stream-dominated alluvial fans in the Himalayan foreland basin system fringe the topographic front of the fold-thrust belt in the intermegafan areas. These fans have areas of ∼10 2 km 2 and slopes of ∼0.5°. The proximal parts of both types of fans are dominated by extremely coarse (boulder-cobble) bedload that is in transit mainly during the monsoon. The prevalence of fluvial megafans in the modern and Miocene Himalayan foreland and in the Upper Cretaceous–lower Tertiary stratigraphic record of the Cordilleran foreland suggests that these types of deposits may be the volumetrically largest gravel accumulations in nonmarine foreland basin systems.

Stratigraphic base level and fluvial architecture: Ericson Sandstone (Campanian), Rock Springs Uplift, SW Wyoming, USA

The concept of stratigraphic base level, or the ratio between accommodation and sediment supply (A/S ratio), has been used to analyse the Rusty and Canyon Creek Members of the Campanian Ericson Sandstone in the Rock Springs Uplift, SW Wyoming, USA. The Ericson Sandstone was deposited under fluvial to estuarine conditions in a foreland basin setting influenced both by Sevier‐style (thrust belt) tectonism and by more local, Laramide‐style, foreland uplifts. The depositional setting was situated several tens to a few hundred kilometres from the nearest shoreline. Therefore, sea level change at the contemporaneous shoreline probably had little, if any, influence on the development of the sedimentary architecture.The Rusty Member shows an alternation between incised valleys filled by multi‐storey estuarine channel sandstones showing palaeoflow to the south and delta plain sediments with single‐storey channels with no evidence of tidal influence, which show palaeoflow to the east. This cyclicity is interpreted as recording repeated uplift of the Wind River Range to the north, causing valley incision and reduction of the A/S ratio. During quiescent periods, the A/S ratio increased allowing the valleys to fill and delta plain conditions to be subsequently re‐established because of increased sediment supply from the thrust belt in the west.A regional unconformity at the base of the Canyon Creek Member truncates the Rusty Member, and represents a significant reduction of the A/S ratio caused by Laramide tectonic uplift. The Canyon Creek Member is a multi‐storey, multi‐lateral fluvial channel sandstone, where channel preservation and thickness increase upwards, suggesting an increase of the A/S ratio. The Canyon Creek Member channels are interpreted to have been sinuous, meandering channels from the observed sedimentary structures and fill patterns, despite their sand‐rich nature. It is argued that grain size is a poor indicator of channel planform, and that there was very low preservation potential for fine material because of a relatively low A/S ratio.The top of the Canyon Creek Member is a regionally correlative surface marking an abrupt increase of the A/S ratio. This surface is termed an expansion surface, denoting an abrupt increase in accommodation. The overlying Almond Formation shows a single‐storey alluvial architecture with a very high preservation of fine‐grained material. An assumed correspondence in time of the Late Absaroka thrust phase in the Sevier belt to the west and the formation of the sharp top of the Canyon Creek Member suggests that the thrust phase caused a basin‐wide abrupt increase of subsidence that changed the alluvial architecture.As an alternative to sequence stratigraphic nomenclature defined for strata controlled by shoreline movements, a scheme relating systems tracts and surfaces to changes in stratigraphic base level is proposed. Such a scheme is useful where correlations to shoreline strata are ambiguous or cannot be made, or where tectonics and climate are important controls.

Structure of the Mount Raymond transverse zone at the southern end of the Wyoming salient, Sevier fold-thrust belt, Utah

The Mount Raymond transverse zone (MRTZ) forms the east-west-trending boundary between the Wyoming salient of the Sevier fold-thrust belt and the Uinta/Cottonwood arch in north-central Utah. Major faults in the zone dip 40° to 45° north. Our structural analysis indicates that the MRTZ contains both contractional and extensional structures. The contractional structures (thrusts and related folds) initially formed as part of a southeast-verging, northeast-trending thrust system. This system gradually curved and merged to the north with the east-verging Absaroka thrust system in the apex of the Wyoming salient. The contrast in trend between structures in the MRTZ and those in the apex of the Wyoming salient reflects the initial curvature of the salient. This curve formed because the stratigraphic sequence involved in thrusting thinned gradually southward, toward the axis of a proto-Uinta arch. The present east-west trend of the MRTZ developed during Laramide uplift of the Unita/Cottonwood arch, an event which tilted the faults into their present position. Cenozoic crustal extension subsequently reactivated segments of the MRTZ. In sum, the evolution of the MRTZ illustrates how post-thrusting processes can affect the map-view geometry of thrust belts to create transverse zones.

Thrust sheet rotation and out-of-plane strains associated with oblique ramps: An example from the Wyoming salient U.S.A.

Movement over oblique thrust ramps results in local thrust sheet rotation and principal strains which deviate from the transport plane. Resulting folds, faults and deformation fabrics are superposed on those associated with displacement on frontal ramps. The South Fork thrust, a hangingwall imbricate of the Absaroka thrust sheet, SW Wyoming, has a steeply-dipping (~70 °). small-displacement (~300 m), oblique ramp in the hangingwall and footwall, which makes an angle of 25 ° to the regional transport direction. Two penetrative deformation events are recorded by folding, stylolitic cleavage, and calcite twinning strains in the deformed hangingwall and footwall. The first is E- to SE-directed layer-parallel shortening prior to or during motion of the Absaroka thrust sheet. The second event indicates NE-directed shortening unique to the oblique ramp, at a high angle to ramp strike. At the trailing frontal ramp-oblique ramp intersection, folds in the footwall and normal faults in the hangingwall reflect local strike-parallel shortening and extension, due to out-of-plane deflection of the wallrock during displacement. The out-of-plane strains arc consistent with those predicted by kinematic and mechanical models. In the hangingwall of the oblique ramp, cross-strike fractures and bedding are rotated ~30° counterclockwise about a vertical axis with respect to structural domains to the north and south due to differential displacement on the South Fork thrust. Stylolitic cleavage and caleite strains unique to the oblique ramp show a similar change in attitude indicating the rotation was the latest recorded stage of deformation. In general, if an oblique ramp is shallowly-dipping, the hangingwall may move from the lower flat, up the ramp, and onto the upper Hat. If an oblique ramp has a steep dip, as in the case of the South Fork thrust, relatively greater sliding and bending resistance may impede displacement up the ramp and upper flat. Further transport of the thrust sheet is then accommodated by footwall deformation and hangingwall rotation.

Late Cretaceous-Paleocene synorogenic sedimentation and kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming

Integration of cross-cutting structural relationships, overlapping sedimentary units, new conglomerate provenance data, and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of Late Cretaceous through Paleocene thrust faulting in the northeast Utah-southwest Wyoming part of the Sevier thrust belt. These data indicate a general eastward progression of deformation that was punctuated by local out-of-sequence and hinterlandward-verging events. Provenance data delimit a sequential restoration of a regional cross section. The principal thrust systems in northeast Utah and southwest Wyoming are the Willard, Ogden, Crawford, Absaroka, and Hogsback thrusts. The Willard is the westernmost, structurally highest, and oldest of the thrusts; it carries a unique section of thick Proterozoic sedimentary rocks, as well as Paleozoic rocks, and was folded during displacement on younger thrust systems. The next youngest thrust system is the Ogden, which comprises several basement-rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the Wasatch Range. Growth of the Wasatch culmination took place during Coniacian through Paleocene time, contemporaneous with sequential displacement on the frontal Crawford, Absaroka, and Hogsback thrusts. Total shortening in this part of the thrust belt was ∼100 km, and structural relief of ∼25 km developed in the area of the Wasatch culmination. During Coniacian through Paleocene time, thrusting followed an overall eastward progression that was interrupted by local out-of-sequence and hinterlandward-verging events. Several episodes of synchronous displacement on two or more thrusts can be demonstrated. Shortening occurred in three main episodes. The first episode (∼89-84 Ma) involved ∼33 km of shortening on the Crawford thrust and its footwall imbricates. Approximately 19 km of structural relief on the basement-cover contact developed in the area of the Wasatch culmination. The second episode (∼84-62 Ma, with a break between ∼75-69 Ma) involved ∼30 km of shortening, mainly on the Absaroka thrust, and development of an additional ∼6 km of structural relief in the culmination. The third episode of shortening (∼56-50 Ma) took place on the Hogsback thrust, involved ∼21 km of horizontal shortening, and produced no significant increase in structural relief in the culmination. Long-term rates of shortening ranged between 3.0 mm/yr and 6.6 mm/yr. These three episodes of shortening produced three large accumulations of synorogenic conglomerate, totaling ∼3 km in thickness. The Henefer Formation and Echo Canyon and Weber Canyon Conglomerates were deposited during Crawford thrusting. The Evanston Formation was deposited during and after Absaroka thrusting, and the lower conglomeratic part of the Wasatch Formation was deposited during and after Hogsback thrusting. Most of the sediment in these synorogenic units, however, was derived from repeated uplift of the Willard thrust sheet and from the eastern flank of the Wasatch culmination in the rear part of the thrust belt. Only local, minor accumulations were derived from the frontal ramp anticlines. Sediment accumulation and structural deformation were generally out-of-phase. Periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin. Periods of structural inactivity were marked by accumulation of aerially widespread, braided-river conglomerate on top of the thrust belt. One exception to this pattern is the Henefer-Echo Canyon-Weber Canyon conglomerate deposit, which contains evidence of progressive deformation in close proximity to the tip of the Crawford thrust. Comparison of the sequential restoration with the Late Cretaceous subsidence history and isopach patterns in the distal foreland basin of western Wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the Wasatch culmination. Growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge.

Extensional reactivation of thrust faults accompanied by coseismic surface rupture, southwestern Wyoming and north-central Utah

Extensional reactivation of thrust faults north of the Uinta Mountains in Utah and Wyoming is accompanied by Quaternary surface faulting, tilting, and monoclinal folding. The 40-km-long Bear River fault zone consists of down-to- the-west, right-stepping, en echelon scarps, each about 3.0 to 3.5 km in length, striking N20°W to N20°E. Scarp-derived colluvial deposits record two Holocene surface ruptures with net vertical tectonic displacements ranging from 5 m per event (West, 1989, in press). Radiocarbon ages indicate surface faulting events occurred at about 4.6 ka and 2.4 ka. The length of surface rupture and net slip per event in the Bear River fault zone imply a seismic potential comparable to the Lost River fault, Hebgen Lake fault, and the Wasatch fault zone. A late Quaternary normal scarp is also coincident with the leading edge of the Absaroka thrust fault 7.2 km west of the Bear River fault zone. Scarp-derived colluvial deposits record one surface faulting event over a length of at least 5.0 km. The age of surface rupture is coeval with the Bear River fault zone, ∼2.4 ka. Normal faults displacing Pleistocene surfaces/deposits and tectonic tilt indicate that the Hogsback thrust east of the Bear River fault zone was reactivated in a normal sense but now may be inactive due to subsequent development of the Bear River fault zone. Cumulative normal throw of 200± m on the Hogsback fault may be responsible for postulated tectonic separation of the Bear River and Green River drainage basins Neotectonic deformation results from east-west extension superposed on the Hogsback and Absaroka thrust plates. Extensional reactivation of thrust faults caused propagation of new normal faults over stress points, particularly at the Hogsback ramp to flat transition, and at the thrust leading edges. The ages of surface rupture and cumulative throws indicate normal reactivation of the Hogsback and Absaroka thrusts, and propagation of the intervening Bear River fault zone proceeded from east to west opposite initial development of the thrusts in Late Cretaceous to Paleocene time. Normally reactivated thrust faults and propagation of listric normal faults accompanied by surface rupture have significant implications for the tectonic/seismogenic development of the eastern Basin and Range transition zone.

Whitney Canyon-Carter Creek Field: Gas Production from Carbonate Reservoirs in a Thrust Belt Structural Setting, Western Wyoming, U.S.A.

Located in the Fossil basin area of the Wyoming thrust belt, giant Whitney Canyon-Carter Creek field has in place reserves of approximately 4.5 tcf of gas, 125 MMBO (condensate), and 24 million long tons sulfur. It is the largest gas field in the U.S. Rocky Mountains. Hydrocarbons are trapped in large, reverse faulted anticlinal closures that formed completely within the Absaroka thrust plate during Laramide deformation. These structures are ramp anticlines that developed when the Absaroka plate was thrust eastward over ramps in the underlying fault plane. Production is sour natural gas and condensate mainly from Paleozoic reservoirs. The most significant are dolomitized carbonate reservoirs of the Mississippian Mission Canyon and Lodgepole formations and the Ordovician Big Horn Dolomite. The Pennsylvanian Weber Sandstone and the Triassic Thaynes Formation have minor production. Source rocks are subthrust Cretaceous shales that were placed in the oil generation window after thrusting and subsidence. The economically most important reservoir is the Mission Canyon Formation with 79% of the total gas in place. Intercrystalline and moldic porosity was created by dolomitization and subsequent partial solution of mud-supported sediments during early diagenesis. Structural deformation fractured the reservoir, but also created a diagenetic environment that allowed calcite, anhydrite,more » and dolomite cements to sporadically plug all porosity types. Discovery of Whitney Canyon-Carter Creek field began by identifying a large, potentially productive area within the Fossil basin.« less

Reservoir Characterization and Geostatistical Modeling of an Eolian Reservoir for Simulation, East Painter Reservoir Field, Wyoming

The East Painter structure is a doubly plunging, asymmetric anticline formed on the hanging wall of a back-thrust imbricate near the leading edge of the Absaroka Thrust. The Jurassic Nugget Sandstone is the productive horizon in the East Painter structure. The approximately 900-ft-thick Nugget is a stratigraphically complex and heterogeneous unit deposited by eolian processes in a complex erg setting. The high degree of heterogeneity iwthin the Nugget results from variations in grain size, sorting, mineralogy, and degree and distribution of lamination. The Nugget is comprised of dune, transitional toeset, and interdune facies, each exhibiting different porosity and permeability distributions. Gacies architecture results in both vertical and horizontal stratification of the reservoir. Adequate representation of reservoir heterogeneity is the key to successful modeling of past and future production performance. In addition, a detailed geologic model, based on depositional environment, must be integrated into the simulation to ensure realistic results. Geostatistics provide a method for modeling the spatial reservoir property distirbution while honoring all data values at their sample location. Conditional simulation is a geostatistical technique that generates several equally probably realizations that observe the data and spatial constraints imposed upon them while including fractal variability. Flow simulations of multiple reservoirmore » realizations can provide a probability distribution of reservoir performance that can be used to evaluate risk associated with a project caused by the imcomplete sampling of the reservoir property distribution.« less

Thrust-Generated Fan-Delta Deposition: Little Muddy Creek Conglomerate, SW Wyoming

ABSTRACT The upper Cretaceous Little Muddy Creek conglomerate in southwestern Wyoming is an often-cited example of a thin-skinned, thrust-derived conglomerate that has been used to date absolute and relative timing of thrust sheet motion in the Idaho-Wyoming-Utah fold and thrust belt. Despite the regional tectonic significance of the Little Muddy Creek conglomerate, no quantitative sedimentological or compositional analyses are available in the literature. Sedimentological analysis indicates that the Little Muddy Creek conglomerate was produced by forelandward (eastward) progradation of a marine fan-delta that emanated from the thrust ridge above the old Absaroka thrust. Fan-delta deposits are overlain by nearshore, inner-shelf, outer-shelf and slope deposits which record sedimentation as motion on the old Absaroka thrust began to wane. A two-dimensional provenance modeling technique described herein compares hypothetical, synorogenic-sediment compositions with actual compositional data from the Little Muddy Creek conglomerate. The two-dimensional modeling is a refinement of previous, one-dimensional models in that it is based upon the reconstruction of syn-thrusting hanging wall and footwall geometries. The provenance mod ling shows that compositional data from the Little Muddy Creek conglomerate record uplift followed by gradual cessation thereof on the old Absaroka thrust. Sedimentological and compositional interpretations are integrated, allowing for an evaluation of the tectonic controls on deposition of this thin-skinned, thrust-derived conglomerate.

Post-Eocene West-Stepping Thrust Faulting in the Utah-Wyoming Thrust Belt: Implications for the Genesis of the Cretaceous Interior Seaway: ABSTRACT

Field relations suggest that thrust faulting began in western Wyoming and eastern Utah with movement on the Absaroka thrust, possibly during the Late Cretaceous. It seems likely that the Darby thrust became active shortly thereafter. Intraplate deformation of the Absaroka package occurred during or after the Eocene, resulting in belts of folds and faults along the Commissary, Tunp, and Crawford trends. This Absaroka intraplate deformation resulted from compression produced by movement on the Ogden thrust to the west. The Ogden thrust is exposed along the Wasatch fault between Ogden and Salt Lake City, Utah and extends as far east as the Devil's Slide area, west of Croyden Utah. This thrust has not previously been recognized. The Precambrian crystalline rocks of the Wasatch Mountains are carried on the Ogden thrust along with Paleozoic and Mesozoic sedimentary rocks that crop out east of the range. The Willard thrust moved after the Ogden and initiated still farther to the west. The Willard includes Proterozoic and lower Paleozoic rocks not recognized on any of the more easterly thrust sheets. The unique occurrence of oil and gas within the Absaroka package is related to many factors including its initial geometry and the remigration of fluidsmore » associated with the post-Eocene modification of this geometry. The source of pre-Maestrichtian sediments to the Cretaceous Interior Seaway becomes problematic as a result of this analysis, as does the assumption that the seaway was a foredeep related to flexural loading by the thrust belt.« less

Isotopic Exchange During Tectonic Veining: Example from Absaroka Sheet in Wyoming Overthrust Belt: ABSTRACT

Multiple stages of tectonic veining are preserved in the Mississippian Madison Group within the Absaroka thrust sheet. Three groups of veins have been defined on the basis of structural relationships and stable isotopic character. Group I veins consist of mineralized stylolite and joint surfaces which are oriented normal to both bedding and tectonic transport. Group II veins are filled fractures oriented normal to bedding and parallel to transport. Group III veins occur only in thin bedded limestones of the lower Madison Group and are mineralized bedding-parallel slip surfaces. Calcite twinning strains from each group indicate that, with rare exceptions, all veins formed pretectonically or syntectonically.

Thermal History of Utah-Wyoming Thrust Belt: Implications for Hydrocarbon Maturation: ABSTRACT

During the last three years, the authors have been engaged in collecting and analyzing thermal data from the Pineview, Anschutz Ranch, Cave Creek, and Anschutz Ranch East oil and gas fields in the Utah Wyoming thrust belt. These data include more than 200 corrected bottom-hole temperatures, 660 thermal-conductivity measurements on 15 geologic formations, and 10 vitrinite-reflectance measurements. Preliminary modeling results indicate that a constant thermal state in this part of the thrust belt is incompatible with vitrinite-reflectance data and that near-surface heat flow at the time of major movement on the Absaroka thrust (about 70 m.y.a.) was approximately 40 mW/m/sup 2/, 30% lower than present day. A model involving warming beginning 45 m.y.a. is consistent with vitrinite-reflectance data, present-day heat flow, and geologic and tectonic constraints. This thermal model is used to calculate the temperature history of Cretaceous source rocks, which, in turn, is used as input to a kinetic model of oil and gas generation. Oil and gas generation is predicted to occur in two stages. The first stage is initiated at time of thrusting and lasts from 1 to 5 m.y. Oil-generation rates at the Pineview field are between 1 and 10% converted kerogen per m.y. during thismore » time. The second phase of hydrocarbon generation is associated with Tertiary warming and is characterized by oil-generation rates on the order of 1% converted kerogen per m.y. for the last 30 m.y. These results depend on the validity of empirical models that relate vitrinite reflectance to temperature and time. They are also sensitive to uncertainties in thrusting kinematics, thickness of eroded overburden, ancient surface temperature, thermal properties of eroded formations, and local variations in temperature due to heat refraction.« less

Presence and Control of Evaporite Top Seals on Occurrence and Distribution of Hydrocarbon Traps: Main Fairway, Central Overthrust Belt, Wyoming and Utah: ABSTRACT

In the Central Overthrust belt fairway, there are 18 known productive formations ranging from Cambrian to Cretaceous in age. Four units - the Ordovician Big Horn formation, Mississippian Mission Canyon formation, Jurassic Nugget formation, and Jurassic Twin Creek Formation - account for 99% of a reported 1.4 billion bbl of oil estimated recoverable reserves discovered to date. A critical factor necessary for significant hydrocarbon accumulations is the presence of effective evaporite seals above these reservoirs. Major accumulations exist in anticlines associated with Paleozoic or Jurassic cutoffs in the hanging wall of the Absaroka thrust with recognized source rocks occurring in the Cretaceous footwall section. Vertical migration is required to source the overlying reservoirs. Migration pathways are via hanging wall cutoffs of these reservoirs or through fractures in the cores of the structures. Hanging wall reservoirs either lacking top seals or having top seals disrupted by extensive fracturing due to folding or faulting, permit vertical migration into higher, more effectively sealed reservoirs. In the Overthrust producing fairway, anhydrites of the Mission Canyon formation and Gypsum Spring Member of the Twin Creek formation as well as the Jurassic Preuss salt are the three effective seal units. Analysis of Overthrust belt traps suggestsmore » that the presence and ductile nature of these evaporite seals are important controlling factors for the significant hydrocarbon accumulations in this highly deformed terrain.« less

Siphon‐induced intermittent discharge

Periodic Spring discharges cold water from an unenterable cave in the Madison Limestone along the axis of the Periodic anticline in the Absaroka thrust sheet of the Salt River Range of western Wyoming. As shown on the cover, the spring “turns on and off with variable cyclicity during low discharge stages. The intermittent yield is not the result of hydrothermal activity; rather, it is caused by the mechanical operation of a siphon in the cave behind the spring.

Thrust fault geometry of the Snake River Range, Idaho and Wyoming

The Snake River Range is located at the north end of the Absaroka fault system in the Idaho-Wyoming-Utah thrust belt, and it adjoins the Teton Mountains. Nine imbricate sheets of the Absaroka system form a shingled array of overlapping thrusts. Most fault nomenclature in the range dates from early, discontinuous, reconnaissance studies wherein names were applied from the top thrust down whether or not the thrust sheets separated by this method are, or ever were, connected as mechanical blocks. Fault names are revised here to consider the lateral continuity of sheets as the major criteria for defining the block to which a name should be applied. The Absaroka thrust and St. John thrust are recognized to be equivalent different parts of a major transfer zone, using the continuity of sheets as the main criteria. The map pattern of the range is dominated by major re-entrants to the west in the traces of the St. John and Absaroka thrusts. The re-entrants in both major faults are caused by folding in their foot walls related to lateral ramping of each thrust upward to the southeast. The folding of the St. John thrust above the Indian Creek Culmination also folds the thrust sheets above it, helping to mark the emplacement of the imbricate sheets as being from top to bottom and west to east. The Absaroka system thrusts formed from 20 to 40 km west of their present position. They are folded by underlying faults of the Jackson or, farther south, the Darby system, indicating that the Absaroka system was emplaced into its present position by motion on these underlying faults. Discussions concerning mechanical interaction between the thrust belt and a foreland buttress (ancestral Teton or Targhee uplift) concentrate on intensities of deformation and changes in geometry adjacent to the proposed buttress. The individual imbricate geometries and regional changes in trend are more related to a changing stratigraphic package and the lateral thrust ramps than to geographic proximity to any proposed buttress.