Wyoming Foreland Research Articles

Paleogeographic reconstruction of the Eocene Idaho River, North American Cordillera

Eocene Lake Gosiute in southwestern Wyo- ming was progressively fi lled in by volcani- clastic sediment between 49.6 and 47.0 Ma. The source of this material has long been thought to have been the Absaroka volcanic province, immediately north of the greater Green River Basin. Lead isotope compositions of sandstone from this interval, however, are consistent with derivation from the Challis volcanic fi eld. The 40 Ar/ 39 Ar ages of single de- trital K-feldspar crystals from greater Green River Basin sandstones are nearly identical to 40 Ar/ 39 Ar eruptive ages for volcanic rocks of the Challis volcanic fi eld (49.8-45.5 Ma), but we also identify Mesozoic and Proterozoic crystals that are consistent with cooling ages for rocks that were likely exposed in the Idaho segment of the North American Cordillera during the Eocene. Most of these rocks are traversed by or are up-gradient of a major Eocene paleovalley in central Idaho. The sud- den appearance of Challis-derived sediment in the greater Green River Basin indicates that a major river, here named the Idaho River, connected the interior of the North American Cordillera to the greater Green River Basin. This connection requires 500 km to reach from central Idaho to the greater Green River Ba- sin. The Idaho River probably carried detritus that was stripped from distant uplifted moun- tains above the active Challis volcanic fi eld as far south as the Piceance Creek Basin, suggest- ing a total length of at least 1000 km. Middle Eocene metamorphic core complexes in the northern Cordillera likely produced a major highland in the headwaters of the Idaho River, which generated river systems that drained both eastward into the Wyoming foreland and westward into the Oregon Coast ranges.

Montana transform: A tectonic cam surface linking thin- and thick-skinned Laramide shortening across the Rocky Mountain foreland

The Montana transform is here defined as the northern boundary of the Wyoming Laramide foreland. This sinistral-transpressional shear zone followed a crustal-scale structure that defined the southern margin of the Mesoproterozoic Belt basin of western Montana. The structural zone transferred clockwise tectonic rotation of the thick-skinned Wyoming foreland to clockwise rotation of thin-skinned thrust plates of the Montana thrust belt. Traction above the Farallon plate, which rotated northeastward beneath the Wyoming foreland, may have driven the rotation of the basement-involved Laramide ranges. The ancestral crustal structure that controlled the transform did not coincide exactly with a small circle of rotation about the Euler pole for the Wyoming foreland. Instead, the transform rotated eccentrically, like a cam surface, and generated sufficient sinistral transpression against the Belt Supergroup to thrust it out of its basin.

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.

Evolution of the Wamsutter Arch, Wyoming Foreland, USA: Influence on Formation of Stratigraphic Traps in Upper Cretaceous Almond Shoreline Sandstones: ABSTRACT

Evolution of the Wamsutter Arch, Wyoming Foreland, USA: Influence on Formation of Stratigraphic Traps in Upper Cretaceous Almond Shoreline Sandstones: ABSTRACT

Intrabasinal tectonic control on fluvial sandstone bodies in the Cloverly Formation (Early Cretaceous), west‐central Wyoming, USA

AbstractTemporal and spatial changes in alluvial sand‐body geometry and palaeodispersal patterns of the lower part of the Cloverly Formation (Early Cretaceous) in west‐central Wyoming suggest differential uplift within the developing medial to distal foreland basin and existence of subtle structurally controlled topography nearly 60 Myr before classic Laramide uplift in the Wyoming foreland province. This intraforeland structural topography exerted a significant control on dispersal and deposition of widespread gravels of Neocomian‐Aptian age.Surface studies have established the presence of two conglomeratic sandstones in the Cloverly Formation in the western Wind River Basin. Both conglomeratic sandstones are chert arenites, but the lower conglomeratic sandstone is characterized by dark chert pebbles derived from western and southwestern extrabasinal sources and was deposited by braided rivers, whereas the upper conglomeratic sandstone is characterized by intraformational clasts derived locally from adjacent floodplains and deposited by NE‐flowing rivers of moderate sinuosity which carried a higher proportion of suspended load in stable bank‐confined channels.Surface and subsurface mapping of sandstone‐body geometry in the lower dark chert‐pebble conglomeratic sandstone, together with palaeocurrent analysis of trough cross‐bedding, reveal a major NE‐flowing trunk river system in the area. Lithological correlation of surface sections to adjacent well logs, together with regional format correlation of fine‐grained intervals in well logs up to 120 km E of the outcrop belt, enable detailed mapping of the trunk system. The trunk system was approximately 5–10 km wide, and may have been confined by subtle structural topography developed by recurrent differential uplift along NE‐trending fractures in Archaean basement rocks. The fractures are along strike with diabase dike swarms and faults mapped in Archaean basement rocks of the Laramide Wind River uplift. The lower chert‐pebble sandstone is absent for a lateral distance of at least 30 km between these lineaments, including an area of at least 1000 km2, indicating the existence of a low NE‐SW‐trending interfluve perpendicular to the axis of the modern Wind River Range.Early Cretaceous development of structural topography and partitioning of the medial to distal foreland basin of west‐central Wyoming were most likely controlled by tectonic reactivation and differential uplift along fractures in Archaean basement rocks in response to early thrust loading and intraplate stresses during initial subsidence of the foreland basin.

Structural Interpretation of the Horse Center Anticline, Western Margin of the Bighorn Basin, Wyoming

Horse Center anticline is an asymmetrical structure on the western flank of the Bighorn basin. This type of basin-flank anticline has created structural traps for hydrocarbons throughout the foreland region. While no commercial hydrocarbons have been discovered at Horse Center, significant shows have been found in the Paleozoic section. Cretaceous and Jurassic sands and shales are exposed on the flanks of the structure, with the red beds of the Triassic Chugwater exposed in the core. Seismic data indicate that the Precambrian basement forcing block moves along a high-angle reverse fault. Resultant compressional folding of the overlying sedimentary section creates volumetric adjustments along detachment zones in less competent units. One such feature, a cross-crestal structure, can be observed at the surface, where bedding plane slip in Jurassic shales offsets the southern crest of the anticline. Although the fold axis trends northwest-southeast, typical of many Wyoming foreland structures, there is a distinct shift in vergence from southwest to northeast along the axis of Horse Center. Detailed analysis of well data and seismic data also confirms this change in asymmetry. This change may indicate the presence of a 'compartmental' fault, localized over a zone of weakness in the Precambrian basement. Surface mapping indicatesmore » that north of this change, there is an anticlinal/synclinal structural development on the western side of Horse Center. These paired structural suggest a possible 'plunging out' of anticlines, reflecting an en echelon arrangement of structures at depth, rather than a continuity along the axis of Horse Center, as has been previously mapped.« less

Interior ramp-supported uplifts: Implications for sediment provenance in foreland basins

The composition of syntectonic conglomerates in foreland-basin sequences is strongly controlled by the lithology of strata exposed to erosion in uplifted thrust plates. Consideration of the styles and mechanisms of uplift in thrust belts, combined with knowledge of sediment provenance and dispersal in eroding thrusted terranes, leads to the concept of interior ramp-supported uplift as a potential mechanism for generating syntectonic conglomerates in foreland basins. Interior ramp-supported uplift occurs when an older, inactive thrust plate that is part of the upper plate of a younger, active thrust is transported over a ramp in the younger thrust. Uplift and folding of the inactive thrust plate re-establish the older thrusted terrane as a sediment source area. This mechanism is utilized to account for the deposition of a thick accumulation of quartzite-clast conglomerate (Harebell and Pinyon Formations) in the Sevier foreland basin of northwestern Wyoming. Other hypotheses of uplift-erosion-deposition suggested to explain the origin of these deposits include (1) basement-involved uplift, (2) progressive clast recycling through multiple episodes of fluvial transport during uplift on successive thrust plates, and (3) direct transport from the toe of active thrusts. Although these mechanisms all undoubtedly operate in eroding thrusted terranes, each has limitations to its potential validity as source for quartzite debris in the northwestern Wyoming foreland basin. The concept of interior ramp-supported uplift alleviates many of the inconsistencies of other models and is compatible with observations elsewhere in the Cordilleran thrust belt. The interior uplift model also implies that uplift of an older, inactive thrusted terrane over ramps of active thrusts may be the primary factor in the development of topographic relief in fold-thrust belts.

Post-Laramide (Oligocene) uplift in the Wind River Range, Wyoming

In late Oligocene time, the crest of the Wind River Range was uplifted from an erosion surface that formed during Laramide (Late Cretaceous-early Eocene) deformation. This uplift created the highest peaks in the Wyoming foreland and is indicated by rejuvenation of the range as a sediment source and by the physiography of the peaks. Differential movement of basement blocks in the core of the range is identified by apatite fission-track data. We distinguish this deformation from earlier Laramide compression and Neogene normal faulting and speculate that it occurred when Precambrian shear zones were reactivated in response to local flexural stress in the Laramide subsidence-uplift couple.

Laramide Basin Subsidence and Basement Uplift in Rocky Mountain Foreland of Wyoming: ABSTRACT

The basement-cored ranges of the Rocky Mountain foreland have attracted geologists' attention since the time of early exploration of the western United States and have been the subject of numerous structural studies since early in this century. Until the advent of hydrocarbon exploration, however, the basins got little attention. Even today, our knowledge of the uplifts far exceeds that concerning basin genesis. The most recent work shows that uplift of the ranges and subsidence of the basins are intimately related and suggests that viewing the subsidence-uplift couple as the unit of deformation in the foreland is most likely to lead to a better understanding of the timing and kinematics of the Laramide orogeny. The Wind River Range in western Wyoming is an excellent natural laboratory for studying a Laramide uplift. A COCORP seismic profile provides geometric control, and tectogenic sediments record the history of uplift and erosion. The stratigraphy and provenance of these sediments indicate a complex Laramide and later tectonic history for the range and identify the timing and position of individual faulting events. These events are (1) main uplift of the range by motion on the Wind River fault and the formation of an erosion surface of low relief (Late Cretaceous through early Eocene), (2) elevation of this erosion surface as much as 3,000 ft (914 m) by motion on imbricates and associated tear faults in the hanging wall of the Wind River fault (end of early Eocene), (3) collapse of the ti of the Wind River fault into sedimentary fill of the Green River basin (between middle Eocene and late Oligocene), (4) uplift of the crest of the range by nearly 3,000 ft (914 m) forming the highest peaks in the Wyoming foreland (late Oligocene), and (5) collapse of the southern part of the range along normal faults (Neogene). Basin modeling in two distinctly different structural settings points to several driving mechanisms for subsidence in Laramide basins. Subsidence of the northern Green River basin was a flexural response to sediment loading and the intracrustal and topographic loads imposed by uplift of the adjacent Wind River Range. In contrast, the Hanna basin subsided when a rigid crustal block rotated downward as the Rawlins uplift was raised on the other end. Both flexure and rigid block rotation likely are operative to varying degrees in most Laramide basins. A schematic cross section through central Wyoming suggests that deep basins, where both rotation and tectonic loading are important, support structurally low ranges. Where rotation is not an important component of subsidence, basins support structurally high ranges. Furthermore, because basin subsidence and basement uplift are genetically linked, both indicate the timing of Laramide deformation. End_of_Article - Last_Page 1523------------

Rejection of Recharge Water from Madison Aquifer Along Eastern Perimeter of Bighorn Artesian Basin, Wyoming

ABSTRACTApproximately half of the perimeters of Wyoming foreland artesian basins are characterized by structural continuity of the Paleozoic aquifers between the recharge areas and the interior parts of the artesian basins. A significant percentage of the ground water which circulates through such recharge areas is rejected through springs because there is a marked decrease in transmissivity basin‐ward from the recharge areas. The transmissivity contrasts have developed since the recharge areas became morphologically differentiated from the basin interiors during post‐Laramide time. Secondary enhancement of permeabilities is occurring in the recharge areas through dissolution of the rock matrix and cement, and in some locations the rocks were tectonically fractured. In contrast, the rates of dissolution within the basin interiors are substantially decreased, or processes of recrystallization, cementation, or compaction are operating to destroy permeability. The result is that in the case treated here, net recharge to the basin interior is a small fraction of measured stream losses in the headwaters of the recharge area.

Fault Severed Aquifers Along the Perimeters of Wyoming Artesian Basins

ABSTRACTThe mountain uplifts which border the major artesian basins of Wyoming are asymmetric antiforms bounded on one flank by large displacement Laramide thrust faults. These thrusts sever the hydraulic continuity of aquifers, thereby creating separate circulation systems in the hanging wall and foot wall blocks. Fault severing of the Paleozoic aquifers can be identified by (1) potentiometric discontinuities across the faults, (2) water quality contrasts across the faults, and (3) geothermally heated waters in the foot wall blocks.Isolated but active circulation systems develop in the hanging wall blocks in which good permeabilities and good quality water prevail. In contrast the foot walls are generally characterized by poor permeabilities and poor water qualities.Fault severing of the Paleozoic aquifers in the Wyoming foreland province is a significant factor because nearly half of the basin perimeters are fault severed. The result is that exploration for large‐volume, good‐quality supplies must focus on the hanging wall blocks in the fault severed environments.

Stratigraphy and Depositional Environment of Upper Mississippian Big Snowy Group, Bridger Range, Montana: ABSTRACT

Isopachs and lithofacies of the Big Snowy Group in the Bridger Range of southwestern Montana reflect subtle shifting of tectonic elements along an ancient structural lineament. The group comprises a transgressive series informally divided into the Kibbey Formation (with two members) and the Lombard facies, which is equivalent to Otter and Heath Formations of the Big Snowy Group farther east in central Montana. The lower Kibbey member was deposited in a sabkha environment and is composed of algal laminated dolostone with desiccation features and evaporite solution breccias deposited at the leading edge of the transgressing sea. Siliciclastic intertidal channels that developed on the sabkha were restricted to a low-lying area that developed in the central part of the range. The upper Kibbey includes a regressive shoreface deposit composed of sandstone at the northern and southern ends of the range in contrast to mudstone and siltstone that dominate in the center of the range where deeper water and lower energy conditions prevailed. Ultimately, the Kibbey sabkha and shoreface transgressed out of the area leaving a partially restricted shelf lagoon. Shale and lime mudstone of the Lombard facies were deposited in quiet water at the center of the range. To the north and south, bioclastic wackestone, packstone, and grainstone were deposited under shoaling, higher energy conditions within the lagoon. All three units of the Big Snowy are thickest where deeper water lithofacies occur, indicating that subsidence was greatest in the central part of the Bridger Range and that this subsidence was an important factor on the control of lithofacies distribution. Similar tectonic influence on sedimentation in this area is evident in other Paleozoic formations. The depositional changes in the central part of the range are coincident with the southern margin of the Precambrian Belt embayment along the Ross Pass fault zone, and seem to indicate a zone of deep crustal weakness along the transition from the fold and thrust belt to the Wyoming foreland deformation province. End_of_Article - Last_Page 850------------

Tectonic loading and subsidence of intermontane basins: Wyoming foreland province

Research Article| August 01, 1985 Tectonic loading and subsidence of intermontane basins: Wyoming foreland province E. Sven Hagen; E. Sven Hagen 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 Search for other works by this author on: GSW Google Scholar Mark W. Shuster; Mark W. Shuster 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 Search for other works by this author on: GSW Google Scholar Kevin P. Furlong Kevin P. Furlong 2Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 Search for other works by this author on: GSW Google Scholar Author and Article Information E. Sven Hagen 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 Mark W. Shuster 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 Kevin P. Furlong 2Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1985) 13 (8): 585–588. https://doi.org/10.1130/0091-7613(1985)13<585:TLASOI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. Sven Hagen, Mark W. Shuster, Kevin P. Furlong; Tectonic loading and subsidence of intermontane basins: Wyoming foreland province. Geology 1985;; 13 (8): 585–588. doi: https://doi.org/10.1130/0091-7613(1985)13<585:TLASOI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Results of two-dimensional flexural modeling of the northern Bighorn and northern Green River basins in the Wyoming foreland province suggest that these basins formed as flexures in response to loading by basin-margin uplifts and basin sedimentary sequences. The northern Bighorn Basin subsided due to loading by the Beartooth uplift along its western margin. The northern Green River Basin developed as a result of concurrent loading by the Wyoming thrust belt to the west and the Wind River uplift to the east. Tectonic loading from basement-involved uplifts played a major role in subsidence and sedimentation, as evidenced by isopach patterns within each basin. Lithospheric flexural rigidities of 1021 to 1022 newton metres (N·m) can adequately explain subsidence in both basins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geometry of Wind River Thrust, Wyoming: ABSTRACT

A thick wedge of Precambrian crystalline rocks of the Wind River Mountains was thrust far across sediments of the Green River basin during the Laramide orogeny. Dip of the thrust zone decreases with depth to form a broadly curved, convex-downward surface. Seismic data also show strong local curvatures that produce a scalloped pattern in the leading edge of the thrust wedge and divide the wedge into lobate segments that are about 8 mi (13 km) wide along strike. The explanation for this scalloped pattern of folding may be lengthening of the unconfined wedge margin by extension during thrusting. Curvature of the thrust surface confirms that deformation of Precambrian rocks occurred by a foldlike mechanism. Local curvatures can also explain low apparent-dip angles of some oth r thrust faults in the Wyoming foreland. End_of_Article - Last_Page 932------------

Nacimiento Uplift and Its Similarity to Foreland Uplifts with Associated Production: ABSTRACT

The Nacimiento mountain front, east flank of the San Juan basin, is a well-documented foreland uplift. Based on detailed surface geology, its northern flank shows strong similarity to the Laramie Range and Owl Creek Mountain-Casper arch foreland uplifts of Wyoming. Petroleum has been discovered beneath these two uplifts by wells drilled through thrusted Precambrian rocks. Recent exploration for petroleum trapped beneath Rocky Mountain foreland uplifts has provided a wealth of geologic and geophysical data not previously available. These recently published data, both from wells and reflection seismic profiles, show surface geology integrated with subsurface geology. Northern Nacimiento uplift surface geology is so similar to these other well-documented foreland uplifts that subsurface anticlines and/or faulted closures are very probably similar to productive subsurface structures under Wyoming's foreland uplifts. Such structures, if present under Nacimiento foreland uplift, could contain significant quantities of hydrocarbons considering the prolific production from stratigraphic traps in the immediately adjacent San Juan basin. End_of_Article - Last_Page 945------------

Laramide Sedimentation and Tectonic Development Along Southern Margin of Wind River Range, Wyoming: ABSTRACT

Surface observations along the southern margin of the Wind River Range indicate that motion on the Wind River and Continental faults controlled depositional patterns and lithologic character of early Eocene syntectonic sediments. The stratigraphic sequence consists of alluvial plain, lake margin, deltaic, and alluvial fan units. Subsequent movement on the Wind River fault warped this sequence into a monocline 2 mi (3.2 km) long. This structure dies out to the northwest and southeast along the Wind River fault and is overlain by undeformed middle Eocene sediments. Clast composition and paleocurrent studies of post-early Eocene conglomerates indicate that there were additional sources for coarse clastics formed by subsequent uplift in the Precambrian core of the range. Tectonic implications of these interpretations are: (1) early motion on the Wind River fault controlled the margin of Eocene Lake Gosiute and generated a distal sediment source to the east; (2) last motion on the Wind River-Continental fault system was episodic and principally over by the end of early Eocene; (3) the Continental fault, now a post-Miocene collapse End_Page 531------------------------------ feature, was a tear fault with a vertical component up on the north; (4) the Wind River fault is a zone consisting of segments that moved separately; and (5) there are at least two unmapped fault zones along which the core of the Wind River Range was uplifted. These interpretations suggest that compression in the Wyoming foreland continued significantly later than early Eocene. An imbricate thrust model is proposed to accommodate these observations and interpretations. End_of_Article - Last_Page 532------------

Horizontal Compression and a Mechanical Interpretation of Wyoming Foreland Deformation: ABSTRACT

If the basement fault-controlled style of deformation in the Wyoming foreland is dominated by elastic response of the upper lithosphere, and the deformation in the foreland is genetically linked to the horizontal compression characteristic of the thin-skinned thrust belt to the west, then concepts of continuum mechanics can be combined with results of experimental rock mechanics to suggest the following. (1) Basement faults initiate at the basement surface, propagate downward at an approximate 35° dip, and die at a depth dependent upon the magnitude of elastic shortening. Displacement on these faults necessarily decreases with depth. The faults are not expected to be appreciably curved in cross section. (2) Foreland structures develop early as fault-cored folds of small amplitude (< 1,500 m, 4,900 ft), with selected ones developing to large amplitudes (up to 13,000 m, 43,000 ft). Regions where the entire lithosphere has not failed (early stage) show only small-scale structures (e.g., Colorado Plateau), whereas regions where the lithosphere has experienced through-going failure will show small intra-basinal structures (early) isolated by more widely spaced large basin-margin structures (late). This bimodal size distribution of structures is present in the Wyoming foreland. In this study, horizontal compression as a sole causal mechanism can be combined with accepted mechanical concepts to produce a plausible model which adequately explains the regional features of Wyoming foreland deformation. End_of_Article - Last_Page 1354------------

Timing of Deformation in Overthrust Belt and Foreland of Idaho, Wyoming, and Utah

A review of the timing and displacement evidence of the major structures of the western Wyoming Overthrust belt and foreland shows there is a progression in thrust displacement, apparent duration of motion, and palinspastic position of thrust traces from west to east. Those toward the west moved farther for an apparently longer period of time and are more widely spaced in their restored positions than those toward the east. However, average thrust velocities are all on the order of 0.5 ± 0.5 cm/yr (0.2 in./yr). Foreland events are in part synchronous with thrust belt events and had an effect on them. Although dating precision varies widely on major normal faults, present evidence does not contradict the generally held view that all normal faults postdate the youngest thrusting.

Investigation of the geomagnetic field polarity during the Jurassic

A large number of sedimentary rock formations on the Colorado Plateau and in the Wyoming foreland have been investigated paleomagnetically in an attempt to determine the geomagnetic polarity during the Middle and the early Late Jurassic, a period frequently postulated to have been of constant normal polarity. A survey of published paleomagnetic litature shows this postulate to be without grounds. A wide range of sedimentary lithologies, both red and nonred, were investigated. Most formations displayed a complex, multivectorial magnetization, even to very high stability ranges. However, a strikingly large amount of reversed polarity was observed and relatively little normal Jurassic polarity. Because of the widespread geographic data base, the multitude of lithologies, and the amount of time represented, it is tentatively concluded that the data at least partially reflect the character of the geomagnetic field during the Jurassic. Weak intensities in all formations, and the instability, may support the marine magnetic anomaly hypothesis of weak geomagnetic field intensity for this time. These data do not support the hypothesis of a long period of normal polarity during the Jurassic. They in turn, suggest that the Jurassic field might have frequently been of reversed polarity.

Structural Problems Pertaining to Petroleum Exploration in Wyoming: ABSTRACT

Two structural provinces exist in Wyoming. These provinces are the overthrust belt of western Wyoming marginal to an earlier trough, and the mountain range and intermontane basin province over the old foreland. The tectonic provinces were outlined by Cambrian time, and reflected in the later depositional history. The attitude of the major fault planes within the overthrust belt is a major problem. The attitude near the toe of the thrust is known locally but at depth is hypothetical. Attitude of the thrustplanes will affect the amount of lateral displacement of rock masses in relation to their original sites and environments of deposition, and hence their potential as a source of petroleum. The amount and direction of displacement will affect the degree of cementation to be anticipated in reservoir rocks. Continuity of reservoir rocks and their hydrostatic systems have been interrupted by the thrust planes. The extent of available areas for free migration of hydrocarbons is dependent upon structural conditions. The foreland province of Wyoming covers a large area and contains the majority of the state's oil fields. Localization of the major tectonic features in the province has been controlled by several factors such as: pre-Laramide fracture systems, possibly as old as pre-Cambrian; heterogeneity in the crystalline basement; and Laramide flexures independent of inherited control. Lesser structural features localize as the result of adjustment of rocks to available space in the developing intermontane End_Page 948------------------------------ basins. Differentiation between different types of controlling influences may lead to recognition and prediction of a basic and systematic arrangement of structural features. It appears that the controlling factors varied with time. Faults in the foreland province are of several types. These include primary fractures in the basement complex which control the localization of folds of both major and minor dimensions, faults secondarily related to the growth of folds; faults developed due to local space accommodation; and rather extensive late fractures related to a period of regional tension. Problems arise in attempting to differentiate between systems at any single locality. The relative age of structural traps in Wyoming influences their effectiveness. Some problems related to time and rate of structural development are: nature and amount of the hydrostatic head in aquifers; time of effective closing of a trap; subsequent tilting of a trap; time of faulting relative to accumulation. Criteria for the age of folds should be investigated. Basic understanding of the mechanics of deformation will broaden the frontiers in exploration. Interpretation of new data is best made on the basis of an understanding of old data. Critical review of the following problems in structural geology is needed to expand our thinking. (1) Is there a curve which best fits the majority of cross sections of Wyoming foreland folds? (2) Is parallel folding too often assumed in constructing cross sections? (3) Further study of the minor faults and their patterns in folds, and (4) quantitative study of the kind and amount of fracturing in folds relative to the amount of potential reservoir void space developed. End_of_Article - Last_Page 949------------