Minor Thrust Faults Research Articles

The role of post-collisional strike-slip tectonics in the geological evolution of the late Neoproterozoic volcano-sedimentary Guaratubinha Basin, southern Brazil

The Guaratubinha Basin is a late Neoproterozoic volcano-sedimentary basin included in the transitional-stage basins of the South American Platform. The aim of this study is to investigate its tectonic evolution through a detailed structural analysis based on remote sensing and field data. The structural and aerogeophysics data indicate that at least three major deformational events affected the basin. Event E1 caused the activation of the two main basin-bounding fault zones, the Guaratubinha Master Fault and the Guaricana Shear Zone. These structures, oriented N20-45E, are associated with well-defined right-lateral to oblique vertical faults, conjugate normal faults and vertical flow structures. Progressive transtensional deformation along the two main fault systems was the main mechanism for basin formation and the deposition of thick coarse-grained deposits close to basin-borders. The continuous opening of the basin provided intense intermediate and acid magmatism as well as deposition of volcaniclastic sediments. Event E2 characterizes generalized compression, recorded as minor thrust faults with tectonic transport toward the northwest and left-lateral activation of the NNE-SSW Palmital Shear Zone. Event E3 is related to the Mesozoic tectonism associated with the South Atlantic opening, which generated diabase dykes and predominantly right-lateral strike-slip faults oriented N10-50W. Its rhomboidal geometry with long axis parallel to major Precambrian shear zones, the main presence of high-angle, strike-slip or oblique faults, the asymmetric distribution of geological units and field evidence for concomitant Neoproterozoic magmatism and strike-slip movements are consistent with pull-apart basins reported in the literature.

Heat flow, strong near-fault seismic waves, and near-fault tectonics on the central San Andreas Fault

The main San Andreas Fault strikes subparallel to compressional folds and thrust faults. Its fault-normal traction is on average a factor of γ=1+2μthr(1+μthr2+μthr), where μthr is the coefficient of friction for thrust faults, times the effective lithostatic pressure. A useful upper limit for μthr of 0.6 (where γ is 3.12) is obtained from the lack of heat flow anomalies by considering off-fault convergence at a rate of 1 mm/yr for 10 km across strike. If the fault-normal traction is in fact this high, the well-known heat flow constraint of average stresses of 10–20 MPa during strike slip on the main fault becomes more severe. Only a few percent of the total slip during earthquakes can occur at the peak stress before dynamic mechanisms weaken the fault. The spatial dimension of the high-stress rupture-tip zone is ∼10 m for γ = 3.12 and, for comparison, ∼100 m for γ = 1. High dynamic stresses during shaking occur within these distances of the fault plane. In terms of scalars, fine-scale tectonic stresses cannot exceed the difference between failure stress and dynamic stress. Plate-scale slip causes stresses to build up near geometrical irregularities of the fault plane. Strong dynamic stresses near the rupture tip facilitate anelastic deformation with the net effects of relaxing the local deviatoric tectonic stress and accommodating deformation around the irregularities. There also is a mild tendency for near-fault material to extrude upward. Slip on minor thrust faults causes the normal traction on the main fault to be spatially variable.

The northward tectonic transport in the southern Apennines: examples from the Capri Island and western Sorrento Peninsula (Italy)

We analyzed a thrust fault system located in the western Sorrento Peninsula and Capri Island (southern Italy) where several mesoscale structures related to the main thrusts, such as Riedel shear planes, overturned folds, minor thrust and back-thrust faults, suggest a dominant northward tectonic transport. Major and minor thrust faults, generally characterized by a ramp-flat geometry, involved the Mesozoic Apennine carbonates, the Middle Miocene foredeep, and the unconformable thrust-top basin deposits. The biostratigraphic analysis of calcareous nannoplankton assemblages on the thrust-top basin sediments indicates an age not older than late Tortonian. We propose that this out-of-sequence thrusting stage was related to a regional tectonic event widespread in the entire southern Apennines, probably occurred in the Pliocene time simultaneously with the activity of deep-seated thrust faults that involved the buried carbonates of the Apulian platform. These out-of-sequence thrust faults, here referred to as “envelopment thrusts,” were enucleated in a lower structural level with respect to the allochthonous wedge, representing the W–E segments of large regional arcuate structures.

Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (∼2.2–2.0 Ga), western Burkina Faso

We have investigated three greenstone belts (Boromo, Houndé, and Banfora) and associated granitoid terrains, which form part of the Eburnean orogen, situated in western Burkina Faso. These belts expose tholeiitic basalts (basal unit) followed by calc-alkaline intermediate predominantly effusive volcanic and sedimentary suites geochemically reminiscent of present-day volcanic island arc environments. The basal mafic unit probably corresponds to a juvenile arc crust or oceanic plateau. It contains unusual megacrystic tholeiitic basalts, allowing us to correlate the western margin of the Boromo belt with the eastern margin of the Houndé belt. These two N–S trending belt-parallel tholeiitic units are interpreted as limbs of a crustal scale anticline, intruded and partially obliterated by tonalite–trondhjemite–granodiorite (TTG) and granite intrusions. Three deformation events (D1–D3) can be distinguished in western Burkina Faso. The first deformation phase (D1) operated under an E–W to WNW-oriented compression. Regional greenschist to lower amphibolite facies metamorphism and intense folding characterize early-Eburnean deformation phases, during which time the crust was thickened by lateral shortening of volcanic island arcs and concomitant magma input. The crustal-scale antiform between the Boromo and Houndé belts is attributed to the D1 event. Shallow water detrital Tarkwaian-type sediments were deposited during the late D1 event within the Houndé belt, in a belt-parallel basin extending for 400 km. The subsequent D2 phase overprints the structural grain of the study area, and is best visible in airborne magnetic data. It is characterized by N to NE-trending transcurrent shear zones, which are considered preferred host structures for gold mineralization. We suggest that the newly formed and thickened crust reached the maximum thickness supportable by a weak and hot mantle during the D1 phase, and the pure shear dominated compressional regime switched to simple shear dominated transpression during the subsequent D2 phase. Granitoid diapirism played an important role at all stages of the Eburnean crustal growth processes in particular through early volume addition to the newly formed orogen and through later accommodating part of the lateral shortening. Pluton emplacement contributed to the greenstone belt structuration at local scales; however, the regional scale system geometry was controlled by coaxial shortening of the viscous volcanic units (basalts, gabbros, and andesites) of the greenstone belts, supported by coeval magma input. The last D3 deformation, which is either late-Eburnean or perhaps even Pan-African in age, is characterized by shallow N or S dipping minor thrust faults or an E-W trending steeply dipping spaced crenulation cleavage and kink folds, occurring mainly in highly anisotropic lithologies across the study region.

Devonian–Carboniferous paleogeography and orogenesis, northern Yukon and adjacent Arctic Alaska

Surface and subsurface data from northern Yukon document a northward facies transition from shelf carbonates to basinal graptolitic shales and cherts from Late Cambrian to Early Devonian time. Parts of this north-facing continental margin were deformed during separate orogenic events of Early Devonian and Early Carboniferous ages. The first event, the Romanzof Orogeny, is identified in exposures across northwestern Yukon, in adjacent northeastern Alaska, and locally in the subsurface of the Alaska North Slope. It resulted in tight folds, north-directed thrust faults, and intrusion by Late Devonian posttectonic granitic plutons. Notwithstanding the thrust-fault orientations, southward diminution of deformation intensity combined with facies variations suggest that tectonic transport was generally southward. Evidence for an Early Carboniferous event is preserved in the northern Richardson Mountains and locally in the subsurface of the Mackenzie Delta region. It consists of detached open folds and minor thrust faults. Geological and geophysical data from northern Yukon document the location and orientation of the Early Carboniferous deformation front, and define a regional tectonic transport direction toward the south or southeast. This event is a distal foreland element of the Ellesmerian Orogeny (sensu stricto) of the Canadian Arctic Islands and is distinct from the Romanzof event in age, intensity, and extent. Endicott and Lisburne group strata, deposited on a southwest-facing subsiding shelf, overstep rocks deformed by the Romanzof event even as Ellesmerian deformation encroached from the north.

Neogene structures overprinting Palaeozoic thrust systems in the Andean Precordillera at 30°S latitude

Field studies along a 30°S latitude transect of the Precordillera in NW Argentina indicate that Neogene to recent deformation caused high-angle faulting that uplifted and breached a Palaeozoic east-verging thrust stack. Palaeozoic structures crop out in the higher linear ranges, and intramontane Neogene basins occur in the intervening valleys. The Palaeozoic rocks include three tectonic units with contrasting structural styles and differences in stratigraphy. The Western Allochthon includes clastic Ordovician rocks metamorphosed to lower greenschist facies and folded by large-scale asymmetric, east-verging folds with a well-developed axial planar cleavage. This unit overthrusts a Siluro-Devonian flysch-type succession of the Central Imbricate thrust system and associated fault propagation folds in which the thrusts merge into a basal thrust beneath Ordovician carbonates. Middle to Upper Carboniferous sediments unconformably overlie structures of this imbricate system. In the Frontal Unit minor thrust faults are interpreted to explain the low-angle unconformity within Palaeozoic rocks. Neogene deformation causes only a minor overprint of the Palaeozoic structure, and large-scale folding in the Frontal Unit appears to be mostly of Neogene age. The new data indicate that Neogene intracontinental shortening may be smaller than previously proposed, providing new perspectives on the present orogenic architecture along this Andean section.

Gently dipping fracture zones in Paleoproterozoic metagranite, Sweden: Evidence from reflection seismic and cored borehole data and implications for the disposal of nuclear waste

The Svecokarelian orogen in the coastal area north of Stockholm, central Sweden, is characterized by a complex deformation belt. In this belt, bedrock domains strike WNW‐NW, dip steeply, and are affected by high ductile strain. These domains anastomose around tectonic lenses that are up to a few tens of kilometers in length and are a few kilometers wide. The bedrock within the lenses is folded and, in general, affected by lower ductile strain. Site investigations for the disposal of highly radioactive nuclear waste are presently in progress within one of these lenses. Approximately 16 km of high‐resolution (10 m source and receiver spacing, 100 channels) seismic data were acquired in 2002 along five separate profiles (2 to 5 km in length) within this lens. In the southeast, reflection orientations are well determined where profiles cross one another. Here, there is a nearly one to one correlation of distinct reflections on the seismic sections with hydraulically conductive fracture zones in cored boreholes, indicating that these fracture zones control the reflectivity. However, smaller lenses of amphibolite within the fracture zones may enhance the reflectivity. The reflections dip gently to the SSE and SE, and the zones in the boreholes are dominated by fractures that show the same dip direction or are subhorizontal. A working hypothesis for the gently dipping fracture zones suggests that these structures formed as minor thrust faults during the later stages of the Svecokarelian orogeny. Mineral fillings and coatings along fractures in these zones indicate later reactivation. The current stress field, a high intensity of fractures in the zones at a high angle to σ3, high in situ stresses in the bedrock, and rapid glacial unloading with the development of high differential stresses may explain the large volume of groundwater in these zones. The reflection seismic data, in combination with the verifying work presented in the cored borehole data, have steered the subsequent focus in geoscientific studies to the northwestern part of the candidate area.

The Carboneras Fault Zone (southeastern Spain) revisited with Ground Penetrating Radar – Quaternary structural styles from high-resolution images

AbstractThe Carboneras Fault Zone (CFZ) represents an active set of sinistral strike-slip faults in the Betic Cordilleras of southeastern Spain. It constitutes a major segment of the ‘Trans-Alboran shear zone’ during the Cenozoic, striking NE-SW. The CFZ separates the Cabo de Gata Block (Neogene volcanics) against Neogene basinal sediments and the metamorphic basement of the Alpujarride Complex.Three sites along the CFZ were examined with Ground Penetrating Radar techniques. Radar surveying was complemented by structural studies. Shallow-depth high-resolution imaging of Tyrrhenian beach terraces exhibited both vertical and minor horizontal offsets in the Rambla Morales site in the south. A sinistral strike-slip fault associated with minor thrust faults in a positive flower structure was detected in the middle segment along the La Serrata ridge, sealed by a caliche of late Pleistocene age (> 10 ka). The Playa de Bolmayor section yielded sub-surface evidence for several faults probably related to recent activity of individual fault strands. Our results suggest a distributed tectonic activity of the CFZ during the Late Quaternary.

Evidence for a strong San Andreas fault

Stress measurements in deep boreholes have universally shown that stresses in the Earth’s crust are in equilibrium with favorably oriented faults with friction coefficients in the range 0.6‐0.7 and with nearly hydrostatic pore-pressure gradients. Because of the lack of any fault-adjacent heat-flow anomaly as predicted by a conductive model of frictional heating, the San Andreas fault has long been thought to be an exception, i.e., far weaker than this standard case. Borehole stress measurements near the San Andreas fault have failed to confirm this weak-fault hypothesis, being either inconclusive or in conflict with it. Directions of maximum horizontal stresses reported to be nearly fault normal in central California are now known not to be regional stresses but a result of active folding within folds that have been rotated 20°‐30° clockwise from their original orientations. Everywhere in southern California it is observed that the maximum stress directions rotate to smaller angles (30°‐60°) with the San Andreas, within 20 km of it. The sense of this rotation is opposite to that expected from the weak-fault hypothesis and indicates that the shear stress on the San Andreas is comparable in magnitude to all other horizontal stresses in the system. In the “big bend” section of the fault, this rotation is predicted from a transpressional plate-boundary model in which the San Andreas is loaded by a deep shear zone with a locking depth of 10 km. If the adjacent minor thrust faults are assumed to obey Byerlee friction, the crustal-average shear stress on the San Andreas in that region must be in the range 100‐160 MPa, regardless of the pore pressure in the fault. These stresses are many times greater than permitted by the weak-fault hypothesis. In the more transcurrent regions farther south, the San Andreas shear stress will be smaller than this estimate, but similar stress rotations observed there indicate that the San Andreas cannot be weak relative to minor faults in that region. These stress rotations can only be consistent with the weakfault hypothesis if it were assumed that all faults in California were equally weak, which is known to be untrue. The conclusion is that the heat-flow model is flawed, probably in its assumption that all heat transfer is governed by conduction.

Shale tectonics and deformation associated with active diapirism: the Jerudong Anticline, Brunei Darussalam

The Jerudong anticline provides an outcrop example of multiple phases of diapir growth and interaction with the country rock. The N–S-trending anticline overlies a high-angled basement fault zone that was episodically active, moving predominantly sinistrally in response to transpressive stresses. The earliest stage of deformation in the study area was mid-Miocene E–W to NE–SW-trending growth faulting and shale-diapir growth. Early normal faults exerted an important influence on the large- and small-scale bedding geometries and facies changes of syn-tectonic shallow-marine, shoreface and tidal strata. In particular there are laterally rapid facies changes and differential rotation of strata across growth faults. Diapir activity is indicated by the presence of shale dykes which commonly intrude normal fault planes, and concordant shale intrusions. Shale intrusions, associated minor thrust faults and normal faults were rotated during a late Miocene–early Pliocene phase of folding related to transpression. Few shale dykes were intruded during this phase, probably because the horizontal principal stresses were relatively large compared with the pore-fluid pressure. At the end of the transpressive phase, shale dykes were intruded into steeply dipping beds as a result of stress relaxation. Continued uplift and erosion elevated overpressured horizons to a point where hydraulic fracturing reached the surface and Holocene-age mud volcanoes were developed.

Deformation history of a synorogenic sedimentary wedge, northern Cilento area, southern Apennines thrust and fold belt, Italy

In the northern Cilento area of the southern Italian Apennines, a deformed turbiditic succession of Langhian to lower Tortonian age (the Cilento unit) unconformably overlies tectonic units (Liguride and Sicilide) derived from an oceanic accretionary complex and, to the east, overlies Mesozoic–Cenozoic carbonates belonging to the Apulian continental paleomargin. The turbiditic succession, representing the sedimentary infill of a Miocene foreland basin developed over the flexured southwestern margin of the Apulian plate, shows a complete record of the deformation at the macroscopic and mesoscopic scales. Structures postdating synsedimentary (slump) features in the Cilento unit include Neptunian dikes and extensional veins perpendicular to bedding, all related to high pore-fluid pressures associated with burial. Early tectonic structures consist of minor thrust faults, often conjugate and at a low angle to bedding, produced by initial layer-parallel shortening. The main regional structure mapped in the area consists of a moderately inclined to recumbent (hinterland-vergent) backfold. This structure probably developed by shortening of the strata caught between shear surfaces that cut obliquely across bedding, within the framework of a noncoaxial progressive deformation. The main shear surface constituting the upper boundary to the shear zone is represented by a regional backthrust which brings Mesozoic-Cenozoic carbonates onto the terrains of the Cilento unit, forming a triangle zone where backfolding of the latter occurred. Minor open domes and basins deform both limbs of the main regional structure, probably as a result of progressive shear deformation and, possibly, of deformation above underlying thrust culminations. It appears that high pore-fluid pressures must have been important not only during the development of pretectonic (i.e., burial-related) structures, but also during the main tectonic deformation. Abnormally high fluid pressures, reducing the effective stress and favoring intergranular movements, suppressed cleavage formation and pervasive strain during the main folding event. Later in the deformation sequence, a decrease in pore-fluid pressure probably favored local cleavage development during refolding. The deformation sequence outlined above provides a case history of the structural evolution of synorogenic clastic deposits progressively deformed and incorporated within a thrust belt. This study, documenting the deformation of a sedimentary wedge dominated by backfolding associated with hinterland-vergent reimbrication, should contribute to a better understanding of the wide range of deformation sequences and processes characterizing the tectonic evolution of thrust-belt–foredeep systems.

STRUCTURAL AND TECTONIC SYNTHESIS FOR THE PERTH BASIN, WESTERN AUSTRALIA

The Perth Basin is localised by reactivation of Neoproterozoic shear zones on the western margin of the Archaean Yilgarn Craton in Western Australia. While Ordovician to Silurian sandstones were deposited in the northern Perth Basin, the earliest sediments elsewhere are Middle Carboniferous to Permian in age. A sinistral transtensional regime, during which the main architecture of the basin was established, developed during NE‐SW extension between Greater India and Western Australia in the Permo‐Triassic. NW‐SE shortening with continued NE‐SW extension resulted in sinistral transpression in the late‐Early to Middle Triassic. Sag‐phase sedimentation in the LateTriassic followed this oblique rifting event.An analogy may be made between the Perth Basin and the Permo‐Carboniferous to Jurassic Karoo basins in southern and central Africa and Madagascar. Deposition of the Karoo sequence took place within pull‐apart and transtensional basins resulting from sinistral reactivation of basement shear zones. The Indian Gondwana Supergroup, and an equivalent sequence in Antarctica, were deposited within normal fault‐bounded graben. The Late Paleozoic to Early Mesozoic formation of the Perth Basin, the Karoo basins of Africa and Madagascar, and the Gondwana basins of India was due to intraplate stress resulting from convergence along the Panthalassa margin of Gondwanaland. Late‐Early to Middle Triassic compressional events in all basins mark terminal collision along the Panthalassa margin.The latest Triassic to Early Jurassic marks a new rifting event due to regional east‐west to WNW‐ESE extension, producing dextralplus normal displacements on many NW‐to NNW‐striking structures, and essentially normal displacements on north‐ to NNE‐striking faults. NW‐SE extension during the Upper Jurassic culminated in the NW separation of Greater India and the formation of oceanic crust in the Neocomian, and resulted in the superposition of structures developed in a dextral transtensional regime. Conjugate strike‐slip faults and minor thrust faults formed during the post‐Neocomian, with approximately north‐south compression and east‐west extension. Precambrian gneisses, granitoids and metasediments of SW Western Australia have been deformed during basin development, and dole rite dyke intrusion during the Carboniferous to Middle Triassic is restricted to these terranes.The sequence of events proposed herein for the Perth Basin is in agreement with the tectonic framework of a large portion of Gondwanaland. A better knowledge of the structural history of the Perth Basin may aid the interpretation of seismic and remote‐sensing data in the search for further hydrocarbon occurrences.

Modes of extension at oceanic spreading centers: evidence from the Solea graben, Troodos ophiolite, Cyprus

The Solea graben in the Troodos ophiolite, Cyprus, represents a preserved Cretaceous-age axial valley formed during a brief period of amagmatic seafloor spreading. Structural analysis of all crustal levels of the graben reveals two periods of deformation; an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the Tertiary-Recent. Amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike-plutonic contact and oriented subparallel to ophiolite pseudostratigraphy. Individual fault segments are essentially planar but link to approximate listric geometries locally. Bulk structural rotation of fault blocks bounding the graben occurred about a NNW-trending axis, parallel to the present graben trend. Average dips in the sheeted complex within the central portions of the graben suggest 40–50° of structural rotation, although this is a minimum figure because present dike dips are not a simple function of rotation magnitude. Tensor analysis of fault populations associated with extensional deformation shows that σ 3 was oriented approximately perpendicular to the graben axis. Late, near-axis extensional deformation segmented the detachment fault along NNW-trending normal faults. Obduction-related uplift and doming of the Troodos ophiolite is expressed in the Solea graben by minor thrust, reverse and strike-slip faults principally developed during reactivation of pre-existing low-angle fault zones formed during seafloor deformation. Tensor analysis of this later phase of faulting shows that σ 1 was oriented approximately N—S and that σ 2 and σ 3 alternated position during the deformation. The style of deformation documented in the Solea graben supports recent speculation for Basin and Range-style detachment faulting at slow-spreading ridge crests with low magma budgets, such as parts of the Mid-Atlantic Ridge near large-offset transform faults. Natural cross-sectional exposure of the Solea graben shows how extensional structures might link at depth beneath such slow-spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large-scale (ore-forming) hydrothermal circulation cells.

Experimental Models of Extensional Forced Folds (1)

We have used single-layer and multilayer clay models to study the development of forced folds above normal faults. Our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer-parallel detachments. In single-layer clay models, extensional forced folds are upward-widening monoclines. Anticlinal axial surfaces dip in the same direction as underlying master normal faults, and synclinal axial surfaces dip in the opposite direction of master normal faults. Most secondary faults are upward-steepening normal faults. If master normal faults are steeply dipping, however, many secondary normal faults become high-angle reverse faults at shallow depths. The propagation and linkage of secondary faults into through-going normal faults terminates the development of extensional forced folds. More folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping. Most dipping beds and secondary faults are preserved in the hanging walls of the through- oing normal faults. In multilayer clay models with layer-parallel detachments, extensional forced folds are also upward-widening monoclines. Slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge. Slip on overlying detachments accommodates minor thickness changes associated with upward-widening of the fold. Secondary faults include low-angle normal faults near the anticlinal axial surface, minor thrust faults near the synclinal axial surface, and high-angle normal faults above the detachment edge. The model-predicted deformation patterns are similar to those of extensional forced folds from the Gulf of Suez and offshore Norway. This similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field, well, and seismic data.

Geology and Petroleum Potential of Flathead Region, Flathead County, Montana, and Southeastern British Columbia: ABSTRACT

Discovery of oil seeps in the Flathead region led to wildcat drilling by 1902. Exploration on the U.S. side stopped by 1910 because of the formation of Glacier Park, and the fact that the oil seeps were in Precambrian sedimentary rocks. Exploration continued through the 1930s on the Canadian side with drilling in the vicinity of major oil seeps. Exploration in Canada from the 1950s to present has included seismic work and six wildcat tests in the Flathead region. A land play has been going on in northwestern Montana for the past several years and recently released seismic data have demonstrated major structures that apparently involve Paleozoic rocks. Plans for drilling have been announced. The Glacier-Waterton and Flathead region are on the Lewis thrust plate. At the Sage Creek, British Columbia, oil seeps, drilling in 1952 penetrated the Lewis fault after drilling 4,400 ft (1,341 m) of Precambrian sedimentary rocks, and then drilled a strongly faulted sequence of upper Paleozoic carbonates and sandstones with several oil shows. The Precambrian rocks on the Lewis plate in the U.S. thin into Canada, eventually to zero where Paleozoic and Mesozoic rocks are carried by the Lewis fault. An area of hundreds of square miles of Paleozoic and Mesozoic outcrop, including the Fernie basin, is present immediately across the international boundary from areas of Montana which have been mapped as containing great thicknesses of Precambrian sediments. In the northern Whitefish Range o Montana, nearly 40 mi (64 km) from the leading edge of the Lewis thrust, is 30 mi2 (78 km2) of Paleozoic and Mesozoic outcrop containing several petroliferous units. This sequence is the only part of the extensive Paleozoic and Mesozoic outcrop of southeastern British Columbia which extends into the U.S., but it is important in understanding the involvement of Paleozoic and Mesozoic rocks in complexly faulted northwestern Montana. The Whitefish Range Paleozoic and Mesozoic sequence is cut by several minor thrust faults as well as having been overthrust by the Hefty plate, present now as klippe on the highest peaks of Paleozoic rocks. The Couldry and Tuchuck faults cut these rocks a few miles to the west. Between the Whitefish Range and the Livingstone Range of Glacier Park lies the Kishenehn basin, a graben to half-graben formed by the listric normal Flathead fault on the Lewis plate. This basin is filled with Oligocene Kishenehn formation, a fluvial End_Page 426------------------------------ to lacustrine sequence, which may be as thick as 16,000 ft (5 km) on the east side, thinning to a few thousand feet or to zero on the west where fault bounded. The Kishenehn formation contains lignite and oil shale beds which may be buried deep enough in parts of the basin to generate hydrocarbons; recent drilling discovered oil and gas shows at less than 200 ft (61 m). The presence of oil and gas shows in many formation as well as the presence of major structures containing porous host beds indicates that this area has an excellent potential for the discovery of hydrocarbons. End_of_Article - Last_Page 427------------

Jointing and minor tectonics of the S. Crop of the South Wales Coalfield between Machen and Bridgend

AbstractObservations on joints, minor faults and mineral veins at 47 localities along the south and south east crop of the S. Wales Coalfield indicate the presence of a joint system comprised of up to 6 sets of fractures striking at 350°, 330°, 290°, 270°, 250° and 210°. The joint sets are parallel to minor faults of strike and dip‐slip types, whilst minor thrust faults indicate an early phase of sub‐horizontal compression. The joint system is thought to be of tectonic origin and developed early in the deformational history of the area in response to a N‐S compression. Later, dilational episodes facilitated the infilling of mineral veins, with the W.N.W. trending set probably indicating the latest phase of dilation in the area. All structures are considered to be of Armorican age, whilst the marked similarity in the orientation of the joint system to that of the North Crop of the Coalfield suggests that the brittle deformation was relatively homogeneous on both flanks of the Coalfield.

Minor thrust faults in the Avonian of the North Crop of the South Wales Coalfield

AbstractSmall‐scale thrust faults are described from the North Crop of the South Wales Coalfield in strata of Avonian age. Two main sets occur which strike at east‐west and north‐south respectively, and show apparent stratigraphic zonation. It is suggested that the sets were initiated during the early stage of deformation of the area in response to primary and secondary resolved compressional forces.

Ouachita Mountain Core Area, Montgomery County, Arkansas

The purpose of this study was to investigate in detail the core area of the Ouachita Mountains of southwestern Arkansas. As work progressed it was discovered that High Peak and Wheeler Mountain ridges are not homoclinal, as originally mapped by Miser and Purdue, but rather are anticlinal. According to our interpretation there are only two outcrop areas of Collier shale in the Crystal Mountains: those along upper Collier Creek and upper Huddleston Creek. This fact is stratigraphically significant in that no dark bluish gray limestone was found by the writers in these remaining two outcrop areas of Collier shale. Another significant change in the stratigraphy is that of the thickness and lithologic character of the Crystal Mountain sandstone, which Miser and Purdue d scribed as almost entirely sandstone and as being 850 feet thick. Pitt discovered that the measurable interval of the Crystal Mountain sandstone in the Crystal Mountains is only 180-320+feet. Lithologically the measured interval consists of two massive sandstone beds, 30-80 feet in thickness, separated by about 100 feet of thin beds of siltstone and shale. Locally as much as 50 feet or possibly slightly more of interbedded sandstone and shale makes up the uppermost member in the Crystal Mountains. In the Mount Ida area north of the Crystal Mountains the Crystal Mountain sandstone ranges in thickness from 280 to 370 feet because the uppermost member is thicker than it is in the Crystal Mountains. No conglomerate or other suggestion of an unconformity was found at the base of the Crystal Mountain sandstone; the writers therefore believe that the Collier shale is probably Ordovician in age rather than Cambrian. The outcrop of supposed Collier shale north of Wheeler Mountain and near the town of Mount Ida is actually Mazarn shale. This Mount Ida area is one of sharp anticlinal folds with gentle westward plunge. Most of the Crystal Mountain sandstone ridges have been overturned southward. Faulting is rare throughout the Mount Ida area. The few observed faults are tear faults, but their presence suggests that minor thrust faults also may be present.

The Chief Oxide-Burgin area discoveries, East Tintic District, Utah, A case history--Part 1, U. S. Geological Survey studies and exploration, by T. S. Lovering, D. R. Cook, and H. T. Morris; Part 2, Bear Creek Mining Company studies and exploration, by J. B. Bush and D. R. Cook

The Burgin shaft is in the Chief Oxide area of the E. Tintic district, Utah, and is about a mile E. of any previously known ore bodies; workings from it are currently developing a substantial amount of commercial Pb-Zn ore in several blind ore bodies that lie in folded Paleozoic carbonate rock concealed beneath a blanket of Eocene lava. This area was mapped by Tower and Smith of the U.S. Geological Survey in 1897 and again by Lindgren and Loughlin in 1911, but no detailed work was done until after 1943 when a field party headed by T.S. Lovering began a study of the entire E. Tintic district. The history of the development of exploration concepts is summarized under Historical Summary. The E. Tintic mining district is in the E.-central part of a N.-trending fault-block mountain range near the eastern margin of the Great Basin; dominant structures of the range are a N.-trending syncline on the W. and a less well exposed anticline on the E. Both folds are cut by an intersecting system of transcurrent strike-slip faults and by minor thrust faults and normal faults of moderate displacement. A strong W.-dipping thrust fault cuts the anticline a short distance E. of the fold axis, but is hidden by Eocene lava throughout the E. Tintic district. The pre-Tertiary rocks range from Lower Cambrian to Upper Mississippian and exceed 7,000 ft. in total thickness; they are dominantly marine limestone and dolomite except for a thick basal Cambrian quartzite. The Tertiary rocks are chiefly dacitic lavas and pyroclastic deposits that are intruded by moderately persistent dikes and irregular bodies of monzonite and quartz porphyry. Nearly all the faulting and folding took place before the extrusion of the lavas on a rugged Eocene erosion surface. At about the time of the intrusion of monzonitic magma, many of the faults in the Paleozoic rock were re-opened and in the overlying lava some fracturing took place which was later accented by hydrothermal alteration. Most of the ore mined in the E. Tintic district has come from Pb-Zn-Ag replacement bodies in shattered Jasperoidized hydrothermal dolomite at the intersection of low angle faults and steep mineralized NE.-trending cross fractures. The U.S. Geological Survey studies of 1943 to 1957 concentrated on detailed mapping of geology and alteration in the E. Tintic district, together with field and laboratory studies of the relation of alteration to stages of mineralization and ore deposition. Trenching and core drilling were carried on after World War II to aid in interpreting the subrhyolite geology, and the Chief Oxide area was 1 of 4 localities tested by drilling. Study of the fossils, lithology, and alteration shown here in a deep drill core, together with the knowledge of the regional geology, led to an essentially correct interpretation of subrhyolite structure in the strongly discordant underlying Paleozoic rocks in which a mineralized tear fault cuts a strong thrust fault, and to the conclusion that ore stage mineralization was present in substantial amounts in the Paleozoic rocks below the Chief Oxide alteration patch in the quartz latite lava. Subsequent geothermal and geochemical work strengthened this conclusion, and the recent development work of the Bear Creek Mining Company, which sank the Burgin shaft, has shown the presence of Pb-Zn-Ag ore of commercial grade in substantial amount in blind ore bodies below the lava blanket in the Chief Oxide area.

Stratigraphic and Structural Traps in Big Piney-La Barge Area, Wyoming: ABSTRACT

The Big Piney gas and oil field, located in T. 26-30 N., R. 112-114 W., Lincoln and Sublette counties, Wyoming, lies within the Green River Basin, east of the north-south trend of the Wyoming Range. Oil and gas are contained in lenticular and blanket sands ranging in ages from Tertiary to Jurassic, namely, Wasatch, Mesaverde, Baxter, Frontier, Muddy, and Nugget formations. Drilling in this area began with the discovery of oil by the Wyoming Reserve Petroleum Corporation, from the Wasatch (Almy) sediments at the La Barge oil field in 1924. Promotional drilling ventures continued off and on until oil was discovered in the Mesaverde formation at North La Barge field by Circle Oil Company in 1930. Wyoming Petroleum Corporation's Budd No. 1, after a blow-out in 1938, stimulated an unpredicted lease play in the immediate area. With the advent of a pipeline for natural gas, development drilling programs were initiated by major and independent oil companies in 1954. Variations in sedimentation, offshore bars, and thickness changes to the northeast, and south, provide updip gas and oil reservoirs in Wasatch sediments. Alphabetic nomenclature has been ascribed to various sands within this Wasatch section. Mesaverde sediments, although widespread, are not only influenced by an unconformity and faulting, but also by facies changes not unlike the overlying Wasatch sediments. Consistent with the overlying Mesaverde and Wasatch sediments, the Baxter formation contains varying shaly sand conditions which, coupled with imbricate thrust faulting, further complicate subsurface studies adjacent to the Darby-La Barge thrust complex. Commercial gas zones have been defined at and adjacent to La Barge oil field. The Frontier formation is delineated into subthrust and overthrust members by the steep attitude of the La Barge thrust fault. Five benches are defined in the Second Frontier, four of which are lenticular in outline. The Second Bench of the Second Frontier provides the only true blanket sand in this entire interval. Muddy production appears to be controlled by variable sand conditions and permeability barriers. To date only two isolated productive areas have been defined. Nugget sediments are undoubtedly widespread through the Big Piney-La Barge area. Oil accumulation at Tip Top and Hogsback is structurally controlled but complicated in part by minor thrust faults. Deeper Phosphoria, Tensleep, and Madison horizons have not been penetrated east of the Darby-La Barge thrust complex. End_of_Article - Last_Page 957------------