Gravity Faults Research Articles

New Ulm Field: An Example of Cretaceous Shelf-Slope Instability in East Texas: ABSTRACT

The New Ulm field in Austin County, Texas, is an example of the structural and stratigraphic complexity above the Cretaceous Edwards shelf margin of east Texas. Deep wells and improved seismic data provide documentation of structural patterns and deepwater facies not previously considered in Gulf Coast reservoir play modeling. Study of the data implies the Late Cretaceous to Eocene section was deposited along a shelf-slope break. Late Cretaceous, pre-Midway sedimentation was affected by structurally induced slope instability, and consequent gravity faulting and slumping resulted in an irregular sea-floor surface. Paleocene Midway sands were carried onto this surface by storm-generated density currents where the uneven topography caused deposition in constructional channels. Continued deposition of the fluvio-deltaic Wilcox on this surface caused faulting and folding by differential compaction. The folds are minor and the faults small and steep, not like the typical large growth faults of the Gulf Coast. Upper Wilcox sediments were progressively less disturbed as the region stabilized. New Ulm field production includes gas from the Midway Formation and oil and gas from the Wilcox Group. Midway reservoirs are stratigraphic, consisting of turbidite channel sandstones; Wilcox reservoirs are structural, consisting of fluvio-deltaic sandstones within faulted anticlines. This study adds evidence to data describing shelf-slope geology along the Edwards margin. The setting can be a new type of hydrocarbon play in the Gulf Coast. End_of_Article - Last_Page 516------------

Structure and evolution of the Sacrofano-Baccano caldera, Sabatini volcanic complex, Rome

Sacrofano eruptive center, in the eastern part of the Sabatini volcanic complex, was active between 0.5 and 0.09 m.y. ago. The Baccano geothermal area lies at the western edge of Sacrofano caldera. Sedimentary substrata that form the reservoir for this geothermal system were exposed at the surface at the beginning of volcanic activity. The volcanic history of the Sacrofano center can be divided into three stages: 1. (1) Construction of the Sacrofano pyroclastic edifice by predominantly Strombolian activity. 2. (2) Collapse of Sacrofano caldera following eruption of pyroclastic flows. 3. (3) Development of the Baccano explosive center at the western edge of Sacrofano caldera and collapse of Baccano caldera. Hydrovolcanic activity that began at the end of the first stage of Sacrofano can be explained by a drop in the magma level in the conduit that relieved hydrostatic pressure in the aquifer. As intense fracturing associated with caldera collapse penetrated the carbonate reservoir, the entrapped water flowed towards the conduit to balance the pressure change. The close link between tectonism and volcanism in the Sacrofano center suggests that eruptions there may have been triggered by gravity faults related to regional tectonics.

The Denholm landslide, Saskatchewan. Part I: Geology

The Denholm landslide, whose surface is composed of scarps, ridges, and elongated depressions, is 160 m high, 2000 m wide, and up to 100 m thick. The shear zone is in silty, montomorillonitic clay of the upper part of the Lea Park Formation and Upper Colorado Group unit. The Upper Cretaceous Judith River Formation and the Quaternary Empress, Sutherland, and Saskatoon groups were affected by the landslide. Although these sediments were fractured and gravity faulted by tension when the landslide moved, they can be readily traced through the landslide, particularly the upper part. The scarps (gravity faults), ridges (horsts), and elongated depressions (grabens) are the surface expression of tension resulting from the stretching of beds during the landslide.The movement of the landslide is thought to have started when the North Saskatchewan spillway eroded to the level of the present shear zone about 11 000 years ago (established by radiocarbon dating) and is believed to have stopped in recent time. During this time, it moved about 390 m across the North Saskatchewan River alluvium at an average rate of 35 mm per year. As the landslide moved across the valley, it encountered deposition of alluvium at an average rate of about 2.4 mm per year which resulted in the curved shear zone on the alluvium. Keywords: retrogressive landslide, shale-alluvium, displacement, rate, age.

Reconnaissance and Economic Geology of Copper Mountain Metamorphic Complex, Owl Creek Mountains, Wyoming: ABSTRACT

The Copper Mountain metamorphic complex lies within a westerly trending belt of Precambrian exposures known as the Owl Creek Mountains uplift. This mountain range lies along the southern edge of the Big Horn basin, separating that basin from the Wind River basin to the south. Rocks of the metamorphic complex are exposed at Copper Mountain, at Wind River Canyon, and apparently continue several miles to the west on the Wind River Indian Reservation. The metamorphic complex at Copper Mountain is part of a larger complex known as the Owl Creek Mountains greenstone belt. Until more detailed mapping and petrographic studies can be completed, the Copper Mountain area is best referred to as a complex, even though it has some characteristics of a greenstone belt. The metamorphic complex is amphibolite-grade metamorphosed supracrustal rocks that have been migmatized along the north and south margins of the complex by the intrusion of leucogranitic stocks and batholiths. The regional trend of the metamorphic complex is N50° to 80°E, and regional foliation dips steeply to the south. The dominant structure along the southeastern margin of the belt is a synform. The supracrustal rocks are quartz, hornblende, plagioclase gneisses and schists, quartzites, para-amphibolites, pelitic schists, cordierite schists, iron formation, quartz-mica schists and gneisses, and intercalated orthoamphibolites. At least three episodes of Precambrian deformation have affected the supracrustals, and two have disturbed the granites. Prior to the intrusion of granite, the metamorphic complex experienced coaxial folding of the metasediments. Following the initial folding of the supracrustals, the Copper Mountain belt was intruded by leucogranite about 2.7 b.y. ago. This intrusive event is believed to be responsible for prograde metamorphism, as well as a second phase of deformation. Portions of the supracrustal belt were mineralized during the waning stages of the intrusive event. Some tungsten-bearing veins and calc-silicates were produced. Auriferous and cupriferous fracture-fill veins were formed, followed by emplacement of simple pegmatites. A final Precambrian deformation event was preceded by a weak thermal event expressed by retrogressive metamorphism and restricted metasomatic alteration. During this event, a second phase of pegmatitization was accompanied by hydrothermal solutions. During the Laramide orogeny, Copper Mountain was again modified by deformation. Laramide deformation produced complex gravity faults and keystone grabens. Uranium deposits were formed following major Laramide deformation. The genesis of these deposits is attributable to either the leaching of granites or the leaching of overlying tuffaceous sediments during the Tertiary. Production of metals and industrial minerals has been limited, although some gold, copper, silver, tungsten, beryl, feldspar, and lithium ore have been shipped from Copper Mountain. A large amount of uranium was produced from the Copper Mountain district in the 1950s. End_of_Article - Last_Page 1341------------

Remnants of a pre–Late Jurassic ocean in northern Turkey: Fragments of Permian-Triassic Paleo-Tethys?

Research Article| October 01, 1980 Remnants of a pre–Late Jurassic ocean in northern Turkey: Fragments of Permian-Triassic Paleo-Tethys? A. M. CELAL ŞENGÖR; A. M. CELAL ŞENGÖR 1Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 Search for other works by this author on: GSW Google Scholar YÜCEL YILMAZ; YÜCEL YILMAZ 2Jeoloji Bölümü, Yerbilimleri Fakültesi, Istanbul Üniversitesi, Vezneciler, Istanbul, Turkey Search for other works by this author on: GSW Google Scholar IHSAN KETIN IHSAN KETIN 3Jeoloji Kürsüsü, Maden Fakültesi, Istanbul Teknik Üniversitesi, Maçka, Istanbul, Turkey Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (10): 599–609. https://doi.org/10.1130/0016-7606(1980)91<599:ROAPJO>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 A. M. CELAL ŞENGÖR, YÜCEL YILMAZ, IHSAN KETIN; Remnants of a pre–Late Jurassic ocean in northern Turkey: Fragments of Permian-Triassic Paleo-Tethys?. GSA Bulletin 1980;; 91 (10): 599–609. doi: https://doi.org/10.1130/0016-7606(1980)91<599:ROAPJO>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 SocietyGSA Bulletin Search Advanced Search Abstract An ophiolitic suture zone and associated active continental margin sequences of mid-Jurassic age are found in northern Turkey exposed in inliers surrounded by an extensive Upper Cretaceous to Eocene volcanic blanket. The pre–Late Jurassic rocks of the eastern Black Sea Mountains (eastern Pontide tectonic zone) consist of two distinct lithologic associations: a “continental” assemblage represents a Permian to Early Jurassic north-facing magmatic arc, whereas an “oceanic” assemblage, including a locally metamorphosed ophiolite suite overlain by deep-sea sediments, is believed to represent the vestiges of an oceanic realm that existed north of the arc during the ?Permian to Jurassic interval. During the mid-Jurassic, the oceanic assemblage underwent penetrative deformation and was overthrust by the continental assemblage. The latter has not been penetratively deformed except for a wide zone of intense cataclasis along its basal thrust. The southern part of the continental assemblage was affected by east-west gravity faulting and basaltic and some trachytic volcanism during the Early Jurassic. We interpret the Permian to mid-Jurassic geological record of the eastern Pontides as the expression of the progressive contraction of a Permian-Jurassic ocean with a south-dipping subduction zone. Its closure resulted in the overthrusting of the continental assemblage onto the oceanic assemblage. Regional considerations suggest that the suture forms a part of an orogenic belt stretching from the mid-Jurassic South Rhodope Orogen through the early to mid-Mesozoic orogenic zones of the peri-Black Sea regions, northern Iran, Afghanistan, central Tibet, and China. This orogenic belt resulted from the closure of Permian-Triassic Paleo-Tethys. The oceanic assemblage we describe is believed to be a part of the floor of this ocean, whereas the continental assemblage is viewed as a part of a previously defined Cimmerian continent. 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.

Cretaceous and Cenozoic faulting in eastern North America

Cretaceous and Cenozoic faults in eastern North America provide evidence of modest intraplate tectonic activity in this region. Thrust and gravity faults are found; strike-slip movement is not demonstrable but could have occurred in some cases. Fault movements are dated by offset Cretaceous, Tertiary, and Quaternary sediments, Cretaceous kimberlite, and Pleistocene glacial stria. Cretaceous and Cenozoic displacements range from about 1 mm to several tens of metres. The faults are found in the central Mississippi River valley, the southeastern and northeastern United States, and eastern Canada. Correlations among historic seismicity, fault plane solutions, and the Cretaceous and Cenozoic faulting in the central Mississippi River valley and the southeastern United States suggest a close causal relationship. The quality of the correlation between historic seismicity and Cretaceous and Cenozoic faulting in the northeastern United States and eastern Canada varies considerably from area to area; hence the relation there between Cretaceous and Cenozoic faulting and contemporary seismotectonics is uncertain. Sharp offsets in loosely consolidated sediments, the presence of slickensides and fault gouge, and the involvement of Precambrian or Paleozoic bed rock provide the best evidence that faulting was associated with tectonic earthquakes. The displacement of near-surface deposits implies that surface faulting has occurred, although it has not been documented for modern earthquakes in eastern North America. At least some of the displacements occurred along pre-existing faults. The number and amounts of Cretaceous and Cenozoic movements on each fault have not been determined, but on one fault a displacement of 0.6 m during a short time span suggests that an earthquake of about magnitude 6 was associated with the displacement. The largest Cretaceous and Cenozoic displacements must represent more than one movement. The presence of thrust and gravity faults and the inconsistent trends of the faults imply that stresses that vary spatially and (or) temporally, rather than a single stress field acting steadily throughout the entire North American plate, are responsible for causing this intraplate tectonic activity.

Collision of irregular continental margins: Implications for foreland deformation of Alpine-type orogens

Research Article| December 01, 1976 Collision of irregular continental margins: Implications for foreland deformation of Alpine-type orogens A. M. Celal Sengör A. M. Celal Sengör 1Geology Department, University of Houston, Houston, Texas 77004 Search for other works by this author on: GSW Google Scholar Geology (1976) 4 (12): 779–782. https://doi.org/10.1130/0091-7613(1976)4<779:COICMI>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation A. M. Celal Sengör; Collision of irregular continental margins: Implications for foreland deformation of Alpine-type orogens. Geology 1976;; 4 (12): 779–782. doi: https://doi.org/10.1130/0091-7613(1976)4<779:COICMI>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 The collision of irregular continental margins during an Alpine-type orogeny may give rise to a series of deformational events in the forelands, the tectonic style of which may be critically dependent on complete or incomplete suturing of the two colliding continents. Dilatational structures such as mafic dikes, gravity faults, and grabens may form at those points where the continents first meet. Later, as the suture lengthens, large-scale wrench faults develop in the foreland area. When the compression caused by the orogen ceases, these foreland structures become inactive. The large-scale grabens and wrench faults observed in the forelands of the Alpine and northwestern European Variscide belts may be examples of such collision-related foreland structures. If the model presented in this paper is approximately correct, then it may be used to connect some intraplate phenomena to interplate tectonism. 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.

Hydrocarbon potential of marginal basins bounded by an island arc

The Japan Basin is an example of a western Pacific marginal basin that is bounded by an island arc. Such basins were produced by an extensional process that involves formation of new, hot sea floor below basins that are opening as newly created arc systems migrate away from the continental borderland or older arcs. Rapid deposition of sediment on the hot basin floors allows development of steep geothermal gradients in these sedimentary prisms. In these sedimentary basins, the relationship between temperature and strata age is such that the kerogen-hydrocarbon transformation occurs in young, shallow strata. The sealing effect of turbidite deposits and the presence of numerous gravity faults ensures entrapment conditions. Hydrocarbon potential of these basins can be evaluated by considering the age of basin opening, sedimentation rates adjusted for compaction, and heat-flow data.

Fracture Patterns on Kuh-e Asmari Anticline, Southwest Iran

Fracturing at air-photo scale on the Kuh-e Asmari anticline presents a simple pattern comprising two tension-fracture sets which clearly are related to Zagros folding. The absence of any marked shear-fracture sets is puzzling. Like all anticlines, this structure has its own inherent complexities. The identification of greater fracture lengths over azimuth classes corresponding to longitudinal fractures in the eastern part of the structure is suggestive of a relation to a larger, basement-controlled lineament. The recognition of such pre-Zagros features on reservoir structures may define zones of enhanced production potential. Photo-scale fractures show maximum densities over the plunging noses and central parts of the structure in that order. The contrast between the diversified small-scale, structure-unrelated patterns derived in an earlier study of the same anticline and the simple photo-scale pattern is most marked. Differing modes and times of genesis for the two scales of fracturing are proposed. The superficial nature of gravity faulting and dip-slope slumping, so prevalent in southwest Iran, suggests that fault and fracture patterns on surface structures may not be characteristic of their buried oil field counterparts. Such superficial features on surface Asmari structures results from unloading consequent on the stripping of the overlying Fars Group.

The Cambrian and Ordovician rocks of the Migneint area, North Wales

The stratigraphy, faulting, mineralization and metamorphism of the Upper Cambrian and Ordovician rocks are described. Newly erected formation names are the Afon Gam Formation (Upper Cambrian), the Carnedd Iago, Serw, Llyn Conwy and Rhiw Bach Formations (Ordovician). Two important unconformities, corresponding to the pre-Arenig and pre graeilis erosions widely recognized in North Wales, are described. They are controlled by major n-s gravity faulting. Many of the faults were active as the stratified rocks were being laid down, and intrusions were injected at the same time. This gave very variable deposits and many disconformities in the Lower and Middle Ordovician. An evolutionary relation-ship between the four principal intrusive rock types is proposed to explain the restriction of certain types to particular stratigraphic horizons. Possibly the first British record of hollandite is recorded.

A FILTERING TECHNIQUE FOR INTERPRETING THE GRAVITY ANOMALY GENERATED BY A TWO‐DIMENSIONAL FAULT

In order to combine the advantages of a direct quantitative interpretation with an efficient regional‐residual separation, an interpretation technique based on characteristic curves derived through digital bandpass filtering has been developed for the two‐dimensional gravity fault model. Although the two‐dimensional fault model suffers from the lack of characteristic parameters, reasonably accurate direct interpretations are possible even in the presence of regional fields whose long wavelength spectral components overlap those of the anomaly. The characteristic curves given here were designed for use on relatively large deep structures with depths and thicknesses ranging from 0.8 to 8 km; however, the slight modifications necessary to accommodate any range are presented in detail.

Age and Evolution of Salt Basins of Southeastern Mexico

The oldest salt basin of southeastern Mexico, the Isthmian salt basin, usually is referred by most authors to the Triassic-Jurassic. However, it is now possible to date the oldest salt basin as pre-Kimeridgian and not older than early Oxfordian (Divesian). This conclusion is based on new data from wells and from studies of the Todos Santos Formation, a unit which has been the object of many studies. A second basin, with salt beds between Late Jurassic strata, below, and Early Cretaceous strata, above, was penetrated in the Turipache No. 1 well near Tuxtla Gutierrez, Chiapas. Information from the La Pita No. 1 and Tortugas No. 1 wells in Guatemala indicates that this basin extended through southern and central Guatemala--perhaps into western Belice (British Honduras). The development of two salt basins of different ages--one on the northwest and the other on the southeast--suggests a northwest to southeast marine transgression along an arcuate line of embayments marginal to the Gulf of Mexico. The marginal position of the basins supports the concept that, before Cretaceous time, the Gulf was a land (shield) area and that the salt basins formed before the foundering of the Gulf was completed at the end of the Mesozoic. The tectonic history of the area includes both Mesozoic and Tertiary crustal movements. Transcurrent faults are present south and southwest of the Yucatan platform; tension or gravity faults characterize the Tertiary basins of the Gulf coastal plain and the eastern side of the Yucatan Peninsula. The transcurrent movement phase is a Cretaceous-early Tertiary event; the tension-movement phase is a middle to late Tertiary event.

Some possible mechanisms for gravity and thrust faults under oceanic trenches

Seismic reflection profiles taken across several trenches of the Pacific suggest that gravity faulting is the dominant type of tectonic deformation within the trenches. Studies made of the directions of oceanic crustal movement suggest, however, that the oceanic crust is absorbed into the mantle beneath island arcs by thrust faulting. First-motion studies of earthquakes occurring beneath the trenches suggest horizontal extension of the crust over the sea-floor level and crustal compression at about 30 km below and beneath the trench axes and the island arcs. To accommodate the concept of underthrusting of the oceanic crust under island arcs with gravity-fault deformation in the trenches, a transformed thrust mechanism is suggested as one of a possible number of mechanisms whereby a gravity fault located at the trench surface would become a thrust fault at depths of 30 to 40 km beneath the island arc.

Gravitational Tensional Faulting and Tectonic Denudation: DISCUSSION

Cloos9 (ibid., v. 52, p. 420-444, 1968) paper on the concept of down-basin gravitational sliding of sedimentary prisms to explain the pattern of Gulf Coast faults may offer a possible solution to theproblem of 9tectonic denudation9 (de Sitter, 1964, p. 239). In areas where the mechanism of gravitational sliding may have been operative, belts of superficial foreland folding and thrust faulting may have developed. On associated uplifts, areas of tensional spreading along many gravity faults may have occurred rather than denudation. Extension on the uplift would match shortening by thrust faulting and folding in the foreland areas.

THE TECTONIC SIGNIFICANCE OF MESOSCOPIC SUBFABRICS IN THE SOUTHERN ROCKY MOUNTAINS OF ALBERTA AND BRITISH COLUMBIA

Deformation throughout much of the southern Rocky Mountains was characterized by brittle failure in a strongly anisotropic layered sequence of non-metamorphic rocks. On a megascopic scale, the overall structure is dominated by an interlocking system of imbricate thrust plates that have moved relatively eastward or northeastward and upward. On a mesoscopic scale, the principal elements in the fabric of these rocks are fractures that are statistically parallel or perpendicular to the bedding, or else intersect it at preferred angles of approximately 25° or 70°. During deformation many of these fractures obviously were kinematically active, as discrete surfaces of slip that became slickensided, as zones of dilation that became filled with vein minerals, or as surfaces of pressure solution that are now marked by stylolites. Each of these fractures provides a partial record of the kinematics of some stage of the deformation, even when they are considered individually rather than as components in a fracture array whose symmetry is related to that of the movement picture during deformation. Each defines a unique line of slip and axis of rotation for slip, or a unique direction of relative extension or compression. Collectively, they provide a direct and succinct record of the kinematic history of an individual fabric domain, and a sound basis for dynamic analyses of deformation.Some preliminary results of a reconnaissance study of these mesoscopic subfabrics illustrate their tectonic significance.A movement picture can be established for the deformation that occurs within an individual thrust plate during its development and translation. Kinematic relationships between and among the interlocking thrust plates can. be studied.Within a broad area centered along the prominent structural reentrant that crosses the Rocky Mountains near Crowsnest Pass, two different movement pictures occur in superposition. Movement about both northerly and north westerly trending axes can be outlined on the basis of the mesoscopic subfabrics of rocks which, on a megascopic scale, have either a northwesterly or a northerly trending fabric axis. Movement patterns for the deformation associated with each of two regional structural salients converge in the vicinity of the reentrant.The mesoscopic subfabrics associated with transverse faults in parts of the Front Ranges outline a pattern of movement which indicates that they did not originate as tear faults related to the translation of the thrust plates, but instead are probably older gravity faults, whose orientation may be controlled by the fabric of the Hudsonian basement extending beneath the mountains from the Canadian Shield.

EXPERIMENTAL ANALYSIS OF FRACTURE PATTERNS

All existing fractures can be imitated in four scale models which show tension or shear fractures with or without rotational deformation. These four experiments can easily be produced by deforming soft clay which rests on a moving square of wire cloth. En echelon zones are easily produced by placing clay above the edge of a moving tin plate. Tension and shear fractures open in the flexure zone and are unfailing indicators of relative displacement. Normal or gravity faults occur preferably above domes and anticlines or other areas of distension. Salt-dome patterns may readily be imitated by pushing a plug into a clay model from below. A systematic study of fracture anomalies may prove useful in the detection of areas of uplift above salt domes. Longitudinal and transverse normal faults in North dome of the Kettleman Hills are readily imitated in a clay model and may be due to vertical uplift with or without lateral compression. The en echelon arrangement of the Kettleman Hills is probably not due to north-south movement toward a weak zone trending northwest. True en echelon folds would trend west-northwest but not nearly north-northwest. The echelon arrangement may be only the alternation of folds or may be due to northeast-southwest movements, possibly in combination with subcrustal blocks and faults. The San Andreas-Garlock fault system can be imitated only if considerable rotation has opened the angle between shear planes from near 60° to 90° or to 120°. Asymmetry of the fault pattern suggests clockwise rotation. A more complicated experiment in which three blocks are placed as suggested by Ralph Reed seems to come nearer to a fitting imitation but may still include too large an area. The value of experimentation lies in experience gained by the experimenter rather than in the exact duplication of known fracture patterns. The intense occupation with mechanical principles and the close visible relationship between cause and effect is sobering as well as stimulating.

The major fault zone of central Vestspitsbergen

Summary A deformation belt runs southwards from Wijdefjorden through the length of central Vestspitsbergen, a distance of over 190 miles. From the stratigraphical summary several factors emerge that are relevant to the structural history. 1. The pre-Downtonian rocks are crystalline schists with a general N.-S. trend resulting from an earlier orogeny. 2. There was a basin of sedimentation in the Billefjorden area in Lower and Middle Carboniferous times. 3. The Carboniferous to Lower Permian succession is concordant in the central portion of this basin. 4. A strip of coarse conglomerates overlying an old fault-line suggests weak faulting activity in Middle Carboniferous times. Fault activity occurred in Upper Devonian times, probably with a strong compressive phase in the earlier part which was followed by weaker gravity faulting 1 near the end of the period. Again in Tertiary times, either gravity faults or flexures formed, depending on the competence of the rocks affected. Parallelism with the trend of the pre-Downtonian metamorphic rocks is striking and renewed dislocations along the Upper Devonian fractures seem to be common. It is concluded that the major Upper Devonian fractures are not gravity faults. Two theories of genesis are possible : firstly, transcurrent faulting with associated upthrust wedges; secondly, high-angle thrusting with a later phase of gravity faulting. The data available appear to favour the second.

Overthrusting in Arbuckle Mountains, Oklahoma

Until recent years, geologists have looked upon the Arbuckle Mountains of south-central Oklahoma, as a single, unit uplift, formed at one period within Pennsylvanian time, and considerably modified by normal or gravity faulting. The uplift is now shown to be a structural complex composed of two major anticlines--the Hunton-Tishomingo arch and the Arbuckle anticline--with axes trending northwest and southeast, and these folds are shown to have been formed at different periods. Pre-Atoka (pre-Pottsville) rocks in the Hunton-Tishomingo arch are overlapped by all younger Pennsylvanian beds, indicating the age of the uplift as early Pennsylvanian. In the Arbuckle anticline, pre-Pontotoc (pre-Monongahela) rocks are essentially conformable with all the lower Paleozoic formations, thus indicating the late Pennsylvanian age of that fold. Evidence is also presented to show that extensive thrust faulting accompanied the late movements, that many of the faults are probably of the low-angle, compression type, that the Arbuckle anticline was thrust a considerable distance northward, overriding the Hunton-Tishomingo arch, and that the latter also experienced considerable thrusting.

Active Faults near Whitebird, Idaho

Fissures which developed near Whitebird, Idaho, in November, 1927, were under observation until November, 1928. There are two larger displacements which are essentially parallel, and several smaller ones, all of which occur in a gently sloping ter-rane made up of lava flows and interbedded sediments. One of these faults has a known length of 8,200 feet, and an inferred length of 13,600 feet. Displacement was measured from time to time by means of a series of stakes set on each side of the fissures. A horizontal movement of 7.35 feet and a vertical movement of 3.5 feet represent the amounts of displacement in a year's time. The fissures may border gigantic landslides, or may be gravity faults, but a study of the evidence points to the latter.