Middle Paleozoic Rocks Research Articles

Late Mesozoic sediment provenance on Georges Bank: Enlargement of river drainages to the Atlantic Ocean in the Late Jurassic–Early Cretaceous

The detrital mineralogy of Upper Jurassic to Lower Cretaceous sandstones from the Continental Offshore Stratigraphic Test G-2 well in the Georges Bank Basin was studied to better understand the varying supply of clastic sediment to the basin. Heavy minerals separated from cuttings were identified by chemical analyses, zircons were dated, and Raman spectroscopy was applied to identify titania polymorphs (rutile, anatase, and brookite). Upper Jurassic (Oxfordian) sands contain tourmaline, fluorapatite, Mn-almandine and grossular garnets (derived from the central Maine and coastal Maine belts), and pumice and trachytes reworked from volcanics in the Long Island platform. Zircon geochronology (300–450 and 550–700 Ma) supports sources from peri-Gondwanan Appalachian terranes. Lower Cretaceous heavy minerals show that Oxfordian supply was diluted to 30%, mostly by additional ilmenite, zircon, tourmaline, and staurolite, probably by components from central and northern Maine and New Brunswick. Rutile and anatase from metapelites are more common than in the Oxfordian, where mafic igneous sources predominate. Lower Cretaceous zircons include a few Mesoproterozoic grains that might be reworked from middle Paleozoic sedimentary rocks in the Gander terrane of northern Maine and New Brunswick, although subhedral grains together with rare chromite may indicate minor or intermittent supply from the Sable River draining Labrador. Heavy minerals provided important provenance information not available from zircon geochronology alone. Progressive enlargement of the catchment area resulted from an increasingly humid climate in the latest Jurassic and active mostly northeast-trending faults in the Early Cretaceous. This resulted in greater supply of petrographically more-mature sand to the basin.

Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes, Baikal Region)

Overall petrologic and geochemical data indicate that the early Paleozoic magmatism in the Olkhon area of the Baikal Region exhibits diverse types of granitoids, whose time of formation is estimated at a narrow age interval of 500-465 Ma. This magmatism was responsible for the formation of both autochthonous gneiss-migmatite-granitoid suites (Sharanur complex) and multiphase intrusions (Aya complex) emplaced into the upper horizons of the continental crust. In major-element chemistry, K2O/Na2O values, and rare-element composition the migmatite-plagiogranites and calc-alkaline and subalkaline granitoids of the Sharanur complex are similar to the host gneisses and schists, as they were likely derived from melting of the ancient metamorphic substratum of the Olkhon series. In new isotope-geochemical characteristics (ICP MS method) the Sharanur granitoids are close to the first-phase biotite granites of the Aya massif, whose further geochemical evolution was governed mainly by intrachamber magmatic differentiation leading to the production of second-phase leucogranites enriched in HREE and HFSE (in particular, Ta and Nb) and depleted in Sr, Ba, Eu, Li, and LREE. The origin of the autochthonous and intrusive granitoids is related to early Paleozoic collision events within the Olkhon metamorphic terrane, while the formation of syncollisional granitoids is best explained by both melting of the crust protolith (Sharanur complex) and magmatic differentiation (multiphase Aya intrusion). All mineralogical and geochemical characteristics indicate that these granitoids are distinguished from rare-metal pegmatoid granites and Li-F and Rb-Be-Nb pegmatites, whose vein bodies crosscut the granitoids, and are regarded as middle Paleozoic rocks, which mark the transition to within-plate magmatism in the Baikal Region.

Fossils from seabed bedrocks: Implications for the nature of the acoustic basement of the Mendeleev Rise (Arctic Ocean)

The geological information on the structure of the Mendeleev Rise in the Arctic Ocean is scarce due to inaccessibility of this offshore area; therefore its origin is still disputable. Two expeditions of the multi-year programme of deepwater research are devoted to solving this problem. The programme is based on integrated seafloor studies using a research submarine for recovering bedrock samples from seafloor outcrops under subglacial conditions.Sampling was performed in three polygons in the southwestern, central and northern parts of the Mendeleev Rise, where according to seismic data, an acoustic basement crops out at the seabed.Sedimentary and igneous rocks were sampled, with similar rock types in all three of the polygons. The fossils were extracted from sedimentary rocks. The detailed study of crinoids, conodonts, acritarchs, brachiopods, foraminifers, dinocysts, spores and pollen was carried out. Obtained data allowed the dating of 13 samples. Among sedimentary rocks exposed on steep slopes of the Mendeleev Rise, stratigraphic units of three ages were identified: Late Ordovician-Silurian, Middle-Late Devonian and Early Cretaceous. The Upper Ordovician-Silurian rock association mainly occurs in lower parts of the sampled slopes and consists of alternating dolomites, quartzose sandstones and limestones. Carbonate rocks with clastic components similar to those in quartzose sandstones as well as quartzose sandstones with dolomite cement were encountered. Partly dolomitised varieties were observed among limestones; the same spectrum of bioclasts is found in dolomites and limestones. The Middle-Upper Devonian (upper Givetian – Famennian) rock association occurs mainly in upper parts of the slopes and consists of alternating limestones and sandstones. The Lower Cretaceous (Barremian-Aptian) sandstones occur in the upper part of the slope of the Trukshin Seamount (the north of the Mendeleev Rise).New geological data demonstrates that lower and middle Paleozoic rocks of the Mendeleev Rise were deposited in the shallow-water marine environments of the epicontinental basin. Paleozoic deposits form the upper part of the acoustic basement of the Mendeleev Rise. Overlaying Lower Cretaceous sandstones are the basal layer of the stratified Mesozoic-Cenozoic sedimentary cover of the Mendeleev Rise.

Early Visean paleogeography of northern Siberia: New evidence of rift to drift transition along the eastern margin of Siberia

In the Late Devonian–Early Carboniferous the eastern part of Siberia was affected by multi-stage rifting, which led to formation of a Late Paleozoic–Mesozoic passive margin. Four sections of the Lower Visean strata were studied in the northern part of the Kharaulakh Ridge, along the Lena River and Bykov Channel (north-east of Siberia). The studied sedimentary succession comprises two lithologically contrasting units: coarse-grained turbidites and hemipelagic cherts. Deposition occurred in a marine basin bounded by steep slopes, which is a key geomorphological feature facilitating deposition of coarse-grained turbidites. Within the studied sandy turbidites, several different zones of submarine sand-rich fans have been recognized. Integrated sedimentological and petrographical analyses allow us to interpret that the nearby uplifts composed of Proterozoic–Middle Paleozoic rocks were the main provenance areas for the Visean clastic rocks. Those uplifts were formed as a result of the latest stages of rifting in the study region that occurred in the earliest Visean, following termination of much of the tectonic activity during the Tournaisian. A similar rock association has been described previously from the central and southern part of the eastern margin of Siberia suggesting that the rift to drift transition occurred approximately at the same time across East Siberia and is of Early Visean age.

Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia

Lower Paleozoic successions of the Gobi-Altai zone of southern Mongolia record an abrupt facies transition from deposition of predominantly fine-grained uppermost Ordovician through lowermost Devonian carbonate and marl facies to deposition of coarse clastic strata of the Lower Devonian Tsakhir Formation. The Tsakhir generally fines upward from alluvial-fan cobble and pebble conglomerate to interbedded coarse- and fine-grained marine siliciclastic and carbonate strata, which were deposited within a tectonically active basin. The marine strata, deposited in a storm-influenced proximal to distal fan delta, include unusual event beds that grade from pebble conglomerate to hummocky cross-stratified grainstone and sandstone. These beds represent sediment emplaced by gravity flows during flood events and reworked by large gravity waves associated with storm events. The interpreted link between flood deposition and storm wave reworking supports a hyperpycnal flow interpretation for these deposits. The sudden facies transition at the base of the formation represents the sedimentological and stratigraphic signature of Early Devonian tectonism in the Gobi-Altai zone. The general upward-fining pattern of the Tsakhir is interpreted as a response to the creation of accommodation space at a greater rate than progradation of the fan delta, in large part due to tectonic subsidence, although some component of eustasy may have been involved. The production of steep relief and deposition of associated volcanics suggest a transition from relatively passive deposition to active tectonics in this region during the Lochkovian to Pragian stages of the Early Devonian. We herein introduce the term “Tsakhir event” for this important tectonic transition. Range-bounding faults for this event are not preserved, but alluvial-fan deposition, the development of unconformities, renewed subsidence, and magmatism throughout the Gobi-Altai zone all suggest syndepositional tectonism. Detrital zircon spectra from both Ordovician and Devonian strata contain Archean to Paleozoic ages. Minor differences between Ordovician and Devonian samples suggest changes in source regions and/or transport paths prior to, and after, the Tsakhir event. The paleoenvironmental setting of the Tsakhir Formation requires short transport distances, and thus the age spectrum of a sample from this formation represents proximal basement rocks of the Shine Jinst region of the Gobi-Altai zone. Basement rocks are not exposed in the Shine Jinst region, but the wide variety of ages in all of the detrital spectra suggest a nearby continental source. Our detrital age spectra contain peaks that coincide with basement ages and magmatic events on the adjacent Mongolian microcontinent and also have strong similarities with recently published spectra of nearby landmasses in Neoproterozoic to Paleozoic paleogeographic reconstructions, namely, Siberia, North China, eastern Gondwana, and Tarim. These similarities extend to spectra of late Neoproterozoic to middle Paleozoic rocks throughout Gondwanaland and also Siberia, illustrating the somewhat limited utility of detrital spectra for determining the tectonic affinities of crustal blocks at this time in Earth history.

Age of felsic volcanism in the Selitkan zone of the Khingan-Okhotsk volcanoplutonic belt, Russian Far East

The eastern margin of the Asian continent was an arena of intense magmatism in the Late Mesozoic (Fig. 1). It contains several volcanoplutonic belts: Mongol‐Transbaikal, Bol’shoi Khingan, Umlekan‐Ogodzha, Nizhnyaya Zeya, Stanovoi, Khingan‐Okhotsk, Okhotsk‐Chukotsk, and Eastern Sikhote Alin. These belts are attributed to the different geodynamic settings at the ocean‐continent boundary or episodes of within-plate activity [2‐5]. Despite significant attention devoted to these volcanoplutonic belts, lack of sufficient geochronological and geochemical information hampers the development of a reliable tectonic scenario for the Asian margin evolution. This paper discusses results of geochronological investigations of felsic volcanic rocks from the Selitkan zone of the Khingan‐Okhotsk volcanoplutonic belt and proposes a more reliable correlation of magmatic occurrences in different parts of the area considered. The Selitkan zone is one of the largest structures of the Khingan‐Okhotsk volcanoplutonic belt [6] and extends over 150 km at a width of 30‐45 km. If we take into consideration comagmatic plutonic rocks, its area becomes two times greater. The volcanic rocks of the Selitkan zone are underlain by Aptian‐Albian coarseclastic rocks [7]. These two rock sequences, in turn, overlie the strongly dislocated Middle and Late Paleozoic oceanic rocks of the Galam terrane of the accretionary wedge of the Mongol‐Okhotsk fold belt [4].

Position of the Blyb Complex in the pre-Mesozoic crustal structure of the Front Range Zone in the Northern Caucasus

A number of Variscan nappe complexes were recognized in the Late Mesozoic structure of the Front Range Zone of the Greater Caucasus in the 1970s. They consist predominantly of greenstone units and override one another in a consecutive order. The only exception is the upper, Atsgara Nappe, which is composed of crystalline schists, amphibolites, and microgneisses. Crystalline schists, gneisses, amphibolites, and other rocks of the so-called Blyb Complex occur at the base of the nappe packet. The affinity of crystalline rocks of the Blyb Complex to one of the upper Variscan nappes is substantiated in this paper. The Middle Paleozoic rocks, which originally were located below the Blyb Complex in the Front Range structure, overrode its rocks along the surface of the Blyb Thrust Fault in the Early Triassic. Since that time, the crystalline rocks of the Blyb Complex have occupied the lowermost position in the structure of the Front Range. The absence of Upper Paleozoic rocks in the footwall of the thrust fault is due to the fact that, in the Late Paleozoic, the area underlain by the Blyb Complex was an inlier and a source of clastic material. The hanging wall of the Blyb Thrust Fault may be traced farther southward into the Main Range Zone, where it most likely consists of the Laba Group and other rocks. As has been established previously, the lower portion of the Laba Group consists of analogues of the Middle Paleozoic successions of the Front Range Zone, while its upper portion consists of crystalline schists of the Lashtrak Nappe, which occupy a position similar to that of the Atsgara Nappe metamorphic rocks. These relationships suggest that the rock complexes of the Front Range Zone could have undergone repeated displacements due to post-Variscan (Indosinian) tectonic events and overrode crystalline rocks in the Main Range Zone and more easterly areas. Owing to the uplift of the Central Caucasus, they are now eroding. The difference in the metamorphic grade of the Blyb Complex and the rocks of the Atsgara and Marukha nappes is due to the fact the Blyb Complex lies close to the root zones of nappes or belongs to different nappe sheets. The Blyb Thrust Fault pertains to the Indosinian faults that played the main role in the formation of the Front Range structure.

Seismo-Geological Models of Stratigraphic Sequences and Their Application to Oil and Gas Exploration in West Siberia

In west Siberia, large structural prospects for oil exploration have been exhausted. Further discoveries are expected to be made in small to medium structural traps and in lithostratigraphic traps. Prospecting and exploration for hydrocarbons in lithostratigraphic traps is based on new seismo-geological models of hydrocarbon-bearing sequences. In middle Paleozoic rocks, small hydrocarbon accumulations have been found in structural highs; the reservoirs are composed of fractured vuggy carbonates. Seismostratigraphic criteria for prediction of such traps are uncertain. The Lower-Middle Jurassic sequence consists of continental and partially of marine clastic rocks. The sequence is believed to contain considerable hydrocarbon resources in lithostratigraphic traps in basal sandstones and in pinch-out zones of Lower Jurassic sandstones. In the upper part of the sequence, oil pools have been discovered in channel sandstone bodies. Two different areas of oil accumulations are found in the Callovian-Oxfordian sequence. The Bazhenov Formation bituminous shales of Volgian-Berriasian contain sporadically distributed fractured-porous reservoirs. Accumulations in these reservoirs are characterized by irregular oil saturation. The Neocomian sequence consists of rocks deposited on the shallow-water shelf and on the basinal slope. Seismo-geological data indicate that the sequence is prospective for discovery of hydrocarbons in lithostratigraphic traps. The clinoform consists of laterally prograding strata;more » most prospective are frontal and depocentral parts of the strata which contain sandstone reservoirs of medium to poor quality. Lithostratigraphic traps in the shelf sequence can be found in the marginal parts of the paleoshelf where regional pinch-out zones and sandstones lenses are common. The Aptian-Cenomanian sequence deposited under terrestrial and coastal conditions is dominantly gas prone. New hydrocarbon discoveries in this sequence are expected in structural traps.« less

Magnetism of South Mongolian Middle Paleozoic ophiolites

This paper presents new paleomagnetic data from South Mongolia, Middle Paleozoic rocks were studied from three different tectonic zones within the South Mongolian fold belt. From north to south these are the Gobi-Altai zone (in terms of paleotectonic position, the ‘north’ shelf of the paleobasin), the Trans-Altai zone (ophiolite complexes) and the South Gobia zone (the ‘south’ shelf). Several paleomagnetic field tests, including conglomerate, baked contact and fold tests, were carried out to determine magnetic stability. In addition, principal component analysis of the stepwise thermal demagnetization data was used. The ophiolites are characterized by three ancient components. One magnetic component is pre-folding and directed to the south with a very gentle inclination (declination ( D) = 180–210°, inclination ( I) = −6 ⋯ +2°, K = 120). Two post-folding magnetic components were observed also. The first post-folding magnetic component has D = 282.4°, I = −36.4°, k = 30.3, and the second has D = 318.6°, I = 33.0°, k = 60.5. The final magnetic component is very close to the Permian paleomagnetic direction expected from the North China platform. The first post-folding component of magnetization may have been acquired during the Variscan accretion in Early Carboniferous time when the basin was closed. The South Mongolian paleo-ocean was located in equatorial latitudes near the Tarim block, the South China platform and probably the North China platform in the Devonian.

Structural geology of the upper plate of the Bullfrog Hills detachment fault system, southern Nevada

An extremely distended terrane containing two detachment faults and an overlying complex of normal faults is exposed in the Bullfrog Hills, southern Nevada. Shallow crustal rocks have been extended along the detachment faults by listric and planar-rotational normal faults. The detachment faults define three structurally discordant plates. The lower detachment fault separates a lower plate of metamorphosed Late Proterozoic rocks from an overlying middle plate, composed of slivers of lower and middle Paleozoic clastic and carbonate rocks. The middle-plate rocks are brecciated and essentially unmetamorphosed, and the stratigraphic succession is incomplete and highly attenuated. The upper detachment fault separates the middle-plate rocks from an upper-plate succession of block-faulted Miocene volcanic, volcaniclastic, and sedimentary rocks. Miocene rocks of the upper plate dip at moderate to steep angles into the upper detachment fault, or, where the middle plate has been tectonically removed, into the lower detachment fault. The rocks are broken, tilted, and repeated in blocks bounded by normal faults that terminate against, or flatten and merge into, the upper detachment fault or, where the middle plate has been removed, the lower detachment fault. The normal faults in the upper plate are (1) planarrotational faults that form imbricate map patterns and (2) listric faults that are characterized by oval and horseshoe map patterns. These fault map patterns may be due to (1) curvilinear intersection of a listric or planar fault with an antithetic fault, (2) complex intersection of two or more faults of different ages, (3) rotated listric faults, (4) extremely rotated planar normal faults that resemble listric faults, (5) nearly flat-lying normal faults, or (6) topographic and erosional effects. Attenuation of the Late Proterozoic and Paleozoic strata indicates large movement on the detachment faults; the upper plate has been extended more than 100% and possibly more than 275% locally. The geometry of the normal faults and the repetition and dip direction of the Miocene rocks indicate that major extension, at least of the upper plate, was west-northwest-east-southeast and occurred mostly between about 10 and 8 Ma.

The structural geometry and evolution of foreland thrust systems, northern Virginia

Seismic reflection data reveal that the structural geometry of the central Appalachians of northern Virginia consists of three distinct thrust systems. Each thrust system is characterized by a unique internal geometry.The Blue Ridge thrust sheet is a composite thrust sheet composed primarily of imbricated Precambrian crystalline rocks. It over-rode Cambrian-Ordovician carbonates and formed a sheared, basement-cored fault-bend fold. Thrusts within the sheet may be Taconic and earliest Alleghanian, whereas final thrusting and emplacement of the sheet were probably slightly younger but still early Alleghanian. The North Mountain thrust sheet is characterized by imbricated Cambrian-Ordovician carbonates that are deformed into large-amplitude mode II fault-bend folds and fault-propagation folds. Rocks of this sheet were transported more than 60 km across a similar section of carbonates. The leading edge of the North Mountain thrust sheet was deformed into a fold with a mode II fault-bend fold geometry and was juxtaposed against middle Paleozoic rocks. The middle Paleozoic rocks occur in a ramp across which displacement along the North Mountain thrust was transferred to a higher detachment. More than 60 km of cover rocks displaced during the emplacement of the North Mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western Valley and Ridge province or were backthrust above the sheet. The timing for imbrication and emplacement of the North Mountain thrust sheet is probably Main Phase Alleghanian. The Lower Carbonate duplex extends from beneath the Blue Ridge and North Mountain thrust sheets, westward across the western Valley and Ridge province. The Lower Carbonate duplex is characterized by imbricated Cambrian-Ordovician carbonates that form low-amplitude fault-bend folds. This thrust system also probably formed during Main Phase Alleghanian deformation.

Petroleum Potential of Lower and Middle Paleozoic Rocks in Nebraska Portion of Mid-Continent: ABSTRACT

Central North America during the Paleozoic was characterized by northern (Williston) and southern (Anadarko) depositional regimes separated by a stable Transcontinental arch. Nebraska lies on the southern flank of this arch and contains the northern zero edges of the lower and middle Paleozoic rocks of the southern regime. Most of these rocks are secondary dolomites with zones of excellent intercrystalline porosity. The Reagan-LaMotte Sandstones and the overlying Arbuckle dolomites are overlapped by Middle Ordovician rocks toward the Transcontinental arch. Rocks equivalent to the Simpson consist of a basal sand (St. Peter) and overlying interbedded gray-green shales and dolomitic limestones. An uppermost shale facies is present in the Upper Ordovician (Viola-Maquoketa) eastward and southward across Nebraska. The dolomite facies extends northward into the Williston basin. The Silurian dolomites, originally more widely deposited, are overlapped by Devonian dolomites in southeastern Nebraska. Upper Devonian rocks exhibit a regional facies change from carbonate to green-gray shale to black shale southeastward across the Mid-Continent. Mississippian carbonates overlap the Devonian westward and northward across the Transcontinental arch. Pennsylvanian uplift and erosion were widespread, producing numerous stratigraphic traps. Sands related to the basal Pennsylvanian unconformity produce along the Cambridge arch. Arbuckle, Simpson, Viola, and Hunton production ismore » present in the Forest City basin and along the Central Kansas uplift. Although source rocks are scarce and the maturation is marginal, current theories of long-distance oil migration encourage exploration in the extensive lower and middle Paleozoic reservoirs in this portion of the Mid-Continent.« less

Petroleum potential in arctic North America and Greenland

Abstract Sedimentary deposition in Arctic North America and Greenland, in Phanerozoic time, occurred north of the Precambrian Canadian Shield and extended from the Wandel Sea in the east to the Chukchi Sea in the west. Seven geological provinces considered to be highly prospective for oil and gas exist within the region. Lower Paleozoic sediments may exceed 3000 m (10 000 ft) in the several separate basins in the Arctic Interior Platform of the Central Stable Region. To the north of the Arctic Interior Platform, lower Paleozoic sediments thicken into the Franklinian Geosyncline, the second geological province. Twelve thousand metres of lower and middle Paleozoic rocks are folded and faulted along a sinuous belt extending from the Wandel Sea in northeastern Greenland to Prince Patrick Island in the western Canadian Arctic Islands. Unconformably overlying the Franklinian Geosyncline are two successor basins, the third and fourth geological provinces considered in this report. The Sverdrup Basin in the Canadian Arctic Islands contains over 13 000 m (43 000 ft) of upper Paleozoic, Mesozoic and Tertiary sediments. The Wandel Sea Basin in and adjacent to northeastern Greenland, analogous to the Sverdrup Basin, contains a similar complement of upper Paleozoic, Mesozoic and Tertiary sediments. Onshore in Greenland, a composite thickness of 3000 m (10 000 ft) of sediments has been described and offshore, beneath the continental shelf, sediments in the basin may thicken to 10 000 m (33 000 ft). The Arctic Slope of Alaska, the fifth province, exhibits characteristics of a successor basin that has been rotated, translated or otherwise displaced from the Arctic Ocean Basin. Beneath a thickness of some 10 000 m (33 000 ft) of upper Paleozoic, Mesozoic and Tertiary strata is a folded and faulted lower and middle Paleozoic terrane similar to the Franklinian Geosyncline «Fold Belta. The southern margin of this province, now containing the Brooks Range, was folded and faulted during the Laramide Orogeny. A sixth province, the Arctic Coastal Plain, extends along the continent's northern margin from Alaska, to northern Greenland. Strata ranging in age from Jurassic through Pleistocene thicken abruptly from a featheredge to more than 12 000 m (39 000 ft) as they prograde into the Arctic Ocean Basin. The seventh major geologic province is the Baffin Bay Basin, which contains some 10 000 m (33 000 ft) of Mesozoic and Tertiary sediments. With the exception of the Wandel Sea Basin, each of the geological provinces has had its petroleum potential evaluated from well-data. The Arctic Slope of Alaska has hydrocarbon reserves in the order of 1.6 × 109 m3 (10.0 Bbbl) of oil and 750 × 109 m3 (26.5 Tcf) of natural gas. Recoverable reserves for the Beaufort Sea-Mackenzie Delta region of the Arctic Coastal Plain are tentatively considered to be 0.15 × 109 m3 (943 MMbbl). The potential natural gas reserves for the Beaufort Sea-Mackenzie Delta region are in the range of 0.25 × 1012 m3 (8.83 Tcf). The Sverdrup Basin has yet to have significant reserves of oil indicated but the potential reserves of natural gas range from a minimum of 0.45 × 1012 m3 (15.89 Tcf) up to what will probably prove to be more than 565 × 109m3 (20 Tcf) with effective delineation drilling. The Franklinian Geosyncline contains both oil and gas but no significant reserves have been developed. The Wandel Sea Basin, Baffin Bay Basin and Arctic Stable Platform have yet to produce indications of significant pooled hydrocarbons. The undiscovered resource potentials of the Arctic Slope and contiguous Arctic Coastal Plain of Alaska have been estimated to have median values of 3.0 × 109 m3 (18.9 Bbbl) of oil and 1.39 × 1012m3 (49.08 Tcf) of natural gas. The undiscovered resource potentials of the Beaufort Sea-Mackenzie Delta region of the Arctic Coastal Plain have expected values of 1.35 × 109m3 (8.45 Bbbl) of oil and 2.91 × 1012m3 (102.75 Tcf) of natural gas. The combined resource potential of the Arctic Interior Platform, Franklinian Geosyncline, Sverdrup Basin and Baffin Bay Basin have expected values of 0.82 × 109 m3 (5.16 Bbbl) of oil and 2.98 × 1012 m3 (105 Tcf) of natural gas. The resource potential of the Arctic Islands Coastal Plain has not yet been effectively evaluated because the greater portion of the region lies beneath constantly shifting pack-ice and data required for a meaningful assessment are not available. It is possible however, that the Coastal Plain in the Arctic Islands area may have resource potentials as great as any other previously assessed Arctic province. Formal estimates of the resource potentials of northern Greenland and the Wandel Sea Basin are not available but on the basis of comparable geology, values can be expected to be proportional to those of the Arctic Interior Platform, the Franklinian Geosyncline and the Sverdrup Basin, respectively.

Lower Ordovician through Lower Devonian cratonic margin rocks of the southern Great Basin

Research Article| May 01, 1981 Lower Ordovician through Lower Devonian cratonic margin rocks of the southern Great Basin RICHARD H. MILLER; RICHARD H. MILLER 1Allison Center, Department of Geological Sciences, San Diego State University, San Diego, California 92182 Search for other works by this author on: GSW Google Scholar GEORGE A. ZILINSKY GEORGE A. ZILINSKY 1Allison Center, Department of Geological Sciences, San Diego State University, San Diego, California 92182 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD H. MILLER 1Allison Center, Department of Geological Sciences, San Diego State University, San Diego, California 92182 GEORGE A. ZILINSKY 1Allison Center, Department of Geological Sciences, San Diego State University, San Diego, California 92182 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1981) 92 (5): 255–261. https://doi.org/10.1130/0016-7606(1981)92<255:LOTLDC>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 RICHARD H. MILLER, GEORGE A. ZILINSKY; Lower Ordovician through Lower Devonian cratonic margin rocks of the southern Great Basin. GSA Bulletin 1981;; 92 (5): 255–261. doi: https://doi.org/10.1130/0016-7606(1981)92<255:LOTLDC>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 Lower and middle Paleozoic rocks in the southeastern Great Basin include thin sequences of gray, generally unfossiliferous dolostones that were deposited in cratonic margin paleoenvironments. Age-diagnostic conodonts recovered from a 295-m-thick section of Mountain Springs Formation indicate that Lower and lower Middle Ordovician, Upper Ordovician, and upper(?) Lower Devonian rocks occur in the section. Distinct disconformities provide evidence for a mid-Ordovician hiatus and a Silurian through Early Devonian hiatus. These cratonic margin rocks thicken, become lithologically more diverse, and form a more temporally continuous section of miogeosynclinal rocks 15 to 20 km to the west. This narrow, northeast-trending facies change is a linear geologic element that is offset ∼40 km by the Las Vegas Valley shear zone. 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.

Middle Paleozoic Sedimentation and Paleogeography of Southern Great Basin: ABSTRACT

Based on detailed analysis of conodont distribution, middle Paleozoic rocks in the southern Great Basin have been divided into refined time intervals. These include late Middle through Late Ordovician; early to middle Early Silurian; late Early Silurian; Late Silurian; and Early to early Middle Devonian. Rocks of these intervals consist of fine-grained limestone, fine to medium-grained dolostone, and lesser amounts of silty limestones and impure calcareous shales. Based on the distribution of lithofacies, deposition occurred on an eastern craton margin and within inner- and outer-shelf regions on the Cordilleran miogeocline. Offset of these regions can be used to estimate movement along major faults (e.g., the Death Valley-Furnace Creek fault zone). The craton margin was a region of supratidal to very shallow subtidal environments during part of this time interval. Regression and erosion of the craton margin during Early and Late Silurian time provided a source of carbonate mud as well as silt to sand-sized quartz which were redeposited on the shelf areas in the west. An unconformity separates Upper Ordovician rocks from overlying Lower Devonian rocks. Intertidal to subtidal environments with nearly continuous deposition existed on the inner shelf, providing evidence of regional regression during early and middle Early Silurian and Late Silurian time. The outer shelf was an area of complex environments including platforms and local basins. Uplift and erosion in Late Silurian time affected part of the outer shelf. In the northern nyo Mountains an unconformity separates upper Lower Silurian rocks from overlying Lower Devonian rocks. End_of_Article - Last_Page 418------------

Structural geology of the Nenana River segment of the Denali fault system, central Alaska Range

Research Article| September 01, 1977 Structural geology of the Nenana River segment of the Denali fault system, central Alaska Range ROBERT G. HICKMAN; ROBERT G. HICKMAN 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 95706 Search for other works by this author on: GSW Google Scholar CAMPBELL CRADDOCK; CAMPBELL CRADDOCK 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 95706 Search for other works by this author on: GSW Google Scholar KIRK W. SHERWOOD KIRK W. SHERWOOD 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 95706 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT G. HICKMAN 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 95706 CAMPBELL CRADDOCK 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 95706 KIRK W. SHERWOOD 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 95706 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (9): 1217–1230. https://doi.org/10.1130/0016-7606(1977)88<1217:SGOTNR>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 ROBERT G. HICKMAN, CAMPBELL CRADDOCK, KIRK W. SHERWOOD; Structural geology of the Nenana River segment of the Denali fault system, central Alaska Range. GSA Bulletin 1977;; 88 (9): 1217–1230. doi: https://doi.org/10.1130/0016-7606(1977)88<1217:SGOTNR>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 The Denali fault system, one of the major tectonic elements of southern Alaska, forms an arc 2,100 km long across southern Alaska. In the central Alaska Range, the system consists of a northern Hines Creek strand and a southern McKinley strand, 30 km apart, which divide the area into northern, central, and southern terranes. There is evidence for at least two episodes of deformation in the northern terrane, four in the central, and two in the southern during Paleozoic and Mesozoic time. During each, the inferred axis of maximum compressive strain was subhorizontal and about north-south, but the direction shifted to north-northwest–south-southeast during a late Paleocene–Eocene folding episode. Tectonic stability during Oligocene–middle Miocene time was followed by differential uplift of crustal blocks during late Miocene–Pliocene time.The Hines Creek fault may preserve a record of the early history of the fault system. Strong contrasts between lower and middle Paleozoic rocks juxtaposed along the fault suggest large dextral strike-slip displacement, but major convergent movement cannot be ruled out. Movement throughout the Hines Creek fault ceased by middle Cretaceous time, but local dip-slip movements continued into the Cenozoic era. The McKinley fault is an active dextral strike-slip fault, with Cenozoic offset of probably at least 30 km and possibly much greater. Mean Holocene displacement rates are 1 to 2 cm/yr. These rates would produce a 30-km offset in 1.5 to 3.0 m.y., or a 400-km offset in 20 to 40 m.y.The Denali fault may be part of a transform fault system connecting the Juan de Fuca Ridge and the landward extension of the Aleutian subduction zone. However, it is more likely that the fault forms the northern boundary of a small lithospheric plate caught between the Pacific and American plates. 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.

Are the Ordovician Northern Appalachians and the Mesozoic Cordilleran System Homologous?

Recent syntheses presume that the Ordovician northwestern Appalachians and the Mesozoic Cordilleran system are genetically similar, as is indicated by subduction zones interpreted in the northern Appalachians. Actualistic models for the proposed relationships have commendably been suggested, and a predictive model that outlines a transition from present Atlantic-type to present Pacific-type margins has been presented. The syntheses complicate the model, however, by using in turn the lower Paleozoic Appalachians and Caledonides as a model for a possible present-to-future Atlantic margin. This assumes that the Appalachian-Caledonian belt is ensimatic and contains subduction zones, two quite essential features of cordilleran-type orogenic belts. Faunal paleogeographic, stratigraphic, petrologic, and structural evidence in the northern Appalachians nevertheless indicates a probable sialic basement overlain by lower and middle Paleozoic rocks that are stratigraphically and structurally continuous across the proposed subduction zones, and in which extension as opposed to compression predominates. These considerations indicate that a simatic basement is conjectural in the Appalachians and that subduction zones are very doubtful. Consequently the lower Paleozoic northern Appalachians and the Mesozoic Cordilleran system do not seem homologous. The principal alternative to subduction is partial closure of the Appalachian geosyncline by flexural folding of the sialic basement and the geosynclinal contents in a brief although major Devonian Acadian orogenic event, preceded by recumbent folding and gravity sliding from intrageosynclinal geanticlines in lesser orogenic events distributed over much of the earlier Paleozoic. The most nearly actualistic model of the northern Appalachian orogen seems to be in the Mediterranean region, where still-foundering continental crust and related extensional features coexist with Benioff zones and related contractional features.

Initial Deposits in the Cordilleran Geosyncline: Evidence of a Late Precambrian (&lt;850 m.y.) Continental Separation

Upper Precambrian and Lower Cambrian strata in western North America are exposed in a narrow slightly sinuous belt extending from Alaska and northern Canada to northern Mexico, a distance of 2,500 mi. Within this belt, the strata thicken from 0 ft on the east to 15,000 to 25,000 ft in areas 100 to 300 mi to the west. The basal unit of this sequence is a diamictite (conglomeratic mudstone) which is widely distributed but discontinuous and is generally considered to be of glacial origin. Overlying sedimentary rocks consist predominantly of siltstone, shale, argillite, quartzite, and conglomerate. Tholeiitic basalt forms thick and widespread units near the base of the sequence but is sparse higher in the section. The distribution pattern and lithologic characteristics of the upper Precambrian and Lower Cambrian sequence fit the recently developed concept that thick sedimentary sequences accumulate along stable continental margins subsequent to a time of continental separation. The depositional pattern of the sequence is unlike that of underlying rocks, a relation consistent with the idea of a continental separation cutting across the grain of previous structures. The pattern is, on the other hand, similar to that of overlying lower and middle Paleozoic rocks, suggesting that the diamictite and post-diamictite rocks were the initial deposits in the Cordilleran geosyncline. Thick units of volcanic rock near the bottom of the sedimentary sequence indicate volcanic activity related to the thinning and rifting of the crust during the continental separation.

New Hypothesis of Early Jurassic Paleogeography and Sediment Dispersal for Western United States

Recent studies of both composition and sedimentary structures of Upper Triassic and Lower Jurassic clastic rocks in the Cordillera provide evidence for a modified paleogeographic reconstruction. The presence of marine Triassic rocks in north-central Nevada and of Jurassic mature quartz sandstones in western Nevada and southeastern California indicates that no continuous Mesocordilleran geanticline completely separated the miogeosynclinal and eugeosynclinal belts during early Mesozoic time. In southern Nevada, Upper Triassic and Lower Jurassic sedimentary strata considered typical of miogeosynclines are interstratified with conglomerates and volcaniclastic rocks derived from a rising volcanic island arc in the west in California. The only adequate source of pure quartz sand to form the Navajo and associated formations appears to have been Triassic and upper Paleozoic sandstones of the western craton in Montana and Alberta, where Middle Jurassic strata are regionally disconformable on middle and lower Paleozoic rocks. Crossbedding orientation data indicate that sand was transported south from the region parallel with the cratonic margin, and thence southwesterly into western Nevada and California. A broad marine carbonate bank lay farther northwest in Nevada. Much of the deposition of the pure sands probably was as very shallow marine sand waves, as well as in coastal eolian dunes formed by onshore winds.

Structural development of the Paleozoic rocks of western Gaspé, Quebec

Cambro-Ordovician greenschists and phyllites in the Shickshock Mountain area are characterized by a foliation S1 almost completely parallel to layering S0, and a mineral lineation apparently axial to obscure tight folds F1. Absence of these structures from Lower to Middle Ordovician argillites immediately to the north of the Shickshocks suggests that F1 folding and metamorphism occurred earlier than at least part of the Early Ordovician Epoch.The dominant folds F2, which affect all Lower and Middle Paleozoic rocks, trend northeast and include the Gaspé synclinorium with a Siluro-Devonian axial zone. F2 are upright to inclined, and an axial-surface fracture cleavage S2 forms a major fan divergent upward across the synclinorium. F2 hinges are generally low plunging; but some are curved as much as 90°, perhaps because of variable dip-slip on S2.Discontinuous folds F3 exposed only in Ordovician rocks on the north coast, have steep axial surfaces striking eastward oblique to northeast-trending S2 cleavage. Small kinks, F4, virtually restricted to the Shickshock schists with S1, form conjugate sets with steep to vertical northwest-trending kink planes. Later (?) low-dipping kink bands in Devonian argillites displace steep S2 sub-horizontally.The strain during F1 folding is not clear. F2, F3, and F4 possibly formed while near horizontal compression across axial surfaces progressively changed azimuth clockwise. The low-dipping kink bands may reflect late lateral expansion associated with uplift.