Early Paleozoic Continental Margin Research Articles

Granulite facies metasedimentary rocks: Insight into the formation of the East Kunlun continental arc

The metamorphism and dynamic processes of the Early Paleozoic continental margin arc in the East Kunlun orogen are still unclear. This study investigated the metasedimentary rocks with granulite facies in the Qingshuiquan area in the eastern section of the East Kunlun orogen. The kyanite is not found in the pelitic granulite. The phase equilibrium modeling indicates a middle-pressure type with metamorphic conditions of 0.7 to 0.8 GPa and 800 to 840°Cfor pelitic granulite. UPb dating of zircons from metasedimentary rocks yielded metamorphic ages of 490-480 Ma and detrital zircons ages of 1700-1500 Ma. REE patterns of the metasedimentary rocks are enriched LREEs and distinct negative Eu anomalies, the εNd values vary from −1 to −16. These results imply that the Precambrian supracrustal rocks were buried into the middle-lower crust level and granulite facies metamorphism occurred in Late Cambrian-Early Ordovician.Synchronous (520-480 Ma) magmatic rocks in Qingshuiquan and adjacent areas indicate that subduction of the Proto-Tethyan oceanic crust led to the formation of arc magmatic rocks. Meanwhile, it induced some sedimentary rocks to underthrust or return flows into the middle-lower crust of the upper plate and underwent granulite facies metamorphism.

A visage of early Paleozoic Japan: Geotectonic and paleobiogeographical significance of Greater South China

AbstractThe current state of knowledge on the early Paleozoic evolution of Japan is reviewed. Although early Paleozoic Japan marked the foundation of long‐lasting subduction‐related orogenic growth throughout the Phanerozoic, details of this have not been fully revealed. Nevertheless, U‐Pb dating of zircons both in Paleozoic granitoids and sandstones is revealing several new aspects of early Paleozoic Japan. The timing of the major tectonic change, from a passive continental margin setting (Stage I) to an active one (Stage II), was constrained to the Cambrian by identifying the oldest arc granitoid, high‐P/T blueschist, and terrigenous clastics of arc‐related basins. Ages of recycled zircons in granitoids and sandstones provided critical information on the homeland of Japan, i.e. the continental margin along which proto‐Japan began to grow. The early Paleozoic continental margin that hosted the development of an arc‐trench system in proto‐Japan had cratonic basement composed mostly of Proterozoic crust with a minor Archean component. The predominant occurrence of Neoproterozoic zircons in Paleozoic rocks, as xenocrysts in arc granitoids and also as detrital grains in terrigenous clastics, indicates that the relevant continental block was a part of South China, probably forming a northeastern segment of Greater South China (GSC) together with the Khanka/Jiamsi/Bureya mega‐block in Far East Asia. GSC was probably twice as large as the present conterminous South China on mainland Asia. Paleozoic Japan formed a segment of a mature arc‐trench system along the Pacific side of GSC, where the N–S‐trending Pacific‐rim orogenic belt (Nipponides) developed with an almost perpendicular relationship with the E–W‐trending Central Asian orogenic belt. The faunal characteristics of the Permian marine fauna in Japan, both with the Tethyan and Boreal elements, can be better explained than before in good accordance with the relative position of GSC with respect to the North China block during the late Paleozoic.

Wave gradiometry and its link with Helmholtz equation solutions applied to USArray in the eastern U.S.

AbstractWave gradiometry is an array processing technique utilizing the shape of seismic wavefields captured by USArray TA stations to determine fundamental wave propagation characteristics. We first explore a compatibility relation that links the displacement spatial gradients to seismogram displacements and velocities through two unknown coefficients, and . These coefficients are solved for through iterative, damped least squares inversion to provide estimates of phase velocity, back azimuth, radiation pattern, and geometrical spreading. We show that the coefficient corresponds to the gradient of logarithmic amplitude, and the coefficient corresponds approximately to the local wave slowness. and vector fields are interpolated to explore a second compatibility relation through solutions to the Helmholtz equation. For most wavefields passing through the eastern U.S., we show that the coefficients are generally orthogonal to the coefficients. Where they are not completely orthogonal, there is a strong positive correlation between and changes in geometrical spreading, which can be further linked with areas of strong energy focusing and defocusing. We finally obtain isotropic Rayleigh wave phase velocity maps for 15 periods between 20 and 150 s, by stacking results from 37 earthquakes, which show a velocity change along the approximate boundary of the early Paleozoic continental margin. We also observe two low‐velocity anomalies, one centered over the central Appalachians where Eocene basaltic volcanism has occurred and the other within the northeastern U.S., possibly associated with the Great Meteor Hotspot track.

Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada

Research Article| July 01, 2007 Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada Sébastien Castonguay; Sébastien Castonguay 1Geological Survey of Canada, GSC-Québec, 490 rue de la Couronne, Québec G1K 9A9, Canada Search for other works by this author on: GSW Google Scholar Gilles Ruffet; Gilles Ruffet 2Geosciences Rennes, CNRS-Université de Rennes 1, Campus de Beaulieu, Avenue Général Leclerc, Rennes Cedex 35042, France Search for other works by this author on: GSW Google Scholar Alain Tremblay Alain Tremblay 3Département des Sciences de la Terre et de l'Atmosphère, Université du Québec à Montréal, C.P. 8888, Succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (7-8): 978–992. https://doi.org/10.1130/B26046.1 Article history received: 25 May 2006 rev-recd: 01 Feb 2007 accepted: 02 Feb 2007 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Sébastien Castonguay, Gilles Ruffet, Alain Tremblay; Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada. GSA Bulletin 2007;; 119 (7-8): 978–992. doi: https://doi.org/10.1130/B26046.1 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 Geochronologic 40Ar/39Ar data of fabric-forming metamorphic minerals, in conjunction with structural and metamorphic studies, are being increasingly used to constrain the deformation and tectonometamorphic evolution of polyphase low-grade orogens. Careful data interpretation is needed to extract meaningful age constraints from neo- and recrystallized minerals affected by isotopic disturbances. In the southern Quebec Appalachians, the Sutton Mountains anticlinorium exposes the metamorphic core of the early Paleozoic continental margin of Laurentia. Orogenesis in this part of the Appalachians was the result of tectonic events that have been classically attributed to the combined effects of the Middle to Late Ordovician Taconian and the Middle Devonian Acadian orogenies; however, evidence of separate and distinct Silurian–Early Devonian tectonism has also been recently documented. Laser step-heating 40Ar/39Ar data on single-grains of muscovite from polydeformed greenschist facies samples of the Sutton Mountains anticlinorium indicate that these tectonometamorphic events are heterogeneously preserved as a prograde Taconian event at ca. 456 Ma and an Acadian overprint at ca. 390 Ma. The integration of 40Ar/39Ar age spectra analyses with structural relationships provides precise age constraints on the duration, propagation, and evolution of Silurian–Early Devonian hinterland-directed deformation as it migrated across the anticlinorium, including back thrusting from ca. 433 Ma to ca. 420 Ma and extensional faulting from ca. 417 Ma to ca. 405 Ma. Along the Laurentian margin of the northern Appalachians, such Silurian–Early Devonian tectonism is currently attributed either to the collapse of the Taconian orogen triggered by the delamination of the subducted slab or to the outboard accretion of peri-Gondwanan terranes during the Salinic orogeny. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Early Paleozoic transform-margin structure beneath the Mississippi coastal plain, southeast United States

A geophysical transect across the central Gulf of Mexico coastal plain shows that the early Paleozoic continental margin of southern Laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late Paleozoic Ouachita orogeny and formation of the modern Gulf of Mexico coastal plain. The thickness of the crystalline crust decreases abruptly across the margin over a distance of ∼50 km, from 35 km beneath the Black Warrior foreland basin to 10 km beneath the Ouachita fold-and-thrust belt. This abrupt decrease in crustal thickness is similar to modern transform margins, but very different from most rifted margins, which display much more gradual transitions in crustal thickness. The geophysical data indicate an absence of synrift intrusive and volcanic rocks, underplated mafic rocks at the base of the crust, and abnormally thick oceanic crust adjacent to the margin. The lack of these features is also characteristic of modern transform margins. Combined with transects across the margin farther west, the data confirm previous suggestions that the central Gulf of Mexico coastal plain overlies an ∼800-km-long transform segment of the late Proterozoic–early Paleozoic southern Laurentian continental margin that extends continuously from western Arkansas to southeast Alabama.

Gravity constraints on lithosphere flexure and the structure of the late Paleozoic Ouachita orogen in Arkansas and Oklahoma, south central North America

Spectral analysis of Bouguer gravity anomalies in western central Arkansas and eastern Oklahoma indicates that the thickness of the crust in the Ouachita fold and thrust belt increases from 38 km in the western Ouachitas to 44 km in the eastern Ouachitas. The change in crustal thickness occurs near the western end of the Broken Bow uplift and coincides with an abrupt decrease in the flexural rigidity of the lithosphere from 1.8×1024 N m in the western Ouachitas to 5.0×1023 N m in the eastern Ouachitas. The flexural rigidity in the western Ouachitas is similar to values determined in the Appalachian fold and thrust belt and coincides with the depth of the 450°C isotherm predicted by conductive cooling models for the thermal evolution of the early Paleozoic southern Laurentian rifted continental margin. The thick crust in the eastern Ouachitas results in lithosphere that is anomalously weak for rifted continental crust of this age. The thicker crust is attributed to an eastward transition from a rift segment to a transform segment of the Paleozoic continental margin. A layered density model derived from the gravity data shows that strata interpreted to be deformed Ouachita facies rocks are thickest in the eastern Ouachitas and are consistent with a greater amount of shortening in the central thrust belt in Arkansas as compared to Oklahoma. The opposite relationship is observed in the frontal Ouachita province, where shortening appears greater in Oklahoma. The cross‐strike changes in the locus of shortening, crustal thickness, flexural rigidity, and the inferred transition from rift to transform segments of the early Paleozoic continental margin all coincide with the location of a previously hypothesized zone of diffuse rightlateral shear located at the western end of the Benton uplift. Flexural modeling indicates that the load required to produce the observed Bouguer gravity low in the Arkoma foreland basin trends parallel to the Benton and Broken Bow uplifts but is located 114 to 276 km farther south. In the western Ouachitas, the position of the load coincides with the northern edge of the Sabine uplift and is interpreted to mark the southern extent of Ouachita facies rocks that were emplaced on the Laurentian continental margin and/or attached remnant oceanic crust.

Deep Exploration Possibilities along the Ouachita Trend

The late Paleozoic Ouachita orogeny produced a sinuous deformed belt that extends from Alabama through Mississippi, Arkansas, Oklahoma, Texas, and finally southward into Mexico. Recent geophysical studies have provided important new data that make it possible to reevaluate exploration possibilities along the Ouachita trend. A deep seismic study alo profile extending from the Ouachita Mountains in southwestern Arkansas into northwestern Louisiana has provided a surprisingly clear image of the early Paleozoic continental margin in the area. This margin is largely undeformed and suggests that the Ouachita orogeny involved little if any shortening of continental crust. Along with gravity data, this result implies that Ouachita thrusts are thin-skinned structures that include features such as the Benton and Broken Bow uplifts. Using these results as a base from which to build, the author has been able to trace this margin southeastward into Mississippi and westward through Texas and into Mexico. Although some production has been established in Ouachita facies rocks, gravity anomalies and limited amounts of seismic data suggest that two subthrusts plays have potential. In addition, there are areas where completely new subthrusts basins are possible. The second play is the early Paleozoic continental margin and rift basins that formed duringmore » the early stages of its development. His results to data indicate that these features are not greatly deformed and are at economically drillable depths in many areas. However, the area south of the Paleozoic continental margin should be avoided because the thickness of Ouachita facies rocks exceeds 10 km.« less

Paleozoic continent-ocean transition in the Ouachita Mountains imaged from PASSCAL wide-angle seismic reflection-refraction data

Research Article| February 01, 1989 Paleozoic continent-ocean transition in the Ouachita Mountains imaged from PASSCAL wide-angle seismic reflection-refraction data G. R. Keller; G. R. Keller 1Department of Geological Sciences, University of Texas, El Paso, Texas 79968-0555 Search for other works by this author on: GSW Google Scholar L. W. Braile; L. W. Braile 2Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar G. A. McMechan; G. A. McMechan 3Center for Lithospheric Studies, University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688 Search for other works by this author on: GSW Google Scholar William A. Thomas; William A. Thomas 4Department of Geology, University of Alabama, Box 1945, Tuscaloosa, Alabama 35487 Search for other works by this author on: GSW Google Scholar Steven H. Harder; Steven H. Harder 5Department of Geophysics, Texas A & M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar Wen-Fong Chang; Wen-Fong Chang 3Center for Lithospheric Studies, University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688 Search for other works by this author on: GSW Google Scholar W. G. Jardine W. G. Jardine 2Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar Author and Article Information G. R. Keller 1Department of Geological Sciences, University of Texas, El Paso, Texas 79968-0555 L. W. Braile 2Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 G. A. McMechan 3Center for Lithospheric Studies, University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688 William A. Thomas 4Department of Geology, University of Alabama, Box 1945, Tuscaloosa, Alabama 35487 Steven H. Harder 5Department of Geophysics, Texas A & M University, College Station, Texas 77843 Wen-Fong Chang 3Center for Lithospheric Studies, University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688 W. G. Jardine 2Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1989) 17 (2): 119–122. https://doi.org/10.1130/0091-7613(1989)017<0119:PCOTIT>2.3.CO;2 Article history First Online: 02 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 G. R. Keller, L. W. Braile, G. A. McMechan, William A. Thomas, Steven H. Harder, Wen-Fong Chang, W. G. Jardine; Paleozoic continent-ocean transition in the Ouachita Mountains imaged from PASSCAL wide-angle seismic reflection-refraction data. Geology 1989;; 17 (2): 119–122. doi: https://doi.org/10.1130/0091-7613(1989)017<0119:PCOTIT>2.3.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 A wide-angle reflection-refraction experiment, sponsored by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), was conducted in the Ouachita Mountains area of southwestern Arkansas and northwestern Louisiana. This experiment employed 400 state-of-the-art seismic recorders and overlapped the southern one-third of the COCORP (Consortium for Continental Reflection Profiling) deep seismic reflection profile in the area. A wide variety of data processing and interpretation techniques was employed to derive an Earth model from these data. The model depicts a preserved early Paleozoic continental margin buried beneath allochthonous Paleozoic strata and younger sedimentary rocks. The southern part of the model indicates the presence of oceanic or highly extended continental crust overlain by about 15 km of mostly Paleozoic sedimentary rock. These results are consistent with little if any shortening of crystalline continental crust during the Ouachita orogeny. 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.

Structural Relations Between Marfa, Marathon, Val Verde, and Delaware Basins of West Texas: ABSTRACT

The Marfa, Marathon, Val Verde, and Delaware basins and related uplifts formed the major structural elements of the southwestern continental margin of North America during the Paleozoic. In contrast with the relatively simple relationships where the southern Oklahoma aulacogen intersects the Ouachita orogenic belt, structural relationships in the area of these basins are very complex. Various geologic evidence points to an allochthonous Marathon basin. However, a prominent gravity anomaly is associated with the Ouachita system as it extends from western Arkansas through Oklahoma and Texas into northern Mexico. If this anomaly is the signature of the early Paleozoic continental margin, then the location of the Marathon basin with respect to this anomaly suggests lateral di placements have been only on the scale of tens of kilometers. The Delaware basin seems clearly analogous to the Anadarko basin in that it formed as a result of reactivation of a major crustal flaw (not necessarily a rift). This reactivation was a result of the Ouachita orogeny. The Marfa basin is also flanked by a linear gravity high and basement uplift. The relationship of this anomaly to the gravity high associated with the Ouachita system suggests that the Marfa basin may be more analogous to the Delaware basin than foreland basins such as the Ft. Worth and Arkoma. A prominent gravity high that extends into northern Mexico is associated with the Devil's River uplift, and the relationships between this feature, the Val Verde basin, and adjacent structures suggest major deformation on a crustal scale. End_of_Article - Last_Page 272------------

Evidence for two pulses of glaciation during the late Proterozoic in northern Utah and southeastern Idaho

A record of glaciation during late Proterozoic time is preserved in a number of localities extending from the Sheeprock Mountains, Utah, to Pocatello, Idaho, and from the Park City area 40 km east of Salt Lake City to the Deep Creek Range along the Utah-Nevada line. Over much of this area, the glacial deposits and associated rocks thicken westward and form the basal part of a miogeoclinal wedge that accumulated near the late Proterozoic and early Paleozoic continental margin. In the east, such deposits are thin and rest on Archean basement or rocks of Proterozoic Y age; in the west, they are part of thicker sequences in which deposition apparently continued without significant interruption from late Proterozoic into Cambrian time. In many places, the original continuity between the western and eastern parts of the depositional wedge has been obscured by thrusting of Cretaceous and early Tertiary age that carried the thick basinal sequences eastward over those deposited on the continental platform. Recent mapping of Fremont Island in Great Salt Lake, the Wasatch Range between Ogden and Brigham City, and the Sheeprock Mountains shows that glacial episodes represented either by diamictite or by dropstones enclosed in finegrained laminated beds are separated by as much as 1,000 m of non-glacial deposits, including black slate, alternating graywacke and siltstone, quartzite, and conglomerate. Using reasonable sedimentation rates for such deposits and by comparison with modern analogues, we infer that two episodes of glaciation, each probably consisting of multiple advances and retreats, were separated by a non-glacial interval of a few hundred thousand to a few million years' duration. Correlation of the allochthonous, miogeoclinal glacial deposits with the single glacial unit present in autochthonous and parautochthonous platform sites is uncertain, but our interpretation of sedimentary facies and paleogeography suggests that only the younger of the two episodes recorded in the allochthon is represented by the diamictites of the autochthon.

Pre-Orogenic and Synorogenic Diagenesis and Anchimetamorphism in Lower Paleozoic Continental Margin Sequences of the Northern Appalachians in and Around Quebec City, Canada.

ABSTRACT Thermal maturation of organic matter in shales and illite crystallinity reveal a regional southeastward increase in grade from the late middle and late stages of diagenesis to epimetamorphism in early Paleozoic continental margin deposits of the Quebec Appalachians near Quebec City. Four diagenetic or low-grade metamorphic zones are distinguished: Zone I representing the late middle stage of diagenesis with a mean random reflectance (0%) of asphaltic pyrobitumen between 1.0 and 1.5 per cent and illite crystallinity (IC) between 5.5 and 8.5 mm (0.72 and 1.10°2) zone II (late diagenetic stage) with 0 varying between 1.5 and 2.8 per cent and IC between 3.2 and 5.5 mm (0.42 and 0.72°2) or up to 8.0 mm (1.04°2) for black shales, zone III (anchimetamorphic zone) with 0 varying between 2.8 and 3.8 per cent and IC between 1.8 and 3.2 mm (0.23 and 0.422) and zone IV (epizone) with 0 greater than 3.8 per cent and IC less than 1.8 mm (less than 0.23°2). In the northwest (zones I and IIA) reflectance and illite crystallinity increase, within individual nappes, as a function of age or depth of burial. Farther southeast (zones IIB to IV) no such dependence is observed; instead, diagenetic grade increases regionally in a southeastward direction. Diagenesis in zones I and IIA is either in part or entirely pre-orogenic and has been transported during nappe movement, because in the stack of nappes near Quebec City diagenetic grade decreases downward from the higher to the lower tectonic units. Depth of burial of Cambrian deep-water clastic rocks of the Chaudiere Nappe (highest tectonic unit) was estimated as 6 to 7 km on the basis of reflectance (1.71 to 2.30%, diagenetic zone IIA) and an assumed paleogeothermal gradient of 30°C/km. This burial depth may have been achieved entirely by syndepositional overburden. For the Middle Ordovician Quebec Promontory Nappe (lowest exposed tectonic unit) a reflectance of 1.01 to 1.15 per cent (diagenetic zone I) requires 4 to 5 km of overburden, part of which was probably provided by the higher tectonic nappes, because younger sedimentary rocks of sufficient thickness that could have once overlain these Middle Ordovician deposits are not known in the region. Diagenesis in zone IIA, therefore, was most likely formed entirely before orogenesis and nappe transport; in zone I it was probably raised after nappe emplacement. The grade of diagenesis and metamorphism reached in zones IIB to IV are interpreted as related to regional synorogenic heating in conjunction with the Taconic orogeny. Thermal maturation levels in zone I indicate that the rocks may not have passed the oil window, which is of interest for petroleum exploration in Quebec.

Subaqueous Gravity-Displacement Products: ABSTRACT

All sediments deposited on subaqueous slopes are affected by the tangential component of the earth's gravity. Among the kinds of gravity-displacement processes are subaqueous rock falls, slumps, and debris flows. Turbidity currents are likewise processes affected by the slope, but are not a subject of discussion here. Individual blocks of rock move down a slope from reefs and coastal cliffs; they result from bioerosion, the effects of storm, or ordinary gravity. Incoherent slumps generate a body of sediment so thoroughly mixed and churned that nearly all traces of stratification are obliterated. Debris-flow deposits containing large blocks are known as olistostromes. In places, such olistostromes have been mistaken for melanges, a mixture of huge blocks of diverse kind and provenance dispersed in pervasively sheared and fine-grained matrix which forms a special kind of tectonic breccia. Deep-water rubble of shallow-water carbonate rocks, usually angular, interstratified with dark deep-water marine shale is known as brecciola. Brecciolas accumulate at the toe of the slope. On the upper part of the s ope accumulate sediments resulting from coherent slumps which have moved down a slope with their strata still preserved. Deposits of slope-influenced sediments may also result from contour-following currents (contourites) which travel along the lower parts of slopes. Many examples of ancient gravity-displacement products have been reported from the rock record, among them the so-called melanges (in fact, olistostromes) of Turkey, slope-fan-basin-plain deposits of the Appalachians, and basinal deposits in the Delaware basin of west Texas. In the Taconic sequence of the Appalachians, as in many other hinge-line deposits, only the lower slope and base-of-slope portion of the early Paleozoic continental margin has been preserved. End_of_Article - Last_Page 453------------

Exploration Research Along Ardmore-Anadarko Basin Trend: ABSTRACT

Consideration of southern Oklahoma geologic history in the context of plate-tectonic analogies to present continental margins suggests several avenues of investigation that help explain some oil and gas accumulations and could lead to more discoveries. Postulated origin as the abandoned arm (aulacogen) of a rift triple junction in the late Precambrian and early Paleozoic suggests the potential for fault-controlled sedimentation and early generation of oil and gas by magmatic heating. Collision-related late Paleozoic deformation suggests displacement of early Paleozoic reservoirs by wrench faulting and the formation of traps by wrench-controlled thrust faulting. The search for fracture reservoirs involves facies relationships to the precollision continental margin, fractur ng during collision, and prediction of open fractures based on stress orientation related to formation of the Gulf of Mexico. Position of the early Paleozoic continental margin with its unrealized potential for oil and gas accumulation remains an enigma concealed by late Paleozoic emplacement of the Ouachita thrust complex unknown distances over the edge of the early Paleozoic continental edge and subsequent burial by Mesozoic sediment during formation of the Gulf of Mexico. End_of_Article - Last_Page 540------------

40Ar/39Ar incremental-release ages of hornblende and biotite from Grenville basement rocks within the Indian Head Range complex, southwest Newfoundland: their bearing on Late Proterozoic – Early Paleozoic thermal history

Hornblende and biotite from autochthonous basement rocks within the Indian Head Range complex of southwest, insular Newfoundland record undisturbed 40Ar/39Ar release spectra with average total-gas ages of 880 Ma (hornblende) and 825 Ma (biotite). These gas-retention ages date the times when this segment of the western Appalachian basement terrane cooled below hornblende and biotite argon retention temperatures (~500 °C and ~300 °C respectively) following culmination of the ~ 1150–1100 Ma Grenville metamorphism. Although these results indicate that elevated temperatures were maintained for a prolonged period following the Grenville thermal peak, once initiated, cooling must have been relatively rapid because hornblende and biotite record generally similar total-gas dates.The undisturbed release spectra of minerals within the Indian Head Range complex indicate that this segment of the western Appalachian basement terrane was not affected by Paleozoic metamorphism. This is consistent with recent tectonic models that indicate that the overlying Humber Arm allochthon was emplaced into its present position as a cold, already assembled structural unit. Lack of Paleozoic metamorphism within the Indian Head basement rocks is also compatible with suggestions that the obduction site of the Bay of Islands ophiolite lay considerably east of the Early Paleozoic continental margin of North America.

A Slope-Fan-Basin-Plain Model, Taconic Sequence, New York and Vermont

ABSTRACT Environmental reconstruction for the Cambrian portion of the Taconic Sequence, New York and Vermont, indicates a depositional environment analogous with a modern slope fan-basin-plain model. Carbonate sediment and generally coarse quartz sand were removed from the Cambrian shelf and deposited with muds of the slope, now slates and siltstones, by a variety of processes at work on the slope and within submarine canyons. The shelf-derived sediment can be divided into six main lithofacies, each bearing the imprint of the principal process or processes involved in its deposition. These include: (1) carbonate clast conglomerates (inferred products of debris flow), (2) massive, coarse sandstones (apparent deposits of fluidized sediment flow and grain flow), (3) graded sandstones and limeston s (presumed turbidites), (4) parallel-laminated sandstones and limestones (probable turbidites), (5) thin, structureless micrites (inferred deposits of vertical settling-out of suspension), and (6) current-ripple-laminated limestones and sandstones (thought to be the products of reworking by contour-following bottom currents or submarine overbank levee deposits). All of these processes were working together or in opposition. Analysis indicates that only the lower slope and base-of-slope portion of the early Paleozoic continental margin has been preserved in the Taconic Sequence.

Early Paleozoic continental margin sedimentation, trilobite biofacies, and the thermocline, western United States

The Upper Cambrian Whipple Cave Formation and Lower Ordovician lower House Limestone of eastern Nevada contain algal stromatolites, flat-pebble conglomerate, lime grainstone, and other carbonate rocks inferred to represent shoal-water depositional environments. Coeval parts of the Hales Limestone, 170 km to the west, consist of dark-colored lime mudstone and wackestone interbedded with coarse-textured alloch-thonous gravity-flow and slump deposits interpreted to have formed in a deeper water slope environment. The Whipple Cave Formation and some allochthonous gravity-flow deposits of the Hales Limestone contain trilobite genera typical of the North American Faunal Province, whereas in situ deeper water limestones of the Hales contain trilobite taxa that are widespread in northwestern and southern China. An abrupt biofacies change between shelf and deeper water sites may be analogous to major changes with depth shown by Holocene marine isopod crustaceans in low latitudes, where the permanent thermocline is a major barrier to faunal dispersal. Thus, caution should be exercised in using faunal resemblance data for early Paleozoic plate-tectonic reconstructions unless the habitat and marine climatic preferences of the faunas are known.