Showing papers on "Permian published in 1991"

Multicyclic History of the Northern India Continental Margin (Northwestern Himalaya) (1)

Abstract: The geologic evolution of northern India is best recorded in the stratigraphic succession of the Zanskar Range (northwestern Himalaya), which represents the most complete transect through this ancient continental margin. Sedimentary history began in the late Proterozoic, and recorded a late Pan-African orogenic event around the Cambrian-Ordovician boundary, when the Gondwana supercontinent was eventually assembled. The following long period of epicontinental deposition in shallow seas linked to palaeo-Tethys lasted until the Early Permian, when a neo-Tethyan rift began to open between paleo-India and the Cimmerian microcontinents. Neo-Tethyan history can be subdivided into two sedimentary megasequences, both recording a major tectonic and magmatic event in the lower part. The first one began with breakup in the Late Permian and lasted until the end of the Jurassic. The second one started in the Early Cretaceous with the final detachment of India from Gondwana and the opening of the Indian Ocean, and ended with the India-Eurasia collision in the Early Eocene. The two megasequences can be in turn subdivided into six transgressive/regressive supersequences bounded by tectonically enhanced unconformities. Basal sandstone units of Early Permian, Late Permian, Norian, Callovian, Early Cretaceous, and Paleocene age are invariably associated with oolitic ironstones or reworked glauco-phosphorites, and mark the transgre sive part of each supersequence. Next, condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing-upward marly units capped by regressive platformal carbonates. The six tectono-eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of India from the rest of Gondwana, and document the combination of plate/microplate reorganizations and eustatic, climatic, and oceanographic changes in the Tethyan domain. After the onset of collision between India and Asia close to the Paleocene/Eocene boundary, obduction of the remnants of the neo-Tethys ocean floor onto the Indian margin began, and the latter underwent multiphase deformation with fold-thrust shortening followed by heating and extension. After the main metamorphic event, ophiolitic nappes were re-thrusted and finally emplaced with their sedimentary sole on top of the passive-margin succession.

Pre-Mesozoic Geology of Iberia

Abstract: I Introduction.- to the Pre-Mesozoic Geology of Iberia.- II Cantabrian and Palentian Zones.- 1 Introduction.- 2 Stratigraphy.- 2.1 Introduction.- 2.2 The Asturian-Leonese Domain (Cantabrian Zone).- 2.3 The Palentine Domain (Palentian Zone).- 2.4 Carboniferous Pre-Stephanian Rocks of the Asturian-Leonese Domain (Cantabrian Zone).- 2.5 Carboniferous of the Palentian Zone.- 2.6 Stephanian and Permian Basins.- 3 Structure.- 3.1 Cantabrian Zone.- 3.2 Palentian Zone.- 4 Igneous Rocks.- 5 Metallogenesis Zones.- III West Asturian-Leonese Zone.- 1 Introduction.- 2 Stratigraphy.- 3 Structure.- 4 Igneous Rocks.- 5 Distribution and Characteristics of Hercynian Metamorphism.- 6 Metallogenetic Characteristics.- IV Central-Iberian Zone.- 1 Introduction.- 2 Autochthonous Sequences.- 2.1 Stratigraphy.- 2.1.1 Introduction.- 2.1.2 Ante-Ordovician Stratigraphy.- 2.1.3 Post-Cambrian Palaeozoic Stratigraphy.- 2.2 Structure.- 2.3 Granites as a Paradigm of Genetic Processes of Granitic Rocks: I-Types vs S-Types.- 2.4 Distribution, Characteristics and Evolution of Metamorphism.- 2.5 Metallogeny.- 3 Allochthonous Sequences.- 3.1 Structure in the Northwest of the Iberian Peninsula.- 3.2 Metamorphic Evolution of the Allochthonous Complexes from the Northwest of the Iberian Peninsula.- V Ossa-Morena Zone.- 1 Introduction.- 2 Stratigraphy.- 2.1 Precambrian.- 2.2 Rift Phase (Cambrian).- 2.3 Passive Margin Phase (Ordovician-Silurian-Devonian).- 2.4 Synorogenic Phase (Upper Devonian-Carboniferous-Lower Permian).- 3 Structure.- 4 Igneous Rocks.- 5 Metamorphism.- 6 Metallogeny.- VI South Portuguese Zone.- 1 Introduction.- 2 Stratigraphy and Synsedimentary Tectonism.- 3 Structural Outline.- 4 Metamorphic Evolution of the South Portuguese/Pulo Do Lobo Zone.- 5 Metallogenesis in the Iberian Pyrite Belt.- VII Sedimentary and Faunal Domains in the Iberian Peninsula During Lower Paleozoic Times.- VIII Geodynamic Evolution of the Iberian Massif.

Sequence stratigraphy of carbonate-evaporite basins: models and application to the Upper Permian (Zechstein) of northeast England and adjoining North Sea

Abstract: The successions of carbonate-evaporite basins, which are periodically cut off from the world ocean, are considered in terms of their sequence stratigraphy. Two basin types are distinguished: those where incomplete drawdown takes place and those showing complete drawdown and wholesale basin desiccation. It is argued that, following relative sea-level falls, sequences begin with the evaporites, precipitated as marginal lowstand gypsum wedges in the first basin type and as lowstand basin-centre halite fills in the second. The evaporites pass up into carbonate sediments which are mainly deposited on shallow-water platforms around the basin during transgressive and highstand systems tracts. The concepts and models developed are applied to the Zechstein (Upper Permian) strata of northeast England and adjoining North Sea. Seven sequences are proposed, with some containing thin parasequences. The sequence approach to carbonate—evaporite basins, which considers both basin margin and basin centre facies, gives a more encompassing perspective of sedimentation than the traditional cycle concept, with its emphasis on the basin centre.

Why was there a delayed radiation after the end‐Palaeozoic extinctions?

Abstract: Data from widespread dysaerobic facies, carbon/sulphur ratios and cerium anomalies suggest that the early Triassic was a time when anoxic conditions spread widely over epicontinental seas. These conditions, associated with marine transgression following the latest Permian regression, are likely to be a prime cause of the mass extinction of Palaeozoic marine faunas. The occurrence of many Lazarus taxa in the Middle and Upper Triassic indicates, however, that the extinctions at the end of the Permian were less severe than has been widely assumed, and that the turnover from Palaeozoic to Mesozoic faunas was considerably extended in time, being finally accomplished only after the end‐Triassic mass extinction event.

Thermal history of Michigan Basin and Southern Canadian Shield from apatite fission track analysis

Abstract: Apatite fission track ages and confined-length distributions were collected from 38 basement outcrop and 5 basement drillcore samples in order to reconstruct the Phanerozoic thermal history of the Michigan Basin and southern Canadian Shield. The apatite data indicate two periods of thermal activity in the region: Triassic heating/cooling that affected the basin and adjacent shield and Cretaceous or post-Cretaceous heating/cooling that primarily affected the basin. The magnitude, timing, and cause of Cretaceous thermal activity cannot be identified with the present data. Model calculations suggest that some of the shield samples and probably all of the basin samples were heated to temperatures of at least 90C just prior to relatively rapid cooling in the Triassic. Available stratigraphic and geochemical constraints suggest that these elevated temperatures were the result of burial by an additional 2-5 km of late Paleozoic (probably Pennsylvanian and Permian) sediments. It is likely that the basin was buried during the Alleghenian Orogeny as observed for the adjacent Appalachian Basin.

A reassessment of the 'Hunter-Bowen Orogeny': Tectonic implications for the southern New England fold belt

Abstract: All Late Permian deformation structures in the southern New England Fold Belt can be ascribed to a single but complex compressive tectonic event, the Hunter‐Bowen Orogeny, operative 265–250 Ma ago. Early Permian rifting of the Carboniferous arc and fore‐arc of the Tamworth Belt and the region directly west of it produced the Sydney Basin and subsidiary meridional troughs in a backarc environment. Initial east‐west compression in the Late Permian produced a series of meridional folds and subparallel faults (D1) above a westward‐propagating decollement. With continued east‐west compression, upward propagation of thrusts culminated in the southwest‐directed Hunter‐Mooki Thrust System (D2), associated with megascale sinistral shear that resulted from pinning and rotation of the Hunter Thrust about an axis located near Maitland. East‐west directed stress release on the upper allochthonous plate of the thrust resulted in the development of meridional backthrusts. The final major deformation (D4) reactivated the...

The structure of the Precambrian and Lower Paleozoic basement of the Central Andes between 22° and 32°S. Lat.

Abstract: Three major diastrophic cycles, defined by their structural style and their spatial distribution are recognized in the Andean Basement of this region. The oldest structures are related to the Panamerican Orogeny (500 to 700 m. a.) which produced in the Central Craton multiply deformed complexes of schists, gneisses and granites, that are covered discordantly by unmetamorphosed Cambrian and Ordovician beds. West of the Central Craton Ordovician sedimentary beds are folded with a simple structural style and intruded by granites. Both the sedimentry beds and the granites are covered discordantly by undeformed Devonian sequences. The folding of the Ordovician is attributed to the ocloyic phase of the Caledonian movements. West of the ocloyic belt is another foldbelt consisting of strongly folded Devonian beds attributed to the chanic phase (hercynian). The chanic belt is intruded by carboniferous and permian granites and covered discordantly by Carboniferous and Permian sequences.

Lithofacies and cyclicity of the Yates Formation, Permian Basin; implications for reservoir heterogeneity

Abstract: Siliciclastics of the Yates Formation (Permian, upper Guadalupian) are significant hydrocarbon reservoirs in the United States Permian basin. Subsurface and outcrop data show that the most porous lithofacies occur in a clastic-dominated middle shelf and that evaporitic inner shelf and carbonate outer shelf equivalents are mostly nonporous. Lithofacies relations and much of the heterogeneity in Yates reservoirs are related to the stacking of depositional sequences (i.e., siliciclastic-carbonate alternations and sandstone-argillaceous siltstone alternations) in response to three orders of orbitally forced, low-amplitude, eustatic variation. In general, siliciclastics dominated the Yates shelf during lowstand parts of asymmetric, 400-k.y. sea level fluctuations, whereas carbonates were deposited during sea level highstands. The character and position of sand depocenters on the Yates shelf during these lowstands were controlled by a longer duration third-order sea level variation. Shorter duration cycles controlled the heterogeneity within the 400-k.y. depositional sequences. The variation in cycle packaging, lithology, and reservoir quality between the Central Basin platform and Northwest shelf may be a response of eustatic variation on parts of the shelf with different slopes or subsidence profiles. The lithofacies described from the Yates Formation and the depositional model proposed to explain the stratigraphy may be valuable as analogs in other basins containing mixed siliciclastic-carbonate settings.

Asian and south-western Pacific continental terranes derived from Gondwana, and their biogeographic significance

Abstract: The recent recognition of numerous small geological terranes in the Indo-Pacific region has revolutionised our understanding of geological and biogeographic processes. Most of these terranes rifted from Gondwana. The Shan-Thai terrane rifted from Australia in the Permian and collided with Indo-China in the Triassic. Parts of Sumatra and Kalimantan may have rifted from Australia in the Cretaceous and carried an angiosperm flora north. Other terranes, now dispersed in South-East Asia and in the Pacific were, at various times in the Cenozoic, part of the Australian continent. Faunal and floral mobilism to Fiji via the Solomons and Vanuatu was probably not difficult up to the late Miocene.

40Ar/39 dating of the Siberian Traps, USSR: Evaluation of the ages of the two major extinction events relative to episodes of flood-basalt volcanism in the USSR and the Deccan Traps, India

Abstract: 40Ar/39Ar incremental-heating studies have been carried out on three whole-rock specimens from the Siberian Traps. A basalt lava flow from the lowermost horizon yields an age of 238.4 ±1.4 Ma (1σ error). A second basalt lava flow from the top of the section, ∼800 m above the first specimen, yields an age of 229.9 ±2.3 Ma, indicating that the duration of volcanism was ∼5-10 m.y. A doleritic dike intrusive into the lower parts of the Siberian Traps contains excess argon and yields an isochron age of 234 ±7 Ma. Critical reexamination of relevant radiometric data relating two separate episodes of flood-basalt volcanism to global faunal extinctions suggests the volcanic event forming the most voluminous sections of the Deccan Traps, India, was coincident to within ±1 m.y. with the time of the Cretaceous/Tertiary boundary. However, the onset of volcanism in the Siberian Traps apparently occurred at a time postdating that of the Permian/Triassic boundary.

Plant fossils in geological investigation : the palaeozoic

Abstract: Devonian palaeogeography and biostratigraphy carboniferous and permian palaeogeography carboniferous and permian biostratigraphy palaeogeography palaeoclimatology

Stratigraphy and reservoir potential of glacial deposits of the Itarare Group (Carboniferous-Permian), Parana Basin, Brazil

Abstract: Drilling in the Parana basin of Brazil in the mid-1980s discovered gas and condensate in the Itarare Group, and showed that glacial deposits in Brazil can contain hydrocarbons. The reservoir potential of the Carboniferous-Permian Itarare Group of the basin is analyzed using new subsurface data from 20 deep wells drilled in the early to middle 1980s. Central to the analysis was the construction of over 3000 km of cross sections based on more than 100 wells, the description of more than 400 m of core, and study of 95 thin sections. Subsurface exploration and mapping of the Itarare are greatly aided by the recognition of three recently defined and described formations and four members, which are traceable for hundreds of kilometers. These units belong to three major glacial cycles in which the pebbly mudstones and shales are seals and glacially related sandstones are reservoirs. The best sandstone reservoirs in the deep subsurface belong to the Rio Segredo Member, the uppermost sandy unit of the Itarare. The Rio Segredo Member is the best petroleum target because it is overlain by thick seals and massive pebbly mudstones and shales, and because it is shallower and less compacted than underlying, more deeply buried sandstones. This member has little detrital matrix and much of its porosity is secondary, developed by carboxylic acid and CO[2] generated when Jurassic-Cretaceous basalts, sills, and dikes were intruded into the Parana basin as Gondwana broke up.

Late carboniferous to permian sedimentation in inner-mongolia, china, and tectonic relationships between north china and siberia

Abstract: A belt of deformed Permo-Carboniferous sedimentary rocks in Inner Mongolia represents an orogenic zone between the North China and Siberian blocks. The belt apparently contains accretionary terranes thrust onto the margins of both blocks, with scattered ophiolite/blueschist suites indicating the consumption of oceanic crust at least slightly older than the age of deformation. Upper Carboniferous to Lower Permian strata in central Inner Mongolia show both sedimentological and paleontological differences from Middle and Upper Permian rocks. In comparison with pre-Middle Permian rocks, younger Permian strata: (1) contain less laterally extensive limestone and more lensoidal carbonate in clastic sections; (2) contain a higher percentage of coarse clastic material; (3) grade upward from marine to terrestrial deposits; (4) contain fauna indicating more restricted circulation, particularly toward the western part of the belt; and (5) contain fauna that are consistent with, but do not require, northward movement ...

New Biostratigraphic Age Dates from the Lake Rukwa Rift Basin in Western Tanzania: Geologic Note (1)

Abstract: The Rukwa rift basin in western Tanzania is a segment of the western branch of the East African rift system. Three major stratigraphic units have been recognized in the basin (from oldest to youngest): Karroo Supergroup, the Red Sandstone, and the Lake Beds. These units have traditionally been assigned the following ages, respectively, based upon lithologic correlations: Permian-Triassic, Jurassic-Cretaceous, and Tertiary. Recently, microfloral assemblages collected from sidewall cores and cuttings from two deep exploration wells drilled in the basin have provided new data on the age of these formations. The biostratigraphic data suggest the formations should be assigned the following ages: Karroo Supergroup, Late Permian; Red Sandstone, Neogene(?) (Miocene-late Pliocene) and the Lake Beds, late Pliocene-Holocene. The new age dates indicate that the post-Karroo to pre-Miocene history of the basin is one of relative quiescence and the modern rift is a relatively young feature. This interpretation is more consistent with the tectonic histories of the other basins of the Western rift.

Paleomagnetism of the Moenkopi and Chinle Formations in central New Mexico: Implications for the North American Apparent Polar Wander Path and Triassic magnetostratigraphy

Abstract: In central New Mexico, red sedimentary rocks unconformably overlying Permian carbonates of the San Andres Formation have been correlated with the Early-Middle Triassic Moenkopi and Late Triassic Chinle Formations of the Colorado Plateau. Paleomagnetic samples from Triassic sections exposed on basement cored uplifts along both the east and west side of the Rio Grande rift near Albuquerque yield, upon thermal and chemical demagnetization, well-defined, high unblocking temperature, dual-polarity magnetizations carried by hematite. The characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding-parallel magnetization polarity zonation. The Moenkopi and lowermost Chinle formations produced paleomagnetic poles respectively at 57.6°N-100.3 °E (N=36 sites, K=74.1, A95=2.8°) and 60.8°N-88.9°E (N=17 sites, K=130.3, A95= 3.1°). These data plus previously published and additional results from the underlying Permian strata suggest that portions of central New Mexico have experienced a small clockwise rotation (i.e., less than 10°) similar to that of the Colorado Plateau with respect to the North American craton. The paleomagnetic directions of the Chinle Formation and related strata in eastern New Mexico document about 12° (great circle distance) of rapid apparent polar wander during mid-Carnian to late Norian times along a track which contains other cratonic poles of similar age. We present a preliminary magnetic polarity time scale for the Triassic that incorporates the present New Mexico data and previously published data, mostly form continental red bed sequences. This magnetic polarity scale provides a basic framework which can be tested with future data from Triassic sections where additional biostratigraphic control exists.