Shelf Carbonate Facies Research Articles

Oligocene larger benthic foraminifera and sedimentation of the Burdur Basin, SW Anatolia, Turkey

This study focuses on the Oligocene successions that contain predominantly siliciclastic, limestones with larger benthic foraminifera in the Burdur Basin of southwest Anatolia, Turkey. The Burdur-1 and Burdur-2 sections in the Oligocene deposits are described sedimentologically and were sampled for lithology, biostratigraphy, microfacies analysis and fossil content. Larger benthic foraminifera were mainly recovered from limestones in the upper part of the Burdur-2 section. Thin-section analysis of the larger benthic foraminiferal assemblage reveals that the upper part of the Burdur-2 section includes Nummulites fichteliMichelotti, 1841, Operculina complanata (Defrance, 1822), Nephrolepidina praemarginata (Douvillé, 1908), Eulepidina sp., Planorbulina sp., Amphistegina sp., and Asterigerina sp. This foraminiferal association represents the Shallow Benthic Foraminifera Zone 22 (SBZ 22) of Rupelian-early Chattian age. Sedimentological analysis in the Burdur-1 and Burdur-2 sections has revealed an association of massive-unorganized conglomerate, parallel-bedded sandstone, massive mudstone, graded-massive conglomerate, graded-massive sandstone, well-sorted organized conglomerate, well-sorted parallel-bedded sandstone and microfossil-bearing parallel-bedded limestone facies. Alluvial fan /river, fan-delta, beach and shallow shelf carbonate facies communities were identified by lateral and vertical correlation of facies. It is concluded that the large Burdur Fan developed in a graben basin that opened and was flooded by a branch of the Tethyan Ocean that invaded southwest Anatolia during the Oligocene.

Sequences, discontinuities and water stratification in a low-energy ramp: the Early Albian sedimentation in central Tunisia

Based on a precise biostratigraphic framework stated previously, the detailed sedimentological study of eight sections of the early Albian succession of Central Tunisia made possible to refine the modalities of the Albian transgression and the related behavior of sedimentary system on the Tunisian margin. Major early Albian transgressive pulses occurred (i) at the base of the L. tardefurcata standard ammonite zone, (ii) around the top of the same zone, (iii) during the D. mammillatum standard zone, and (iv) near the top of the latter ammonite zone. They resulted in the progressive flooding of the Tunisian margin emergent since the Aptian boundary, in the disappearance of emergence evidences through time, in the southward backstepping of the carbonate shelf facies, and eventually in the homogeneization of the outer shelf marly sedimentation in the study area. The low energy of deposition of the early Albian sediments in this part of the South-Tethyan margin may be due to the deflection of winds and storms by the Coriolis forces toward the North-Tethyan margin, creating a clockwise gyre in this part of the ocean. This circulation may have enhanced upwelling currents likely responsible for phosphate mineralization, and for the oxygen depletion of deep waters, which progressively invaded the Tunisian margin, giving way to organic-rich deposits of late early Albian age. The early Albian sedimentation is then interrupted by a major hiatus of middle Albian to early late Albian age.

Stratigraphic correlation of the Late Cretaceous Simsima Formation United Arab Emirates and Akveren Formation, northwest Turkey

Latest Cretaceous (Campanian-Maastrichtian) microfossils are used to correlate the carbonate rocks of the Simsima Formation in the northeastern part of the Arabian Peninsula (Northern Oman Mountains, United Arab Emirates and Oman) with the Akveren Formation in Kandira (northwest Turkey, near Black Sea region). Both formations have characteristically rich planktonic foraminiferal and calcareous nannofossil faunal assemblages that permit the recognition of the Globotruncanella havanensis Zone and Quadrum sissinghii Zone CC22. The palaeontological data is used to build an appropriate palaeoenvironmental model for the latest Cretaceous Aruma Group in the Oman Mountains foreland basin. The study reveals that the Late Cretaceous formations of UAE and Turkey can be divided into an open marine carbonate shelf facies (planktonic foraminifera/calcareous nannofossil biomicrite) and a shallow-marine carbonate facies (rudistids, coralline algal foraminiferal biomicrite).

The application of ichnofabrics towards bridging the dichotomy between siliciclastic and carbonate shelf facies: examples from the Upper Jurassic Fulmar Formation (UK) and the Jubaila Formation (Saudi Arabia)

An initial study of the ichnofabrics of the Upper Jurassic (Kimmeridgian) Jubaila Formation of Saudi Arabia shows that the ichnofabrics are closely matched to the relatively well-described ichnofabrics of the contemporary Fulmar Formation of the UK Continental Shelf (North Sea), in respect of the lower shoreface/offshore transition facies to offshore facies. The ichnology and ichnofabrics of the Lower Jubaila Formation show that deposition took place on an open-marine platform on the Arabian craton subject to periodic storm activity, but under a persisting equilibrium between sediment accumulation and subsidence. This is consistent with the moderately deep-marine foraminiferal assemblages and the presence of calcareous nannofossils. Cyclicity is absent, though storm beds may be grouped, in contrast with the genetic sequences present in the rift and halokinetic scenario of the North Sea. In contrast with the siliciclastic setting hardgrounds (with Gastrochaenolites ), more common firmground omission surfaces, and micritic mudstones with Chondrites and Zoophycos are notable features of the carbonate facies. In siliciclastic successions (parasequences) the latter ichnotaxa are generally regarded as having been deposited in rather deeper water, but in the carbonate Jubaila Formation are interpreted as being associated with local areas of lower turbulence. Likewise, the hardgrounds and firmgrounds, which have not been traced laterally, are tentatively regarded to be of local significance.

On the Interpretation of Shallow Shelf Carbonate Facies and Habitats: How Much Does Water Depth Matter?

ABSTRACT Many stratigraphic studies of ancient shallow-water carbonate platforms focus interpretation on the primacy of changes in accommodation, inferred by assuming a direct and deterministic link between depositional facies and water depth. The purpose of this study is to test this assumption explicitly and quantitatively by exploring relations between substrate type (classified on the basis of both habitats and facies) and the water depth at which they occur on a modern well-studied shallow rimmed shelf margin. GIS analysis of digital benthic habitat and bathymetry maps of the shallow ( 8 m) South Florida shelf illustrate that most substrate types occur across a range of water depths. Similarly, at greater water depths, habitats and facies are more constrained (more dominance of one class, more deterministic), with generally decreasing dominance (more diversity) at shallower water depths. At a scale of shelf-to-basin transects, facies and habitats clearly are related to water depth. Yet, on this narrower ( 10 km) shallow shelf margin, at the scale of this study, water depth and habitats and facies are not uniquely related or linked. This general lack of correspondence between bottom type and water depth may be a manifestation of landscape disequilibrium, a state in which habitats and facies do not fully reflect ambient environmental conditions, perhaps recording the influence of rapid change in sea level. Alternatively, it may reflect the impact of variables other than water depth. Although more general consequences of these results for other depositional settings remains to be evaluated, they do emphasize the fact that variables other than bathymetry may significantly influence the ecological and sedimentologic attributes of depositional surfaces. They do allow for the quantification of some of the potential uncertainties inherent in the interpretation of analogous ancient platforms, and can serve to focus future observations on the causes and consequences of vertical and lateral heterogeneity in ancient stratigraphic sequences.

Upper Silurian heliolitine corals, Canadian Arctic: Taxonomic method, keys to identification, and biogeographic relationships

Taxonomy of heliolitine corals, and tabulate corals in general, benefits from analysis of larger assemblages of coralla that approximate populations. Quantitative data derived from such assemblages allow intraspecific variation to be tested rigorously and illustrated, and similar, co-occurring species to be discriminated using normal patterns of continuous population variation. Field collection and analysis necessarily involve recording and comparing assemblages from the full spectrum of sedimentary facies available at any one stratigraphic level and from the maximum available stratigraphic range. The large published taxonomic literature on heliolitine corals is a daunting obstacle to identification at the species level because a substantial part appears to involve taxonomic splitting to a degree inconsistent with variation known in modern coral species. The heliolitine corals studied occur in Ludlow-age ramp and shelf carbonate facies in the Boothia Uplift region in the southern Arctic archipelago of Canada. The “population” approach resulted in discrimination of 18 morphotaxonomic species, belonging to five genera and four families. A few select morphologic characters, incorporated in simple identification keys, are particularly helpful in distinguishing the 11 heliolitid and four stelliporellid species present, and are useful in screening the voluminous literature on species of Heliolites. Six of the species compare closely to Silurian species reported from central Kazakhstan, West Siberia, and North China; few are shared with other regions. The principal affinities are consistent with occurrence in the Uralian-Cordilleran paleogeographic region in the Silurian northern hemisphere. Eight species do not appear to be recorded in published literature elsewhere and are apparently endemic.

Post-glacial sea-level control and sequence stratigraphy of carbonate–terrigenous sediments, Wanganui shelf, New Zealand

Abstract Shallow-marine, mixed carbonate–terrigenous sediments have been accumulating in water depths of 20–110 m on Wanganui shelf, central-western New Zealand, during the latest Quaternary post-glacial sea-level rise since about 18,000 years BP. Cores show an upward change from coarse, mostly terrigenous-dominated sands and gravels, to shell-rich sediments, and then to terrigenous-dominated muddy, very fine sands and sandy muds. These post-glacial carbonate–terrigenous sediments are a modern analogue for similar episodes of cyclothemic shelf deposition that characterise onland Plio–Pleistocene sections in central-western New Zealand. Like their ancient counterparts, the latest Quaternary sediments can be interpreted in a sequence stratigraphic context. A transgressive systems tract, 4–15 m thick, overlies a transgressive erosion surface evident on seismic reflection records, and is represented by slightly shelly coarse sands and gravels at the base of several cores, and as palimpsest surficial deposits on the modern shallow shelf out to a depth of about 50 m. The overlying shelly deposits, both shell-rich and matrix-rich, are equivalent to mid-cycle or backlap shellbeds described elsewhere. They comprise a diverse in situ or locally transported bivalve molluscan and bryozoan biota typical of temperate-latitude cool-water carbonate shelf facies. These condensed section shellbeds are up to at least 1.5 m thick, gradationally overlie the transgressive sediments in cores, and occur as modern surficial shelf facies between about 25 and 100 m water depth. Cores are locally capped by up to a few metres of terrigenous sands or muds that represent highstand systems tract deposits fed onto the shelf from opposing onshore and offshore directions. In the nearshore region, North Island-derived felsic fine sands are beginning to prograde seawards (westwards) across the shellbeds, while in the offshore zone, South Island-derived micaceous very fine sands and muds are encroaching upon, and covering, the shellbeds from the west and south.

The stratigraphy and depositional context of a temporary exposure in the Anstruther Formation (Strathclyde Group), Anstruther Wester, Fife

Synopsis A temporary exposure in the Anstruther Formation (Strathclyde Group: Viséan, Asbian) behind the now-demolished Anstruther Wester swimming pool uncovered 13 m of strata within a cyclical sedimentary succession. A total of 68 m of this succession were logged from an arbitrary 10 m below the base of the Johnny Dow’s Pulpit sandstone to 5.2 m above the Billow Ness Marine Band. The measured section includes two complete and two incompletely measured cycles representing variations on a primitive ‘Yoredale’ pattern, in which the transgressive marine shale–carbonate shelf facies is largely suppressed in favour of upward-coarsening progradational prodelta and delta front, and aggradational delta plain facies. Each cycle commenced in a non-marine to marginal marine coastal environment in which shales, thin dolostones (some oncolitic), and ‘musselband’ shales and dolostones were deposited (the mussels are non-marine bivalves). Shales were succeeded by progradational channelized delta front facies, and aggradational delta plain facies also including channel-fill sandstones, typically with pedogenic tops. Variations in cycle thickness relate partly to fluctuations in the supply of sand-grade sediment, and partly to contemporary subsidence, possibly related to tectonic processes. Comparisons are made with the late Brigantian Lower Limestone Formation at St Monans, which together with the Anstruther Formation forms part of a transgressive megasequence initiated late in the Devonian with continental, alluvial, redbeds.

Sedimentation rates, basin analysis and regional correlations of three Neoarchaean and Palaeoproterozoic sub-basins of the Kaapvaal craton as inferred from precise U–Pb zircon ages from volcaniclastic sediments

Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of 2588±6 Ma and 2549±7 Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of 2516±4 Ma for the upper Gamohaan Formation and 2555±19 for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.

Sedimentology of 40 000 year Milankovitch‐controlled cyclothems from central Hawke's Bay, New Zealand

ABSTRACTCyclothemic sedimentary rocks of the Plio‐Pleistocene Petane Group outcrop extensively in the Tangoio block of central Hawke's Bay, New Zealand. They are products of inner to mid‐shelf sedimentation and were deposited during glacio‐eustatic sea level fluctuations along the western margin of a shallow, pericontinental seaway located in a forearc setting. The succession consists of five laterally continuous cyclothems, each containing a fine grained interval of silt and a coarse grained interval of siliciclastic sand ± gravel or limestone. Five sedimentary facies assemblages comprising 20 separate facies have been recognized. Coarse grained intervals of cyclothems were deposited mostly during relative sea level lowstands and contain up to four facies assemblages: (1) a non‐marine assemblage (with three component facies, representing braided river and overbank environments); (2) an estuarine assemblage (with three component facies, representing tidal flat and mud‐dominated estuarine environments); (3) a siliciclastic shoreline assemblage (with six component facies, representing greywacke pebble beach, shoreface and inner shelf environments); and (4) a carbonate shelf assemblage (with four component facies, representing tide‐dominated, inshore and shallow marine environments). Fine grained intervals of cyclothems were deposited during sea level highstands when the Tangoio area was generally experiencing mid‐shelf sedimentation. This produced an offshore assemblage consisting of four component facies. The distribution of facies assemblages during relative sea level lowstands was dependent upon proximity to the shoreline, the type and rate of sediment supply to the basin, and shelf hydrodynamics. Carbonate shelf facies dominate coarse grained intervals in Cyclothems 3–5, but siliciclastic shoreline and non‐marine facies dominate in Cyclothems 1 and 2. The abrupt change from siliciclastic to carbonate sedimentation during relative sea level lowstand deposition is thought to have been induced by rapidly falling interglacial to glacial sea level accentuated by regional tectonic shoaling. This caused most of the terrigenous sediment supply to bypass the Tangoio area. Consequently, carbonate sediment accumulated in inshore and shallow marine settings. Facies assemblages rarely show lateral interdigitation, but are vertically stratified over the entire Tangoio block. Facies successions in each cyclothem preserve a record of relative sea level change during deposition of the Petane Group and are consistent with a Plio‐Pleistocene sea level change in eastern New Zealand of c. 75–150 m, i.e. approximately the magnitude suggested for Late Quaternary glacio‐eustatic sea level changes.

STRATIGRAPHY OF THE CHIKKIM AND FATU LA FORMATIONS IN THE ZANGLA AND ZUMLUNG UNITS (ZANSKAR RANGE, INDIA) WITH COMPARISONS TO THE THAKKHOLA REGION (CENTRAL NEPAL) : MID-CRETACEOUS EVOLUTION OF THE INDIAN PASSIVE MARGIN

Detailed biostratigraphic study of mid-Cretaceous Tethys Himalayan pelagic units shed new light on the evolution of the northern India passive continental margin. After a major episode of alkaline magmatism recorded from Zanskar to Nepal in the Early Cretaceous, condensed glauco-phosphorites were deposited during the Rotalipora subticinensis Subzone, and were overlain unconformably by pelagic mudstones during the Rotalipora ticinensis to Rotalipora appenninica Zones (Fatu La Formation in Zanskar and Muding Formation in Nepal). Throughout the Tethys Himalaya drowning of clastic shelves thus occurred with the same modalities and synchronously in Late Albian time. The base of the Chikkim Formation in Zanskar is younger, and overlies palimpsest arenites and reworked glauco-phosphorires deposited during the latest Cenomanian Whiteinella archaeocretacea Zone. Hiatuses several million years long, mainly coeval and similar in duration in the Chikkim and Fatu La Formations, characterize the Cenomanian and the Late Turonian to Campanian sections throughout the Zanskar Range. These hiatuses are ascribed to continuous resuspension of pelagic sediments on the upper slope, caused by the action of strong eastbound oceanic currents. Much higher accumulation rates were recorded in the early-middle Turonian, when deposition occurred at grearer depths below the mudline. Accumulation rates increased greatly in the Maastrichtian, when offshore marls with sparsc phosphate nodules were gradually rcplaced by inner shelf carbonate facies at the close of the Cretaceous. The studied succession shows a mirrorlike repetition of sedimentary facies, interpreted as part of a 65 to 70 my long Cretaceous supersequence. Sedimentary evolution reflected a complex interplay of global and regional phenomena, including the extensional and magmatic episodes leading to thc final fragmentarion of Gondwana-Land and opening of the Indian Ocean, a long-term tendency to eustatic rise, changing paleooceanographic circulation, and oceanic anoxic events.

Sequence Stratigraphy of the Smackover Formation in the Northern Half of the Manila Embayment, Southwestern Alabama

ABSTRACT The Smackover Formation in the northern half of the Manila Embayment, southwestern Alabama, ranges from 100 to 540 ft (30 to 165 m) thick and encompasses two genetic packages of facies. These genetic packages developed within a single cycle of relative sea-level change during the Late Jurassic (specifically late early Oxfordian to early Kimmeridgian) and comprise parts of a single depositional sequence. The base of the lower genetic package is the sharp contact between terrestrial clastic facies of the underlying Norphlet Formation and shelf carbonate facies in the Smackover. The lower Smackover genetic package (=transgressive systems tract) includes high-energy grainstones, shelf packstones and wackestones, and shelf-margin and patch-reef algal boundstones. The contact between the upper and lower genetic packages is a maximum-flooding surface delineated by a sharp facies change to a set of ramp carbonate facies. The upper genetic package (=prograding highstand systems tract) includes, in stratigraphic order, ramp packstones and wackestones, lagoon al wackestones and mudstones, and high-energy grainstone and clastic tempestite facies. The top of the upper genetic package is the sharp transition to anhydritic sabkha facies in the overlying Buckner Anhydrite Member of the Haynesville Formation.

Maastrichtian to early Tertiary stratigraphy and palaeogeography of the Central and Northern Oman Mountains

Abstract Maastrichtian to Tertiary sedimentary rocks outcrop extensively around the periphery of the Oman Mountains. They form part of a geologically distinct suite of strata deposited after obduction of the Semail ophiolite and thrusting of the Hawasina and Sumeini imbricates onto the eastern edge of the Arabian plate. The lithostratigraphy of Maastrichtian and early Tertiary rocks of the central and northern Oman Mountains is revised and a number of new formations are erected. The proto-Oman Mountains were emergent in the early Maastrichtian and were subject to subaerial erosion and weathering. NW and W of the mountains, gradual subsidence followed during the Maastrichtian, allowing for formation of a transgressive, onlapping sequence represented by the Qahlah (fluviatile to shallow marine terrigenous clastics) and Simsima (shallow shelf carbonate) Formations. More rapid subsidence NE of the mountains produced fan delta and submarine debris flow/turbidite apron sequences comprising the Al Khawd and Thaqab Formations respectively. A low-angle unconformity separates Tertiary strata from older units. During the Palaeogene, basinal and slope facies with debris flows and turbidite sequences were deposited directly upon the unconformity surface in rapidly subsiding basins NW (Muthaymimah Formation) and NNE (Ruwaydah Formation) of the Oman Mountains. Elsewhere, shallow carbonate shelf facies were extensively developed in less rapidly subsiding areas during the Palaeogene. The Palaeocene and early Eocene is represented by the Jafnayn Limestone Formation. This is succeeded and overlapped by a variety of later Eocene units. ENE and E of the mountains early Eocene regression is marked by restricted facies of the Rusayl Formation. Overlying the Rusayl Formation are shallow, open shelf limestones of the Seeb Formation which is rich in alveolinid and nummulitic foraminifera. On the Arabian side of the mountains the Rusayl Formation is not developed and the Jafnayn Formation is succeeded either by the Seeb Formation or fine-grained, faunally restricted limestones and marls of the Fahud Beds, which represent intra-shelf facies.

High-energy, inner shelf carbonate facies along a tide-dominated non-rimmed margin, northwestern Australia

High-energy, inner shelf carbonates occur within 1 to 2 km of the mainland along the southern part of the Northwest Shelf (a non-rimmed carbonate shelf) along northwestern Australia. Coarse-grained sands and gravels are deposited on and adjacent to a chain of subtidal and intertidal rock platforms which lies oblique to regional NE-SW directed tidal currents. Intertidal and subtidal sand sheets, less than 50 cm thick, predominate along the margins and interior of the chain, and subtidal linear to sinuous sand bodies up to 10 m thick and 4 km in length form banner banks to the northeast of rock platforms and reefs. Other features include narrow fringing and patch coral reefs, calcareous red-algal reefs, and intertidal through subtidal coral shingle ridges. Fleshy algae are abundant on open, sand-covered and bare rock platforms as well as forming a mantle on relict coral reef frameworks. Supratidal through subtidal sediments are comprised of skeletal (molluscs, foraminifera, bryozoans, corals, calcareous red algae) and rock fragments, the latter derived from rock platforms; calcareous chlorophytes (e.g. Penicillus and Halimeda) are not important sediment contributors. In general, sediments are not representative of the present-day turbid and fleshy algal-dominated subtidal environment. The predominantly coarse-grained lithofacies (<3 wt.% mud) and configuration of sand bodies are controlled by (1) vigorous tidal currents (up to 170 cm s −1) established via unimpeded across-shelf transport of oceanic waters, in the absence of a shelf margin rim, and tide ranges of 2–4 m; (2) interaction of shelf waters with rock platforms and reefs along the inner shelf creating strong currents; (3) passing cyclones; and (4) sediment reworking within normal strong wind, tide and wave regimes. High-energy carbonate sand shoals along this tide-dominated inner shelf are far removed from the shelf-slope break, facies distribution being similar to nearshore facies of a distally steepened carbonate ramp.

Cenozoic Sedimentary and Deformational History of Hispaniola, 1: Southeastern Cordillera Central: ABSTRACT

The Cordillera Central approximates an elongate (220 km), elevated (>3 km), thrust-bounded anticline cored by Cretaceous-Paleogene arc rocks and uplifted during Miocene to recent time by convergent strike-slip movements between the North American and Caribbean plates. The southeastern termination of the anticline plunges beneath a thick (>6 km), well-exposed marine clastic sedimentary sequence. Because uplift-related faulting is minimal in this hinge region of the Cordillera Central anticline, the authors have carried out detailed mapping of the area to determine (1) relation of Cretaceous-Paleogene arc basement to overlying Cenozoic sedimentary cover, and (2) Cenozoic deformational history of arc and basin sequences. Mapping has clearly distinguished three superimposed Cenozoic basins lying on arc basement. The lowest basin (basin 1) is Paleocene-Eocene in age and consists of alternations of arc-derived turbidies with interbedded pelagic limestone and red mudstone. In apparent conformity above this basin is an approximately 4-km thick marine clastic sequence of medial Eocene through early Miocene age (basin 2). These sediments consist of fining-upward turbiditic sequence derived from the northwest and northeast. Arc basement and overlying basins 1 and 2 were shortened approximately 25% in a short-lived, northwest-southeast-directed compressional event that resulted in the formation of large open synclines and tightlymore » folded and faulted anticlines with fold amplitudes of 1-6 km. Underformed, medial Miocene sediments of a mixed clastic and carbonate shelf facies (basin 3) unconformably overlie the folded latest Oligocene-early Miocene rocks of basin 2 and thus constrain the age of folding as early to middle Miocene.« less

Tectonic Development and Hydrocarbon Potential Offshore Troms, Northern Norway: ABSTRACT

The study area, offshore Troms, is located from 16 to 25°E long. and 70 to 72°N lat. The geologic border between continental and oceanic crust is defined by a dramatic increase in water depth in the western part of the area. The area with water depth less than 500 m (1,640 ft) covers approximately 6,000 km2 (2,300 mi2). In this area, several highs and basins are defined. The Troms and Hammerfest basins are situated between the Loppa high on the north and the Troms-Finnmark basement high on the south. The southwestern continuation of the Troms basin is a strongly faulted depression, the Harstad basin. Thinned continental crust under the Troms basin is covered with more than a 15 km (9.3 mi) thick sedimentary sequence of Paleozoic and younger rocks. Early Permian evaporites have formed a few large diapiric structures. The main rifting phase in the Troms basin occurred during Late Jurassic-Early Cretaceous. Several thousand meters of Cretaceous claystones were subsequently deposited. During early Tertiary, a second rifting phase resulted in sea floor spreading west of the studied area. During the cooling stage, the southwestern Barents Sea acquired a westward dip. Tertiary sediments onlap the Upper Cretaceous unconformity from the west. Quaternary sediments lie directly on Cretaceous sediments in the eastern part of the area. The north-south-oriented Ringvassoy-Loppa fault complex separates the Troms from the Hammerfest basin. In the Hammerfest basin, the Permian is developed as a shallow shelf carbonate facies. Tertiary and Mesozoic sedimentation was dominated by clastics. The Harstad basin is dominantly filled with Jurassic and younger sediments, and Paleozoic sediments are thought to underlie. Minor amounts of diapirism are present. Several compressional structures, formed by mid-Cretaceous strike-slip forces, are associated with the Troms-Finnmark fault complex. The Loppa high is a triangular-shaped structure covered by relatively thin Jurassic to Quaternary sediments. The southeastern part is an inverted Triassic basin. Shallow shelf carbonates define a good seismic marker in the upper Paleozoic. Seismic control of the area offshore Troms, at present, is generally a 2 by 4 and 2 by 2 km grid. More detailed surveys cover the licensed areas, totaling 2,200 km2 (850 mi2). The first exploratory well was spudded in spring 1980, and 15 wells, dominantly wildcats, have now been drilled. Five gas discoveries have been made in the western Hammerfest basin, two of them with recoverable reserves in the range of 50-100 × 109 Sm3 (1.76-3.52 tcf) gas. All discoveries have been made in Early to Middle Jurassic sandstones. Rocks of Triassic and Permian age will be increasingly important toward End_Page 1206------------------------------ the east and north. In the Troms basin, these sub-Cretaceous rocks are deeply buried, so younger reservoir zones have been drilled. Jurassic and younger rocks will be the main targets in the virgin Harstad basin. The gas has been generated from Lower Jurassic coals and shales. These, and Upper Jurassic shales with oil source potential, will probably also be the source for most hydrocarbons to be discovered in the future. Some deep Paleozoic structures will be dependent on older source rocks. The hydrocarbon potential of the relatively well-known Troms and Hammerfest basins is considered low and moderate, respectively. More promising today are Paleozoic structures on the Loppa high and structures in the Harstad basin. End_of_Article - Last_Page 1207------------

Carbonate shelf environments and faunal communities in the Upper Bringewood Beds of the British Silurian

The Upper Bringewood Beds of the type Ludlow Series preserve a lateral suite of carbonate shelf facies. From west to east, the shelf edge is represented by three parallel belts of sediments: thin-bedded biocalcarenites (downslope deposits), biosparrudites (bank deposits), and thin-bedded biocalcarenites (overwash deposits). Further east, the inner shelf is represented by highly bioturbated calcisiltites, calcilutites, and mud-supported biocalcarenites. The shelf edge was a high-energy environment with high species diversity and dense populations of crinoids and the pentamerid brachiopod Kirkidium. Low-energy mud environments of the inner shelf contained a low-diversity community of tetracorals, ostracords, Bryozoa, and the brachiopod Atrypa. The composition and distribution of benthic communities across the shelf was determined by substrate, palaeogeographic relations, and the historical factor of previous communities in the area.

The eastern part of the Tasman Orogenic Zone

The eastern part of the Tasman Orogenic Zone (or Fold Belt System) comprises the Hodgkinson—Broken River Orogen (or Fold Belt) in the north and the New England Orogen (or Fold Belt) in the centre and south. The two orogens are separated by the northern part of the Thomson Orogen. The Hodgkinson—Broken River Orogen contains Ordovician to Early Carboniferous sequences of volcaniclastic flysch with subordinate shelf carbonate facies sediments. Two provinces are recognized, the Hodgkinson Province in the north and the Broken River Province in the south. Unlike the New England Orogen where no Precambrian is known, rocks of the Hodgkinson—Broken River Orogen were deposited immediately east of and in part on, Precambrian crust. The evolution of the New England Orogen spans the time range Silurian to Triassic. The orogen is orientated at an acute angle to the mainly older Thomson and Lachlan Orogens to the west, but the relationships between all three orogens are obscured by the Permian—Triassic Bowen and Sydney Basins and younger Mesozoic cover. Three provinces are recognized, the Yarrol Province in the north, the Gympie Province in the east and the New England Province in the south. Both the Yarrol and New England Provinces are divisible into two zones, western and eastern, that are now separated by major Alpine-type ultramafic belts. The western zones developed at least in part on early Palaeozoic continental crust. They comprise Late Silurian to Early Permian volcanic-arc deposits (both island-arc and terrestrial Andean types) and volcaniclastic sediments laid down on unstable continental shelves. The eastern zones probably developed on oceanic crust and comprise pelagic sediments, thick flysch sequences and ophiolite suite rocks of Silurian (or older?) to Early Permian age. The Gympie Province comprises Permian to Early Triassic volcanics and shallow marine and minor paralic sediments which are now separated from the Yarrol Province by a discontinuous serpentinite belt. In morphotectonic terms, a Pacific-type continental margin with a three-part arrangement of calcalkaline volcanic arc in the west, unstable volcaniclastic continental shelf in the centre and continental slope and oceanic basin in the east, appears to have existed in the New England Orogen and probably in the Hodgkinson—Broken River Orogen as well, through much of mid- to late Palaeozoic time. However, the easternmost part of the New England Orogen, the Gympie Province, does not fit this pattern since it lies east of deepwater flysch deposits of the Yarrol Province.

Correlation and Paleogeography of Upper Part of Helderberg Group (Lower Devonian) of Central Appalachians

Physical stratigraphy and environmental relations in the upper part of the Helderberg Group have clarified the interpretation of biostratigraphic data in a previously enigmatic area of Appalachian Lower Devonian correlation. The Licking Creek Limestone, a marine carbonate shelf facies, is subdivided and its members (lower, Cherry Run Member; upper, Little Cove Member) are traced along the eastern outcrop belt from south-central Pennsylvania southward to central Virginia. Toward the northwest the Cherry Run increases in thickness and intertongues with the lower two thirds of the Shriver Chert, an offshore calcareous cherty silt and mud which occupies the depositional basin axis. The Little Cove rises stratigraphically toward the northwest and is traced into the upper part f the Shriver Chert. Lithologic and faunal differences of the equivalent units are caused by the differing, but time-equivalent, adjacent environmental settings (shallow shelf-basin axis). Lithostratigraphic evidence indicates that the upper contact of the Licking Creek-Shriver strata rises in the section in a northwesterly (basinward) direction and that the Licking Creek of the eastern outcrop belt intertongues with the lower two thirds of the Shriver in its type area. Biostratigraphic evidence indicates that faunal elements are added to the upper part of this interval as the contact rises. This evidence also suggests that the Licking Creek of the eastern outcrop belt and the lower two thirds of the Shriver are Helderbergian and the upper third of the Shriver is Deerparkian. Paleogeographic reconstruction of these correlations shows that (1) shoreline and facies boundaries are slightly oblique to the outcrop belt; (2) shallow carbonate facies tended to migrate westward (basinward) during this time, indicating a regressive trend just prior to Oriskany Sandstone deposition; and (3) the basin margin pre-Oriskany unconformity does not extend over large areas of the depositional basin itself. These conclusions are of potential significance for gas exploration in the upper Helderberg and overlying Oriskany Sandstone in relating subsurface stratigraphic facies and structure to the observed surface trends and correlations.

Pennsylvanian-Permian Miogeosyncline to Nearshore Shelf Carbonate Facies Transition, Clark County, Nevada: ABSTRACT

Pennsylvanian and Lower Permian carbonates of Clark County, Nevada, were studied at Arrow Canyon (AC), Frenchman Mountain (FM), and Azure Ridge (AR). Distinct facies changes are observed from the AC section (2,193 ft, Bird Spring Group, restricted, miogeosynclinal) where limestones are 65.5% micrite to FM section (1,055 ft, Callville Formation, outer shelf) where limestones are 57.2% pelsparites, biosparites, or oosparites to AR (779 ft, Callville Formation, inner shelf) where limestones are 50.9% pelsparites, biosparites, and oosparites. The marked change in facies provides evidence for support of a dual terminology, Bird Spring Group for the basin facies and Callville Limestone for the shelf. The Bird Spring-Callville interval is subdivisible into a lower eastward thinning member (1,634 ft, AC; 634 ft, FM; 322 ft, AR) characterized by abrupt vertical fluctuations in facies (AC 14.3, FM 8.0, AR 15.2 fluctuations per 100 ft) causing the steplike outcrop appearance characteristic of the interval. Gray to buff weathering and lack of quartz silt distinguish the lower member from the arenaceous buff to brown upper member. The latter contains several quartz sandstone units (zero AC, 41.7% FM, 51.1% AR) in which the size of the fine ( End_of_Article - Last_Page 350------------