Shallow-water Carbonate Platform Research Articles

Sedimentary Infilling of an Open Seaway: Bawihka Channel, Nicaraguan Rise

ABSTRACT Recent oceanographic cruises to Bawihka Channel have revealed the dominant sedimentary products along and within this open seaway separating two carbonate platforms and the mechanisms and style of infilling. This feature is a seaway 125 km long and 8 km wide through which pass strong currents associated with the regional western-boundary-current system. This seaway is in a tropical setting near a tectonically active plate boundary making it a suitable, comparative counterpart to the much better known and well-studied seaways to the north associated with the Bahama Banks. Multichannel seismic data indicate that the long and narrow Bawihka Channel evolved from a broad, open basin 50 km wide in the Eocene by basin-margin aggradation and progradation. Basin infilling was asymmetric, with the western margin becoming stabilized and aggrading by the middle Miocene while progradation to the west along the eastern margin was accelerated. Strengthening of the western boundary current during this time via tectonic foundering of the Nicaraguan Rise megabank, as well as by stronger easterly trade winds due to climatic deterioration, stimulated this style of basin-margin development and led to narrowing of the seaway. High-resolution seismic data reveal that more recent infilling has been dominated by debris-flow deposits, turbidites, large displaced blocks, and widespread algal-bioherm growth. These sediment-gravity-flow deposits, particularly the debris flows and displaced blocks, may be a result of the active tectonic setting of the northern Nicaraguan Rise. The distribution of sediment textures and constituents as well as the widespread presence of well-cemented bioherms and hardbottoms results from strong currents flowing through the seaway. Shallow-water surface sediments are dominated by coarse Halimeda fragments, while base-f-slope and channel-axis sediments are finer grained and contain higher concentrations of planktonic forams and pteropods. Piston-core data indicate a late Pleistocene or early Holocene phase of turbidite infill followed by a phase of pelagic sedimentation. The pelagic-infill phase resulted from decreased turbidite frequency and lack of shallow. water sediment production on the bank tops during early sea-level flooding. Midslope mounds, interpreted to be algal bioherms, and associated midslope basins also retard downslope movement of shallow-water sediments. The infilling of Bawihka Channel is significant in that it reveals how rapid and under what conditions two shallow-water carbonate platforms might coalesce into a larger bank. It also reveals the potential complexity of sedimentary facies and stratigraphy that may lie beneath the interiors of large carbonate banks.

Source-Rock Distribution Model of the Periadriatic Region

The Periadriatic area is a mosaic of geological provinces comprised of spatially and temporally similar tectonic-sedimentary cycles. Tectonic evolution progressed from a Triassic-Early Jurassic (Liassic) continental rifting stage on the northern edge of the African craton, through an Early Jurassic (Middle Liassic)-Late Cretaceous/Eocene oceanic rifting stage and passive margin formation, to a final continental collision and active margin deformation stage in the Late Cretaceous/Eocene to Holocene. This evolution occurred in a transtensional and transpressional framework resulting from the oblique separation and convergence of the African and European plates. Extensive shallow-water carbonate platform deposits covered large parts of the Periadriatic region in the Late Triassic. Platform breakup and development of a platform-to-basin carbonate shelf morphology began in the Late Triassic and extended through the Cretaceous. On the basis of this paleogeographic evolution, the regional geology of the Periadriatic region can be expressed in terms of three main Upper Triassic-Paleogene sedimentary sequences: the platform sequence (sequence A), the platform to basin sequence (sequence B), and the basin sequence (sequence C). These sequences developed during the initial rifting and subsequent passive-margin formation tectonic stages. Three distinct episodes of source-bed deposition can be identified. Middle-Upper Triassic to Lower Jurassic (Liassic) organic-rich, oil-prone and mature source beds generally are structurally controlled, and are closely associated with transtensional intraplatform basins formed in the early continental rifting stage. The principal Triassic source basins and most of the surface hydrocarbon indications and economically important oil fields of the Periadriatic region are associated with sequence B areas. The second episode of organic-rich rocks deposition includes both shallow and deeper water facies of Jurassic-Cretaceous age. Thin, immature, deep-water epioceanic organic-rich marls and shales are generally bathymetrically controlled. These units are related to basin-wide anoxic events in the Tethys ocean and its submerged continental margin, and are associated with sequence B areas. Middle-Upper Cretaceous shallow-water, oil-prone facies occur in isolated anoxic basins within carbonate platforms; these source sequences are generally thicker and are associated with sequence A areas. No major hydrocarbon accumulation can be directly attributed to the Jurassic-Cretaceous epioceanic and intraplatform source rock sequences. The third episode of source bed deposition characterizes the final active margin deformation stage, and is represented by Upper Tertiary organic-rich terrigenous units, mostly gas prone. These are essentially associated with turbiditic and flysch sequences of foredeep basins, and have generated the greater part of the commercial biogenic gases of the Periadriatic region.

A fossil carbonate contourite drift on the Lower Ordovician palaeocontinental margin of the middle Yangtze Terrane, Jiuxi, northern Hunan, southern China

The Early Ordovician (Tremadocian) in part of Jiuxi, northern Hunan Province, comprises a succession of deep-water carbonate sediments deposited on a palaeocontinental margin at the southern edge of the Yangtze Platform. A distinctive elongate mound-like form, some 350–450 m thick, can be recognized lying between shallow-water platform carbonates and deeper-water mudstones. This elongate body, here called the Jiuxi drift, is made up dominantly of sediments interpreted as contourites on the basis of their mid to base-of-slope location, alongslope current indicators, features of traction flow processes coupled with intense bioturbation, and distinctive contourite sequences (typically 30–80 cm thick). Five contourite facies are recognized: calcilutites, calcisiltites, calcarenites, calcirudites (possibly contourite lag deposits), and bioclastic contourites. Hemipelagites and turbidites make up only a small proportion of the drift, which accumulated at an average sedimentation rate of 38 m/m.y. Large-scale cross-stratified units found in parts of the calcilutitic contourite section are believed to result from seafloor development of mudwaves and/or erosional furrows under the influence of a semipermanent bottom-current regime.

Submarine Cementation of Subsurface Pliocene Carbonates from the Interior of Great Bahama Bank

ABSTRACT Thick intervals of early Pliocene subtidal carbonate sediments in the subsurface of northwestern Great Bahama Bank were lithified by submarine cement. This cementation, subsequently altered by meteoric diagenesis, occurred over a broad expanse of the platform interior. It shows that submarine cements can lithify thick packstone and grainstone intervals in the interior of large shallow-water carbonate shelves and platforms given open circulation, moderate to slow sedimentation rates, and minimal accumulation of fine sediments. A submarine origin for these cements is inferred from direct and circumstantial evidence, including: (1) regional depositional and diagenetic setting; (2) fabric and habit similar to modern submarine cements; (3) alteration of cements from mineralogically unstable precursors indicated by (a) abundant inclusions, (b) relict precursor fabric, and commonly (e) yellowish coloration, (d) undulose crystal extinction, and (d) selective leaching of specific generations of cement or cement types; (4) sutured mid-pore crystal contacts; (5) precipitation coeval with accumulation of internal marine sediment; and (6) association with organisms requiring a firm substrate for attachment. These considerations may help other workers recognize submarine cements in ancient carbonate rocks.

Early Cretaceous Platform-Margin Configuration and Evolution in the Central Oman Mountains, Arabian Peninsula

The Hajar Supergroup (Middle Permian-Lower Cretaceous) of northeastern Oman records rifting and development of a passive margin along the edge of the Arabian platform facing Neo-Tethys. The Jurassic and Lower Cretaceous part, comprising the Sahtan, Kahmah, and Wasia groups, was deposited during the maximum extent of the broad epicontinental sea landward of this margin. These limestone units reach a total of 1500 m in thickness and they correlate with the principal hydrocarbon reservoirs of the Arabian Peninsula. The trace of the Jurassic and Cretaceous margin in northeastern Oman followed a zigzag series of rift segments, resulting in promontories and reentrants that changed in position through time in response to the configuration and differential motion of underlying rift blocks in an overall extensional regime. Synsedimentary normal faulting with 600 m of throw occurred locally during the Middle Jurassic, whereas during the Late Jurassic, the margin was subjected to erosion from variable uplift of up to 300 m before subsiding to below storm wave base. This uplift may have been caused by compression from oceanic crust that obducted along the southeastern side of the platform. Subsequently, Lower Cretaceous strata document varying rates of block subsidence. The Lower Cretaceous succession in the central Oman Mountains and adjacent subsurface began with regional drowning around the Jurassic-Cretaceous boundary. The succession in the east (Saih Hatat) records a single regressive sequence, culminating in the progradation of the shallow-water carbonate platform by the Cenomanian. However, the succession in the west (Jebel Akhdar and interior) is dominated by shallow-water carbonate facies, but punctuated by a second regional drowning in the late Aptian. A third, Late Cretaceous drowning terminated deposition of the Wasia Group in the Turonian and was caused by convergence of oceanic crust and foreland basin formation. Although previously ascribed to eustatic sea level rises, the two Early Cretaceous drowning events can be explained as consequences of the tectonic behavior of the platform. The earlier event (Rayda Formation) occurred when first uplift tilted the eastern side of the platform and induced a relative sea level stillstand and widespread evaporative conditions in the interior (Hith anhydrite), and then the eastern side subsided rapidly to below storm wave base, forming a broad embayment starved of periplatform sediment. By contrast, the second drowning (Nahr Umr Formation) was due to detrital influx generated by uplift on the west side of the platform. Both tectonic episodes led to a reduction in the calcium carbonate generating capacity of the epicontinental sea, causing shallow-water fa ies to be abruptly overlain by deeper water facies. The effects from these two drownings eventually diminished, and shallow-water environments reestablished themselves. The overall succession appears independent of eustatic sea level change.

Genetic modelling for banded iron-formation of the Hamersley Group, Pilbara Craton, Western Australia

The banded iron-formation (BIF) of the Hamersley Group, Pilbara Craton, Western Australia, particularly from the well studied Dales Gorge Member, is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60,000 km 2, enabling reasonable estimates for the annual input of components to the depository. In the model of this paper, varying supply of materials for the medley of mesoband types, particularly of iron and silica in the oxide BIF, can be accommodated by the interaction of two major oceanic supply systems: (1) surface currents and (2) convective upwelling from mid-oceanic ridge (MOR) or hot-spot activity, both modified by varied input of pyrochastic material. (1) The surface currents were saturated in silica and carried minimal iron due to photic precipitation, but were periodically recharged by storm mixing. Precipitation from them gave rise to the banded chert-rich horizons, including the varves, whose regular and finely laminated iron/silica distribution resulted from seasonal meteorological influences. (2) Precipitation from convection driven upwelling of high iron solution from MOR or hot-spot activity periodically overwhelmed the delicate seasonal patterns of (1) to produce the iron-dominated mesobands. A wide range of intermediate mesoband types resulted where the deep water supply was modified by varied MOR activity, or by partial blocking of upwelling waters by surface currents (such as by the present El Niño). During these periods of oxide-dominated BIF, silica was deposited from saturated solution mainly by evaporative concentration, and iron by oxidation due to photolysis and photosynthetically produced oxygen. Superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources, changing the meteorological and depositional conditions. Occasional input of extremely fi ash during BIF precipitation produced mesoband (cm) scale variations involving increased carbonate-silicate precipitation. Sustained volcanic periods resulted in S-macroband deposition (chert-carbonate-silicate BIF, with shale), gradually returning to the dominant hematite-magnetite-chert BIF as the volcanic input waned. During volcanic periods, the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron, and by photosynthetic production of oxygen, was modified by turbidity in the atmosphere (aerosols and dust) and in the water (colloids from reactive ash). S Surface-precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone, augmenting the iron content of the zone. Precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub-photic zone exceeded their respective solubilities. During volcanism, the increased availability of nutrients, particularly phosphorus, to surface waters increased the organic contribution despite lower light values, leading to an almost total absence of ferric iron oxides in the S macrobands (i.e. no magnetite or hematite). Cooling of warm, silica-saturated sea-water during these periods of “olcanic winter” increased the ratio of precipitation of silica to iron, which, however, was still controlled by seasonal conditions. Intermediate concentrations of organic matter, insufficient to totally convert the ferric compounds either during precipitation or diagenesis, resulted in overgrowths of magnetite on hematite, and eventually in the substantial conversion of hematite to magnetite, where higher temperatures were achieved during low-grade regional metamorphism. Changes in sea-level to explain facies changes in BIF are not required in this model, but are not excluded. The preferred conditions are for a very low oxygen to anoxic atmosphere, a much higher level of MOR activity than at present, the presence of photosynthetic plankton, the absence of si silica-secreting organisms, and a deep sea-water temperature higher than 20°C. However, none of these conditions is essential to the model. A narrow carbonate bank is postulated for part of the Fortescue River Valley area during Marra Mamba Iron Formation times (basal Hamersley Group), with BIF precipitation on either side. The reef is postulated to have grown northward becoming a major shallow-water carbonate platform on the Pilbara continent during upper Marra Mamba Iron Formation and Wittenoom Dolomite times, but ceased to play an important role in subsequent periods.

The Lower Cretaceous Mesogean benthic ecosystems: palaeoecologic aspects and palaeobiogeographic implications

Douvillé's Mesogée was based on an organismic assemblage and therefore defined mainly as a palaeobiogeographic entity. The palaeoecologic analysis of Douvillé's Lower Cretaceous reference taxa, as well as that of a lot of other taxonomic groups, and the discussion on their palaeobiogeographic meaning, according to the concepts of modern marine biogeography especially the bathymetric/thermic relationships, result in the definition of three main organismic assemblages whose Mesogean significance is discussed. The key reference taxa and associated biota indicate that the definition of the Lower Cretaceous Mesogée implies characteristic palaeoenvironmental conditions. This definition, substantial for the whole Cretaceous, means that Mesogée is not simply a palaeobiogeographic entity but also an ecosystem corresponding to the area of distribution of warm, shallow-water carbonate platforms.

Sedimentologic and paleotectonic evolution of carbonate platforms on a segmented continental margin: example of the African Tethyan margin during Turonian and Early Senonian times

During Turonian and early Senonian times, there were extensive shallow-water carbonate platforms on the northern margin of the African craton (Eastern Algeria, Central Tunisia and adjacent offshore areas), at the edge of the Tethys ocean. The development of these carbonate platforms (mid-Turonian to late Coniacian) was especially controlled by the segmentation of the African (Saharan) margin by extensional fault systems, as evidenced by block-faulting, volcanic activity and halokinetic movements. From the mid-Turonian to early Coniacian times, the carbonate platforms underwent an incipient drowning, indicated both by their sedimentologic evolution (rimmed-shelves to carbonate ramps) and the landward shift of facies belts. A partial drowning of carbonate platforms during late Turonian times was probably induced by climatic and/or oceanographic changes. The late Coniacian is the period of continuation and further extension of deepening events with the widespread development of deep ramp facies. At the Coniacian-Santonian boundary, the sinking of the Saharan margin induced an abrupt increase of subsidence rates and the complete drowning of carbonate platforms, covered by deep-water argillaceous facies. This renewal of tectonic activity at the Coniacian-Santonian boundary is recorded in many areas in North Africa (“Intra-Senonian phase”).

Metamorphism and polygenesis of the Madem Lakkos polymetallic sulfide deposit, Chalkidiki, Greece

Metamorphism and polygenesis of the Madem Lakkos polymetallic sulfide deposit, Chalkidiki, Greece

Recognition and Significance of Parasequences and Parasequence Sets in Leonard Carbonate Reservoirs, West Texas

Although hydrocarbon production from reservoirs of Leonardian age is sizable (cumulative production from the Leonardian on the Central Basin Platform (CBP) alone totals 1.4 billion bbls), recovery efficiencies from these restricted, shallow-water platform carbonate deposits are among the lowest in west Texas, averaging only about 19%. Detailed examination of the Leonard section in the Dollarhide and Monahans fields on the CBP indicates that poor recovery from these reservoirs is a function of extreme vertical heterogeneity produced by high-order oscillations of relative sea level. The Leonard sequence is composed of thin (1- to 2-m thick), upward-shallowing parasequences, each of which is marked at its base by a prominent marine flooding surface. Parasequences are, in turn, packaged into 15- to 20-m parasequence set defined by variations in parasequence facies stacking patterns. Reservoir porosity is typically preferentially developed in very thin (less than 1 m) zones of grain-rich subtidal facies within parasequences, although porosity may also be encountered in capping tidal-flat deposits. Because these facies display distinct pore and permeability characteristics, with subtidal facies usually exhibiting the highest permeabilities, development of accurate reservoir models for simulation and exploitation depends on the ability to distinguish and map these deposits. Recognition and correlation of parasequencesmore » and parasequence stacking patterns in parasequence sets provide a powerful tool for more accurate mapping of facies and attendant porosity development and facilitate development of improved models necessary for efficient exploitation of these highly heterogeneous reservoirs.« less

Biomineralization and Magnetization of Sediments

Although nearly 60 mineral-like compounds are known to be synthesized biochemically by living organisms, only two (magnetite Fe{sub 3}O{sub 4}, and greigite Fe{sub 3}S{sub 4}) have ferromagnetic properties capable of providing a stable remanent magnetization to sediments. In particular, the magnetotactic bacteria are now known to provide a significant supply of fine-grained, single-domain magnetite to surface sediments in environments ranging from deep-sea sediments to terrestrial soils. In many such environments, including the deep-sea sediments shallow-water platform carbonates, and perhaps soils, the fossilized bacterial magnetosomes are clearly the source of the stable remanent magnetization (NRM) upon which paleomagnetic studies and magnetic polarity stratigraphies are based. The crystal structures of these bacterial magnetofossils, revealed through the use of high-resolution TEM techniques, confirms both their biological origin and documents some of the postdepositional dissolution and diagenetic effects to which they are subjected. Recent discoveries have revealed an amazing diversity of magnetite-precipitating micro-organisms, many of which produce up to several hundred times more fine-grained magnetite per cell than known previously. These organisms include some of the largest bacterial cells yet found, which are up to 15 micrometers in length and contain several thousand magnetite crystals in their magnetosome chains. The contribution of these organismsmore » to the magnetization of sediments is as yet unknown.« less

Depositional cycles and source rock development in an epeiric intra-platform basin: the Hanifa Formation of the Arabian peninsula

The Upper Oxfordian to Lower Kimmeridgian Hanifa Formation was deposited in an intra-platform basin on the broad epeiric shallow-water carbonate platform that extended over much of the Middle East during the Mesozoic. The formation consists of organic-rich carbonate mudstones, bioturbated mudstones and sulphates. The organic-rich mudstones are considered to be one of the major hydrocarbon source rocks in the Middle East oil province. The correlation of sedimentary facies shows a hierarchical stacking pattern of transgressive-regressive depositional cycles within the Hanifa Formation. Since these cycles occur at a basin-wide scale, it is assumed that they are related to relative changes of sea level. The superimposed scales of cyclicity determine the occurrence of source rock and sulphates. Major source rock development occurs in transgressive-dominated cycles. Sulphates, on the other hand, are more common during periods of overall regression.

Deep-water basins within the Mesozoic carbonate platform of Argolis, Greece

In the past the classic Argolis area of the central southern Hellenides was interpreted as comprised of essentially in situ carbonate platforms and basins, or, alternatively of far-travelled thrust sheets derived from a Mesozoic ocean basin sited well to the east (Vardar ocean). Our fieldwork, however, demonstrates that at two key localities an important unit of Middle Triassic–Early Jurassic deep-water redeposited carbonates (Asklipion Unit) passes laterally into shallow-water platform carbonates (Pantakrator Unit). The deep-water limestones represent basins within the platform, rather than far-travelled thrust sheets, while other slices of deep-water limestone record a platform margin, further east. The basins originated during Middle Triassic (Ladinian) rifting of the Neotethys and are floored by extrusive rocks of intermediate composition. Highly condensed Ammonitico Rosso pelagic limestones accumulated on an inferred volcanic seamount during the Mid–Late Triassic (Anisian–Carnian). The platforms slowly subsided in Early Jurassic time, while the basins partly filled with redeposited carbonates (Adhami Limestone). Shallow-water carbonates locally transgressed the margins of the deeper-water basins. The platforms were submerged in the Mid-Jurassic (Toarcian–Bathonian), with the deposition of pink pelagic Ammonitico Rosso, followed by marked subsidence and ribbon radiolarite deposition below the carbonate compensation depth (e.g. Koliaki Chert). In early Late Jurassic time (Kimmeridgian), the area was regionally compressed (‘Eohellenic’ phase), and the intraplatform basins, founded on thinned continental crust were inverted, and thrust up to several kilometres over the adjacent platform units. The deformed continental margin then flexurally subsided, ahead of emplacing Jurassic ophiolites, largely olistostromes (Migdhalitsa Unit). After taking account of up to 90 degrees clockwise palaeorotation of assumed Neotectonic age, kinematic indicators and facies trends suggest the ophiolite was derived from a Pindos ocean basin originally sited to the west. Early Cretaceous transgression resulted in the deposition of shallow-water limestone and was followed by Late Cretaceous deep-water limestone, then terrigenous flysch accumulation in a foreland basin related to final suturing of the Neotethyan ocean. Later, the area was dissected by a series of mainly E–W trending, high-angle Neotectonic normal faults, related to the Aegean back-arc extensional system.

Evolution of the Blake Plateau Basin from Rift Basin to Carbonate Platform to Present Eroded Continental Borderland: ABSTRACT

Multichannel seismic profiles, well and submersible diver samples, and GLORIA long-range sonar data disclose the evolution of the Blake Plateau Basin (BPB) off the southeastern US. The BPB contains strata more than 16 km thick and is roughly 350 km wide. At the postrift unconformity, the northern and western sides of the basin are relatively steep, whereas the southern and eastern sides slope more gently. The entire region probably was a shallow-water carbonate platform during most of the Late Jurassic and Early Cretaceous; the depocenters of the BPB slowly migrated westward during this period, probably due to thermal or magmatic variations in the lithosphere. Although subsidence slowed during the Late Cretaceous, carbonate deposition failed to maintain the surface near sea level, and chalks and marls blanketed the sinking platform. During the Cenozoic, the Gulf Stream scoured the top of the present plateau and restricted clastic deposition to the westernmost part of the BPB region where a narrow shelf is formed. Southerly deep water thermohaline currents have eroded the old continental slope/reef front into a steep escarpment except for an area having relict canyons. Deep-water erosional retreat of platform deposits forms steep, smooth or stepped cliffs east of the northern andmore » central BPB; but to the south, erosional cliff retreat occurs by collapse of huge (<10 km) blocks, probably initiated by faults that bound the south side of the BPB. These faults may be reactivated due to differential loading caused by growth of the Bahama platform.« less

Zechstein reservoirs in The Netherlands

Abstract Thick, shallow-water gypsum banks, formed during Zechstein times at the site of regional palaeohighs along the margins of the Southern Permian Basin, acted as foundations for shallow-water carbonate platforms. The carbonate platform margins, in particular their northern to northeastern windward margins, were characterized by deposition of grainstones dominated by aragonitic ooids, pisoids, intraclasts and bioclasts, while muddy, stromatolitic carbonates and nodular anhydrites accumulated on tidal flats and in lagoons behind the platform margins. Usually the platforms are surrounded by thick slope deposits consisting of carbonate mudstone and redeposited intervals, but locally the platforms deepened gradually and changed, without intervening slope deposits, into a shallow-marine facies devoid of ooids. The slope aprons grade basinwards into thin successions of finely laminated carbonate. Porosity creation in Zechstein carbonates was due to meteoric-water leaching and to subsurface leaching by carbon dioxyde-enriched formation waters during burial. Porosity destruction was mainly caused by anhydrite pore plugging and subsurface calcitization. The best reservoir properties are found in the unrestricted platform facies and in parts of the slope facies. Fractures are important in Zechstein carbonates; their presence is a prerequisite for high production rates in gasfield wells.

Triassic corals and spongiomorphs from Hells Canyon, Wallowa terrane, Oregon

Twenty-one species of corals and three species of spongiomorphs occur in a series of richly fossiliferous, molluscan-dominated beds with silicified bioclasts in the Upper Triassic Martin Bridge Limestone of Hells Canyon, Oregon. Two of these,Maeandrostylis grandiseptusandRecticostastraea wallowaensisare new species.Recticostastraeais designated as a new genus.The fauna is early Norian and occurs in the island arc Wallowa terrane, one of many tectonostratigraphic terranes in western North America. Like other examples, it appears to have developed independently of the North American craton and to have links with Wrangellia. The fossil corals and spongiomorphs are para-autochthonous, occurring in a series of tempestite beds. They are interpreted to have inhabited a shallow-water carbonate platform that developed around a tropical island arc following cessation of volcanic activity. The corals and spongiomorphs are associated with abundant gastropods and a diverse epifaunal suspension-feeding bivalve fauna. Relative to the corals, branching spongiomorphs,Spongiomorpha ramosa, are more abundant and occur with relatively common branching, sheet to plate-like, colonial corals. Solitary corals are relatively rare. The associated bedded limestone includes a variety of shallow-water microfacies but throughout the Hells Canyon sequence, reef structure is absent.Together, the 24 coral and spongiomorph taxa show mixed paleogeographic affinities with Upper Triassic faunas known only from alpine regions of the western Tethys (five species), the Pamir Mountains, U.S.S.R. (two species), and the island of Timor (one species). Five additional species are pan-Tethyan and exceptionally cosmopolitan, but 11 species (45.8%) occur only in displaced terranes. Of these, a significant component (six species) is endemic to the Wallowa terrane. At least four Hells Canyon taxa, previously thought endemic to North American terranes, have recently been reported from the high-latitude Koryak terrane of northeastern U.S.S.R., a displaced tropical volcanic terrane of the northwestern Pacific. For Triassic corals, this is the first example of a clear link between western Pacific and eastern Pacific terranes. Less similarity exists with the Wrangell Mountains, Alaska, where identical age lower Norian silicified corals and spongiomorphs are known.

Palaeogeographic Significance of Clay Mineral Distributions in the Inferior Oolite Group (Mid Jurassic) of Southern England

AbstractFive areally distinct mineral assemblages are recognized in the Inferior Oolite of S. England. In each area, vertical (stratigraphic) variations are insignificant. The five assemblages comprise varying proportions of illite, illite-smectite, kaolinite, chlorite and kaolinite-smectite, the mixed-layer clays being largely poorly crystalline and randomly interstratified. A predominantly detrital rather than authigenic origin is suggested for most of the clays. Shallow-water platform carbonates contain kaolinite with illite and illite-smectite. Kaolinite decreases in abundance away from former mid-Jurassic land areas, the deeper shelf and more basinal facies being dominated by illite and/or illite-smectite. Possible volcanic contributions to clay suites are suggested but cannot yet be fully evaluated. The palaeogeographic usefulness of clay mineral suites is confirmed, even in carbonate-dominated systems.

Geology and Reservoir Characteristics of Carbonate Buildup in Giant Bu Hasa Oil Field, Abu Dhabi, United Arab Emirates

Bu Hasa field in Abu Dhabi was the first giant oil field in the Arabian Gulf to produce from the Lower Cretaceous Shuaiba Formation (Aptian). The field has a productive area of about 155,673 ac (63,047 ha.) and 12.6 billion bbl of oil reserves. The formation is composed mainly of rudistid and algal sediments, rudistid mounds having overlain and built up the topography of an algal platform. The position and elevation of the platform edge, combined with rising eustatic sea level, created a tendency toward both vertical and lateral growth, and regulated the form and distribution of the rudist accumulation. The Shuaiba Formation has good reservoir quality ranging in porosity from 18-25% and with average permeability exceeding 100 md. Nine reservoir units (A-I), classified by their porosity, permeability, and lithology, can be clearly defined from the logs. Eleven main lithofacies have been defined in the Shuaiba from detailed core descriptions. These lithofacies were deposited in three main phases. The earliest phase consisted of chalky mud-supported sediment with some algal colonies, followed by a middle phase characterized by a thick algal platform. During this stage of deposition, a shallow-water carbonate platform developed over the south and central part of the field, grading into deeper, open-marine sedimentation toward the north. The late phase was characterized by rudistid buildups (mainly caprinids and caprotinids). The growth of the thick algal platform in the south and central areas controlled the initial distribution of the main rudistid buildup; toward the north, small rudistid reefs are sparsely developed and present only n the latest stage on high-energy shoals.

El Margen Atlántico Ibérico al W de Galicia. Evolución en régimen extensional y sedimentación. (Resultados preliminares del Leg. 103, Ocean Drilling Program.)

Leg 101 of the Ocean Drilling Program (ODP) was devoted discovering the tectonic and sedimentary evolution of the Atlantic Margin of the lberian Peninsula. A transect of five sites, with a total of 14 drill-holes was undertaken to the South of the Galicia Bank on the seaward edge of the margin. The data obtained revealed a complex history of subsidence and rifting preceding the initiation of sea floor spreading between Newfoundland and Iberia. The main findings include: 1) The Upper Jurassic-Lowermost Cretaceous shallow-water carbonate platform are the first Messozoic deposits at the margin. The «basement seismic reflector» is made-up of these carbonates. 2) The platform drowning, tilting of fault blocks and rapid subsidence preceded the spreading by as much as 25 million years. 3) A ridge of serpentiniced peridotites is located near the boundary between the oceanic and continental crusts. 4) The seismic reflector «S» does not, as widely believed represent a ductile-brinle boundary within the continental crust but is instead a reflector at the base of the synrift sediments.

Reconstructing Paleoenvironments of Jameson (Strawn) Reef Field, Coke County, Texas: ABSTRACT

The Jameson reef is an elongate, strike-trending mud buildup that occurs within the Middle Pennsylvanian (Desmoinesian) Strawn reef trend of west-central Texas. It consists primarily of carbonate wackestones, mudstones, and packstones with smaller units of Chaetetes framestones and grainstones. Microfacies analyses indicate that the buildup was deposited on a warm, shallow-water carbonate platform characterized by downslope mud accumulations and upslope calcarenite beaches and islands. The shape of the buildup was controlled by ocean currents, which spread the sediments laterally and downdip. The sediments were exposed to early marine and meteoric diagenesis as well as late subsurface diagenetic processes. Marine diagenesis includes primarily micritization of component grains. Leaching occurred in the meteoric phreatic environment following micritization. The leaching produced intergranular, intragranular, moldic, and vuggy porosity. A later transgression placed the buildup in the meteoric phreatic environment that induced cementation, partially filling pore spaces. The grainier rocks were cemented to a greater degree than the wackestones. Dolomitization occurred in the mixing zone where dolomite rhombs generally filled pores in the micrite matrix. Diagenesis culminated with stylolitization in the deep subsurface reducing porosity. The middle zone of wackestone makes up the best reservoir rock mainly because selective cementation of the grainier rocks occurred in the meteoric phreatic environment. End_of_Article - Last_Page 1423------------