Shallow-water Carbonate Platform Research Articles

Petroleum Potential and Stratigraphy of Holitna Basin, Alaska: ABSTRACT

The Holitna basin, an interior Alaskan basin, is flanked by Cambrian to Cretaceous sedimentary rocks that have been highly folded and faulted. Gravity mapping and modeling indicate up to 15,000 ft of sedimentary section is present within the basin. Cambrian rocks consist of trilobite-hash lime mudstone, red siltstone, and basinal chert. Ordovician through Devonian basinal facies rocks consist of platy limestone to graptolitic shale with minor interbeds of limestone turbidites and turbidite-channel debris flows. Ordovician through Middle Devonian shallow-water platform carbonate rocks prograde over the basinal rocks and are composed of algal boundstone buildups with associated inboard lagoonal, oolitic shoal, and tidal-channel deposits. Toward the interior of the platform, restricted subtidal and intertidal to supratidal carbonate rocks were deposited. Upper Devonian to Permian platform carbonate deposition occurred to the east of the older platform rocks and conformably(?) over basinal rocks. Minor mixed carbonate-clastic deposi ion occurred into Triassic time. The Cretaceous Kuskokwim Group is in fault contact with and/or unconformably overlies the Paleozoic carbonates. This unit varies from lithic-rich conglomerates to marine turbidite deposits. Maturation values for Cretaceous and Paleozoic rocks are within the oil window, with most of the shales showing a thermal alteration index (TAI) from 2 to 3. Organic carbon content exceeds 3% in some samples, however, deep surface weathering has resulted in low hydrocarbon values. The lithic-rich Cretaceous sandstones are well indurated, poorly sorted, and are considered to have low reservoir potential. Partly dolomitized, shallow-water Ordovician to Devonian carbonate rocks are the best potential reservoir rocks exhibiting vuggy porosities greater than 10% and good intergranular porosity. End_of_Article - Last_Page 308------------

Sedimentology of the Kuruman and Griquatown Iron-formations, Transvaal Supergroup, Griqualand West, South Africa

Abstract Iron-formation was deposited as a distal facies of ferruginous carbonate turbidites in an open shelf environment in front of a shallow-water carbonate platform at the time of deposition of the Campbellrand carbonate sequence. A subsequent transgression resulted in the deposition of open shelf iron-formation on top of the Campbellrand carbonate platform. Progradational sedimentation coupled with shoaling followed, and an iron-formation sequence represented by the Kuruman and Griquatown Iron-formations was deposited. This sequence consists from the base upwards of stacked open-shelf cycles of altered volcanic ash stilpnomelane lutite beds and autochthonous banded ferhythmite units; toe-of-slope greenalite—siderite rhythmites; platform slope greenalite—siderite rhythmites with grainflow bands; platform edge sideritic orthochemical and allochemical iron-formations; epeiric sea orthochemical and allochemical sideritic, hematitic and greenalitic iron-formations; supratidal disclutites and lacustrine banded greenalite lutite. Landwards, the lacustrine felutites were followed by deltaic chloritic claystone, siltstone and quartz wacke of the Koegas Subgroup. Autochthonous ferhythmites of the Kuruman Iron-formation reach a maximum development in a basin near Prieska, whereas orthochemical and allochemical units are more abundant on the Kaapvaal craton. Iron mineral and chert microbanding in the ferhythmites is attributed to seasonal changes in Eh and pH in the depository and may be related to biological activity. Chert mesobanding in the iron-formations is essentially of an early diagenetic origin.

Petrology and depositional history of a Middle Ordovician carbonate platform: Chickamauga Group, northeastern Tennessee

The Middle Ordovician of northeastern Tennessee comprises two stratigraphically and paleoenvironmentally discrete sedimentary packages. In the northwest, the Chickamauga Group, which can be further subdivided into nine formations, is primarily composed of shallow-water carbonate-platform deposits formed during the transgression of the early Middle Ordovician sea. Water depths normally were no greater than about 50 m, usually less. Greatest depths were probably present in the southeastern parts of the platform, where deeper-water (below wave base) reef complexes developed. Farther southeast, partially equivalent Middle Ordovician deposits, comprising at least five distinct formations, exhibit considerably more lithologic and paleoenvironmental diversity. These deposits include a relatively thin basal sequence of shallow-marine carbonates overlain by deep-water, hemipelagic carbonates and turbiditic shales and siltstones. Maximum depths of water in this region were on the order of 700 m. The earliest recognizable stage in the history of this paleoenvironmentally complex sequence is associated with the development of a shallow-water carbonate platform in the southeastern-most part of the area (Whiterockian time). Subsequent rapid downwarping in this area resulted in the drowning of the platform and the formation of a starved, deep-water, foreland basin in which hemipelagic carbonates and related slope deposits began to accumulate. The final phase of tectonic movement in the area was associated with the nearly isochronous flooding of exposed Lower Ordovician sediments (Knox Group) to the northwest. This event, which marked the beginning of deposition of the Chickamauga Group, was associated with the first introduction of eastwardly derived turbidites into the basin. Continued infilling of the basin was initially coupled with a gradual deepening trend on the carbonate platform to the northwest. Eventually, the basin was filled to the point at which terrigenous material began to be effectively transported across the basin and onto the platform. Associated with this terrigenous influx, an initial deepening, related to a reduction in carbonate production, was followed by gradual shallowing on the platform. Continued terrigenous input and shallowing ultimately resulted in the development of terrigenous-carbonate tidal flats throughout the area. The Tennessee Middle Ordovician contains a sequence of depositional facies that detail the developmental history of a foreland basin in an active margin setting. Recognition of similar sequences elsewhere may be used to infer analogous depositional settings and tectonic styles.

Deep Sea Drilling Project, Leg 77, southeastern Gulf of Mexico

In January 1981, R/V Glomar Challenger drilled five holes in the southeastern Gulf of Mexico to provide ground data for extensive seismic surveys and to document the pre-Tertiary history of the Gulf. Holes 535 and 540 were drilled in a basinal terrane for maximum penetration of the Cretaceous-Tertiary sequence. Rhythmic alternations of light bioturbated and dark laminated carbonaceous limestone represent the Early Cretaceous interval. Some of the dark layers are rich but immature oil source rocks. The limestones resemble the Blake-Bahama Formation in the North Atlantic but their stratigraphic age overlaps in part with the Hatteras Shale. Late Cretaceous rocks are almost totally missing in the basin sites and the Cenozoic section consists of chalk and marly carbonate ooze. Holes 536,537, and 538A were drilled on high-standing fault blocks. Hole 537 recovered phyllite that records 40 Ar/ 39 Ar plateau ages of about 500 m.y. and is overlain by an Early Cretaceous deepening sequence of alluvial to littoral elastics and oolitic-oncolitic limestones, capped by a thin sequence of Cretaceous and Cenozoic pelagics. In Hole 538A, basement consists of mylonitic gneiss and amphibolite, intruded by several generations of diabase dikes (that is, “transitional” crust). 40 Ar/ 39 Ar dates of hornblendes and biotite from the regional metamorphic rocks suggest a 500-m.y. (“Pan-African”) age with mild late Paleozoic thermal overprint. 40 Ar/ 39 Ar whole-rock dates from the dikes suggest intrusions between 190 and 160 m.y. ago. Basement is covered by a thin layer of pelagic chalk, followed by Early Cretaceous skeletal-oolitic limestones and, finally, Cretaceous-Tertiary pelagics. The oolitic-oncolitic limestones at both sites represent either parts of a shallow-water carbonate platform or platform talus deposited in deep water. Hole 536 bottomed in shallow-water dolomite (Jurassic or Permian), overlain by middle Cretaceous skeletal limestones with shallow-water biota and intercalations of pelagic chalk, interpreted as Cretaceous talus at the foot of the Campeche Bank. Cretaceous-Tertiary chalk and carbonate ooze cap the sequence. Among the most significant results of the leg are: (1) recovery of “transitional” crust with early Paleozoic (Pan-African) metamorphic rocks, (2) recovery of Early Cretaceous deep-water limestones with immature petroleum source beds, (3) recovery of mid-Cretaceous platform talus resembling the reservoirs in the Poza Rica and probably some of the Reforma fields of Mexico, and (4) discovery of a Late Cretaceous hiatus of 30 m.y. that corresponds approximately to the “mid-Cretaceous unconformity” recognized widely on seismic records in the Gulf of Mexico.

Stratigraphy and geochemistry of accreted fragments of the ancestral pacific floor in southern south America

The outer Pacific archipelago of southern Chile has allochthonous fore arc rocks that were accreted prior to the Late Jurassic along the ancestral Pacific margin of the southern South American sector of Gondwana. The fore arc rocks in the Archipielago madre de Dios (50°-50° 47′S) are divisible into three assemblages: the Tarlton Limestone, a sequence of massively bedded Upper Carboniferous to Lower Permian limestone; the Duque de York Complex, a sequence of flysch-like sedimentary rocks; and the Denaro Complex, an association of pillow basalt, bedded chert, detrital calcareous rocks, and shale. The Denaro Complex is interpreted as a sequence of the late Paleozoic ancestral Pacific floor. The Tarlton Limestone may represent a shallow water carbonate platform also built on a basaltic substrate located within the ancestral Pacific realm. The basalt units of the Denaro Complex have tholeiitic major element patterns similar to those of modern day “MOR” and “within plate” basalts. Chemical data for the Denaro Complex allow for the partial discrimination of biogenic, hydrothermal, hydrogenous, and detrital components within the sedimentary units, and facilitate comparison with modern day sediments as well as with other ophiolite sequences. Chemical data together with stratigraphic and structural field relations suggest that the evolution of the Denaro Complex and Tarlton Limestone involved seafloor spreading, hydrothermal activity, seamount volcanism, carbonate reef development, and convergence of the oceanic realm with the margin of Gondwana.

Early Cretaceous Sedimentation and Tectonics in Southeastern Arizona: ABSTRACT

Sedimentary strata of Aptian to Albian age are widespread in southeastern Arizona, southwestern New Mexico, and northern Mexico. These rocks comprise a transgressive/regressive marine/nonmarine basinal sequence, locally over 3,000 m thick, of sandstone, shale, limestone, and conglomerate mapped as the Bisbee Group. In general, the nonmarine lower third of the sequence is composed of coarse clastic alluvial fan and basin-fill deposits of local origin and rests unconformably on rocks of Jurassic to Precambrian age. These syntectonic sediments were deposited during regional southwest-northeast extension where local fault-block uplifts bounded by northwest-trending normal faults created a basin-and-range paleogeography with isolated clastic-filled basins and mountain ranges r mmed by alluvial fans. The marine and marginal-marine shales and limestones of the middle third are restricted to the southeastern part of the region and represent the northwestern end of a shallow marine sea (Bisbee Sea) which advanced northwestward across northeast Mexico from the Jurassic rift basins of the Gulf of Mexico. Shallow-water platform carbonates were deposited over a wide area, with coral-algal-rudist patch reefs localized over paleo-structural highs. The upper part of the Bisbee Group is the regressive facies of this sea and is composed primarily of nonmarine deltaic (fan delta), lacustrine and fluvial sandstones, and siltstones. This facies reconstruction suggests that differential vertical displacements along northwest-trending normal faults controlled the regional variations in thickness, lithology, and grain size in the Bisbee Group. End_of_Article - Last_Page 549------------

The paradox of drowned reefs and carbonate platforms

Shallow-water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation, and benthonic carbonate production ceases. Drowned platforms are common in the geologic record, but they present a paradox if one considers rates of processes involved. During the early Holocene, when sea level rose at rates of 6,000 to 10,000 µm/yr (= mm/1,000 yr), most reefs and platforms were outpaced by the rising sea. During the late Holocene with sea level rising 500 to 3,000 µm/yr in the Atlantic-Caribbean area, reefs and platforms started to recover, built to sea level, and prograded seaward, 1,000 µm/yr is thus a conservative estimate of the average growth potential of modern reefs and platforms. Independently, accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year. The growth potential of 1,000 µm/yr exceeds any relative rise of sea level caused by long-term processes in the geologic record. Newly formed ocean crust subsides at a maximum of 250 µm/yr, basin subsidence averages 10 to 100 µm/yr, and sea level rises due to increased sea-floor spreading amount to less than 10 µm/yr. Rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning. The geologic record shows examples of both of these processes. Global mass extinctions of reefs and platforms occurred in the middle Cretaceous (eustatic rise due to submarine volcanism or desiccation of a small ocean basin?) and the Late Devonian (global crisis of ocean environment, extraterrestrial cause?). Drowning controlled by regional tectonics prevailed in the Jurassic and Early Cretaceous of the Tethyan realm, and the drowning of Mesozoic platforms in the western North Atlantic seems to have been dictated by plate-tectonic drift to higher latitudes.

Geologic History of U.S. Eastern Continental Margin South of Cape Hatteras: ABSTRACT

The continental margin off the southeastern United States contains two major basins, the Blake Plateau Basin off Florida and Georgia and the Carolina Trough off South and North Carolina. The Blake Plateau is a large, equidimensional basin that probably is underlain by relatively thick rift-stage crust. It probably was filled dominantly by shallow-water carbonate platform deposits. Reef and carbonate-bank buildups occurred near its seaward edge and the platform deposits interfingered with continental facies deposits near its landward edge. Reef building was interrupted in Barremian time and ended after an Albian-Aptian pulse. Subsequently, deposition did not keep pace with subsidence, and a deep-water plateau was formed. Onset of Gulf Stream flow across the inner Blake Pla eau during the Paleocene prevented seaward progradation of the continental shelf across the plateau, and major deep-water erosion removed the old continental slope, creating a steep cliff, the Blake Escarpment. The Carolina Trough is a long, narrow basin underlain by a narrow zone of rift-stage crust, much thinner than that beneath the Blake Plateau basin. This thin crust, presumably the result of major stretching of continental crust in the rift stage of ocean opening, floated deep isostatically and formed a salt-depositing basin early in continental-margin history. Later sediment loading caused the salt to flow into a series of slope diapirs, and withdrawal of the salt resulted in major subsidence of the block of sediment overlying the trough; movement occurred along growth faults. End_of_Article - Last_Page 1660------------

Some Questions about the Development of Upper Devonian Carbonate Buildups (Reefs), Western Canada.

ABSTRACT Some of the key difficulties concerning the development of Upper Devonian carbonate buildups ('reefs') in the Alberta Basin are: recognizing facies and setting up members and formations that reflect as closely as possible the primary depositional patterns. establishing local and regional time markers. determining the causes of carbonate buildup initiation and localization. duration and significance of diastems during carbonate buildup development. determining the factors responsible for termination of the carbonate buildups. style of basin sedimentation and filling, including factors controlling the Duvernay-Perdrix anoxic basin. In order to help resolve these problems and arrive at suitable buildup and basin models, information from both the subsurface and Rocky Mountain outcrops must be utilized in conjunction with data and concepts derived from Holocene, Pleistocene and other Phanerozoic reefs. The Swan Hills and Leduc carbonate buildups of central and western Alberta have a similar pattern of development: beginning with a shallow-water carbonate platform often above an unconformity or abrupt shift in facies, followed by biohermal and reef complex growth on local carbonate shoals. This pattern represents a major transgressive sequence starting with a slow rise of sea level, followed by a more rapid rise, and finally a period of sea-level stability or still-stand. Carbonate sedimentation kept pace with sea-level changes and probably exposure was frequent, resulting in more gaps than rock record within individual buildups. The main factor that controlled carbonate buildup location and development appears to be sea-level fluctuations. The major changes in sea level seem to be basin wide and probably are epeirogenic or eustatic in origin. Later the amount of terrigenous and carbonate sediment deposited in the basin, controlled by both rates of supply and sea-level changes, also had a major influence on the location and development of the carbonate buildups as irregular carbonate shelf sequences of limited and irregular distribution extended successively basinward. The dominant north-northeast-south-southwest trends of carbonate buildup and platform margins suggest the presence of linear flexure zones across which slight differential subsidence probably occurred in the underlying crystalline basement. End_Page 315------------------------

Llandoverian and Wenlockian brachiopods from the Canadian Cordillera

Llandoverian and Wenlockian age brachiopods are described from five localities in the southern and northern Canadian Cordillera. The majority of brachiopods were collected at, or close to, the shallow water platform carbonate – deep water graptolitic shale and limestone facies transition. Most brachiopods are members of moderate to deep water brachiopod communities referred to the Dicaelosia–Skenidioides Community. Genera which are common in the fauna include Skenidioides, Dicaelosia, the smooth atrypid Cryptatrypa, and rhynchonellids Ferganella and Rhynchotreta, and at one locality only, pentamerids.Newly described species consist of Ancillotoechia pentaforma, Cryptatrypa glaberconcha, C. rotunda, Epitomyonia sekwiensis, E. tripseptata, and Ferganella chattertoni.

Clastic Accumulation of Sulphate Evaporites in Deep Water

ABSTRACT The European Zechstein is a Late Permian group of formations that consists of shale, limestone and dolomite, anhydrite, rock salt, and potash salts in cyclic alternation over 1,000 m thick. Some of the anhydrite formations contain (a) beds with a distinct fining-upward texture, sharp bases and gradational tops, here interpreted as turbidites; (b) other beds with tightly packed clasts of anhydrite up to one meter in size, here interpreted as mass-flow breccias, and (c) bedded anhydrite with folds and concave sliding planes which may represent slumps. The thickness variation of the sulphate body along with the facies pattern in the overlying and underlying carbonates suggests that the sulphates were piled up in a platform similar to a shallow-water carbonate platform along the basin margin. The sediments described in this paper were all found along the steep basinward slope of this platform. Depositional structures similar to those in the Zechstein have been described in the Messinian gypsum formation (Miocene, Italy) Here the evaporites were deposited on pre-existing topography and did not create the relief by their own accumulation as in the Zechstein. Both the Zechstein and the Messinian sulphate bodies, however, were shaped by a similar interplay of localized precipitation in shallow water (or adjacent sabkhas), reworking and downslope transport as clastic material, and not by in situ precipitation alone. During the evaporite stages sulphates accumulated in all parts of the basin from the marginal tidal flats to the basin interior.

Late Ordovician-Early Silurian glaciation and the Ordovician-Silurian boundary in the northern Canadian Cordillera

The growth and subsequent melting of the latest Ordovician-earliest Silurian glaciers in north and south Africa were responsible for a global transgression and regression. The shallow-water platform carbonates of the Cordilleran and Arctic regions of North America encompass a stratigraphic break apparently equal in duration to the glaciation. The transgression due to melting of the ice sheets was probably rapid. The break in sedimentation is without discordance and is recognized by subtle but abrupt lithologic changes and particularly by rocks with middle or late Llandoverian fossils resting directly on those with Late Ordovician fossils. Deep-water graptolitic shales, however, show a complete or nearly complete sedimentary sequence across the boundary.

Reefs, Platforms, and Basins of Middle Cretaceous in Northeast Mexico

The late Mesozoic depositional cycle in eastern Mexico began with redbed deposition in the Jurassic, included deep- and shallow-water marine carbonates (Late Jurassic to Late Cretaceous), and concluded with terrigenous detritus from the deformation of the Sierra Madre Oriental (Eocene). Rudist-fringed, shallow-water carbonate platforms attained their maximum areal extent and facies contrast with the surrounding carbonate basins in the Albian-Cenomanian. The resultant lithofacies are (1) a suite of platform carbonates; (2) dark, cherty, pelagic, basinal limestones; and (3) basin-margin carbonates consisting of platform-derived breccias and bioclastic rocks interlayered with pelagic limestones. Scale and relief of the Mexican platforms compare with those of the Capitan reef and the Bahama Banks. Steep-sided, high-relief platforms provide optimum conditions for basin-margin debris accumulations, but the necessary and sufficient combination of conditions for debris accumulation and for platform development is unsolved.