Shelf Cycles Research Articles

Deepening-Upward Subtidal Cycles, Murray Basin, South Australia

ABSTRACT Decimeter-scale, deepening-upward cycles are the basic depositional motif of Oligocene-Miocene Murray Supergroup limestones in the Murray Basin, southeastern Australia. Cycles formed in this large intracratonic basin on a centripetal, temperate-water epeiric ramp. They reflect generation by trophic resource-influenced carbonate production under mostly transgressive conditions. Listed from the base up, each cycle is ideally composed of five parts: Part A--biotically depauperate carbonates reflecting relatively shallow-water, restricted, variably stressed highly mesotrophic environments; Part B--increasingly biotically diverse limestones recording progressively more physical energy and less mesotrophic conditions upward; OM1--a conspicuous hardground to firmground surface formed during late transgression to stillstand during which wave sweeping and reworking contributed to omission and lithification; Part C--relatively biotically diverse, epifauna-dominant sediments that were highly abraded during periods of condensed sedimentation under marginally oligotrophic conditions; and OM2 (cycle boundary)--a rarely conspicuous surface representing arrested sedimentation and variable cementation as trophic resources increased and conditions for carbonate production deteriorated. Ten cycle types are grouped into four major styles; clay cycles, mollusc cycles, echinoid cycles, and bryozoan cycles. These are interpreted to form a lithological continuum from inner restricted terrigenous locales to outer, open marine, bryozoan-colonized environments. Deepening-upward cycles were profoundly sensitive to autogenic factors such as nutrient influx, terrigenous content and turbidity, hydrodynamic energy, water temperature, salinity, and water depth manifested in this intracratonic setting. Although these cycles share a similar sedimentological motif and hydrodynamic energy-based cycle-capping process with open shelf, epicratonic, shallowing-upward subtidal cycles of Tertiary age, they formed in a very different way reflecting the distinctive intracratonic environment in which they formed. Murray Basin cycles preserve a record of relative sea-level rise whereas open shelf cycles accumulated mostly during relative sea-level fall.

Lithofacies and sedimentary cycles within the Late Dinantian (late Brigantian) of Fife and East Lothian: is a sequence stratigraphical approach valid?

ABSTRACTThe late Brigantian topmost parts of the Pathhead Formation (Aberlady Formation in East Lothian) and the succeeding Lower Limestone Formation crop out widely in Fife and East Lothian. The successions include nine deltaic, coastal floodplain and marine shelf cycles (cyclothems), of which the lowest examined terminates the Pathhead and Aberlady Formations and the remaining eight constitute the Lower Limestone Formation.The cyclothems conform broadly to the ‘Yoredale” transgressive/regressive pattern in which a transgressive marine shelf phase is succeeded by delta progradation and terminates with a fluvial delta plain phase. Cycles may combine to form compound cyclothems up to more than 50 m thick, in which a basal, typically complete initial cycle of Yoredale pattern is succeeded by up to five base-absent minor cycles. These are thinner, more variable and less laterally persistent units in which the marine phase is weakly represented or absent.Cyclothems reflect successive marine flooding events, possibly under eustatic control, succeeded by delta progradation and, ultimately, leading to extensive palaeosol formation, including coal seams. Sedimentation and palaeosol formation were partly controlled by fault-induced differential subsidence and are likely to have been related to autocyclic processes. Local uplift and subsidence associated with vulcanicity, as at Kinghorn and Elie, have led to thickening or thinning of sediments accumulated in a given time period.Initial cycles initiate longer-period allocycles, corresponding broadly to third-order Exxon Production & Research (EPR) Type 1 sequences having a periodicity of around 1 Ma, within the Milankovitch orbital band. Two parasequences constitute each initial cycle: a lower, initiated on a marine flooding surface, and an upper, bounded by the base of the lowest thick sandstone in the cycle; cyclothem bases and sequence bases thus alternate. Parasequences and sequences are less well defined in minor cycles due to the problem of tracing the combined disconformity and soil profile of the underclay beyond the edge of channel sandstones. Minor cycles were controlled primarily by short-period autocyclic sedimentary and, or, tectonic processes, including delta-lobe switching and differential subsidence.Although we have attempted to interpret the deposits of Fife and the Lothians in terms of sequence stratigraphy, we are not fully convinced that the patterns of associated changes widely recognised within the framework of sequence stratigraphy can be confidently applied in succesions in which autocyclic changes feature strongly in an area undergoing active basin subsidence associated with strike-slip faulting. There is no doubt that some of the cyclicity discerned in the late Brigantian successions of eastern Scotland was related to eustatic sea level changes, which gave rise to the widespread limestone platforms or marine bands. The formation of eight cyclothems within the 2·5–3·5 Ma of late Brigantian suggests a cyclicity of about 400 ka, which corresponds to the long period eccentricity cycles of Milankovitch rather than the 0·5–5·0 Ma of third-order EPR cycles.

Upper Carboniferous cyclic shelf deposits, Kapp K�re Formation, Bj�rn�ya, Svalbard: response to high frequency, high amplitude sea level fluctuations and local tectonism

Upper Carboniferous cyclic shelf deposits, Kapp K�re Formation, Bj�rn�ya, Svalbard: response to high frequency, high amplitude sea level fluctuations and local tectonism

Upper Carboniferous cyclic shelf deposits, Kapp Kåre Formation, Bjørnøya, Svalbard: response to high frequency, high amplitude sea level fluctuations and local tectonism

The upper Bashkirian-Moscovian Kapp KIre Formation is well-exposed in coastal cliff sections along the west coast of Bjørnøya, Svalbard. It is composed of stacked cycles of nixed siliciclastics and carbonates in the lower Bogevika Member and of cyclic shelf carbonates in the overlying Efuglvika Member. The uppermost Kobbebukta Member consists of shelf carbonates and syntectonic conglomerates and sandy turbidites. The shift in cycle types reflects an overall transgression of the region during the Moscovian combined with renewed tectonic activity and uplift of eastern Bjørnøya during the late Moscovian. Twelve carbonate facies and 6 siliciclastic facies are distinguished. The carbonate facies range from intertidal dolomitic mudstones with pseudomorphs after gypsum to subwavebase, intensely bioturbated wackestones. Most carbonates are deeper subtidal facies and shallow marine carbonate facies are only common in the transgressive part of mixed siliciclastic-carbonate cycles of the Bogevika Member. Incorporating the effects of high amplitude, high frequency glacioeustacy and active extensional tectonism, a dynamic model is developed to explain the spatial variability of facies observed within the Kapp Kke Formation. Observations from Bjørnøya are placed within the context of the regional structural and stratigraphic framework so that significance of the study to ongoing exploration efforts in the Barents Sea can be evaluated. Most important, our observations suggest that dolomitized, porous carbonate buildups are most likely to be found in the upper Moscovian succession in areas where accommodation space increased temporarily due to local tectonism.

Shelfbreak gullies; products of sea-level lowstand and sediment failure; examples from Bowser Basin, northern British Columbia

ABSTRACT The transition from cyclothemic shelf to shale-dominated slope deposits (Callovian to Oxfordian) that accumulated along the northern convergent margin of Bowser Basin is delineated by spectacular, conglomerate-filled channels, or gullies. The gullies, representing the shelf-slope break, formed in two stages: (1) Slumping of upper slope-outer shelf sediment created topography, which determined (2) the course of fluvial, distributary-type channels and focused gravel-dominated sediment transport during relative sea-level lowstands. Gully fill consists predominantly of debris-flow deposits, locally incorporated into small (lowstand) fan deltas that prograded from gully margins. Lowstand fluvial channels overlie shelf cycles, incise the underlying highstand deposits, and are overlain by fossiliferous, transgressive sandstone. However, the shelfbreak gullies deposits are up to 10 times thicker than those of the associated lowstand fluvial channels. Although the lowstand fluvial channels likely acted as bypass conduits for gravel and sand during times of low sea level, the initiation and overdeepening of the shelfbreak gullies by slumping probably took place during both high and low relative sea levels. Deposition within gullies may also have continued during relative highstands, from sediment in storage on the outer shelf. The implications for sequence stratigraphic models are: shelf-break gullies can form at any stage of sea level fluctuation; gully thickness may not be related to the magnitude of lowstand incision on the adjacent shelf; and gullies formed by sediment gravity failure do not provide quantitative information about the magnitude of sea-level fall.

PROVENIÊNCIA E IDADES MODELO Sm/Nd DAS ROCHAS SILICICLASTICAS ARQUEANAS DOS GREENSTONE BELTS DE FAINA E SANTA RITA, GOIÁS

The Faina and Santa Rita greenstone belts are two N60°W trending synclinoria separated by a N30°E strike-slip fault and rest allochtonous on the adjacent Uva and Caicara granite-gneiss complexes. The belts are made up of lower metakomatiites, followed by metabasalts and thick metasedimentary sequences deposited under contrasting paleogeographic settings. In Faina, the sequence consists of two complete shelf cycles, the first resting on basalts by an erosional uncoformity. In Santa Rita, the basalts give gradually place to carbonaceous metashales, unconformably overlain by metarhythmites. Provenance based on trace element geochemistry, mineral chemistry of chlorite and muscovite, source-area modeling and REE elements indicate that protoliths of the first shelf cycle of Faina and the carbonaceous metashales of Santa Rita formed under the influence of a source area dominated by mafic and ultramafic rocks, whilst during the sedimentation of the second shelf cycle and the metarhythmites of Santa Rita the source-area was dominated by TTG granitoids. Sm-Nd model ages of the lower sedimentary packages vary between 3.0 and 2.8 Ga and the upper between 2.7 and 2.6 Ga. These intervals coincide, respectively, with the estimated age of the underlying komatiites, and with the Sm-Nd, Rb-Sr e U-Pb ages of the main granitoids and gneisses of the adjacent complexes. The same data indicate that, during the sedimentation of the first shelf cycle of Faina and the carbonaceous metashales of Santa Rita, both basins had independem source-areas, but shared the same source during the upper sections. Weathering nature and intensity of the source-area calculated by means of geochemical data suggest that the dramatic change of provenance from the lower to the upper sections is due to the shift from tectonically stable to unstable regimes, interpreted as resulting from the emergence of the island are whose roots are represented by the adjacent granite-gneiss complexes

Streptognathodus (Conodonta) succession at the proposed Carboniferous-Permian boundary stratotype section, Aidaralash Creek, Northern Kazakhstan

Streptognathodus Pa elements dominate conodont faunas from the proposed Carboniferous-Permian boundary stratotype at Aidaralash Creek, northern Kazakhstan. The phyletic development of this genus provides the means for subdividing the late Gzhelian to early Sakmarian prodeltaic section (Beds 9-37) into six zones. The preferred Global Stratotype Section and Point (GSSP) for the base of the Permian System is 27 m above the base of Bed 19 at the first occurrence of Streptognathodus isolatus Chernykh, Ritter, and Wardlaw, 1996. This arbitrarily chosen point in the evolutionary continuum of nodose streptognathodids is recognizable in basinal rocks elsewhere in the Ural trough (lower part of Bed 16 at Usolka) as well as the cyclic shelf succession of the American Midcontinent (Glenrock Limestone Member of the Red Eagle Limestone).Eight new species of Streptognathodus occur in the Aidaralash collections: S. bellus n. sp., S. costaeflabellus n. sp., S. flexuosus n. sp., S. glenisteri n. sp., S. longilatus n. sp., S. rectangularis n. sp., S. sigmoidalis n. sp., and S. tenuialveus n. sp. In addition, three morphotypes with limited occurrence (S. sp. A-C) are left in open nomenclature. New collections of topotype material from the nearby Tabantal River section permit clarification of S. barskovi (Kozur).

Lower Cisco Formation (Pennsylvanian- Virgilian) Paleokarst, Wolf Flat Field, Palo Duro Basin, Texas: ABSTRACT

Wolf Flat field produces from karstified Lower Cisco Formation at the rimmed northeast shelf margin. Mounds, peripheral skeletal debris, and oolitic facies form the vertical/lateral succession. Mounds are composed of crinoids, bryozoans, corals and algae. Skeletal packstone/grainstones surround mounds, With back-mound wackestone. Ooid grainstone formed in high-energy passages onto the shelf and cap shallowing-upward cycles. Karstification resulted from two 100+ ft. (30+ m) sea level drops, that dropped in a step-by-step manner to form [approximately]20 porous intervals that extend through all facies and lithologies in the field. Aragonitic mound and skeletal grainstone/wackestones experienced intense dissolution. Calcitic ooid grainstones dissolved less and form cave roofs and floors. Dissolution was in two phases: first, small vertically oriented pipes formed, resembling a [open quotes]swiss cheese[close quotes] texture; second, was intense lateral dissolution that created a [open quotes]sponge[close quotes] texture, with porosities up to 60-70%. Eventually, collapse breccias formed, with some clasts transported in the karst system. Porosity was reduced by vadose silt, breccia clasts, speleothem cements, transgressive shale infilling, and burial saddle dolomite. Portions of the field nearest the rimmed margin were dolomitized. [delta][sup 18]O is within the range of early meteoric diagenesis and [delta][sup 13]C is negative and becomes more negative towardmore » the exposure surface.« less

Depositional Cycles as a Shelf-to-Basin correlation Tool, Capitan Reef Margin - Ideas from Stratigraphic Computer Modeling: ABSTRACT

Correlations from the Yates shelf through the Capitan reef margin and into the Bell Canyon of the deep Delaware Basin (Guadalupian, Permian Basin) are not known with great detail due to limited biostratigraphic control and the inability to trace beds or time lines from the cyclic shelf deposits, through the massive reef and foreslope, and into basinal siliciclastics. The presence of a strong hierarchy of depositional cycles on the shelf and in the basin suggests that cycles may be useful as a correlation tool. A Yates shelf-to-basin correlation is proposed based on matching 3rd- and 4th-order cycles recognized with a stratigraphic computer model and on cores and outcrop. We believe thick, sand-dominated cycles were deposited in the basin during long-term lowstands; whereas, thin, organic-rich lime mudstone-dominated cycles were deposited during long-term highstands. The correlation scheme hinges on equating 3rd-order condensed intervals of organic mudstone in the basin with thick carbonate {open_quote}banks{close_quote} deposited ad the lower and upper Yates boundaries. Within this tentative large-scale framework, there is a reasonably good match between 4th-order shelf and basin cycles. Some ambiguity remains, however, due to the possibility of missed cycle beats in both the shelf (thin or no cycles, erosion) and basin (nomore » or minor condensed mudstones resulting in sand on sand, erosion) during long-term lowstands. Computer modeling suggests siliciclastics bypassed the Yates shelf and were delivered to the basin as turbidites during high-frequency (5th-order) lowstands, but the amount of bypass during any lowstand was controlled by longer-term cycles. The large-scale basinal cycles are slightly more symmetrical than their shelf counterparts; typically showing a gradual increase in clean sandstone and decrease in siltstone and organic mudstone away from a condensed interval, and the converse towards the next condensed interval.« less

Upper Carboniferous cyclic shelf sedimentation, North Greenland and the Barents Sea: ABSTRACT

Upper Carboniferous cyclic shelf sedimentation, North Greenland and the Barents Sea: ABSTRACT

The interactions of sea-level change, terrigenous-sediment influx, and carbonate productivity as controls on Upper Cambrian Grand Cycles of western Newfoundland, Canada

The Upper Cambrian stratal record in western Newfoundland is complex and reflects the interplay and feedback between (1) changes in third-order sea level, (2) episodic incursion of terrigenous fines onto the paleoshelf, and (3) productivity levels of the ancient carbonate factory. Mixed carbonate and terrigenous clastic rocks of western Newfoundland are configured in large-scale, third-order stratal patterns called (such cycles are tens to hundreds of meters in stratigraphic thickness and appear to span millions of years). Grand Cycles are divided here into ribbon half-cycles (composed principally of ribbon rock) and oolitic half-cycles (dominated by oolite). Stratigraphic distribution of shale in these rocks, however, is not everywhere coincident with large- and small-scale patterns in carbonate lithofacies, and it suggests that the incursion of terrigenous clay and silt onto the ancient platform must be evaluated separately, in part, from the dynamics of carbonate accumulation. Areal and stratigraphic distribution of rock cycles record shifts in facies belts on the ancient platform, presumably in response to third-order changes in accommodation potential. Meter-scale cycles in carbonate lithofacies are of two types: peritidal cycles that fine upward, and shelf cycles that coarsen upward. Thick successions of shelf cycles composed of ribbon rock are interpreted as highstand deposits and are separated (by a sequence boundary marked by peritidal cycles) from lowstand deposits comprising thick successions of ootite shelf cycles. The former mid-Grand Cycle transitions of previous workers are thereby reinterpreted in this study as sequence boundaries. The long-term position of sea level, and thus the degree of platform inundation, influenced the health of the platform carbonate factory. During times of low sea level, the platform was bathed in water more saline than open-ocean sea water, and it had a robust non- skeletal carbonate factory that was resilient to poisoning by episodic influx of terrigenous fines onto the shelf. In contrast, the carbonate factory during highstands was less resilient to terrigenous poisoning and was susceptible to localized drowning by the reduction in the rate of carbonate-sediment production. The thick shale units are therefore interpreted to represent the coincidence of (and interaction between) (1) incursion of terrigenous fines onto the ancient platform and (2) a weakened highstand carbonate factory unable to recover from this terrigenous poisoning.

3D Seismic lnterpretation in Jarn Yaphour Field, Abu Dhabi

Summary A major 3D seismic project covering about 425 sq miles project coveringabout 425 sq miles [1100 kM] was conducted at Jarn Yaphour to define thestructural geometry, particularly faulting, better and to resolve stratigraphiccomplexities associated with the field's principal reservoirs. Datainterpretation included detailed geoseismic mapping of nine horizons andseismostratigraphic analysis of three intervals of interest. This paperdiscusses the interpretation techniques applied to the 3D seismic data, usingboth conventional and interactive methods, and the results achieved. Theproject, which has revealed extensive details about the structural andstratigraphic aspects of the Jarn Yaphour field, derived a reservoir-depthmodel for the Thamama Zone B, the main hydrocarbon-bearing reservoir. Thismodel and the seismostratigraphic anomalies identified suggested new areas toexplore. Introduction The Jam Yaphour field is a tectonically complicated, extensively faulted, northwest/southeast-trending, asymmetric, anticlinal structure in the easternpart of Abu Dhabi's coast (Fig. 1). Of the 10 wells drilled in the field, thelast two were based on the 3D seismic survey and proved the use of thetechnique. Fig. 2 illustrates the stratigraphic succession in the area and the nineinterfaces selected for geoseismic mapping. The stratigraphic column iscomposed mainly of cyclic shelf carbonate/evaporite sequences with minorclastic sedimentation phases of shales and sandstones. Seals of differentcapacities are seen throughout the succession, which comprises lime mudstones, shales, and anhydrites. The main reservoir in the field is the Thamama Zone B (Early CretaceousKharaib formation), which is oil- and gas-bearing. Secondary-orderreservoirs-the Thamama Zone A and the locally developed algal-coral facies ofthe Bab member-are found in the overlying Shuaiba formation. Before the 3D seismic program, the Thamama Zone B showed pronouncedvariations in hydrocarbon/water contacts in wells reaching 183 ft [56 m]. Thissituation hindered implementation of an adequate field development program andwas the main reason for the 3D geoseismic project. The project had many objectives. The first was to define the structuralgeometry of the field. The second was to highlight stratigraphic featuresassociated with the algal-coral facies developed at the top of the EarlyCretaceous Thamama group and the potential shelf carbonates at the top of thepotential shelf carbonates at the top of the Middle Cretaceous Wasia group. Thethird goal was to derive a reservoir-depth model for the Thamama Zone B and toshow its impact on hydrocarbon entrapment. The last objective was to explainearlier well results and to assess future exploration/delineation plans. plans.3D High Resolution To secure optimum temporal and lateral resolved images, field parameters andprocessing package were carefully selected. processing package were carefullyselected. Subsurface coverage in 41 × 82-ft [ 12.5 × 25-m] units improvedimaging by virtue of the two component fields in migration. One-and two-pass 3Dmigration techniques optimized lateral resolution. High vertical resolution wasattained through a comprehensive deconvolution testing program. A predictivedeconvolution approach and an ancillary frequency deconvolution technique wereapplied separately to satisfy the structural and seismostratigraphic analysisrequirements. In the interpretation phase, the entire 3D seismic data set was loaded ontoan interactive workstation to allow analysis of each in-line, cross-line, andtime slices every 4 milliseconds, the sample rate of the data. The wide rangeof the 3D products and the use of the interactive system resulted in a networkof 16 random traverses along Tracks A through 0 and In-Line 4535 (Fig. 3). These tracks were selected to cross through important structural/stratigraphicfeatures, to provide additional ties, and to obtain better definitions ofsubtle fault patterns and main fault zones. The random patterns and main faultzones. The random traverses were produced in the form of regular verticalseismic sections, Hilbert-attribute displays, and seismic lithologicmodels. Interpretation Techniques and Discussion In addition to the large number of seismic data sets produced by the 3Dsurvey, geological information, wireline logs, and well-test data from ninewells (Wells JY-1 through JY-7, HW-1, and AT-1) were made available. Time and Depth Structural Maps. Synthetic Seismogram Ties. Borehole acousticdata in the form of check shots and sonic and density logs were available forall nine wells except Well HW-1, which lacked check shots. Syntheticseismograms were subsequently generated for each well to performseismogeological calibration, with perform seismogeological calibration, withgood matches being established by wavelet extraction studies.

Field and Modelling Studies of Cambrian Carbonate Cycles, Virginia Appalachians

ABSTRACT Field studies coupled with computer modelling of carbonate cycles in the Elbrook-Conococheague Formations (Middle to Upper Cambrian) in the Appalachians have helped define the mechanisms of formation. The sequence is up to 1.6 km thick and composed of cyclic, peritidal carbonates. These formed on an aggraded, rimmed shelf on a mature, passive continental margin. The cycles (1-7 m thick) have lower parts of subtidal/intertidal carbonate and caps of dolomitic laminite containing minor quartz arenite, shale and breccia. Sedimentation rates for these peritidal carbonate environments exceeded long-term subsidence of the platform (0.01 to 0.1 m/1,000 yr), thus, the shelf stayed near to sea level (i.e., was aggraded). Milankovitch (20,000 to 100,000 yr), low-amplitude sea-level fluctuations (few meters) superimposed on 3rd-order sea-level fluctuation of 1 to 3 m.y. duration and less than 20 m amplitude caused periodic submergence of the gently sloping platform (less than 0.02 m/km slopes), resulting in deposition of over 400 peritidal cycles in about 26 m.y. Cycles that formed during 3rd-order sea-level rise are thick and limestone-rich, whereas those formed during 3rd-order fall are thin, quartzose, and highly dolomitized. The modelling suggests that fewer and thinner cycles formed during the long-term falls compared to the rises, reflecting decreased accommodation. Cycle-frequency decreases into areas of lower subsidence rate. Open-marine cycles typical of the outer shelf and cycles formed during 3rd-order sea-level rise contain digitate bioherms, grainstone, and ribbon carbonates in lower parts. They reflect high subsidence rate and an open-marine setting. Inner shelf cycles and cycles that formed during 3rd-order sea-level fall commonly are thrombolitic, or have thin subtidal parts (grainstone, conglomerate or stromatolite) and reflec the low subsidence rates. Interior areas of Quaternary carbonate shelves are characterized by incipiently drowned facies and pinnacle reefs and are punctuated by soil/caliche horizons and karst surface that reflect 100 m glacio-eustatic sea-level fluctuations. In contrast, many ancient shelves, like the Cambro-Ordovician shelf of the Appalachians, were flat-topped and dominated by cyclic peritidal sequences lacking evidence of high amplitude sea-level fluctuations. Such aggraded shelves may represent the typical state of mature carbonate shelves that formed in response to low-amplitude Milankovitch sea-level fluctuations as opposed to the large-scale fluctuations that typified the Quaternary.

Application of Sequence Stratigraphic Analysis to Thin Cratonic Carbonate-Dominated Shelf Cycles (Upper Pennsylvanian) in Mid-Continent: ABSTRACT

Seismic sequence analysis has proven extremely effective and practical in identifying potential petroleum reservoir plays. This analysis requires defining stratal units, 10s of meters in thickness, bounded by unconformities and translated on seismic traces. A similar approach, using different techniques, is applied to subsurface data to improve the resolution of the sedimentary architecture in thin Upper Pennsylvanian cratonic cycles. These cycles reflect the relative rise and fall of sea level, aspects of which vary between cycles in terms of rate, magnitude, and duration. Understanding aspects of the fall of relative sea level and accordant sediment aggradation and progradation is important in order to recognize the character of the basinward migration of favorable shallow-water carbonate facies tracts. Because of their short duration, individual aggradational and progradational events cannot be resolved via biostratigraphy. Correlatable events, bounded by omission surfaces, can be recognized through detailed lithofacies analysis of cores. Wireline logs provide a basis from which to extrapolate thicker (>0.7 m) core-derived lithofacies. Very high resolution CMP seismic profiling (less than or equal to300-Hz) has been used successfully at penetration depths to about 450 m to distinguish and characterize reflections from beds (1.5 m minimum) within cycles and internal depositional surfaces within thickermore » deposits. Potential exists for using detailed seismic sequence analysis to assist in defining geometry of these component beds at reservoir scale.« less

Storm-Dominated Cyclic Shelf Deposits in Upper Ordovician of Cincinnati Arch Region: ABSTRACT

The Kope and Fairview Formations and the Bellevue Tongue of the Grant Lake Limestone (Upper Ordovician, Cincinnatian Series) exposed in the Cincinnati, Ohio, region represent storm-dominated regressive sedimentation on an intracratonic ramp or shelf. The Kope-Fairview transition zone (25 m) is an outer shelf lithofacies composed of numerous small-scale (1-3 m), shallowing-upward cycles. These cycles consist of two facies: (1) a shale-dominated component characterized by numerous, single-event distal tempestites, and (2) a fossiliferous limestone component characterized by amalgamated, proximal tempestites. Distal storm components contain abundant thin-bedded, parautochthonous packstones, rare autochthonous fossil accumulations, and abundant thin-bedded siltstones that commonly contain tool marks, gutter casts, parallel lamination, small-scale hummocky-cross-stratification, hummocky bed forms, mud-filled scours, and millimeter ripples. Proximal tempestites are thick-bedded, fine to coarse-grained packstones and grainstones that commonly have megarippled surfaces, cross-lamination, and shale lithoclasts, and contain a mixture of fossils showing degrees of abrasion and transport. Lower Fairview cycles are proximal equivalents of underlying Kope cycles, and typically show more scour features, bedding amalgamation, and proximal components. Cycles are laterally continuous across the study area (25 x 15 mi) and exhibit only minor variation. Cycle boundaries appear isochronous and mark the abrupt transition from proximal to distal storm sedimentation. Themore » widespread character and isochronous relationship suggest that these cycles are the product of an allocyclic mechanism involving episodic, but rapid, rises in relative sea level. Deposition of cyclic sequences occurred on a very gently dipping, northward-facing shelf.« less

Variations in Carbonate Shelf Cycles in Response to Appalachian Tectonism: ABSTRACT

Variations in Carbonate Shelf Cycles in Response to Appalachian Tectonism: ABSTRACT

Petrology of Upper Mississippian Denmar Formation, Greenbrier Group, East-Central West Virginia: ABSTRACT

The Denmar Formation of Randolph County is a package of interbedded marine carbonates and siliciclastics deposited on a broad subsiding carbonate platform. Seven major microfacies are recognized: terrigenous siltstone, dolomitized micrite, pelmicrite, biopelmicrite, quartzose pelmicrite, oosparite, and oomicrite. These microfacies characterize three major paleoenvironments: supratidal, intertidal, and shallow subtidal. Four major diagenetic realms are noted: the low supratidal flat containing evaporites and dolomite; the meteoric zone containing blocky calcite spar; the shallow marine phreatic zone with marine cement and micritized fossil fragments; and the burial zone containing mature stylolites. The Denmar Formation represents a sequence of alternating low-energy, shallowing-upward pelmicrite-mudstone and higher energy, shallowing-upward oosparite-grainstone shelf cycles. The pelmicrite-mudstone cycles are mainly composed of intertidal and supratidal sediment. The oosparite-grainstone cycles are mainly composed of low and higher energy subtidal sediments. Diagenesis occurred in both the marine and meteoric zones. Paleoenvironmental reconstruction of east-central West Virginia during deposition of the Denmar is attempted. End_of_Article - Last_Page 1449------------

An Incipiently Drowned Platform Deposit in Cyclic Ordovician Shelf Sequence: Lower Ordovician Chepultepec Formation, Virginia: ABSTRACT

The Chepultepec interval, 145 to 260 m (476 to 853 ft) thick, in Virginia contains the Lower Member up to 150 m (492 ft) thick, and the Upper Member, up to 85 m (279 ft) thick, of peritidal cyclic limestone and dolomite, and a Middle Member, up to 110 m (360 ft) thick, of subtidal limestone and bioherms, passing northwestward into cyclic facies. Calculated long term subsidence rates were 4 to 5 cm/1,000 yr (mature passive margin rates), shelf gradients were 6 cm/km, and average duration of cycles was 140,00 years. Peritidal cyclic sequences are upward shallowing sequences of pellet-skeletal limestone, thrombolites, rippled calcisiltites and intraclast grainstone, and laminite caps. They formed by rapid transgression with apparent submergence increments averaging approximately 2 m (6.5 ft) in Lower Member and 3.5 m (11.4 ft), Upper Member. These submergence increments may have resulted from small (about 1 m; 3.2 ft) relative sea level rise, coupled with subsidence due to loading by water and accumulating sediment. Progradation of tidal flat facies did not occur until subsidence had ceased. Tidal flats were shifted westward from 70 to 380 km (43 to 236 mi) after each submergence event, but in peritidal sequences, had sufficient time to prograde back into the shelf. Deposition during Middle Member time was dominated by skeletal limestone-mudstone, calcisiltite with storm generated fining-upward sequences, and burrow-mixed units that were formed near fair-weather wave base, along with thrombolite bioherms (subwave base to wave agitated shallow water settings). Locally, there are upward shallowing sequences, (subtidal cycles) of basal wackestone/mudstone to calcisiltite to bioherm complexes (locally with erosional scalloped tops). Apparent submergence increments during Middle Member deposition averaged 7.0 m (23 ft), ranging up to 23 m (75 ft) in southeastern belts. Following each submergence, carbonate sedimentation was able to build to sea level prior to renewed submergence. Large submergence End_Page 428------------------------------ events caused tidal flats to be shifted far to the west, and they were unable to prograde out onto the open shelf because of insufficient time before subsidence was renewed, and because the open shelf setting inhibited tidal flat deposition. The Middle Member represents an incipiently drowned sequence that developed by repeated submergence events. Such incipiently drowned shelf sequences are common but poorly documented in the geologic record. They punctuate many aggraded cyclic shelf sequences and may ultimately provide important information for construction of relative sea level curves for Paleozoic. End_of_Article - Last_Page 429------------

Carbonate-Dominated Shelf Cycles in Late Pennsylvanian of Mid-Continent: Intrabasinal and Extrabasinal Controls on Sedimentation and Early Diagenesis: ABSTRACT

The Upper Pennsylvanian (Missourian) Lansing-Kansas City Groups in the subsurface of western Kansas consist of a dozen cyclothems of carbonate and terrigenous clastics deposited on a platform that was gently tilted southward toward the more rapidly subsiding Anadarko basin. Maps of the study area covering western Kansas, derived from several thousand well logs and over 30 cores, describe four cyclothems in the Kansas City Group. Regressive carbonates developed in each of these cyclothems are major petroleum reservoirs in this region and are the focus of this examination. The regressive carbonates thicken southward (basinward) at rates controlled by the tilt of the platform. Local and subregional variations in thickness and facies distribution are affected by local differential subsidence on the shelf, particularly along broad positive areas that closely correspond with previously active uplifts. Relatively thin carbonates having restricted shallow-marine facies are abundant over these positive areas. Subtle flexures along the shelf, especially where the slope increased basinward, were loci for ooid shoal development caused by wave and current action during shallow-water deposition. Facies patterns interpreted from core and log-derived mapping demonstrate that neither the flexures nor the broader positive areas of the shelf were consistently active throughout the deposition of all four cycles studied. Hence, despite remarkable similarities, there are distinct variations between the cycles. Furthermore, not all cycles cover the study area to the same extent because (1) in some cycles, regressive carbonates pinch out along the northern (landward) shelf; (2) shallow restricted marine facies can be displaced southward; (3) the marine black shale, the deepest water phase of the cycle, can be missing or only poorly developed; and (4) intense local variations can occur in the early meteoric freshwater diagenesis that affects all cycles over much of the shelf. Terrigenous clastics in the cycles are composed of thin layers of silty shale and claystone that prograded southward over much of the northern half of Kansas during the regressive phase of each cycle. Clastics from the Ouachitas, important components in cycles in eastern Oklahoma and southeast Kansas, never reached western Kansas until the Virgilian because these sediments were trapped by a relatively deep basin in western and central Oklahoma. Extensive and prolonged subaerial weathering and associated freshwater diagenesis, marked and sharp vertical changes in the lithofacies, and broad repetitive facies patterns in relatively thin carbonates and shales can be explained by glacial eustatic changes in sea level. The Gondwana glacially influenced eustatic sea level changes had a periodicity and magnitude comparable to those of the Pleistocene. Variations in thickness, carbonate facies patterns, and diagenesis were also strongly influenced by second-order intrabasinal processes, including differential subsidence over positive areas and along breaks in slope. End_of_Article - Last_Page 567------------

Significance of Carbonate-Clastic Shoaling Cycles in Mississippian of Northern Arkansas: ABSTRACT

Shoaling-upward carbonate shelf cycles are known to occur in many parts of the world and throughout the geologic record. A typical shoaling carbonate cycle begins with deep-shelf, interbedded, lime mudstones and shale. These are succeeded by open-shelf bioclastic packstones and grainstones, and finally in many places by oolite, the latter representing the culmination of the shoaling process. Subsequent cycles repeat this basic pattern. When correlated across a shelf, the cycles are thought to represent the episodic progradation of a carbonate sand platform that built up to wave base, subsided, and built up again. Upper Mississippian (Chesterian) strata in northern Arkansas consist of alternating limestones and shales that include a set of shoaling-upward cycles. These cycles, occurring within the dominantly carbonate Pitkin Formation, differ from the pattern described above in that they contain a significant terrigenous component. This material exists both as fissile black shale within lower-cycle lime mudstone, and as quartz sand admixed within upper-cycle oolite grainstones. This dual occurrence is thought to be related to movement of the shoreline that accompanied the repeated outbuilding of a carbonate platform. At the beginning of a cycle, the shoreline was distinct from the deeply submerged shelf and lime and terrigenous mud accumulated in a quiet-water setting. Thickness and facies anal sis of the Pitkin indicate an east-west trending shelf with a shoreline located in southern Missouri, perhaps curving south around the Ozark uplift. Eastward increase in shale percentage in the Pitkin suggests transport of fine clastics around the southeast flank of this uplift. The Illinois basin was an active Chesterian depocenter and during periods of maximum transgression fine clastics may have passed through a seaway southwest into northern Arkansas. As shoaling proceeded, the shoreline regressed south with the advancing carbonate sand sheet, cutting off the avenue of fine clastics. Quartz sand, probably derived from a thick sequence of lower Paleozoic sandstones exposed along the uplift by the retreating sea, became admixed with oolite across the newly built platform. These shoalin cycles are the product of slow, steady, subsidence along a northern Arkansas shelf punctuated by episodes of relatively rapid carbonate sand sedimentation and clastic influx. A similar tectonic and depositional setting must have given rise to other such deposits that occur in the geologic record. End_of_Article - Last_Page 453------------