Carbonate Cycles Research Articles

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.

Evolution of Lacustrine Diatomite Carbonate Cycles of Miocene Age, Southeastern Spain: Petrology and Isotope Geochemistry

ABSTRACT Continental formations of Miocene age outcropping in southeastern Spain include beds of diatomites (in economic quantities) irregularly alternating with carbonate beds. The minerology and isotope geochemistry of the alternating carbonates and diatomites indicate variable depositional conditions associated with an overall deepening of a lake. Anhedral dolomite associated with sulfate minerals characterizes the first depositional stage and has heavy oxygen- and light carbon-isotopic values which are consistent with a formation in a restricted environment with highly evaporated waters. Increasing dilution of the water is reflected in the presence of abundant aragonite near the base of the assemblage which passes to calcite near the top. The gradual upward decrease of both 18O and 13C in these minerals indicates a parallel decrease of the evaporation/inflow ratio within the depositional basin. Also, seasonal variations of composition and volume of the water affect both the formation and diagenesi of carbonate minerals.

Superimposed Platform Carbonate Cycles: Eustatic Response of an Aggradational Carbonate Buildup, Middle Triassic of the Dolomites: ABSTRACT

Superimposed Platform Carbonate Cycles: Eustatic Response of an Aggradational Carbonate Buildup, Middle Triassic of the Dolomites: ABSTRACT

Cyclic Transgressive and Regressive Sequences and Their Association with Hydrocarbons, Sirte Basin, Libya: ABSTRACT

The Sirte basin was developed in north Africa between the Tethys Sea and the Saharan shield during Late Cretaceous time and was the site of mixed siliciclastic and carbonate deposition throughout the Tertiary. A series of bioclastic limestones and shales was deposited around the basin rim. Shales were confined to the low-energy zones in the basin, whereas carbonates were deposited on the shelf areas. The Farrud Formation (equivalent to the Beda Formation in the central part) is the main reservoir for oil found in the western portion of the basin. The faunal assemblages and lithologies recognized in this formation apparently reflect a very shallow marine depositional environment. Source rocks are developed in organic-rich, transgressive shales (Dahra and Hagfa). Moldic, fenestral, and intraparticle porosities are the most common types recognized in the carbonate reservoirs of the Farrud Formation. Permeability is developed in part by processes such as dolomitization, leaching, and fracturing in the two progradational, regressive carbonate cycles, resulting in the exceptional Ghani field reservoirs. Hydrocarbons were trapped in these reservoirs due to the presence of a supratidal anhydrite cap rock.

Extent and bathymetry of North American Platform Seas in the Early Silurian

The North American craton is 25% covered by Lower Silurian (Llandovery) rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface. This figure does not take into account extensive post‐Silurian tectonics and erosion. Isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the Silurian. Analysis of interbasin structural arches and outliers provides important clues as to the actual extent of Early Silurian seas. Information on the age of tectonic origin for 15 prominent arches is reviewed. The placement and significance of 11 key outliers is also discussed. Evidence of this kind indicates Early Silurian platform seas were among the most extensive of the Paleozoic. Probably more than 65% of the North American craton was then flooded. Nothing on so vast a scale serves as a recent guide to the past. The Pliocene‐Holocene Bahamian Banks do provide, however, worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies. The original bathymetry of Early Silurian environments may be determined, correlated, and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations. The range of fully integrated facies patterns includes (1) coral‐stromatoporoid/stromatolite/paleosol cycles (Williston Basin), (2) pentamerid brachiopod/coral‐stromatoporoid/stromatolite cycles (Michigan Basin), and (3) stricklandiid brachiopod/pentamerid brachiopod/coral‐stromatoporoid cycles (East Iowa Basin). With shallow bathymetries of 0–30 m the Williston class tends to represent mosaic facies, while with bathymetries of 0–60 m and 10–90 m the deeper‐water Michigan and Iowa classes typically involve blanket deposits. Bathymetry of other basin deposits generally fits one of these three classes. Maximum and minimum stands in sea level during Late Llandovery time are represented by bathymetric maps.

Correlation and Facies Analysis in Exploration for Subtle Traps Within Hunton Group, Oklahoma: ABSTRACT

The bulk of Hunton production to date is associated with rather well-defined structural and/or truncation-style traps. Yet the trapping mechanism in these settings, to a large extent, depends on the development of particular depositional facies within the Hunton Group. Accurate correlation and subdivision of the Hunton require an understanding of the overall depositional environment and history. The depositional model for the Silurian Chimneyhill and Henryhouse formations and the Devonian Haragan and Bois d'Arc formations is a carbonate ramp. Both aggradational and progradational sequences formed, as did several unconformities during periods of erosion and nondeposition. The Frisco, however, was deposited on submerged paleohighs, probably as a mud-mound deposit. Using the foregoing depositional models as a guide, subdivisions of the Hunton, based on regional markers related to changes in sea level between progradational episodes, can be recognized and correlated throughout the Anadarko-Arkoma region. Comparing core data and log signatures, along with applying depositional cycles, permits more detailed correlations as their component facies are recognized by log character. Reservoir-prone facies within the carbonate cycles can then be identified, correlated, and mapped.

Anoxic events, productivity rhythms, and the orbital signature in a Mid‐Cretaceous deep‐sea sequence from central Italy

Albian pelagic sediments from the Umbrian Apennines exhibit rhythmic bedding in outcrop. High temporal resolution (∼ 4‐kyr spacing) sampling of carbonate, Si, and Al along a 1.6‐m.y. interval of core demonstrates that changes in accumulation rate of calcareous and siliceous microorganisms were controlled by insolation cycles. Optical densitometry logging correlates variations of geochemical parameters to episodes of deep‐sea anoxia, indicated by numerous black, laminated zones (“black shales”). Harmonic analysis of data shows oscillations at three frequencies, which correspond closely in estimated duration and ratio to the modern eccentricity and precessional orbital cycles. As in Pleistocene marine sequences, the eccentricity (approximately 100 kyr) term dominates the sedimentary variance in the Umbrian cycles. Anoxic episodes occur in phase with a limestone‐marl repetition and reflect minima of the precessional (21 kyr) cycle. The covariance of carbonate and silica, with estimates of foraminiferal abundances, suggest that carbonate cycles are the result of changes in surface productivity of calcareous organisms rather than an effect of dissolution or diagenesis. Recognition of orbital cyclicity provides a tool for making quantitative estimates of these changes. Productivity during anoxic events was uniformly low (0.4– 0.5 g/cm2/kyr carbonate, 0– .05 g/cm2/kyr silica), but it rose to high levels during oxygenated periods (1– 2.5 g/cm2/kyr carbonate, 0.1– 0.3 g/cm2/kyr silica). Vertical overturning and organic carbon flux decreased during anoxic episodes, but decreased oxygenation of the deep water led to enhanced preservation of the organic matter delivered. The degree of paleoceanographic variability indicated by this study requires that accepted notions of climate stability and oceanic quiescence during nonglacial times be reassessed.

Milankovitch cycles in Neocene deep‐sea sediment

Pelagic carbonate sediments from the world ocean basins commonly show cyclic variations in amount and/or degree of preservation of biogenic calcite, with periodicities of several tens to several hundreds of thousands of years. The direct causes of these cycles are fluctuations in noncarbonate dilution, carbonate production, carbonate dissolution, and/or current winnowing. The overall driving force, however, is variation in the earth's orbital characteristics (Milankovitch cycles) through their influence on global climate and depositional processes. The main inferred climatic effects of orbital perturbations are on global ice volume, global temperature, ocean circulation, and distribution of climatic patterns. Eustatic sea level is directly related to ice volume, as is the rate of erosion of clastic material from continental margins. Changes in volume of sea ice affect the volume and intensity of bottom water flow, which in turn may cause changes in the intensity of sediment winnowing by bottom currents, in the intensity of upwelling of nutrient‐rich bottom waters, and in the depth of carbonate dissolution. Change in productivity of calcareous plankton are difficult to prove as a cause of carbonate cycles but may have contributed to the formation of carbonate cycles off northwest and southwest Africa. Fluctuations in winnowing of fine‐grained components have been demonstrated as a cause of cyclic variations in the coarse‐fraction component of carbonate sediments in the southwest Pacific. Dilution of carbonate by clastic material probably was a major cause of fluctuations in carbonate content of deep‐sea sediments off northwest and southwest Africa. Carbonate dissolution cycles probably are the most common manifestation of fluctuations in bottom water flow. Dissolution cycles are common in Quaternary and Neogene sediments of the North Atlantic, Caribbean, and eastern equatorial Pacific. The main cause of the carbonate dissolution was shoaling of the carbonate compensation depth during the early Neogene in response to climatically induced fluctuations in the thickness of Antarctic Bottom Water.

Five easy questions put to the sea

Since the 1940s progress in oceanography has been rapid and many once puzzling issues have been resolved. Fortunately, increasing understanding has also brought forth increasingly profound questions which, to the greater inspiration of the oceanographer, remain unanswered. Looking back across one third of a century, the author suggests five questions which he would like to see answered as soon as convenient. These are: 1) why did in the Jurassic the carbonate and silica cycles in the ocean suddenly come under biological control?; 2) to what degree did the level of carbon dioxide in the atmosphere and oceans fluctuate and why, and what was the impact on past climates?; 3) when did the abyss first become accessible to life with an oxygen-based metabolism?; 4) is a decipherable record of the past compositions of sea water contained in hydrothermically altered oceanic basalts now preserved on continents and what can it tell us?; and 5) what kind of coastal environments and communities managed to adapt to the most rapid Quaternary sea-level changes?

Depositional Dynamics of Cyclic Carbonates from the Interlake Group (Lower Silurian) of the Williston Basin

A set offour transgressive-regressive cycles is recognizable in the Lower to Middle Interlake Group of central Manitoba. The initial Silurian transgression reached its maximum with development of Virgiana (brachiopod) beds in the Fisher Branch Formation. Three subsequent transgressive peaks are identified by coral-rich layers in the upper part of the overlying Inwood Formation, the whole of theAtikamegFormation, and the middle part of the Cedar Lake Formation. These are separated by strata with flat to domal stromatolitic structures or other very shallow water features occurring in the lower to middle parts of the Inwood Formation, the upper part of the Moose Lake Formation, and much of the East Arm Formation. Regressive phases are marked by thin, reddish-brown shale beds bearing abundant frosted quartz grains. These marker beds are best known from middle and lower levels in the East Arm Formation (v and u2 horizons) and the middle of the Inwood Formation (ul horizon). A similar marker bed separates the Interlake Group from the underlying Stonewall Formation of latest Ordovician or earliest Silurian age. The number and spacing of these cycles suggest a correlation with other carbonate cycles found in the East Iowa, Michigan, and Anticosti Basins. While a Bahamian bank model of mosaic-facies deposition explains some aspects of the shallow-water Manitoban cycles, modification of the Israelsky wedge concept (facies in phase) enhances the interpretation of a vast, dynamically interconnected sea stretching far beyond the confines of the Williston Basin. During the first 10 m.y. of the Silurian, the Williston Basin was not an isolated topographic depression. Available evidence suggests that the region was still one of the shallowest parts of the periodically, extensively flooded North American platform.

THE GEOLOGY AND HYDROCARBON POTENTIAL OF THE EP 104 PERMIT, NORTHWEST CANNING BASIN, WESTERN AUSTRALIA

Current interpretations suggest the presence of five Devonian reef building cycles in EP 104 with a further two in the Ordovician, Nita, and Willara formations. The possible Devonian reefs occur in the NVorral, Lower and Upper Pillara, Virgin Hills, and Nullara carbonate cycles. Minor buildups may be present in the Fairfield and Laurel formations.The Upper Pillara reef complex was subaerially exposed and partly eroded for a period of 10 to 50 million years, following the Early Frasnian 'Tappers Inlet orogeny'. Large potential hydrocarbon reservoirs were created by the secondary mega-porosity formed during this period, and these were sealed by the regional Laurel transgression in the Early Carboniferous. This mode of porosity enhancement is applicable to all the potential reef cycles, as varying degrees of unconformity mark their upper boundaries, generally followed by onlap of sealing lithologies. The Upper Pillara cycle is considered the most prospective of the Devonian carbonate cycles which were the main targets in the first five wells.In response to the success of Home Energy Co. Ltd at Sundown, the last four wells in EP 104 have had Permo-Carboniferous objectives. This drilling resulted in the discovery in 1982 of the small Kora oilfield, believed to have been sourced from the Laurel Formation.The Black Rocks Prospect, in King Sound, is being considered for the first 1986 exploration well. This represents a return to Devonian Pillara reefs, the traditional primary targets in EP 104. As a bonus, drape, accentuated by late collapse of the reefal core, has localized multiple Permo-Carboniferous structural closures, coincident with reefal closure.By comparison with the highly productive Swan Hills and Rainbow-Keg River reefs in Canada the Pillara and Worral carbonate cycles in EP 104 must have considerable potential.

Models for generation of carbonate cycles

Computer modeling provides a quantitative approach to a better understanding of actual carbonate cyclic sequences. To model carbonate cycles, the authors can use water-depth-dependent sedimentation rate for each facies, an initial lag time, linear subsidence, tidal range, and period and amplitude of sea-level oscillation about a horizontal datum. Tidal-flat-capped cycles up to a few meters thick result from low-amplitude sea-level oscillation of a few meters and short lag times. Nonerosive caps reflect sea-level lowering being balanced by subsidence, and basinward migration of the shoreline not exceeding tidal-flat progradation rate. When higher amplitude sea-level oscillations occur, the tidal flats are abandoned on the inner shelf during sea-level fall, because seaward movement of the strandline outpaces progradation rate of flats. Increased amplitude also results in sea-level falling faster than flats can subside, so that disconformities with thick vadose profiles develop. High-amplitude (100 m or more) oscillations result in incipient drowning of platforms and juxtaposition of deep-water facies against shallow-water facies within cycles. Sea level falls before the platform can build to the sea-level highstand, and the shoreline migrates much more rapidly than tidal flats can prograde; thus, cycles are disconformity-bounded and lack tidal-flat caps. 10 references.

Evaporite Carbonate Cycles of the Messinian, Sicily: Stable Isotopes, Mineralogy, Textural Features, and Environmental Implications

ABSTRACT An investigation of the genetic environment of laminar and massive gypsum and intercalated carbonates (Messinian, central Sicily) has been carried out by means of isotopic, textural, and mineralogical analyses. The oxygen and hydrogen isotope compositions of crystal water from the laminar gypsum indicate primary precipitation of the sulfate from marine waters concentrated by evaporation. The isotopic data also show compositional changes due to periodic influx of continental waters into the deposition basin. In the laminar gypsum and interlayered dolostones we find a micritic, near-to or stoichiometric dolomite showing very positive 18O and negative 13C values which are consistent with formation of this carbonate, at a very early diagenetic stage, by precipitation from highly evaporated interstitial waters depleted in SO4-- ions by gypsum cr stallization and concurrent bacterial SO4-- reduction. Open-marine-water incursions probably led to the deposition of carbonate layers overlying the laminar gypsum. These contain abundant aragonite whose textural features and isotopic composition suggest precipitation in shallow and quiet marine waters slightly enriched in 18O by evaporation. The isotopic data of a euhedral and Ca-rich dolomite, formed later than the aragonite, may reflect further influxes of continental waters into the basin. On the basis of 18O and D values, the massive gypsum in the upper portion of the sedimentary sequence seems to have been formed by hydration of anhydrite.

Mixed Siliciclastic and Carbonate Sedimentation Within Spar Mountain Member of Ste. Genevieve Limestone, Hamilton County, Illinois: ABSTRACT

The Spar Mountain Member of the Ste. Genevieve Limestone (middle Mississippian) in Hamilton County, Illinois, consists of 40-60 ft (12-18 m) of interbedded limestones, shales, and sandstones. Five cores and 1,400 electric logs were used to delineate two shallowing-upward carbonate cycles and 2 major clastic pulses within the Spar Mountain. Eight lithofacies representing 6 depositional environments were identified. They are: (A) echinoderm-brachiopod dolomicstone to packstone (outer ramp), (B) ooid-peloidal grainstone (intermediate ramp), (C) skeletal grainstone (intermediate ramp), (D) ooid-molluscan-intraclastic wackestone to grainstone (inner ramp), (E) pelletal-skeletal wackestone (inner ramp), (F) quartzarenite (channelized nearshore), (G) quartz-sublithic arenite to acke (delta platform), and (H) quartz mudstone (prodelta, delta platform). Deposition occurred on a southwest-dipping carbonate ramp, with siliciclastic sediments originating from the northeast. The sequence of facies and their inferred depositional environments record 2 major progradational episodes. Oolitic facies are interpreted to be of tidal-bar belt origin and quartzarenite facies are interpreted to be of delta-distributary channel origin. Their distribution is partially controlled by antecedent and syndepositional topography. Many of these paleotopographic highs are positive features today and yield pinch-out stratigraphic relationships. Paleogeographic reconstructions demonstrate that the primary control on facies distribution was the position of the delta proper along strike. However, depositional topography also influenced sedimentation, particular y in the sand-sized fraction. Using this concept, better prediction of underlying porous buildups (ooid shoals) is possible if thickness of the overlying siliciclastic is known. Within buildups, a complex diagenetic history complicates the distribution of porosity. End_of_Article - Last_Page 301------------

Coral Zonules: New Tools for Petroleum Exploration in Mission Canyon Limestone and Charles Formation, Williston Basin, North Dakota: ABSTRACT

Study of the distribution of corals and rock types in the Mission Canyon Limestone and the lower part of the Charles Formation (Tilston, Frobisher-Alida, and Ratcliffe intervals) in 29 cores from wells in the Williston basin of western North Dakota resulted in recognition of four coral zonules and three regressive carbonate cycles. Although coral diversity and abundance decrease eastward toward the basin margin and upward in the sequence because of the influence of increasingly restricted environments, two of the zonules in the lower part of the Mission Canyon extend into areas of western North Dakota where marker-defined intervals are difficult or impossible to recognize. The Nesson anticline, or a paleotopographic ridge following the same trend, may have been a barrier hat hindered coral development in the east during later Madison deposition. Parallelism between the zonules and marker beds used to define standard intervals employed in subsurface stratigraphic correlation indicated that the marker beds are essentially time lines within the area studied. The first records of Stelechophyllum micrum and S. banffense in Madison rocks in the United States indicate a connection with the Alberta shelf and indicate that North Dakota was probably a part of the Central Western Interior subprovince during Osagean time. End_of_Article - Last_Page 870------------

Paleosols Capping Regressive Carbonate Cycles in the Pennsylvanian Black Prince Limestone, Arizona

ABSTRACT The lower Pennsylvanian (Morrowan to early Derryan) Black Prince Limestone, exposed in southeastern Arizona, consists of shallowing-upward (regressive) carbonate cycles that accumulated on a shallow-shelf platform. A typical complete cycle from the base upward consists of the following fades: mixed skeletal mudstones to grainstones(subtidal shelf, open circulation), mixed skeletal to nonskeletal packstones and grainstones (restricted shelf-lagoon), nonskeletal fenestral mudstones to grainstones (intertidal), nonskeletal cryptalgally laminated, brecciated mudstones to wackestones (supratidal), and paleosol horizons (terrestrial). Two paleosol types occur, calcrete and terra rossa. Calcrete occurs as thin (< 5-10 cm) profiles consisting of laminar micrite, coated grains, aggregate grains, and vadose diagenetic features. Terra rossa paleosols consist of subspherical nodules derived from pedogenic alteration of parent limestones, separated by a reddish, insoluble accumulation ofclays, iron oxides, calcite skeletal debris, and quartz silt. The terra rossa paleosols occur as in situ preserved C horizons and as reworked soil residuum. The first occurrence is marked by parent limestone in completely brecciated into nodules, whereas reworked paleosols consist of nodules enveloped in red matrix. Both calcrete and terra rossa paleosols record subaerial exposure, although the terra rossa paleosols suggest that signi icant amounts of limestone were removed during pedogenesis. The terra rossa paleosols also favor an extrinsic versus anintrinsic mechanism for the origin of the cycles.

Barium Partitioning in Carbonates: Theory and Applications: ABSTRACT

The partition coefficient for Ba2+ into calcite has been established as 0.06 ± 0.01 at 25°C (77°F). The partition coefficient proved independent of rate of precipitation in a series of 19 runs of varying duration. This value is substantially lower than that reported by Kitano in 1971, reflecting our seeding technique, which focused the experiment on crystal growth rather than on spontaneous nucleation and growth. The 0.06 value is compatible with the values reported for Sr2+ (0.05 to 0.14), a cation of identical charge, but closer to Ca2+ in ionic radius. The natural partitioning of Ba2+ was examined in fresh (aragonite) and altered (calcite) corals from the Pleistocene reef terraces exposed on Barbados, West Indies. The Ba2+ concentrations ranged from 8 to 15 ppm in the aragonites and decreased to typical values of 1 to 3 ppm in the calcites. The partitioning of Ba2+ in these samples was quantitatively similar to the attendant partitioning of Sr2+. The partitioning of Ba2+ during the carbonate cycle offers new areas of investigation. The low concentrations of Ba2+ in sedimentary biograins (3 orders of magnitude less than Sr2+) make Ba2+ a sensitive monitor of externally derived, barium-bearing solutions involved in diagenetic reactions. Likewise, t e low concentrations ensure ideal partitioning behavior in diagenetic solutions of low ionic strength. End_of_Article - Last_Page 516------------

A Spectrum of Late Paleozoic Siliciclastic Shelf-Bars, Sacramento Mountains, New Mexico: ABSTRACT

Study of several Pennsylvanian-Permian shelf sandstones in the northern Sacramento Mountains, New Mexico, suggests that siliciclastic shelf-bars were migrating on a high-energy shelf adjacent to the Pedernal uplift. These shelf-bars had sufficient relief, 2 to 6 m (6.5 to 20 ft), to provide the clear-water, agitated conditions requisite to carbonate grain-shoal development in areas of low clastic influx. The carbonate and siliclastic cycles can be End_Page 436------------------------------ explained by a relatively simple progradational cyclic model without invoking major sea level fluctuations. A typical cycle contains 3 major components: (1) shelf-bar stones; and (3) interbar siltstones. Shelf-bar sandstones, 1 to 6 m (3 to 20 ft) thick, are typically overlain by carbonate grainstone/packstone facies 1.5 to 6 m (5 to 20 ft) thick, which are grainstone/packstone facies, 1 to 6 m (3 to 20 ft) thick, which are in turn overlain by siltstones, 1 to 6 m (3 to 20 ft) thick, thus completing the cycle, 4 to 18 m (13 to 59 ft) total thickness. Shelf-bar sandstones in the Holder Formation (Virgilian) are lenticular concavo-convex sand bodies, 2.5 to 6 m (8 to 20 ft) thick, with flat depositional bases. Average grain-size (4^phgr - 0^phgr) and maximum clast size (12 mm, 0.5 in.) increases upward. Seaward dipping (W - NW) accretion foresets, up to 0.6 m (2 ft) thick, and bimodal trough cross-stratification, 10 to 30 cm (4 to 12 in.) thick, characterize lower sandstone facies. Hummocky cross stratification, 7 to 9 cm (2.8 to 3.5 in.) thick, 1.2 to 2.5 m (3.9 to 8 ft) wavelength, is common along accretion foreset surfaces and the tops of many of these sets are rippled and burrowed. Syndepositional deformation structures are common. Missourian and Virgilian shelf-bar sandstones were rapidly deposited by storm-induced currents and were modified by fair-weather tidal oscillatory flow and biological processes. In contrast with Virgilian shelf-bar sandstones, Wolfcampian bars are thinner, 1.5 to 2 m (5 to 6.5 ft) thick, coarser grained, and locally have erosional bases. Bimodal through cross-stratification is well-developed at the expense of seaward dipping accretion foresets which if present, are poorly developed. Hummocky cross-stratification and burrows are rarely present, whereas ripples and syndepositional deformation structures are common. Average grain size coarsens-upward crudely. These Wolfcampian shelf-bars were deposited in shallow water subject to strong tidal currents, which winnowed finer sediments, produced distinct bimodality in sedimentary structures, and prevented the bars from achieving significant depositional relief. Carbonate units, 1 to 5 m (3 to 16 ft) thick, decrease in micrite and increase in abundance of allochems upward in the vertical sequence. Sedimentary structures are similar to those of the associated siliciclastic shelf-bars, indicating a change in sediment-supply without significant change in hydrodynamic conditions. Carbonate units are thinner, 1 to 2 m (3 to 6.5 ft), become less important volumetrically in Wolfcampian sediments as compared with those in Virgilian rocks. The spectrum of high energy siliciclastic shelf-bar sandstones, and the presence of fan-deltaic and fluvial deposits in overlying sediments (Wolfcampian) suggests that the shelf adjacent to the Pedernal uplift continuously prograded into the Orogrande basin. Shifting clastic influx along depositional strike caused drastic local changes in shoreline configuration, longshore currents, and subsequent clastic and carbonate sediment distribution patterns. End_of_Article - Last_Page 437------------

Small-Scale Basin-Slope Carbonate Cycles in Trenton Limestones (Middle Ordovician), Southern Ontario: ABSTRACT

The Middle Ordovician marine transgression is marked by a simple stratigraphic sequence from supratidal and tidal-flat carbonates, through lagoonal and shoal carbonates, into offshore and finally deep-shelf carbonates. Within the offshore carbonate succession, contemporary peninsulas, islands, and shoals complicate the detailed facies distributions, but show features similar to the basin-slope carbonate models proposed for other carbonate deposits, albeit on a smaller scale. Two end-member cycles are End_Page 1453------------------------------ proposed. The proximal cycle consists of coarse pebbly biosparites, commonly showing the characteristics of grain flows, passing up into cross-bedded, fine-grained biosparites, which then are overlain by micrite and calcareous shales. Calcareous shales occur interbedded throughout the cycle. This cycle is interpreted as having been deposited in bypass channels on slopes around the Middle Ordovician islands or shoals. The distal cycle consists of poorly washed biosparite passing up into interbedded micrite and calcareous clay, and is interpreted as being deposited near the base of the slope or in the adjacent basin. Both types of cycles have hardgrounds on their coarser units, indicating long periods of nondeposition and/or erosion after deposition. A close analogy can be made, both in microfacies and depositional environment, with the Holocene Arabian shelf of the Persian Gulf. Furthermore, both have similar tectonic situations--carbonate shelves on ancient shields undergoing collision with a magmatic arc. End_of_Article - Last_Page 1454------------

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------------