Peritidal Sequences Research Articles

Facies and geochemical characteristics of the Middle-Lower Ordovician Yingshan Formation in the Tarim Basin, NW China: Implications for the high-frequency sequence stratigraphy in shallow-water carbonate platform

Shallow-water carbonates from the Yingshan Formation are important target of hydrocarbon exploration in Tarim Basin. Detailed descriptions of outcrop, core, and thin sections, isotopic composition analysis, and trace element analysis were conducted. Seven lithofacies which consists of two facies belt groups were identified and used to interpret deposition environments of Yingshan Formation: (1) peritidal carbonate, with relatively lower δ13C values from −4.2% to −1.9%, mainly represented by intertidal to supratidal facies and restricted subtidal facies and (2) open-marine subtidal carbonate, with higher δ13C values (−1.5% to −0.3%), mainly consists of shoal facies and interbank sea facies. On the basis of the lithology sets, four types of meter-scale cycle model (types A–D) were grouped into peritidal sequences and subtidal sequences. On the basis of vertical lithofacies, cycle stacking patterns, and accommodation variations in Fischer plots, two third-order depositional sequences (SQ1–SQ2) were recognized. The sequence boundary between SQ1 and SQ2 is not only a surface of a positive shift of δ13C values, but also the transitional zone of cycle stacking patterns. The sequences can be further divided into four fourth-order sequence sets: Sq1–Sq4. The lower sequences (Sq1–Sq2) are dominated by peritidal facies and characterized by a progressive decreasing in accommodation space probably indicates a longer-term fall in sea level. The upper sequences (Sq3–Sq4) are mainly dominated by subtidal facies and characterized by the accommodation space with a progressive increasement, likely indicates a rising sea level with a longer term. The facies-controlled reservoirs are mainly present in the Sq3, related to the sequence boundary between SQ1 and SQ2.

Facies evolution and sequence chronostratigraphy of a “mid”-Cretaceous shallow-water carbonate succession of the Apulia Carbonate Platform from the northern Murge area (Apulia, southern Italy)

The “mid”-Cretaceous carbonate succession of the Apulia Carbonate Platform cropping out in northern Murge area (Apulia, southern Italy) is composed of shallow-water carbonate rocks and is over 400 m in thickness. This paper focuses on the lithofacies analysis of this carbonate succession, its paleoenvironmental interpretation, and its sequence-chronostratigraphic architecture. Lithofacies analysis permitted to identify deposits which can be grouped into the following three facies belts: (1) terrestrial facies belt formed by: intraclast-supported paleosoils; solution-collapse breccias; (2) restricted facies belt made up of lithofacies deposited in protected peritidal environments; (3) normal-marine facies belt made up of lithofacies formed in moderate- to high-energy subtidal environments. The detailed study both in outcrops and in thin-sections revealed that, at the bed scale, lithofacies are cyclically arranged and form shallowing-upward small-scale depositional sequences comparable to parasequences and/or simple sequences. The following three small-scale sequence types have been distinguished: (1) subtidal sequences mostly made up of lithofacies formed in the normal-marine open subtidal domain; (2) peritidal sequences made up of lithofacies formed in the restricted peritidal domain; (3) peritidal sequences showing a cap formed by paleosoils. Small-scale sequences are not randomly arranged in the compiled succession but form discrete packages, or sets, that alternate in the sedimentary record. The repetition of such small-scale sequence packages in the succession has been the key to recognize large-scale sequences comparable to third-order depositional sequences. Although sedimentological data are often fragmentary due to late dolomitization, four large-scale sequences have been distinguished. The data support a generalized landward-backstepping of facies belts during transgression, which implies a gradual gain of accommodation culminating with the deposition of a package of small-scale sequences formed by normal-marine subtidal deposits. These mark periods of maximum accommodation space and form the maximum-flooding zones of large-scale sequences. A gradual seaward progradation of facies belts is recorded during highstand conditions, which implies a gradual loss of accommodation culminating with the deposition of a package of peritidal small-scale sequences capped by paleosoils or by solution-collapse breccias. The occurrence of terrestrial deposits marks periods of minimum accommodation on the platform and determines the sequence boundary of large-scale sequences. The large-scale sequences identified in this study fit with the main transgressive/regressive cycles published in the sequence-chronostratigraphic chart of European basins. As a consequence, it is interpreted that changes of the sea level recorded at the scale of European basins played an important role in determining the sequence-stratigraphic architecture of the studied succession. In spite of this, the occurrence of solution-collapse breccias, which implies a significant gap in carbonate sedimentation in between Early and Middle Cenomanian times, may also have an alternative interpretation. In particular, this deposit may represent the local fingerprint of the well-known tectonic phase which, during Late Albian-Early/Middle Cenomanian times, determined the subaerial exposure of large parts of Periadriatic carbonate platforms producing a marked regional unconformity.

Precambrain Fault Systems as Control on Regional Differences in Relative Sea Level Along the Early Ordovician Platform of Eastern North America

Abstract This study proposes that there was regional variation in Early Ordovician subsidence histories along eastern Laurentia controlled by broad differences in the susceptibility of inherited Precambrian crustal structure to reactivation with changing ocean-basin tectonism. Two continental-scale Phanerozoic fault systems (Southern Oklahoma Fault System, Ottawa–Bonnechere Graben) extend into the craton interior from the apices of the Ouachita and Quebec embayments or reentrants, respectively. Their traces identify reactivated Proterozoic fault zones. Previous studies show that rates of subsidence for Early Ordovician sedimentation along the axis of the Southern Oklahoma Fault System are elevated compared to regional rates beyond this structural corridor. A similar history applies to Early Ordovician sedimentation within the Ottawa Embayment (eastern Ontario), along the present axis of the Ottawa–Bonnechere Graben. The Theresa Formation (lower Beekmantown Group) contains a third-order peritidal (siliciclastic-dominated) sequence. Evidence for local tectonism prior to and during its development within the embayment includes: (1) Late Cambrian (pre-Theresa) preferential uplift, differential erosion, and subsidence along the eastern structural boundary of the embayment; (2) differential intrabasinal subsidence during Theresa deposition; (3) an apparent offset in timing of peak flooding within the embayment to that along the St. Lawrence Promontory, ~ 1500 km distant; and (4) absence of a subaerial unconformity in the embayment that otherwise separates eustatic cycles on the promontory, and defines a likely sea-level fall below the shelf margin. In comparison, subsidence within the embayment appears to have matched (at least) the rate of the eustatic fall. Regional differences in Early Ordovician subsidence histories along this margin may illustrate that the Precambrian fault zones were structurally more sensitive than promontories to the developing reconfiguration of oceanic lithospheric elements that would lead to the Taconic orogeny.

Monomineralic carbonate ooid types in the Triassic sediments from Northwestern Bulgaria

Oolitic rocks occur in almost all lithostratigraphic units from the Iskur Carbonate Group (Lower – Upper Triassic) in NW Bulgaria. Primarily monomineralic ooids with homogenous cortices belong to several fabric types: micritic/microsparitic, brickwork, sparry relic, blocky, radial fibrous, radial sparry, shrunken and mottled. In addition, ooids with compound cortical fabrics were distinguished: blockymicritic, micritic-radial and radial-brickwork. Cracked, spalled, broken, abraded, distorted and pitted types were differentiated according to outer deformed shape. Apart from the single individuals polyooids were found in some rocks. The primary ooid fabrics were modified by various isochemical diagenetic processes including microbial micritization, compaction, aragonite transformation, recrystallization, dissolution and cementation. These proceeded in the marine-phreatic, meteoric-phreatic and deep-burial connate diagenetic environments. Only one specific ooid association shows relation with a particular depositional setting. At the very base of the Lower Triassic peritidal sequence (Mogila Fm) mottled and radialbrickwork ooids coexist with bimineralic individuals and broken shapes. The formation of such association is attributed to hypersaline conditions, as this interpretation is based on other sedimentological data and analogy with similar occurrences from the geological record. The original mineralogy of some ooid types was inferred mainly on the basis of petrographic and geochemical data, but actualistic evidence was considered as well. The precipitation of aragonite and high-Mg calcite ooids was proved. Their stratigraphic occurrence conforms well to the global distribution of carbonate ooids through the Triassic period. Thus aragonite ooid precipitation was predominant during Late Olenekian – Middle Anisian times. The main control was high carbonate saturation state of seawater promoting higher precipitation rates for aragonite due to kinetic reasons. In this particular case, the high degree of seawater carbonate saturation was probably further promoted by an increased salinity reflecting intense evaporation in an arid peritidal setting. The contrastingly large predominance of calcitic ooids in the Upper Anisian – Lower Carnian sediments corresponds to the secular increase of calcite abiotic precipititation during the Middle Triassic epoch. This change was related to the appearance of global highstand, more intense seafloor spreading, warmer climate, higher CO2 levels, and consequently, to low carbonate saturation state of seawater favouring calcite precipitation. In this particular case, the seawater reached normal salinity from the Middle Anisian onwards, thus probably further promoting calcite ooid formation. Locally controlled short-term shifts in the carbonate precipitation also took place in the epicontinental Triassic sea. This is inferred from the coexistence or interbedding of primary aragonitic and calcitic ooids, as well as presence of bimineralic ooid cortices. It is suggested that subtle temperature and/or salinity changes ultimately controlled the carbonate saturation state of seawater. Unlike the established trend of primary ooid mineralogy, marine phreatic cements in the studied Triassic rocks were predominantly composed of high-Mg calcite. This discrepancy is generally explained by the lower supply of carbonate ions into interparticle pores, which favoured calcite cementation, although poor preservation of marine aragonite cements is another possibility. Further evidence on the abiotic carbonate precipitation might be obtained from investigation of the limestone micritic matrix.

Lower Carboniferous peritidal carbonates and associated evaporites adjacent to the Leinster Massif, southeast Irish Midlands

Analysis of a 275 m-thick section in the Milford Borehole, GSI-91-25, from County Carlow, Ireland, has revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?-late Chadian to late Holkerian (Visean, Lower Carboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part of the sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. These include a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolite sand. A 68 m-thick, well-developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternating stromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizons containing pedogenic features. In the succeeding Arundian–Holkerian strata, transgressive–regressive carbonate units are recognized. These comprise high-energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstones with minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons. The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands. These strata appear to be coeval with similar evaporite-bearing rocks in County Wexford that are developed on the southern margin of this landmass, and similar depositional facies exist further to the east in the South Wales Platform, south of St. George's Land, and in Belgium, south of the Brabant Massif. The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyan strata. They may also have been a source of high salinity fluids associated with nearby base-metal sulphide deposits. Copyright © 2005 John Wiley & Sons, Ltd.

Early dolomitization and fluid migration through the Lower Carboniferous carbonate platform in the SE Irish Midlands: implications for reservoir attributes

Abstract Shallow-marine, Lower Carboniferous carbonate sequences of the SE Irish Midlands, close to the Leinster Massif, are intensely dolomitized. Fine-crystalline (<50 μm), planar-s (subhedral) dolomite is associated with evidence for evaporites, typical of arid peritidal sequences. However, stable isotope data suggest a diagenetic overprint. Volumetrically more important medium-crystalline (50–200 μm), planar-s and minor planar-e (euhedral) dolomites were precipitated from slightly modified Lower Carboniferous seawater. These dolomites replace open-marine intraclastic and bioclastic packstones and grainstones. Length-slow, fibrous quartz partially replaces crinoids and fills dissolution cavities beneath peritidal strata. Associated dolomites are gradually enriched in 18 O downward through the underlying strata, suggesting vertical brine migration. The widespread occurrence of skeletal material replaced by chalcedony in open-marine wackestones and grainstones further to the west, within the Rathdowney Trend, suggests evaporite cementation in the Zn-Pb mineralized area. Base-metal mineralization in the fractured Waulsortian ‘reservoir’ is associated with chloride-enriched brines (beyond that expected from seawater evaporation alone). The presence of evaporites in the Leinster Massif area suggests a possible source of the excess chloride. The dolomitizing brine may have contributed to the overall chemistry of the Zn-Pb mineralizing fluid and also to the distribution of porosity within the carbonate platform. An Arundian or younger age is suggested for the mineralization, based on the timing of evaporite cement emplacement, and this is compatible with numerical fluid-flow models of brine movement through the carbonate platform. Dolomitization of Lower Carboniferous carbonate rocks of the Irish Midlands is comparable to diagenetic histories of several important dolomite petroleum reservoirs. This study provides an example that may be applied to petroleum exploration in similar geological settings.

Poisson processes of carbonate accumulation on Paleozoic and Holocene platforms

ABSTRACT Cambro-Ordovician carbonate lithofacies units in the Elbrook and Conococheague formations exposed at Wytheville, Virginia, as well as those in many other Phanerozoic peritidal sequences, exhibit exponential thickness frequency distributions. That is, occurrence frequency decreases exponentially with linear increase in unit thickness. Such distributions are characteristic of waiting times between independent Poisson events. This relative frequency of spaces of different size between horizons of lithologic change is what one would expect if the horizons were distributed randomly throughout carbonate successions. Abundances of different lithologies, both as net stratigraphic thickness and as number of occurrences, also decreases exponentially among successively rare sediment types, with each lithology being about 60% as plentiful as the next more abundant rock type. Relations between net thickness and number of occurrences for each facies define a linear trend coincident with a mean thickness for all Wytheville units of 0.48 m, a relation indicating that thickness distribution is independent of facies type. Similar relations are apparent for the horizontal extent of carbonate sediment bodies from the Holocene Florida-Bahama platform. Areal extents of individual facies units (lithotopes) are described by a frequency distribution in agreement with that anticipated for a population of equidimensional facies elements whose diameter distribution follows an exponential frequency distribution. Although regional gradients in sediment texture and composition are also apparent along most transects from platform margin to interior, such frequency distributions indicate that lateral extents of individual sediment reflect a largely stochastic distribution of facies boundaries across this Holocene surface. Lithotope abundances also yield trends of exponentially decreasing dominance among successively subordinate facies, with each being about 70% as extensive the next more abundant sediment type. Relations between areas and abundances for all lithotopes define a covariant trend corresponding to a mean area of 2.2 103 km2 for all Florida-Bahamas lithotopes. We consider several numerical models of stochastic carbonate accumulation; although not demonstrably unique, scenarios incorporating the sequential superposition of randomly placed coniform lithotopes result in thickness and area frequency distributions that are the same as those observed in ancient and modern platform deposits. Such simulations of Poisson processes of sediment accumulation are in general agreement with stochastic models of lithologic heterogeneity that have been more widely applied to petroleum reservoirs and groundwater aquifers. To the as yet unknown degree that peritidal lithofacies area and thickness are correlated, data from Paleozoic and Holocene platforms suggest that carbonate units should exhibit length/height ratios of approximately 105. Given the decimeter scale over which facies are designated in most Paleozoic peritidal successions, these relations predict mean lateral extents on the order of several tens of kilometers, a value in general agreement with the few data that exist on spatial continuities of peritidal lithotopes in Paleozoic carbonate sequences.

Michigan hockey, meteoric precipitation, and rhythmicity of accumulation on peritidal carbonate platforms

Research Article| August 01, 1998 Michigan hockey, meteoric precipitation, and rhythmicity of accumulation on peritidal carbonate platforms Bruce H. Wilkinson; Bruce H. Wilkinson 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063 Search for other works by this author on: GSW Google Scholar Nathaniel W. Diedrich; Nathaniel W. Diedrich 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063 Search for other works by this author on: GSW Google Scholar Carl N. Drummond; Carl N. Drummond 2Department of Geosciences, Indiana-Purdue University, Fort Wayne, Indiana 46805-1499 Search for other works by this author on: GSW Google Scholar Edward D. Rothman Edward D. Rothman 3Department of Statistics, University of Michigan, Ann Arbor, Michigan 48109-1027 Search for other works by this author on: GSW Google Scholar Author and Article Information Bruce H. Wilkinson 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063 Nathaniel W. Diedrich 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063 Carl N. Drummond 2Department of Geosciences, Indiana-Purdue University, Fort Wayne, Indiana 46805-1499 Edward D. Rothman 3Department of Statistics, University of Michigan, Ann Arbor, Michigan 48109-1027 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1998) 110 (8): 1075–1093. https://doi.org/10.1130/0016-7606(1998)110<1075:MHMPAR>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Bruce H. Wilkinson, Nathaniel W. Diedrich, Carl N. Drummond, Edward D. Rothman; Michigan hockey, meteoric precipitation, and rhythmicity of accumulation on peritidal carbonate platforms. GSA Bulletin 1998;; 110 (8): 1075–1093. doi: https://doi.org/10.1130/0016-7606(1998)110<1075:MHMPAR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract On Saturday afternoon March 30, 1996, the University of Michigan hockey team won the 1996 National Collegiate Athletic Association Division I national championship. Durations between Wolverine goals, between opponent goals, and between all goals during the preceding 40-game regular season each describe an exponential distribution in which duration frequency depends only on number of shots on goal and probability of success. Compared to opponent scores, University of Michigan between-goal duration frequencies describe a trend having a steeper slope (University of Michigan shot better) and a higher intercept (University of Michigan took more shots).Over much of the past 100 yr, meteoric precipitation on Ann Arbor occurred during 11 949 days. Time durations of the 6401 precipitation episodes that occurred over this interval, as well as durations of contiguous days of precipitation and contiguous days of drought, each define an exponential distribution in which duration frequency is largely defined by total interval length (35 101 days) and probability of precipitation (34%).Roadcuts near Wytheville, Virginia, provide spectacular exposures of a 303.7-m-thick section of peritidal carbonate in the Middle to Upper Cambrian Elbrook and Conococheague Formations. Stratigraphic durations (thicknesses) of the 527 lithologic units within this sequence, of the 265 “cyclic” upward-shallowing lithofacies associations that can be designated over this interval, and of stratigraphic intervals between recurrences of like lithofacies, also define exponential distributions wherein frequency of stratigraphic recurrence is only dependent on the total thickness and net abundance of designated stratal elements.Frequency of goal scoring, and frequency and/or magnitude of meteoric precipitation can be described in terms of random, independent processes at short time scales. Similarly, exponential distributions of lithologic and “cyclic” thickness frequencies at Wytheville, Virginia (as well as in most other epicratonic peritidal sequences), indicate that meter-scale variation in carbonate deposition was predominantly controlled by stochastic (Poisson) processes that were largely unrelated to recurrent intrabasinal or extrabasinal forcing and/or to periodic (rhythmic) sediment accumulation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A reappraisal of dolomite abundance and occurrence in the Phanerozoic

Critical assessment of dolomite abundance throughout the Phanerozoic suggests that most large-scale dolomite has had an origin related to salinity-elevated seawaters. The distribution of such dolomite is not directly related to periods of major continental flooding but rather to periods when extensive peritidal sequences or large-scale evaporite basins/lagoons existed. The development of such depositional systems is controlled largely by eustasy, paleogeographic/tectonic setting, and paleoclimate. Extensive peritidal sequences are characteristic of mature, passive-margin carbonate platforms that formed during long periods of Earth history generally devoid of major continental glaciation (e.g., late Precambrian through early Paleozoic, and Mesozoic). Sea-level fluctuations of relatively low amplitude during such periods allowed extensive, cyclic peritidal-dominated platforms to develop. Such platforms typically lack raised rims, had depositional surfaces that stayed near sea level for tens of millions of years, and show little evidence of major drops in sea level that would have exposed the shelf for long periods. This would have allowed repeated replenishment of pore waters by Mg-rich marine fluids over very extensive supratidal surfaces. Also, since the cycles are very thin, the downward-moving brines could penetrate several cycles on the inner platform, causing massive dolomitization. In contrast, during major glacial periods (e.g., Middle to Late Mississippian to Early Permian and late Cenozoic) higher-amplitude, ice-driven fluctuations in sea level led to extensive meteoric-water flushing of shallow platforms, but related dolomitization has been of minor significance. Under conditions of elevated Mg/Ca ratio in hypersaline brines, however, large-scale dolomitization could occur in response to the formation of extensive evaporite basins/lagoons. Suitable climatic and paleogeographic/tectonic conditions have allowed such dolomitization at various times during the Phanerozoic, regardless of specific styles of eustatic sea-level change. Lack of massive dolomitization in Holocene carbonates may be because neither extensive peritidal sequences nor large-scale evaporite basins/lagoons are prevalent in modern carbonate environments.

Palustrine Carbonates and the Florida Everglades: Towards an Exposure Index for the Fresh-Water Environment?

ABSTRACT Palustrine carbonates are shallow fresh-water deposits showing evidence of subaqueous deposition and subaerial exposure. These facies are common in the geological record. The intensity of modification is highly variable depending on the climate and the length of emergence. Palustrine limestones have previously been interpreted as marginal lacustrine deposits from fluctuating, low-salinity carbonate lakes, but several problems remain with existing facies models: 1) palustrine carbonates possess a lacustrine biota but commonly display fabrics similar to those of calcretes and peritidal carbonates; 2) the co-occurrence of calcrete horizons and karst-like cavities is somewhat unusual and appears to indicate contemporaneous carbonate precipitation and dissolution in the vadose zone; 3) the dominance of gray colors indicates water-saturation, apparently inconsistent with the evidence for strong desiccation overprint; 4) profundal lake deposits are generally absent from palustrine sequences, and sublittoral facies commonly make up only a small proportion of total thicknesses; 5) no good modern analogue has been identified for the palustrine environment. Analogy with the Florida Everglades suggests a re-interpretation of palustrine limestones, not as pedogenically modified lake margin facies but as the deposits of extensive, very shallow carbonate marshes. The distribution of environments in the Everglades is determined by the local hydrology, reflecting the control of seasonal water-level fluctuations and topography. Climate and topography were the main controls on deposition of ancient palustrine carbonates. As in peritidal sequences, aggradational cycles are capped by a range of lithologies (evaporites, desiccation and microkarst breccias, calcretes, lignite or coal horizons etc.), permitting interpretation of the climate. Careful analysis of lateral facies variations may permit reconstruction of subtle topography. Consideration of the Florida Everglades as a modern analogue for the palustrine environment has suggested the development of an exposure index for fresh-water carbonates.

Carbonate ramp to rimmed shelf evolution: Lower to Middle Cambrian continental margin, Virginia Appalachians

Drill cores through the Lower to Middle Cambrian Shady Dolomite carbonate platform (600 to 1,200 m thick) in the Austinville, Virginia, region have provided critical platform-margin facies relations not exposed in outcrop. The control provided by the closely spaced drill cores has allowed detailed cross sections to be constructed through the margin, which allow the evolution of the carbonate platform from a gently sloping, carbonate ramp to a high-relief, reef-rimmed shelf to be interpreted. The Shady Dolomite records the inception of carbonate sedimentation on the Lower Cambrian passive margin and forms the initial carbonate foundation for the over-lying Cambrian-Ordovician carbonate shelf sequence. Carbonate sedimentation was initiated on a submerged Lower Cambrian clastic shelf, resulting in deposition of a thick sequence (300 m) of deep-ramp, nodular limestones. This facies shallows upward into a carbonate ramp fringed by low-relief, stromatactoid mud-mound buildups (as much as 30 m thick and 3.5 km wide). Downslope these buildups pass into slope conglomerate and deep-ramp carbonate facies; landward of the buildups, cyclic peritidal sequences developed. By latest Early Cambrian time, as the ramp increased in relief, skeletal algal bioherms and lime sands were established on the high-energy margin to form a high-relief (as much as 500 to 700 m), rimmed shelf with thick (>600 m) off-platform deposits immediately seaward of the shelf edge. In the shelf interior, a major regression during Early to Middle Cambrian time resulted in development of a major unconformity and in deposition of red beds. On the more rapidly subsiding outer platform, subsidence rates exceeded sea-level-fall rates, allowing continued upbuilding of the reefal rimmed shelf. This rimmed shelf persisted for about 30 m.y. and influenced carbonate sedimentation throughout the Cambrian until it was destroyed by incipient collision during the Early Ordovician period (Read, in press).

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.

Use of Fischer plots to define third-order sea-level curves in Ordovician peritidal cyclic carbonates, Appalachians

Fischer plots in which cumulative cycle thickness corrected for linear subsidence is plotted against time were originally used in the study of Triassic peritidal sequences to define short-duration departures in relative sea level in the Milankovitch range (104 to 105 yr). However, they also provide a means of assessing magnitude of third-order (1 to 10 m.y.) fluctuations in sea level which, on the basis of their regional correlatability, we ascribe to eustatic events.

Incipiently drowned facies within a cyclic peritidal ramp sequence, Early Ordovician Chepultepec interval, Virginia Appalachians

The Chepultepec interval (145 to 260 m thick) in Virginia, although dominated by peritidal carbonates, contains thick subtidal sequences that formed during repeated, incipient drowning of the Early Ordovician ramp. Cyclic peritidal sequences make up the lower member (up to 150 m thick) and upper member (up to 85 m thick). Subtidal limestone and bioherms make up the middle member (up to 110 m thick) and pass landward into cyclic peritidal facies. Average long-term subsidence rates were up to 4 cm/1,000 yr (mature passive margin rates), and ramp gradients commonly were extremely low (2 cm/km). Computer modeling helped define mechanisms involved in formation of the stratigraphic sequence. The peritidal sequences are upward-shallowing cycles (average period 140,000 to 200,000 yr) of pellet and skeletal limestone, pellet grainstone, thrombolites, intraclast grainstone, and laminite caps. These resulted from periodic sea-level oscillations of several metres amplitude, and possible slow, long-teim (third-order) sea-level fall superimposed on long-term subsidence. Tidal flats were shifted far westward after each sea-level rise but had sufficient time to prograde back across the ramp during sea-level fall. The middle number contains repeated sequences of subtidal skeletal limestone and pellet limestone with storm beds and burrowed-units (formed below fair-weather wave base), local rippled pellet limestone, and thrombolite bioherms (subwave-base to wave-agitated shoals) that may have erosional tops overlain by thin grainstone sheets. Tidal-flat facies are absent. These sequences formed by periodic sea-level rise (more than 12 to 17 m amplitude), which caused incipient drowning of the platform, and migration of tidal flats several hundred kilometres back onto the platform. These flats were unable to prograde onto the outer ramp during regression because of insufficient time and because seaward retreat of the shoreline outpaced progradation of tidal flats. This left them abandoned on the inner ramp. Similar sequences reflecting incipient drowning are widespread in the geological record.

Carbonate Platform Sedimentology, Upper Cambrian Richland Formation, Lebanon Valley, Pennsylvania

ABSTRACT The Richland Formation is exposed on both limbs of the Lebanon Valley nappe in southeastern Pennsylvania. The nappe probably contains the farthest transported carbonate platform rocks of the Cambro-Ordovician continental margin sequence. Intertidal to deep subtidal environments are represented. Intertidal deposits include cryptalgalaminates and tabular-laminated dolomites. Ooid grainstones, in part altered to coarsely crystalline dolomites, are shoal or beach deposits. Subtidal deposits include mudstones, peloid packstone/wackestones, thrombolites, and domal stromatolites. Ribbon carbonates were probably deposited in an open-platform setting below fair-weather wave base. Lithofacies occur in cyclic peritidal, subtidal, and thrombolitic facies associations. Facies sequences on each limb are reciprocally related. Upper-limb rocks occupied a more seaward platform position. The upper limb features a 180-m-thick cyclic peritidal sequence followed by 320 m of deep to shallow subtidal deposits. On the lower limb a 300-m-thick cyclic subtidal sequence is succeeded by 100 m of cyclic peritidal deposits. The contrasting facies successions on each limb imply two major depositional phases. Phase one featured a shelf system with a broad shoal complex (upper limb) situated in front of or beside lagoon or embayment deposits (lower limb). In the second phase, deep subtidal conditions spread across the area of the upper limb while cyclic peritidal facies were established in the lower-limb setting. The upper-limb shoal complex probably was incipiently drowned during a transgression that culminated in a late Cambrian-early Ordovician sea-level highstand.

Comparison of Sequences Formed in Marine Sabkha (Subaerial) and Salina (Subaqueous) Settings--Modern and Ancient

Marine evaporites occurring in modern subaqueous (salina) settings and subaerial (sabkha) settings are different. Subaqueous Holocene evaporites occur as shoaling-upward lacustrine sequences up to 10 m thick. They are evaporite dominated and are composed primarily of bottom-nucleated crystals that may be deposited as massive, laminated, or rippled units. Each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments. In plan view, they show a well-developed bull's-eye pattern with a sulfate center and a carbonate rim. In contrast, subaerial (sabkha) evaporites occur as part of a laterally prograding, shoaling-upward, peritidal sequence in which the supratidal unit is usually no more than 1 m thick. Sabkha sequences are matrix dominated, not ev porite dominated, with the bulk of the sulfate phase occurring as diagenetic nodules, enteroliths, or diapirlike structures. These sulfates were formed during syndepositional diagenesis by replacement and displacement processes. The various facies of the sequence tend to accumulate in belts parallel with the shoreline. Relative to the sea level or the brine level, sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows. Some of the characteristics that distinguish Holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies, even when gypsum has been converted to nodular anhydrite. The distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates.

The stratigraphy of coastal carbonate banks and Holocene sea levels of northern Spencer Gulf, South Australia

Stratigraphic and chronologic studies of the coastal sediments of northern Spencer Gulf in the vicinity of Redcliff are described. Two Pleistocene marine units are recognised beneath an extensive Holocene peritidal sequence. The Holocene sequence is dominated by sediments of the Posidonia australis seagrass facies which form a distinctive carbonate bank fringing the coastline. Peritidal sedimentation commenced prior to 6600 radiocarbon yrs B.P. and a + 2.5 m higher relative sea level existed from about 6000 until around 1700 yrs B.P. The subsequent relative fall to present level probably resulted from local tectonic uplift. After the regression, mangrove and samphire colonization, and beach ridge progradation, occurred over the former seagrass bank.

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

Early Carbonate Fabrics in Silurian Reefs of Gotland, Sweden: ABSTRACT

The Hogklint reefs of Gotland occur within a shallowing-upward sequence and are developed in the outer zones of a carbonate wedge adjacent to a shale belt. Shoal and peritidal sequences are the major carbonates which occur in the inner parts of the wedge and are preferentially associated with erosion surfaces. The Hogklint reefs show carbonate fabrics that are inferred to indicate synsedimentary marine cementation of the reefs. These fabrics are: (1) pseudofibrous calcites that are believed to have replaced acicular/fibrous cements with original aragonite and/or Mg-calcite mineralogy, (2) pelmicsparite-pelmicrite areas, (3) stromatolite-like crusts, and (4) early micritic cements. The reefs also show evidence of subaerial exposure by the occurrence of erosion surfaces at the top of the reefs, dissolution cavities, and vadose crystal silt infills in cavities in the algal biofacies which cap the reefs. End_Page 640------------------------------ The distribution of the various cement types within the reefs can be linked with the shallowing-upward nature of the limestone sequence as there is a predominance of early marine cement fabrics in the middle and upper parts of the reefs, particularly in the algal biofacies. The spatial and sequential nature of the various carbonate diagenetic fabrics show that the Hogklint reefs went through four main diagenetic events. Microstalactitic cements, vadose crystal silts, grain-contact cements, and pseudofibrous isopachous cements are developed within cross-bedded peloidal grainstones at the top of the shallowing-upward sequence and indicate cementation within the freshwater vadose zone and intertidal and/or marine phreatic zone. End_of_Article - Last_Page 641------------