Shallowing-upward Sequences Research Articles

Tempestites recorded as variable Pentamerus layers in the Lower Silurian of southern Norway

The brachiopod Pentamerus oblongus is especially abundant in the Lower Silurian Rytteråker Formation, which occurs widely throughout the Oslo region in southern Norway. Alternating thin limestones and shales typically occur in a shallowing-upward sequence ending in a massive grainstone often rich in tabulate corals and stromatoporoids. In the lower beds, colonization by pentamerid populations was enhanched by prior emplacement of an orthotetacean shell pavement over the clastic sea bed. Individual Pentamerus size decreases with increasing disturbance of populations or their fragmentation as lag deposits nearer the stratigraphic level of the massive grainstone. The frequency of specific storm events and the changing depth to the sea bed may be indexed according to the average size of pentamerid shells in a given population and the degree to which the population is disturbed. Preservation of prominent growth lines (assumed to be annual) suggests storms stirred the sea bed in deeper waters on an 8–10 year cycle. In shallower waters, storms more regularly disturbed the sea bed every 2–3 years. These data corroborate the interpretation that the Pentamerus community inhabited marine waters below fair-weather wave base, but still within reach of storm wave base.

Sedimentology and diagenesis of Upper Mississippian to Lower Permian strata, Talbot Lake area, Jasper National Park, Alberta

In the Talbot Lake area, the Upper Mississippian carbonate sequence comprises, in ascending order, the Turner Valley and Mount Head formations. The Mount Head Formation is informally divided into lower and upper members. The rocks of the Upper Mississippian originated by sedimentation associated with successive shallowing-upward sequences. On the basis of the fossils, lithologies, textural characters, and sedimentary structures, the Turner Valley Formation apparently resulted from sedimentation on open marine shoals and in a lagoon, the lower member of the Mount Head Formation from sedimentation in shallow subtidal to lower supratidal environments, and the upper member of the Mount Head Formation from deposition in upper intertidal to supratidal environments.The paraconformity between the lower and upper members of the Mount Head Formation records a short erosional break. A disconformity between the Upper Mississippian carbonates and the Upper Permian Ishbel Group displays paleotopographic relief and paleokarstic features developed during exposure in post-Mississippian times. The disconformity is overlain by a condensed sequence of Pennsylvanian to Permian strata.Major diagenetic modifications of the Upper Mississippian strata include dolomitization, silica replacement and cementation, carbonate cementation, and dedolomitization. A diagenetic realm comprising sea water, fresh water, and hypersaline brine was probably responsible for some of the complex diagenesis in these rocks.

Depositional Environments and Porosity Development, Strawn Formation (Middle Pennsylvanian), Wagon Wheel and HSA (Penn) Fields, West Texas: ABSTRACT

The Wagon Wheel and HSA (Penn) fields are located on the western flank of the Central Basin platform within the Permian basin. In this area, both siliciclastic and carbonate depositional systems controlled sedimentation during Strawn time. To the east (HSA Penn field) siliciclastic fanglomerate sedimentation dominated. Several fanglomerate lobes, sourced from the Sand Hills uplift to the east, prograded westward across the platform. Interlobe swamps were common along this strand-line setting. Approximately 3 mi westward (Wagon Wheel Penn field), carbonate bank deposition dominated. Siliciclastic sands and clays commonly are intermixed within these carbonates and represent distal reaches of the siliciclastic progradational lobes. Conglomerates occur more rarely within the carbonate sequence. Carbonates from the upper Strawn (HSA 1392-P well, Wagon Wheel Penn field) demonstrate five shallowing-upward sequences. Each sequence consists of a lower subtidal phylloid algal wackestone which shallows upward into a grainstone shoal deposited above effective wave base. Transgression was rapid and marked by the subtidal phylloid algal wackestone facies overlying the grainstone shoal facies at the top of the previous sequence. Where present, chaetetids punctuate the contact between sequences. Porosity occurs as interparticle voids within the nearshore fanglomerate lobes and as leached biomolds in the grainstone shoals of themore » carbonate bank. Phylloid algal-rich facies are tight. Porosity is adversely affected by clastic pulses within the grainstone shoal facies and by late-stage ferroan calcite cement.« less

The early Carboniferous (Courceyan) Middle Hope volcanics of Weston-super-Mare: development and demise of an offshore volcanic high

The Lower Carboniferous (Courceyan), Middle Hope Volcanics of Weston-super-Mare form a coarsening- and shallowing-upward sequence which records the development of a localised, probably non-emergent, offshore volcanic-high above the outer carbonate-ramp deposits of the upper part of the Black Rock Limestone. The succession consists of interbedded pyroclastics, pillow lavas and limestones representing deposition from below storm wave-base to above fairweather wave-base. A study of the different sedimentary facies developed, down the slope of the volcanic apron, revealed volcanic updoming to have been an important control in the development of the buildup. Its eventual submergence to beneath mean wave-base is recorded by the overlying, erosive-based cross-bedded bioclastic limestone which forms a fining- and deepening-upward sequence.

Dolomitization as Control on Reservoir Quality--Case Study: Block III, Lake Maracaibo, Venezuela: ABSTRACT

Paleocene carbonate sequences in Block III, Lake Maracaibo, have always been neglected, even though similar sequences such as those reported in the Smackover Formation, Gulf of Mexico coastal plain, USA, have shown great hydrocarbon potential. Core study in well VLC-950 shows that the Paleocene carbonate sequence consists of four oolitic facies: skeletal-oolitic rudstones (C1), oolitic packstones/grainstones(C2), oolitic grainstones (C3), and sandy oolitic grainstones (C4), all deposited in a prospective shallowing-upward carbonate sand sequence. Diagenetic processes such as micritization, calcite cementation, solution, dolomitization, and compaction have modified the original textures of these facies. It seems that the dolomitization has been the most important in controlling effective porosity. Dolomitization has occurred in the whole sequence. In C1, it is represented by isolated, fine, euhedral crystals. In C2, dolomitization has selectively replaced the oolites with a mosaic of coarse, anhedral crystals. In C3, dolomitization has selectively replaced the calcite cement and to a much lesser extent the oolites. Oolites not dolomitized have been selectively leached, resulting in well-developed oomoldic porosity that forms the reservoirs. In C4, dolomitization is minor, and it is represented by isolated, very fine crystals. Petrophysical analysis revealed that in C3 the porosity value is 19%, permeability is 1.21 md,more » and oil saturation is 80%; in C1, C2, and C4, porosity averages 4.3%, permeability is 0.06 md, and oil saturation is negligible.« less

Depositional mechanisms of storm sedimentation in the Triassic Dürrenstein Formation, Dolomites, Italy

The Dürrenstein Formation (Triassic, Italy) represents an overall shallowing-upward sequence formed by vertical accretion in response to a long-term sea-level stillstand. The investigated upper part of the Dürrenstein Formation represents an inner and an outer shelf. The lithologic composition and sedimentary structures of the inner shelf (poorly sorted onco-intrasparites, greenish pelites and dismicrites) reflect a deposition under a low sedimentation rate, above fair-weather base, interrupted by significant sediment reworking during storms. The outer shelf consists of poorly sorted bioclastic calcarenites. Hummocky cross-bedding and channels typically occur in this facies which formed below fair-weather base. Sedimentary structures indicate a deposition by high-energy storms, responsible for an offshore transport of sediment. The directions of storm currents inferred from paleocurrent measurements (NW-SE; NE-SW), interpreted respectively as an alongshore and an offshore-directed storm current, fit the basic storm pattern described by Swift et al. (1983). The offshore-directed secondary flow in the bottom layer of storm current formed storm-cut gutters, probably as a result of a helical flow pattern. A fining-upward and a coarsening-upward sequence have been recognized. The fining-upward sequence which occurs in the lower portions of the measured stratigraphic sections formed in the outer shelf in response to offshore storm currents. The coarsening-upward sequence occurs in the upper parts of the measured sections and often directly overlies the fining-upward sequence. It represents a deposition under fair-weather conditions in the inner shelf. Vertical relationships between sequences and lithologies are in accordance with a gradual infilling of the basin by vertical aggradation.

Nolley Wolfcamp Unit, Andrews and Gaines Counties, Texas: Classic Shallowing-Upward Sequence: ABSTRACT

Nolley Wolfcamp Unit, Andrews and Gaines Counties, Texas: Classic Shallowing-Upward Sequence: ABSTRACT

Glacially- and tidally-influenced shallow marine sedimentation of the late Precambrian Port Askaig Formation, Scotland

The late Precambrian Port Askaig Formation forms part of the thick succession of metasediments and volcanics of the Dalradian Supergroup of northwest Britain. The glacianegnic Port Askaig Formation is well exposed on the Garvellach Islands in western Scotland and consists of a series of thick and regionally extensive diamictites interbedded with sandstones, siltstones, dolomites and minor conglomerates. Diamictite facies are similar to glaciomarine diamicts described from the Late Cenozoic Yakataga Formation in the Gulf of Alaska, formed by settling of fine-grained suspended sediment and ice-rafting in relatively shallow water (< 250 m) . Two particularly distinctive diamictite units, the Great Breccia and the Disrupted Beds are interpreted as the product of downslope resedimentation events; it is significant that the Port Askaig Formation was deposited during a period of increasing tectonic activity in the Dalradian Basin. Thick and extensive sand sequences interbedded with the diamictite facies are interpreted as tidally influenced shallow marine shelf deposits; sands may have been derived from deltaic sources on the basin margin and transported into the basin during storms. The entire Port Askaig Formation appears to represent an overall shallowing upwards sequence punctuated by smaller scale transgressive (diamictite) and regressive (sandstone) events. Interpretation of the Port Askaig Formation as a succession of glacially and tidally-influenced shelf sediments has significant implications for the reconstruction of palaeoclimatic histories and regional palaeogeographies during late Precambrian time.

Interfingering of evaporites and red beds: An example from the Queen/Grayburg formation, Texas

Interbedded red sandstone, siltstone, mudstone, anhydrite and halite comprise the Queen/Grayburg formation (Guadalupian) of the Palo Duro Basin. These deposits accumulated in a 28,000 km 2 epicontinental basin that was characterized by regional aridity and episodic, nearly basinwide influx of marine-derived hypersaline waters. In each of several recurrent progradational cycles a variety of environments developed in this basin including eolian dunes, interdune areas, sand sheets, saline mud flats, and broad, shallow expanses of marine-derived brine (salinas). Siliciclastic sources were in the west and northwest whereas brines were derived from the south and southwest. Eolian dune deposits are represented by medium- to high-angle (15–35°) cross-stratified, fine-grained sandstone. Interdune areas accumulated flat- to low-angle bedded, wind-rippled strata, which are moderately to well sorted, fine-grained sand with frosted grains and illuviation structures. Sand sheets are characterized by deposits of horizontally bedded, rippled to massive, well-sorted, very fine-grained sandstone with illuviation structures. Sheetwash deposition on sand sheets and mud flats produced sandstones and siltstones with graded bedding, ripples with mud drapes, and intraclasts, and also flaser-bedded siltstones and mudstones deformed internally probably by haloturbation. Saline mud flats produced admixtures of mud and coarsely crystalline halite (chaotic halite-mudstone) and were located on the margins of regionally extensive hypersaline environments (salinas) in which bedded anhydrite and halite were precipitated subaqueously. Queen/Grayburg facies are cyclic and occur in characteristic vertical sequences that comprise, from base to top, bedded anhydrite, bedded halite, chaotic halite-mudstone, fine-grained clastics with displacive halite, siltstone and mudstone with deformed ripples, and eolian clastics. The sharp contacts between these shallowing-upward sequences suggest that transgressions were rapid.

Formation of illitic minerals at surface temperatures in Purbeckian sediments (Lower Berriasian, Swiss and French Jura)

AbstractThe clay-mineralogical assemblages of Purbeckian carbonate sediments of the Swiss and French Jura Mountains are often composed of illite and interstratified illite-smectite. These illitic minerals occur mainly in thin layers of green marls which show evidence of subaerial exposure and mark the top of shallowing-upward sequences. X-ray diffraction, chemical and thermal analyses coupled with transmission electron microscopy suggest that the Purbeckian illitic minerals replaced smectite in intermediate continental-marine environments. The transition from smectite to illite and interstratified illite-smectite probably resulted from repeated cycles of wetting by marine waters and subsequent drying in hypersaline environments, under a hot Purbeckian climate.

Shoreline Depositional Environments of Glen Rose Formation (Lower Cretaceous) in Type Area, Somervell and Hood Counties, Texas: ABSTRACT

ABSTRACT The Comanchean Series in north-central Texas was deposited along the margin of a shallow sea as it transgressed northwestward across the Central Texas platform during early Cretaceous time. A lithologic and paleontologic study of the Glen Rose Formation (uppermost Trinity Division) in its type area was undertaken to define the shoreline sediments deposited during Aptian-Albian time. The studied interval was subdivided into informal units A, B, and C based on lithology and fossil content. The primary focus was on the lowermost Unit A for purposes of facies analysis. Unit A comprises a clastic-to-carbonate transition deposited in a seaward-fining tidal-flat salt-marsh complex. Eight distinct facies were identified, including the calcareous sandstone, calcareous shale, bivalve shale, oyster shale, transitional terrigenous-carbonate, dolomite, bioclastic packstone-grainstone, and shell fragment wackestone facies. These facies were deposited in sand flats; small fluvial creeks; an ecologically complex, mud-dominated intertidal flat; and an subtidal, nearshore lagoon. Thin bioclastic packstone-grainstones and dolomites of the high energy intertidal and supratidal environments regularly interrupt the terrigenous clastic facies. The microfossils are severely restricted to absent, and the macrofossils are dominated by euryhaline bivalves (including Corbula martinae), gastropods, serpulid worm colonies, and dasychladacean algae. Other abundant nearshore features include salt-tolerant land plants, dinosaur tracks, desiccation cracks, and localized concentrations of gypsum and barite. This tidal flat-salt marsh complex received sediments from both terrestrial and marine sources. A broad intertidal zone of vegetated sand and mud flats was dissected landward by small fluvial creeks and seaward by storm beaches and marine inlets. The fine size of the terrigenous clastics indicates a low energy flat with a very gently seaward-dipping substrate. A warm, semi-arid to arid climate favored the development of salt marshes and the precipitation of evaporites. The faunal restriction also suggests the abnormal salinites of tidal marshes. this sequence is considered ocmparable to the Holocene salt marshes on Sapelo Island, Georgia, based on similar seaward-fining substrates, sedimentary structures, and biological components. Unit B (the Thorp Spring Member) consists of a peloidal, molluscan packstone-wackestone facies. The fossil assemblage (including Orbitolina texana) suggests slightly brackish to near normal marine salinites in a shallow, restricted platform lagoon. Thorough bioturbation indicates deposition in the well aerated subtidal zone. Unit C is characterized by coarsening-upward limestones capped with thin intertidal-supratidal deposits, indicated a transitional nearshor lagoon-tidal flat environment. Repeated inundation with marine waters produced stacked, shallowing-upward sequences. The entire studied interval reflects fluctuating sea levels along a line of section nearly normal to the paleoshoreline, and represents the final waning of Glen Rose seas. End_of_Record - Last_Page 588-------

Facies Control on Oil Producing, Naturally Fractured Reservoirs, A Case Study: Block I, Lake Maracaibo, Venezuela: ABSTRACT

In Block I, Lake Maracaibo, oil producing intervals in the Cretaceous have been found to be facies controlled. Detailed studies of the cored sections in various wells have shown that facies Ls2, characterized by an oolitic grainstone, is the main producing reservoir. This facies has been interpreted as deposited as an oolitic shoal in a shallowing-upward sequence. Other facies such as Ls1, represented by a bioclastic rudstone, and Ls, composed of a mudstone, are also present but do not show significant oil production. The presence of subvertical, open, hairline fractures and centimeter-scaled wide, bridged fractures in facies Ls2 can explain its great producing capacity, while the presence of calcite mineralized fractures in Ls1 and absence of fractures in Ls account for the poor producing capability of these facies. In contrast to what has been reported in other case studies, the grainstone facies shows the most fracture density and open fractures. A facies model which predicts the lateral continuity of the facies present in Block I is proposed as a tool for oil exploration and exploitation for this area and other areas with similar sedimentological characteristics.

Depositional Environments and Hydrocarbon Potential of Copper Ridge Dolomite in Union County, Tennessee: ABSTRACT

The Upper Cambrian Copper Ridge Dolomite in Union County, Tennessee, consists of subtidal, intertidal, and supratidal facies that formed in a hypersaline setting. Subtidal facies are dark and thrombolitic; intertidal facies contain thick, planar to wavy laminations, and may be cross-stratified. Supratidal facies are light colored, mud cracked, and thinly laminated. Algal stromatolites, thought to comprise up to 60% of the unit, were not found to be a major rock type in the area studied. Facies in the unit occur as cyclic, shallowing-upward sequences. The lower member consists of complete cycles, whereas the upper member consists of partial cycles. Cycles are a result of low-amplitude sea level oscillations with long oscillation periods and slow rates of subsidence. Partial cycles lack subtidal facies and were probably caused by tidal flat progradation. Nearly all cement in the unit consists of white sparry (saddle) dolomite, and occurs in vugs, fractures, and interparticle voids. White sparry dolomite displays cathodoluminescence that consists of dark to bright to dull zones. This sequence suggests that cementation occurred during deep burial of the unit. Total organic carbon content of carbonate rocks in the unit indicates that the rocks are not a potential hydrocarbon source. Porous zones with upmore » to 12% porosity occur in thrombolitic rocks and packstones to grainstones.« less

Lower and middle Guadalupian Shelf Carbonates, Eastern Margin of Central Basin Platform, Permian Basin, West Texas: ABSTRACT

Lower and middle Guadalupian shelf carbonates serve as the reservoir for a nearly continuous band of oil fields extending 100 mi along the eastern margin of the Central Basin platform of west Texas. Approximately 5 billion bbl of oil have been produced from stratigraphic-structural traps within the Upper Permian (Gaudalupian Series) dolomites of the San Andrea and Grayburg Formations in Upton, Crane, Ector, Pecos, and Andrews Counties, Texas. The San Andrea and Grayburg Formations are cyclical shallowing-upward carbonate sequences of open shelf through sabkha facies whose depositional strike parallels the eastern margin of the Central Basin platform. Porosity and permeability of reservoir rock are governed by diagenetic processes such as dolomitization, anhydrite porosity occlusion, leaching, silicification, and authigenic clay formation. Self sediments are primarily burrowed wackestones and packstones that locally contain pelletal, skeletal, and ooid grainstones. Typical subtidal shelf sediments are capped by algal-laminated dolomite, nodular anhydritic dolomite, and bedded anhydrite. The fauna is normally sparse and dominated by foraminifera and algae. Less common faunal components include pelecypods, crinoids, sponges, Bryozoa, brachiopods, gastropods, and coral that are associated with the development of small scattered patch reefs. Lowering the sea level during the early Guadalpian initiated basinward progradation of San Andresmore » carbonate facies with hydrocarbon reservoirs best developed in shallow self fusulinid wackestones to packstone and oolitic grainstone. Reservoir dolomites of the Grayburg formation are present east of San Andres fields with optimal reservoir properties occurring near the San Andreas outer shelf margin.« less

McElroy Field, Permian Basin, West Texas: Cyclic Sequence Dolomite Reservoir of Central Basin Platform: ABSTRACT

The McElroy field, located in Crane and Upton Countries of west Texas, is one of several carbonate reservoirs developed along the eastern margin of the Central Basin platform. The reservoir rock at McElroy consists of Permian System (Guadalupian Series) dolomite of the Grayburg Formation. The Grayburg dolomite and the underlying San Andres Formation were formed as shallowing-upward carbonate sequences. The carbonate facies are overlain by the Queen Formation, an evaporite-dominated unit that serves as the reservoir seal. The Grayburg Formation carbonates consist of dolomitized lime mudstones, wackestones, pelletal and skeletal packstones, and ooid grainstones deposited in subtidal to supratidal environments. Facies prograde eastward toward the Midland basin, suggesting relative sea level lowering or progressive shallowing. The westward limit of the McElroy field is determined by pinch out of reservoir carbonates that interfinger with supratidal nodular and bedded anhydrite. The reservoir petrophysical characteristics are defined by diagenetic overprinting on primary carbonate depositional fabrics. The principal diagenetic processes include dolomitization, anhydrite occlusion of primary porosity, and leaching. McElroy, which was discovered in 1926, produces approximately 13,500 BOPD. This Chevron-operated field has 2.2 billion bbl of oil in place; 329 million bbl have been produced from nearly 1,600 wells. The field is currentlymore » under waterflood and is being evaluated for potential CO/sub 2/ injection.« less

Structural History and Reservoir Characteristics (Mississippian) of Nesson Anticline, North Dakota: ABSTRACT

Nesson anticline is the largest hydrocarbon productive structure in the North Dakota portion of the Williston basin. Nesson anticline was discovered in 1951, just a few months after discovery of oil in the Williston basin. Fifty-four fields, producing from 14 lower to middle Paleozoic formations, are scattered along the north-south length of the anticline. Nesson anticline fields have produced a total of 377 million bbl of oil, with the Madison Group accounting for two-thirds of the total production. Central southern parts of the anticline were subdivided into nine areas that revealed episodic and independent structural movement since the late Precambrian. All Phanerozoic periods are present within the stratigraphic section. Unconformity-bound, major tectonic-eustatic sequences were mapped along the length of the Nesson anticline, and sedimentary tectonics were documented for the entire Phanerozoic. Greatest amounts of tectonic development of the anticline were during the Devonian to Early Mississippian. Post-Greenhorn, Laramide tectonism was responsible for the last major structural deformation of the anticline. Selected oil fields, productive from the Madison Group, were studied where they are productive from the Mission Canyon Formation and the Rival (Nesson) subinterval. These intervals record sediment infill of a slowly shrinking epeiric sea, as a series of shorelines more » separated by brief transgressions prograded toward the center of the basin. The Mission Canyon Formation can be characterized as a major shallowing-upward sequence, which upsection is (1) shallow open marine, (2) transitional open to restricted marine, (3) restricted marine, and (4) fringing marginal marine. Barrier island and shoreline buildup complexes developed along the shoreline, with bedded evaporites located away from the anticline toward the east and south. « less

Tidal and deltaic controls on carbonate platforms: Glen Dean Formation (Upper Mississippian) of Illinois Basin, U.S.A.

About 1200 vertically oriented carbonate, sandstone and shale samples were collected from twenty-three stratigraphic sections of the Glen Dean Formation in the Illinois Basin. Detailed petrographic investigation using the microfacies approach has led to the recognition of sixteen distinct carbonate microfacies. Their vertical succession and reciprocal relationships in the different sections and their correlation coefficients led to the recognition of an ideal shallowing-upward sequence in turn converted into a horizontal depositional model. The model is a shallow open-marine subtidal carbonate platform consisting of the juxtaposition of two broad banks. The following environments from offshore to onshore were recognized: lower slope, upper slope, outer bank with three sub-bank environments (foraminiferal, crinoidal, and oncolitic), inter bank, inner bank with three sub-bank environments (bryozoan, oncolitic, and oolitic), lagoon. The carbonates display a complex diagenetic sequence with several pathways which includes the following environments: stagnant and active marine phreatic, freshwater vadose, undersaturated and saturated freshwater phreatic, mixing freshwater-marine phreatic, and burial. The petrographic analysis of the siliciclastic rocks led to the recognition of five microfacies: siltstone, quartz arenite, quartz arenite with argillaceous matrix, quartz wacke and shale. A depositional model was developed which consisted of a deltaic complex periodically prograding over the eastern shelf of the Illinois Basin. Consequently, three distinct environments were juxtaposed: delta system, carbonate platform and basin. At times sands and muds cut across the carbonate platform by means of channel systems and were deposited in the basin by gravity flows. The clay mineral assemblage of the shales from the deltaic fringe consists of discrete illite, kaolinite or chlorite, and mixed layer illite/smectite. Kaolinite and chlorite display a north-south zonation due to their physical segregation in the depositional environment with kaolinite in more shoreward and chlorite in more marine environment. The illite/smectite composition ranges from 50 to 85% illite and from 15 to 50% smectite. Its mixed layered structure is short range ordered and/or random. These samples mainly reflect the effects of provenance. The clay mineral assemblage of the shales from the central part of the basin contains long range ordered illite/smectite with the composition of 90–95% illite and 5–10% smectite. These samples mainly reflect the effects of thermal diagenesis due to burial.

The stratigraphy of the Lower Ordovician St. George Group, western Newfoundland: the interaction between eustasy and tectonics

The St. George Group is a ~500 m thick sequence of carbonate rock that accumulated during Early and early Middle Ordovician time in a series of shallow subtidal and peritidal environments near the outer edge of a low-latitude continental margin. Lithological variations, in the form of two megacycles, reflect deposition in response to eustatic fluctuations in sea level preceding and during the early stages of Taconic orogenesis.Strata are grouped into four formations of roughly equal thickness. The newly named basal Watts Bight Formation is a lower sequence of peritidal limestones and dolostones and an upper thicker, commonly dolomitized succession of burrowed carbonates distinguished by large digitate thrombolite mounds. The overlying Boat Harbour Formation (new) is a series of muddy, peritidal, shallowing-upward sequences of limestone and dolostone. A widespread subaerial disconformity near the top of the formation, reflecting eustaic sea-level fall and the end of the first megacycle, is marked by breccia, quartz-pebble conglomerate, paleokarst, and (or) extensive dolomitization and is succeeded by higher energy peritidal limestones called the Barbace Cove Member (new). The succeeding, thick, monotonous Catoche Formation (revised) is a succession of fossiliferous subtidal limestones with scattered thrombolite mounds whose upper part is locally affected by extensive, multigeneration dolomitization and Pb–Zn mineralization. The St. George Group is capped by the newly defined Aguathuna Formation, a stack of peritidal dolostones and minor limestones and shales deposited during a period of repeated exposure and synsedimentary faulting. An erosional disconformity, resulting from regional compressional tectonics and eustatic sea-level fall, locally marks the top of the St. George and the second megacycle.

Evolution of Early to Middle Cambrian Carbonate Platform, Southwest Virginia Appalachians: ABSTRACT

Three core transects through the Early to Middle Cambrian continental shelf margin (Shady-Rome sequence) allow its evolution to be established. The sequence was initiated on a drowned clastic shelf to form a ramp with downslope mud mounds. Overall subsidence rates are estimated to be 5-10 cm/1000 years. Mud mounds consist of rare Archaeocyathid reefs that cap stromatactis MS/WS core facies and form upper parts of shallowing-upward sequences whose basal portion is shale and nodular bedded limestone. Individual mounds (1-30 m thick) are laterally extensive; some are over 3.5 km wide. Depositional slopes on the flanks range from 5.0/sup 0/. Three periods of backstepping of the mud mound sequences resulted from sea level fluctuations (average period approx. = 2 m.y.). Following the second backstep, regressive shelf sands and basinal turbidites formed. Later transgression led to development of mud mounds and patch reefs above the sands. Sea level fall during Rome regression caused widespread deposition of cyclic red beds and carbonates on the slowly subsiding shelf interior. On the more rapidly subsiding outer platform where subsidence rates exceeded sea level fall, an algal reef-rimmed shelf formed landward of earlier ramp buildups. Seaward of the shelf were thickmore » deposits of periplatform talus and foreslope sands. Small-scale shallowing-upward sequences record 10/sup 4/ to 10/sup 5/-year sea level fluctuations superimposed on longer term cycles. The platform was exposed during major regressions, resulting in meteoric diagenesis of the platform as well as the off-platform talus, which provided conduits for deeply circulating meteoric fluids.« less

Platform/Foreslope Facies and Buildup Geometry Resulting from Short-Term and Long-Term Eustatic Sea Level Fluctuations: Latemar Buildup (Middle Triassic), Dolomites, Northern Italy: ABSTRACT

Superimposed short-term and long-term eustatic sea level fluctuations directly controlled Latemar platform stratigraphy and indirectly influenced the deeper water facies and overall buildup geometry. Deeper water facies, the foreslope and toe of slope, are a function of platform submergence (highstand shedding) and subaerial exposure (lowstand lithification and erosion) and thus only indirectly reflect eustatic fluctuations. The Latemar consists of a platform core (3-4 km wide, 700 m thick) with a narrow margin, flanked by foreslope (30-35/sup 0/ dips), toe of slope, and basin deposits. The shallowing-upward platform sequence records a long-term (about 10 m.y.) eustatic sea level oscillation with an amplitude of about 150 m. The lower 250 m marks an initial catch-up phase (subtidal carbonates); the upper 450 m marks the sequential keep-up phase (meter-scale cyclic carbonates). These cycles record platform submergence and exposure caused by short-term (10/sup 4/-10/sup 5/ years) Milankovitch eustatic oscillations superimposed on the long-term trend. Platform submergence and exposure conditions result in contrasting foreslope deposits. During highstands, platform-derived sands bypass the foreslope, accumulating as toe-of-slope graded beds and basin turbidites. During lowstands, sand supply ceases, producing basin hard-grounds. Foreslope megabreccias contain margin-derived boundstone clasts, with only minor platform-derived sands (highstands) and lithified clasts (lowstands). Asmore » the platform margin/foreslope contact is nearly vertical, a progressively increasing volume of foreslope megabreccia was needed to maintain the depositional geometry. This coincides with the most commonly exposed platform interval, suggesting that platform exposure determines buildup flank geometry by controlling megabreccia clast production.« less