Platform Geometry Research Articles

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: depositional system, biostratigraphy and stratigraphic architecture

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: depositional system, biostratigraphy and stratigraphic architecture

Genesis and Dispersal of Carbonate Mud Relative to Late Quaternary Sea-Level Change Along a Distally-Steepened Carbonate Ramp (Northwestern Shelf, Western Australia)

Abstract Accumulation of carbonate mud is minor on the Northwest Shelf (Western Australia), a broad (< 200–300 km wide), distally steepened, tropical carbonate ramp. It is negligible along most of the inner ramp (0 to 50 m water depth) owing to siliciclastic (riverine) input. It is also negligible along the mid (from 50 to 120 m) ramp, and where the entire ramp narrows to < 100 km, owing to constant reworking by storm waves and ocean swell. In these areas, carbonate mud is derived mostly from mechanical degradation of skeletal fragments, but this does not include calcified remains of calcareous green algae (e.g., Halimeda) although these plants grow locally. Only along the outer (120 to 200 m water depth) ramp and slope is there any relatively abundant accumulation of carbonate mud. Here the sediment consists of modern pelagic (foraminifer, pteropod, and nannoplantkon) ooze and aragonitic needle-rich (< 2 μm) micrite, the latter having an age (based on AMS 14C) of ~ 19 ka. Distribution of aragonite micrite defines an extensive (5.0 × 104 km2), yet relatively narrow (< 40 km), tract running ~ 600 km along the platform margin. Crystal morphology, mineralogy, and isotope (C, O) composition identify the micrite to be a seawater precipitate, its production and seaward export having occurred during the Last Glacial Maximum, when lowered sea level transformed the otherwise broad platform into a narrow epicontinental shelf margin, now the modern distal slope. Today, this sediment body lies stranded in deep water, about 200–300 km seaward of the shallow inner ramp. The volume of micrite produced appears equal to the present-day production potential of the Great Bahama Bank. Rapid postglacial rise in sea level, and change in platform geometry, climate, and oceanography, were very influential factors, in addition to water depth and area of submergence, in the production and distribution of aragonite micrite along the Northwest Shelf. In general: (1) epicontinental carbonate platforms can lack substantial micrite accumulation when oceanic current and wave energies sweep unimpeded across a platform, and coastal clastics and absence of shoal barriers prevent development of protective lagoons and carbonate tidal flats; (2) not all highstand carbonate systems are associated with significant amounts of micrite in shallow water environments, nor do they necessarily export large volumes of micrite to the periplatform realm; and (3) some lowstand systems can produce and export volumes of carbonate mud that rival highstand systems.

Three‐dimensional modelling of an isolated carbonate buildup (Triassic, Dolomites, Italy)

AbstractThe Middle Triassic carbonate buildups of the Dolomites show facies similarities with mud mounds but display apparent architectural elements of flat‐topped carbonate platforms. In order to test whether the facies similarities to mounds are also reflected in the internal buildup architecture, a three‐dimensional modelling study of the Middle Triassic Monte Cernera buildup has been carried out. The Cernera buildup exhibits apparent geometries suggesting a mounded platform in the lower and uppermost part of the buildup, separated by an interval with apparent platform geometry and a retrogradational platform interior, which is difficult to explain with a flat top platform model. For this purpose, a number of three‐dimensional models were constructed using the three‐dimensional modelling programme petrelTM. Key geological horizons were constructed based on outcrop measurements, intermediate horizons were calculated in the modelling program, and the intersections of the modelled layers with a digital topography surface were displayed and compared with outcrop photographs. The models were refined stepwise until a best fit with the actual bedding architecture was achieved. The best fit model shows that the mounded geometries in the lower and uppermost part of the buildup are real architectural elements. The intermediate platform stage, about 1·5 km across, had probably retained a mounded top with a relief of up to 50 m, which is difficult to distinguish from an absolutely flat top, but necessary to explain the retrogradational platform interior. The present study shows that Monte Cernera was dominated by mounded geometries at all stages of platform development. The mounded geometry plus facies data suggest that the platform is a deep‐water accumulation, below the zone of intense wave energy, but within the photic zone. The Cernera represents a tropical buildup type, which did not have the capacity to grow into continuously wave‐swept environments because of the small size and the absence of a wave‐resistant energy barrier. Such buildup types are probably common after major crises in earth history, when reef organisms were virtually absent.

Dolomitization: from conceptual to numerical models

Abstract Dolomitization requires not only favourable thermodynamic and kinetic conditions, but also a fluid-flow mechanism to transport reactants to and products from the site of dolomitization. This paper reviews work that seeks to provide a quantitative framework for conceptual models of dolomitization, using analytical and, particularly, numerical simulation models of fluid flow and rock-water interaction. This approach is starting to yield new insights into the major controls on the rate and pattern of fluid flux, and the resultant dolomitization. Three sets of forces can drive the fluid flow required for dolomitization: elevation (topographic) head of meteoric water and/or seawater; gradients in fluid density due to variation in salinity and/or temperature; and pressure due to sedimentological and/or tectonic compaction. However, in many situations individual flow mechanisms may not operate in isolation. Rather fluid flow will commonly be a product of a number of different drives acting simultaneously. The balance between drives will change over time with variations in relative sea-level, climate, platform geometry and palaeogeography (which collectively comprise the critical boundary conditions). The simplistic prediction of dolomite body geometry from a single driving force may be misleading, as fluid flow will critically depend both on the boundary conditions and the distribution of permeability. Indeed, even for single driving forces, model predictions change significantly as simplistic assumptions are relaxed and these key parameters are specified with increasing realism. The coupled modelling of dolomitization reactions within the flow field is less tractable than that of groundwater circulation because the kinetics of dolomitization are less well understood, particularly at lower temperatures. Dolomitization is likely to occur along a reaction front, where a favourable balance is struck between mass transport and reaction kinetics. For instance, in simulations of geothermal convection dolomitization focuses along the 50–60 °C isotherm. Dolomitization reactions are favoured by higher temperatures in deeper zones, but rates are limited by low flow because of lower permeability. Although flow rates are higher in shallow more permeable carbonates, lower temperatures limit reactions. High flow rates during reflux of platform-top brines give rapid dolomitization. This is associated with porosity occlusion in front of and behind the broad zone of replacement dolomitization driven by anhydrite cementation and overdolomitization, respectively. Lithological heterogeneities strongly affect the pattern of dolomitization, which is highly focused within more permeable beds and those with a higher reactive surface area. While we focus here on dolomitization, models can also provide insights into diagenetic processes such as marine calcite cementation and aragonite, calcite and evaporite dissolution by refluxing brines, and by seawater circulation below the aragonite and calcite compensation depths. However, it is important to be aware of the assumptions and limitations of the numerical model(s) used. Particular attention must be paid to specification of boundary conditions, permeability and reactive surface area. The uncritical application of numerical techniques to particular cases of dolomitization is at best uninformative and at worst misleading. Careful application of these techniques offers great promise for well-constrained field problems, with greater inclusion of natural heterogeneity and time-variant boundary conditions. We also need to model feedbacks between diagenesis and porosity-permeability, and to include platform growth in simulations of slower diagenetic processes.

Carbonate systems along nutrient and temperature gradients: some sedimentological and geochemical constraints

Research over the past several decades has clearly demonstrated that changes in the ocean environ- ment have had major impacts on carbonate systems. Changes in climate, ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry, including carbonate saturation state and nutri- ent availability, and thereby have determined boundary conditions for the biota that form carbonate platforms. In turn, the biota determine accumulation rates and facies zonations, thus controlling platform geometry and facies dynamics. In the first section of this paper, we examine how nutrient availability influences carbonate facies associations. We first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations. Then we discuss how carbonate sedimentation can be characterized along nutrient gradients. In the second section, we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems. We highlight the variety of existing proxies and their limitations, and suggest that multiple lines of evidence are needed for valid interpre- tations. Our short review discusses sedimentological, biogenic, and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleo- ceanographic controls over the distribution of carbonate associations. However, it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs. nutrient supplies changes and facies in ancient carbonate sequences.

Orthogonal gough-stewart platforms for micromanipulation

Development of methods to design optimal Gough-Stewart platform geometries capable of meeting desired specifications is of high interest. Computationally intensive methods have been used to treat this problem in various settings. This paper uses analytic methods to characterize all orthogonal Gough-Stewart platforms (OGSPs) and to study their properties over a small workspace. This characterization is used to design optimal OGSPs for precision applications that achieve a desired hyperellipsoid of velocities. Some examples demonstrating the versatility of this theory are discussed.

An integrated geophysical investigation of the upper crust in the epicentral area of the 1980, Ms=6.9, Irpinia earthquake (Southern Italy)

In this paper, we investigate the upper crustal structure of the Irpinia region, Southern Apennines thrust belt, Italy, through analysis and joint interpretation of gravity data, seismic reflection lines and subsurface information from many deep wells. The investigated region includes the epicentral area of the 1980 ( M s=6.9) Irpinia earthquake and is one of the Italian regions with the highest seismic hazard. The upper crustal structure is imaged by modeling a series of 15 SW-trending gravity profiles, spaced about 5 km apart, plentifully constrained by seismic reflection lines and wells, thus reducing the inherent ambiguity of the gravity modeling. Despite of the complexity of the modeled Bouguer anomalies, the application of a calibrating procedure to constrain the range of variability of the density values, as well as the use of geometric constraints, results in a good level of stability in the final density cross-sections, which in fact appear coherent both in the density values and in the geometrical features. The inferred model shows important lateral density variations that can be mostly related to NW-trending geologic structures. High-density bodies delineate carbonate platform thrust sheets and broad antiforms involving Mesozoic basinal rocks, while low-density shallow bodies are associated with Pliocene basins. In addition, important density (i.e. lithological) variations are evident along the strike of the range, the most relevant being an abrupt deepening of the Apulia Carbonate Platform in the southeastern part of the investigated region. In the epicentral region of the 1980 event, we find that the geometry of the high-density, high-velocity carbonates of the Apulia Platform appears correlated with the distribution of the aftershocks and with the P-wave velocity anomaly pattern as inferred from a previous local earthquake tomography. The structural highs of the Apulia Platform correspond to high-velocity regions, where aftershocks and coseismic slip of the mainshock are concentrated. This correlation suggests that the Apulia Carbonate Platform geometry played an important role in the rupture propagation and in the aftershock distribution.

Experimental setup for rapid crystallization using favoured chemical potential and hydrodynamic conditions

The rapid crystallization of KH2PO4 (KDP) from solution is demonstrated at a rate up to ≈ 7.5 mm/day along [100] and 22 mm/day along [001] in a crystallizer of 5 l capacity, using accelerated crucible rotation technique (ACRT) and simulated platform geometry for controlling the hydrodynamic conditions. On an experimental basis we have grown the crystals up to 40 · 43 · 66 mm3 size in about 3 days. Comparative analysis of the main structural and optical properties of crystals grown by conventional and rapid crystallization technique, is discussed.

Sequence organization and growth patterns of late Mesoproterozoic stromatolite reefs: an example from the Burovaya Formation, Turukhansk Uplift, Siberia

The latest Mesoproterozoic Burovaya Formation is a 600–1000 m thick stromatolite/mud dominated carbonate unit demarcated by an erosional unconformity into two subformations containing stromatolite reefs of variable size and geometry. The lower subformation consists of two distinct stratigraphic sequences. The lower sequence records shelf to ramp evolution and includes small-scale stromatolite reefs developed within an upper subtidal shelf settings at the transgressive systems tract and overlying mud-dominated highstand deposits accumulated in lower subtidal to peritidal environments. The second sequence of muddy and fine-grained sediments makes up a lowstand systems tract and documents a progressive structural and environmental differentiation of the Burovaya platform. The upper subformation records a third stratigraphic sequence, containing transgressive and highstand systems tracts represented by reef-bearing and reef-cap facies complexes. The reef-bearing complex characteristic of the transgressive and the bulk of the highstand systems tracts contains a giant (20–25 km wide, 10+ km long, up to 550 m thick) stromatolite reef and surrounding deep-water carbonate mudstones containing sporadic slump block deposits. The reef was located on the distally steepened part of the ramp and contains distinct internal facies organization defined by particular stromatolite microstructure and morphology and prevailing environments. Late highstand development of the Burovaya platform was marked by the termination of reef accretion, infilling of basinal relief by shallow-water carbonate and shale sediments of the cap complex, and subsequent subaerial exposure. The history of the Burovaya reef is indicative of the interplay between reef growth potential and platform geometry defined by regional tectonic and sea-level dynamics.

Examination of the implant—abutment interface after fatigue testing

Purpose. This study examined potential differences in detorque values of abutment screws after fatigue testing when the dimensions between external implant hexagon and internal abutment hexagon were altered or the implant external hexagonal shape was eliminated. Material and Methods. Three subsets (N = 10) of NobelBiocare implants were assessed: (1) standard external hexagon (R), (2) modified hexagon (M), and (3) circular (C) platform geometry. Thirty Procera machined abutments with 25-degree angulated loading platforms were manufactured. Abutments were retained with gold Unigrip abutment screws tightened to 32 N/cm with an electronic torque controller. Vertical scribes across the implant—abutment interface allowed longitudinal displacement evaluation. A carousel-type fatigue testing device delivered dynamic loading forces between 20 and 200 N for 5,000,000 cycles, or the approximate equivalent of 5 years in vivo mastication, through a piston to the abutment platform. Macroscopic and radiographic examination of the implant/abutment specimens was performed. The abutment screws were removed and the detorque values recorded. Bearing surfaces were examined microscopically. Results. No abutment looseness or longitudinal displacements at the implant—abutment interface were noted. Radiographic examination demonstrated no indication of screw bending or displacement. The mean detorque values for R, M, and C were 14.40 ± 1.84 N/cm, 14.70 ± 1.89 N/cm, and 16.40 ± 2.17 N/cm, respectively. The analysis of variance demonstrated significant differences between only designs R and C (P=.031). Conclusion. Increasing the vertical height, or degree of fit tolerance, between the implant external hexagon and the abutment internal hexagon or completely eliminating the implant external hexagon did not produce a significant effect on the detorque values of the abutment screws after 5,000,000 cycles in fatigue testing, or the equivalent of 5 years' of mastication for the implant/abutment specimens evaluated. (J Prosthet Dent 2001;85:268-75.)

Abstract: The effects of extensional tectonics on shallow water carbonate platform facies and geometry: the case of the Upper Triassic Dolomia Principale, Carnian Prealps, northern Italy

Abstract: The effects of extensional tectonics on shallow water carbonate platform facies and geometry: the case of the Upper Triassic Dolomia Principale, Carnian Prealps, northern Italy

Optimization of Six-Degrees-of-Freedom Motion Systems for Flight Simulators

The cueing capabilities of a synergistic e ight-simulator motion system are limited primarily by the maximum translationaland rotational travelallowed by themotion-base.Thistravelcapability,also known astheworkspace, is dictated by the kinematic layout of the motion system. Furthermore, the Jacobian matrix, which maps velocities from platform space to joint space, indicates the dexterity of the mechanism, or the mechanical effort needed by theactuatorsto movetheplatform. To systematically design unconventionalmotion-bases, a methodology hasbeen developed to analyze arbitrary six-degrees-of-freedom motion systems. The approach is based on an optimization programtodeterminetheoptimallayoutofthemotionsystem,giventheworkspaceperformanceobjectivesandthe design constraints. This allows the investigation of unconventional platform geometries and actuator attachment points, thus allowing the designer to tailor the workspace as required by the simulation task, to ensure that a satisfactory dexterity is maintained, and to guarantee that the actuator legs do not interfere mechanically. This paper describes the proposed methodology, and shows examples of its applications, e rst to generic workspaces, and then to the workspace required for the simulation of a large transport aircraft.

Quantifying rates of syndepositional marine cementation in deeper platform environments-new insight into a fundamental process

ABSTRACT Syndepositional marine cementation on carbonate platforms is a fundamental process that results in physical stabilization and lithification of sediments, as well as reduction in primary porosity and permeability which may affect the migration of later diagenetic fluids. Rapid submarine cementation is commonly invoked to explain various aspects of carbonate platform geometry, especially related to the preservation of steep slopes, but the rates typically reported for cementation in deeper parts of the platform are mostly conjectural. In an attempt to quantify the rates of syndepositional cementation in marginal platform environments, substrates containing samples of carbonate sand (ooids) suspended above the sea floor were emplaced at various depths across a modern platform in the Bahamas to evaluate the rates of cement growth through direct observation. Initial results indicate that cementation may lead to partial lithification of carbonate sands within a minimum of 8 months in water depths of up to 60 m. These results provide some of the first direct evidence for rates at which cementation can occur in deeper marginal environments, and confirms earlier hypotheses that very rapid, syndepositional marine cementation may not be limited to very shallow subtidal and intertidal environments.

Three-dimensional computations of solution hydrodynamics during the growth of potassium dihydrogen phosphate: II. Spin down

Three-dimensional, time-dependent flows that occur in the Lawrence Livermore National Laboratory system for rapid growth of potassium dihydrogen phosphate (KDP) crystals from solution are studied using massively parallel finite element computations. The simulation reveals that excellent global mixing occurs during the spin-down phase of a time-dependent stirring cycle. The large scale fluid motions in the radial and axial directions that promote mixing are caused primarily by effects of platform geometry, but are augmented to some degree by the intrinsic tendency of a decelerating rotational flow to reverse direction within Ekman layers that form at the boundaries. Along with Part I of this work [Y. Zhou and J.J. Derby, J. Crystal Growth 180 (1997) 497], which emphasized spin up and steady rotation, significant advances have been made in our understanding of hydrodynamic phenomena in this system.

Airborne electromagnetic systems - 50 years of development

The initial successful test flights of the Stanmac-McPhar fixed-wing airborne EM (AEM) system in Canada during the summer of 1948 can nominally be called the birth of this branch of exploration geophysics. The discovery of the Heath Steele deposit in New Brunswick, Canada in 1954, as a result of an AEM survey, proved to be the catalyst for the development of additional AEM systems and the eventual application of AEM surveys worldwide.

Carbonate ramps, 1973–1996: a historical review

Abstract The carbonate ramp as a type of sedimentary depositional platform was conceived after failed attempts to fit some carbonate facies patterns to shelf models. Ramps were first defined as two-dimensional (2D) depositional surfaces along which facies variations reflect differences in the oceanographic environment from shore to basin. Shelves and ramps were placed at opposite poles without definitions of intermediate platform configurations or environmental subdivisions between shore and basin. The 2D ramp is of limited use in determining slope and surface configuration in the rock record where stacked depositional surfaces are separated by beds of different age and thickness, and the original definition did not anticipate the complexities that would develop as high- v. low-energy and tropical v. temperate carbonate systems became better known. Though the original definition is flexible, modifications were inevitable. Refinements of the concept introduced distal steepening, among other intermediate styles of platform geometry. Environmental subdivisions were proposed for shallow, intermediate, and deeper-water ramp segments. Further studies of ramps and shelves brought to light differences in sediment production and retention on ramps, response of ramps to sea-level change, and the variety of tectonic settings in which ramps occur. Sequence stratigraphic analysis of ramps resolved most of the problems in reconstructing depositional surfaces from 3D sequences in the rock record, high- v. low-energy and temperate v. tropical ramps became well known, and studies on hydrocarbon reservoir characterization focused on diagenesis in a carbonate ramp sequence stratigraphic context. From a paper in a rather obscure regional journal, the ‘carbonate ramp’ has grown to become a world-wide standard.

Synsedimentary Cements as Paleoproterozoic Platform Building Blocks, Pethei Group, Northwestern Canada

Peritidal and subtidal carbonates of the Pethei Group are a complex arrangement of fibrous and micritic synsedimentary precipitates, carbonate grains, and detrital micrite. Synsedimentary precipitates, present largely as stromatolitic laminae, constitute the bulk of the Pethei platform, providing about 75% of the total carbonate. They occur as stromatolitic precipitates (precipitates closely associated with stromatolitic laminae) which constitute about 60% of the rock and encrusting precipitates (precipitates outside stromatolites and between grains, i.e. cement) which constitute about 15% of the rock. The most abundant precipitates are fibrous, in the form of stromatolitic microdigitate stalks (tufa) and masses of radial fans, as well as encrusting pore-filling fans, isopachous rinds, and laminar crusts. Micrite precipitates occur as stromatolitic clotted micrite and encrusting isopachous rinds. The ratio of fibrous to micritic precipitates is 2:1. Stromatolitic precipitates are arranged in three distinct microfabrics: (1) ubiquitous, spar-micrite couplets of fibrous and clotted micrite laminae; (2) deep-water vertical fibrous masses that precipitated over micritic precipitate cores; and (3) supratidal microdigitate crusts of branching fibrous stalks. Micritic cements are interpreted to be of biogenic origin, having precipitated somehow in association with microorganisms. Fibrous precipitates are interpreted to be largely abiotic. Since stromatolitic and encrusting precipitates are petrographicaily similar, they probably share similar origins. There is little evidence of sediment trapping and binding as a mechanism for stromatolite formation. The abundance of synsedimentary precipitates varies with platform geometry: subtidal ramps > subtidal rimmed shelves > subtidal open shelves > peritidal rimmed shelves. While the basic building blocks of the Pethei platform, synsedimentary precipitates, carbonate grains, and detrital micrite, are the same as those in Phanerozoic platforms, their distribution and relative proportions are different. These differences reflect a Proterozoic carbonate factory which extended across the entire platform and which produced carbonate largely by in situ precipitation.

Application of sequence stratigraphy to carbonate reservoir prediction, Early Palaeozoic eastern Warburton Basin, South Australia: ABSTRACT

The Early Palaeozoic Warburton Basin underlies the gas and oil producing Cooper and Eromanga Basins. Postdepositional tectonism created high potential fracture porosities, complicating the stratigraphy and making reservoir prediction difficult. Sequence stratigraphy integrating core, cuttings, well-log, seismic and biostratigraphic data has recognized a carbonate-dominated to mixed carbonate/siliciclastic supersequence comprising several depositional sequences. Biostratigraphy based on trilobites and conodonts ensures reliable well and seismic correlations across structurally complex areas. Lithofacies interpretation indicates sedimentary environments ranging from carbonate inner shelf, peritidal, shelf edge, deep outer shelf and slope to basin. Log facies show gradually upward shallowing trends or abrupt changes indicating possible sequence boundaries. With essential depositional models and sequence analysis from well data, seismic facies suggest general reflection configurations including parallel-continuous layered patterns indicating uniform neuritic shelf, and mounded structures suggesting carbonate build-ups and pre-existing volcanic relief. Seismic stratigraphy also reveals inclined slope and onlapping margins of a possibly isolated platform geometry. The potential reservoirs are dolomitized carbonates containing oomoldic, vuggy, intercrystalline and fracture porosities in lowstand systems tracts either on carbonate mounds and shelf crests or below shelf edge. The source rock is a deep basinal argillaceous mudstone, and the seal is fine-grained siltstone/shale of the transgressive system tract.

Controls on Carbonate Platform Geometries and Facies Stacking Patterns: Influence of Second-Order Sea-Level Fluctuations and Sediment Supply, Upper Devonian, Rocky Mountains, Alberta: ABSTRACT

Controls on Carbonate Platform Geometries and Facies Stacking Patterns: Influence of Second-Order Sea-Level Fluctuations and Sediment Supply, Upper Devonian, Rocky Mountains, Alberta: ABSTRACT

Peritidal Carbonate Platform Growth and Cyclicity in an Early Proterozoic Foreland Basin, Upper Pethei Group, Northwest Canada

ABSTRACT The upper Pethei Group is a 170-235 m thick, largely limestone, succession located in the east arm of Great Slave Lake, northwest Canada. Strata accumulated in a Paleoproterozoic foreland basin during the late stages of marine deposition. Rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages: (1) marginal peritidal complex; (2) back-barrier lagoon; (3) shelf margin; (4) subtidal shelf; and (5) slope and basin. Carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae, water-column precipitation of carbonate mud, and generation and transport of nonskeletal grains (stromaclasts, ooids, oncoids, peloids, and intraclasts). The upper Pethei platform is interpreted to have evolved through three basic platform geometries, peritidal rimmed shelf, subtidal rimmed shelf, and subtidal open shelf, in response to changes in the rate of creation of accommodation space. The peritidal complex and rim behaved in a manner consistent with models of Phanerozoic reefal rims, and alternatively, kept up, gave up, or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation. Three scales of cycle are recognized: (1) overall formation-scale upward-deepening of the platform; (2) laterally traceable, decameter-scale shallowing-upward cycles separated by marine flooding surfaces; and (3) laterally discontinuous, meter-scale shal owing-upward peritidal cycles. The first two cycle types are interpreted to have been allogenic, controlled by changes in eustasy and subsidence rates. Meter-scale cycles are interpreted as mainly autogenic, having formed in an aggradational tidal island model. The upper Petrel platform is similar to other Paleoproterozoic foreland-basin carbonate platforms. Important differences, however, include (1) laterally discontinuous peritidal cycles, (2) lack of widespread sand shoals inboard of the shelf rim, (3) a thick, predominantly carbonate, slope and basin succession, (4) moderate to low platformal micrite content, (5) extensive lagoonal ooid sands, and (6) low contents of terrigenous clastics.