Carbonate ramp depositional systems

Various types of carbonate platforms are characterized by distinctive profiles, facies, and evolutionary sequences. Ramps may be homoclinal or distally steepened, and may have fringing or barrier shoal-water complexes of ooid-pellet sands or skeletal banks. Homoclinal ramps pass seaward into deeper water without major break in slope, and lack deep-water breccias. Distally steepened ramps may be low energy, and characterized by widespread, shallow, subwave-base mud blankets, or high energy with coastal beach/dune complexes and widespread skeletal sand blankets. Slope facies may contain abundant breccias of slope-derived clasts. Rimmed shelves have relatively flat tops, and marked break in slopes at the high energy, shallow-shelf edge where they pass into deep water. Such shelves may be aggraded with peritidal facies extending over much of the shelf, or incipiently drowned, depending on magnitude of sea level fluctuations. They may be accretionary, or bypass types that include gullied slope, escarpment, and high-relief erosional forms. Intrashelf basins occur on some shelves, controlling distribution of reservoir and source beds. Isolated platforms are surrounded by deeper water and may be located on rifted continental margins, or on submarine volcanoes. Most have high-relief rimmed margins. Platforms that have been subjected to rapid sea level rise may be incipiently drowned, and characterized by raised rims, elevated patch or pinnacle reefs, and widespread subwave-base carbonate or fine clastic blankets. Completely drowned shelves develop where the shelf is submerged to subphotic depths, terminating shallow water deposition, and commonly resulting in blanketing of the shelf by deeper water facies. Some margins show extensive down-to-basin faulting that is contemporaneous with carbonate deposition, or associated with thick prograding clastic sequences. The various types of platforms change in response to variations in sedimentation, subsidence or sea level rise, and may form distinctive evolutionary sequences. The relatively few models presented appear to accommodate most geological examples, some of which contain major reservoir facies.

Carbonate Platform Facies Models

Many types of carbonate platforms have been described, from homoclinal ramps to rimmed shelves and a full spectrum of variations in between; the distinction between these different types can be problematic. Nevertheless, classification of carbonate platforms is not just a semantic or academic issue. For example, it is clearly important for the accurate interpretation of seismic images of facies geometry and for assessing the potential of stratigraphic traps. Even though predictive efficiency of conceptual models depends on the degree of comprehension of the genetic factors controlling depositional profiles and the distribution of facies belts, current models for classification of carbonate platforms are basically descriptive and mainly based on depositional profile, size, and attachment to or detachment from a landmass.

A genetic approach considers the variability of depositional profiles among carbonate platforms as a function of the type of sediment that was produced (basically grain size), the locus of sediment production, and the hydraulic energy. Three groups of carbonate-producing biota may be distinguished according to their dependence upon light: (1) euphotic (good light) in shallow, wave-agitated areas; (2) oligophotic (poor light) in deeper, commonly non-wave-agitated areas; and (3) photo-independent biota in all water-depth ranges.

Several platform types in wave-dominated seas can be considered in relation to genetic factors, even when simplifying the many possible scenarios. Euphotic framework-producing biota create rimmed shelves similar to modern reef platforms. Soft-substrate-dwelling biota, which produce gravel-sized carbonate in the shallow euphotic zone, create flat-topped open shelves. Oligophotic gravel-producing biota, such as some larger foraminifera and red algae, generate distally steepened ramps. Mud-dominated carbonate production, in either euphotic or oligophotic zones, generate homoclinal ramps. Carbonate production dominated by photo-independent biota (crinoids, sponges, bryozoans, etc.) above wave base give rise to open shelves or ramps, depending upon grain size, but may produce mounds if carbonate production occurs below the base of wave/current sweeping.

Types of carbonate platforms: a genetic approach

Bioerosion, involving the weakening and breakdown of calcareous coral reef structures, is due to the chemical and mechanical activities of numerous and diverse biotic agents. These range in size from minute, primarily intra-skeletal organisms, the microborers (e.g., algae, fungi, bacteria) to larger and often externally-visible macroboring invertebrate (e.g., sponges, polychaete worms, sipunculans, molluscs, crustaceans, echinoids) and fish (e.g., parrotfishes, acanthurids, pufferfishes) species. Constructive coral reef growth and destructive bioerosive processes are often in close balance. Dead corals are generally subject to higher rates of bioerosion than living corals, therefore, bioerosion and reef degradation can result from disturbances that cause coral mortality, such as sedimentation, eutrophication, pollution, temperature extremes, predation, and coral diseases. The effects of intensive coral reef bioerosion, involving El Nino-Southern Oscillation, Acanthaster predation, watershed alterations, and over-fishing, are re-examined after ~20 years (early 1990s–2010). We review the evidence showing that the biologically-mediated dissolution of calcium carbonate structures by endolithic algae and clionaid sponges will be accelerated with ocean acidification. The CaCO3 budget dynamics of Caribbean and eastern tropical Pacific reefs is reviewed and provides sobering case studies on the current state of coral reefs and their future in a high-CO2 world.

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Bioerosion and Coral Reef Growth: A Dynamic Balance

Analysis, origin and significance of mineral matter in coal: An updated review

This paper presents a depositional model for a temperate, low-energy carbonate ramp based on descriptive studies of five areas around the Balearic Islands of Mallorca and Menorca. This low-energy ramp differs significantly from other present-day temperate carbonate platforms that are primarily high-energy open shelves. It is characterized by the following lithofacies (from shore to basin): (1) lagoon, (2) barrier island, (3) shallow subtidal, (4) inner ramp, (5) middle ramp, and (6) distal ramp. Subaqueous carbonate dunes are present near slope breaks off Menorca and Cabrera, but they are not representative of the entire ramp. Balearic ramp sediments differ in composition, texture, biology, and degree of cementation from those on modern low-energy, tropical ramps. Balearic ramp carbon tes lack ooids and peloids, hermatypic coral buildups, and calcareous green algae (except in one restricted bay where Halimeda is relatively common). Red algal sands and gravels extend to depths of up to 90 m, and are coarser than their strandplain-equivalent lime sands. Skeletal allochems consist of the bryomol-rhodalgal association, marine cementation is rare, and the carbonate fraction of deep-water muds is mainly Mg-calcitic and calcitic in composition. Aragonite is rare. Except for the red alga Peyssonnelia, which is composed of aragonite, it is neither a dominant skeletal constituent nor common as a cement. The Mediterranean Sea off Mallorca and Menorca is a low-energy, temperate, oligotrophic, clear-water environment. The depositional model for the region is an isolated platform configured as a homoclinal ramp. Ancient counterparts of the Balearic ramp are present in the Neogene of the Mediterranean Tethys and the Paratethys, and though the constituents of fossil assemblages vary with time, the biota of the Balearic ramp, such as bryozoans, red algae, echinoderms, and mollusks, ranges from the Paleozoic Era to the present. Echinodermal bryomols passing to basinal muds on carbonate ramps are particularly characteristic of the Early Carboniferous in North America and Europe. This suggests that the Balearic Islands temperate ramp may be more representative of some ancient carbonate sequenc s than either temperate, high-energy, open shelves or tropical ramps in the present oceans.

Temperate carbonates on a modern, low-energy, isolated ramp; the Balearic Platform, Spain

Abstract The pre-1980s literature on modern carbonates was biased toward tropical examples because non-tropical carbonates had not been studied extensively. Though non-tropical carbonates have received considerable attention in the past decade, the variety of low-energy, temperate ramp examples in the literature is limited. In contrast, examples of modern and ancient low-energy, tropical ramps are well represented. They are characterized by a gradual change from calcarenites updip to calcilutites downdip, by a photozoan biota, abundant non-skeletal aragonitic grains, such as ooids and peloids, and by widespread and rapid marine cementation. The Balearic Platform in the western Mediterranean is an isolated, low-energy, temperate ramp. This paper describes the Balearic ramp bathymetry, environmental regimes and depositional facies, which include (from the shoreline seaward): coastal lagoons, beach—dune complexes, inner-ramp seagrass meadows with mixed terrigenous—foraminiferal—molluscan muddy calcarenites, middle-ramp bryozoan—rhodalgal facies and outer ramp clastic—carbonate muds. Biotic and textural characteristics vary with depth, with hydrological conditions and with environmental factors, such as temperature, salinity, light and nutrients. Seagrasses extend across the inner and part of the middle ramp, where the grasses offer shelter to a variety organisms, including epibionts, molluscs, bryozoans, echinoderms and red algae. Most of the beach and dune sediments consist of bioclasts derived from the communities that thrive in the seagrass meadows, but the greatest volume of skeletal carbonates is produced as bryozoan, rhodalgal and molluscan gravels that occur as patchy blankets, primarily on the middle ramp. The Balearic Platform is characterized by an oligotrophic, clear water, microtidal environment. The dominant biota of the Balearic ramp — bryozoans, red algae, echinoderms and molluscs — is common in other non-tropical modern environments as well as in ancient temperate settings. This fact, combined with the absence of aragonitic non-skeletal grains, abundant marine cements and photozoans other than red algae, establishes the modern Balearic ramp as a model for comparison with low-energy, non-tropical ramps in the global rock record.

Present-day temperate carbonate sedimentation on the Balearic Platform, western Mediterranean: compositional and textural variation along a low-energy isolated ramp

The development of Little Cedar Creek field in the eastern Gulf coastal plain of the United States has shown that the current exploration strategy used to find hydrocarbon-productive microbial and high-energy, nearshore carbonate facies in the Upper Jurassic Smackover Formation requires refinement to increase the probability of identifying and delineating these potential reservoir facies. In this field, the petroleum trap is a stratigraphic trap characterized by microbial boundstone and packstone and nearshore grainstone and packstone reservoirs that are underlain and overlain by lime mudstone and dolomudstone to wackestone and that grade into lime mudstone and dolomudstone near the depositional updip limit of the Smackover Formation. Reservoir rocks trend from southwest to northeast in the field area. The grainstone and packstone reservoir is thickest in the central part of the field. The boundstone reservoir is thickest in local buildups that are composed of thrombolites in the southern part of the field and is absent along the northern margin. These reservoir facies are interpreted to have accumulated in water depths of approximately 3 m (10 ft) and in 5 km (3 mi) of the paleoshoreline. In contrast to most other thrombolites identified in the Gulf coastal plain, these buildups did not grow directly on paleohighs associated with Paleozoic crystalline rocks. The characterization and modeling of the petroleum trap and reservoirs at Little Cedar Creek field provide new information for use in the formulation of strategies for exploration of other Upper Jurassic hydrocarbon productive microbial and related facies associated with stratigraphic traps in the Gulf coastal plain.

Upper Jurassic updip stratigraphic trap and associated Smackover microbial and nearshore carbonate facies, eastern Gulf coastal plain

Characterization of carbonate rocks may involve identifying the important pore types which are present. In the past, this task has required detailed petrographic analysis of many core samples. Here, we describe a method which uses nuclear magnetic resonance (NMR) measurements to reduce the amount of petrographic analysis needed for porosity typing of carbonate reservoir rocks. For a rock sample which has been measured with NMR, our method decomposes the log(T2) spectrum into at most three Gaussian-shaped components and gives a set of nine parameters. Two characteristic quantities having geological significance are extracted from the nine parameters. Values of the two quantities are compared with a reference set, established from samples having both NMR and petrographic evaluations of porosity types. We use a Bayesian approach to the classification of the dominant porosity type. Tests of our method on 103 samples show a correct prediction in 60 to 90 percent of the samples. The lower success rate was obtained for samples with five porosity types from three fields; the higher success rate obtained with samples with three porosity types from one well. The use of geologically significant quantities extracted from the decomposition gives comparable success rate to those obtained using a standard, non-geological approach such as canonical variates.

Wayne M. Ahr

Papers

Temperate carbonates on a modern, low-energy, isolated ramp; the Balearic Platform, Spain

Present-day temperate carbonate sedimentation on the Balearic Platform, western Mediterranean: compositional and textural variation along a low-energy isolated ramp

Upper Jurassic updip stratigraphic trap and associated Smackover microbial and nearshore carbonate facies, eastern Gulf coastal plain

Distinguishing Carbonate Reservoir Pore Facies with Nuclear Magnetic Resonance Measurements

Early diagenetic microporosity in the Cotton Valley Limestone of east Texas