Prograding Carbonate Platform Research Articles

Paleoenvironments and stratigraphy of the Valanginian–Hauterivian carbonates of the Arabian Platform: Implications for the recognition of the Weissert Event on the Southern Tethys

On the Arabian Platform, the Valanginian Weissert Event is associated with the sudden drowning of a major north-easterly prograding carbonate platform located on the southern margin of the Tethys Ocean. This was associated with a sharp back-stepping of the main facies belts to the south-west, bringing shallow marine carbonates into the previously restricted platform top. Integrated detailed facies analysis, biostratigraphy (including nannofossil and palynology) and carbon isotopes have been used to reconstruct the evolution of the carbonate shelf during the Valanginian–Hauterivian in Saudi Arabia. During the early Valanginian the study area was dominated by the deposition of dolomitized clay-prone microbial intertidal mudstones, which passed basinwards into low-energy inner ramp limestones. These facies encompass the initial stage of a 3rd order transgressive trend above the regional Lower Valanginian Unconformity. They are sharply overlain by Lithocodium/Bacinella-dominated floatstones, indicating the maximum flooding of the platform during the late Valanginian. These deposits systematically increase in thickness and abundance towards the platform margin and are associated with a broad positive excursion in δ13C (c.+2‰ V-PDB), corresponding to the Weissert Event. The presence of abundant Lithocodium/Bacinella in strata deposited during the major flooding of the platform is consistent with higher nutrient levels and increased upwelling during the late Valanginian. The Valanginian–Hauterivian boundary is marked by a regional unconformity identified through palynology and benthic foraminifera assemblages, and locally confirmed by carbon isotopes. The combination of carbon isotope data with detailed palynology and micropaleontology provided a stratigraphic resolution in line with ammonite biozones published in the western Tethys. The Valanginian–Hauterivian continental palynoflora, described for the first time on the Arabian Platform, is dominated by primitive conifer pollen such as Classopollis suggesting a rather arid climate in agreement with published reconstructions. In addition, the occurrence of various fern and bryophyte spores suggests an increase in humidity along coastline areas, associated with higher continental input into inner ramp settings, therefore affecting the carbonate factory.

Dolomitization of a Miocene-Pliocene progradational carbonate platform by mesohaline brines: Re-examination of the reflux model on Bonaire Island

Neogene dolomites are common in island settings, and they have been used to understand the complex processes of dolomitization. The island of Bonaire was one of the first locations where the concept of reflux dolomitization was applied. Given this historical significance, here we re-evaluate the importance and nature of reflux using a previously unstudied set of outcrops in Bonaire. Mio-Pliocene units in Seru Grandi in northwestern Bonaire show well-defined bodies of dolomite that extend along clinoform surfaces beneath a subhorizontal erosional unconformity. The dolomite distribution suggests early dolomitization as a result of fluids moving downwards through a succession of subtidal facies, as expected from the reflux model. However, non-stoichiometric compositions, weak 18O enrichment, low trace element concentrations and the sedimentological context of the dolomites suggest that refluxing fluid was more likely to be mesohaline in composition rather than hypersaline as previously proposed. In addition, our findings suggest that the fluids did not dolomitize the succession uniformly, but rather elongate bodies of dolomite were developed within selected clinoforms through different events. This variability may be due to temporal changes in the flux, chemistry of reactive fluids and textural changes of original sediment, whereas downdip trends may reflect evolution of magnesium exchange efficiency with distance from the brine source and reactivity of the rock.

The influence of variable progradation to aggradation ratio and facies partitioning on the development of syndepositional deformation in steep-walled carbonate platforms

Abstract One of the remaining questions in carbonate geology and reservoir studies is the origin of steep-walled carbonate platform margins and the role of gravity versus other structural processes in controlling the distribution of fracture networks and failure surfaces. This study is an attempt to document the role of gravity, interacting with stratigraphic architecture in controlling fracture patterns and margin collapse. We employ finite-discrete element models to illustrate that progradation/aggradation (P/A) ratio and facies tract partitioning in steep-walled carbonate platforms affects the development, distribution, and intensity of syndepositional deformation. All models are under the influence of only gravitational loading, where pore pressure is held constant. We utilize a modified Mohr-Coulomb constitutive law with a Rankine Rotating crack tensile corner to capture both tensile and shear brittle failure. Our results illustrate that P/A ratio affects the distribution and intensity of discrete fractures that form in steep-walled carbonate platforms. Our results suggest that deformation is more extensive in a progradational carbonate platform, where shelf edge angles are greatest and the clinoform is thickest. Alternatively, aggradational carbonate platforms experience localized deformation in front of the antecedent shelf edge where the clinoform is thickest and steepest. The introduction of mechanical heterogeneity associated with facies tract partitioning affects the intensity of fracturing in both progradational and aggradational models, with the greatest number of fractures developing in reef facies in both scenarios. Development of brittle deformation under the sole application of gravity and lack of seaward lithostatic confinement is consistent with the syndepositional nature of deformation in these settings. This work illustrates the interplay between carbonate platform geometry, facies distribution, and the resulting syndepositional deformation.

Architecture and paleogeography of the Early Paleozoic carbonate systems in the east-central Tarim Basin (China): Constraints from seismic and well data

Abstract The evolution of the Cambrian-Middle Ordovician carbonate systems in the east-central Tarim Basin, northwestern China, has been investigated using 2D seismic profiles and well data. Interpretation was performed on seismic profiles flattened along an originally horizontal surface (base of the Cambrian), to remove the effects of post-depositional deformations. Six seismic facies differently arranged in six seismic stratigraphic units record four major stages of architectural evolution, from ramp to flat-topped systems, with differences from north to south: (1) the onset of a homoclinal ramp over a clayey unit at the beginning of Early Cambrian; (2) the development of a prograding carbonate platform characterized by restricted (evaporitic) facies in the inner platform domain during Early Cambrian to Late Cambrian; (3) the development of a first prograding and a aggrading (retrograding to the north) platform during Lower Ordovician; (4) the development of a prograding/retrograding to aggrading final platform during Middle Ordovician. The basinward (eastward) progradation of the carbonate platform edge has been reconstructed in time steps, highlighting differences from the north to the south of the study area in terms of amount of progradation, platform-to-basin relief and depositional surfaces, likely controlled by differences in subsidence and environmental factors. The identification of the four major stages in the evolution of the lower Paleozoic succession of the Tarim Basin was likely controlled by changes in the rate of creation/destruction of accommodation space, due to the interplay between eustatic changes and tectonic events. The demise of the Cambrian to Middle Ordovician carbonate system in the study area is associated with a major tectonic event, recorded by a tilting of the succession toward the east (onlapped by sediments coming from the east) and an uplift (bulge) of the western part of the study area.

Seismic stratigraphy of a Lower Cretaceous prograding carbonate platform (Oman) and implications for the eustatic sea-level curve

The Lower Cretaceous of northern Oman consists of a large carbonate platform that prograded over a distance of 280 km (174 mi) in approximately 23 m.y. The seismic stratigraphic interpretation of this system, based on a regional three-dimensional survey (40,000 km2 [15,400 mi2]), revealed a complicated organization of nested clinoforms with changing depositional dips. The tectonic setting and stratal patterns suggest that different orders of sea-level fluctuations, ranging from 20 to 150 m (66–492 ft) in amplitude, have controlled the overall sedimentation. The Lower Cretaceous Lekhwair, Habshan, and Salil Formations form the lithofacies of this carbonate platform that prograded in a northeastward direction from the United Arab Emirates and Oman to northern Oman, filling in the Rayda Basin. The majority of this system is documented in seismic and well data, whereas the final part of the progradation is visible on outcrops in the Oman Mountains. Cored wells and outcrops have been dated with calpionellids, echinoids, and calcareous algae biostratigraphy; 14 sequences have been identified and are grouped into four sequence sets based on their seismic stratigraphic organization. The differences in stratal patterns between the sets are interpreted to be caused by variations in the amplitude of sea-level fluctuations. The sequence sets indicate a general increase of the amplitude of sea-level fluctuations to a maximum of 150 m (492 ft) in the early Valanginian, followed by a general highstand and low-amplitude fluctuations (20 m [66 ft]) in the late Valanginian. The Oman relative-sea-level curve shows similarities with the one proposed for the Vocontian Basin in France, providing strong evidence for the eustatic nature of these fluctuations. A glacio-eustatic control is invoked to explain this global trend.

Does compaction-induced subsidence control accommodation space at the top of prograding carbonate platforms? Constraints from the numerical modelling of the Triassic Esino Limestone (Southern Alps, Italy)

The demise of the high-relief, steep-slope, prograding Ladinian-Early Carnian carbonate platforms of the Esino Limestone (Central Southern Alps of Italy) is marked by subaerial exposure of the platform top associated with different erosional (mainly karst-related), depositional and diagenetic processes (Calcare Rosso). The exposure-related deposits consist of three major facies associations: 1) residual soils with thin lenses of conglomerates with black pebbles, and, locally, weathered vulcanites; 2) chaotic breccia lenses irregularly distributed in the uppermost part of the Esino Limestone carbonate platform, interpreted as collapse breccias in karstic setting: 3) inter-supratidal carbonate cycles with dissolution and development of paleosols and tepee structures.Facies distribution follows the sub-environments of the underlying Esino Limestone. Facies 1 and 2 typically characterize the core of the platform, covering the underlying inner platform facies. Facies 3 instead develops toward the edge of the platform, above reef-upper slope facies of the prograding facies of the Esino Limestone. The thickness of facies 3 decreases toward the core of the platform. Facies distribution reflects differences in the accommodation space and sedimentary processes from the rim (highest accommodation, favouring the deposition of peritidal-supratidal carbonates) to the core (reduced accommodation, causing pedogenesis and karstification) of the carbonate system.The observed thickness changes may be controlled by different factors: 1) syndepositional tectonics, 2) subsidence induced by magmatic activity or 3) differential subsidence controlled by the stratigraphic architecture of the Esino Limestone platform and adjoining basins. As evidence of tectonics was not observed and the presence of volcanic bodies is only documented tens of km away from the study area, the scenario involving the creation of accommodation space by compaction of the basinal sediments (resedimented, fine-grained calciturbidites) during the progradation of the carbonate platform is here investigated. Numerical modelling was performed to verify the compatibility of compaction-induced subsidence with the observed depositional architecture. The models were built to simulate the architectural evolution of the platform by progressively adding layers from deepest to shallowest, while compacting the underlying sediments, in order to evaluate compaction-induced subsidence (and accommodation space for the Calcare Rosso) after the deposition of the youngest platform strata. Modelling results allow us to conclude that the wedge geometry of the Calcare Rosso, deposited on top of the extinct Esino carbonate platform, can be explained by subsidence controlled by compaction of the basinal sediments present below the early-cemented, fast prograding platform slope deposits.

Mechanical stability model of progradational carbonate platform margins under tectonic loads: Deformation of Cretaceous carbonate platforms in the Sierra Madre Oriental fold‐thrust belt (east central Mexico)

AbstractShortening in the Sierra Madre Oriental fold‐thrust belt (east central Mexico) is localized along the margins of Cretaceous carbonate platforms and controlled by mechanical stratigraphy. The platform margins are deformed by imbricate series of thrust ramps, whereas the coeval basins and platform interiors are deformed by map‐scale detachment folds. Here we present a finite element model to evaluate the influence of the boundary geometry and boundary conditions on the style of deformation observed at these basinward progradational platform margins. We calculate the stress distribution in a linearly elastic platform‐basin transition zone under the action of horizontal tectonic stress, taking into account changes of rock mechanical properties across the platform margin, as well as their dependence on direction, and infer the resulting fracture patterns based on the Mohr‐Coulomb failure criterion. Stress concentrations are predicted at the contacts between the massive rocks of the platform margin and the well‐layered rocks of both, the platform interior and the adjacent basin. Brittle failure of the platform border can be mostly attributed to three effects: mechanical coupling between the carbonate platform and a substratum of moderate to low viscosity, variations in layering and texture that governed the mechanical properties of the involved carbonates as well as their dependence on direction, and the development of sharp domain boundary corners associated with progradational facies changes. In contrast, the dip of the basement and a possible taper of the overlying Upper Cretaceous shale toward the basin appear to have little influence on the mechanical failure of the platform margin.

Middle Triassic carbonate-platform break-up and formation of small-scale half-grabens (Julian and Kamnik–Savinja Alps, Slovenia)

In the Julian Alps (Mt. Prisojnik, NW Slovenia) and in the Kamnik–Savinja Alps (Mt. Križevnik, N Slovenia), both of which form part of the eastern Southern Alps, several meters of Upper Anisian pelagic red nodular, radiolarian-rich limestone (Loibl Formation) were deposited on the drowned platform carbonates of the Contrin Formation. The time of the platform drowning is dated with radiolarians and conodonts to the Illyrian, more precisely to the upper part of the Paraceratites trinodosus Ammonoid Zone. The red limestone is overlain by pyroclastics and volcanics (rhyolites) or carbonate (mega)breccia (Uggowitz Formation). The following unit consists of thin-bedded limestone, grainstone and subordinate marl (Buchenstein Formation) deposited during the final filling of the basin from the adjacent prograding carbonate platform (Schlern Formation) in the Ladinian. Map-scale geometry, neptunian dykes, the onset of volcanism, the presence of (mega)breccia and related paleo-escarpments, the lateral variations in thickness and the wedge-shaped geometry of the lithological units provide evidence of syn-sedimentary block faulting and the formation of small-scale, relatively shallow half-grabens within the previously uniform Slovenian Carbonate Platform. This analysis indicates a clear tectonic control over the development of the Middle Triassic stratigraphy. The described extensional event is well correlated and genetically connected with the syn-rift formation of the neighboring Slovenian Basin and other Southern Alpine basins that formed in connection with the opening of the Meliata-Maliac branch of the Neotethys Ocean.

The aftermath of the Carnian carbonate platform demise: a basinal perspective (Dolomites, Southern Alps)

AbstractIn the Dolomites of northernmost Italy the carbonate‐platform growth came to a standstill late in the Early Carnian (Late Triassic). The response to this shutdown of shallow‐water carbonate production in the interplatform basins is largely unknown because erosion has removed most of the soft basinal sediments, giving rise to today's scenic landscape of the Dolomites. Mapping in the central part of the Dolomites and newly available core material has recently revealed a well‐preserved succession of basinal rocks within the Heiligkreuz Hospiz Basin (ital. Ospizio di Santa Croce Basin). In this paper, the regional depositional nature of arrested carbonate platform production is reconstructed by tracing its sedimentological record across the slope and into the basin. The uppermost St. Cassian Formation, the time‐equivalent basinal rocks to the prograding carbonate platforms, is overlain by the Heiligkreuz Formation, whose basal succession was deposited in a restricted and oxygen‐depleted environment immediately post‐dating the platform demise. The succession consists mainly of mudrocks, marlstones, and peloidal packstones, with abundant low‐diversity ostracod and pelecypod fauna and early diagenetic dolomite. C and O isotope values of the basal Heiligkreuz Formation, post‐dating platform demise, average + 2·4 and − 2·4‰, respectively, and largely overlap the isotopic composition of St. Cassian carbonates. A shift toward slightly lower δ13C values in the Heiligkreuz Formation may reflect incorporation of isotopically depleted C released during bacterial sulphate reduction in the Heiligkreuz sediments. Sedimentological, palaeobiological and geochemical indices suggest that near‐normal marine conditions persisted long after the shutdown of shallow water carbonate‐platform growth, although there are clear indications of severely reduced oxygen levels in the restricted Heiligkreuz Hospiz interplatform basin. The Early Carnian platform demise induced a distinct switch in the locus of carbonate production from the shallow‐water platform and slope to the basin floor and a decrease in the availability of dissolved oxygen in the basinal waters. It is inferred that anoxia extended at least temporarily to the top of the carbonate slope, as indicated by the onlap of normal‐marine mounds by dark marlstones of the basal Heiligkreuz Formation.

High-resolution characterization and integrated study of a reservoir formation: the danian carbonate platform in the Aquitaine Basin (France)

This paper provides an example of an integrated multi-scale study of a carbonate reservoir. The Danian Lower R2 carbonate reservoir is located in the South of the Aquitaine Basin (France) and represents a potential underground gas storage site for Gaz de France. The Danian Lower R2 reservoir was deposited as a prograding carbonate platform bordered by a reef barrier. The effects of sedimentary and diagenetic events on the reservoir properties, particularly dolomitization, were evaluated. In this study, the reservoir quality has been assessed by seismic analyses at the basin scale, by log-analysis at the reservoir scale, by petrographic methods and by petrophysical tools at the pore-core scale. Two dolomitization stages, separated by a compaction event with associated fracturing and stylolites, have been identified. These diagenetic events have significantly improved the Lower R2 carbonate reservoir properties. It is demonstrated that the reservoir quality is mainly controlled by the pore-geometry, which is determined by various diagenetic processes. The permeability values of the reservoir range over 4 orders of magnitude, from 0.1 to 5600 mD and the porosity values range between 2 and 42%. Reservoir unit 4 (a karstic dolomite) shows the best reservoir properties with average porosity values ranging between 11.1% and 19.3% and an average permeability ranging between 379 and 766 mD. Reservoir unit 2 (a fine-grained limestone) shows the worst reservoir properties. The cementation factors range from 1.68 to 2.48. The dolomitic crystal carbonate texture (mainly units 3 and 4) shows the highest value of the cementation factor (1.98–2.48) and formation factor (9.54–36.97), which is due to its high degree of cementation. The saturation exponents vary between 1.2 and 3.4. Using these experimental electrical parameters and the resistivity laterolog tool we predicted the water saturation in the various reservoir units. The permeability was predicted by combining the formation factor with the micro-geometric characteristic length. The best fit is obtained with the Katz and Thompson's model and for a constant of 1/171.

Sedimentary patterns in perched spring travertines near Granada (Spain) as indicators of the paleohydrological and paleoclimatological evolution of a karst massif

Perched spring travertines of the Granada basin (South Spain) constitute a perched system with four well-defined steps, which are formed by several facies associations deposited in different sub-environments (travertine pools, dams and cascades). These perched travertines are considered as a freshwater reef system with a facies zonation and stratigraphic architecture closely resembling that of marine reef terraces and prograding carbonate platforms. The travertine deposits have been dated by 230Th/ 234U and 14C methods. As in other Mediterranean areas, the travertine deposition occurred episodically during warm and wet interglacial periods coinciding with isotopic stages 9, 7 and 5, and with the transition between isotopic stages 2/1. During these periods, underground dissolution, large outflow in the springs and subsequent calcium carbonate precipitation occurred. In the same way that evolution of reef systems indicates sea level changes, the geomorphology, age and architecture of perched spring travertine systems may be used to interpret former climatically controlled changes in outflow, in base level marked by the altitude of springs and in the chemistry of spring waters. Thus, aggradation or climbing progradation may indicate an increase of outflow at the spring, progradation with toplap is due to a stable base level and, conversely, dowlapping progradation may signify that the base level was gradually dropping. Therefore, the travertines can be considered semiquantitative indicators of the paleohydrological evolution of karstic massifs and used as an important terrestrial proxy climate record.

Seismic frequency control on carbonate seismic stratigraphy: A case study of the Kingdom Abo sequence, west Texas

Conceptual models and real three-dimensional (3-D) seismic data show that in progradational carbonate platform margin and slope deposits of the Kingdom Abo reservoir of the Permian basin, west Texas, primary seismic reflection events do not necessarily follow clinoformal geologic-time surfaces. The seismic frequency content of the data controls the dip and architecture of seismic reflection events. High-frequency seismic data tend to follow thinner, time-bounded clinoform depositional elements (time-stratigraphic units), whereas low-frequency seismic data tend to image thicker, low-angle lithofacies units (time-transgressive units). In seismic data of moderate frequency, both clinoform units and flat lithofacies units are imaged, creating complex interference patterns that are difficult to interpret.Experiments with models and real data demonstrate that seismic data can be selectively filtered in the signal bandwidth to help distinguish time-stratigraphic units from lithostratigraphic units. Selective filtering alters the dominant frequency of the data to match a desired scale of geologic objects. If there are enough high-frequency components in the seismic data, true clinoform stratigraphy can be imaged even if the data are dominated by lower frequency components.Seismic modeling of outcrop of the Abo sequence in Apache Canyon, Sierra Diablo, west Texas, indicates that a dominant frequency of 100 Hz is needed to recover true clinoform stratigraphy using seismic data. The interpretation of available 3-D seismic data can only partially distinguish time-stratigraphy from lithostratigraphy because of the lack of frequency components greater than 70 Hz in the data. Application of this outcrop model in seismic modeling avoids interpretational pitfalls that can occur if the dominant frequency of the seismic data is not matched to the unit thicknesses that need to be resolved.

A micrite-dominated norian carbonate platform from Northern Calabria (Southern Italy)

This paper is focused on the facies associations of an Early-Middle Norian stratigraphic succession cropping outin the Northern Calabria (Buonvicino, Cosenza). These carbonate deposits, pertaining to the Verbicaro Unit, represent a dolomitized platform characterized by a clear dominance of automicrites over skeletal metazoans. On the basis of the chronostatigraphic data and sedimentary evolution, two stratigraphic units have been distinguished: the Lower Unit (object of the present paper) and the Upper Unit. The Lower Unit consists of an Early-Middle Norian high-relief prograding carbonate platform. The Upper Unit is represented by Middle p.p.-Upper Norian basinal deposits. Three main facies associations, indicative of different depositional settings, have been identified. The inner platform facies association is characterized by automicrite (cauliflower-columnar subordinate planar stromatolites) associated with detrital carbonates (intraclastic breccia, bioclastic grainstone and packstone). Apeculiar facies is represented by megalodontid-bearing beds. Mud-cracked horizons and low-developed teepees occur quite frequently. The margin facies association is dominated by automicrite (planar to low relief stromatolites associated with thrombolitic fenestral boundstone) and detrital carbonates with subaerial exposure features. The slope facies association includes detrital carbonates (breccia/megabreccia) associated with serpulid/sphinctozoan bioconstructions and automicrite (planar stromatolites/thrombolitic boundstone). The whole carbonate body is completely dolomitized; nevertheless the morphology and microarchitecture of carbonate components (cements, grains and automicrite) is still clearly recognizable. The dolomite Mg content ranges from 40 to 48 mole%, sometimes reaching the stoichiometric value. Cements, primary and late, represent a minor component of the rock volume; they occur more frequently on the margin and upper slope setting. Quantitative tacies analyses lead to the following conclusions: The sedimentary facies and the low-diversity fossil associations seem to indicate a stressed marine environment; the platform margin probably flanked an intra-platform basin, not an open marine basin.

Subsurface Geology of a Prograding Carbonate Platform Margin, Great Bahama Bank: Results of the Bahamas Drilling Project

SEPM Special Publication Series No. 70, Robert N. Ginsburg (Editor), 2001, SEPM (Society for Sedimentary Geology), Tulsa, OK, 271 p. ($65 non-members, $40 members) ISBN: 1-56576-077-8. The present volume summarizes the scientific results of the Bahamas Drilling Project, the fruit of an idea born in Robert Ginsburg's Comparative Sedimentology Laboratory at the University of Miami. Two continuous core holes named Unda and Clino were drilled on the western margin of the famous Bahamas carbonate platform to depths of 454 m and 677 m, respectively, and subsequently analyzed by a large team of geoscientists from different institutions that were brought together to participate in an exemplary project in carbonate geology. The volume contains a preface by the editor and 12 papers grouped into five categories: (1) background, (2) lithostratigraphy and chronostratigraphy, (3) diagenesis and geochemistry, (4) petrophysics, and (5) sequence stratigraphy. The volume includes a total of 163 figures, 35 tables, and a 1.2 m2 fold-out in a pocket in the back cover of the book. The comprehensive index lists 235 topics. Robert Ginsburg provides an introductory overview of the scientific background of the Bahamas Drilling Project, and describes log acquisition and project operations in the first main division of the volume. Fundamental questions of carbonate platform progradation are listed along with previous research in the Bahamas as background to the listed objectives of the project. These objectives include: (1) testing the earlier interpretation of some 30 km of platform progradation based on seismic interpretation (Eberli and Ginsburg, 1987, 1989); (2) evaluating the influence of sea level change on platform facies and architecture; (3) seeking lithologic reasons for seismic sequence boundaries; and (4) characterizing depositional facies, diagenesis, and timing. The drilling was accomplished between April and June 1990, and operations are described briefly and illustrated by photos. Following the …

Termination of carbonate slope progradation: an example from the Carnian of the Dolomites, Northern Italy

Abstract The demise of carbonate platforms during Carnian time is a widespread phenomenon of the Western Tethys, but the controlling factors are not well-understood. The Dolomites of the Southern Alps (Northern Italy) are well-known for their large-scale Ladinian and Carnian prograding carbonate platforms. Platform growth stopped during the Early Carnian and both platforms and basinal areas were subsequently covered by mixed carbonate–siliciclastic sediments (Raibl Group). We present a carbonate platform, whose slope progradation came to a halt prior to draping by the siliciclastic sediments. Judging from the height of the clinoforms a platform-to-basin relief of at least 100 m existed during the final stage of progradation. This paleo-slope was colonized by coral patches draped by skeletal packstones to microbial bindstones, forming typically lens-shaped mounds on the steep slope. The formation of mounds on the platform flank was only possible, because the downslope transport of detritus from the platform and/or upper slope was stopped. The sedimentary evolution in the adjacent basin indicates a poorly oxygenated environment with abundant brackish water ostracods of low diversity. Based on the inherited platform-to-basin relief and the lithofacies of the basin fill a local sea-level drop in the order of 60–80 m is postulated. With rising sea level the accommodation space between former basin and platform became filled by shallow-marine, mixed carbonate–siliciclastic sediments. The abundance of epibenthic suspension feeders of the basin fill such as megalodonts suggests increased nutrient supply. The termination of slope progradation was a multiphased process triggered by a stagnation of the regional subsidence, a reduction of water circulation, followed by a sea level-drop, the influx of freshwater and the input of siliciclastics. Fluctuating salinity caused intermittently a biotically stressed environment.

Depositional Geometries and Facies Associations in an Upper Cretaceous Prograding Carbonate Platform Margin (Orfento Supersequence, Maiella, Italy)

ABSTRACT Although progradation of platform margins is a well documented process, its timing with respect to sea-level cycles is controversial. This outcrop study was designed to test the relation between prograding stratal patterns and sea-level cycles, using a sequence stratigraphic analysis and direct investigation of the sedimentological facies associations. The upper Campanian-Maastriehtian Orfento supersequence in the Maiella platform margin is an outstanding outcrop example of a large-scale prograding carbonate wedge, composed of smaller-scale, vertically and laterally stacked sequences. The evolution of the Orfento supersequence was characterized by an initial phase of aggradation, followed by progradation. At the scale of the supersequence, basinal aggradation preceded progradation of hallower-water facies, because excess relief had to be buried prior to progradation. Seven depositional facies were distinguished on the basis of depositional geometries, stratal patterns, and sedimentary features. These facies record different depositional environments on a low-angle ramp, ranging from deep-water pelagic wackestones to wave-dominated upper shoreface grainstones. Facies distributions record the overall transition from pelagic sediments, turbidites, and megabreccias into progressively shallower-water shoreface complexes. The prograding units consist of shingled offlapping sigmoidal complexes, composed of rudstones and grainstones, deposited in shallow, wave-dominated environments, and their deeper-water equivalents. Sigmoidal sequences are separated by erosive unconformities, which bring upper-shoreface facies into direct contact with underlying lower-shoreface or outer-shelf facies, indicating a sea-level fall (forced regression). Meteoric diagenesis associated with the unconformities also supports lowering of relative sea level. Depositional facies patterns within sequences record a relative rise and highstand of sea level. Repetitive downward shifts bring the shoreline successively farther basinward, indicating long-term lowering of sea level during the late stages of progradation, producing downstepping progradation. Our data suggest that changes in relative sea level, acting on a low-angle ramp, control the overall architecture of sequence distribution, whereas changes in sediment supply con rol changes in facies association within each sequence.

Seismic models of a prograding carbonate platform: Vercors, south-east France

The southern Vercors area (south-east France) displays superb exposures of a Lower Cretaceous prograding carbonate platform. These exposures allow a geometrical sequence analysis to be combined with detailed outcrop observations. Two-dimensional seismic modelling techniques were used to determine the seismic response of three of these exposures: the Montagnette, the Archiane Valley and the Rocher de Combau. The outcrops display prograding and retreating series of clinoforms interfingering with fine-grained basinal sediments. Photographs from different angles of view, combined with detailed field observations, were used to construct lithological profiles with a resolution of f few metres. P-wave velocities and bulk densities were measured from cored hand specimens and selected values were assigned to lithostratigraphic units in each profile. The vertical incidence modelling technique was then used to compute migrated time sections of reflectivity. These were convolved with source wavelets of different frequencies to produce the final synthetic seismic profiles. None of the modelled outcrop geometries is correctly portrayed seismically at 25–50 Hz frequencies. Not only do the synthetic seismic profiles show less detail, they also show misleading geometries. In most seismic models, for instance, pseudo-unconformities are present that correspond to rapid changes in dip and in facies in outcrop. Furthermore, the seismic tool is not able to resolve between stratal patterns punctuated by unconformities and those with gradual shifts in facies belts accompanied by complex interfingering relationships. This implies that an unconformity recognized on a real seismic line should not automatically be assumed to represent a true stratal unconformity. Large outcrops are often used to show the validity of sequence stratigraphy, a concept mainly based on the analysis of seismic lines. This study, however, shows that there is no one to one relationship between geometries observed in outcrop and in seismic lines. Seismic modelling of outcrops therefore provides a means for evaluating outcrop geometries in terms of seismic stratigraphy.

Depositional Cycles on a Prograding Reef-Rimmed Carbonate Platform During High-Frequency Sea-Level Fluctuations, Upper Miocene Reef Complex of Mallorca, Spain

Depositional Cycles on a Prograding Reef-Rimmed Carbonate Platform During High-Frequency Sea-Level Fluctuations, Upper Miocene Reef Complex of Mallorca, Spain

Structural and Stratigraphic Evolution of the East Java Sea, Indonesia

Abstract The East Java Sea displays excellent examples of Miocene to Present-Day structural inversion of a Palaeogene extensional/transtensional basin system. The structural geometry of these inversions has been analysed by a series of regional cross-sections which demonstrate significant lateral variation in structural style. This variation is interpreted to be a function of the fault shape and linkage which evolved during the extensional phase of basin development. Prediction of fault geometry has been aided by analogue-modelling of inverted extensional basins. Driving mechanisms of structural inversion have been assessed by modelling the motion of the Eurasian, Indian Ocean, Australian and Pacific plates during the Neogene. The evolution of inversion within the East Java Sea is interpreted to be a function of the propagating collision of the Australian Plate with the Sunda Arc, and the resultant variation in relative convergence rate and direction of the subducting Indian Ocean Plate and the S.E. Eurasian Plate. Stratigraphic evolution is strongly influenced by relative sea level fluctuations, principally eustatic, within overall control of tectonic processes, particularly inversion. The sediment fill is mainly fine-grained with alternating carbonate-dominated and clastics-dominated cycles. The use of wireline logs, seismic data and biostratigraphic data allows a detailed sequence stratigraphy to be developed, with both regionally and locally recognisable highstand, lowstand and transgressive systems. A well-imaged progradational carbonate platform margin exhibits late-stage growth and subsequent inundation which are used to calibrate locally derived relative sea-level curves.

Slope Depositional Processes along Western Great Bahama Bank: A Rapidly Prograding Carbonate Platform

The western margin of Great Bahama Bank has prograded rapidly into the Santaren Channel throughout Quaternary time. Platform growth is accomplished mainly by vigorous outbuilding of the slope - up to 500 m in the last 6-10 ka. Seismic stratigraphy and sediment and rock analysis reveal sharp contrasts in depositional facies and mechanisms of growth between the upper slope (50-160 m) and lower slope (160-600 m). The upper slope is steep (30-60{degree}) and rock with a thin (< 1 m) layer of coarse Halimeda sand. In situ sediment production, in combination with cementation by magnesium clacite, allows this interval to grow as a slowly accreting (meters/1,000 yr), lithified surface. Because of cementation, slope failure is rare but, in some areas, large blocks calved off the upper slope are found on the lower slope (mega-breccia). The lower slope is gentle (10-30{degree}) and consists of thick (up to 90 m) accumulations of fine bank-top sediment. Deposition occurs episodically via settling of sediment plumes swept off the bank during large storms. Lower slope deposits are absent where off-bank transport is barred by shelf-edge islands. In addition, hyperpycnal flows which cascade over the margin both sweep the upper slope and erode lower slope depositsmore » near the depth of onlap. Slope failure in the uncemented lower slope deposits is primarily via slumping and possibly gravity flows. Lower slope deposition occurs only during sea level high stands when the rates of growth may be an order of magnitude higher than those of the upper slope which apparently experiences slow growth throughout most of a sea level cycle.« less