Reef Buildups Research Articles

The tectonic evolution and hydrocarbon potential in the boundaries of Vietnam continental shelf

Recent survey results indicate the presence of Tertiary sedimentary basins with hydrocarbon potential in the boundaries of Vietnam continental shelf. These basins were perhaps formed in the Early Oligocene on the Mesozoic basement of continental crust and were filled with mainly deltaic clastic sediments. From very early Miocene these basins have linked together and became wider. Sediments of marine and prograding delta facies accumulated and widespreaded on the whole continental shelf. At the Middle Miocene to present-day the opening of the Bien Dong Sea (Vietnam East Sea), the regional tectonic subsidence and the eustatic fluctuation had created sedimentary formations of open and deep marine facies with turbidite shale, platform carbonate including reef build-up. Andesite and basalt extrusion occurred in form of dykes resulting from tectonic inversions in Late Oligocene, in the beginning of Middle Miocene, particularly in Late Miocene-Pliocene. These Tertiary sedimentary basins are expected to have hydrocarbon potential and need to be investigated and explorated adequately. The main risk is the drilling technology, the exploitation in the deep sea and high investment cost.

Oligocene–Miocene carbonates in the Reed Bank area, South China Sea, and their tectono-sedimentary evolution

We have analyzed two recently acquired multi-channel seismic profiles across the Reed Bank area, South China Sea. A detailed seismic interpretation coupled with drilling data has proved the occurrence of a wide carbonate platform, developed between the Late Oligocene and the Early Miocene (32–20 Ma). The top of these carbonates is an important regional unconformity, corresponding to a strong and continuous seismic reflector. An age of about 20 Ma is inferred for this reflector, acting as a regional unconformity marking the cessation of seafloor spreading with an erosional/non-depositional hiatus, spanning 3 or 4 m.y. The time interval during which the carbonate platforms formed is concurrent with the opening of the South China Sea. The reconstruction of the tectonic subsidence in this area has shown a decrease in rate of subsidence during the drifting stage (32–17 Ma), indicating that the whole Reed Bank area was in a tectonically stable and shallow water environment for more than 10 m.y., favoring the development of shallow water carbonates. With a sharply increased subsidence rate after the end of spreading (17 Ma) and a continuously increased sea level, the carbonate platforms were drowned and died, except some structural highs with reef buildups, where the carbonate sedimentation continued to Middle Miocene. We suggest that delayed tectonic subsidence in Reed Bank area might have been related to a secondary mantle convection under the rift lasting more than about 10 m.y.

Carbonate platform-margins and reefs distribution using 2-D seismic analysis, Central Tunisia

The seismic characterization of sedimentary facies in a carbonate platform, comprising different types of reefs constructions, is based using two-dimensional (2-D) seismic and borehole data. Reefs of the Aptian Serdj carbonates are shown as mounds of strong chaotic amplitudes that have a high-amplitude continuous reflection at the top. They are sealed by Albian marl and claystone deposits characterized by mid- to low-amplitude, parallel and discontinuous to weak reflections. These buildups were restricted to the outer platform margin of Central Tunisia. Sea level oscillations associated with master fault rejuvenation governed the growth, the distribution and development of these reefs. Their distribution is largely controlled by deep-seated fault-related folds and the topography of underlying structures, representing local domal uplifts. Falls of sea level led to subaerial exposure and the development of a karstified denudation of the carbonate platform. Subsequently, reefs were partially or totally destroyed and then overlain, during the Albian, by marls and claystones of the Fahdene Formation. Their study indicates that reef buildups have important oil and gas exploration potential, not only onshore, but also in offshore, Central Tunisia.

Deep shelf biostrome of Late Permian in South China and its implications for the adaptability of calcisponges to water depth

The Yangtze platform of South China was an area of extensive carbonate accumulation during the Late Permian, with abundant benthic fossils and reef buildups. Its northern margin has long been considered to be bordered by deep shelf sediments rich in radiolarians and ammonoids. However, recent investigations found a sponge biostrome in deeper shelf settings in the uppermost Permian. The biostrome is about 0.45m thick with an organic framework formed by autochthonous calcisponges. In contrast to more diverse shallow marine reef communities at this time, the biostrome was almost entirely constructed by the sponge genus Peronidella as the only frame-building organism. This low biodiversity reflects a relatively deep-water environment. Peronidella individuals in the biostrome are apparently larger than the same genus in shallow platform settings, including the diameters of both the sponge bodies and the central tubular spongocoel, as well as the thicknesses of the body walls. This increased size may be related to the low biodiversity and therefore to reduced competition, with sponge individuals having increased space in which to develop. Biostrome development was terminated by volcanic clay deposition.

Facies heterogeneity of a Kimmeridgian Carbonate Ramp (Jabaloyas, eastern Spain): a combined outcrop and 3D geomodelling analysis.

The Upper Kimmeridgian outcrops exposed around the village of Jabaloyas (NE Spain) have been analyzed to document the facies heterogeneity across the shallow portion of a low angle carbonate ramp. The studied area covers a total surface of 12 square kilometres. Seventeen stratigraphic profiles have provided basic information to carry out the detailed facies analyses presented in this work. The studied unit corresponds to a high-frequency sequence, bounded by discontinuities that can be traced at regional scale. This sequence shows the widespread development of coral-microbial reef buildups with conical to cylindrical shape (up to 19 m thick). The characterization and interpretation of the different sort of inter-reef and post-reef facies resulted in the reconstruction of the sedimentary domains of the studied carbonate ramp, from burrowed mud-dominated skeletal facies (distal middle ramp), to grainy (skeletal, peloidal, oolitic) shoal and backshoal facies (inner ramp). The studied sequence is divided into four sedimentary episodes bounded by sharp facies changes. The lower two episodes include the vertical reef growth during rapid accommodation gain; the upper two episodes are marked by the rapid progradation of the inner ramp facies. The depositional facies and their stacking patterns in the Jabaloyas outcrops are similar to those described in the subsurface Arab D reservoirs. This Late Jurassic carbonate unit forms major hydrocarbon fields in the Middle East. For improving the geological distribution of reservoir heterogeneity in this kind of low angle carbonate ramp system, two 3D models have been created. A Fullfield Model of the overall geometry of the facies belts has allowed the quantification of vertical and lateral extension of the inter-reef and post-reef facies. A Sector Model, which includes individual reef bodies has been used to better simulate reservoir heterogeneity resulting from different diagenetic overprint.

Changes in fossil assemblage in sediment cores from Mingulay Reef Complex (NE Atlantic): Implications for coral reef build-up

CT scanning and descriptions of sedimentary sequences have been conducted to describe the internal structure and define the evolutionary sequence of events leading to the build-up of cold-water coral reef constructed by the azooxanthellate scleractinian Lophelia pertusa. Two vibrocores containing coral clasts and associated fauna embedded in a matrix of marine sediment were taken at the Mingulay Reef Complex, an inshore (~120–190m water depth) L. pertusa reef off western Scotland. The cores were scanned using computed x-ray tomography (CT scan) with the aim of (1) reconstructing and quantifying the relative abundance and preservation of coral fragments in the cores, (2) identifying distinct reef facies, and hence (3) providing data that could lead to a conceptual model of reef growth. U-series dates were obtained on L. pertusa clasts from the two sediment cores to provide a temporal framework of the reef build-up. After exploring a range of methods for processing CT scans, three simple parameters were used to classify different strata within the cores: the overall density of the cores, the number of fossil clasts, and the sum of the area of clasts detected by the CT scan. These parameters were validated by comparing with density measurements from a multi-sensor core logger (MSCL) and physical dissection and analysis of fossils in the cores. From the CT scan analyses, three distinct reef facies were evident: Coral-Rich, Sediment-Rich and Shelly-Coral Hash Layers. The succession and rapid cycle of reef framework-forming L. pertusa development, disturbance and recovery allow us to provide a model of cold-water coral reef build-up including shifts in biodiversity, physical/chemical/biological processes and the sedimentary regime.

Multi-scale characterisation of the Paaratte Formation, Otway Basin, for CO2 injection and storage

A resurgent interest in the gas potential of the Gulf of Papua has been enhanced by the recent announcement of developing new LNG facilities in Port Moresby. Petroleum Geo-Services (PGS) has recently acquired a 6,000-km2Multi-Client 3D (MC3D) survey in the gulf. Although most of the discoveries in PNG are from the highland areas, the gulf is a proven hydrocarbon province with gas/condensate accumulations previously discovered in several reef build-ups. It is thought that sediments from the Papuan Fold Belt produced clastic dominated deltas in the Gulf area. Although this play is untested offshore, there is evidence these sandy facies exist and could contain hydrocarbons. On the first phase of acquisition/interpretation, an efficient screening workflow called Prospect Scanner, developed to highlight areas of AVO effects in large 3D seismic datasets, was tested. It uses pre-stack seismic time migrated (PSTM) gathers to extract AVO attributes that are then inverted to derive relative acoustic and shear impedance volumes. Using idealised cross-plots, the relationship between Vp/Vs ratio and the Ip values gives a good indication of the fluid and lithology of the tested interval. The results are loaded into a conventional interpretation package for interpretation/visualisation. Prospect Scanner has highlighted previously undrilled turbidite and basin floor fan prospects in the region. The Vp/Vs versus Ip cross-plot shows clustering ofthe sand anomaly and good separation from the majority of the plot. These points correspond to the idealised location for a gas sand. Past concerns about lateral prediction of reservoir presence and quality associated with the basin floor fans can be addressed through this workflow.

Amplitude-versus-offset (AVO) screening in frontier basins: an example from the Gulf of Papua, Papua New Guinea

A resurgent interest in the gas potential of the Gulf of Papua has been enhanced by the recent announcement of developing new LNG facilities in Port Moresby. Petroleum Geo-Services (PGS) has recently acquired a 6,000-km2 Multi-Client 3D (MC3D) survey in the gulf. Although most of the discoveries in PNG are from the highland areas, the gulf is a proven hydrocarbon province with gas/condensate accumulations previously discovered in several reef build-ups. It is thought that sediments from the Papuan Fold Belt produced clastic dominated deltas in the Gulf area. Although this play is untested offshore, there is evidence these sandy facies exist and could contain hydrocarbons. On the first phase of acquisition/interpretation, an efficient screening workflow called Prospect Scanner, developed to highlight areas of AVO effects in large 3D seismic datasets, was tested. It uses pre-stack seismic time migrated (PSTM) gathers to extract AVO attributes that are then inverted to derive relative acoustic and shear impedance volumes. Using idealised cross-plots, the relationship between Vp/Vs ratio and the Ip values gives a good indication of the fluid and lithology of the tested interval. The results are loaded into a conventional interpretation package for interpretation/visualisation. Prospect Scanner has highlighted previously undrilled turbidite and basin floor fan prospects in the region. The Vp/Vs versus Ip cross-plot shows clustering of the sand anomaly and good separation from the majority of the plot. These points correspond to the idealised location for a gas sand. Past concerns about lateral prediction of reservoir presence and quality associated with the basin floor fans can be addressed through this workflow.

Platy coral patch reefs from eastern Panthalassa (Nevada, USA): Unique reef construction in the Late Triassic

Upper Triassic (Lower Norian) reefal buildups near Mina, Nevada, represent some of the earliest scleractinian coral reefs from eastern Panthalassa. The small patch reefs (~ 20–40 m high and ~ 50–150 m wide, obvious metre-scale elevation above the surrounding sediments) are from the Luning Allochthon and grew on an inner to middle ramp or in a deep lagoon. The Mina patch reefs were constructed by several different coral ecomorphotypes (platy, tabular, domal/massive, and branching corals) interpreted to have had zooxanthellate symbionts, and record subtle coral zonation within the reefs. Based on modern coral ecomorphotypes, platy to tabular corals at Mina grew in the lower euphotic zone (stressed by low light conditions), and the massive or domal corals inhabited shallower water (possibly above fair weather wave base) and were stressed by wave energy. Unlike most other Late Triassic reef ecosystems where phaceloid branching corals or calcareous sponges constitute the principal bioconstructors, the platy to tabular corals were the primary builders in the Mina patch reefs. The Mina reefs are also unique because cryptic and cavernous internal environments, epibionts, cryptobionts, and thick microbial crusts are rare or absent. The combination of platy coral dominance with the lack of epibionts/cryptobionts/microbial crusts suggests that the Norian reefs from Mina, Nevada represent a unique form of scleractinian reef construction from the Late Triassic.

Cuencas sedimentarias de Costa Rica: evolución geodinámica y potencial de hidrocarburos

The Upper Cretaceous/Cenozoic basins of Costa Ricaare underlain by oceanic crustand located along the western margin of the Caribeean Plate. Since Upper Cretaceous time, an intraoceanic arc was formed along the western margin of the Caribeean Plate. This arc marked the beggining of the subdution of the Farallon Plate in the region and also the creationof the first land bridge joining North and South America. Continued subdution of the Farallon Plate from Campanian until the present, created a convergent marginal tectonic setting; with, from SW to NE: a trench, an outer arc, a forearc region, a volcanic arc and a backarcr egion. Localized platforms of shallow-water carbonates, overlain by extensive cyclical turbidites system, characterized the sediments of most of the basis until the Eocene, when a major tectonic compressive event affected the area. As well, about this time, a major sinistral east-west fault (COSTA RICA TRANSCURRENT FAULT SYSTEM) divided the region into north and south segments and transtensional or pull apart basins developed along this major fault system. In the Oligocene time, the north segment was emerged and subject to erosion, whereas in the south segment the sedimentation continued. From Miocene on, shallow marine deposition was widespread in both sements. In the north segment, and extensional stage in the backarc region, created a structural depression filled with Neogene sediments. About the same time, the south segment experienced compressional tectonicis due to noth-directed movementof the Panama block. Later in the Pleistocene, northeasterly-directed shallow subdution of the thick Cocos Ridge, resulted in adiotional thrusting and the developmentof a foreland basin, both, in the back arc and fore arc regions. Organic-rich, deep water, marginally to termally mature source rock, from Cretaceous age, occur in several basins. Also, source rocks of moderate to high TOC values are present in all basins, in the Tertiary. Potential carbonate reservoirs in some of these basins include shallow water carbonates, rudistic reef buildups and grainstone accumulations. Potential clastic reservoirs include turbidite sandstones, regressive deltaic deposits and near-shore porous sandstones. The presence of potential structural and stratigrafic traps in all basins, confer an important as far as the pretoleum potential is concern.

Genesis and internal architecture of the Middle to Upper Devonian Gwirat Al Hyssan reef-mound (Western Sahara)

On the southern flanks of the Tindouf Basin (Western Sahara) reefal complexes of various sizes were developed, mostly of Givetian age but possibly reaching into the early Frasnian. Surrounding sedimentary rocks were dominantly sandstones, siltstones and marls. The larger reef complexes in the northeastern part of Western Sahara show three reef cycles within the Givetian and lowermost Frasnian, interrupted by marly sedimentation. Farther to the west, more isolated reef structures in open shelf settings occur often showing smaller dimensions. The present study focuses on one of these western reefal build-ups. Besides large-scale reefs dominated by stromatoporoids (NE), several smaller bioherms have been investigated, such as the Gwirat Al Hyssan reef-mound near Smara. Initial reef growth was of late Givetian age as indicated by corals (e.g. Heliolites). The main reef builders were the corals Alveolites, Thamnopora, Aulopora, Frechastraea, Phillipsastrea, and, to a lesser degree, Heliolites, Scoliopora, and Roemerolites. Stromatoporoids also contributed, but were less frequent. Chaetetids (e.g. Rhaphidopora) acted as pioneer stabilizers on bare sediment surfaces. Preservation in life position was frequently observed in both groups of organisms. Crinoids are not rare, but only present as debris. In distinct areas of the reef (depressions) concentrations of brachiopods and small solitary corals occur. The reef-mound has a present elevation of 17 m and measures about 370 m in diameter. Vertical as well as horizontal zonation could be recognized in detail. The initial reef growth started on a submarine shoal of siliciclastic sediments, containing various trace fossils and sedimentary structures such as cross-bedding and wave ripples; interference ripples suggest generation in very shallow water. The initial reef-building organisms were encrusting chaetetids, followed by platy Frechastraea colonies; thamnoporids were also present. The overlying reef limestones consist of different corals and, to a minor degree, stromatoporoids. The latter are sparsely distributed vertically as well as horizontally, depending on their position within the reef. Growth forms are more robust (bulbous) towards higher hydrodynamic conditions at the southern reef front. Reef growth is interrupted by debris limestones of thamnoporid and crinoid bioclasts, intercalated with detrital platy stromatoporoids. Generally, the debris was not transported over long distances, as demonstrated by the presence of relatively large fragments. This ‘debris phase’ (early Frasnian) is overlain by the last documented stage in reef development represented by medium- to thin-bedded coral-rich limestones. The onset of the carbonate production of the Gwirat Al Hyssan reef-mound is maybe related to the global transgressive Givetian–Frasnian Boundary Event.

Microbial framework in Upper Triassic (Carnian) patch reefs from Williston Lake, British Columbia, Canada

The Upper Triassic (Carnian) patch reefs from Pardonet Hill, British Columbia, Canada are the only known Upper Triassic reefal buildups from the craton margin of northwestern Pangaea. These buildups occur within the lower Baldonnel Formation and are characterized by a moderately diverse assemblage of corals, spongiomorphs, crinoids, echinoderms, bivalves, gastropods, brachiopods, and microbialites. Despite the evidence that these buildups had a rigid structure (e.g. brecciation and hardgrounds) and relief above the seafloor (visible in outcrop), the exact nature and extent of the framework has previously been undetermined. This study focuses on the microfacies within these patch reefs and highlights the importance of microbialites in the framework of these diminutive buildups (~ 100 m 3 to 500 m 3). The Pardonet Hill patch reefs were mapped in outcrop to determine their structure, geometry, and the importance of large metazoan framework builders. Samples of the reefal horizons and surrounding strata were evaluated utilizing point count analyses of organisms, cements, lithic grains, and microbial textures in thin section. Quantitative analysis of the microfacies in concert with traditional descriptions of reef construction and reef classification allow a more thorough understanding of organism abundance and reef structure. Despite the presence of corals and spongiomorphs throughout the outcrop, they very rarely form a true, intergrown framework, except at the base of Patch Reef B. Instead, cemented microbialites form a rigid framework in the core of some patch reefs. Detailed examination of the microfacies and reef construction demonstrates the importance of non-skeletal framework builders in the structural fabric of these small buildups. Pardonet Hill Patch Reef B is the first true reef with a microbial framework documented from the Upper Triassic of North America. This microbial–coral framework bears a striking similarity to some Triassic and Upper Jurassic coral–microbial reefs from the Tethys, although the depositional environment of the Pardonet Hill patch reefs is likely shallower and more energetic than the Tethyan reefs.

Stratigraphic trapping potential in the Carboniferous of North Africa: developing new play concepts based on integrated outcrop sedimentology and regional sequence stratigraphy (Morocco, Algeria, Libya)

Abstract The lower Carboniferous (Tournaisian to Visean) of North Africa is characterized by cycle-stacks of predominantly shelfal to marginal marine sandstones and limestones, thick shelfal mudstones and less common but important interbedded fluvio-deltaic sandstones. The cyclic sedimentation pattern continues into the Mid Carboniferous (Serpukhovian to Bashkirian), when mixed siliciclastic–carbonate sequences give way to tropical carbonates, before an abrupt return to continental deposits in the upper Carboniferous (Bashkirian to Gzhelian). The alternation of widespread shallow marine and more discrete fluvial reservoirs with interbedded offshore mudstone seals is interpreted to result from high-frequency, high-amplitude Carboniferous glacio-eustatic sea-level changes. The large base-level changes during that time, combined with climatic conditions that produced high amounts of terrigenous mud, provided favourable conditions for the development of stratigraphic traps in the clastic-prone lower Carboniferous, while the advent of tropical carbonates produced reefal buildups in the Mid Carboniferous. Four stratigraphic trapping types are recognized: (1) truncation traps in which reservoir units were eroded on subaerially exposed proximal palaeohighs and thick underlying transgressive and highstand systems tract (TST and HST) mudstones form the bottom-seal and the rapid transgression of the offshore facies forms the top-seal; (2) pinchout traps of lowstand wedges on the flanks of distal palaeohighs, which were only affected by subaqueous reworking of previous TST–HST mudstones and were buried during the subsequent transgression; (3) incised valleys of the lowstand systems tract (LST), filled with thick fluvial and tidal sandstones, cutting either into TST–HST mudstones in the lower Carboniferous, or into exposed carbonate platforms in the Mid Carboniferous; (4) Waulsortian-type reefal buildups of the Mid Carboniferous. The four trapping types are discussed using selected outcrop examples, and are placed into regional sequence stratigraphic context of the Carboniferous depositional systems and sequence development of North Africa. These concepts can be readily applied to the subsurface and offer significant potential for new plays across North Africa.

Climatic and tectonic control on the distributionof the Oligocene reefs of the Tertiary Piedmont Basin (NW Italy)

The Tertiary Piedmont Basin (TPB) stretches along the Piedmont-Liguria border and is a late- to post-orogenic basin that evolved in a piggy-back position on the Monferrato thrust belt. Its depositional story is strongly controlled by tectonic and eustatic events. The basin was filled with mainly marine sediments (upper Eocene-upper Miocene), which unconformably overlie the Ligurian Alps and the Northern Apennines. The early stage of sedimentation of TPB records a transgressive phase (time-transgressive from the eastern to the western sectors, upper Eocene-upper Oligocene), characterized by the deposition of alluvial fan and fan delta siliciclastic conglomerates and sandstones, marine shallow-water coarse to fine grained siliciclastic sediments, and reef limestones.The TPB reefal buildups, which developed close to the paleoshoreline, have a lower Rupelian-middle Chattian stratigraphic distribution and are restricted to the central part of the southern margin of the TPB. The Valzemola-San Bernardino reef is the younger and the westernmost settlement, while the Val Lemme reef represents the older and easternmost one.The depicted stratigraphic and geographic distribution may be the result of a combined climatic and tectonic control. The lack of reef settlements west of Valzemola-Bric S. Bernardino may be due to climatic condition unfavourable for the reef development (temperature cooler than those of the early Rupelian-middle Chattian time interval). The absence of reefs in the area to the east of Val Lemme may be the consequence of bottom instability and high sedimentation rates (unfavourable conditions for the reef development) caused by the syn-sedimentary tectonic activity along the Scrivia fault, during the time interval spanning from the late Eocene to the early Oligocene (notwithstanding the favourable climatic conditions).Finally, the age of the older reefal complex, i.e. early Rupelian (Val Lemme), gives an obvious constraint to the age of the end of the N-vergent thrusting phase of the Voltri Unit onto the flysch units and its sedimentary cover.

Palaeozoic low-oxygen, high-latitude carbonates: Silurian and Lower Devonian nautiloid and scyphocrinoid limestones of the Anti-Atlas (Morocco)

High latitude Silurian and lowermost Devonian basinal limestones of northern Gondwana are characterised by mass accumulations of orthocone nautiloids, bivalves and planktonic crinoids (scyphocrinoids). Extensive exposures in the Anti-Atlas of Morocco reveal this basinal facies passes up-gradient into an iron-rich bioclastic facies associated with distal shelf siliciclastics and into crinoid–brachiopod dominated bioclastic blankets and shoals. The limestones formed in a cool-water setting in which upwelling of nutrient-rich waters during sea-level lowstands is interpreted to have triggered the abundance of nautiloids and scyphocrinoids, preservation of organic matter in under- and overlying graptolite shales as well as iron-precipitation in the neighbouring, shallower-water sub-oxic zone. The nautiloid limestones proved to be good correlation markers on a regional to global scale. Contemporaneous siliciclastic supply into the neighbouring facies belt as well as new biostratigraphic data are taken as evidence that these limestones can be correlated to 2nd-order eustatic sea-level lowstands and the following transgression. The low oxygen basinal facies disappeared more or less abruptly during the Early Devonian (Pragian) and was replaced by a well oxygenated basinal lowstand facies, suggesting the end of upwelling. At the same time, the first reefal build-ups (coral-biostromes and mud-mounds) appear in North Africa, indicating a significant rise in water temperatures. The change from low oxygen lowstand mass accumulations to well oxygenated basinal facies with a concomitant rise in water temperature is in accordance with recent models of a cool Silurian to earliest Devonian (Lochkovian) ocean with oscillating oxic to anoxic (ventilated to poorly circulated) oceanic states and a warm ocean with anoxic to euxinic (poorly ventilated to stagnant) ocean waters from the beginning of the Pragian onwards.

Upper Jurassic Platform succession with characteristics of a deeper water intraplatform trough (Mt. Svilaja, Croatia)

From the middle part of the Late Jurassic (Kimmeridgian), the hitherto uniform depositional environment of the Adriatic carbonate platform began to show differentiation. In the central part of the platform two intraplatform troughs were formed, one of which stretched from the Karlovac area southward and is known as the Lemeš Trough. The depositional processes that took place within this trough can be observed in the sedimentary succession of Mt. Svilaja. Based on investigated facies characteristics five lithofacies units were distinguished, representing three paleoenvironmental units: (1) shallow subtidal zone below the fair-weather wave-base; (2) intraplatform trough; (3) shallow subtidal zone above the fair-weather wave-base. The environmental changes are related to the tectonically-controlled retrogradation of the shallow subtidal platform, when the deeper-water intraplatform trough area, connected with the open Tethys realm, spread over the sunken part of the platform. Progressive infilling of the trough with bioclastic material led to the re-establishment of the shallow subtidal environment, and the area of the previously existing intraplatform trough was occupied by coral-hydrozoan reefal buildups.

Seismic stratigraphy, tectonics and depositional history in the Halk el Menzel region, NE Tunisia

In the Halk el Menzel area, the proximal- to pelagic platform transition and related tectonic events during the Upper Cretaceous–Lower Miocene have not been taken into adequate consideration. The integrated interpretation of outcrop and subsurface data help define a seismic stratigraphic model and clarify the geodynamic evolution of the Halk el Menzel block. The sedimentary column comprises marls and limestones of the Campanian to Upper Eocene, overlain by Oligocene to Lower Miocene aged siliciclastics and carbonates. Well to well correlations show sedimentary sequences vary considerably in lithofacies and thicknesses over short distances with remarkable gaps. The comparison of sedimentary sequences cut by borehole and seismic stratigraphic modelling as well help define ten third order depositional sequences (S1–S10). Sequences S1 through S6 (Campanian–Paleocene) are mainly characterized by oblique to sigmoid configurations with prograding sedimentary structures, whereas, sequences S7–S10 (Ypresian to Middle Miocene) are organized in shallow water deposits with marked clinoform ramp geometry. Sedimentary discontinuities developed at sequence boundaries are thought to indicate widespread fall in relative sea level. Angular unconformities record a transpressive tectonic regime that operated from the Campanian to Upper Eocene. The geometry of sequences with reduced thicknesses, differential dipping of internal seismic reflections and associated normal faulting located westerly in the area, draw attention to a depositional sedimentary system developed on a gentle slope evolving from a tectonically driven steepening towards the Northwest. The seismic profiles help delimit normal faulting control environments of deposition. In contrast, reef build-ups in the Eastern parts occupy paleohighs NE–SW in strike with bordering Upper Maastrichtian-Ypresian seismic facies onlapping Upper Cretaceous counterparts. During the Middle–Upper Eocene, transpressive stress caused reactivation of faults from normal to reverse play. This has culminated in propagation folds located to the west; whereas, the eastern part of the block has suffered progressive subsidence. Transgressive carbonate depositional sequences have predominated during the Middle Miocene and have sealed pre-existing tectonic structures.

Interaction of the Zagros Fold–Thrust Belt and the Arabian-type, deep-seated folds in the Abadan Plain and the Dezful Embayment,SW Iran

The Dezful Embayment and Abadan Plain (SW Iran) contain major parts of the remaining Iranian oil reserves. These oil provinces are characterized by two types of structural closure: very gentle N–S- to NE–SW-trending basement-cored anticlines (Arabian-type highs) in the SE; and open to tight, NW–SE-trending thrust-related folds in the NE (Zagros Fold–Thrust Belt; ZFTB). Most deep-seated anticlines are upright and symmetrical in Cretaceous and older units. In some cases they reveal steep faults in their core which, in the light of regional observations, suggest that the basement is involved in the faulting. Untested plays around these anticlines include reefal build-ups, debris flows, truncated sedimentary sections and onlapping clastic units. The ZFTB shows a classic structural style, with overall shortening reflected in thrust displacement declining from the Dezful Embayment towards the frontal zone in the Abadan Plain. The Early Cambrian Hormuz Salt represents the fundamental sole for the fold–thrust belt and locates major fault-propagation folds in the southwestern Dezful Embayment. These folds represent the main petroleum target of the area. Another important unit is the Mid-Miocene Gachsaran Formation. This detachment reveals both in-sequence and out-of-sequence thrusting. Interaction of deep-seated anticlines and fold–thrust structures results in thrust imbrications and formation of duplexes within the Gachsaran Formation when thrusts abut deep-seated anticlines. Above the crest of the anticlines, thrusts are forced up-section into syn-tectonic deposits, whereas the forelimb reveals out-of-the-syncline thrusts. Several petroleum plays are identified in such zones of structural interaction, including anticlines above buttress-related duplexes, out-of-sequence imbricate thrust fans with associated folds above major anticlines, truncation of footwall layers below potentially sealing thrusts, and sub-thrust anticlines.

Differential compaction as a control on depositional architectures across the Maiella carbonate platform margin (central Apennines, Italy)

The role of differential compaction as a control in the creation of accommodation and on compaction-modified depositional features and stratal geometries across the Maiella platform margin, has been investigated through a combined analysis of seismic scale outcrops, porosity evaluation and modeling. Geologic evolution and large exposures make the platform margin of the Maiella an ideal place to investigate the effects of differential compaction. A high-relief cemented carbonate platform, a deep basin filled with highly compactable deposits, and a prograding grain-rich succession sealing morphologic differences across the platform margin, represent suitable features for promoting differential compaction. Stratal relationships across the platform margin exhibit evidence of differential compaction-induced effects, such as basinward divergence and thickening of strata, updip pinch-out of wedge-shaped stratal packages, and an anticline hinge. Porosity analysis and modeling indicate that, through progressive loading, mechanical and chemical processes act in concert to destroy most of the depositional porosity. Mechanical compaction appears to have played the greatest part in the total budget of compaction. However, chemical compaction seems to have played a prominent role in the formation of geometrically consistent depositional profiles during progradation. Due to differential compaction across the platform margin a compaction hinge formed concomitantly with the beginning of progradation, producing a basin-facing monocline characterized by the progressive steepening of basinward stratal dips. The resulting compaction-induced stratal deformation, together with sea level changes, controls the distribution, and depositional timing of wedge-shaped stratal packages during late Cretaceous and Paleocene and the distribution of coral–algal reef buildups, during the late Eocene–early Oligocene. The development of the compaction hinge usually follows the progressive increase of loading, but a decrease in compaction dissipation may be caused by overburdening of compactable deposits. This will cause a delayed compaction-induced subsidence, whose effects will be produced after deposition, i.e. during loading interruption. This mechanism is thought to have an important role in the timing of compaction-induced subsidence throughout the end of Cretaceous and the early Tertiary, and it is considered, together with sea level, as the main controlling process of the occurrence and distribution of downslope sediments, during a long-lasting period of platform emersion.

Echinoid fauna from a Late Oligocene (Chattian) reef at Damon Mound, Brazoria County, Texas

The echinoid fauna collected from the Late Oligocene (Chattian) reef at Damon Mound, Brazoria County, Texas, includes at least seven species. The reef can be divided into four zones, in ascending order: reef drape, Porites thicket, reef core, and back-reef. The reef drape represents the deepest water and includes the echinoid species Clypeaster marinanus, Agassizia mossomi, and Lovenia alabamensis. The overlying zone consists of interbedded calcilutites and Porites coral thickets, and includes the species Echinometra prisca, C. marinanus, Clypeaster cf. oxybaphon, and Brissus exiguus. The reef core includes Prionocidaris cojimarensis and E. prisca. The shallow back-reef deposits include only C. marinanus. The Damon Mound fauna is more closely related to tropical Oligocene faunas of Mexico and the Antilles than to Gulf and Atlantic Coast faunas of the United States of the same latitude. During the Oligo-Miocene, Damon Mound and similar salt dome reef buildups in the northwestern Gulf of Mexico may have been havens for tropical species that could not have otherwise survived at that latitude.