Platform Top Research Articles

Unravelling the depositional origins and diagenetic alteration of carbonate breccias

Carbonate breccias dissociated from their platform top counterparts are little studied despite their potential to reveal the nature of past shallow-water carbonate systems and the sequential alteration of such systems. A petrographic and stable isotopic study allowed evaluation of the sedimentological and diagenetic variability of the Cenozoic Batu Gading Limestone breccia of Borneo. Sixteen lithofacies representing six facies groups have been identified mainly from the breccia clasts on the basis of shared textural and compositional features. Clasts of the breccia are representative of shallow carbonate platform top and associated flank to basinal deposits. Dominant inputs are from rocky (karstic) shorelines or localised seagrass environments, coral patch reef and larger foraminiferal-rich deposits. Early, pre-brecciation alteration (including micritisation, rare dissolution of bioclasts, minor syntaxial overgrowth cementation, pervasive neomorphism and calcitisation of bioclasts and matrix) was mainly associated with marine fluids in a near surface to shallow burial environment. The final stages of pre-brecciation diagenesis include mechanical compaction and cementation of open porosity in a shallow to moderate depth burial environment. Post-brecciation diagenesis took place at increasingly moderate to deep burial depths under the influence of dominantly marine burial fluids. Extensive compaction, circum-clast dissolution seams and stylolites have resulted in a tightly fitted breccia fabric, with some development of fractures and calcite cements. A degree of facies-specific controls are evident for the pre-brecciation diagenesis. Pervasive mineralogical stabilisation and cementation have, however, led to a broad similarity of diagenetic features in the breccia clasts thereby effectively preserving depositional features of near-original platform top and margin environments. There is little intra-clast alteration overprint associated with subsequent clast reworking and post-brecciation diagenesis. The diagenetic-, and to an extent depositional- and clast-characteristics of the Batu Gading deposits are diagnostic of breccia origins. The predominance of: early and pervasive stabilisation of calcitic components, pervasive compaction resulting in a fitted texture, and paucity of meteoric dissolution or cementation effects are collectively all indicators of slope deposition and lithification. These features are comparable with other regional and global examples of submarine slope breccias, and in particular those also from syntectonic settings (Wannier, 2009). The results of this study, along with regional analogues, suggest the potential for reworked carbonate debris in slope settings to be a viable way of investigating carbonate platform variability and their subsequent alteration in the absence of preserved platform top or margin deposits.

The mid-Cretaceous Natih Formation in Oman: A model for carbonate platforms and organic-rich intrashelf basins

The Natih Formation in Oman is an exceptionally well–exposed and studied example of a large (>1000 km [621 mi]) epeiric, shallow marine, tropical carbonate platform system with adjacent organic-rich intrashelf basins. It serves as an outcrop analog for many of the mid-Cretaceous giant oil reservoirs as well as for unconventional reservoirs in the Mesozoic of the Arabian plate. A key feature of the Natih Formation is its strict hierarchical stratigraphic organization at multiple orders of depositional sequences (third to fifth), which was first demonstrated in outcrop and subsequently adopted in the seismic and log data sets of the nearby subsurface. It has provided a high-resolution time framework for the robust sequence stratigraphic model that has been developed for an area of 6000 km2 (2317 mi2). Deposited in a calm tectonic setting, the Natih Formation provides an excellent example to demonstrate the principles of carbonate systems’ response to eustatic sea level change and some fundamental differences with siliciclastic systems. Key aspects that are addressed include the influence of the rate of sea level rise on the transition from ramp to platform, the creation of organic-rich intrashelf basins through differential accumulation rates, changing facies models within third-order sequences, and the importance of different platform top channel types. The economic relevance of this case study is the improved prediction of reservoir, seal, and source rock facies distribution and the associated early diagenetic overprint, insights that have been applied in the reservoir modeling of nearby oil fields as well as in exploration-scale studies.

The Sierra del Cuera (Pennsylvanian microbial platform margin) in Asturias, north Spain

ABSTRACT Stratal geometry and lithofacies architecture from carbonate outcrops provide critical information for subsurface prediction of rock types, their spatial distribution, and reservoir quality. Such observations guide and support exploration, appraisal, development, and production strategies. The Pennsylvanian Sierra del Cuera (SDC) carbonate platform outcrop of the Cantabrian Mountains in north Spain offers such information through a seismic-scale cross section of a microbial, unrimmed, and high-relief platform to basin transect, deposited during an icehouse period. The SDC outcrops provide alternative platform margin models to those developed from coral–algal-dominated Neogene carbonate systems and serve as direct analogs for supergiant fields such as Tengiz, Kashagan, and Karachaganak in the Pricaspian Basin of Kazakhstan and potentially for other high-relief platforms with microbial margins. Key features of the SDC platform are (1) a flat-topped platform with a rollover into a steep (30°–40°) and high-relief (600–850 m [1960–2790 ft]) depositional slope and (2) the presence of a microbial boundstone factory from the platform break to nearly 300 m (984 ft) paleowater depth on the slope, stabilized by pervasive marine cementation. The SDC represents a nonactualistic type of carbonate platform margin where the microbial carbonate factory on the upper slope controlled the rate of progradation instead of the platform top–sourced sediment input. These features have significant implications for the interpretation of the evolution and controlling factors of carbonate depositional systems in outcrops and in the subsurface.

Spatial and temporal distribution of microbial carbonates, skeletal and non-skeletal grains in a Pennsylvanian carbonate platform (Valdorria, Northern Spain)

The Valdorria carbonate platform, in northern Spain, features a well-preserved and continuous outcrop of a Bashkirian platform to basin transect. During the Asatauian (late Bashkirian), the platform growth changed gradually from a progradational to an aggradational mode, recording 10 most likely short-eccentricity-controlled cyclothems. The facies of the platform-top and slope have been mapped, and the architecture of each cyclothem schematically reconstructed. The distribution of microbial carbonates (microbially mediated precipitates), skeletal (grazers/burrowers, corals/filter feeders, algae, foraminifera, Osagia-like oncoids and Thartarella-Terebella worm tubes) and non-skeletal grains (faecal pellets) has been quantified for the transgressive–regressive periods of deposition (transgressive, maximum flooding, early and late regressive intervals) of each cyclothem.This study shows that the distribution of microbial carbonates, skeletal and non-skeletal grains along a carbonate platform transect is variable through time and mainly governed by a set of measurable interconnected factors regulating the local palaeoenvironments: the water depth and the wave energy (facies) along the platform profile (inner, outer, break, slope) during periods of sea-level fluctuations (transgressive to regressive intervals of deposition). Auloporid corals, siliceous sponges, phylloid algae and Osagia-like oncoids are characteristic of a low-energy environment situated from estimated palaeo-water depths of 25 to 80metres below sea level (mbsl onward) that formed in the outer platform during the maximum-flooding intervals. Anthracoporella–Archaeolithoporella boundstone is characteristic of the moderate-energy environments created in the horizontal inner-platform during the transgressive intervals. Microbially mediated precipitates are reliable indicators of the slope and Masloviporidium(?) indicator of low-energy environments of the upper slope. On the platform-top, Ungdarella and Donezella are more abundant during the early regressive intervals than during any other interval. Stacheoids are mainly present in the water depth range of 10 to 60mbsl, tournayellids from 15 to 35mbsl and archaediscids from 15 to 75mbsl, all in low- to moderate-energy conditions. Rugose corals are common in the water depth range of 5 to 35mbsl, either during the transition from the early to the late regressive intervals, as solitary forms or forming boundstone, or during periods of higher sea-level stand, as solitary forms in floatstone. Fenestellids, fistuliporids and trilobites occur mainly in the slope, but are also common in the platform top during the maximum flooding and/or the early regressive intervals. Archaeolithoporella is a reliable indicator of the platform top deposits, but its abundance is closely linked to the presence of adequate substrates. A high abundance in the assemblage of gastropods, endothyrids, palaeotextulariids, bradyinids and fusulinids characterizes the platform top shallow-water coated-grain grainstones formed under high-energy currents during the late regressive intervals. Tetrataxids act as encrusters on microbial precipitates of the slope or on phylloid algae and Archaeolithoporella of the platform top, but are also observed as nuclei of coated grains. Brachiopods are more abundant in low- to moderate-energy and 35 to 55mbsl environments that prevailed during the transgressive intervals. Faecal pellets were commonly observed in low-energy environments, from 50 to 325mbsl, at the platform break and in the slope, and the same applies for Thartarella-Terebella worm tubes that are virtually absent in high-energy deposits. Finally, crinoids, lasiodiscids and tuberitinids are randomly distributed and are not indicative of any specific environment.The here well-constrained distribution patterns of microbial carbonates, skeletal and non-skeletal grains in Valdorria outcrop hold the potential to serve as a reference for interpretation of depositional environments of other platforms using biota associations and depositional texture, particularly in regions where the platform geometry is not preserved or is not fully exposed.

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.

Presalt stratigraphy and depositional systems in the Kwanza Basin, offshore Angola

The Lower Cretaceous presalt section in the Kwanza Basin contains an excellent petroleum system that includes “synrift” strata (Barremian) overlain by a “sag” interval (Aptian) and capped by the Loeme Salt. The upper synrift is generally limestone with widespread mollusk packstones and grainstones (coquinas) deposited in a fresh-to–moderately saline (alkaline) lake. The sag interval is characterized by carbonate platforms and silica-rich isolated buildups formed in highly evaporated, highly alkaline lakes. Shrubby (dendritic), microbially influenced boundstones and intraclast–spherulite grainstones accumulated in shallow water on platform tops. Microbial cherts were deposited as organic buildups on large, isolated structural highs basinward (west) of platforms, and they apparently formed at low temperatures in very alkaline lake water. Shrubby boundstones and microbial cherts have vuggy pores that are primary and result in high permeability. Wackestones and packstones with calcitic grains (mainly spherulites) in dolomite or argillaceous dolomite were deposited in slightly deeper, low-energy sag environments. In addition, clays (especially stevensite) precipitated out of the silica-rich, highly alkaline lake waters. During sag deposition, calcite precipitated on the shallow lake floor with morphologies that ranged from spherulites to shrubs and included a continuum of intermediate forms. Spherulites probably precipitated just below the sediment–water interface. Spherulites and shrubby calcites are commonly recrystallized. Spherulites floating in stevensite probably formed in deeper lacustrine environments. Organic-rich mudstones were deposited in even deeper lacustrine environments in synrift and sag intervals, and they are likely the source of most hydrocarbons in this system. These interpretations are supported by seismic, core, petrographic, and stable isotope data.

Morphologies and depositional/erosional controls on evolution of Pliocene-Pleistocene carbonate platforms: Northern Carnarvon Basin, Northwest Shelf of Australia

The detailed morphologies, evolution and termination of latest Neogene tropical carbonate platforms in the Northern Carnarvon Basin (NCB), on the passive margin of the Northwest Shelf (NWS) of Australia, defined based upon mapping using 3D seismic images, reveal the history of local/regional oceanographic processes, fluctuations in relative sea-level and changing climate. Cool-water carbonate deposition, dominant during the early-middle Miocene, was followed by a siliciclastic influx, which prograded across the NWS beginning in the late-middle Miocene, during a period of long-term global sea-level fall. The resulting prograding clinoform sets, interpreted as delta lobes, created relict topographic highs following Pliocene termination of the siliciclastic influx. These highs constituted multiple favorable shallow-water environments for subsequent photozoan carbonate production. Resultant platform carbonate development, in addition to being a response to cessation of siliciclastic influx and the existence of suitable shallow-water substrate, was also influenced by development of the warm-water Leeuwin Current (LC), flowing southwestward along this margin.Four flat-topped platforms are mapped; each platform top is a sequence boundary defined by reflection onlap above and truncation below. Successive platforms migrated southwestward through time, along margin strike. All platforms exhibit predominantly progradational seismic geometries. Mapped tops are ≥10km wide. Seismic evidence of karst on three of four platform tops, e.g., v-shaped troughs up to 50m deep and ~1km wide, and broader basins with areas up to 20km2, suggests episodic subaerial exposure that may have contributed to platform demise. Platform 4, the most recent, is unique in having interpreted biohermal build-ups superimposed on the progradational platform base. The base of these interpreted patch reefs now lies at a water depth of ~153m; therefore, we suggest that these reefs developed post-LGM (~21Ka); Subsequent reef drowning occurred, possibly as a result of accelerated subsidence and turbidity associated with formation of adjacent sediment drifts and fluctuations in strength of the LC during the late Pleistocene. Progressive drowning and termination of platforms from northeast to southwest along strike may have resulted from unknown combinations off climate factors, supplemented by differential compaction of the deltaic substrate and/or differential tectonic subsidence caused by mantle induced downwelling.

The Extent and Patterns of Sediment Filling of Accommodation Space On Great Bahama Bank

Abstract Great Bahama Bank (GBB) is the modern example of a flat-topped, isolated carbonate platform. It is a major modern location of carbonate deposition that stands behind much of the understanding of modern processes of carbonate sedimentation, serves as a training venue for academia and industry, is the basis for numerous geological models, and is commonly used as a reservoir analog. GBB also provides valuable insight into the extent and patterns of sediment fill of accommodation space atop an isolated carbonate platform. Satellite imagery (Landsat TM and ETM+) and an extensive set of water-depth measurements (n = 5,723) were used to map bathymetry across GBB and derive a digital terrain model (DTM). Analyzing the extent, patterns, and nature of sediment fill of accommodation space was facilitated by partitioning a depositional facies map on the basis of the DTM to show that 18% of accommodation on the GBB is “overfilled and filled” (emergent to −1.5 m water depth), 52% is “underfilled” (−1.5 to −6.0 m), and 30% is “unfilled” ( −1.5 m water depth) and therefore the greater part of available accommodation (88,000 km2) remains incompletely filled with sediment. Areas of filled accommodation mostly extend platformward from the western coastlines of islands, which in turn are preferentially distributed along the eastern (windward) margin of the GBB. Seventy percent of sediment in the areas of filled accommodation is rudstone, high-energy grainstone, grainstone, and mud-poor packstone. Although dominated by grainstones (45%), since it occupies so much space (55,000 km2), the underfilled sector also contains the most heterogeneous facies mosaic and by definition, therefore, the greatest and most complex lateral facies variations. Although islands are numerous (n = 1,430) they occupy only 8%, or 8,700 km2, of the platform top. Despite their small proportional occupancy of the platform, islands play a direct role in the accumulation of muddy fabrics and exert a sphere of influence over the character of sedimentation for many tens of kilometers from their coastlines. Further, since islands represent the portion of the bank where accommodation has been overfilled, their rarity emphasizes the challenges that such a large platform faces in filling accommodation space, even given the diverse grain factories producing carbonate sediment (mud precipitation through whitings, ooids and reefs along the platform margins, skeletal sediments and nonskeletal grains such as fecal pellets, peloids, and pelletoids). Two factors can be evoked to explain the inability of GBB to fill accommodation space. First, falling sea level during the Pleistocene appears to have repeatedly aborted the filling process. By analogy, it will not be able to do so in the Holocene either, a situation exasperated by the second factor, which is that most of the Holocene GBB, particularly areas away from islands, presently lack any appreciable filling. Instead, facies analysis suggest reduced sedimentation and enhanced cementation. Through regional mapping, the work delivers a platform-wide assessment of how accommodation is filled and the facies responsible for the filling. Amongst other trends, by highlighting how the thickest accumulations of Holocene sediment are skewed to the windward margin of the GBB and related to the presence of islands, whereas the thinner accumulations, which dominate the platform interior and are more grainy but also more heterogeneous than the thicker deposits, these data might contain useful guidelines as to how depositional cycles might vary laterally in ancient systems.

Halokinetic sequences in carbonate systems: An example from the Middle Albian Bakio Breccias Formation (Basque Country, Spain)

In diapir flanks, unconformity-bounded sedimentary packages associated with gravity-driven deposits, controlled by the ratio between the rates of sediment accumulation and diapir growth can be interpreted in the context of halokinetic sequences. The Bakio Breccias Formation (Basque Country, Spain) corresponds to redeposited carbonate deposit that developed in response to the Bakio diapir growth during the Middle Albian. These deposits provide on of the rare documented example of carbonate-dominated halokinetic sequences. The Bakio Breccias Formation consists of an alternation of clast- and matrix-supported breccias, calcirudite, calcarenite and marl, deposited along the flanks of the diapir. The description and the analysis of the Bakio Breccias Formation lead to a new model for carbonate-dominated halokinetic sequences. These sequences differ from their siliciclastic counterpart because sediment accumulation rate is controlled by carbonate platform growth on the topographic relief top of the diapirs, while sediments are preferentially deposited in the mini-basins adjacent of the diapirs, in siliciclastic settings. During transgressive system tract, carbonate platform are able to keep up with the sea level rise and to aggrade on top of the diapirs, forming thick and resistant roof, which is assumed to limit the diapir growth and thus to favour the development of halokinetic sequences with low angle unconformities (wedge halokinetic sequences). During late highstand system tract deposition (and lowstand system tract if present), platform progradation results in high sediment accumulation in the adjacent depocenters, loading the autochthonous salt layer and promote diapir growth and creation of topographic relief. In addition, if the diapir roof reaches emersion, karstification of the carbonate platform top may also favour roof destruction and diapir growth. Depending on the thickness of the roof developed previously and the amplitude of the sea level fall, the halokinetic sequences with the emersion and the karstification of the carbonate platform may display high angle unconformities (hook halokinetic sequences). Furthermore, gravity-driven deposits are assumed to be more common in carbonate-dominated halokinetic sequences, compared to their siliciclastic counterparts, since carbonate platform aggradation creates steep slopes on the diapir margins, leading to the partial collapse of the margin, even when limited diapir growth occurs. The carbonate-dominated halokinetic sequence model proposed here is an important tool for the prediction of potential reservoir distribution, seal and hydrocarbon migration in flanks of salt diapirs where carbonate platform developed.

Wings, mushrooms, and Christmas trees: The carbonate seismic geomorphology of Central Luconia, Miocene–present, offshore Sarawak, northwest Borneo

Central Luconia is a geological province of the Sarawak Basin, offshore northwest Borneo, characterized by extensive development of Miocene to Holocene carbonate buildups. The buildups are up to 2-km (1-mi) thick and reveal complex seismic geomorphologies including out-building, in-building, carbonate-siliciclastic intercalations, and coalescence of neighboring carbonate bodies. Intermittent carbonate and siliciclastic deposition is interpreted to have been governed by frequently oscillating sea level and variable siliciclastic input. Location of the buildups with respect to deltaic facies and the seismic expressions of marginal carbonate strata, locally known as wings, are used here to classify carbonate seismic geomorphologies. Buildups encompassed by deltaic topsets are said to reside onshelf and are flanked by marginal carbonate deposits conformable with the onlapping siliciclastics and interpreted as thin carbonate platforms deposited in situ over prominent erosional unconformities and flooding surfaces. Such marginal facies are described on the basis of their appearance as open wings. In contrast, buildups downlapped by prodelta foresets or bottomsets, or those draped by pelagic sediments, are said to reside offshelf and are characterized by high relief, steeply dipping marginal carbonate strata discordant with respect to the relatively flat-lying platform carbonates as well as the onlapping siliciclastics. These marginal strata have an appearance of closed wings and are interpreted as slope aprons composed of carbonate material derived from the productive platform tops and deposited off-platform in deeper water. Carbonate geometries range from contracting-upward pinnacles to expanding-upward platforms in onshelf settings, whereas buildups located offshelf have mostly contracting-upward or aggrading morphologies. Buildups deposited onshelf have generally maintained a similar height of depositional slopes as they evolved, reflecting rates of background siliciclastic deposition approximating those of carbonate growth. In contrast, depositional slopes of offshelf buildups have become progressively higher, owing to limited siliciclastic input. Observations presented here on carbonate geomorphology and adjacent siliciclastic stratigraphy provide important clues for the understanding of the geology of the province. Because carbonates and siliciclastics intercalate frequently, siliciclastic biostratigraphy can be used to precisely date and correlate geographically remote carbonate-reservoir intervals and intrareservoir depositional and diagenetic events such as marine transgressive or exposure surfaces or karst networks. Conversely, basin paleobathymetry can be measured by the size of carbonate clinoforms and the thickness of buildups established over flooded siliciclastic surfaces. Deposition of low-relief carbonate platforms over siliciclastic substrates indicates shallow-marine conditions, as opposed to the prevalent notion of kilometer-scale paleobathymetry. These interpretations have important implications for understanding of the sedimentology and lithology of the ambient basinal siliciclastics, with fundamental applications to hydrocarbon exploration.

Facies, geometry and growth phases of the Valdorria carbonate platform (Pennsylvanian, northern Spain)

AbstractThe 14 km wide Valdorria outcrop (Pennsylvanian, northern Spain) is one of the few examples of entirely exposed flat‐topped and high‐relief carbonate platforms reported in the fossil rock record. Laterally and vertically traceable stratal patterns expose three phases of growth. Phase I is a 430 m thick platform to slope succession that prograded over 6 km, and is dated as Early Bashkirian (Akavasian–Askynbashian). Phase II aggraded and prograded, exhibiting 180 m thickness of cyclical platform top deposits, dated as Late Bashkirian (Asatauian). Phase III is a mound‐shape structure that developed over the platform top of Phase II as a new phase of platform nucleation. It is 535 m thick and 2 km wide, and dated as Late Bashkirian (Asatauian–Transition interval). The observed changes of growth styles during platform evolution, from a prograding to an aggrading–prograding system, and a rapid aggradational phase, are inferred to be controlled by flexural subsidence in the active Cantabrian foreland basin, at the Variscan orogenic front. The metre‐scale shallowing‐upward cycles of the platform top are most probably due to glacioeustasy, as evidenced by well‐recorded subaerial exposure surfaces superimposed on subtidal deposits, and by a stratal pattern recurrent in a short interval of about 160 kyr. Observations of outcropping Bashkirian cyclothems in an isolated carbonate system, devoid of siliciclastic input, are relevant for a better understanding of the impact of high‐frequency sea‐level fluctuations on the carbonate factory. Moreover, progradation of the platform margin during Phase I reaches a rate of 2500 m/Myr, and 1810 m/Myr during Phase II; rates that are high when compared to other Pennsylvanian examples. The aggradation rate of 447 m/Myr calculated for the Late Bashkirian–Transition interval (Phases II and III; uncorrected for compaction, missing beats and erosion) is uncommonly high in comparison to coeval Pennsylvanian examples. The platform exhibits a self‐nourishing prograding microbial boundstone‐dominated slope. Thus, the slope‐shedding model applies well to Valdorria. However, Phase II recorded eustatic variations able to inhibit the slope microbial boundstone factory during low sea‐level stands; this is marked by common slope red‐stained breccias synchronous to platform top subaerial exposure phases. Contrarily, periods of relative high sea‐level and rapid subsidence in Phase III registered a greater development of cemented microbial boundstone. These observed, partly opposing relationships of sea‐level stands, shedding modes and slope architecture provide an improvement of the currently used slope‐shedding model. The overall architecture of the Valdorria outcrop compares well with that of other contemporaneous platforms, such as Sierra del Cuera and Bolshoi Karatau. Valdorria shares the high‐relief and flat‐topped, steep slopes, cyclothemic patterns and occurrence of karst features with the Pricaspian Basin platforms (Tengiz, Karachaganak and Kashagan), with minor variations in facies distribution of the internal platform. Furthermore, the continuous seismic‐scale outcrop of Valdorria, together with its isolated setting and asymmetrical growth, makes it a very good candidate for potential subsurface analogues of hydrocarbon‐bearing systems.

Diagenesis and fracturing of a large-scale, syntectonic carbonate platform

The influence of coeval tectonics on carbonate platform development is widely documented, yet the diagenesis of such syntectonic platforms is barely evaluated. An outcrop, petrographic and geochemical study details here for the first time the diagenesis of the Tonasa Limestone Formation developed in an extensional regime in central Indonesia. This equatorial carbonate system was affected by block faulting, tilt-block rotation, differential uplift and subsidence throughout its Eocene to Early Miocene history (Wilson, 1999; Wilson et al., 2000). The Tonasa carbonate platform is dominated by alteration in shallow to deeper burial depths by fluids with predominantly marine precursor origins. Mechanical and chemical compaction features are common, as are a range of mainly burial-related granular mosaic, blocky and equant calcite cements. Earlier marine cements and meteoric influences are rare, being highly localised to block faulted highs and/or bathymetrically upstanding platform margin areas. Early marine micritisation of allochems was common on the platform top. Tectonic uplift together with a major oceanic throughflow current are thought to be key influences on localised karstification, meteoric diagenesis and marine cementation. The distribution and orientation of faults, fractures and calcite veins together with evidence for their relative timing are the strongest manifestation of tectonism coeval with diagenesis. There is concordance in the orientation and timing of structures affecting the Tonasa Platform with those basin-wide, with the potential for reactivation of pre-existing basement fabrics. Tectonic subsidence, including fault-associated differential subsidence, controlled the degree of burial diagenesis impacting different areas of the platform. A predominance of burial diagenetic features and dearth of earlier marine or meteoric cementation is seen in other Tertiary equatorial platforms and is partly attributed to: (1) predominance of non-framework building larger benthic foraminifera and/or algae that are prone to remobilisation, have low production rates and limited potential to build to sea level, and (2) high runoff due to the equatorial humid climate contributing to lowered marine salinities in SE Asia. Underlying tectonic reasons for the preponderance of a “regional” diagenetic signature over a “syntectonic” one, fracturing excepted, are: (1) development on the flanks of a backarc basin not on typical continental crust, (2) key platform influencing structures are oblique to the main extensional direction in the basin, and (3) development in an overall subsiding tectonic regime, post-dating basin initiation. The aim here is that this study will contribute to understanding diagenetic alteration of syntectonic carbonate platforms, and those from equatorial regions.

LATE MOSCOVIAN TO EARLY KASIMOVIAN FUSULINES FROM THEÁNDARA MASSIF, PICOS DE EUROPA (PENNSYLVANIAN, CANTABRIAN ZONE, NORTHERN SPAIN)

Extensive outcrops of fusuline-rich limestone strata of Pennsylvanian age occur in the Eastern or Andara Massif of the Picos de Europa mountains in Northern Spain. The stratigraphic section is composed of the upper part of the Picos de Europa Formation, a carbonate succession deposited on a broad platform top in a distal foreland-basin margin setting, and the unconformable overlying Las Llacerias Formation, which represents carbonate-ramp strata deposited during the deformation of the underlying Picos de Europa platform. The tectonic setting allowed the accumulation of a very thick stratigraphic succession during the late Pennsylvanian, when the basin was located at paleoequatorial latitudes in the gulf of the western Palaeo-Tethys Ocean, adjacent to the eastern coast of Pangea. An in-depth stratigraphic and micropaleontological study involving six sections and several isolated localities revealed an interval spanning the uppermost Moscovian to the lower-middle Kasimovian. This study has allowed us to establish a composite section, more than 400 m thick, recording the uppermost part of the Fusulinella Zone ( F. alvaradoi subzone), the Protriticites Zone and the Montiparus Zone. Forty-one fusuline taxa are described, including four new species, Protriticites grajalensis, P. quasiovoidalis, P. benshae, and Pseudotriticites cantabricus. The composition of the fusuline assemblages differs from those of the Carboniferous in key areas of Eastern Europe, and reveals their affinity to Central Asian assemblages as part of a separate biogeographic province. In spite of these paleobiogeographic differences, the observations on the fusuline evolutionary lineages, as well as the presence of a few common forms, made it possible to establish an approximate correlation between the Andara and the Eastern European successions. We also report a previously unknown thrust fault, refine the stratigraphy and age of the successions outcropping in the Andara Massif, and determine the diachronism of the base of the unconformable Las Llacerias Formation.

Patterns in the distribution of Aptian rudists and corals within a sequence-stratigraphic framework (Maestrat Basin, E Spain)

The ecological zonation of, and environmental controls on rudist and coral assemblages on carbonate platforms of the Old World have received more attention for Late Cretaceous examples than for their Early Cretaceous counterparts. This study accordingly investigates the vertical and lateral distribution of Aptian rudist bivalves and scleractinian corals on a carbonate platform succession from the western Maestrat Basin (E Iberian Peninsula). Here, colonial corals grew profusely on an isolated platform top environment during an earliest highstand stage of a long-term trend of relative sea level, as well as on marly slope settings during higher-frequency transgressive pulses. During the later highstand stage within a longer-term relative sea-level cycle, a facies belt dominated by autochthonous rudist bivalves overlaid the coral meadow that had developed on the isolated platform top. The internal part of this carbonate platform with rudists is dominated by slender elevator caprinids such as Caprina parvula, whereas requieniids and polyconitids predominate in the external zone. The abundance of caprinids in the internal platform is remarkable given that caprinid lithosomes of late Early Aptian age are usually rare in the northern margin of the Tethys. The proliferation of caprinids in this case was probably favoured by the apparently more isolated nature of the carbonate platform. On the slopes, the coral communities that flourished during higher-frequency transgressive pulses are overlain by carbonates with rudists, mainly requieniids, shed from the platform top during normal and forced regressive higher-frequency changes of relative sea level. Accordingly, the vertical change from coral-dominated to rudist-dominated facies in both platform top and slope settings records progradation. To decipher the long-term relative sea-level changes that controlled the deposition of this carbonate succession, a sequence stratigraphic analysis was performed. Two depositional sequences including a late Early Aptian (intra Dufrenoyia furcata Zone) forced regressive stage of relative sea level, which subaerially exposed and incised the Early Aptian succession to a depth of 21m, were recognised. The incisions were back-filled with peritidal deposits during the subsequent marine onlap. The rudist- and coral-bearing carbonates were deposited along platform top to slope profiles lacking a barrier margin, and hence, lagoon environments.

Sequence stratigraphy after the demise of a high-relief carbonate platform (Carnian of the Dolomites): Sea-level and climate disentangled

Sedimentary facies analysis aided by quantitative 3D georeferenced field data is applied to constrain the sequence stratigraphy of a complex stratigraphic interval in the Late Triassic of the Dolomites. This multidisciplinary approach was the key to disentangle the timing of climatic change vs. sea-level fluctuation and their effects on shallow water carbonate depositional systems. The “Carnian Pluvial Event”, a global episode of climate change worldwide documented at low latitudes, involved increased rainfall and possibly global warming. This climatic event begins before a drop of sea-level and caused the demise of microbial-dominated high-relief carbonate platforms that dominated the Dolomites region, and was followed by a period of coexistence of small microbial carbonate mounds and arenaceous skeletal–oolitic grainstones. A subsequent sea-level fall brought to the definitive disappearance of microbialites and shallow water carbonates switched to ramps dominated by oolitic–bioclastic grainstones. The crisis of early Carnian shallow water carbonate systems of the Dolomites generated a geological surface similar to a drowning unconformity, although no transgression occurred. As the high-relief microbial carbonate systems characterized by steep slopes switched to gently inclined oolitic–skeletal–siliciclastic ramps, basins were rapidly filled. The change of carbonate depositional systems was associated with an increase in siliciclastic input, in turn triggered by the onset of a humid climatic event and only later to a sea-level drop. This evolution of carbonate systems cannot be interpreted in the light of sea-level changes only: climate change, and consequent ecological changes in the main carbonate producing biotas, induced significant modifications in depositional geometries. This case study may serve as a conceptual model for the sedimentary evolution of carbonate systems subject to ecological crisis that do not evolve in platform drowning because, despite a drop in shallow water carbonate production, a combination of low subsidence and/or sea level drop maintains the platform top at shallow depth.

Tectonic setting and stratigraphic architecture of an Early Cretaceous lacustrine carbonate platform, Sugar Loaf High, Santos Basin, Brazil

Abstract The Sugar Loaf High is an extensive basement structure within the Santos Basin, offshore Brazil, in which all of the elements of the pre-salt hydrocarbon play are present. Following a clastic-dominated early synrift phase, the later synrift and sag phases are characterized by an extensive non-marine carbonate platform over the basement high. The study area within this new 3D seismic dataset contains carbonates stratigraphically equivalent to the reservoirs within the Lula and Sapinhoá fields. Analysis of the 3D seismic data shows that the pre-salt carbonates onlap the basement of the Sugar Loaf High and form aggradational geometries on a platform that has a proximal to distal width of about 100 km and an interpreted relief of some 900 m. The carbonate platform has an aggradational and mounded basinward-facing gullied margin with slope deposits. Progradational units are imaged on the platform top that may be either carbonate or clastic in composition. The platform is preserved beneath evaporites that form the regional seal within the Santos Basin. Platforms of this size are previously unknown from lacustrine settings outside the pre-salt stratigraphy of the Santos and Campos Basins, Brazil.

A study on low strain integrity testing of platform-pile system using staggered grid finite difference method

To study the three-dimensional characteristics of wave propagation in platform-pile system, a three-dimensional computation model for transient vibration of platform-pile-soil system is established. Based on initial and boundary conditions, the numerical solution of this model is obtained. A MATLAB program is compiled through using staggered grid finite difference method. The dynamic response of the integrate pile in platform-pile-soil system is got under vertical impact force, and the reliability and feasibility of the numerical simulation are corroborated by comparing calculation result with measured data of low strain integrity testing of platform-pile system. The optimal sensor location at platform top is studied. The results show the position distancing the pile center 0.5R~0.6R (R is pile radius) is the optimal sensor location, which the line between sensor location and pile center parallels the short side. It plays a certain role in reducing three-dimensional interference through increasing shear wave velocity of surrounding soil and appropriately increasing the ratio of characteristic wavelength to pile radius. In addition, contact area has less influence on low strain integrity testing of platform-pile system.

Mapping bathymetry and depositional facies on Great Bahama Bank

AbstractSatellite imagery and an extensive set of water‐depth measurements have been used to map and critically evaluate the magnitude and patterns of bathymetry across Great Bahama Bank. Descriptions of previously collected sediment samples were combined with satellite imagery to map and refine the interpreted distribution of surficial carbonate sediments (depositional facies). Data reveal that 60% of Great Bahama Bank lies in 5 m or less of water. The deep portion occurs mainly in a generally east–west trending area in the southern portion of the platform. The re‐evaluation of the facies reveals that Great Bahama Bank is essentially a very grainy platform with muddier accumulations primarily in the lee of Andros Island. This area of Great Bahama Bank also experiences currents related to an excursion of the Florida Current onto the platform top; possibly enhancing sediment mud production through the generation of whitings. Sediment equivalents to mudstones, wackestones and mud‐rich packstones cover 8%, 5% and 14%, respectively, of the platform top, whereas sediment equivalents to mud‐poor packstones, grainstones and rudstones account for 20%, 45% and 3% of the surface area. Boundstones (reefs) were not specifically mapped in this study due to the resolution of the mapping. There is a poor relationship between the occurrence of the depositional texture and water depth, in that the grainier sediment types are abundant across the full range of water depths. The most abrupt lateral facies changes portrayed on the facies maps are observed leeward of islands, areas which also hold the highest diversity in facies type. The majority of the islands on the platform align with the north‐west/south‐east strike of the platform margin and these islands, in turn, exert control on the shape and orientation of facies belts that develop in proximity to them. For this reason, regions of the platform that contain principal islands host facies belts that align with the principal axis of the platform, whereas for regions lacking islands, the facies belts adopt an east–west trend consistent with prevailing winds and currents. There is a clear trend that the wide southern portion of the platform hosts the most continuous expanses of grain‐rich sediments.

Barnes Maze Testing Strategies with Small and Large Rodent Models

Spatial learning and memory of laboratory rodents is often assessed via navigational ability in mazes, most popular of which are the water and dry-land (Barnes) mazes. Improved performance over sessions or trials is thought to reflect learning and memory of the escape cage/platform location. Considered less stressful than water mazes, the Barnes maze is a relatively simple design of a circular platform top with several holes equally spaced around the perimeter edge. All but one of the holes are false-bottomed or blind-ending, while one leads to an escape cage. Mildly aversive stimuli (e.g. bright overhead lights) provide motivation to locate the escape cage. Latency to locate the escape cage can be measured during the session; however, additional endpoints typically require video recording. From those video recordings, use of automated tracking software can generate a variety of endpoints that are similar to those produced in water mazes (e.g. distance traveled, velocity/speed, time spent in the correct quadrant, time spent moving/resting, and confirmation of latency). Type of search strategy (i.e. random, serial, or direct) can be categorized as well. Barnes maze construction and testing methodologies can differ for small rodents, such as mice, and large rodents, such as rats. For example, while extra-maze cues are effective for rats, smaller wild rodents may require intra-maze cues with a visual barrier around the maze. Appropriate stimuli must be identified which motivate the rodent to locate the escape cage. Both Barnes and water mazes can be time consuming as 4-7 test trials are typically required to detect improved learning and memory performance (e.g. shorter latencies or path lengths to locate the escape platform or cage) and/or differences between experimental groups. Even so, the Barnes maze is a widely employed behavioral assessment measuring spatial navigational abilities and their potential disruption by genetic, neurobehavioral manipulations, or drug/ toxicant exposure.

Component-specific petrographic and geochemical characterization of fine-grained carbonates along Carboniferous and Jurassic platform-to-basin transects

Fine-grained carbonates are present throughout much of the geological record and are widely used as geochemical archives, even though their origin and diagenetic pathways remain poorly understood. Here, petrographical and geochemical properties of granulometrically separated component spectra of marine mudstones sampled along two proximal-to-distal transects (Carboniferous of Spain and Jurassic of Morocco) are documented. These settings represent end members in terms of platform geometry, steep flanked versus gentle ramp, and the aragonite versus calcite sea mode. The data from Spain reveal a bimodal organization of microcrystalline carbonate isotope values from platform top and slope and toe-of-slope settings. The data from Morocco lack a clear spatial and bathymetrical pattern. The significance of the complex, site-specific biological and physico-chemical parameters is emphasized. Mudstones have been separated in granulometric fractions of 8–5, 5–3 and <3μm respectively, and resulting particle classes are described and interpreted in terms of their origin and diagenetic pathways. Fine-grained carbonate particles from both sites show remarkably similar size and crystallographic features. Their isotopic composition reflects the volumetrically proportion and component-specific geochemical signature of each particle class. Decreasing particle size classes are characterized by decreasing isotope values. This might be due to an enhanced diagenetic reactivity of the finest micritic particles to diagenetic processes. This implies that stratigraphic differences in mean fine carbonate grain sizes may trigger shifts in isotope values. Mean bulk and mean component-specific isotope ratios from the two case settings differ by about 0.5‰ for carbon and 0.7‰ for oxygen. The results shown here are of general significance for those concerned with fine-grained carbonates-based chemostratigraphy and environmental analysis.