Platform Geometry Research Articles

Micro-seismic measurements of cliff motion under wave impact and implications for the development of near-horizontal shore platforms

Few high-resolution measurements of process-form interactions have been taken on rock coasts, but recent studies in California have shown that portable seismometers enable useful proxy measurements of wave-energy delivery to cliffs. Here we describe measurements over 20days of high frequency ground motion of cliffs formed in sedimentary (flysch) rocks at Okakari Point, north of Auckland, New Zealand. Three sensors were located in a shore-normal array inland from the cliff top and a fourth sensor was bolted to a ledge 2m above the cliff toe. The nearshore wave field in front of the cliff and shore platform was monitored using a shore-normal array of 5 wave gauges. The instrumentation provided measurements of wave-energy delivery and consequent ground motion, including the first observations of motion at the top and bottom of cliffs. Results showed that horizontal ground motion is dominant at the cliff top, whereas vertical motion is dominant at the cliff toe. Power spectra show that several high frequency peaks occur in data from the cliff toe, whereas a single, broader peak frequency occurs at the cliff top resulting from signal modification as seismic waves pass through tens of metres of cliff rock. A 100m wide shore platform at the cliff toe fundamentally controls the patterns of observed energy delivery. The shore platform is nearly horizontal, elevated close to high water level, and abruptly plunges into water >10m deep at its seaward edge. As expected, the magnitude of ground motion at all sensors is greatest during larger waves. Measurements further show that ground motion, both at the bottom and top of the cliff, is strongest at low tide and weakest at high tide. This observation is opposite to that noted at Santa Cruz, where ground motion was greatest at high tide. At Okakari Point the most significant high frequency ground motions occur at low tide when waves are forced to break (sometimes violently) against the seaward edge of the shore platform. Four distinctive frequency peaks between 1 and 50Hz increase in magnitude as tidal stage drops, implying that wave breaking against the outside edge of the shore platform represents an important source of vibration. A detailed understanding of the energy source (e.g. short duration shock pressures) and rock resonance is not provided by this study. However, quantifying the spatial and temporal patterns of energy delivery places strong emphasis on the important role of shore platform geometry in filtering wave-energy delivery to the cliff. During the 20-day experiment most wave energy was delivered to the outside edge of the shore platform, not the cliff toe. The geomorphic role of high-frequency shaking from wave impacts remains to be clearly demonstrated, but if wave impacts are capable of eroding rock then the data from this study imply that under present conditions the outside edge of the shore platform may be subject to higher erosion rates than the cliff toe. It is possible that the shore platform is currently being destroyed rather than created, but a longer programme of measurements is required to test this notion.

Wave transformation on a sub-horizontal shore platform, Tatapouri, North Island, New Zealand

Waves are generally considered to be an important control on shore platform evolution but there have been very few detailed studies of wave dynamics on rocky coasts. This study provides a detailed description of wave transformation processes across a shore platform near Gisborne, New Zealand. A field experiment was conducted on an intertidal, 250 m-wide platform that is characterised by a sharp seaward edge. Mean platform elevation is 0.7 m below mean sea-level (MSL) and platform gradient is 0.3°. Four nearly equally spaced pressure gauges were deployed over a 24-h period during fair-weather swell conditions characterised by a mean wave height of 0.4–0.5 m and wave period of 7 s. Wave energy at the seaward edge of the platform was dominated by incident swell waves with peak frequency at 0.09 Hz. At all tidal stages waves broke at the seaward edge of the platform, but changing tidal stage exerted a strong control on the rate of wave energy attenuation across the platform. The relative importance of waves at wind-wave frequencies increased in the centre of the platform as the tidal stage and water level increased, whereas infragravity waves became dominant toward the cliff toe. Three different hydrodynamic zones were distinguished across the shore platform; (i) a breaker zone close to the seaward edge; (ii) a propagation/shoaling zone in the central region of the platform; and (iii) a zone of energy dissipation across the inner part of the platform. The spatial dimensions of the three zones varied with changing tidal stage, enabling different wave processes to operate across different parts of the platform at different times. The experiment showed that wave-height attenuation is strongly controlled by tidal stage, ranging between 93% attenuation of wave height during the initial flooding stage to 44% attenuation at high tide. Wave height was found to be depth-limited, with the ratios of H max and H m0 to water depth calculated as 0.7 and 0.4, respectively. The findings imply that platform geometry has an important influence on wave processes on shore platforms. The geomorphic implications of gravity and infragravity waves are discussed.

Sequence development in an isolated carbonate platform (Lower Jurassic, Djebel Bou Dahar, High Atlas, Morocco): influence of tectonics, eustacy and carbonate production

AbstractSix sequences, each with a distinct shape, stratal anatomy, bounding surfaces, lithofacies composition and stacking patterns, were identified in the Djebel Bou Dahar isolated carbonate platform which developed in a half‐graben in the High Atlas marine rift basin during the Early Jurassic. Sequences are believed to have a complex, multi‐process origin as the result of tectonic processes (regional subsidence and fault‐block rotation) overprinting the eustatic signal. In addition, changing carbonate producers with shifting centres of deposition and relative accumulation potential influenced the filling and anatomy of the accommodation space that had been created by the hybrid tectonic and eustatic processes. Sequences I and II are tabular in shape and interpreted to primarily reflect the combined processes of uniform tectonic subsidence and eustacy which resulted in a ramp to shelf carbonate system with lagoon and tidal flat lithofacies. Onset of a rifting episode around the base of the Lotharingian and fault block rotation were responsible for the development of the angular unconformity with the underlying Sequence II, and the progressive development of a shallow‐water isolated platform and the wedge‐shaped depositional Sequences III to VI during the Lotharingian and Pliensbachian. Concurrent variations in palaeoceanographic conditions and in the carbonate factory caused a shift towards a flat‐topped platform geometry with high‐energy coated grains and sub‐photic sponge‐microbial deposition around the margin. The aggradational to retrogradational character of Sequences III, IV and VI is inferred to be a result of the combined effect of in situ carbonate production coupled with high rates of creation of accommodation space in the hanging wall. Environmental effects on carbonate production were the most probable cause for drowning of the Djebel Bou Dahar platform during the Toarcian but also influenced the filling and geometry of Sequences IV to VI. This study suggests that the commonly accepted model of pulse‐driven sequence‐development in half‐grabens may not be fully applicable for the Djebel Bou Dahar, where a more continuous rotation at sequence scale, associated with subsidence pulses of the Djebel Bou Dahar block, seems to have interacted with eustacy and carbonate production to generate sequences. Continuous rotation is suggested by the presence of regular patterns of diverging strata and numerous minor exposure surfaces in lagoonal deposits of Sequences V and VI. In addition, this study demonstrates that eustacy and variations in the carbonate factory might be as significant as tectonics in sequence development and platform drowning in carbonate platforms within rift basins. This observation could imply that the effects of eustacy and the carbonate factory might not have been correctly recognized in other similar carbonate platforms due to a tectonic‐biased interpretation.

Integration of sedimentology and ground‐penetrating radar for high‐resolution imaging of a carbonate platform

AbstractGround‐penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south‐east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground‐penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground‐penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground‐penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea‐level fluctuations controlling the Upper Jurassic carbonate production.

Ringing and springing response of triangular TLPs

Certain class of offshore structures exhibit highly intense nonlinear behavior called springing and ringing. Dynamic response of compliant structures like Tension Leg Platforms (TLP) under impact and non-impact waves responsible for ringing and springing phenomenon is of large interest to marine engineers. This paper develops a mathematical formulation of impact and non-impact waves and discusses the method of analysis of TLPs of triangular geometry under these wave effect; response of square and equivalent triangular TLPs are compared. Heave response in square TLPs show bursts but there are no rapid build-ups; gradual decays are seen in most cases looking like a beat phenomenon while such results are not predominantly noticed in case of equivalent triangular TLPs. Ringing, caused by impact waves in pitch degree-of-freedom and springing, caused by non-impact waves in heave degree-of-freedom in both the platform geometries are undesirable as they pose serious threat to the platform stability. Analytical studies conducted show that equivalent triangular TLPs positioned at different water depth are less sensitive to these undesirable responses thus making it as a safe alternative for deepwater oil explorations.

Investigating Carbonate Platform Types: Multiple Controls and a Continuum of Geometries

Abstract Current classifications of carbonate platforms use depositional gradient as the main criterion for separating systems into two end-member types, ramps and flat-topped platforms (FTPs). However, many examples do not conform to this simple classification. To investigate why this is and to better understand probable controls on platform development, we have used a series of 2D numerical forward model runs to investigate how sediment production, diffusional sediment transport, and other controls such as tectonic subsidence, antecedent topography, and relative sea-level oscillation interact to determine platform geometry. Modeling results reaffirm that rates of down-dip sediment transport relative to rates of autochthonous production are a critical factor in maintaining a ramp profile in stable cratonic settings under a constant rate of relative sea-level rise. Type of carbonate production versus water-depth curve, for example euphotic versus oligophotic, is not a significant control in our model cases. Both euphotic and oligophotic production profiles produce FTPs when diffusion coefficients are low relative to production rates, and ramps when diffusion coefficients are relatively high. These results suggest a continuum of platform types, ranging from transport-dominated, low-gradient systems at one end of the spectrum, to in situ accumulation dominated systems at the other. A system may be transport-dominated because high-energy processes are able to break down and transport even bound sediment, or because carbonate factories produce only sediment that is easily transportable under even low-energy conditions. Time evolution is also probably important. Initially low gradient systems will, in the absence of sufficiently high sediment transport rates, tend to evolve towards high-gradient flat-topped steep-margined platforms. Many observed or inferred platform geometries are therefore likely to be transient forms, and this could complicate interpretation. Investigating how basin bathymetry and style of subsidence control platform geometry suggests that, in transport-dominated systems, strata simply drape the underlying topography, and that pre-existing breaks of slope and differential fault subsidence are a stronger control on platform geometry in in situ accumulation dominated systems. Rotational subsidence tends to create transport-dominated systems during rotation as the topographic gradient increases and transport rate increases and outpaces in situ production rate. Relative sea-level oscillations tend to move the locus of sediment production laterally along any slope present on the platform, distributing the sediment accumulation across the whole width of the platform, suppressing progradation and steepening, and so favoring development of low-gradient systems. Based on all these results, we suggest that a simple cutoff classification into ramp and flat-topped platform types can still be useful in some circumstances, but a more meaningful approach may be to describe and predict platform strata in terms of a multiple-dimension platform parameter space containing a continuum of geometries controlled by sediment production, sediment diffusion coefficient, antecedent topography, differential subsidence effects, relative sea-level oscillations and perhaps other as yet unappreciated controls.

Electrochemical performance of a DNA-based sensor device for detecting toxic algae

This work illustrates the electrochemical performance of a DNA-based sensor device for detecting toxic algae. This biosensor uses an electrochemical detection of the species in a sandwich hybridisation. A thiol (biotin) labelled capture probe was immobilized onto gold (carbon) electrodes. Synthetic positive control DNA was applied to the sensor and allowed to hybridize to the capture probe. A signal probe with a horseradish peroxidase (HRP) label was then applied, followed by an antibody to the HRP and a substrate. The electrical signal obtained from the redox reaction was proportional to the amount of DNA applied to the biosensor, which in turn would be proportional to the number of cells harvested when applied to real samples. Optimization of the hybridization process was already achieved in a previous work. Elucidation of the different steps of the fabrication process from the electrochemical point of view, proof of concept with different algal species and evaluation of the influence of the transducer platform geometry and material in the biosensor analytical performance are the main achievements reported here.

Reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection

Reactive transport models (RTMs) permit quantitative investigation of diagenesis and its effects on reservoir quality. The RTM TOUGHREACT is used to investigate diagenesis in an isolated platform driven by geothermal (Kohout) convection of seawater, which has been invoked to explain dolomitization during early burial. Previous short (0.1 m.y.) RTM simulations suggested that convection can drive dolomitization, mostly at greater than 50C, and anhydritization, but complete dolomitization requires greater than 30–60 m.y. Our more extended RTM simulations (30 m.y.) indicate significant nonlinearities in the system, consistent with high-temperature experiments, with parts of the platform completely dolomitized within 10–15 m.y. As dolomitization proceeds, the process becomes predominantly flux controlled, with development of a wedge-shaped dolomite body, which thins from the margin to the interior, at considerably shallower depth and cooler temperatures (20–30C) than suggested by short simulations. Dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms. Sediment permeability and reactive surface area, commonly inversely related, are key controls. Dolomitization is limited to the margin of low-permeability muddy platforms despite a high reactive surface area. Dolomitization of more permeable grainy platforms is limited by a lower reactive surface area, occurring only in the platform core due to widespread cooling. Sedimentary layering produces a complex diagenetic stratigraphy, dolomitization favoring more reactive beds at shallow depth where permeability is not limiting, but switching to more permeable beds at depth. Bank-marginal fracturing limits dolomitization of the platform interior, whether the fractures are baffles or conduits for flow.

Porosity development, diagenesis and basin modelling of a Lower Cretaceous (Albian) carbonate platform from northern Spain

Abstract This paper describes an integrated field, petrographic and geochemical study of an Albian carbonate platform from the Basque-Cantabrian basin (northern Spain). It examines the distribution and evolution of porosity and the relationships between facies variations, sequences and variations in diagenesis. These platform carbonates were deposited on the footwall crests of active tilted blocks formed during continental rifting related to the Mesozoic opening of the Bay of Biscay and North Atlantic. The studied units document an early Albian aggradational steep-sloping platform and a late Albian, low-gradient expansive shelf separated by a hiatal unconformity spanning the middle–early late Albian ( c. 5 Ma). The late Albian platform unit also exhibits a major internal unconformity. Platform geometries and facies architecture were mainly controlled by tectonics, hydrodynamic energy level and water depth. Petrographic, cathodoluminescence and geochemical analyses suggest that early meteoric diagenesis developed during sub-aerial exposure in strata below these two major unconformities. The platform carbonates have been affected during burial by a number of diagenetic processes that include four phases of dissolution, several fracture generations, and six cement sequences with development of at least 13 calcite and dolomite cement zones (Z0–Z12). A contrasting diagenetic response from the different platform environments illustrates the role of primary sediment composition and unconformity development in controlling porosity and cement distribution. Limestone stabilization and cementation were relatively early processes that were mostly completed within the first kilometre of burial depth beneath the depositional surface. Below this burial depth, fluid circulation was concentrated along restricted pathways (fractures and fault zones). Migration of hydrothermal-related fluids along fault zones created localized dolomite patches and large-scale porosity associated with cavities and collapse breccias, but did not significantly increase the small-scale porosity.

From ramp to platform: building a 3D model of depositional geometries and facies architectures in transitional carbonates in the Miocene, northern Sardinia

The depositional geometry and facies distribution of an Early Miocene (Burdigalian) carbonate system in the Perfugas Basin (NW Sardinia) comprise a well-exposed example of a transition from a ramp to a steep-flanked platform. The carbonate succession (Sedini Limestone Unit) is composed of two depositional sequences separated by a major erosional unconformity. The lower (sequence 1) records a ramp dominated by heterozoan producers and the upper (sequence 2) is dominated by photozoan producers and displays a gradual steepening of the depositional profile into a steep-flanked platform. This paper shows the process of creating a digital outcrop model including a facies model. This process consists of combining field data sets, including 17 sedimentary logs, and a spatial dataset consisting of differential global positioning system data points measured along key stratigraphic surfaces and sedimentary logs, with the goal of locking traditional field observations into a 3D spatial model. Establishing a precise geometrical framework and visualizing the overall change in the platform geometry and the related vertical and lateral facies variations of the Sedini carbonate platform, allows us to better understand the sedimentary processes leading to the geometrical turn-over of the platform. Furthermore, a detailed facies modeling helps us to gain insight into the detailed depositional dynamics. The final model reproduces faithfully the depositional geometries observed in the outcrops and helps in understanding the relationships between facies and architectural framework at the basin scale. Moreover, it provides the basis to characterize semiquantitatively regional sedimentological features and to make further reservoir and subsurface analogue studies.

Sequence stratigraphy and architecture of a late Early–Middle Aptian carbonate platform succession from the western Maestrat Basin (Iberian Chain, Spain)

The attributes of a ‘four-systems-tract’ sequence are at times difficult to identify in outcrop-scale carbonate successions. Poor exposure conditions, variable rates of sediment production, erosion and/or superposition of surfaces that are intrinsic to the nature of carbonate systems frequently conceal or remove its physical features. The late Early–Middle Aptian platform carbonates of the western Maestrat Basin (Iberian Chain, Spain) display facies heterogeneity enabling platform, platform-margin and slope geometries to be identified, and provide a case study that shows all the characteristics of a quintessential four systems tract-based sequence. Five differentiated systems tracts belonging to two distinct depositional sequences can be recognized: the Highstand Systems Tract (HST) and Forced Regressive Wedge Systems Tract (FRWST) of Depositional Sequence A; and the Lowstand Prograding Wedge Systems Tract (LPWST), Transgressive Systems Tract (TST) and subsequent return to a highstand stage of sea-level (HST) of Depositional Sequence B. An extensive carbonate platform of rudists and corals stacked in a prograding pattern marks the first HST. The FRWST is constituted by a detached, slightly cross-bedded calcarenite situated at the toe of the slope in a basinal position. The LPWST is characterized by a small carbonate platform of rudists and corals downlapping over the FRWST and onlapping landwards. The TST exhibits platform backstepping and marly sedimentation. Resumed carbonate production in shelf and slope settings characterizes the second HST. A basal surface of forced regression, a subaerial unconformity, a correlative conformity, a transgressive surface and a maximum flooding surface bound these systems tracts, and are well documented and widely mappable across the platform-to-basin transition area analyzed. Moreover, the sedimentary succession studied is made up of four types of parasequence that constitute stratigraphic units deposited within a higher-frequency sea-level cyclicity. Ten lithofacies associations form these basic accretional units. Each facies assemblage can be ascribed to an inferred depositional environment in terms of bathymetry, hydrodynamic conditions and trophic level. The architecture of the carbonate platform systems reflects a flat-topped non-rimmed depositional profile. Furthermore, these carbonate shelves are interpreted as having been formed in low hydrodynamic conditions. The long-term relative fall in sea-level occurred during the uppermost Early Aptian, which subaerially exposed the carbonate platform established during the first HST and resulted in the deposition of the FRWST, is interpreted as one of global significance. Moreover, a possible relationship between this widespread sea-level drop and glacio-eustasy seems plausible, and could be linked to the cooling event proposed in the literature for the late Early Aptian. Because of the important implications in sequence stratigraphy of this study, the sedimentary succession analyzed herein could serve as an analogue for the application of the four-systems-tract sequence stratigraphic methodology to carbonate systems.

Effect of the Leakage Flows and the Upstream Platform Geometry on the Endwall Flows of a Turbine Cascade

This work describes the effect that the injection of leakage flow from a cavity into the mainstream has on the endwall flows and their interaction with a large pressure surface separation bubble in a low-pressure turbine. The effect of a step in hub diameter ahead of the blade row is also simulated. The blade profile under consideration is a typical design of modern low-pressure turbines. The tests are conducted in a low speed linear cascade. These are complemented by numerical simulations. Two different step geometries are investigated, i.e., a backward-facing step and a forward-facing step. The leakage tangential velocity and the leakage mass flow rate are also modified. It was found that the injection of leakage mass flow gives rise to a strengthening of the endwall flows independently of the leakage mass flow rate and the leakage tangential velocity. The experimental results have shown that below a critical value of the leakage tangential velocity, the net mixed-out endwall losses are not significantly altered by a change in the leakage tangential velocity. For these cases, the effect of the leakage mass flow is confined to the wall, as the inlet endwall boundary layer is pushed further away from the wall by the leakage flow. However, for values of the leakage tangential velocity around 100% of the wheel speed, there is a large increase in losses due to a stronger interaction between the endwall flows and the leakage mass flow. This gives rise to a change in the endwall flows’ structure. In all cases, the presence of a forward-facing step produces a strengthening of the endwall flows and an increase of the net mixed-out endwall losses when compared with a backward-facing step. This is because of a strong interaction with the pressure surface separation bubble.

Different platform and tube geometries and atomization temperatures in graphite furnace atomic absorption spectrometry: Cadmium determination in whole blood as a case study

In the present work the performance of different platform and tube geometries and atomization temperatures in graphite furnace atomic absorption spectrometry was investigated, using the determination of Cd in whole blood as an example. Grooved, integrated and fork platforms as well as atomization temperatures between 1200 °C and 2200 °C were investigated in a longitudinally heated graphite atomizer and compared with the performance of a transversely heated furnace. In the longitudinally heated furnace the increase of the atomization temperature in the studied range resulted in an increase of matrix effects for all platform geometries. The integrated platform exhibited slightly lower sensitivity and increased multiplicative interferences in comparison to the other two platform designs. Interference-free Cd determination was possible with all types of platforms and 1200 °C as the atomization temperature as well as with grooved and fork platforms at 1700 °C. On the other hand, lower atomization temperatures resulted in poorer limits of detection, due to the longer integration time needed. No matrix effect was observed at any atomization temperature using the transversely heated atomizer; in addition, limits of detection were better than those observed with the longitudinally heated atomizer. Best values were around 0.02 μg L − 1 with the latter atomizer compared to values around 0.02 μg L − 1 with the former one.

3D stratigraphic forward modelling for analysis and prediction of carbonate platform stratigraphies in exploration and production

Stratigraphic forward modelling (SFM) has been used to answer important questions in stratigraphic analysis. This has been demonstrated through generic modelling of synthetic data and numerous two-dimensional (2D) real data case studies, but the method has rarely been used to further our understanding of specific, real three-dimensional (3D) carbonate data sets by quantitative comparison. This paper applies CARBONATE-3D, a SFM program, to a hydrocarbon reservoir and outcropping carbonate platforms at both exploration and production scales. We show the value of SFM in the exploration and production of hydrocarbons through predicting facies distributions and stratigraphic geometries between wells, and by analysing and answering fundamental questions about the formation of specific carbonate strata. First, the model is tested using the 3D outcrops of a Miocene attached carbonate platform at Níjar, SE-Spain where 78% of mapped facies are matched in the simulations. Second, SFM of an 80,000 km 2 Jurassic ramp in NE Spain (exploration scale), predicts new facies in areas without data, leads to an improved understanding of ramp evolution, modification of the interpreted sea-level curve and re-interpretation of the main sediment transport direction. Application to the isolated platform reservoir Judy Creek (Devonian, Canada) exemplifies the advantages of applying SFM at reservoir scale. The simulations yield information on platform geometries and facies distributions, an improved understanding of the factors that control platform evolution, and propose a revised sea-level history. Wider applications of SFM in exploration include assessment of the possibilities and probabilities of facies distributions and evaluation and ranking of different interpretations of platform evolution. At the reservoir scale facies probability distributions and layer-geometries derived from SFM can serve as input into static reservoir models to guide geostatistical simulations.

A Pennsylvanian microbial boundstone-dominated carbonate shelf in a distal foreland margin (Picos de Europa Province, NW Spain)

A Pennsylvanian (Bashkirian–Moscovian) high-rising, microbial boundstone-dominated carbonate platform (> 12,000 km 2) developed in the external margin of a Variscan foreland basin. Intensely studied in the northern domain (Cuera area), the stratigraphy and sedimentology of the southern domain (the Picos de Europa Province) is here revisited. Distribution of the Bashkirian facies (Valdeteja Formation) defines a ramp/low-angle platform with an internal zone (central and eastern sectors of the Picos de Europa Province) consisting of huge amounts of massive, cement-rich microbial boundstones, surrounded by an external zone comprising lower-slope boundstone-derived breccias and basinal lime mudstones. From the Bashkirian–Moscovian transition to the early Kasimovian (Picos de Europa Formation), the carbonate platform moderately aggraded acquiring a high-rising and flat-topped geometry and then prograded rapidly into the external zone, where platform-derived carbonate sediment and clay, sourced from the orogen, were deposited. Seismic-scale exposures of the western margin of the shelf show angular-bedding relationship between basin/toe-of-slope, slope and platform-top stratal domains; with slope inclination up to 32° and a topographic relief of 550 m. The platform evolution above described for the Picos de Europa Province significantly differs with respect to the Cuera area, where platform mostly prograded during the Bashkirian and subsequently aggraded during early Moscovian. The evolutionary differences between these two carbonate domains, which are geographically close, are interpreted as changes in tectonic subsidence distribution, acting as the major factor controlling the large-scale platform geometries in the active foreland basin context. Finally, the tectonic overload due to the emplacement of the Cuera thrust sheets during the Moscovian–Kasimovian transition changed the platform growth style from progradational to aggradational and consequently the stacking pattern of the platform-top cycles in the Picos de Europa Province.

3D seismic analysis of an Upper Palaeozoic carbonate succession of the Eastern Finnmark Platform area, Norwegian Barents Sea

This paper presents 3D seismic data to enhance our understanding of the seismic stratigraphy and buildup developments of the Finnmark Platform. We focus on carbonate buildup distributions and platform geometry in space and upper Palaeozoic time. Multi-attribute analysis, neural network techniques and the use of inverted impedance data reveal seismic anomalies that can be linked with significant geological geometries such as buildups, subtle prograding clinoforms, karstified surfaces or platform breaks. The seismic expressions show a platform development mainly driven by climatic changes and differential subsidence. Seismic data indicate the first progradation of a low angle warm-water carbonate ramp during Mid Carboniferous times. A gradual change from a low angle (mixed) carbonate ramp to a more restricted carbonate shelf with a marked break and slope occurs during Upper Carboniferous/Lower Permian (Gipsdalen Group). Large buildups on the platform are identified both as individual mounds and unique continuous complexes (e.g. network of polygonal ridges as well as a 30-km long barrier complex). Multi-attribute analysis reveals a distinct karst event on top of the Gipsdalen Group. Climatic cooling during the Lower Permian (Bjarmeland Group) resulted in an even more restricted platform as seen in the seismic data with developments of 200-m high bryozoan mounds in deeper waters of the platform slope. Further cooling during Mid Permian (Tempelfjorden Group) resulted in a major chert event with development of a series of spiculitic chert mounds.

Accommodation and Sedimentary Architecture of Isolated Icehouse Carbonate Platforms: Insights from Forward Modeling with CARB3D+

Abstract A fundamental control on the evolution of carbonate platforms is accommodation, the space available for sediment deposition. Here we adopt a sensitivity-analysis approach, using the process-based forward model CARB3D+, to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy, subsidence, and surface dissolution. Sediment production, the mechanism invoked to fill accommodation, is also investigated. Fourth-order Milankovich-driven sea-level cycles dominate icehouse carbonate sedimentology, with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present. Both overwhelm higher-frequency, lower-amplitude fifth-order cycles, which are rarely preserved on the platform top. Subsidence is a fundamental control on sedimentary stacking patterns, sequence-boundary frequency (SBF), and platform geometry, with rapid subsidence increasing the preservation of platform-top sequences. Most icehouse platforms are subject to extended periods of platform-top exposure. Missed beats, when the platform top remains emergent at a sea-level maximum, are common and create non-Milankovich SBFs. Furthermore, during platform emergence, surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event, with surface dissolution rates in humid climates comparable to moderate rates of subsidence. However, accommodation generation by surface dissolution is self-limiting, in as much as accommodation generated by dissolution is rapidly refilled on flooding. Sediment production is a key control on unfilled accommodation, which our simulations suggest is considerable on icehouse platforms. Increased shallow-water (< 3 m depth) sediment production enhances intra-platform bathymetric relief between the margin and interior; this is driven in large part by a higher sedimentation rate at the margin than in the interior. Platforms can be flooded to depths up to 45 m during marine transgressions, and accommodation often remains unfilled. There are two distinct phases of platform-top sedimentation. During the transgression and early highstand, platform margins accumulate at a greater rate than interior sediments, generating differential intra-platform bathymetry. Subsequently, accommodation in the interior fills by forced regression and progradation from the margins into the platform interior. Numerical forward models can augment our understanding of icehouse carbonate-platform evolution by identifying the nature of the sedimentary response to individual controls. They can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms, such as the large hydrocarbon fields in the North Caspian and S.E. Asia.

Lower Cretaceous carbonate platform of the eastern Levant (Galilee and the Golan Heights): stratigraphy and second-order sea-level change

The marine succession of the late Early Cretaceous carbonate platform of the eastern Levant was located at the southern border of the Tethyan Ocean. The studied areas of Galilee and the Golan Heights represent a transect across the shallow inner platform. The investigations were focused on carbonate platform biostratigraphy and facies analysis. A precise stratigraphic interpretation of the Upper Barremian–Albian succession was compiled on the basis of the definition of larger benthic foraminifer biozones allowing subdivision of the succession into six well-dated intervals. Detailed analyses of facies and sedimentological parameters allow interpretation of platform development, in connection with platform geometry, as well as relative third-order and second-order sea-level changes. The observed facies patterns reflect a depositional geometry changing from a ramp to a flat-topped platform during Late Barremian–Middle Albian times. Three second-order depositional sequences were recorded in the studied mid-Cretaceous succession of the eastern Levant (MCEL-1 to MCEL-3). The facies analysis clearly indicates their origin in relative sea-level variations. The stratigraphic framework allows biochronostratigraphic calibration of these depositional sequences (MCEL-1: Upper Barremian–Lower Aptian, MCEL-2: uppermost Lower Aptian–middle Upper Aptian, MCEL-3: middle Upper Aptian–Middle Albian) and correlation with stratigraphic charts of local and regional sequences. The second-order sequence boundaries and maximum flooding surfaces correlate with basic sea-level variations recorded on the Arabian Plate, here interpreted as originally eustatically enhanced by subsidence. Tethyan signals are recorded around the Aptian/Albian boundary. A noticeable deepening in the upper Lower Aptian correlates with an extended platform drowning during the Selli Level (Oceanic Anoxic Event 1a: OAE 1a). Orbitolinid beds at this level suggest a deepening facies and a possible response to nutrient enhancement developing at the continental margin during OAE 1a. Eight third-order depositional sequences were observed in the Upper Barremian–Albian interval. They comprise successions of the inner ramp facies from open marine to restricted lagoons or tidal flats. The age and frequency of the Upper Barremian–Lower Aptian sequences correlate with those observed on the Pacific guyots or the Arabian Plate suggesting influence by regional sea-level cycles.

Sequence Stratigraphic Development of the Neoarchean Transvaal carbonate platform, Kaapvaal Craton, South Africa

The similar to 2.67 to similar to 2.46 Ga lower Transvaal Supergroup, South Africa, consists of a mixed siliciclastic-carbonate ramp that grades upward into an extensive carbonate platform, overlain by deep subtidal banded iron-formation. It is composed of 14 third-order sequences that develop from a mixed siliciclastic-carbonate ramp to a steepened margin followed by a rimmed margin that separated lagoonal environments from the open ocean. Drowning of the platform coincided with deposition of banded iron-formation across the Kaapvaal Craton. The geometry and stacking of these sequences are consistent with more recent patterns of carbonate accumulation, demonstrating that Neoarchean carbonate accumulation responded to subsidence, sea level change, and carbonate production similarly to Proterozoic and Phanerozoic platforms. The similarity of carbonate platform geometry through time, even with significant changes in dominant biota, demonstrates that rimmed margins are localized primarily by physiochemical conditions rather than growth dynamics of specific organisms. Stratigraphic patterns during deposition of the Schmidtsdrift and Campbellrand-Malmani subgroups are most consistent with variable thinning of the Kaapvaal Craton during extrusion of the similar to 2.7 Ga Ventersdorp lavas. Although depositional patterns are consistent with rifting of the western margin of the Kaapvaal Craton during this time, a rift-to-drift transition is not required to explain subsidence. Heating and thinning during Ventersdorp time can produce the observed thermal subsidence from similar to 2.7 to similar to 2.45 Ga if the thermal diffusivity of the craton was moderately low.

Lithological control on the elevation of shore platforms in a microtidal setting

AbstractThe shore platforms on Shag Point, southern New Zealand, are quasi‐horizontal surfaces and are developed between supratidal and low water spring levels. A range of morphologies occur, with more exposed platforms having a distinct low‐tide cliff, in contrast to low‐tide surfaces where the seaward edge is buried beneath rubble and macro‐algal growth. The platforms range in width from 20 to 80 m and are eroded into Late Cretaceous/Early Tertiary fine marine sandstones and mudstones. Shore platforms have formed in two principal lithological units: a homogeneous unit that is characterized by few discontinuities, and a fractured unit with joints spaced about 0·5 m apart. Rock hardness is low in both units (L‐type Schmidt hammer rebound values of 31 ± 4), and there is little systematic variation in values between the two units in which platforms have developed. Case‐hardened concretions within the sandstone are significantly harder than surrounding rock and cause local relief of metre scale as the spherical diagenetic features are eroded from the bedrock. They do not, however, appear to affect broad‐scale platform geometry. Joints within the bedrock are a primary control on platform elevation. Platforms formed in jointed rock occur at the lower portion of the intertidal zone, in contrast to platforms formed in unjointed bedrock, in which horizontal surfaces occur at or above mean high water spring tide level. Rock structure, therefore, appears to be the primary determinant factor of platform geometry at Shag Point. Copyright © 2006 John Wiley &amp; Sons, Ltd.