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Meter-Scale Vertical and Lateral Facies Variability in a Sequence Stratigraphic Framework: Example from Shallow-Marine Carbonates of the Middle Jurassic Izhara Formation (United Arab Emirates)

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Abstract

A sedimentary bed is classically defined as a distinct layer of sedimentary rock that has a relatively uniform composition. Several outcrop-based studies have shown that facies within individual beds can vary laterally on a scale of around 100 m. As facies transitions are important criterions used as a proxy for the depositional environment and often to infer sequence stratigraphic trends in subsurface studies where data is limited to one-dimensional wells, this observed facies heterogeneity has implications for both paleoenvironmental studies and sequence stratigraphy. In this study, we investigate whether sedimentary and facies heterogeneities known to occur at the hundred meters scale are also present at the meter to tens of meters scale in a well-preserved facies mosaic deposited on a carbonate ramp (Wadi Naqab, Izhara Formation, Lower Bajocian, northern UAE), where a robust, outcrop-based sequence stratigraphic framework exists. A bed set was logged and mapped across a 120 m long curving cliff face; combined with thin section analysis, the data allowed the reconstruction and quantification of facies heterogeneity at this location. Results reveal a large amount of lateral facies transitions at the meter scale. Lithofacies types have a probability of less than 70% of being laterally continuous over 12 to 18 meters, representing the highest amount of lateral facies heterogeneity so far reported in an ancient example. The case study reveals intra-bed facies transitioning attributed to spatially heterogeneous biogenic carbonate production as well as to syndepositional homogenizing and sieving processes occurring within shallow-marine depositional environments in ancient as well as in modern analogous systems. A series of continuous hardgrounds, previously interpreted as flooding and exposure surfaces, provide an independent sequence stratigraphic framework that demonstrate that the existence of small-scale lateral facies heterogeneities complicates interpretation of the vertical stacking pattern of facies. This confirms that the best practice is to limit sequence stratigraphic interpretations based on facies trends to larger stacking patterns (>10 meters). Meter-scale vertical patterns in carbonates often do not represent a proxy for base level changes, as illustrated in our study; therefore, fine-scaled, high-resolution sequence stratigraphy or lateral correlations are not attainable in carbonate sequences based on lithostratigraphy.

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Seismic-scale outcrops of lower Pennsylvanian steep-margined carbonate platforms in northern Spain (Sierra de Cuera, Cantabrian Mountains) provide the opportunity to link depositional facies to platform-interior architecture. Lithofacies character, vertical stacking patterns, and spatial distribution of depositional facies and shallowing- upward cycles were investigated to provide a semiquantitative depositional model for inner-platform strata. A 70-m-thick succession of eight shallowing-upward cycles (2.5-15 m thick) was studied across a 2-km-wide transect. Cycles consist of algal bioherms and skeletal packstone (lithofacies association B) deposited in an open marine and sub-wave-base environment, which are locally lateral to crinoid-dominated packstone (lithofacies association D). Lithofacies association B developed on a transgressive, one-meter-thick high-energy coated-grain grainstone (lithofacies association A) and is overlain by open marine to restricted lagoonal deposits (lithofacies association C) indicative of decreased paleo-water depth. Cycle boundaries are marine flooding surfaces occurring at the base of lithofacies A and the top of lithofacies C. Petrographic and outcrop evidence of subaerial exposure is conspicuously lacking. Cycle and lithofacies thickness varies laterally, and all the lithofacies can be either continuous for the 2 km width of investigation or discontinuous, terminating within tens to hundreds of meters. In most of the cycles, the lithofacies succession changes laterally because of lateral facies transitions (between lithofacies B and D) and pinching out of lithofacies A and C. Meter-scale variations in depositional topography are related to the stacked lens-shaped bioherms assigned to lithofacies B and appear to control the thickness and lateral continuity of the overlying facies. Sierra de Cuera strata do not resemble other Pennsylvanian cycles driven by high-frequency, high-amplitude glacio-eustasy, which are commonly characterized by subaerial exposure surfaces developed on subtidal deposits, except for the absence of peritidal facies and the presence of low-relief algal bioherms on the platform top. The necessary accommodation to enhance the growth of bioherms on the platform top was probably due to high-frequency (100-240 ky) moderate-amplitude (nearly 40 m) sea-level fluctuations combined with subsidence rates of 70-140 m/My. Tectonic subsidence might have been responsible for the lack of evidence of subaerial exposure surfaces in the examined Moscovian strata. The studied inner-platform lithofacies character and architecture were controlled by the interplay of high-frequency changes in accommodation and the presence of meter-scale depositional topography attributed to lateral variations of carbonate accumulation.
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In outcrops, shallow‐water carbonates often form thick, vertically stacked, metre‐scale, exposure‐bounded depositional packages, here called carbonate cycles. Because the facies within carbonate cycles can be tied precisely to water depths, they are considered ideal for reconstructing past sea‐level changes. There is, however, increasing evidence that these depositional cycles are unreliable recorders of both the frequency and the amplitude of orbitally driven sea‐level fluctuations. Carbonate cycles record only a fraction of the amplitude of a sea‐level cycle: a portion of the rise and nothing of the fall. Cores through the Pleistocene cycles on Great Bahama Bank illustrate this shortcoming. Although the amplitudes of the last nine sea‐level changes are each a hundred metres or more, the thickness of the cycles varies from a few metres to ca 15 m. The lack of correlation between sea‐level amplitude and cycle thickness is not eliminated for cycles deposited during times of lower sea‐level amplitudes, for example, the Cretaceous. Upper Cretaceous cycles on the Maiella platform margin document the irregularly filled accommodation space and the resultant variability in cycle thickness and frequency. Uncertainties in assessing the frequencies of sea‐level changes from shallow‐water carbonate cycles are caused by ‘missed beats’ and metre‐scale oscillations of sea‐level within highstands that potentially produce cycles of very short duration. The random amplitude variability during the last 57 glacio‐eustatic sea‐level changes illustrates the difficulty of assessing ‘missed beats’, where a sea‐level fluctuation is not recorded because the sea‐level rise does not reach the platform top. ‘Missed beats’ are also produced by the depositional topography that is created by irregularly filled accommodation space. As a result, variable numbers of cycles are deposited across the platform. Further complicating orbital frequency analyses are decametre‐scale oscillations of the sea‐level during highstands. The amplitude of these sub‐orbital sea‐level oscillations (up to 17 m within the last interglacial, Marine Isotope Stage 5e) are sufficient to expose shallow platforms like Great Bahama Bank and subsequently produce an additional depositional cycle with similar facies successions. The combined effects of missed beats and oscillations within highstands are likely to produce cycles and hiatuses of variable duration that are difficult to extract from the rock record. Consequently, estimates of the orbital forcing mechanisms and frequencies from ancient shallow‐water carbonate cycles carry large uncertainties.
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The principles and purposes of stratigraphic well correlation in carbonate sedimentary systems are defined and discussed within the context of static reservoir modeling. The challenge of well correlations is to relate the heterogeneities measured at core and well scales to the spatial heterogeneities at reservoir and flow unit scales. The introduction of a priori knowledge in the process of stratigraphic well correlation is critical to support the stratigraphic rules and to establish a coherent geological and petrophysical concept. The links between well correlation and geostatistics are discussed with regard to the stationarity hypothesis and property trend analysis. We stress that wells are incomplete and biased samples of the geological reality, which is not dependent, unlike the dynamic reservoir behavior, on the well numbers, location, and spacing. Stratigraphic rules are applied as a function of the well spacing relative to the geological reality. A simple trigonometric method, combining angle of base profile, paleobathymetry, and well spacing, is introduced to check the validity of the well correlation in carbonate ramp-like systems. Two models, based respectively on outcrop and subsurface with seismic data, are discussed in detail to show the combined influence of the data set, sedimentary systems, and diagenetic transformations on stratigraphic well correlations.
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In the Causses platform (south-east France), Late Hettangian to Sinemurian deposits were interpreted previously as shallow-water carbonate ramp deposits. A new look at these deposits has shown a fault-controlled mosaic carbonate platform that is different from the carbonate ramp models. Within the platform mosaic, 15 lithofacies have been recognized, which are organized in four facies associations, including peritidal, restricted shallow sub-tidal, sand dunes and sub-tidal shelf facies associations. The rapid lateral and vertical facies changes, and the lack of consistent landward or seaward direction indicated by the pattern of facies changes, question the existence of a shoreline suggested by the traditional models for this region. Instead, the facies organization and cycle stacking pattern suggest deposition in a mosaic of intertidal islands between which sub-tidal restricted or open conditions could coexist in very close proximity. Such a platform mosaic would have been defined by tectonic activities along normal faults which segmented the shallow-water Causses platform.
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Each simulation algorithm, including Truncated Gaussian Simulation, Sequential Indicator Simulation and Indicator Kriging is characterized by different operating modes, which variably influence the facies proportion, distribution and association of digital outcrop models, as shown in clastic sediments. A detailed study of carbonate heterogeneity is then crucial to understanding these differences and providing rules for carbonate modelling. Through a continuous exposure of Bajocian carbonate strata, a study window (320 m long, 190 m wide and 30 m thick) was investigated and metre-scale lithofacies heterogeneity was captured and modelled using closely-spaced sections. Ten lithofacies, deposited in a shallow-water carbonate-dominated ramp, were recognized and their dimensions and associations were documented. Field data, including height sections, were georeferenced and input into the model. Four models were built in the present study. Model A used all sections and Truncated Gaussian Simulation during the stochastic simulation. For the three other models, Model B was generated using Truncated Gaussian Simulation as for Model A, Model C was generated using Sequential Indicator Simulation and Model D was generated using Indicator Kriging. These three additional models were built by removing two out of eight sections from data input. The removal of sections allows direct insights on geological uncertainties at inter-well spacings by comparing modelled and described sections. Other quantitative and qualitative comparisons were carried out between models to understand the advantages/disadvantages of each algorithm. Model A is used as the base case. Indicator Kriging (Model D) simplifies the facies distribution by assigning continuous geological bodies of the most abundant lithofacies to each zone. Sequential Indicator Simulation (Model C) is confident to conserve facies proportion when geological heterogeneity is complex. The use of trend with Truncated Gaussian Simulation is a powerful tool for modelling well-defined spatial facies relationships. However, in shallow-water carbonate, facies can coexist and their association can change through time and space. The present study shows that the scale of modelling (depositional environment or lithofacies) involves specific simulation constraints on shallow-water carbonate modelling methods.
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A series of 19 paleofacies maps have been generated for given time intervals between the Late Permian and Holocene to reconstruct the depositional history of the Arabian Plate. The succession of changing lithological sequences is controlled by the interplay of eustacy and sediment supply with regional and local tectonic influences. The Mesozoic paleofacies history of the Plate is, in its central and eastern portion east of Riyadh, strongly influenced by an older N-trending, horst and graben system that reflects the grain of the Precambrian Amar Collision and successively younger structural deformations. The late Paleozoic Hercynian orogenic event caused block faulting and relative uplift and resulted in a marked paleorelief. This jointed structural pattern dominated the entire Mesozoic and, to some extent, the Cenozoic facies distribution. The relationship between producing fields and the paleofacies maps illustrates the various petroleum systems of particular times and regions.
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The primary location of this publication on ResearchGate is: https://www.researchgate.net/publication/279778628_Arabian_Plate_Sequence_Stratigraphy_GeoArabia See linked data here for various chapters: https://www.researchgate.net/publication/279778628_Arabian_Plate_Sequence_Stratigraphy_GeoArabia
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Although the general criteria for recognition and environmental interpretation of different carbonate facies are well-established, a predictive understanding of the areal extent and spatial patterning of facies bodies and why they might organize into facies belts or facies mosaics is poorly constrained. To explore patterns and process dynamics of facies on isolated carbonate platforms, quantitative analysis of thematic maps derived from remote sensing images of 27 Holocene atolls of the Paracel and Spratly chains in the South China Sea explores variability within and among platforms. On these systems, most annular shelf-margin reefs are less than 500 m wide on both chains; inboard of the reefs, reef sand aprons range up to 500 m (Spratlys) and 1000 m (Paracels) wide. Around individual platforms, Spratly Chain sand apron widths are wider to the north-west, whereas apron widths in the Paracel Chain are more symmetrical; collectively, data indicate log-normal width-exceedance probability distributions. Platform-interior patch reefs include area-exceedance probability distributions and gap size distributions (lacunarity) consistent within chains, but distinct between the chains. To understand the processes underlying distinct distributions, simulations explored distinct growth scenarios. Results suggest that differences may represent distinct process classes: proportional growth processes with multiplicative random effects (reef sand aprons - belts), versus non-linear, size-proportional growth of randomly aged and distributed elements (patch reefs - mosaics). The probabilistically distinct sizes and spatial patterns of geomorphic elements within these general process classes are interpreted to represent 'variations on themes' related to the different impacts of tropical storms, winter cold fronts and circulation in each chain. The results highlight fundamentally different growth patterns impacting the sizes and distribution of facies belts and mosaics on isolated carbonate platforms. Because these types of bodies ultimately construct stratigraphy, the themes could be applied to understand and predict variability in the architecture of subsurface reservoir analogues.
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Article
Three textural features seem especially useful in classifying those carbonate rocks that retain their depositional texture (1) Presence or absence of carbonate mud, which differentiates muddy carbonate from grainstone; (2) abundance of grains, which allows muddy carbonates to be subdivided into mudstone, wackestone, and packstone; and (3) presence of signs of binding during deposition, which characterizes boundstone. The distinction between grain-support and mud-support differentiates packstone from wackestone—packstone is full of its particular mixture of grains, wackestone is not. Rocks retaining too little of their depositional texture to be classified are set aside as crystalline carbonates.
Article
A fundamental question in the correlation of 1-D sedimentologic data is whether to use a layer-cake or shingled correlation approach. The resulting reservoir geometry has important implications for the characterization of reservoir heterogeneities and fluid flow. On the Saiq Plateau in Oman, epeiric carbonate ramp deposits of the Triassic Sudair Formation are well exposed and can be investigated in detail over several kilometers. There, reservoir heterogeneities on different scales have been documented by creating various outcrop wall panels and 2-D correlations. Multi-level architectural elements with different depositional geometries were discovered, which were linked to a sequence-stratigraphic hierarchy consisting of three levels. Level 1: A “layer-cake”-type stratigraphic architecture with minor thickness variations over several kilometers becomes apparent when correlating fourth-order cycle set boundaries. Level 2: The correlation of fifth-order cycle boundaries reflects horizontally continuous geometries, within which, however, internal grainstone layers were discovered to be arranged in a shingled fashion. Muddy layers in between these shingles illustrate sixth-order mini-cycle boundaries. Level 3: Within sixth-order mini-cycles another scale of a shingle-like architecture can be observed. Amalgamated cm-thick grainstone units form thin wedges with subtle but clearly inclined dipping geometry. Fourth-order cycle sets and fifth-order cycles can be traced over several kilometers, and therefore assumed to be related to allocyclic stratigraphic processes. The internal shingle geometries within fifth-order cycles are traceable over 100s of meters and presumably reflect an autocyclic lateral migration of a shoal complex. Cm-thick shingling grainstone wedges within sixth-order mini-cycles are interpreted as storm-related spill deposits. Their event-driven character is reflected by frequent amalgamation and reworking of the preceding deposits. The results of this study of epeiric carbonate ramp deposits suggest that a “layer-cake” correlation approach is appropriate when correlating 10s of m-thick grainstone units over a distance of several kilometers. However in the documented example, these thick grainstone units consist internally of small-scale architectural elements, which show inclined geometries and require a shingled correlation approach. These small-scale heterogeneities within an overall “layer-cake” architecture might have an impact on fluid flow in similar subsurface reservoirs and should be taken into account for detailed reservoir correlations and static reservoir models.
Article
Cambro-Ordovician carbonate lithofacies units in the Elbrook and Conococheague formations exposed at Wytheville, Virginia, as well as those in many other Phanerozoic peritidal sequences, exhibit exponential thickness frequency distributions. That is, occurrence frequency decreases exponentially with linear increase in unit thickness. Such distributions are characteristic of waiting times between independent Poisson events. This relative frequency of spaces of different size between horizons of lithologic change is what one would expect if the horizons were distributed randomly throughout carbonate successions, Abundances of different lithologies, both as net stratigraphic thickness and as number of occurrences, also decreases exponentially among successively rare sediment types, with each lithology being about 60% as plentiful as the nest more abundant rock type. Relations between net thickness and number of occurrences for each facies define a linear trend coincident with a mean thickness for all Wytheville units of 0.48 m, a relation indicating that thickness distribution is independent of facies type. Similar relations are apparent for the horizontal extent of carbonate sediment bodies from the Holocene Florida-Bahama platform, Areal extents of individual facies units (lithotopes) are described by a frequency distribution in agreement with that anticipated for a population of equidimensional facies elements whose diameter distribution follows an exponential frequency distribution, Although regional gradients in sediment texture and composition are also apparent along most transects from platform margin to interior, such frequency distributions indicate that lateral extents of individual sediment reflect a largely stochastic distribution of facies boundaries across this Holocene surface. Lithotope abundances also yield trends of exponentially decreasing dominance among successively subordinate facies, with each being about 70% as extensive the next more abundant sediment type. Relations between areas and abundances for all lithotopes define a covariant trend corresponding to a mean area of 2.2 x 10(3) km(2) for all Florida-Bahamas lithotopes, We consider several numerical models of stochastic carbonate accumulation; although not demonstrably unique, scenarios incorporating the sequential superposition of randomly placed coniform lithotopes result in thickness and area frequency distributions that are the same as those observed in ancient and modern platform deposits. Such simulations of Poisson processes of sediment accumulation are in general agreement with stochastic models of lithologic heterogeneity that have been more widely applied to petroleum reservoirs and groundwater aquifers, To the as yet unknown degree that peritidal lithofacies area and thickness are correlated, data from Paleozoic and Holocene platforms suggest that carbonate units should exhibit length/height ratios of approximately 10(5). Given the decimeter scale over which facies are designated in most Paleozoic peritidal successions, these relations predict mean lateral extents on the order of several tens of kilometers, a value in general agreement with the few data that exist on spatial continuities of peritidal lithotopes in Paleozoic carbonate sequences.
Article
This study analyzes the three-dimensional variability of a 20-meter-thick section of Pennsylvanian (Missourian) strata over a 600 km2 area of northeastern Kansas, USA. It hypothesizes that sea-level changes interact with subtle variations in paleotopography to influence the heterogeneity of potential reservoir systems in mixed carbonate-silidclastic systems, commonly produdng build-and-fill sequences. For this analysis, ten lithofacies were identified: (1) phylloid algal boundstone-packstone, (2) skeletal wackestone-packstone, (3) peloidal, skeletal packstone, (4) sandy, skeletal grainstone-packstone, (5) oolite grainstone-packstone, (6) Osagia-brachiopod packstone, (7) fossiliferous siltstone, (8) lenticular bedded-laminated siltstone and fine sandstone, (9) organic-rich mudstone and coal, and (10) massive mudstone. Each facies can be related to depositional environment and base-level changes to develop a sequence stratigraphy consisting of three sequence boundaries and two flooding surfaces. Within this framework, eighteen localities are used to develop a threedimensional framework of the stratigraphy and paleotopography. The studied strata illustrate the model of "build-and-fill". In this example, phylloid algal mounds produce initial relief, and many of the later carbonate and silidclastic deposits are focused into subtle paleotopographic lows, responding to factors related to energy, source, and accommodation, eventually filling the paleotopography. After initial buildup of the phylloid algal mounds, marine and nonmarine siliciclastics, with characteristics of both deltaic lobes and valley fills, were focused into low areas between mounds. After a sea-level rise, oolitic carbonates formed on highs and phylloid algal facies accumulated in lows. A shift in the source direction of siliciclastics resulted from flooding or filling of preexisting paleotopographic lows. Fine-grained silidclastics were concentrated in paleotopographic low areas and resulted in clay-rich phylloid algal carbonates that would have made poor reservoirs. In areas more distant from silidclastic influx, phylloid algal facies with better reservoir potential formed in topographic lows. After another relative fall in sea level, marine carbonates and silidclastics were concentrated in paleotopographic low areas. After the next relative rise in sea level, there is little thickness or fades variation in phylloid algal limestone throughout the study area because: (1) substrate paleotopography had been subdued by filling, and (2) no silidclastics were deposited in the area. Widespread subaerial exposure and erosion during a final relative fall in sea level resulted in redevelopment of variable paleotopography. Build-and-fill sequences, such as these, are well known in other surface and subsurface examples. Initial relief is built by folding or faulting, differential compaction, erosion, or deposition of relief-building facies, such as phylloid algal and carbonate grainstone reservoir fades, or silidclastic wedges. Relief is filled through deposition of reservoir-fades siliciclastics, phylloid algal fades, and grainy carbonates, as well as nonreservoir facies, resulting in complex heterogeneity.
Article
In recent years, considerable research has been undertaken in order to gain a better quantitative understanding of morphometric patterns within modern carbonate depositional systems. The industrial application of the scaling/juxtaposition relationships derived from the Modern to subsurface Cenozoic carbonate reservoirs appears relatively straightforward, given that many key biota are common to both. However, the direct application of Modern sedimentary insight further back into the geologic rock record is more controversial, given the enormous changes in the biota, climate, sea level, water chemistry and so on, that have taken place. To justify such an approach, we contend that similar morphometric patterns should be observed in both the Modern and ancient data. In the Norwegian Barents Sea, numerous seismic surveys have imaged Upper Palaeozoic carbonate buildups arranged in polygonal networks, or reticular patterns. These patterns are observed in both warm photozoan and cool water heterozoan carbonate stratigraphies, and are developed atop platforms founded on stable shelves, in tectonically active settings and platforms developed over basinal evaporites. GIS mapping of multiple seismic horizons allows the Palaeozoic reticulated morphology to be numerically compared to that mapped in Alacranes Reef from QuickBird satellite imagery. QuickBird’s meter-scale resolution allows identification of subtle cross-platform trends, such as windward-leeward differences in the packing density of ridge-and-pond complexes, which can be correlated with the kilometre-scale patterning extracted in the Barents subsurface. Despite different controls and architecture, the patterning of reticular networks is statistically inseparable between the two systems, once the meter-scale Modern dataset is down-sampled to seismic resolution. While other controls cannot unequivocally be ruled out, these results suggest that biotic self-organization is a fundamental driver of sedimentary patterns on carbonate platforms. To our knowledge, this is the first quantitative comparison of morphometric patterns from the Modern and Palaeozoic that clearly reveals similar patterns of self-organization. For the depositional environments considered, the findings suggest that juxtaposition rules, facies proportions and scaling relationships extracted from the Modern can successfully be applied to the ancient.
Article
Carbonate sequence stratigraphy is founded on the principle that changes in relative sea-level are recorded in the rock record by the accumulation of sediment with relative water depth-dependent attributes. While at the scale of a shelf to basin transect, facies clearly arrange by water depth, the relation blurs for depths <40 m, the most vigorous zone of carbonate production. The reason for this change in behaviour is two-fold. Firstly, in shallow water, the intrinsic processes of storm and wave reworking influence the seabed through submarine erosion and sediment redistribution. Secondly, facies diversity tends to be greater in shallow water than deep water because of a greater diversity in grain producers. Remote sensing imagery, field observations and hydrodynamic models for two reef-rimmed shore-attached carbonate platforms in the Red Sea show neither water depth nor energy regime to be reliable indicators of facies type when considered in isolation. Considered together, however, the predictive power of the two variables rises significantly. The results demonstrate it to be an oversimplification to assume a direct link between palaeo-water depth and depositional diversity of subtidal lithofacies, while highlighting the importance of hydrodynamics in directing the accumulation of carbonate sediments in the shallow photic zone. While the size distributions of facies extents in the two focus areas follow power laws, no direct relation between the lateral continuity of the facies belts and water depth or wave height is reported. The work is relevant for the interpretation of metre-scale subtidal carbonate cycles throughout the geological record by demonstrating how caution must be applied when inferring palaeo-water depths from depositional facies.
Article
Satellite 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). The 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 reevaluation 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 the greatest current velocities 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 relation 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. This article is protected by copyright. All rights reserved.
Article
This study, based in the Haushi-Huqf area of central east Oman, aims to characterize the controls on facies distribution and geometries of some of the best preserved examples of Lower Cretaceous tidal flat facies within the Tethyan epeiric platform. Field, petrographic and geochemical data were acquired from the Barremian–Aptian Jurf and Qishn formations that crop out in a 500 × 1000 m2 butte, thus allowing for pseudo three-dimensional quantitative data acquisition of the dimensions and spatial distributions of discontinuity surfaces and sedimentary bodies. The interpretation presented here suggests that the main processes impacting sedimentation in the Lower Cretaceous peritidal environment of the Haushi-Huqf were transport and erosion processes related to storm waves and currents. The vertical evolution of the carbonate system is organized into six types of metre-scale depositional sequences, from subtidal dominated sequences to supratidal-capped sequences, which are bounded by regional discontinuity surfaces. At subaerial exposure and submarine erosion surfaces associated with a base level shift, sedimentary horizons along the entire depositional profile are cut by scours possibly created by storm events. Chemostratigraphy allows correlation between the Haushi-Huqf and the age-equivalent sections logged in the interior of the platform in Oman. The correlation suggests that the change from subtidal to intertidal depositional sequences during the late highstand is coeval with the development of rudist dominated shoals on the shelf. This study is the first to discuss the controls on Lower Cretaceous peritidal carbonate cyclicity of the Arabian epeiric platform. The results presented here also offer a unique quantitative dataset of the distribution and dimensions of peritidal carbonate shoals and storm scours in a regional sequence stratigraphic context.
Article
The sedimentology of a Recent carbonate mound is investigated to further our understanding of mound building communities, surface and subsurface mound sediments, and the overall sediment budget of mounds. Nine sedimentary facies of the surface of Tavernier mound, Florida Keys are described. These sediments are composed of Neogoniolithon, Halimeda, Porites, mollusc and foraminiferal grains, and lime mud. Muds rich in aragonite and high magnesian calcite show little mineralogical variation over the mound surface. Geochemical evidence suggests that the mud is mainly formed from breakdown of codiacean algae and Thalassia blade epibionts. Production rates of the facies are established from in situ growth rate experiments and standing-crop surveys. Annual calcium carbonate production is c. 500gm⁻², intermediate between reef and other bay and lagoonal environment production rates in the Caribbean. The internal structure of the mound, studied from piston cores and sediment probes, indicates that seven facies can be identified. Five of these can be related to the present-day facies, and occur in the upper part of the mound (gravel-mound stage). The remaining two facies, characterized by molluscs and aragonite-rich muds, occur in the lower part of the mound (mud-mound stage), and are most similar to facies from typical Florida Bay mud mounds. Mangrove peats within the mound indicate former intertidal areas and C¹⁴ dates from these peats provide a time framework for mound sedimentation. The mound appears to have formed because of an initial valley in the Pleistocene surface which accumulated mud in a shallow embayment during the Holocene transgression.
Article
This study presents the results of the first quantitative analysis of surface sediments along multiple transects of the Arabian Gulf shoreline of Abu Dhabi, United Arab Emirates. The natural coastline of this low-angle carbonate ramp depositional system is being anthropogenically modified at an unprecedented rate. As this system is much-employed as a Recent analogue for many ancient depositional sequences, it is imperative that sedimentological characteristics of natural facies occurrences are accurately recorded for employment in future applications. We establish distinct relationships between the geomorphic setting and sediment characteristics. Siliciclastic grains admixed to this system are naturally sourced from the Pliocene Ghayathi Formation and from aeolian sediment influx during dust storms. This natural source of siliciclastic detritus is being increasingly superseded by siliciclastic material derived from anthropogenic activities. Evaporite minerals are similarly being contributed from anthropogenic sources. The occurrence of mud to silt grade sediments is primarily controlled by the presence of a sediment-binding microbial mat that prevents the removal of fine material during high energy events. Similarly, the presence of a halite crust in the upper tidal flats and supratidal zone prevents aeolian deflation. This study did not establish any clear relationship between coastal geomorphic setting and the bioclastic components present in the sediments. This finding has major implications for the application of bioclasts in fine-tuning palaeoenvironmental interpretation of ancient carbonate ramp coastal systems.
Article
It is the purpose of this paper to demonstrate, on the basis of bed-by-bed tracing of stratigraphic features, that highly discontinuous facies mosaics also developed during times of moderate- to low-amplitude accommodation variations associated with Middle Cambrian transitional greenhouse conditions. Stratigraphic, geometric, and sedimentologic data from a carbonate interval 35--50 m thick in the upper part of the Jangle Limestone Member (Middle Cambrian) of the Carrara Formation, SE California, provide the opportunity to document the lateral extent of cyclic facies in 1.8 km of laterally continuous outcrop. Facies, their contacts, and parasequence boundaries were walked out and lateral variations documented. Correlations were then attempted to nearby ranges in a regional cross section extending approximately 55 km along depositional strike.
Article
The San Andres Formation (Permian, Guadalupian) of the Permian basin is representative of carbonate ramp reservoirs in that it has highly stratified character, complex facies and permeability structure, and generally low recovery efficiencies of 30% of original oil in place. The approach used here to describe carbonate ramp reservoirs such as the San Andres Formation produces detailed reservoir models based on integration of sequence stratigraphic analysis, petrophysical quantification through definition of rock fabric flow units, and fluid flow simulation. Synthesis of these subdisciplines clarifies which aspects of the geologic-petrophysical model are most significant in predicting reservoir performance and ultimately in understanding the location of remaining oil saturation. On the basis of sequence stratigraphic analysis, three scales of cyclicity are recognized: depositional sequences, high-frequency sequences, and cycles. Two-dimensional black oil fluid flow models illustrate that (1) major differences in sweep efficiency and fluid flow performance are predicted when linear interwell interpolations are compared with actual interwell-scale geologic structure as determined by outcrop geologic and petrophysical mapping, (2) an understanding of static geologic/petrophysical conditions provides only a partial understanding of reservoir performance defined by the interaction of these static properties and dynamic properties of fluid flow interaction within the flow unit architecture, and (3) because of the orderly distribution of high- and low-permeability facies within cycle stacks of high-frequency sequences, this larger scale of geologic description can give a reasonable first-order approximation of fluid flow patterns and early breakthrough. 87 refs., 33 figs., 1 tab.
Article
Although the general factors influencing the occurrence and distribution of oolitic systems are well known, details of their landscape-scale patterns and formative processes are less systematically explored. By reviewing published studies and presenting new data and insights, this review systematically examines a suite of Holocene systems from the Bahamas in which patterns and processes are characterized and explicitly linked, with the goal of providing more realistic and accurate models for variability, heterogeneity, and predictability in these systems and their ancient analogs. Sedimentological, geomorphic, and hydrodynamical data document that each ooid shoal is unique in detail, yet shoals share common elements. Within bar forms, systematic trends in grain size, type, and sorting are related to spatial and temporal variability in wave, tide, and current energy. These trends are driven by connections among sedimentology, hydrodynamics, and geomorphic forms, as they shape, and concomitantly are shaped by, each other. Similarly, due to feedbacks, a limited number of bar forms occur among shoals, including tidal sand ridges, parabolic bars, transverse shoulder bars, tidal deltas, and sand flats; the dominant bar form broadly corresponds with flow velocities within and among shoals. Among Bahamian tidal shoals, the mean grain size of ooids, maximum bar heights, and the width of the oolitic facies belts are all positively correlated. Collectively, these results illustrate the mode and magnitude of variability in shapes, orientations, and sedimentology of Holocene oolitic bodies, interpreted to be driven by self-organization and autogenic processes. These perspectives provide testable qualitative and quantitative insights that can be used to understand and predict the character of ancient analogs.
Article
The writings of Johannes Walther (1860-1937) have been neglected in the west and his Law of the Correlation (or Succession) of Facies has been ignored or misstated in many textbooks of stratigraphy. Walther should be recognized as a pioneer stratigrapher-sedimentologist, important as both a world traveller and explorer of modern sedimentary environments (deserts, reefs, laterites), and as a theorist. His main theoretical contributions were his championing of the actualistic method for the study of fossils and sedimentary rocks and his founding of the science of comparative lithology. Comparative lithology was seen by Walther as the analogue for sedimentary rocks of comparative anatomy for fossils. It has been neglected in the West until the recent revival of the concept of facies models. Walther's Law was the key concept within comparative lithology, and was originally stated as follows: "The various deposits of the same facies areas and similarly the sum of the rocks of different facies areas are formed beside each other in space, though in cross-section we see them lying on top of each other. As with biotopes, it is a basic statement of far-reaching significance that only those facies and facies areas can be superimposed primarily which can be observed beside each other at the present time." In Russia, Walther's writings appear to have had a greater influence than they have had in Europe and America. They have been partly responsible for the development there of "lithology" as a branch of the geological sciences separate from stratigraphy or petrology.
Article
Many stratigraphic studies of ancient shallow-water carbonate platforms focus interpretation on the primacy of changes in accommodation, inferred by assuming a direct and deterministic link between depositional facies and water depth. The purpose of this study is to test this assumption explicitly and quantitatively by exploring relations between substrate type (classified on the basis of both habitats and facies) and the water depth at which they occur on a modern well-studied shallow rimmed shelf margin. GIS analysis of digital benthic habitat and bathymetry maps of the shallow (less than or equal to 8 m) South Florida shelf illustrate that most substrate types occur across a range of water depths. Similarly, at greater water depths, habitats and facies are more constrained (more dominance of one class, more deterministic), with generally decreasing dominance (more diversity) at shallower water depths. At a scale of shelf-to-basin transects, facies and habitats clearly are related to water depth. Yet, on this narrower (similar to 10 km) shallow shelf margin, at the scale of this study, water depth and habitats and facies are not uniquely related or linked. This general lack of correspondence between bottom type and water depth may be a manifestation of landscape disequilibrium, a state in which habitats and facies do not fully reflect ambient environmental conditions, perhaps recording the influence of rapid change in sea level. Alternatively, it may reflect the impact of variables other than water depth. Although more general consequences of these results for other depositional settings remains to be evaluated, they do emphasize the fact that variables other than bathymetry may significantly influence the ecological and sedimentologic attributes of depositional surfaces. They do allow for the quantification of some of the potential uncertainties inherent in the interpretation of analogous ancient platforms, and can serve to focus future observations on the causes and consequences of vertical and lateral heterogeneity in ancient stratigraphic sequences.
Article
Compositional characteristics of limestones are fundamental to the interpretation of depositional environments, with an assumption that preserved grain assemblages preserve sufficient evidence of initial sediment input to enable delineation of environments. It is acknowledged, however, that a range of early diagenetic processes (including encrustation, fragmentation, abrasion, microboring, dissolution, cementation, and recrystallization) may influence grain preservation potential, and thus bias the composition of resultant grain assemblages. This study examines the effects and relative importance of a range of early diagenetic processes across;a fringing reef at Discovery Bay, north Jamaica, Physical processes (abrasion and fragmentation) are most important at shallow fore-reef sites, whilst back-reef sites are dominated by biochemical processes, including microboring, recrystallization, biofilm-related calcification, and dissolution. Grain susceptibility to each process is highly variable and influenced by mineralogy and skeletal structure. Coral fragments, for example, are most susceptible to the effects of microboring, whereas Halimeda plates are most susceptible to dissolution. Extensive alteration of grain assemblages is predicted within the back-reef, where dissolution of coral and Halimeda occurs because of intense microboring and pore-water undersaturation. Fore-reef assemblages, by contrast, remain relatively unaltered. Results have implications both for improved ecological interpretation of carbonate sedimentary sequences and for understanding of how and why carbonate microfacies developed.