This study interprets the stratigraphic evolution of the mid-Cretaceous Urgonian Carbonate Platform sensu lato exposed in the northwestern Subalpine Chains of southeast France within a framework of dynamic environmental and climatic changes including relative sea-level fluctuations. Macroscopic observations of stratal geometries, especially stratal packaging within the succession revealed by differential weathering patterns have been integrated with the petrographic examination of limestone samples to assist in the interpretation of the carbonate succession. Seven hundred and twenty-eight rock samples were examined under the microscope as either thin sections or acetate peels and twenty five component parameters were used to define sediment composition within the data set. New and emerging analytical techniques have been used to interpret these data rather than the more traditional static fixed facies belts. Sequence stratigraphic concepts have been used to interpret platform evolution within the context of relative sea-level changes. In order to define microfacies and identify underlying environmental gradients computer optimized Jaccard's similarity coefficients matrices have been calculated between samples and component parameters using two computer programs SedUtil and JaccMat run on an IBM compatible Pentium personal computer. Computer optimized Jaccard's similarity coefficients similarity matrices are able to model both gradational and abrupt changes in facies composition in response to palaeogeographic and stratigraphic controls. Computer optimized similarity matrices between limestone compositional parameters from a series of logs show differences in the spatial and temporal nature of variations in sediment composition in response to environmental changes across the platform. The contrasts between the configurations of optimized similarity matrices are interpreted as indicating a palaeogeography at the beginning of the Urgonian characterised by a more strongly subsident basin to the southeast of the field area connected to the topographically-higher, tectonically more stable parts of the platform by low-angle slopes; the basin was eliminated by the end of the Urgonian. In addition optimized similarity matrices calculated between samples have been used to assist in defining eight-eight conventional microfacies assemblages which are described and illustrated using photomicrographs and pie-diagrams of modal composition. Despite the apparently conformable nature of the succession as observed in individual exposures, changes in the stratigraphy of the Urgonian platform sensu lata as traced along two southwest-northeast transects constructed through the Aravis and Bargy Chains indicate the existence of a low-angle distally steepened ramp topography at the top of the Hauterivian basement that sloped into a more strongly subsident basin. The Urgonian succession is divided into two third-order depositional sequences termed Depositional Sequence 1 and 2. In Depositional Sequence 1 all three systems tract are developed whereas Depositional Sequence 2 only contains transgressive and highstand systems tracts. The lowstand systems tract of Depositional Sequence 1 preserved in the southwest of the Aravis and Bargy Subalpine Chains is volumetrically significant, being dominated by hemipelagic deposits that shallow laterally to intertidal deposits to the northeast and also contain calciturbidites and two spectacular megabrecciahorizons. The two megabreccia horizons are interpreted as thin surficial collapse horizons formed by net extension on a very-low angle, previously kinematically stable slope, during fourth-fifth order relative sea-level falls. The lower sequence boundary of Depositional Sequence 1 passes laterally from being conformable in the southwest (deep-water basin) to being a major unconformity in the northeast ('inner' platform). However, this sequence boundary appears planar and concordantly bedded in the main and it is only in the northeastern Bargy Chain that it is observed as a erosional angular unconformity. The top of Depositional Sequence 1 is marked by the end of rudist lagoonal sediments and the deposition of beach facies which contain evidence of grain dissolution, overlain by Orbitolinidae foraminiferal rich horizons. By the top of Depositional Sequence 1 the low angle slope to the southwest had been totally subdued by sediment infilling and/or a decrease in the rate of differential subsidence. Depositional Sequence 1 was terminated by a comparatively minor relative sea-level fall followed by an immediate return to a major third-order relative sea-level rise and the basal transgressive deposits of Depositional 2. Depositional Sequence 2 re-establishes rudist sediment lagoonal deposition on the platform. It is often difficult to differentiate the transgressive and highstand systems tracts from one another in Depositional Sequence 2 and the boundary may be transitional. The upper sequence boundary of Depositional Sequence 2 is marked by a subaerial karst overlain by phosphatic greensands interpreted as reflecting a major relative sea-level fall and subsequent reflooding. Factors unique to carbonate depositional systems and their implications for the use of 'traditional' sequence stratigraphic models for carbonates are also discussed.

The Lower Carboniferous ('mid'-Courceyan to early Chadian) Black Rock Limestone ramp succession of SW Britain, accumulated along the southern margin of the Welsh-Brabant Massif during the early stage of development of the SW European foreland basin. Two depositional cycles are recognized which culminate in the subaerial exposure of those sequences in a more landward setting. The lower depositional cycle includes the Barry Harbour Limestone Formation, which contains a variety of storm-related sedimentary structures including hummocky cross-stratification. This is conformably overlain by the tidally influenced, cross-stratified Brofiscin Oolite Formation which locally displays evidence of subaerial exposure. Seven sedimentary lithofacies are identified in these two formations, which record a distal to proximal, shallowing-upward trend that passes from beneath mean wave-base to above fairweather wave-base. The upper depositional cycle of the Friars Point Limestone Formation includes the volcanics at Middle Hope, the Waulsortian facies at Castlemartin and Cannington Park, and is capped by the (herein named) Portishead Paleosol Beds (near Bristol). Above the basal transgressive sequence, the Friars Point Limestone consists of two distinct offshore ramp facies belts. The more proximal inner ramp facies belt was more strongly influenced by storm-related currents and exhibits tubular tempestites (storm sediment-filled burrows). The finer-grained, outer ramp facies belt is extensively bioturbated and preserves few features indicative of deposition from storm-related currents. The effects and relative timing of differential subsidence was elucidated by using the well documented biostratigraphy to separate the succession into isochronous units. The onset of pronounced differential subsidence was signified by volcanic activity at Middle Hope (Weston-super-Mare). The volcanic tuffs form a coarsening- and shallowing-upward sequence which records the development of a localised, probably non-emergent, offshore-volcanic high. Recognition of a major eustatic sea level fall at the end of the Courceyan and during the early part of the Chadian, is revealed by studying the decline of the Waulsortian reefs in an offshore ramp setting and evaluating the regional significance of the Portishead Paleosol Beds. Overall, the development of the Black Rock Limestone ramp was the result of the interplay between eustacy and tectonism. A full petrographic and geochemical analysis of the Waulsortian dolomites at Castlemartin, including the use of stable isotopes revealed that dolomitization occurred during burial, most probably as a result of basin dewatering.

The Late Triassic platform carbonates of the Transdanubian Range, Hungary were part of a passive margin platform at the southwestern end of the Triassic Tethys now occurs in a single fault-bounded terrain. The Hungarian platform is made up of meter-scale, precessional (~20 k.y.) carbonate cycles. It contains a lower unit, the Main Dolomite Formation (600-1500m thick), which is totally dolomitized. It is overlain by the Transitional Unit (150-400m thick). The overlying Dachstein Limestone is up to 800m thick. The platform is a cyclic succession of subtidal carbonate, laminated tidal flat limestone or dolomitic caps, and reddish or greenish paleosols or reworked paleosols. The Triassic was a time of global greenhouse conditions and Milankovitch climate forcing has been well documented from lakes and off-shelf facies. The Triassic Hungarian carbonate platform records an imperfect Milankovitch eustatic signal. They lack the bundling of 5 precessional cycles into 100 k.y. eccentricity cycles or 20 cycles/400 k.y. bundle. This is interpreted to be due to many missed beats evidenced by caliches and paleosols, and thick amalgamated subtidal carbonates. These result from precessional sea-level fluctuations either not flooding the platform, or flooding it too deeply to allow shallowing up to sea-level in one precessional beat. Spectral analysis of the Hungarian carbonates was used to compare the amplitude spectra of different time series including lithology, gamma ray, self potential and neutron density. The spectra based on lithology were compared to synthetic spectra generated by computer from platforms subjected differing Milankovitch signals. Most dolomitization of the Hungarian carbonates occurred early in tidal flat settinfs during each high frequency cycle. Intertidal-supratidal dolomites are fine grained, Fe²⁺ and Mn²⁺ rich and slightly enriched in δ¹⁸O compared marine calcite cement, and formed from weakly to moderately reducing marine waters. Subtidal dolomites are slightly coarser grained, low in Fe²⁺ and Mn²⁺ and have heaviest d¹⁸O signatures, indicating more evaporative oxidizing brines beneath flats. Repeated emergence stabilized the dolomites to low Sr²⁺ and Na⁺ types similar to Cenozoic dolomites. Later, coarse-grained dolomites with very low Mn²⁺ Fe²⁺ and light δ¹⁸O signatures were formed along the platform margin by thermally driven, warm oxidizing marine water associated with Jurassic rifting of the Pennini Ocean (Neo-Tethys). The overall vertical distribution of early dolomite on the platform does not reflect long term ecstasy. Instead the regional stratigraphic trends in climatically sensitive sediments, as well as stable isotopes, suggest that intense dolomitization of the lower platform reflects a semi-arid, hot subtropical setting and megamonsoonal climate. Global cooling and increased humidity toward the latest Triassic and Early Jurassic, inhibited pervasive early dolomitization, leaving the upper platform little dolomitized. / Ph. D.

The Late Jurassic Arab D Formation in Saudi Arabia hosts the some of the world’s largest hydrocarbon reservoirs including Ghawar, the world’s largest oil field, and Khurais, the world’s largest supergiant to come into production in the last 5 years. Despite the vast oil reserves within the Arab D, and the central role of this reservoir at Ghawar in making up short-falls in global production, our understanding of the much fundamental characterization work both in terms of modern sequence stratigraphic reservoir frameworks and linked structural/fracture characterization. This study of Arab D reservoir at Qatif, immediately to the north of Ghawar, provides one of the first looks at a modern sequence analysis of this producing interval and illustrates that porosity zonations, and ultimately flow unit architecture may be substantially different than currently in use. The Arab D of the Arabian Plate is a carbonate ramp system of exceedingly low angle (<1o) developed during a low-eustatic-amplitude greenhouse Milankovitch setting. Combined macroscopic and petrographic data analysis allowed recognition of nine depositional facies: 1) spiculitic wackestone, 2) Planolites-burrowed wackestone, 3) bioturbated skeletal-peloidal packstone, 4) pelletal packstone, 5) peloidal-skeletal grain dominated packstone, 6) peloidal-skeletal grainstone, 7) skeletal-ooids grainstone, 8) cryptalgal laminites and 9) anhydrite. The depositional facies defined are used to interpret three facies tracts from deep to shallow across the ramp profile: 1) low energy sub-storm wave base (SWB) dominated facies that may illustrate disaerobic tendencies, 2) high energy within-fair-weather-wave-base ramp-crest or mid-ramp facies including foreshore and upper shoreface oolitic and skeletal grainstones that define one of the key reservoir pay zones and 3) back-barrier tidal flats consisting of cryptalgal laminites, sabkha-type anhydrites, and salina-type anhydrites. Three high frequency sequences are defined: QSEQ 1 is asymmetrical, dominated by subtidal lithofacies; and QSEQ 2 and QSEQ 3 are symmetrical and record a complex history of the fill on an intrashelf basin. Detailed cycle-scale correlations using core-based cycles and wireline log patterns allowed a cycle-scale correlation framework to be established that illustrates a north to south progradation of the Arab D reservoir strata, building landward from the Rimthan Arch. Diagenetic features observed in the Arab D reservoir include fitted fabric (chemical compaction), dolomitization, and cementation. These features play a major role altering reservoir quality properties as they essentially control fluid flow pathways which ultimately alter primary porosity and permeability. / text

No description available.

Manito Lake is a large, perennial, Na-SO4 dominated hypersaline lake located in the northern Great Plains of western Canada. Significant water level decrease over the past several decades has lead to reduction in volume and surface area. Today, the lake is 15% of its mid -20th century volume and 46% of its former area. This decrease in water level has exposed large areas of nearshore microbialites. These organosedimentary structures have various external morphologies, vary in mineralogical composition, and show a variety of internal fabrics from finely laminated to massive and clotted. These features range from small, mm-scale, finely laminated encrustations to large, reef-like structures up to 5 m high and over 500 m long. Although there is relatively little consistent lateral variability in terms of morphology, the structures do vary significantly with elevation in the basin. Petrographic evidence confirms a strong biological involvement in the formation of these structures. Nonetheless, inorganic and trapping mechanisms may also play a role. Dolomite, aragonite, and calcite are the most commonly found minerals in these structures, however, monohydrocalcite, magnesian calcite, hydromagnesite, dypingite, and nesquehonite are also present. The calcite is a pseudomorph after ikaite, which forms an open porous dendritic and shrub-like fabric, comprising the interiors of massive shoreline microbialite mounds and pinnacles. These ikaite pseudomorphs are encased in millimeter to centimeter-scale laminated dolomite-aragonite rinds. Radiocarbon dating and stable isotope analysis have indicated microbialite formation began about 2200 yBP in a shallow, productive, saline and cold lake. Over the next 900 years, the microbialites record a transgressing lake in a cool climate, which corresponds to a period not previously documented in this region but is referred to as the Dark Ages Cold Period, which has been documented in other parts of the Northern Hemisphere. This is followed by 500 years of warmer conditions coinciding with the Medieval Climate Anomaly. Starting about 600 years ago the lake experienced a dramatic decrease in level resulting in formation of extensive carbonate pavements, cemented siliciclastics, rinds, and coatings.

Manito Lake is a large, perennial, Na-SO4 dominated hypersaline lake located in the northern Great Plains of western Canada. Significant water level decrease over the past several decades has lead to reduction in volume and surface area. Today, the lake is 15% of its mid -20th century volume and 46% of its former area. This decrease in water level has exposed large areas of nearshore microbialites. These organosedimentary structures have various external morphologies, vary in mineralogical composition, and show a variety of internal fabrics from finely laminated to massive and clotted. These features range from small, mm-scale, finely laminated encrustations to large, reef-like structures up to 5 m high and over 500 m long. Although there is relatively little consistent lateral variability in terms of morphology, the structures do vary significantly with elevation in the basin. Petrographic evidence confirms a strong biological involvement in the formation of these structures. Nonetheless, inorganic and trapping mechanisms may also play a role. Dolomite, aragonite, and calcite are the most commonly found minerals in these structures, however, monohydrocalcite, magnesian calcite, hydromagnesite, dypingite, and nesquehonite are also present. The calcite is a pseudomorph after ikaite, which forms an open porous dendritic and shrub-like fabric, comprising the interiors of massive shoreline microbialite mounds and pinnacles. These ikaite pseudomorphs are encased in millimeter to centimeter-scale laminated dolomite-aragonite rinds. Radiocarbon dating and stable isotope analysis have indicated microbialite formation began about 2200 yBP in a shallow, productive, saline and cold lake. Over the next 900 years, the microbialites record a transgressing lake in a cool climate, which corresponds to a period not previously documented in this region but is referred to as the Dark Ages Cold Period, which has been documented in other parts of the Northern Hemisphere. This is followed by 500 years of warmer conditions coinciding with the Medieval Climate Anomaly. Starting about 600 years ago the lake experienced a dramatic decrease in level resulting in formation of extensive carbonate pavements, cemented siliciclastics, rinds, and coatings.

The Aneth Field in the Paradox Basin (SE Utah) has produced nearly 500 MMbbls of oil from phylloid-algal and oolitic carbonate reservoirs of the lower and upper Desert Creek (Paradox Formation, Middle Pennsylvanian) sequences, respectively. The oil resides in a 150 to 200 foot-thick isolated carbonate platform located in a distal ramp setting on the southwest margin of the Paradox Basin. The horseshoe-shaped platform is roughly 12 miles in diameter with an aerial extent of approximately 144 square miles. Evaluation of the platform-to-basin transition on the leeward (southern) margin of the Aneth Platform, the focus of this study, was made possible through Resolute Energy's 2017 donation of well data and core to the Utah Geological Survey Core Research Center. The lower Desert Creek sequence ranges from 50 to 100 feet in thickness and produces from a succession of phylloid-algal, boundstone-capped parasequences in the Aneth Platform. The upper Desert Creek sequence is generally thinner across the platform and is characterized by a succession of oolite-capped parasequences, except on the southern margin of the platform where it ranges from 80 to 115 feet in thickness. The upper Desert Creek thick resulted from southward shedding of platform-derived carbonate sediment and lesser amounts of quartz silt and very fine sand off the low-angle southern platform margin slope. A nine-mile-long, north-south-oriented stratigraphic panel constructed from log and core data permits characterization of thickness and facies trends through the upper Desert Creek from platform (north) to slope to distal basin (south) in the Ratherford unit. In the southern margin, five novel facies for the Aneth Field were analyzed, described, and interpreted using a sequence stratigraphic framework, all of which represent deposition on a gravity-influenced platform-edge slope. It is interpreted that the slope facies association was deposited during transgression and highstand and was generally a result of oversteepened slopes as a function of the carbonate factory on the platform being highly productive. Slope and basin facies range from proximal rudstone and floatstone to thin, graded distal turbidites, the latter of which extend at least five miles into the basin. Compaction of the muddy and fine-grained allochthonous sediment followed by pervasive calcite and anhydrite cementation has destroyed any primary porosity in the platform-derived slope-to-basin sediments.

The Bahamas Island archipelago grows by the precipitation and secretion of calcium carbonate. A majority of this growth is by lateral accretion of shoreline sedimentary deposits. Previous research is not clear on whether the growth is largely due to eustasy, sediment input from catastrophic events, or a combination of both. The Bahamas is an ideal location for studying Holocene carbonate generation and deposition, but there is limited research on the analysis of strandlines in relation to lateral accretion. Carbonate strandline deposits are commonly classified as low-energy beach ridge deposits. Previous researchers have primarily focused on ooid shoals and subtidal regions. Understanding the mechanisms of platform and shoreline growth in the Bahamas is important for creating petroleum reservoir analogs for exploration. We use ground penetrating radar (GPR) to image and interpret the internal fine-scale stratigraphy of Bahamian carbonate strand plains and thereby constrain our understanding of the processes by which the islands grow. Although GPR has been used extensively to analyze the interior of clastic strandline deposits across the world, tropical carbonate settings have received little attention. We are the first to utilize GPR to study strand plains in Crooked Islands, The Bahamas, our primary location for 2D GPR data acquisition. We integrate our interpretation of these data with a 3D GPR data volume collected on Pleistocene eolianites on San Salvador Island, The Bahamas. We used a GSSI (Geophysical Survey Systems, Inc.) bistatic 400-MHz antenna with a field frequency filter of 100"“800 MHz for all datasets. GPR allowed visualization of the interior of the strand plains down to a depth of about 2 m with high resolution. Data processing was performed using state-of-the-art petroleum industry techniques (e.g., gain control, deconvolution, migration, seismic attribute computation) to better visualize the reflectivity. Our data constrains a model that the lateral accretion of carbonate sediment preserved in strandline was deposited in a combination of storm processes and gradual sediment progradation, rather than one or the other. Our conclusions help determine that The Bahamas is ideal for GPR imaging of strandlines due to being assessable, high data quality, no clastic influence, and a dry environment during parts of the world

Observations of modern carbonate depositional environments and their accompanying depositional models have been used for decades in the reconstruction and interpretation of ancient carbonate depositional environments and stratigraphic successions. While these Holocene models are necessary for interpreting their more ancient counterparts, they inherently exclude important factors related to the erosion, diagenesis, and ultimate preservation of sediments and sedimentary structures that are ubiquitous in shallow marine carbonate environments. Andros Island, Bahamas is an ideal location to examine the validity of Holocene conceptual models, where geologically young (Late Pleistocene) limestones can be studied immediately adjacent to their well-documented modern equivalents. For this study, two 3D ground-penetrating radar (GPR) datasets (200 MHz and 400 MHz) were collected at a schoolyard in northwest Andros. These surveys reveal the geometries and internal characteristics of a peloidal-oolitic sand wave and tidal channel in unprecedented detail. These two prominent features are underlain by low-energy lagoonal wackestones and packstones, and are bordered laterally to the northwest by wackestones-packstones intermixed with thin sheets of peloidaloolitic grainstone. A deeper radar surface is observed at approximately 6 m depth dipping gently to the west, and is interpreted to be a karstified exposure surface delineating the base of a complete depositional sequence. Interpretation of the 3D radar volumes is enhanced and constrained by data from three cores drilled through the crest and toe of the sand wave, and through the tidal channel. This study is the first of its kind to capture the complex heterogeneity of a carbonate depositional package in three dimensions, where various depositional environments, sedimentary structures, and textures (mudstone to grainstone) have been preserved within a small volume.The results from this study suggest that the degree of vertical and lateral heterogeneity in preserved carbonate successions is often more complex than what can be observed in modern depositional environments, where sediments can generally only be observed in two dimensions, at an instant in time. Data from this study demonstrate the value of using two overlapping GPR datasets at differing resolutions to image the internal characteristics of a complete carbonate depositional package in three dimensions. From these datasets, a depositional model similar to other Holocene and Pleistocene carbonate depositional models is derived.