Lateral Facies Research Articles

Stromatolites: a guideline for development of a carbonate ramp, Basantpur Formation, Neoproterozoic Simla Group in Lesser Himalaya, India

Outcrop-based facies analysis of the Proterozoic Basantpur Formation, Simla Group in the Lesser Himalaya was combined with the stromatolites morphometry and sea-level fluctuation to delineate the stages of carbonate ramp development. On this basis, a vertical profile depositional model (Basantpur type) has been developed. Facies associations and variation in the patterns of microbial growth along with the sea-level fluctuations have contributed to the identification of the development of a tide-influenced carbonate ramp. Different stromatolitic structures (mega-, macro- and microstructures) are documented in the dolomudstones and dolosiltstones along with fenestral structures and their depositional facies together with evidences of marine transgression which leads to development of carbonate ramp where inner-mid-outer-ramp subenvironments are recognised. The “Basantpur”-type model is therefore unique in that it deals with lateral facies variation due to shift in shore line along with fluctuations in accommodation space on a carbonate ramp owing to fluctuations of sea level. This model will probably find its applicability in similar carbonate ramps.

Deformed cross-stratified deposits in the Early Pleistocene tidally-dominated Catanzaro strait-fill succession, Calabrian Arc (Southern Italy): Triggering mechanisms and environmental significance

The Early-Pleistocene Catanzaro strait-fill succession consists of large-scale tidal sets, accumulated in a tectonically confined basin during a phase of rapid relative sea-level rise. It crops out mainly in the present-day Catanzaro Trough where numerous field sections supported the characterization of the vertical and lateral facies variations and the documentation of a variety of soft-sediment deformation structures, exposed throughout their vertical and lateral extents. The soft-sediment deformation structures (SSDS) are the result of liquefaction and fluidization processes that deformed cross-laminations and other primary structures into folds, fluid-escape structures, and structureless expanses. Three different groups of SSDS have been documented in the cross-stratified deposits of the Catanzaro strait. The detailed description of these soft-sediment deformation structures in a depositional context established by facies analysis enables interpretation in terms of possible trigger mechanisms. Consistent relationships between the occurrence of distinctive SSDS and specific tidally dominated facies have been established, indicating a probable autogenic origin for the soft-sediment deformations. Liquefaction and fluidization features are interpreted as the result of increases in water pore-pressure, induced by overloading. In particular, two types of overloading agents are hypothesized, which affect the lee and stoss sides of the migrating dunes in distinct events, and inducing the deformation of foreset laminae or sets of cross-strata, respectively.

Discrimination, correlation, and provenance of Bed I tephrostratigraphic markers, Olduvai Gorge, Tanzania, based on multivariate analyses of phenocryst compositions

The chronology of Pleistocene flora and fauna, including hominin remains and associated Oldowan industries in Bed I, Olduvai Gorge, Tanzania, is primarily based on 40Ar/39Ar dating of intercalated tuffs and lavas, combined with detailed tephrostratigraphic correlations within the basin. Although a high-resolution chronostratigraphic framework has been established for the eastern part of the Olduvai Basin, the western subbasin is less well known due in part to major lateral facies changes within Bed I combined with discontinuous exposure. We address these correlation difficulties using the discriminative power of the chemical composition of the major juvenile mineral phases (augite, anorthoclase, plagioclase) from tuffs, volcaniclastic sandstones, siliciclastic units, and lavas. We statistically evaluate these compositions, obtained from electron probe micro-analysis, applying principal component analysis and discriminant analysis to develop discriminant models that successfully classify most Bed I volcanic units. The correlations, resulting from integrated analyses of all target minerals, provide a basin-wide Bed I chemostratigraphic framework at high lateral and vertical resolution, consistent with the known geological context, that expands and refines the geochemical databases currently available. Correlation of proximal ignimbrites at the First Fault with medial and distal Lower Bed I successions of the western basin enables assessment of lateral facies and thickness trends that confirm Ngorongoro Volcano as the primary source for Lower Bed I, whereas Upper Bed I sediment supply is mainly from Olmoti Volcano. Compositional similarity between Tuff IA, Bed I lava, and Mafic Tuffs II and III single-grain fingerprints, together with north- and northwestward thinning of Bed I lava, suggests a common Ngorongoro source for these units. The techniques applied herein improve upon previous work by evaluating compositional affinities with statistical rigor rather than primarily relying on visual comparison of bivariate plots.

Stratigraphic Architecture of a Transtensional Continental Basin In Low-Latitude Semiarid Conditions: The Permian Succession of the Central Orobic Basin (Southern Alps, Italy)

Abstract Continental successions in fault-controlled extensional–transtensional basins store a wide range of clastic deposits from various depositional environments, representing a challenging target for hydrocarbon exploration. Recent advances in field-based facies analysis and genetic interpretation of these sediments show that these successions are characterized by great variability in terms of volume, geometry, facies, and stacking patterns. This variability derives from a number of factors that govern the sedimentary processes, such as tectonic setting, basin size and morphology, type and amount of sediments, and climate. The study focuses on a well-exposed Early Permian continental succession (Southern Alps, northern Italy), which was deposited in an extensional–transtensional tectonic regime under a semiarid to arid climate. The study area is characterized by a thick (more than 1000 m) succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs (Pizzo del Diavolo) Formation resting on up to 800 m of prevailing pyroclastic flows (Cabianca Volcanite). These units are unconformably covered by the Upper Permian alluvial sediments of the Verrucano Lombardo. Sedimentological features reflect a proximal fan system where mass-flow phenomena dominate, a distal fan-terminal setting with prevailing sandy sheet-flow processes and a silty floodplain environment, where sheet-flood events dominate. Laterally continuous siltstones and dark shales with carbonate nodules and layers of microbial carbonates in the most depressed part of the basin indicate alternating conditions of shallow lake and desiccated floodplain. Syndepositional tectonic activity is documented in the succession by rapid vertical and lateral facies changes. In this study, we present a data-constrained reconstruction of the depositional architecture of the basin, a description of depositional processes and depositional environments, and a model of facies distribution. Outcrop data, integrated with laboratory analyses, provide a detailed understanding not only of the small-scale elements of the system (sub-seismic scale) but also of the entire basin, focusing on how those elements are interrelated. The studied succession can be considered an interesting outcrop analogue for subsurface extensional–transtensional basins, potentially useful to predict reservoir properties for similar subsurface settings.

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

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

Depositional controls on sediment body architecture in the Eagle Ford/Boquillas system: Insights from outcrops in west Texas, United States

A detailed sedimentological study of the middle Eagle Ford/Boquillas (outcrop analog of subsurface producing strata) units was conducted on numerous roads cut along US Highway 90 in Val Verde County and on outcrops in Big Bend National Park, Brewster County, Texas, using field-based petrographical and high-resolution image capture methods (light detection and ranging and GigaPans). This study demonstrates that vertical and lateral facies distribution is controlled by the interaction of sediment productivity under the influence of bottom currents below storm wave base. Vertical cyclicity is the result of alternating periods of lower primary productivity and relatively low sediment accumulation rates (globigerinid argillaceous wackestones) and shorter periods of high primary productivity and higher accumulation rates (pelagic grainstones), driven by the absence or presence of iron from volcanic ash deposition. Lateral variations are controlled by the deposition and reworking of pelagic sediment under the influence of below–storm wave base bottom currents. Pelagic grainstones accumulated in the form of isolated barchanoid hydraulic dunes, sand ridges, coalesced sand ridges, and sand sheets and less commonly as continuous beds. Detailed measurements show that pelagic grainstones have no more than 50% continuity, and ash beds have 72% continuity. Application of sequence stratigraphic principles needs to be done with caution because the deeper-water depositional setting is affected not by sediment input from a shallow-water benthic carbonate factory but by pelagic sediment from the open-marine environment subject to bottom current reworking. Packages of strata may be more reliable for longer-distance correlations.

Geoelectrical and hydrogeological study to delineate the geological structures affecting the groundwater occurrence in Wadi El Khariq Basin, Northwest El Maghara, north Sinai, Egypt

Wadi El Khariq has a vast area that can be reclaimed. Upper Jurassic carbonates represent the main aquifer. The Desert Research Center has a pilot station in Wadi El Khariq that suffers from water deficiency. Therefore, the drilling of new wells is needed. As fault zones make fracture conduits for groundwater, detecting these faults is a vital process. Therefore, this study aims to trace the existing subsurface structures and their impact on groundwater occurrence. The interpretation of the results of the vertical electrical soundings leads to the detection of five geoelectrical layers. The third and fourth layers correspond to fractures and argillaceous limestone represents the water-bearing layers. The interpretation of the results of the eight 2D electrical resistivity tomography profiles gave more illustrative information about the lateral facies changes, the structures effect along the study area, distribution of the different types of water-bearing limestone, and the depth to their top. The deduced structures from electrical study are represented by the faults F1–F6. The faults F1 and F2 are confirmed from both 1D and 2D imaging profiles. The fault plains of F1, F2, F5, and F6 are continuous between the measured imaging profiles. The 2D imaging profiles revealed that the clay content increase eastwards. The correlation results of the natural gamma and lithological logs of the drilled wells as well as the constructed hydrogeological profiles revealed that the area of study affected by another group of faults (F6–F12). These faults affect on the groundwater occurrence and quality. These faults lead to the formation of graben and horst structures, lateral facies changes, variation in layers thickness, and the occurrence of the fractured limestone layer lying beneath the calcareous clay at variable depths. Most of the detected faults in the study area have NW–SE trends coinciding with the lineation analyses of the faults and fractures of El Khariq basin. The fractured limestone aquifer in the study area is recharged via the percolation of the surface runoff water of local precipitation on Gable El Maghara catchment area through the fracture lines, fault planes and bedding planes besides paleowater of the paleo-rainy seasons. The depth to water varies from 60 to 114 m. The groundwater salinity of the wells drilled in El Khariq plain varies greatly due to the effect of the lithological nature of the water-bearing formation and the well design. It varies from 2335 to 12548 mg/l. The discharge value (Q) varies from 60 to 12 m3/h. This variation is related to the extension and type of the effective fracture system affecting the concerned aquifer.

High-resolution stratigraphy and facies architecture of the Upper Cretaceous (Cenomanian–Turonian) Eagle Ford Group, Central Texas

Rock-based studies of the Eagle Ford Group of Central Texas demonstrate that mudrock deposition is more complicated than previously supposed. X-ray diffraction, x-ray fluorescence, total organic carbon (TOC), and log data collected from eight cores and two outcrops demonstrate that bottom-current reworking and planktonic productivity are primary depositional controls, acting independently from eustatic forcing. Central Texas Eagle Ford facies include (1) massive argillaceous mudrock, (2) massive foraminiferal calcareous mudrock, (3) laminated calcareous foraminiferal lime mudstone, (4) laminated foraminiferal wackestone, (5) cross-laminated foraminiferal packstone–grainstone, (6) massive bentonitic claystone, and (7) nodular foraminiferal packstone–grainstone. High degrees of lateral facies variability, characterized by pinching and swelling of units, lateral facies changes, truncations, and locally restricted units, are observed even at small lateral scales (50 ft [15 m]). At 10 mi (16 km) and greater lateral spacings, core and geochemical data significantly underestimate intraformational facies variability. Approximately 73% of units can be successfully correlated across a distance of 500 ft (152 m), 35% are traceable across 1 mi (1.6 km), and only 16% of beds are correlative across 10 mi (16 km). Geochemical proxies (enrichment in molybdenum and other trace elements) indicate that maximum anoxia occurred within the Bouldin Member despite being composed of the most calcareous and high-energy facies. Comparison of total gamma ray (GR) logs to computed GR logs is requisite, because GR alone may provide misleading determination of facies, TOC content, depositional environment, and sequence stratigraphic implications.

Not so simple “simply-folded Zagros”: The role of pre-collisional extensional faulting, salt tectonics and multi-stage thrusting in the Sarvestan transfer zone (Fars, Iran)

The Sarvestan plain is bounded by highly elevated anticlines associated with thrusts or transpressional faults and hosts the NNW–SSE Sarvestan transfer zone. Surface and subsurface geological data, and 22 seismic lines allowed us to reconstruct the 3D geometry of the area. Mixed layer illite–smectite and 1D burial and thermal modelling were used to constrain the complex geological evolution of the Sarvestan plain where inherited structures strongly controlled the geometry of syn- to post-collisional contractional structures. Paleozoic–Mesozoic rifting related extension generated E–W and NNW–SSE normal fault systems. Such faults were associated with changes in the thickness of the sedimentary cover. Lateral facies changes were later induced by the Cretaceous obduction of ophiolites, cropping out some tens of km north of the study area. During the Miocene the footwall and the hanging wall of the Sarvestan Fault had different thermal evolution. This is tentatively explained by flow of Cambrian salt from the plain area towards the hanging wall of the Sarvestan Fault, associated with salt diapirism during Lower–Middle Miocene time. Salt tectonics is invoked also to explain, at least in part, the development of the overturned anticline in the hanging wall of the Sarvestan Fault. An early phase of contractional deformation occurred in the Middle Miocene (since 15My, i.e., after the deposition of the Agha Jari Fm) generating the E–W oriented folds buried below the plain, likely inverting inherited normal faults. The erosion of these structures was followed by the deposition of the Bakhtiari Fm conglomerates in Middle–Late Miocene times. A later phase of contractional tectonics generated the thrust faults and the anticlines bounding the Sarvestan plain some 6–5My ago. The Sarvestan dextral transpressional fault, that likely acted as a strongly oblique ramp of the Maharlu thrust, mainly structured in this period, although its activity may have continued until present.

Advanced cuttings analysis provides improved completion design, efficiency and well production

R ecent case studies of lateral well completions have highlighted the apparent ineffectiveness of geometric stage performance with regards to fracture initiation and stimulated reservoir volume (Far et al., 2015; Ashton et al., 2013; Ganguly and Cipolla, 2012). It has been observed that poor stage performance can be attributed to varying rock type compositions along the borehole, due to lateral facies changes and/or wellbore porpoising. The inefficiencies linked to non or poor performing stages can result in higher costs, associated with equipment and materials, and limit the production potential of wells. This case study highlights how the application of an optimized hydraulic fracture stimulation plan, by honouring lateral well geological heterogeneities, improved completion efficiency and well production within the Cleveland Sandstone formation, Oklahoma, USA. Optimized completions Designing optimized hydraulic fracture stimulation in a lateral well requires an understanding of the near-well bore geomechanical properties and the near-well bore and farfield stresses along the entire lateral. Such reservoir characterization is normally developed from geomechanical and petrophysical analyses using wireline or logging-while-drilling (LWD) services that include acoustic and borehole- image logs. Unfortunately, economic considerations can often inhibit or prohibit the use of logging techniques and hence the complete characterization of the reservoir, especially in the current low oil and gas price environment. A practical and convenient alternative for reservoir characterization is to use the commonly available cuttings samples to provide gross characterization of a vertical or lateral well. Utilizing Automated Mineralogy (AM) techniques, such as CGG’s RoqSCANTM, (Ashton et al., 2013) provides comprehensive data from these cuttings which includes mineralogy, texture and rock properties throughout the entire length of the well. RoqSCAN analysis can be performed either in the laboratory or at the well site to provide near-real-time reservoir navigation and data to optimize the design of completions that will be performed soon after the well reaches total depth (TD).

A Miocene-restricted platform of the Zibane zone (Saharan Atlas, Algeria), depositional sequences and paleogeographic reconstruction

Depositional sequences and paleogeographic evolution of the Miocene deposits have been studied in the Zibane zone (Saharan Atlas, Algeria) located at the north of the African platform. During the Miocene, this region corresponded to a fault-bounded collapse area and filled by diversified deposits, showing important lateral facies and thicknesses variations. The studied deposits are divided into five depositional sequences separated by major unconformities. These depositional sequences are well developed in the whole basin and testify a paleogeographic differentiation from E–W, induced by a set of NW-SE-trending old faults inherited from the Atlasic orogeny. The organization and the development of those sequences make it possible to correlate them better to the basin scale, which is integrated in a model of restricted platform intersected by NW-SE faults where the tectonic-sedimentation duality is predominant. These new data point to a paleogeographic evolution different from the one usually admitted environment for this region during Miocene time and plead in favour of a reconsideration of the depositional environments of the post-Burdigalian formations in the Zibane zone of the Algerian Atlasic domain.

Evidence for SEDEX-style mineralization in the 1.7 Ga Tawallah Group, McArthur Basin, Australia

The Paleoproterozoic McArthur Basin (McArthur Group) of northern Australia hosts world-class sedimentary ‘exhalative’ (SEDEX) McArthur type Zn–Pb deposits, which are largely hosted within a sequence of 1.64Ga pyritic carbonaceous shales deposited in an extensional rift setting. A well-known example of these is McArthur River (or Here's Your Chance [HYC] Zn–Pb–Ag deposit). The ~1.78Ga McDermott and ~1.73Ga Wollogorang formations (Tawallah Group) both contain carbonaceous shales deposited in similar environments. Our observations suggest the carbonaceous facies of the Wollogorang Formation were deposited under mostly euxinic conditions, with periodically-high concentrations of sedimentary pyrite deposition. The carbonaceous shales in the older McDermott Formation contain considerably less early pyrite, reflecting a mostly sulfide-poor, anoxic depositional environment. Localized fault-bound sub-basins likely facilitated lateral facies variations, which is evident from synsedimentary breccias.The presence of evaporitic oxidized facies within the McDermott and Wollogorang formations, alongside evidence for synsedimentary brecciation in reduced shales are favourable criteria for SEDEX-style base metal deposition. Both formations overlie volcanic units, which could have been sources of base metals. Detailed X-ray petrography, new geochemical data and sulfur isotope data from historical drill cores indicate multiple horizons of stratiform and sediment breccia-hosted base metal sulfide within carbonaceous shale units, with high-grade Zn concentrations. A close association between sphalerite and ferromanganean dolomite alteration draws comparisons with younger SEDEX mineralization at HYC. Additionally, SEDEX alteration indices, used demonstrably as a vector to the younger orebodies, indicate the sedimentary rocks analyzed in this study are marginally below the ore window when compared to the overlying mineralized stratigraphy.Our data imply that localized active circulation of metalliferous brines occurred in the Tawallah Group basin. High-grade sulfide deposition in reduced facies alteration may represent distal expressions of larger SEDEX-style deposits. Furthermore, abundant pyrite and high molybdenum in the Wollogorang Formation suggest the global oceanic sulfate concentration was sufficient by ~1.73Ga to engender intermittent but strong bottom-water euxinia during shale deposition, thus providing a robust chemical trap for base metal sulfide mineralization.

Sea–land geology of Marettimo (Egadi Islands, central Mediterranean sea)

ABSTRACTWe present a 1:10,000 scale geological map of Marettimo Island and its offshore (Egadi Archipelago, central Mediterranean Sea). The map was achieved by integrating a new geological survey with data from recent, marine, geological and geophysical surveys acquired along the adjacent continental shelf. The island is composed of a Mesozoic, mostly carbonate platform succession, which is overlain by continental to coastal marine Quaternary deposits. Extensional tectonics have affected the carbonate platform since the Late Triassic producing an initial increase of accommodation space that was filled by interbed breccias, marls and calcareous marls. During the Jurassic, a NE-SW-directed normal fault-bounded structural lows where thick beds of megabreccias accumulated followed upwards by cherty limestone and bedded cherts. During the building of the chain, the compressional tectonics generated a southeast-verging tectonic wedge, displaced by subsequent transpression. During the Quaternary, repeated sea-level changes shaped a rough erosional surface along the inner shelf and controlled the lateral facies variation of the continental and marine coastal deposits along the present day coastline.

Volume calculation of subsurface structures and traps in hydrocarbon exploration — a comparison between numerical integration and cell based models

AbstractThe volume calculation of geological structures is one of the primary goals of interest when dealing with exploration or production of oil and gas in general. Most of those calculations are done using advanced software packages but still the mathematical workflow (equations) has to be used and understood for the initial volume calculation process. In this paper a comparison is given between bulk volume calculations of geological structures using trapezoidal and Simpson’s rule and the ones obtained from cell-based models. Comparison in calculation is illustrated with four models; dome ‐ 1/2 of ball/sphere, elongated anticline, stratigraphic trap due to lateral facies change and faulted anticline trap. Results show that Simpson’s and trapezoidal rules give a very accurate volume calculation even with a few inputs(isopach areas ‐ ordinates). A test of cell based model volume calculation precision against grid resolution is presented for various cases. For high accuracy, less the 1% of an error from coarsening, a cell area has to be 0.0008% of the reservoir area

Stratigraphic evidence for development of Aptian intrashelf basin in the Zagros area, eastern Fars Province, SW Iran

?The Lower Cretaceous of the Arabian Platform is renowned for the development of intrashelf basins. The study area is located in eastern Fars Province on the northeast Arabian plate and has experienced this environmental condition. The present study investigated two surface sections (Genau and Anguru) of the Dariyan Formation and compared the results with three neighboring wells in eastern Fars (eastern Zagros Fold Belt) in southern Iran to determine the development of rudist-bearing carbonates and intrashelf basin sediments. The Dariyan Formation was deposited in the Aptian as shallow-to-deep water carbonate facies. Lagoon, rudist banks/barriers, and open marine/basinal facies and six microfacies types have been distinguished. In the Genau section, the upper formation contains shallow rudistid limestone separated from the orbitolinid limestone of the lower formation by a deep pelagic unit. The pelagic sets disappear toward the Anguru section, but the rudistid unit remains. Abrupt changes were observed from shallow rudist facies to deep planktonic foraminifer facies. Comparison of the surface and subsurface sections (wells) suggests an intrashelf carbonate depositional model for the Dariyan Formation in the eastern Fars area. The term "Genau intrashelf" is proposed for this intrashelf basin surrounded by a shallow ramp. This intrashelf appears to be a northern subbasin of the Bab basin of the southern Persian Gulf. Extension of the Genau intrashelf subbasin and development of rudists around the margins of this intrashelf can be defined by tracing changes in the lateral facies of the Dariyan Formation.

Facies and facies association of Piramboia Formation in the region of Descalvado (SP)

A Formação Piramboia é uma unidade litoestratigráfica da Bacia do Paraná que tem sido alvo de diversos estudos em razão de sua grande importância como componente primordial no Sistema Aquífero Guarani e no sistema petrolífero Irati-Piramboia, atuando como excelente rocha-reservatório. Historicamente, essa unidade tem sido alvo de diversas controvérsias quanto à idade, relações de contato com as unidades sotoposta e sobreposta e paleoambiente deposicional. Entretanto, é comumente considerada de idade triássica e produto da deposição em sistemas eólicos úmidos, com abundância de interdunas úmidas e fácies fluviais subordinadas. Neste trabalho, a Formação Piramboia foi caracterizada por meio de técnicas como análise de fácies, associação vertical e lateral de fácies e arquitetura deposicional realizadas a partir de levantamentos nas frentes de lavra da Mineração Jundu, em Descalvado, nordeste do estado de São Paulo. Foram descritas cinco fácies para a Formação Piramboia na região de estudo: St, Sh, Sm, Sr e Gt, geradas por meio de processos sedimentares do tipo carga de fundo, a maioria sob regime de fluxo inferior. Além disso, foram reconhecidas quatro associações de fácies que permitem estabelecer quatro elementos arquitetônicos presentes no canal fluvial principal: Complexo de barras de canal, constituído pelas macroformas de acresção vertical (FM), forma de leito arenosa (SB) e forma de leito do tipo barras conglomeráticas (GB); Depósito de enchentes, constituído pelos lençóis de areia laminados (LS); Depósitos de fluxos hiperconcentrados e depósitos eólicos. Os resultados indicam que a Formação Piramboia na região de estudos é o registro da sedimentação em rios entrelaçados com depósitos de dunas e interdunas subordinados que caracterizam interação fluvio-eólica

Fracture distribution along an Upper Jurassic carbonate ramp, NE Spain

Understanding the distribution of natural fractures in sedimentary systems is of high relevance for the exploration and production of fluids in the subsurface. This study focuses on a Kimmeridgian mixed siliciclastic-carbonate ramp system, which is part of the Jurassic limestones outcropping in the northeastern Iberian Chain. The study area is located north of the Ricla village, fifty kilometers southwest of Zaragoza. The outcrop stretches over six kilometers in length, it allows for recording detailed fracture patterns and facies variations. A GIS-based software-package 'DigiFract' is used to measure and digitize fractures in vertical outcrops. Fracture orientation measurements obtained from vertical and horizontal outcrops are used to create a conceptual three-dimensional image of the fracture distribution of the mixed ramp system. The fracture data are processed by integrating outcrop logs and sample-data obtained from thin-sections and rock property analysis. The continuous facies belts of the shallow low-angle ramp system show limited lateral variations. The studied sediments are subdivided in four main facies tracts (FT's): (FT-1) Bioclastic siltstones, (FT-2) alternating sandy limestones and marls, (FT-3) cross-bedded and channelled oolitic-bioclastic sand- and grainstones and (FT-4) coralgal float-to mudstones with eventites. Measured fracture orientations of the vertical and horizontal outcrops are identical throughout the entire exposure. Two main fracture sets are identified, the first set (Set 1) has a N–S direction and the second set (Set 2) has a NE–SW direction. The lateral homogeneity of the sedimentary system, thus facies, layer thickness and slope angle, can be translated to the observed fracture patterns. Within one single facies belt, fractures tend to behave the same in the proximal, middle and distal part of the ramp system. However, vertical facies variations are an important factor for the measured vertical fracture-heterogeneity. Fine-grained mud-supported facies correspond to periods of a sea-level highstand; coarse-grained cemented facies on the other hand are related to a sea-level lowstand. The physical contrast of the sediments caused by sea-level fluctuations forces fractures to solely concentrate in the brittle layers. Fracture density and termination patterns observed on this ramp stand in contrast to fracture geometries observed in flat-topped carbonate platforms. Lateral facies heterogeneity and platform anatomy of flat-topped carbonate platforms are key parameters for the eventual fracture distribution. For the studied mixed ramp-type system the vertical facies variations are key parameter.

Evidence of early halokinesis in the Zechstein Group suggests the formation of Permian-Triassic carbonates build-ups offshore UK (Quad. 20-21)

The Upper Permian Zechstein Group comprises sequences of carbonate and evaporites, which extend over most of the onshore areas in North- West Europe, from Britain to Poland, and across the central and southern North Sea, bringing important economic value to hydrocarbon exploration. The nature and distribution of the evaporitic facies are key factors influencing the deposition, thermal evolution and the trap¬ping mechanism in the overburden section, as well as the sealing of the Early Permian-Carboniferous units under¬neath. The Zechstein Group includes source rocks from anoxic shale and microbialites, reservoirs from shallow water carbonate and several levels of seals from anhydrites and halite (Karnin et al., 1992; Cooke-Yarborough 1994; Slowakiewicz et al., 2013). The early salt movements have been tracked back to the Triassic and Jurassic (Glennie and Higham, 2003 and references therein), under different tectonic regimes between the Jurassic-Cretaceous rifting, to the Paleogene inversion, active in the remote foreland of the Alpine Orogeny. The lateral facies distribution has been largely described as a result of different subsidence rates and climatic fluctuation in the Zechstein Basin (Figure 1). Well data confirms the complex architecture of this basin at different scales (Geluk, 2000).

Discriminant Analysis of Elemental Data To Differentiate Formations of Like Facies Vertically Across An Unconformity and Laterally Across A Paleotopographic Divide

Abstract Distinguishing different lithostratigraphic formations that are in contact with each other can be problematic when they are of like facies, appearance, and provenance. For example, formations of like facies that are vertically separated by a subtle unconformable surface are difficult to distinguish, especially if multiple candidate surfaces exist. Similarly, time-equivalent formations of like facies that were separated into distinct subbasins by a topographic divide may also be challenging to distinguish. In this study, formations of this nature were discriminated using statistical discriminant analysis of X-ray fluorescence (XRF) elemental data that included both mobile and immobile elements. This discrimination has important consequences for stratigraphic analysis. For example, accurate placement of an unconformable surface within a stratal succession is important because it affects the interpreted thickness of units and sequences in contact with that surface. Unit thickness influences the interpretation of basin subsidence and its timing, depositional style of basin fill, paleotopography, and diagenesis. When developing discriminant models, data must be collected from outcrop or well cores where formations bounding the unconformity or topographic divide are known. The models next establish and high-grade key elemental differences between the known formations. The models can then be used to classify samples from unknown formations to one of the known formations. In our study, samples from three lithostratigraphic formations were used to build two models: one grouped by facies trends, and one grouped by geographic location or tectonic feature. Results demonstrate that discriminant analysis of elemental XRF data can be used to distinguish these formations both vertically and laterally. Our models relied on both mobile and immobile elements to make the discrimination, implying that as long as sandstone formations of similar provenance have experienced different diagenetic histories (i.e., cements and alteration products) they can be distinguished in the rock record. This study applied the methodology of discriminant function analysis to identify the J-1 unconformity on Utahs Colorado Plateau. The J-1 unconformity is a regional unconformity that separates the Early and Middle Jurassic. Three formations of similar facies, appearance, and provenance are all in contact with this unconformity. The Early Jurassic Navajo Sandstone underlies the Middle Jurassic Page Sandstone in the eastern part of the study area, and the Middle Jurassic Temple Cap Formation in the western part of the study area. In much of the central part of the study area the stratigraphic position of the J-1 unconformity is unknown and the overlying formation is less well established. The J-1 unconformity appears to have been a baffle or barrier to fluid flow in much of this area. Also, paleotopography on the Navajo Sandstone appears to have separated the overlying formations into two subbasins. Thus, all three formations had different diagenetic histories that facilitated the discriminant modeling. The methods used in this study can be applied to similar sedimentary packages that were likewise partitioned vertically by fluid-flow divides such as unconformities and hiatuses, or laterally by topographic, hydrodynamic, structural divides, or lateral facies changes.

Paleocene stratigraphy in Egypt

The Egyptian Paleocene is widely distributed with vertical and lateral facies changes geographically separated and subject to different tectonic and sedimentary regimes. Five coeval facies associations of the Paleocene outcrops are identified and named from south to north: Garra El-Arbain, Nile Valley, Farafra, Sinai, and Southern Galala. Ten Paleocene third-order depositional sequences (Ds Da1 to Ds Th9) are tentatively distinguished in Egypt. These are bounded at their base and top by ten sequence boundaries (Eg.Da-1, Eg.Da-2, Eg.Da-3, Eg.Da-4, Eg.Da-5, Eg.Da-6, Eg.Se-7, Eg.Th.-8, Eg.Th.-9, and Eg.Eo-10). The relative ages and correlation of the Paleocene depositional sequences are based on planktonic foraminiferal biostratigraphy. Comparison of identified Paleocene sequences in and outside Egypt are referred to eustatic sea-level changes and partly to regional tectonics events, which have caused hiatuses of variable durations and different configurations of Paleocene sedimentary regimes from place to place.