Facies Description Research Articles

Argilluviation, Calcification, and Silicification in Palaeosols from the Late Cretaceous of Brazil

Palaeosols are soils that have formed in past landscapes. They are used in palaeoenvironmental analyses and reconstitutions, as in stratigraphic correlations, because they indicate surface stability and represent moments of pause in erosion and deposition. In the Marília Formation (Upper Cretaceous of Bauru Basin), the palaeosols exhumed and buried were developed on sandstones and present argillic (Btk, Bt and Bss), hydromorphic (Cg), and calcic (Bk) horizons with different cementation degrees. During the Maastrichtian of the Bauru Basin, these palaeosols were conditioned by calcification, silicification, and argilluviation. The objective of the study was to analyse the relationship and palaeoenvironmental significance between the processes of argilluviation, calcification, and silicification in palaeosols developed in the proximal, medial, and distal zones of distributive fluvial systems (DFS) of the Upper Cretaceous of Brazil. Eleven sections (S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, and S11) were described, comprising forty-three profiles. The description of facies and identification of architectural elements, characterisation of palaeosols, and micromorphological and geochemical analyses were performed according to the specialised literature. We identified well-drained (Bt horizon) and poorly drained or hydromorphic (Cg horizon) pedotypes that represent alternated periods of drier (Bkm horizon) and higher humidity (Bss horizon). The profiles were classified as Luvisols (Bt and Btk horizons), Calcisols (Bk and Ck horizons), Arenosols (C-horizons), and Vertisols (Bss-horizons). The palaeosols of the Marília Formation, Maastrichtian of Bauru Basin, developed in a distributive fluvial system (DFS) dominated by braided rivers and ephemeral lakes. The palaeosols with Bk, Btk, Bss, Bg, and Bt horizons suggest climate semi-arid during the Maastrichtian, with changes in precipitation and evapotranspiration patterns. Calcification and argilluviation were the main formation process responsible for the genesis and evolution of palaeosols during the Maastrichtian. Silicification, responsible for forming silcretes in some profiles, occurred subsidiary to the calcification process in semi-arid climate conditions and may have been affected by diagenetic processes. Calcification, responsible for the diversity of calcretes in the Marília Formation, was the most abundant and intense process and reflected periods with more arid conditions in the general context of the semi-arid palaeoclimate of the Late Cretaceous of the Bauru Basin. Calcification and silicification, processes that occurred in a conditioned, contemporary, or subsequent way in a precipitation-dissolution relationship, preceded the argilluviation process in the evolutionary framework of the Maastrichtian of the Basin. The argilluviation constitutes a process of superimposition over the calcification, favouring the formation of profiles with Btk, Bss, and Bt horizons. The argilluviation process indicates a high-water circulation rate in the profile, suggesting increased precipitation, desilication, leaching, and weathering. That process allows the advancement of pedoplasmation, the generation of textured coatings features, and the formation of grano-, mono- and porostriated b-fabrics at the micromorphological level. At a certain time, the calcification, silicification, and argilluviation coexisted, concomitantly or subsequently, during the variations of the semi-arid palaeoclimate that predominated in the Maastrichtian of the Bauru Basin.

Deciphering Permian wetland deposits of the Parnaíba Basin through an integrated study of lithofacies and palynofacies in Western Gondwana

During the Permian, the northwestern portion of the supercontinent Gondwana experienced significant climate changes, among which the increasing aridity in the Parnaíba Basin, located between Brazil's northern and northeastern regions. The marine influence associated with the Panthalassa Ocean, evident in the Silurian and Devonian deposits, was progressively attenuated in the late Carboniferous, culminating in extensive continental deposits during the Permian and the Early Triassic in this region. In this paper, we present the first palynofacies analysis of the Permian Pedra de Fogo Formation, conducted concurrently with the description and association of sedimentary facies from outcrops located in the state of Maranhão, southwest of the Parnaíba Basin. A total of 15 m of sedimentary succession was described, and 28 samples were selected for palynofacies analysis. As a result, we identified six sedimentary facies, which were grouped into two distinct facies associations: Fa1 – Central Lake and Fa2 – Playa Lake. The palynofacies analysis considered the sedimentary organic matter, classifying it into amorphous organic matter, phytoclasts (subdivided into eight subgroups), and palynomorphs (with six subgroups). Integrating the results enabled an understanding of the depositional context, which is closely related to water dynamics and climate seasonality. The shallow lake system interpreted during the wet phases recorded interruptions in the normal sedimentation of the central lake due to turbidite deposits associated with ephemeral rivers. Conversely, during dry phases, salt precipitation occurred with subsequent substitution and aerial exposure of the surface when the water balance was negative. Eodiagenetic events were inferred and are considered throughout the article. The alternation of the hydrological regime allowed the deposition in proximal, transitional, and distal environments, creating a distinctive mosaic of wetland landscapes. The data presented indicate humid conditions within a context of increasing aridity in the studied location, particularly in the upper part of the Pedra de Fogo Formation.

Stratigraphic-chemostratigraphic assessment of early dolomitization and associated phosphogenesis of the Ediacaran-Cambrian Salitre Formation, Irecê Basin, Brazil

During the Precambrian − Cambrian transition, the occurrence of phosphogenesis and the rise in atmospheric oxygen levels are closely linked to the proliferation of life. However, the mechanisms associated with these phenomena and their underlying causes are not well understood. The Irecê Basin represents the remnants of intracratonic marine environments that document the Neoproterozoic sea flooding of West Gondwana. This study presents an integrated analysis of sedimentary facies and chemostratigraphy through high-resolution sampling of pristine limestones and dolomites from the Salitre Formation in east-central Brazil. The aim is to explore the relationship between environmental settings and phosphatized stromatolites by combining detailed facies descriptions with isotopic and geochemical data. A total of fifteen lithofacies types are identified and grouped into seven major facies associations. These associations reveal a depositional stacking pattern defined by flooding surfaces and transgressive–regressive cycles, allowing for the division of the Salitre Formation into two depositional sequences. The basal sequence includes sedimentary phosphorite deposits intimately associated with widespread dolomitization levels at the top. Here, we describe a combination of two major processes for the dolomitization model of the Salitre Formation, with a contribution of evaporative environment settings related to a sabkha, and the influence of a meteoric mixing zone. Furthermore, three stages of phosphogenesis have been identified, with two of them interpreted as early diagenetic deposition associated with stromatolites and early cementation, and the last one associated with a late stage of phosphogenesis. The combined analysis of REY patterns, 87Sr/86Sr ratios, and geochemical results suggests a significant continental influence on both dolomitization and phosphogenesis. The identification of depositional settings and stratigraphic patterns for these deposits is crucial for the understanding of phosphogenesis during the Ediacaran, which may have been influenced by the continentalization of the marine environments forced by the intracratonic marine setting and the tectonic evolution at the end of the Neoproterozoic Era.

Re-evaluating Marine Isotope Stage 5a paleo-sea-level trends from across the Florida Keys reef tract

Unraveling how Global Mean Sea Level (GMSL) fluctuated during past warm periods can improve our understanding of linkages between sea-level fluctuations, orbital forcing, and ice-sheet dynamics. Current estimates of GMSL for Marine Isotope Stages (MIS) 5a and 5c — two warm intervals following the relatively well-documented MIS 5e — contain meters of uncertainty and fewer data due to several challenges. These challenges include concealment of datable in-situ coral facies by MIS 1 deposits and inaccessibility due to submergence by modern sea level. We present a comprehensive dataset based on U–Th dating and stratigraphic correlation of 23 cores totaling over 170 m of recovered coral-reef deposits across the tectonically stable Florida Keys Reef Tract (FKRT). Following detailed facies descriptions, 34 in-situ, minimally altered aragonitic coral samples (≤2.7% calcite) below the Holocene-Pleistocene boundary were targeted for U–Th geochronology. Fourteen closed-system coral U–Th ages from MIS 5a include the commonly used sea-level indicator Acropora palmata, but also the massive coral taxa Pseudodiploria strigosa, Siderastrea siderea, Orbicella spp., and Porites astreoides. Dating yielded ages in the range of 88–81 ka (average 2σ uncertainty of less than 200 years). These ages suggest MIS 5a reef initiation at ∼88 ka BP, a peak near 83 ka with minimum elevations between −6.0 ± 0.5 and −5.6 ± 0.5 m MSL (2σ uncertainty and subsidence-corrected), and reef termination and sea-level fall by ∼81 ka BP. Notably, the range of peak MIS 5a relative sea-level estimates of −6.5 to −5.1 m MSL are more than 2 m shallower (higher) than previous estimates of −11 to −9 m. Our higher resolution regional sea-level reconstruction across four subregions of the Florida Keys reef tract aligns with changes in July insolation at 65° N: a trend that most other records, such as deep-sea sediments, do not have the accuracy and precision to resolve. Three massive coral samples from MIS 5c, consisting of Pseudodiploria clivosa, and Orbicella spp., yielded ages in the range of 104 to 99 ka (average 2σ uncertainty less than 200 years); however, because only one sample met the closed-system criteria, our ability to estimate MIS 5c sea level is relatively limited. More empirical estimates of sea-level from the MIS 5a and MIS 5c intervals based on numerical dating of reliable local sea-level constraints are critical for GMSL calculations and relating changes in sea-level amplitude and timing to global ice volume modeling and glacio-isostatic effects, all of which can improve predictions of future sea-level changes in coastal regions.

Neogene isolated carbonate platform of the Rio Grande rise (southwest Atlantic ocean)

Isolated carbonate platform (ICP) deposits have been extensively studied due to their high sensitivity to sea-level variations and hydrocarbon reservoir potential. Oceanic seamounts/plateaus such as the Rio Grande Rise (RGR) off south-east Brazil, are significant sites for investigating drowned ICPs, as they record the existence of former oceanic volcanic islands. The Geological Survey of Brazil dredged hundreds of samples from the RGR summit, comprising a condensed section with phosphatization, ferromanganese (Fe–Mn) crusts, and associated carbonate rocks. Our study investigates the sedimentary facies of carbonate substrate rocks collected from the borders of the Cruzeiro do Sul Lineament of RGR. Eight facies were identified: Planktonic foraminifera mudstone (Pfm), Planktonic foraminifera wackstone (Pfw), Bioclastic wackstone (Bw), Red algae packstone (Rp), Red algae grainstone (Rg), Foraminifera grainstone (Fg), Red algae bindstone (Rb), and Bioclastic rudstone (Br). Based on facies descriptions and fossil index ages, a paleoenvironmental reconstruction was proposed, indicating the development of a carbonate isolated platform during tectonic quiescence in the Oligocene-Miocene associated with a volcanic island that emerged during the RGR exposure in the Eocene. These deposits likely represent a weakly protected carbonate platform featuring a lagoon associated with barrier coralgal reefs, algal shoals, and patch reefs. A semi-drowning phase during the Pliocene-Pleistocene, characterized by the strong influence of ocean currents and deposition of mixed planktonic and shallow carbonate particles, ensued until complete drowning in the Pleistocene-Recent, marked by pelagic/reworked sedimentation. Platform shrinking and cessation may correlate with the Miocene onset of modern carbonate platforms influenced by currents, leading to the present-day substrate composed of carbonate and Fe–Mn duricrust covered by dunes of winnowed pelagic particles. The RGR's former ICP resembles the south-west Atlantic islands/atolls.

Depositional model of the Holocene coquinas - Albardão platform, southern Brazil

For the past decade, giant deepwater oil discoveries in the pre-salt section of the Campos and Santos basins of Brazil, have brought significant attention to offshore exploration activities along the South Atlantic margins. The prolific Cretaceous coquina deposits in these basins are part of the pre-salt rock record and constitute an effective but complex and heterogeneous hydrocarbon reservoir difficult to predict and model. Parting from this context, an evaluation of the sedimentological, structural and taphonomic criteria for coquinas are essential to better understand and predict the facies distribution and depositional models of the pre-salt coquinas strata. Based on this premise, the present work aims to genetically interpret 133 mixed carbonate-siliciclastic bottom sediments of the Albardão shelf – a modern marine coquina analogue, using facies description, investigating the relationship with hydrodynamic forces, and accessing the influence of morphology and structural framework on their deposition. From these analyses, we recognized a hybrid facies, three modern carbonate facies in analogy to the carbonate rock classification and four siliciclastic facies. These eight facies were then grouped into three facies associations representing high, moderate, and low energy facies. The high energy facies association comprises rudstones (Rf) and grainstones (Gf) with highly fragmented bivalve shells and barnacles abundantly present in the beach system, above the fair-weather wave base limit (FWWB). These facies also occur offshore on bathymetric highs above the storm wave base limit (SWB) but display less reworking than the coastal high energy facies above the FWWB due to wave shoaling. The moderate energy facies association consists of hybrid sand (Hs), sand (S) and muddy sand (mS) occurring between the FWWB and SWB limits in the offshore transition zone with extensive winnowing action and low rate of reworking. The low energy facies association includes sandy mud (sM), mud (M) and micritic mud (Mc), characterized by the decantation of the fine sediments below the offshore SWB limit. The results confirm a bottom facies distribution controlled by depth, shelf profile morphology and energy from incident waves. The fragmented rudstone and fragmented grainstone facies are the best-recognized reservoirs with both having high porosity and high permeability.

High‐frequency palaeoenvironmental changes in the mixed siliciclastic–carbonate sedimentary system from a lower Permian restricted basin (West Gondwana, southern Brazil)

ABSTRACTGlobal climatic and palaeogeographical changes generated a siliciclastic–carbonate system with high organic matter accumulations in a shallow sea during the lower Permian in Western Gondwana. The 60 m thick mixed siliciclastic–carbonate succession (Irati Formation and the base of the Serra Alta Formation) from the Paraná Basin represents a singular record of the interplay between carbonate production and siliciclastic input, providing a window of opportunity to integrate large‐scale depositional architecture with facies and geochemical analyses. The detailed study of cores supported by outcrop columnar sections revealed a siliciclastic‐dominated, retrogradational to aggradational facies, and a carbonate‐dominated, mixed aggradational to progradational facies composing three depositional sequences that record an outer‐ramp and mid to upper‐ramp. An integrated approach based on the description of facies and microfacies, organic geochemistry and mineralogical composition, indicated high‐frequency palaeoenvironmental changes during the evolution of this restricted basin. Climate changes, resulting in humid and dry phases, produced ideal conditions for high organic matter production and dolomite formation. The high organic matter production (humid phases) in addition to the restricted condition was responsible for the anoxic bottom waters that were widespread due to the low angle and homoclinal platform. Nutrient‐rich freshwater inflows in the anoxic and hypersaline restricted basin created a density‐stratified water column causing low‐salinity surface and anoxic bottom water, which allowed planktonic life, typical of lakes, such Botryococcus braunii, to flourish in the photic zone and sulphur bacteria to populate below the chemocline. Microbial activity induced primary dolomite precipitation (dry phases) and widespread formation of synsedimentary dolostone. Freshwater inflow and marine incursions are reflected in the organic matter accumulation (kerogen types I and II, respectively), generating bituminous shale with high total organic carbon (5 to 27 wt.%).

Sedimentological and ichnological variations in fluvio‐tidal translating point bars, McMurray Formation, Alberta, Canada

ABSTRACTSedimentological and ichnological descriptions of fluvio‐tidal translating point bars are rare, and complex physico‐chemical processes make highly detailed but concise facies descriptions challenging. Herein, mesofacies are defined to describe and interpret three ancient translating point bars from the Lower Cretaceous McMurray Formation, Alberta, Canada. Twenty‐three mesofacies are defined, based on their recurring sedimentological and ichnological characteristics. These mesofacies form the building blocks of beds and bedsets that make up three depositional facies. Facies 1 reflects sand dune migration at the channel base, which grades into inclined heterolithic stratification of Facies 2 and 3. Facies 2 occurs in the centre and seaward portions of the translating point bars and records tide‐dominated deposition of sand and muddy sand during periods of reduced river discharge. Ichnological suites and bioturbation intensities in these beds reflect persistent but variable brackish‐water conditions, fluctuating deposition rates and the deposition of mud. Mud beds are derived from flows with high suspended‐sediment concentrations. Tidally derived mud beds are typically bioturbated with trace fossil suites indicative of slow deposition rates and brackish‐water conditions. Mud deposited during elevated river discharge is burrowed after the dewatering of the bed. Facies 3 occurs at the landward apex of the translating point bar and is marked by sand‐rich and mud‐rich dune deposits with abundant soft‐sediment deformation, indicative of elevated flow velocities and deposition rates. Bioturbation is rare and sporadically distributed owing to unstable substrates. The distribution of the facies reflect the hydrodynamic variations that occurred vertically and laterally across the bar in response to temporal variations in fluvial and tidal flow interaction, as recorded by their mesofacies. The detailed facies analysis strongly suggests that deposition of the three McMurray Formation translating point bars occurred in proximity to the turbidity maximum zone of a fluvio‐tidal channel system.

Hydrochemical Facies Description to Assess the Groundwater Quality in Southern Part of Haditha District, Iraq

The current study explains the hydrochemical processes controlling the variance in the hydrochemical facies for eighteen samples obtained from the study area. The groundwater samples were analyzed for the major ions "cations and anions" data in mg/l, total dissolved solids (TDS) in mg/l, electrical conductivity (EC) in μS/cm, pH unit less, and temperature in °C. Piper trilinear (three-line) diagram indicates the overall of samples belong to class 1 (Ca2+, Mg2+, Cl-, SO42-), category I (SO42- - Cl- and Ca2+ - Mg2+) and permanent total hardness (calcium chloride type). The contribution of cations in the study area was different percentages (dominant cation sodium (Na+)), while the sulfate (SO42-) is the dominant ion of the groundwater in the study area. The analytical values showed that overall the samples were brackish water and high enrichment salts within the impermissible limits of the World Health Organization standards. Irrigation parameters and water quality index were calculated for samples to assess water for agricultural and drinking uses for the inhabitants of the study area. The basic exchange is the exchange of sodium (Na+) and potassium (K+) ions in water with magnesium (Mg2+) and calcium (Ca2+) ions in materials which are exposed to weathering.

Archaeomagnetic analyses on fumiers burned under controlled experimental conditions

The improvement of the archaeomagnetic dating method requires compiling new and older data of the Earth's magnetic field (EMF) variations for the last millennia. Combustion events from fumier sequences have been proposed as good directional EMF recorders. However, they are subjected to diverse taphonomical processes and how these affect the archaeomagnetic record has not yet been studied. In order to evaluate it, here we report the first archaeomagnetic and rock-magnetic results on samples from experimentally recreated fumiers since 2014 under controlled conditions. A facies description with unprecedent resolution was used to study the variation of magnetic properties in depth. Rock-magnetic analyses indicate a homogenous magnetic mineralogy dominated by pseudo-single domain magnetite as main carrier in all facies, with not very high and similar contribution of the finest (superparamagnetic) grains. The low values of anisotropy of magnetic susceptibility (AMS) indicate that the studied sample set is mainly isotropic. The directional behaviour in well-preserved burned facies (here described as G, LM and DGB), are jointly characterized by highly reversible thermomagnetic curves, high Koenigberger (Qn) ratio values and intense, univectorial and normal polarity orthogonal NRM demagnetization diagrams. On the contrary, specimens affected by mechanical alteration processes are less magnetic and show anomalous directional behaviours. The high thermomagnetic reversibility of ashes indicates that they reached ca. 600–700 °C, in line with the thermocouples’ data. Temperatures of 460 °C were obtained for the DGB facies (subyacent black carbonaceous facies). Sampling of ashes located on the top of these combustion events should be avoided for archaeomagnetism. Being just beneath the last stabling episode they are the most prone to undergo mechanical alterations and do not preserve well the Earth's magnetic field direction. Despite their unlithified nature and the multiple taphonomic processes that fumier sequences may undergo, under certain quality criteria, they are valid geomagnetic field recorders providing both information of archaeological and geophysical interest.

Pre-salt carbonate cyclicity and depositional environment: NMR petrophysics and Markov cyclicity of lacustrine acoustic facies (Santos Basin, Brazil)

The Pre-salt strata are idealized as a lake-dominated breakup depositional sequence recorded between the South American and African continental margins to which there is no present-day analog. Most of the complexities of the Pre-salt reservoir rely on addressing carbonate cyclicities at different scales using limited cored intervals where well-log and seismic correlation is still unclear. In this case, the application of borehole image logs (BHI) and nuclear magnetic resonance (NMR) techniques allows not only the scale correlation and extensive data acquisition but also to establish a more robust relationship between acoustic facies and their petrophysical significance. For that, this work combines BHI and NMR data to propose a feasible scheme of depositional acoustic facies for the Barra Velha Formation in the Santos Basin. To verify the statistical significance of the acoustic facies’ vertical organization, this work applied the Markov chain analysis along a c. 500 m stratigraphic interval, deriving four carbonate cycles associated with shallowing and reworking lacustrine cyclicities. Abrupt changes in cyclicity patterns were adopted as stratigraphic markers to subdivide the succession into lowermost, intermediate, and uppermost intervals that record the depositional history of deep, shallow, and emergent lake environments, respectively. By comparing the thickness variation of the Markovian cycles (i.e., accommodation space) with the seismic onlap curve variation (i.e., lake level) throughout the Pre-salt interval, a larger-scale superimposed cycle was unraveled, suggesting that accommodation space of the studied Pre-salt paleolake was likely controlled by fluctuations in the lake level. Finally, this work aimed to explore the applicability of diagnostic criteria for the description and interpretation of depositional acoustic facies, their petrophysical significance, and cyclicity patterns in the Barra Velha Formation.

Facies and Hydrocarbon Reservoir Rock Characterization of the Paleozoic Rocks of Peshawar Basin, Northwest Pakistan

The present study details the facies description and hydrocarbon reservoir characterization of the Paleozoic rocks of Peshawar Basin, northwest Pakistan. The outcrop samples from the Cambrian-Devonian rocks along the famous Nowshera-Risalpur Road and Turlandi Village Section were investigated. The analysis of outcrop data revealed significant information regarding the facies and their depositional environments. Based on its detailed sedimentological data, it is believed that the Ambar Formation is deposited in shallow shelf-tidal flat settings, while the protolith of the Misri Banda Quartzite came from the wide beaches. The protolith of the Panjpir Formation was deposited in the shelf conditions, which shows an overall shallowing when moving up the section from the argillites and phyllites to crinoidal limestone. The Nowshera Formation was deposited as a reef complex on the shelf edge having reef core, reef breccia and a back-reef lagoon. The techniques of petrography, XRD and SEM were used for the bulk geochemical composition of the rocks focusing on their matrix, mineralogy, micro-porosity and pore-filling materials. The presence of micrite, goethite, kaolinite and illite as intergranular mass, dolomitization-induced porosity, twin cleavage plane and high dissolution porosity in the Ambar Formation can provide significant pore space to the reservoir fluids migration. Hematite with minor kaolinite, illite and chlorite as alteration products of unstable framework grains as intergranular mass and a deep seated burial diagenesis has minimized the reservoir potential in Misri Banda Quartzite. The coarse and fine calcite, dolomite, chlorite and kaolinite occur as pore-filling material between the framework grains while the high intensity of intragranular dissolution and micro intergranular connecting porosity in the Nowshera Formation makes it a promising reservoir.

Role of depositional environment on clay coat distribution in deeply buried turbidite sandstones: Insights from the Agat field, Norwegian North Sea

The preservation of reservoir quality in deeply buried sandstones largely depends on the presence of grain-coating clays that can inhibit quartz overgrowth and cementation. These grain-coating clays are the focus of today's hydrocarbon exploration as they enable new oil and gas discoveries in sedimentary basins. However, the origin and distribution of these clays in turbidites are poorly documented in scientific literature. Our study addresses this knowledge gap by investigating the mechanisms and parameters governing the clay coat development in turbidite sandstones, with a focus on the Agat Formation (Fm) in the Norwegian North Sea. Petrographic observations, scanning electron microscopy, and X-ray diffraction analyses were used to identify the origin of clay minerals and their distribution, as well as to understand the diagenetic evolution of the sandstone and its impact on reservoir quality. Based on the sedimentary facies description, the Agat Fm comprises four principal members deposited in various depositional environments, ranging from distal lobe fringe to amalgamated proximal lobes and weakly confined channels. Chlorite and kaolinite are the main authigenic clay minerals in these sandstones, associated with a variable amount of inherited detrital clays consisting in Fe-bearing kaolinite and mixed-layer minerals composed of illite and hydroxyl-interlayered clay minerals formed in acidic soil environments. Well-developed chlorite coats are associated with the highest reservoir quality (φ > 10%), while discontinuous chlorite coats are linked to extensive quartz cementation and poor reservoir quality (φ < 5%). Samples with abundant pore-filling chlorite also exhibit poor reservoir quality (φ < 7%). Proximal lobe deposits and weakly confined channels show well-developed chlorite coats, whereas discontinuous chlorite coats are mainly found in proximal lobe deposits associated with high velocity escaping fluids during dewatering. Pore-filling chlorites are typically present in distal lobe fringes and levee deposits. Chlorite in the Agat Fm is mainly derived from the replacement of inherited detrital clays via an intermediate berthierine precursor forming during early diagenesis. The dissolution of Fe-rich grains such as biotite and ferric detrital clays under reducing conditions governed the kinetics of berthierine/chlorite growth. The late diagenesis is characterized by extensive feldspar dissolution in a closed diagenetic system, leading to the precipitation of kaolinite and quartz cement. The presence of chlorite coatings around detrital grains preserved the porosity from extensive quartz cementation during deep burial, highlighting the role of chlorite in maintaining the quality of deeply buried turbidite reservoirs.

Biostratigraphy and facies description of Middle Triassic rift-related volcano-sedimentary successions at the junction of the Southern Alps and the Dinarides (NW Croatia)

Biostratigraphy and facies description of Middle Triassic rift-related volcano-sedimentary successions at the junction of the Southern Alps and the Dinarides (NW Croatia)

A terminal Messinian flooding of the Mediterranean evidenced by contouritic deposits on Sicily

ABSTRACTThe evolution of marine gateways and sea straits exerts major control on bottom current depositional systems. A well‐known interval in geological history characterized by frequent changes in marine connectivity is the Messinian Salinity Crisis (5.97 to 5.33 Ma) when the Mediterranean allegedly experienced major (&gt;1 km) sea‐level drawdown followed by a catastrophic marine replenishment at the base of the Zanclean. Controversy exists around the timing and mode of this event as unambiguous flood deposits have so far never been drilled or recognized in outcrops. In the Sicilian Caltanissetta Basin (Italy), the Messinian/Zanclean boundary is directly underlain by the Arenazzolo Formation. This 5 to 7 m thick sandy sedimentary interval may reveal a genetic link with the abrupt refilling of the Mediterranean, but at present a detailed study to understand its origin is lacking. In this work, the Arenazzolo Formation at Eraclea Minoa has been studied by a multi‐method approach, employing detailed facies description, grain‐size analyses, petrographic analyses and palaeocurrent analyses. Palaeogeographical reconstructions and facies associations show that the Arenazzolo Formation sands were deposited on the northern flank of the Gela thrust front by persistent bottom currents, flowing parallel to the regional slope physiography, during a transgression. It is hypothesized that these currents are associated with the active circulation of surface and intermediate water masses coeval with a terminal Messinian flood, when basin margins overtopped and a reconnection between western and eastern Mediterranean was created. The Arenazzolo Formation is a unique example of a contouritic deposit formed by bottom currents that establish during the reconnection of major isolated water bodies.

Sedimentary processes and paleoenvironment reconstruction of the Barra Velha formation, Santos Basin, Brazilian pre-salt

The Barra Velha carbonates are widely known for their complexity regarding facies, diagenetic features, and depositional history. The absence of recent analogues representing the depositional setting of these rock formations leaves many uncertainties concerning the sedimentary process that generated them. The present work aims to propose a model for the depositional environment of the Barra Velha Formation based on sedimentary features identified through facies description and interpretation. The employed data consists of 645 m of core descriptions, 2300photomicrographs, 1201 plug samples, and 44 thin sectionsdistributed throughout 34 wells in two Santos Basin oil fields. Fourteen facies were identified and classified into four categories: in situ, microbial, reworked, and altered. The in situ facies are composed of spherulites, shrubs, and mud. Sedimentological evidence points out that spherulites and shrubs precipitated within layers of mud, as a post depositional process. Microbial facies have several characteristics that refer to stromatolites and occur laterally and vertically associated with in situ facies. The spatial association between them indicates that in situ and microbial facies formed in the lower energy portions of the lake. Reworked facies were formed by grain fragmentation and transportation, indicating an energetic lake-littoral zone, where grainstones and packstones formed in the shallowest portions, and wackestones formed in the less energetic portion of the littoral zone. The altered category comprises the weathering profiles. These profiles resemble soils and developed in response to periods of strong evaporation, when lake brines would become dense enough to infiltrate into the groundwaters, leading to the precipitation of the dolocretes in the mixing zone. The spatial distribution of the facies corroborates the sedimentological interpretations. In situ and microbial facies are frequently found in the structurally lower regions of the study area. In contrast, reworked facies and weathering profiles are more common on structural highs.

Depositional framework and reservoir characteristics of microbial carbonates in the fourth member of the middle Triassic Leikoupo Formation, western Sichuan Basin, South China

Microbial carbonate reservoirs are the main petroleum exploration targets in the upper submember of the fourth member of the Middle Triassic Leikoupo Formation in the western Sichuan Basin. To improve understanding of the distribution and reservoir characteristics of microbial carbonates, detailed facies descriptions and reservoir characterizations were performed under the framework of cyclostratigraphy. Ten facies can be identified and grouped into four facies associations in two transgressive-regressive cycles, within an epeiric marine carbonate platform in the western Sichuan Basin. In Cycle 1, Emei taphrogeny caused the formation of the Longmenshan subaqueous paleo-uplift, which formed a relatively restricted tidal flat environment and developed large-scale microbialites on its eastern side. In the central area, wavy to domal stromatolitic dolowackestone (F3) and thrombolitic dolomudstone (F4) are developed in the intertidal and subtidal of the central area with steep substrate gradients and restricted seawater circulation. Laminated microbial dolowackestone (F2) is mainly developed in the intertidal of the southern area with low substrate gradients and restricted seawater circulation. Intraclast dolograinstone (F6) are widely developed in the subtidal shoals of the northern area, which is frequently disturbed by tides and waves with better seawater circulation. In Cycle 2, increasing relative sea level weakened the barrier of Longmenshan subaqueous uplifts to waves, the restricted subtidal environment developed the intraclast dolograinstone (F6) and intraclast packstone-grainstone (F7), occupying the platform with better seawater circulation and higher energy. The framework or inter-clot pores of F2, F3 and F4, interparticle pores of F6, and vugs resulting from syn-depositional dissolution are the dominant pore types. High-quality reservoirs are mainly distributed in the regressive microbialites (F2–F4) of Cycle 1 in the central area and intraclast dolograinstone (F6) of Cycle 1 and Cycle 2 in the northern area. The porosity of microbialites and intraclast dolograinstone reservoirs mainly ranges from 2.26% to 10.00%, with a mean value of 4.96%. The permeability mainly ranges from 0.01mD to 13.98mD, with a mean value of 4.63mD. This study proposed a depositional framework of microbialites and evaluated its control on reservoir quality, which is of great significance for the further study of microbial carbonate reservoirs.

Impact of dataset size and convolutional neural network architecture on transfer learning for carbonate rock classification

Modern geological practices, in both industry and academia, rely largely on a legacy of observational data at a range of scales. However, widespread ambiguities in the petrographic description of rock facies reduce the reliability of descriptive data. Previous studies have demonstrated a great potential for the use of convolutional neural networks (CNNs) in the classification of facies from digital images; however, it remains to be determined which of the available CNN architectures performs best for a geological classification task. We evaluate the ability of top-performing CNNs to classify carbonate core images using transfer learning, systematically developing a performance comparison between these architectures on a complex geological dataset. Three datasets with orders of magnitude difference in data quantity (7000–104,000 samples) were created that contain images across seven classes from the modified Dunham Classification for carbonate rocks. Following training of nine different CNNs of four architectures on these datasets, we find the Inception-v3 architecture to be most suited to this classification task, achieving 92% accuracy when trained on the larger dataset. Furthermore, we show that even when using transfer learning the size of the dataset plays a key role in the performance of the models, with those trained on the smaller datasets showing a strong tendency to overfit. This has direct implications for the application of deep learning in geosciences as many papers currently published use very small datasets of less than 5000 samples. Application of the framework developed in this research could aid the future of deep learning based carbonate classification, with further potential to be easily modified to suit the classification of cores originating from different formations and lithologies.

Evolution of submarine channel morphology in intra-slope mini-basins: 3D-seismic interpretation from offshore Niger delta

The morphological characteristics of a submarine channel system, within buried intra-slope mini-basins in the offshore Niger Delta have been investigated on 3D seismic data. The studied submarine channel named Amaku Channel Levee System (ACLS) initiated after avulsion of a feeder channel called Amaku Major System (AMS). The ACLS runs across three folds (A, B, and C), related to mobile shale deformation, being underlain by mass transport complexes (MTCs), lobes, and levees. From three defined channel reaches (upper, middle, and lower reaches), we have used seismic facies description, channel architecture characterization and quantitative analyses to decipher the spatial and temporal evolution of the channel levee system. Our results reveal that the western channel margin was controlled by structural barriers associated with mobile shale deformation, while the eastern channel margin was influenced by depositional barriers linked to levees and mass transport complexes (MTCs). We argue that the depositional barriers are in some cases more efficient than the structural barriers in controlling channel pathways, since pre-channel sediments confine the channel elements and divert them against positive reliefs. Seismic observations show that channels migrate from folds to structural lows causing flow stripping processes. Downstream variations of morphological parameters (such as sinuosity, width, and levee width), indicate that pre-existing deposits prevented these basin-ward migrations, driving straightening and narrowing of the channel system. This study reveals that submarine channels can undergo dramatic morphological and stratigraphic changes in structurally complex areas because of the configuration of sidewall reliefs, which is controlled, in turn, by the interplay between structural deformation and pre-channel deposition.

Digital Outcrop Models applied to high-resolution stratigraphy in braided fluvial systems: A study case at Jaicós formation in Parnaíba Basin, NE Brazil

The visualization, analysis and interpretation of sedimentary data can be optimized with techniques such as the Digital Outcrop Models (DOMs) generated with the Structure from Motion – Multi-View Stereo (SfM-MVS) technique. In this paper, to demonstrate the usage of DOMs in the characterization of the depositional architecture of a braided fluvial system, we developed a case study in Jaicós Formation, Parnaíba Basin, NE Brazil, where a high-resolution stratigraphic analysis of a braided fluvial system was performed, and the potentials and limitations of the method were evaluated. This multi-scale reservoir analysis allowed the identification, description and interpretation of facies, bounding surfaces and architectural elements. In addition, it was also possible to predict quantitative attributes of the fluvial system that deposited this sequence and evaluate the potential of this formation as a reservoir.