This study presents a detailed investigation into the microfacies, geochemistry, and depositional environments of carbonate rocks from the southern Benue Trough and eastern Dahomey Basin. This ...analysis involved a combination of techniques, including X-ray diffraction, X-ray fluorescence, scanning electron microscopy with energy-dispersive X-ray spectroscopy and thin-section petrographic examination. Samples from the Igumale Formation of the southern Benue Trough and samples from Ewekoro Formation of the eastern Dahomey Basin were analyzed, offering insights into their mineral compositions, elemental distributions, and depositional environments. In each basin, two carbonate microfacies were identified. The two microfacies found in the Igumale Formation were recognized as micritic bioclastic packstone and sandy bioclastic wackestone. In the Ewekoro Formation, the two microfacies recognized were bioclastic packstone and sparitic peloidal wackestone. The X-ray diffraction analysis of samples from both basins revealed a dominance of calcite. X-ray fluorescence analysis showcased calcium oxide and calcium as the predominant oxide and elements in both basins with varying concentrations. Scanning electron microscopy-energy-dispersive X-ray spectroscopy results unveiled the elemental composition with calcium and silicon emerging as major constituents. The photomicrographs suggest distinct depositional conditions for both samples with those from the Igumale Formation showing evidence of shallow marine and that of the Ewekoro Formation suggesting calm marine environments comprised of fined grained carbonate rocks containing fossils. This study has provided detailed understanding on the nature, microfacies types, mineralogical composition, depositional environment and diagenetic processes of Cretaceous-Paleocene carbonate rocks within the studied basins.
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•Carbonate microfacies characterization was performed in selected sedimentary basins in Nigeria.•Integration of carbonate microfacies and a sedimentological approach was used to establish paleodepositional conditions.•The use of petrographic study was used to investigate sedimentology, microfacies association and the paleoenvironment of the carbonate facies.•The use of geochemical index was used to reveal the original sources of the carbonate sediments in the basin.•Integration of SEM and XRD data for understanding the carbonate mineral formation processes, crystal growth and its morphology.•Integration of sedimentological, petrological and geochemical data is key in highlighting poor to fair reservoirs, bioclastic packestone to sandy bioclastic wackestone deposited in shallow marine carbonate environment.
Carbonate minerals have precipitated from seawater for at least the last 3.8 billion years, but changes in the physical and chemical properties of carbonate rocks demonstrate that the nature, loci, ...and causes of this precipitation have varied markedly through time. Biomineralization is perhaps the most obvious time-bounded driver. However, other changes in the sedimentological character of Archean and Proterozoic carbonates offer an under-considered record of fluctuations in Earth’s chemical, biological, and physical systems before and during the rise of animal life. Geobiologists and geochemists have successfully used large geochemical datasets to track changes in Earth’s surface chemistry through time, but inconsistencies between proxies make the recognition of clear spatial-temporal patterns challenging. In order to place new constraints on the environmental history of Archean and Proterozoic oceans, especially with regard to redox state, we present for the first time a high-resolution database of global Precambrian and Cambrian carbonate sedimentation.
Our initial datasbase, named Microstrat, comprises carbonate-bearing successions dating from c. 3.4 billion to ~500 million years old; these include more than 130 carbonate-bearing formations, digitized at the highest stratigraphic resolution possible and classified by environment of deposition. Lithofacies details are recorded at the finest scale available, including a range of microbial and other depositional fabrics, mineralogy, environment of deposition, age and location. We interrogate the Archean, Proterozoic, and Cambrian carbonate records, testing both long-standing and novel hypotheses about changes in Earth’s carbonate system through time. Changes in carbonate sedimentology illuminate global changes in bioturbation and microbial community structure; the depth-dependent timing of marine oxygenation and the transition from anaerobic to aerobic respiration at and within the seafloor; and temporal changes in patterns of dolomitization. Shifts in both the loci of carbonate precipitation and in markers of animal activity are consistent with time- and depth-dependent trends in ocean oxygenation. We also note changes through time in the morphologies of stromatolites and thrombolites, which we suggest reflect secular shifts in carbonate precipitation, clastic sediment transport, and eukaryotic evolution. Observations from the database and petrographic studies confirm that the nature and extent of Proterozoic dolomite contrasts with that of dolomite in the Phanerozoic, consistent with dolomite being an early or even, in some cases, primary precipitate prior to the Cambrian Period. Because this database preserves environmental context and the stratigraphic relationships of carbonate features rather than tracking the occurrence of single features through time, it permits new and detailed analyses of the physical sedimentology of Precambrian carbonates and correlations in time and space among different features.
We also consider how to build upon this initial dataset. Capturing and analyzing sedimentological data is difficult. Analysis becomes more challenging when data have been collected by multiple researchers working across Earth’s entire history and surface. Field observations have a great deal of power to describe and explain changes in Earth’s surface environments, yet they are often reported in idiosyncratic ways that make it difficult to compare them, obscuring their true explanatory power. Microstrat is an effort to pool idiosyncratic sedimentological data in hopes of illuminating Archaean, Proterozoic, and Cambrian carbonate depositional environments, yet it is unavoidably limited. We describe how other researchers can help to build Microstrat into a community-curated database by reporting and sharing data.
The Capiru Formation records supracrustal metassedimentary rocks within the Southern Ribeira Belt. These rocks underwent heterogeneous deformation and metamorphism, resulting in an incomplete ...greenschist facies paragenesis. They are tectonically interbedded with strata that exhibit well-preserved primary structures. In the Morro Grande synform region, there is a continuous 150 m outcrop of metadolomites with preserved sedimentary records and was described in a stratigraphic profile (1:500) subsequently detailed (1:50). Dolomite, minor amounts of quartz, traces of ilite, graphite and zircon constitute the mineral assembly, defined by XRD, petrography and SEM-EDS. Distinct preserved facies are recognized by morphostructures: (1) lamina (parallel, discontinuous, crenulated); (2) stromatolites (homogeneous columnar, club-shaped, conical conophyton-like, pseudocolumnar, parallel-branching, divergent-branching, delicate-branching); (3) thrombolite (rugous) and (4) flat pebble conglomerate. Single-isotope data revials different microbial patterns, particularly in stromatolites (−1.57 to −0.40‰ δ13C; −8.21 to - 3.94‰ δ18O) and lamina (−1.89 to 1.29‰ δ13C; −7.32 to −3.55‰ δ18O). Facies associations suggest a shallow sea with a regressive trend. Tectonofacies are characterized by massive, venulated or brecciated carbonates. Chemostratigraphic profile is divided into two major intervals. At the bottom, the first does not exhibit a specific isotopic trend and can be subdivided in heterogeneous post-deformation venulas (Unit I, −1.75 to 0.40 δ13C; −2.34 to −8.16 δ18O), thrombolythic signals (Unit II, −1.43 to 0.40‰ δ13C; −6.35 to −4.11‰ δ18O), and facies variation, mainly supratidal (Unit III, 1.89 to 1.29‰ δ13C, −7.32 to −3.31‰ δ18O). Upwads in the profile, the second interval consists of microbial facies, displaying a more uniform signature with depleted values of δ13C and δ18O along with a slight enrichment marking the transition from intratidal to supratidal environments (Unit IV; −1.32 to −0.81‰ δ13C; - 8.17 to −6.25‰ δ18O) and supratidal lagoons (Unit V; −1.35 to −0.41‰ δ13C; −8.20 to −5.49‰ δ18O). Preserved EPS, graphite, and ilite confirm low-grade metamorphism, with clumped isotope thermometry temperatures ranging from 206.07 to 307.58 °C. Calcite is secondary, filling porosity and present in recrystallized facies. Isotopic signature of recrystallized facies differs from microbial ones, with the most depleted values (−2.16‰ δ13C; −14.02‰ δ18O) and lower formation temperature (122.29 ± 7.07 °C), indicating a late restricted event. This dolomitic succestion within Capiru Formation provides evidence of a shallow marine paleoenvironment with diverse microbial activity during its deposition.
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•Overview of a poorly studied Capiru Formation carbonate sequence.•Carbonate facies and associations of a 150 m preserved neoproterozoic microbial sequence.•Morphological, isotopic and chemical characterization of preserved facies.•Paleoenvironmental depositional model proposal for Rio Branco unit.
In the Upper Cretaceous Balcones Igneous Province in South and Central Texas, numerous small volcanoes developed during Austin Chalk and Anacacho deposition that are similar to the recent Surtsey ...volcano offshore of Iceland. The volcanoes formed topographic highs on the deep-water (500–900 ft 150–275 m), earlier-drowned Comanche Platform. The Elaine field in South Texas is associated with one of these relict volcanic highs and integration of a 3D-seismic dataset and cores permitted a detailed analysis of the deeper-water carbonate depositional systems of the field and associated carbonate reservoir development. This reservoir description extends the understanding of petroleum systems associated with relict, open-marine, deep-water, volcanic highs. In the shallow water at the top of the volcanic topographic highs, a carbonate factory developed that produced coarser-grained carbonate sediments that sourced the deeper-water flanks and toe-of-slope of the mound. Finer-grained carbonate chalk was deposited in the open-marine, deep-water environment adjacent to the volcanic mound. The coarser-grained carbonate sediments sourced from the mound top were transported downslope by gravity-flow processes that formed debrites and hyperconcentrated density-flow deposits. The gravity-flow deposits are composed of a mixture of shallow- and open-marine, deep-water biota indicating they are not shallow-water deposits, but are deeper-water mound-flank and toe-of-slope deposits.
In the Anacacho Elaine field, the coarser-grained carbonates (i.e., skeletal packstones and grainstones) associated with the volcanic mound are the oil-producing reservoirs. The Elaine volcanic mound itself is tight and not productive. The Anacacho chalk immediately above the mound are also tight and form the top seal for the Elaine field.
Associated with the relict volcanic mounds in the Balcones Igneous Province are a spectrum of carbonate plays. The relict volcano can be a reservoir, but not all the relict volcanic mounds are porous. The coarser-grain carbonates on the top, flanks, and at the base of the volcanic mounds are reservoirs in the area of the Balcones Igneous Province. The chalks deposited penecontemporaneously on the open-marine, deep-water platform can also be tight carbonate reservoirs.
•Integration of 3D-seismic data and cores to define volcanic mound paleogeomorphology•Carbonate lithofacies associated with volcanic mounds.•Recognition of depositional environments associated with volcanic mounds.•Hydrocarbon reservoirs associated with volcanic mounds in open-marine settings.
The Cenomanian-Turonian Mishrif Formation at the H field in southeastern Iraq displays a wide range of rock fabrics that result in highly variable reservoir quality and flow properties. Reservoir ...quality in this field is controlled by the complex interplay of depositional and diagenetic factors. Depositionally, seven main productive facies are recognized, which are organized into six sequences; reservoir quality occurs primarily in the grain dominated high energy shoal, shallow open marine, and rudist biostrome facies. A major change in stratigraphic style occurs midway through the reservoir, with a lower portion that is dominated by laterally continuous upward shoaling cycles (that are often shoal or tidal flat capped) and an upper part that is more heterogeneous and dominated by lagoonal and back-shoal facies. While diagenesis has overprinted and altered this initial framework, karsting - resulting in both enhanced reservoir quality due to leaching and reduced reservoir quality due to cementation, brecciation and fines infiltration - associated with sequence boundaries within and at the top of the Mishrif Formation represents the major flow-significant diagenetic effect in the reservoir. Here, diagenesis has followed the stratigraphic pattern established at the time of deposition, and reinforced its impact on reservoir quality. Mishrif reservoir quality is ultimately controlled by sea-level history, in that high energy, grain dominated textures and facies associated with sea-level highstands have the best reservoir quality, while meteoric diagenesis has produced low reservoir quality (and reservoir boundaries) at karsted exposure surfaces that formed during sea-level falls. Recognition that reservoir quality, both good and bad, results from congruent depositional and diagenetic effects ultimately driven by the sea-level history of the succession provides an important key to understanding and predicting reservoir quality in the H field and in other carbonate fields with similar depositional and diagenetic histories.
•The Mishrif Formation at the H field in Iraq comprises six major sequences.•Sequence boundaries show significant evidence of subaerial exposure and karsting.•Stratigraphically, the lower Mishrif is dramatically different from the upper.•Reservoir quality is driven by the interplay of depositional and diagenetic factors.
The sedimentary and stratigraphic features of the low-gradient slope carbonate palustrine system of the Fortanete Formation (northeast Spain) during the early Campanian have been studied. The ...succession of the Fortanete Fm provides depositional record of a progressive transition to continental settings in an area that was predominantly marine for most of the Late Cretaceous. Palustrine systems can be developed in a wide variety of environments, however, modern analogues are scarce. The Fortanete Fm thus presents a great opportunity to study the stratigraphy, the sedimentary features and isotopic composition of a palustrine–marine influenced system. A total of 10 stratigraphic sections distributed along a 40 km wide and 80 km long area have been carried out in order to decipher the facies distribution in the system. This study led to the differentiation of three palaeoenvironments based on predominant lithofacies and sedimentary structures: (1) marshes and shallow low-gradient slope lakes; (2) ephemeral lakes, lake margins and prairies with variable water level and marine influence; and (3) mudflats with evaporites and fine siliciclastic input. The Fortanete Fm serves as an excellent case study for palustrine environments with the presence of typical palustrine features such as pseudomicrokarst, brecciation, black pebbles, desiccation cracks, nodules, Microcodium, and fenestral fabric. Most palustrine systems are associated with continental freshwater settings, whereas the Fortanete Fm displays evidence of intermittent marine influence in the distal studied sections. Carbon and oxygen isotope composition is within the range of typical freshwater lake carbonates and also corresponds to isotope composition of palustrine carbonates with marine influence. The Fortanete Fm shares a number of characteristics with freshwater continental and freshwater modern analogues. Comparison with these systems provides new insights on how modern analogues can be used to decipher the complexity of the processes developing these systems in the past.
•Fortanete Fm deposited in a low-gradient system with marshes and ephemeral lakes•Strong pedogenic features that were capable of developing pseudomicrokarst•Ancient analogue of the Florida Everglades due to the marine influence evidence•Supports palustrine carbonates being complex mosaics of fresh-water environments
The Santos Basin mesophotic zone comprises one area of 104,915 km2, occupying 78% of the entire basin. Santos Basin is one of the most under stress and sensitive marine regions in the Brazilian ...territory due to several major ports, Oil & Gas industry, and coastal marine environmental management areas. Scientific literature recognizes that rhodolith beds and biogenic rigid-bottom patches occur on the shelf, but because lack of high-resolution seismic data, these seabed features are not yet wholly mapped at Santos Basin. However, an unprecedented gathering of historical depth sounding points and surface sediment data collection, in conjunction with an extensive field campaign of seabed sampling and sub-bottom shallow seismic records, points out new evidence of biogenic sediment distribution reefal structures in the Santos Basin. The patches of calcareous algae crusts, terraces mounds, pinnacles, and free-living rhodolith beds compose the carbonate facies on the outer shelf. Rugged bottom relief and pinnacles up to 10 m in height correlate with carbonate gravels and sandy areas. Bioconstructions are more common northward and gradually decrease southward. Geomorphological features such as rugged bottom relief might indicate the occurrence of mesophotic bioconstructions off Paraná, and Santa Catarina State coast. The mesophotic bioconstructions have evolved through four stages: 1- LGM fully sposed shelf, 2- sea-level transgression and bioconstrutions pioneers, 3- influence of the La Plata plume at the southern sector, and 4- establishment of mud patches in the northern sector due to local input.
The study presents a detailed record of foraminiferal faunas from CK2 core, a 928.75 m long core drilled in the center of Xisha Islands, South China Sea, for biostratigraphy and palaeoenvironmental ...reconstruction of the study area. Coupling foraminiferal biostratigraphy and bulk-sediment strontium-isotope chronologic age revealed that carbonate deposition occurred from Early Miocene (~19.6 Ma) to present. Examination of 281 petrologic thin sections and 246 unconsolidated sediment samples revealed 141 foraminiferal taxa, among which the foraminiferal abundance and simple diversity peaked in the Middle Miocene and Pliocene to early Pleistocene strata. Nine foraminiferal assemblages (FAs) were distinguished, documenting eight distinct facies that indicated five intervals in the development of Xisha Islands. At the initial interval (19.6–17 Ma), a coral, coralline algal and shallow-water larger foraminifers (FA1 and FA2) association colonized the shallow platform facies, forming an aggrading sequence in response to tectonic subsidence and sea-level rise. Platform development slowed from 17 to 10.19 Ma, dominated by the miliolids and small benthic foraminifers as well as larger benthic foraminifers (FA3 and FA4) representing lagoonal to reef-flat facies with agradation to progradation (backreef infilling) associated with sea-level fall. At 10.19–4.35 Ma, a deepening sequence was indicated by the change from shallow-water benthic foraminifers (FA5) upward to deeper dwelling species (FA6), as a result of climatic cooling and abrupt sea-level fluctuation. As sea level rose, the platform was drowned from 4.35 to 2.18 Ma, as indicated by abundant planktic foraminifers and large, flat benthic foraminifers (FA7 and FA8). At 2.18–0.089 Ma, the platform aggraded rapidly with sea level fall indicated by the appearance of thick benthic foraminifers (FA9) and increased coral. The close association between the biofacies and regional paleobathymetry, which was in phase with the long-term global sea-level change, suggest that foraminiferal morphology and assemblage composition are ideal parameters for paleobathymetric and paleo-depositional environmental interpretations.
•Nine foraminiferal assemblages were distinguished, documenting eight carbonate biofacies.•Two major transgressive-regressive palaeobathymetry cycles were recorded since about 19.6 Ma.•Five development phases of Xisha carbonate succession since Early Miocene were revealed.•Eustasy, tectonics, paleoclimatic and paleooceanographic change are the main factors affecting the carbonate production.
The Upper Jurassic Jubaila Formation in central Saudi Arabia exhibits laterally coeval environments from the southwest to the northeast. These coeval environments involve inner, mid-, and outer ...ramps. The inner ramp occurs at the Al Haddar section in the extreme southwest and consists of mixed clastic-carbonate facies and are represented by intercalations of ferruginous calcareous quartz arenite, ferruginous sublitharenite, and burrowed dolomicrite. In the mid-ramp setting, there were changes from lateral facies in the Riyadh and the Wadi Huraymila areas to carbonates characterized by sandy lime-mudstones, dolosparie, peloidal olitic packstones, and coralline bioclastic wackestones to packstones. These are intercalated with sandy lime-mudstones that might be deposited in highly agitated environments. The outer-ramp setting occurred in the farther northeast Al Uyaynah area. It is characterized by reduced thickness and is composed of lime-mudstones, dolomitic marls, and pelletal bioclastic wackestones with thin beds of dolomitic marls. The lateral and vertical microfacies distribution in the Jubaila Formation are arranged vertically into four 3rd-order cycles that are correlate laterally overall the studied area, which are overprinted by oscillations in eustatic sea-level during the deposition of Jubaila Formation.
•The Jubaila Formation in Saudi Arabia exhibits laterally coeval environments from the southwest to the northeast.•These coeval environments involve inner, mid-, and outer ramps.•Four 3rd-order cycles are overprinted by oscillations in eustatic sea-level.
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