Depositional Palaeoenvironment Research Articles

Mineralogical, Petrographical and Geochemical Properties of the Late Oligocene Coal Seam (Seam-VI): Insights into Elemental Enrichments and Palaeodepositional Environment (İbrice field, Thrace Basin)

The Thrace Basin hosts several mineable coal seams (upwards numbered I to VII) within the coal-bearing Late Oligocene-age Danişmen Formation. For the first time, the properties of the Seam-VI in the İbrice (Malkara) area have been investigated in order to evaluate elemental enrichments and palaeoenvironmental conditions. The xylite-rich and mineral-rich lithotypes were identified from the studied coal samples representing the whole-coal thickness. The coal samples, on a dry basis, display variable ash yields (13.2–63.0%), total C (21.2–57.8%), and total S (0.9–4.1%) contents. The minerals identified using XRD-whole rock analysis include clay minerals, quartz, pyrite, calcite, dolomite, and feldspars. Siderite is only identified in a single sample. Aragonite is detected in two samples, which contain fossil shell remains. The SEM observations agree with XRD data, and apatite, barite, biotite, pentlandite, sphalerite, Ti-oxide, and zircon were also identified as accessory phases. Huminite is the predominant maceral group in the studied samples, while liptinite and inertinite group macerals display variable proportions. The mean %Rr values of ulminite macerals are around 0.40-0.41±0.01-0.02%, indicating relatively low-rank coal. This study implies that the precursor peat-mires of the Seam-VI were mainly developed under forested mire conditions, where woody peat-forming plants were abundant, and occasionally the contributions of herbaceous plants were important. The coexistence of syngenetic pyrite and carbonate minerals (e.g., siderite and carbonate mineral bands) could be related to the development of weakly acidic to neutral conditions within palaeomires. Furthermore, the predominance of clay minerals and the presence of detrital accessory minerals (e.g., apatite, pentlandite, and Ti-oxides) can also be indicators of clastic influx into palaeomires. Although aluminosilicate minerals (e.g., clay minerals) are abundant in dominant phases, SEM-EDX data shows that accessory minerals mainly control the elemental enrichments in the Seam-VI. The Mn enrichments in the samples seem to be controlled by Mn-bearing siderite micronodules. Considering the presence of sulphide mineralization and metallic ore deposits in the Strandja and Rhodope massifs, enrichments of Cr, Co, Ni, Ge, and Mo are related to As-bearing pyrite grains and accessory sphalerite and pentlandite grains within clay mineral aggregates in the samples. In addition, the Sr-bearing barite overgrowths around feldspar grains and syngenetic carbonate mineral bands could also cause Sr enrichments. Overall, clastic influx ratios into palaeomires and pH conditions during peat accumulation controlled the elemental enrichments in the Seam-VI.

Depositional palaeoenvironment of the Middle Jurassic (Aalenian) ooidal ironstones in Labino-Malkin zone (north-western Caucasus)

This paper investigates the conditions of formation of the Middle Jurassic iron-bearing strata, in the Labino-Malkin zone in the North-West Caucasus. The mineral composition and geochemical features of the rocks indicate variable conditions of rock formation. The predominance of clay and carbonate minerals in the sedimentary sequence, as well as Ce-anomaly values ranging from 0.9 to 1.4 indicate mainly suboxygenic conditions of sedimentation. Three intervals of anoxia characterised by U, Mo and V enrichment, the presence of siderite and a large amount of pyrite macroframboids, and the formation of phosphorites were identified in the studied section. Oxygen depletion of seawater is the result of upwelling. Formation of oxide minerals was promoted by oxygen saturation of the environment due to sea transgression or meteoric water inflow, which is confirmed by isotope analysis indicators (δ18Ocarb from −11.6‰ to −1.2‰).Eight chemical and two clastic lithofacies were identified based on the mineralogical, geochemical and textural-structural features of the studied strata. Depending on the material input into the basin, different rock types were formed. The constant value of detrital elements Ti and Al indicates a constant input of terrigenous material from the continent. The abundant input of terrigenous material favoured the formation of sandstones and clay minerals. Decomposition of organic matter favoured the formation of limestones and phosphorites due to the input of phosphate- and carbonate-ions into the bottom layers. The presence of iron in seawater favoured the deposition of ironstones. Depending on the level of oxygen and carbon in the bottom environment of the basin, ironstones with siderite, clay or clay-carbonate cement were formed. Calcite and barite contain fluid inclusions of hydrocarbons and hydrogen sulphide gas, indicating the influence of submarine fluids on rock formation. Although the first explorers of the area assumed that the sources of metals come from the continent, the detection of fluid inclusions in the calcite cement, increased content of hydrothermal elements, and the presence of rare minerals such as galena, sphalerite, arsenopyrite, argentotetrahedrite, nugget silver, and coronadite, allows to consider hidden fluid venting as one of the sources of metals for the ooidal ironstones.

Benthic Foraminifera from the Lower Miocene Sediments of Murachbann, Western Kachchh, Gujarat, India: Implications for Inferring Palaeoenvironment

A thorough taxonomic study was carried out for the first time using high-resolution microscopy from Neogene sections of western Kachchh, India. In the present study, 25 species of benthic foraminifera belonging to 3 classes viz. Globothalamea, Tubothalamea, Nodosariata and 4 orders Rotaliida, Textulariida, Miliolida and Polytmorphinida have been identified from limestone and silty-claystone of Murachbann section belonging to lower Miocene. This taxonomic study provides the distribution of benthic foraminifera species from Murachbann section. The attempt is made to interpret depositional palaeoenvironment based on this distribution of the studied foraminifera. Keywords: Morphology, Taxonomy, Benthic Foraminifera, Limestone, Palaeoenvironment, Chhasra Formation

Facies and trace fossils of the Upper Cretaceous Sillakudi Formation (Cauvery Basin, S. India) and their palaeoenvironmental significance

The Sillakudi Formation is a sandstone-dominated succession bounded by unconformities. It represents the oldest unit of the Ariyalur Group deposited during the Campanian under fully established passive margin depositional conditions. The present study is an attempt to interpret the depositional palaeoenvironment of Sillakudi Formation based on its trace fossil content, supported by sedimentological evidences. Detailed sedimentological and ichnological observations were undertaken in multiple traverses and spot locations across the Sillakudi Formation. The Sillakudi Formation is subdivided into four facies associations, each represented by a distinct ichnology and environment of deposition. Unit 1 comprises conglomerates grading upwards into pebbly sandstones almost devoid of trace fossils except a few Planolites isp. indicating a high-energy foreshore-to-shoreface environment. Unit 2 consists of glauconitic sandstones with Ophiomorpha nodosa, O. annulata, Thalassinoides isp. and Skolithos linearis, belonging to the Skolithos ichnofacies indicating deposition in shoreface to offshore transition environments. Unit 3 comprises pebbly to very coarse-grained massive sandstones, characterised by an almost monospecific Skolithos ichnofacies indicating rapid colonisation of beds in a delta-front associated debris flows and calcareous sandstones alternating with claystones containing a diverse Cruziana ichnofacies with an ichnoassemblage comprising of Trichichnus isp., Scolicia prisca var. laminites, S. ?prisca, S. vertebralis, Palaeophycus tubularis, Planolites beverleyensis, P. montanus, Taenidium isp., Thalassinoides suevicus, Ophiomorpha isp., Chondrites isp. and Phycodes isp., pointing towards deposition in calmer, possibly dysoxic outer shelf prodeltaic environments. Unit 4 has pebbly to gritty cross-bedded sandstones exhibiting the development of networks of Thalassinoides isp. and Ophiomorpha isp., indicating shallow sub-tidal to intertidal conditions of deposition. The overall succession is interpreted to show initial deepening followed by shallowing.

Organic matter enrichment and palaeoenvironmental comparison of Alinian fine-grained sedimentary rocks in lake facies

The Yan'an Formation in the Ordos Basin and the Xintiangou Formation in the Sichuan Basin are typical lake facies sedimentary formations developed in the Alinian period. They contain a large amount of organic-rich fine-grained sedimentary rocks and are the main sites for shale oil and gas enrichment. In this study, geochemical tests, including major, trace, and rare earth elements and TOC tests, were carried out on the fine-grained sedimentary rocks of the shallow lake Yan'an Formation and deep lake Xintiangou Formation to reconstruct their depositional palaeoenvironments, and to explore their organic matter enrichment mechanisms in different depositional environments under lithological differences. The conclusions of this study are as follows: 1) The total organic carbon (TOC) of fine-grained sedimentary rocks increases with increasing mud content and decreasing grain size; 2) The palaeoenvironments of the fine-grained sedimentary rocks in the shallow and deep lake facies are generally consistent, but the redox conditions and deposition rate of the water column in the deep lake facies are stronger than those in the shallow lake facies, while the input of terrigenous detrital, palaeosalinity, and stagnant environment are relatively weaker; 3) The organic matter enrichment mode of lake facies fine-grained sedimentary rocks is a typical palaeoclimatic and palaeoproductivity-dominated “Production mode”; 4) The difference in depositional environments under lithological differences is mainly related to productivity factors, such as palaeoclimate (CIA) and palaeoproductivity (P/Ti). The results of this study help to understand the formation mechanism of lake facies fine-grained sedimentary rocks, distinguish the hydrocarbon generation potential of different lithologies, and provide a basic geological theoretical basis for the exploration of shale oil and gas in the study area.

Detection of Early-Middle Devonian biotic crises in East Gondwana, Paraná Basin, Brazil: An organic geochemical approach

Changes detected in the physicochemical conditions of the depositional palaeoenvironment of the Ponta Grossa Formation (Early-Middle Devonian), East Gondwana, were correlated with significant biotic crises that preceded the great Devonian extinction event (Frasnian-Famennian), culminating in the extinction of the Malvinokaffric fauna. The present study identified Zilchov, Daleje, Choteč, and Kačák biotic crisis events through geochemical analyses. Fifteen outcrop shale samples were analysed for the distributions of well-established geochemical parameters, such as those based on saturated and aromatic biomarkers and polycyclic aromatic hydrocarbons. Parameter results were correlated to polar compound distributions using atmospheric pressure photoionisation (APPI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in positive ionisation mode. A decrease in the summation of n-alkanes, αβC30 hopane, and gammacerane concentrations was observed at the Pragian-Emsian interval, related to the Zilchov event and the first decline of the Malvinokaffric fauna. At the Late Emsian, a severe change in paleoenvironment depositional conditions was detected by the increase in αβ C30 hopane, tetrahydrophenanthrene and gammacerane concentrations, suggesting water column stratification. Moreover, Tricyclic Index values indicated a change to a more freshwater lacustrine environment, which was confirmed by the occurrence ofn-alkyl cyclohexanes series. In addition, high percentages of O1 and N1 class compounds indicated that this interval was probably composed of a mixture of bacterial and terrigenous organic matter. These results indicated a large inflow of fresh and continental waters due to a regression related to the Daleje event and the second decline of the Malvinokaffric fauna. A significant decrease of the pristane/phytane ratio, summation of n-alkanes, αβ C30 hopane and gammacerane concentrations was observed, along with the absence of N1 compounds at the Emsian-Eifelian interval, representing a rapid transgression, corresponding to the Choteč event. At the Eifelian-Givetian interval, the detection of n-alkanes and biomarker low concentration values, low pristane/phytane values (anoxia), as well as the non-detection of N1 class compounds, indicated a great transgression related to the Kačák event and the disappearance of the Malvinokaffric fauna.

Towards an improved discrimination and correlation of Permian-Lower Triassic sediments in Central Europe: A chemostratigraphic approach

The stratigraphic classification and correlation of Permian-Lower Triassic clastic beds in the Central European Basin System traditionally rely on geophysical, bio-, and lithostratigraphic methods. However, in basin margin areas, the accuracy of these tools is diminished due to the heterogeneous distribution of stratigraphic markers, lack of index fossils, and greater climatic and tectonic disturbance. To overcome these limitations, this study introduces a more versatile, multifaceted and chemostratigraphy-based approach. By integrating results from bulk X-ray fluorescence analysis, sandstone petrography, Raman heavy mineral analysis, lithofacies characterisation and borehole gamma-ray logs, we produce robust multi-stratigraphic schemes for selected sequences in the Franconian Basin (SE Central European Basin System). Hierarchical chemostratigraphic zones, comprising five chemozones and sixteen subzones, were designed using key elemental ratios (Ga/Rb, Rb/K, Na/K, Ca/Mg, Cr/Ti, Th/Ti, Nb/Th, Ti/Nb, P/Ti, Zr/P, Zr/Y), whose changes through time reflect modifications in the mineralogical composition of sedimentary rocks. To ensure precise interpretations of geochemical data, we established links between mineralogical and chemical components through statistical processing and direct comparison with mineralogical assemblages. Our results demonstrate that chemostratigraphic boundaries can identify major changes in palaeo-depositional environments and sediment sources induced by eustatic, tectonic and climatic disturbances. For example, changes in the P/Ti and Zr/P ratios across the CP3-CT1 document the increased abundance in apatite-rich granitic detritus, resulting from the exposure of new source lithologies in the catchment area due to Early Triassic synsedimentary tectonism. Since these perturbations had a regional impact on sedimentation, their geochemical fingerprints are correlated across the basin, regardless of lithofacies variability or distance between sections. Moreover, integrating characteristic geochemical signals into multi-stratigraphic schemes enables more precise placement of stratigraphic boundaries, identification of locally deposited/preserved stratigraphic markers, and overall refinement of outdated classification schemes. The chemostratigraphic approach introduced in this study offers a solution to the limitations of conventional stratigraphic tools, allowing robust characterisations and correlations of Permian-Triassic sequences in the marginal regions of the Central European Basin System. In addition, the multi-analytical workflow for processing geochemical data, refining stratigraphic schemes and attributing geochemical anomalies to environmental perturbations represents a further step towards more precise applications of chemostratigraphy to the depositional record.

Facies analysis, petrography and textural characteristics of the onshore Paleogene-Neogene Lawin Basin, Perak, Peninsular Malaysia: Insights into palaeodepositional environment and provenance

Sedimentological features of the onshore Paleogene-Neogene Lawin Basin are of importance in better understanding the basin evolution and geodynamics. The N-S trending Lawin Basin, which is situated within the Western Belt of Peninsular Malaysia, is an extensional basin that formed during the sinistral movement of the Bok Bak Fault zone. This study evaluates the palaeodepositional environment, reveals the provenance, and interprets the transport dynamics of the mini-basin by integrating facies, petrographic, and grain size analyses. The basin lithologies can be broadly divided into conglomerate and sandy facies, with the former consisting of clast-supported conglomerate, matrix-supported conglomerate, cross-stratified pebble conglomerate, and bedded pebbly conglomerate, whereas the latter comprises massive pebbly sandstone, horizontally bedded sandstone, planar cross-bedded sandstone, and ripple laminated sandstone. The lithologies can be grouped into three main facies associations, including massive conglomerate, stratified conglomerate, and fluvial sandstone deposits, which reflect alluvial fan (debris/gravity flow deposits), braided gravel bar, and channel bar elements, respectively. Petrographic data indicate that the conglomerates are clast- and matrix-supported and were derived from a proximal source. The QFL range of detrital framework grains reveals that sandstones are composed of quartz (43–80 vol%), feldspar (9–54 vol%), and lithic fragments (2–16 vol%). The sandstones are coarse-grained, poorly-sorted, and sub-rounded to sub-angular in texture. They are arkose, sublitharenite, and lithic arkose in composition. QtFL/QmFLt ternary diagrams show that the sandstones are primarily derived from a provenance area comprising continental block material, including uplifted basement rocks and transitional continental deposits. This study proposes that the sandstones had a nearby granitic source (Bintang) and a debris source derived from recycled sedimentary rocks (the Baling Group sediments). Grain size parameters ranges (mean, −2.37 to 0.74 Φ; sorting, 1.22 to 2.07 Φ; skewness −0.21 to 0.91 Φ, kurtosis, 0.66 to 1.98 Φ, and average mean-sorting index of −0.24) are consistent with immature to sub-mature sedimentation deposited within a high-energy fluvial environment.

PALYNOFACIES AND SEDIMENTOLOGY OF HB-001 WELL

The palynofacies and sedimentological analyses of the sedimentary succession of the HB-001 well were carried out to establish biozones and the palaeodepositional environment. Ninety-six samples of ditch cutting within intervals of depth of 1250 -4011 m were analyzed. Most of the previous researchers established biozones with alpha-numeric method. The use of palynomorphs and palynofacies analyses to delineate paleoenvironment of deposition has been applied to a limited extent in the Niger Delta Basin in Nigeria. The analysis produced fairly diverse and moderately abundant pollen and spores with high recoveries of palynomaceral 1 and 2 and minimal retrievals of palynomaceral 3 and 4. The textural, lithologic, as well as wireline log data point out that the whole studied interval in the HB-001 well fit in to the Agbada Formation. Late Miocene age was assigned on the bases of the analysis of stratigraphic age range of indicative palynological markers for example Zonocostites ramonae, Verrucatosporites sp., Laevigatosporites sp., Monoporites annulatus, Pachydermites diederixi and Psila,tricolporites crassus. Two assemblage zones were established in the well with the use of the International Stratigraphic Guide for the biozones establishment. The two recognized palynostratigraphic zones are Cyperaceaepollis sp. – Nympheaepollis clarus, and Stereisporites sp Zones. Lower delta plain, pro-delta and delta front depositional environments have been deciphered for well with the use of the palynofacies association, palyno-ecological groupings, and sedimentological features. Bodies of sand that signify sub-environments inside these settings are deposited in sequences. Every sequence begins with a transgressive stage, after that considerable regression. The palyno-ecological groupings of the retrieved palynomorph taxa revealed that the well intervals studied were deposited under alternating wet and dry paleoclimatic settings. The zones of dry climate presented high account of montane and savannah taxa and low incidence of rainforest, mangrove and freshwater taxa. The zones of wet climate signified increased account of rainforest, freshwater and mangrove taxa and lower incidences of montane and savannah taxa.

Lower Permian Gondwana sequence of Rajhara (Daltonganj Coalfield), Damodar Basin, India: floristic and geochemical records and their implications on marine ingressions and depositional environment.

Early Permian floral diversity and palaeodepositional environment of the Lower Permian Rajhara sequence of Damodar Basin have been studied based on mega-, microfossil and geochemical proxies. Even though Gondwana sediments are generally considered as fluvio-lacustrine deposits, recent studies indicate marine inundations with patchy records. Here in the present study, an attempt has been made to address the changeover from fluviatile to shallow marine conditions and also to address the palaeodepositional aspects. Luxuriant vegetation during deposition of the Lower Barakar Formation generated thick coal seams. The macroplant fossil assemblage shows Glossopteridales, Cordaitales and Equisetales comprising one palynoassemblage with the dominance of bisaccate pollen grains having glossopterid affinities. However, Lycopsids are absent in the megafloral record and are represented in megaspore assemblage. The present floral assemblage led to envisage the presence of dense forest with swampy conditions and prevalence of warm and humid climate during the deposition of Barakar sediments. Correlation with the coeval Indian assemblages and those from other Gondwanan continents also supports an Artinskian age and reveals a stronger affinity with flora of Africa than that of South America. Biomarker analysis reveals low pristane/phytane values (0.30-0.84), noticeable absence of hopanoid triterpenoids and long-chain n-alkanes that is attributed to the obliteration of organic compounds and subsequent alteration of composition due to thermal effect. The high chemical index of alteration, A-CN-K plot and PIA also suggest severe denudation under a warm/humid climate. The V/Al2O3 and P2O5/Al2O3 indicated freshwater-near-shore conditions. However, signature of possible marine influence is identified from Th/U and Sr/Ba ratios resulted from the eustatic fluctuations during Permian.

Palynological Study and Age Dating of the Early Miocene Dukku Claystone Type Section of Gwandu Formation, Sokoto Basin, Northwestern Nigeria

Outcrop samples were collected from Dukku area for the purpose of subjecting them to palynological analysis in order to determine the relative age of Gwandu Formation and palaeoenvironment of deposition of the deposit. Field and laboratory methods were carried out in order to obtain lithological sequence description of the outcrop. The samples obtained went through processes of acid digestion with hydrochloric acid, hydrofluoric acid and separation of macerals by floatation with the use of zinc bromide. The prepared slides were observed under microscope for palynomorph contents. Diagnostic palynomorphs that are stratigraphically important for palynozonation were retrieved such as Belskipollis eligans, Verrutricolporites laevigatus. Striamonocolpites sp., Retibrevitricolporites sp., Retibrevitricolporites obodoensis, Retistephanocolpites sp., Cingulatisporites ornatus, Longapertites marginatus and Proteacidites sp. The marker fossils belong to Magnatariatites horwardii zone; Zone P600 and Subzone P630 and C1 Zone of Nigerian Niger Delta sedimentary basin zonal schemes, dated Early Miocene age. Dinoflagellate forms recovered include Batiacasphaera sp., Senegalinium sp., Andalusiella polymorpha Selenophemphix nephroides, Andalusiella sp., Andalusiella laevigata, Histrichosphaera turocona and undifferentiated dinoflagellate cysts of peridinacean forms that belong to marginal marine setting.

Late maastrichtian- early eocene dinoflagellate cyst biostratigraphy and aleoenvironmental study of sediments in Akukwa-1 well in the Anambra Basin

Dinoflagellate cyst biostratigraphy and palaeoenvironment of deposition of sediments penetrated by Akukwa-1 Well was carried out with a total of 253ditch cuttings using the conventional maceration technique for recovering of acid insoluble organic-walled microfossils from sediments. Sedimentological description delineated five (5) lithological units within the well, which included those of Nkporo Shale, Mamu Formation, Ajali Sandstone, Nsukka Formation and Imo Shale. A total of 14 genera and 35 species of dinoflagellate cyst were identified. The recovered dinocysts were used to establish palaeoenvironment which ranged from marginal to shallow marine, the forms were also used to propose four (4) informal dinoflagellate cyst zonations labelled (A-D) based on the first and last occurrences of two or more species. They included Zone A (Apectodinium paniculatum zone), occurring at 7990ft(Maastrichtian), zone B(Paleocystodinium australinium zone) occurring between 7990ft -4000ft (Upper Maastrichtian), zone C(Apectodinium homomorphum zone) occurring between 4000ft -2550ft (Upper Maastrichtian), and zone D(Homotryblum palladium zone) occurring between 5550ft-1520ft(Paleocene- Early Eocene). The erected dinocyst zones were correlated with the existing biozones in tropical areas of Africa and northern South America.

Bio-environmental interactions and associated hydrocarbon generation in a saline lake basin: A case study of the Palaeogene interval in the Dongpu Sag, eastern China

• Different hydrocarbon-generating organisms occur in different saline palaeoenvironments. • The palaeoenvironment affects the hydrocarbon-generating organisms and source rock potential. • Source rocks deposited in saline environments have a low oil generation threshold and are characterised by multi-stage oil generation. • Saline environments form source rocks with a good organic matter type and high hydrocarbon generation potential. Saline lake basins are important Cainozoic petroliferous basins in China. There are significant differences in the palaeoenvironments of these basins. Different biological assemblages resulted in varying hydrocarbon generation characteristics and potential. Therefore, the bio-environmental interactions are key to understanding the oil generation from terrestrial organic matter. In this study, we investigated the organic petrology and geochemistry of the Palaeogene Shahejie Formation in the Dongpu Sag, Bohai Bay Basin, China. The depositional palaeoenvironment varied from saline to freshwater. Cyanobacteria and coccolithophores were deposited in the saline environment, and green algae and higher plant debris were deposited in the freshwater to brackish water environments. The algal biomass in the sediments increased with salinity. Thermal and pyrolysis experiments revealed differences in the hydrocarbon generation potential of the source rocks deposited in different sedimentary environments. The source rocks deposited in the saline water environment have a high hydrocarbon generation potential. Two immature to low-mature oil generation peaks were observed, which are mainly due to the pyrolysis of coccolithophores, cyanobacteria, and dinoflagellates. Only one oil generation peak was observed for source rocks deposited in the freshwater environment, which is mainly due to the pyrolysis of dinoflagellates and green algae. The hydrocarbon generation potential and two stages of oil generation of the source rocks deposited in the saline environment may be due to the type of hydrocarbon-generating organisms and the catalytic effect of the high-salinity diagenetic environment. The results show that the sedimentary environment of saline lake basins affects the hydrocarbon-generating organisms and hydrocarbon generation process.

FACIES ANALYSIS AND PALAEODEPOSITIONAL ENVIRONMENT OF THE TANJONG FORMATION IN THE KALABAKAN AREA, SOUTHEAST SABAH

An integrated approach is adopted to study the sedimentary deposits of the well-exposed Early to Middle Miocene Tanjong Formation in the Southeastern part of Sabah. The integrated sedimentological and palynological data will provide a better insight into the depositional architecture of the area. This work presents the first detailed sedimentological facies analysis of the Tanjong Formation within the Kalabakan area. The integration of facies analysis, ichnological and palynological data provide a better evaluation of the organic matter (OM) origin and the reconstruction of the palaeodepositional model. An extensive outcrop-based sedimentological facies analysis was performed on eight (8) well-exposed outcrops with studied stratigraphic sections. Seven facies associations are classified in the Tanjong Formation from twelve lithofacies elements which have been interpreted from the fluvialdeltaic to shallow marine environment: FA1 - Floodplain facies association; FA2 – Fluvial channel facies association; FA3 - Coastal swamps facies association; FA4 – Tidal flat facies association; FA5 – Delta front facies association; FA6 – Mouth bar facies association; FA7 - Upper shoreface facies association. The palynomorphs of the studied coal samples yielded abundant mangrove and freshwater types of pollen assemblage, while the analysed mudstone samples suggest deposition within a lower coastal plain setting with proximity to the marine environment. The presence of Florschuetzia levipoli (Sonneratia caseolaris) and Florschuetzia trilobata suggest an age not older than the Early Miocene and/or younger. The reconstructed palaeodepositional environment model is essential in improving the predictability of the petroleum system mechanisms and the hydrocarbon potential for conventional petroleum exploration.Keywords: Sedimentology, facies association, fluvial-deltaic, shallow marine, geochemical, palynology

Palynomorph composition and palynofacies analysis of the Cenomanian Bahariya Formation in the north Western Desert of Egypt: Depositional palaeoenvironment and sequence stratigraphic implications

Abstract This study investigates the Bahariya Formation using 39 rock-cutting samples from the Abu Gharadig Basin in the north Western Desert, Egypt. An approach combining palynomorph composition and palynofacies analysis with lithological composition and geophysical gamma ray log values is used to assess biostratigraphic age constraints, prevalent depositional palaeoenvironments, and relative sea level changes. A moderately diverse assemblage comprised mainly of terrestrial palynomorphs versus minor content of marine palynomorphs is identified. Marker spore and pollen taxa are used to name three interval zones of early Cenomanian, and early-middle Cenomanian ages. Quantitative distribution of particulate organic matter using cluster analysis reveals two palynofacies assemblages: an older assemblage dominated by phytoclasts (mainly wood particles); and a younger assemblage characterised by moderate abundances of amorphous organic matter and phytoclasts. Palynofacies analysis suggests deposition of the Bahariya Formation in fluvio-deltaic to inner neritic shelf environments. Based on the pronounced stratigraphic variations in the particulate organic matter composition, including terrestrial:marine palynomorph ratio, lithological and gamma-ray log data, three complete transgressive-regressive sequences and one incomplete sequence and their systems tracts are defined. The transgressive-regressive trends reflect 3rd order sequences. Additionally, successive changes in relative sea level indicate a continuous rise at the topmost part of the succession.

Origin and palaeodepositional environment of evaporites in the Bala sub-basin, Central Anatolia, Türkiye

ABSTRACT Bala is located in Central Anatolia and is a sub-basin of the Tuzgölü basin, which consists of many basins formed in response to the closure of the Neotethys ocean. The evaporites in Bala sub-basin formed under the influence of regional uplifting during the Middle Eocene period with gradual shallowing in response to the structural compressional regime. The Bala sub-basin comprises four units from bottom to top: the Lower Unit (BU1) consisting of mudstone, claystone with limestone layers; the Evaporites (BU2) containing dominant gypsum with anhydrite; mixed siliciclastic-carbonate-evaporitic deposits (BU3); and the Upper Unit (BU4) containing conglomerate-sandstone-claystone and siltstones. The evaporitic unit, which are the main subject of study consists of primary selenitic gypsum, anhydrite, locally celestite crystals and secondary gypsum formed due to the anhydritization of primary gypsum and then hydration of these anhydrites. The results obtained from mineralogical-petrographic, geochemical and isotopic (δ18O and δ34S) investigations indicate that these evaporites were deposited in shallow marine to sabkha environments dominated by arid conditions. The presence of primary selenite crystals, the anhydrite with nodular, chicken-wire, and mosaic structures support precipitation occurred in a partially saline shallow water. The excessive thickness of the evaporitic accumulation, chaotic masses including gypsum breccia, salt diapirs and dome structures are attributed to tectonic processes acting during the formation of the Central Anatolian basins.

Optical and geochemical assessment of petroleum source rock potential and palaeodepositional environments of Cretaceous mudstones, Koum Basin, North Cameroon

Limited organic matter (OM) characterisation studies on Cretaceous mudstone successions in the Koum Basin have resulted in a significant knowledge gap in the source rock potential; a petroleum system prerequisite. Field mapping and lithofacies analysis of outcrop sections point to fluvial channel and marginal to profundal lake deposits. Representative mudstone samples were evaluated using multiproxy organo-petrographic and bulk geochemical analyses to assess the kerogen type, thermal maturity, hydrocarbon generation potential, and deduce the paleodepositional environment. Results reveal that ∼90% of the samples are organic-lean and ∼10% organic-rich with TOC content of 2.81–6.91 wt%. Rock-Eval pyrolysis S2 yields of 14.50–54.18 mg HC/g rock and hydrogen index (HI) 784–846 mgHC/gTOC, suggest excellent potential for oil generation. High S1 values of 1.18–1.45 mg HC/g rock and low production indices (PI) of 0.02–0.03 indicate in situ generated free hydrocarbons. PI and Tmax of 438°C–443 °C, consistent with measured vitrinite reflectance (Ro% = 0.28–0.53%), Spore Colour Index (SCI = yellow to golden brown) and Thermal Alteration Index (TAI = 1.5 to 2.6) on the Chevron scale, portraying an immature to marginally mature OM. Oil-prone kerogen Type I-II dominates and to a lesser extent gas-prone Type III, derived from algal-terrestrial organic facies, typified by the predominance of amorphous OM 51.2%, liptinites 24.6%, vitrinite 16.4% and inertinite 7.8%. Based on lithofacies, and palynological evidence, a fluvio-lacustrine depositional setting with limited marine influence, under fluctuating warm humid to hot semi-arid paleoclimates, is inferred. A comparison with surrounding petroleum-producing basins further enhances the understanding of the Koum Basin's hydrocarbon potential within the West Central African rift system context.

A comment on overlooked storm sensitivity of the carbonate factory recorded in the Mississippian Mobarak Formation (Alborz Mountains, Iran)

New interpretations of depositional palaeoenvironments in the Mississippian (Lower Carboniferous) Mobarak Formation (Alborz Mountains, Iran) suggest a significant and persistent influence of storms. This deviates from previous conclusions that these deposits recorded mounds, patch reefs, and extensive lagoons deposited under stagnant environmental conditions. We here clarify and discuss the origin and nature of this misconception by explaining “unexpected” observations that are informed by outdated interpretations of the depositional environment of the Mobarak Formation. This evaluation offers the context required for appropriately interpreting and correlating Mississippian depositional records across the Alborz Basin.

Geology, Palaeodeposition and the Involvement of Rhyolite Melts in the Petrogenesis of the Tabenken Coal Seam in the North West Region of Cameroon

The discovery of patches of a coal deposit and other associated sedimentary and volcanic rocks in Tabenken North West Cameroon has raised the question of the geology and palaeoenvironment of that area. This Region, which is predominantly characterized by a granitoid basement of Precambrian age is in most parts overlain by Cenozoic basalts, hawaite, mugearite, trachyte and rhyolites. Volcanic outpours modified the geomorphology of the area into a series of hills and valleys. We investigated the geological setting in view of reconstituting the palaeodepositional environment of the Tabenken Coal Seam. Field studies show that the coal occurs in form of inclusions within sandstones, high grade coal bed, massive beds exposed by landslides and in alkali rhyolites. The results of Ultimate analyses of the coal indicate bituminous coal with 58% Carbon, sulfur content as low as 0.12% and ash content of 17%. The occurrence of a well stratified dark volcanic ash bed in the area is interpreted to be an interactive product of the explosive volcanic activity and weathering. Field examination of the area suggests that it was a micro-continental sedimentary palaeo-basin which was later infilled with Cenozoic volcanic outpours which probably modified the chemistry of the coal to meta-anthracites. The actual ages of the coal as well as the associated sedimentary units have not been established, meanwhile, the volcanism started some 31 ma ago.

Dactyloidites ottoi (Geinitz, 1849) in Bahamian Pleistocene carbonates: a shallowest-marine indicator

Abstract The rosetted trace fossil Dactyloidites ottoi ( Geinitz, 1849 ) is reported and described for the first time from late Pleistocene (MIS 5e) carbonates of the Bahama Archipelago in shallowing-upward, shelly calcarenites from Great Inagua and Great and Little Exuma islands. The distinctive, fan-shaped D. ottoi specimens from the Bahamas, while not preserved in fine detail and not revealing a shaft, compare favourably in shape and size with specimens from other localities around the world, including the oldest well-documented specimens from the Jurassic of Argentina. D. ottoi is interpreted as a fodinichnion formed by the activity of a deposit-feeding worm, probably a polychaete, consuming marine-plant remains within host sediment. The late Pleistocene palaeodepositional environment of these carbonate sediments is interpreted as within the lower foreshore–upper shoreface zone in full marine, tropical waters. This discovery of D. ottoi marks an addition to the Bahamian shallow-marine ichnocoenose within the Skolithos ichnofacies.