Depositional Setting Research Articles

The late Neogene distribution of C 3 –C 4 plants in the Himalayan Foreland Basin: insight from the δ 13 C values and sedimentological architecture of the Siwaliks

Recent studies emphasize that, in addition to climate-driven forces, sediment grain size and depositional setting with respect to the mountain front significantly influenced the abundance of late Neogene C 3 –C 4 plants in the Himalayan Foreland Basin (HFB). The contrasting depositional settings of the Siwalik Group exposed across the western, central and eastern HFB therefore provide an ideal opportunity to understand the influence of sedimentary architecture on the distribution of C 3 –C 4 plants in palaeolandscapes. To this end, we generate new δ 13 C soil carbonate data from the Siwaliks of the Katilukhad region (12 Ma to 6 Ma) of the Kangra sub-basin and synthesize these data with compiled sedimentological data and δ 13 C values of organic matter, soil carbonate and n -alkane data from the western to eastern HFB Siwalik Group. Our comparison suggests that the rate and magnitude of the positive shift in the 13 C/ 12 C ratios were higher in the floodplain-dominated Siwaliks. Despite an existing conducive climate for the growth of C 4 plants in the late Neogene, the channel-fill-dominated Siwaliks favoured C 3 plants over C 4 plants in the eastern HFB.

Fluid Inclusion, Rare Earth Element Geochemistry, and Isotopic (O and S) Characteristics of the Ardakan Barite Deposit, Yazd Province, Iran

The stratabound barite mineralization occurs in the Ardakan deposit as patches and veins in the dolomites and limestones of the Middle Triassic Shotori Formation. Rare-earth element (REE) geochemistry, O and S isotopes, and fluid inclusion data were used to identify the mode of barite formation. Barite is associated with subordinate fluorite and quartz and, to a lesser extent, with sphalerite, malachite, chrysocolla, and iron and manganese oxide-hydroxides. Barite contains a very low ∑REE concentration (14.80–19.59 ppm) and is enriched in light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs). The low ∑REE content and the Ce/La ratio (4.0–6.5) indicate a hydrothermal (terrestrial) origin of the barite. Similar to barite, the ∑REE content in fluorite is low (0.14–6.52 ppm) and suggests a sedimentary setting. The Tb/Ca versus Tb/La diagram also indicates a hydrothermal origin of fluorite. The δ34S values in the barite (+27.9 to +32.4‰) indicate that the sulfur most likely originates from evaporites and/or connate waters from the Late Precambrian to the Lower Cambrian. The δ18O values (+15.9 to +18.1‰) in the barite show that the oxygen originated either from Late Precambrian–Lower Cambrian evaporites or from basinal brines with slightly higher δ18O values than the evaporites. The salinity and homogenization temperature ranges of the aqueous fluid inclusions in barite, fluorite, and quartz (0.88–16.89 wt% NaCl equivalent and 90–270 °C, respectively) reveal that the mineralizing fluids were formed from basinal brines with the participation of heated meteoric water. From this, it is concluded that the Ardakan barite deposit was formed by the meeting of heated, ascending sulfate-bearing meteoric water and cooler, Ba-bearing connate water trapped in the overlying Middle Triassic dolomites and limestones. The Ardakan deposit belongs to the structure-related class and the unconformity-related subclass of barite deposits.

Identifying tectonic settings of porphyry copper deposits using zircon trace elements – A semi-supervised machine learning method

Porphyry copper deposits (PCDs), as main sources of Cu resources worldwide, have attracted great attention for both scientific research and mining purposes in the past decades. The formation of PCDs is closely related to magmatic-hydrothermal system formed in both (continental/oceanic) arc and non-arc environments. Many low-dimensional geochemistry discriminants were developed for identifying the tectonic environments where ore-related magmas formed. However, long-term geological processes may obscure the geochemistry information recorded in rocks and minerals, which inevitably introduces uncertainties for identifying tectonic environments when using the low-dimensional discriminant diagrams. Here, we proposed a semi-supervised machine learning (ML) approach based on zircon chemistry data (spanning 16 elements) to identify the tectonic environments of PCDs. The results show that the semi-supervised models perform better in identifying the tectonic environments compared to the conventional low-dimensional discriminants. Moreover, as another benefit of big data analysis, ML model can clarify the systematic geochemistry differences of igneous zircons formed in different tectonic environments. The results suggest that (U+Th)/Yb and LREE/HREE are the top two important zircon chemical features for distinguishing PCDs formed in arc or non-arc environments. Specifically, zircons from non-arc show higher (U+Th)/Yb compared to that of continental arc, while zircons from island arc show lowest (U+Th)/Yb and LREE/HREE. Finally, the semi-supervised learning model was applied to the Dexing porphyry copper deposit to approach the longstanding debate regarding the underlying metallogenic setting. The results support the viewpoint that the Dexing porphyry copper deposit was formed in an intracontinental (non-arc) environment related to asthenosphere upwelling.

Osmoregulation in Alligatoroidea: shifting the paradigm untethers biogeographic questions

ABSTRACT A pervasive maxim in herpetology is that alligators (Alligatoridae) are poor salinity regulators relative to their taxonomic kin, crocodiles (Crocodylidae), evidenced by the purported lack of osmoregulatory abilities of modern Alligator and caimans, which has led to the assumption that freshwater obligation is synapomorphic across the clade, to include extinct members of Alligatoridae, or even Alligatoroidea. Presented here is a brief review of osmoregulation and the fossil record of alligatoroids and a suggestion that salt-tolerance may have been more widespread in the group than is widely assumed, based upon the observations that 1) modern alligatorids are increasingly reported from brackish and marine environs and possess osmoregulatory abilities attained through different regulatory pathways than their crocodile counterparts, and 2) fossil members of the group are routinely (or exclusively) found in or near marine depositional settings, suggesting plausibility that earlier members of the clade were more ocean-going than their extant descendants. Starting with fewer assumptions about the osmoregulatory abilities of extinct taxa may help solve continuing problems in eusuchian biogeography, namely, the dispersals of alligatoroids into Europe and Asia, and multiple dispersal events to and from pre-Interchange South America. Secondary loss of salinity tolerance in Alligator may be adaptive in continental habitats.

Rapid sulfurization obscures carotenoid distributions in modern euxinic environments

Anoxygenic phototrophic bacteria (green and purple sulfur bacteria) thrive in anoxic environments where light penetrates a sulfide-containing (euxinic) water column. Genomic data and photosynthetic bacterial carotenoid pigments should provide complementary information on the spatio-temporal dynamics of anoxygenic phototrophs in modern euxinic environments. In turn, these contemporary depositional settings often serve as analogues for ancient counterparts. However, in some modern environments, DNA-informed patterns of phototrophic sulfur bacteria occurrence do not match distributions of their carotenoid inventories. One possible explanation for these seemingly incompatible observations is that the rapid sulfurization of carotenoids and incorporation into macromolecules via multiple carbon–sulfur bonds prevents or confounds their detection by conventional means. Here, to evaluate this conundrum, we revisit some representative contemporary euxinic environments where anoxygenic phototrophic bacteria have mostly been detected based on genomic analyses. Although free intact carotenoids are sporadically detected in surface sediments, their distributions do not reveal a complete picture. Exogenously sourced fossil carotenoids (e.g., paleorenieratane) is an additional complication. Carotenoid inventories obtained by desulfurization with Raney nickel, on the other hand, stand in stark contrast to those present as free lipids. In particular, sulfur-linked carotenoids present in euxinic lake sediments provide a more complete picture of compositions of anoxygenic sulfur bacterial communities and account for discrepancies reported in previous studies. We observe a closer alignment between genomic data and patterns of sulfurized carotenoids and, importantly, our results highlight how sulfurization serves as a pathway for the rapid modification of highly functionalised lipids and their sequestration into the macromolecular component of sediment extracts.

Organic geochemistry of Palaeogene coals from Greenland and Svalbard

AbstractThe organic geochemistry and coal petrology of Palaeogene coals from northeast Greenland (Thyra Ø Island and Kronprins Christian Land) and central Spitsbergen (Longyearbyen and Grumantbyen) were studied using Rock–Eval and gas chromatography–mass spectrometry, as well as microphotometry and maceral group analyses. Bulk data and biomarker distributions of the coals demonstrate a low coal rank for both, but a lower coalification degree of coals from Greenland (0.49–0.55% VRr) compared to those from Svalbard (0.68–0.75% VRr). Maceral group analyses revealed relatively similar distributions with a strong predominance of vitrinite. The generally high abundance of hopanoids (hopanes/hopenes and hopanoic acids) implies a strong bacterial reworking of the organic matter, whereas sulphur occurrences indicate a marine influence after organic matter deposition. A great variety of higher plant biomarkers was detected in all coals. Distinctive compounds recorded in the coals are aliphatic and aromatic diterpenoids as well as partly hydrogenated picenes, suggesting strong input of conifers and angiosperms. Pristane/phytane ratios indicate that the organic matter in the ancient swamps was deposited in an oxic, fluvio-deltaic setting at both sites. This study provides a detailed geochemical investigation of understudied coals from northeast Greenland. Moreover, it enhances our understanding of probably interrelated Palaeogene depositional settings from Greenland and Spitsbergen in terms of their palaeoecology, primary input into coal swamps, and individual thermal history. Graphical abstract

Ostracod turnover during the Carnian Pluvial Episode (Late Triassic) in the Western Neotethys

The Carnian Pluvial Episode (CPE) caused a biotic crisis and turnover in terrestrial and marine ecosystems in the Western Neotethyan realm. The expansion of deltaic systems on land and the increased siliciclastic input affected the distribution of benthic ostracods, both nearshore and offshore. Diversity changes of Carnian ostracod faunas from seven successions in the Transdanubian Range (TR), Western Hungary tracked the effects of the CPE, despite its distal hemipelagic depositional setting and position within an intraplatform basin. At the onset of CPE in the early Julian 2, the composition of the studied benthic ostracod assemblage was similar to that of the late Julian 1 communities albeit with decreased abundance of smooth bairdiids and healdiids. The ratio of healdiids and smooth bairdiids is likely related to the detrital supply with bairdiids colonizing primarily in intervals with decreased terrestrial input and carbonate platform progradation. During the maximum expansion of CPE in late Julian 2, the predominance of platycopid Bektasia indicates a shallowing trend, infilling of the Carnian basins and stressed environment due to clastic input and salinity stress. The Julian–Tuvalian boundary is characterized by high extinction rates among marine invertebrates simultaneously with the occurrence of a unique low diversity but abundant brackish water ostracod community with Simeonella and Renngartenella that can be traced along the entire Western Neotethys and northern Gondwana shelf. The appearance of this community is primarily linked to salinity variations during the CPE and the formation of restricted basin habitats in the TR with water stratification where the special community lived or was transported there with storms or river outflow. During the Tuvalian, fully open marine conditions were re-established with increased carbonate productivity in the platforms, favouring the colonization of stable shallow marine habitats with the appearance of ornate bairdiids in the ostracod faunas.

Reservoir heterogeneities due to diagenesis in Jurassic Samana Suk Formation Kahi section Nizampur Basin North West Himalayas Pakistan

The porosity and permeability determine the reservoir quality of sedimentary rocks, which is fundamentally influenced by both depositional and diagenetic processes. The Jurassic carbonates are targeted in many regions for oil and gas exploration. The current study is carried out to elaborate on the diagenetic alterations and their effect on reservoir properties. A thick outcrop of the Jurassic Samana Suk Formation is studied in the Kahi section of Nizampur Basin Northwest Himalayas, Pakistan, to study and relate the trend of lithological variations and depositional settings. Field investigation revealed that the Samana Suk Formation is extensively distributed in the area and primarily made up of interbedded limestone and dolomite units. The original unaltered limestone has a thick-bedded and oolitic to bioclastic nature. Different types of dolomites have been recognized based on colour contrast and sedimentary features. Moreover, saddle dolomite cement, calcite cementation, and mechanical and chemical compaction have also been observed. The petrographic studies show different types of diagenetic alterations that affected the Samana Suk Formation, including micritization, bioturbation, mechanical and chemical compaction in the form of fractures and stylolites, various calcite cementation, which includes the various cement types that range from isopachous, blocky, granular equant, fibrous, and dog tooth cementation, along with dissolution that occurred in different diagenetic realms. Pyritization was rarely observed. Moreover, different phases of dolomites were identified, ranging in size and shape, i.e., finely crystalline to coarse crystalline and planar euhedral to non-planar anhedral. The stable oxygen isotope values of these dolomites show depletion from original marine signatures and suggest burial-related fault-controlled dolomitization events. Overall, diagenetic processes like dissolution, fracturing and dolomitization increased the reservoir potential. On the contrary, the overburden and cement precipitation result in a decrease in the reservoir properties.

Assessment of the source rock potential in the Sirka and Giddi collieries of South Karanapura coalfield, Jharkhand, India: Insights from megaflora, palynology, and geochemistry

Permian deposits in the Indian Peninsula have long been a significant source of coal and have great potential for hydrocarbon exploration. Here we present results of megafloral, palynological, and geochemical analysis of the Late Artinskian-Kungurian sediments in the South Karanpura coalfield to assess hydrocarbon generation potential, kerogen types, depositional settings, and thermal maturation. The results suggest anoxic to oxic depositional environments with fluctuating water levels, influenced by terrestrial inputs. The Sirka colliery is considered to be most favorable for hydrocarbon generation due to the palaeodepositional setting dominated by flooded palaeomires. The dominance of degraded organic matter and the rarity of opaque phytoclasts suggest type II/III to type III kerogen material in the palaeomire of the Srika succession, characterized by low-energy dysoxic to anoxic conditions. The thermal maturation values (Tmax 429℃) and the production index (0.01–0.02) indicate that the Sirka area has immature kerogen, but the Giddi colliery has a relatively higher Tmax (average 435℃) placing the studied sample within the mature zone. However, due to deposition in the oxidized swamp, Giddi C has poor potential for hydrocarbon generation, showing that type III/IV material has charcoal input into the sediments. Our findings contribute to global knowledge of coal formations’ oil and gas storage capacity, which has implications for energy resource assessment and exploration strategies.

Spatial heterogeneity in nutrient utilization during the end-Devonian ocean anoxic event: a case study of the Western Canada sedimentary basin

The Devonian-Carboniferous (D-C; 359 Ma) boundary is marked by widespread deposition of organic-matter-rich black shales associated with the Hangenberg mass extinction event. The Exshaw Formation spans the D-C boundary in the Western Canada Sedimentary Basin (WCSB) and includes the basal Exshaw Shale deposited under broadly anoxic waters. The sediments at the base of the Exshaw Shale were deposited synchronously during a transgressive event across the WCSB, spanning the geographic variability of the basin. The variable Corg content of the shale was affected by local nutrient upwelling and paleotectonic features impacting water depth and circulation. To characterize the link between paleogeography and nutrient cycling, geographic (N = 20 locations) and stratigraphic (N = 6 locations) trends of δ13Corg and δ15Nbulk were examined throughout the WCSB, representing a range of depositional settings. The δ15Nbulk values range between 0.0 and 6.3‰ and δ13Corg from −29.5 to −26.8‰. Phytoplankton production in focused upwelling zones acquired a relatively 15N-depleted signature through isotopic fractionation during nutrient assimilation, and the residual nutrient pool was 15N-enriched. The advection of surface waters away from the location of upwelling supported additional phytoplankton growth and the deposition of sediments with higher δ15N values. The stratigraphic sections include black laminated and burrowed mudrock sequences that record changes in paleoredox conditions, water depth, and tectonism over time. Up-core from the base of the Exshaw, the Corg content decreases and simultaneously δ15Nbulk increases, suggesting a decrease in eutrophic conditions. Variable δ13Corg and δ15Nbulk trends demonstrate that there is no “type” isotopic profile spanning the D-C boundary in the WCSB.

Palaeogeography and tectono‐stratigraphic evolution of the Aptian Ezanga‐Loémé evaporites along the proximal domain of the south Gabon‐Congo‐Cabinda margin

AbstractDuring the Early Cretaceous, massive evaporite accumulations formed in the opening South Atlantic. However, the depositional model of these salts is still poorly constrained at the scale of the West African margin. The present study focuses along the proximal domain of the south Gabon‐Congo‐Cabinda margin and is based on (i) log interpretations of 246 wells crossing undeformed to weakly deformed evaporite intervals and (ii) a structural characterization of the basement. The evaporites show 11 regional evaporite depositional cycles (CI–CXI) bounded by meter‐thick shale beds. The cycles display alternating meter‐scale carnallite‐halite beds that can be correlated over several hundred kilometres, and CVI, CVII, CVIIIa and CX culminate in localized tachyhydrite accumulations. Cross section correlations and isopach maps help to understand the palaeogeographical evolution of each cycle and depositional environments that evolved from relatively deep at the base of cycles, to very shallow at their top. CI formed a mosaic of halite‐prone depocenters deposited in pre‐salt topographic relief. CII and CIII were deposited uniformly over a flattened basin in a highly extended brine pond. From CIV to CVIIIa, the stratigraphic architecture of the salts was shaped by freshwater inflow sourced from the north and possible basement movements. This setting, together with an increased confinement of the proximal domain from the distal one with basin drawdown, favoured the development of depocenters with perennial subaqueous conditions and extreme salinities, in which more than 70 m of tachyhydrite accumulation could locally be preserved. From CVIIIb to CXI, the basin returned to a flat depositional setting without well‐developed depocenters and with increasing subsidence westwards. Marine influx increased starting from CX, allowing the deposition of sulphate beds. The salt section is capped by anhydrite deposits interbedded with clastic and dolomite, before the final marine invasion of the basin. For the first time, this study provides a large‐scale depositional tectonostratigraphic setting of the Aptian salts in the proximal domain of the West African margin. The results are of interest for K‐Mg salts exploration resources in the Aptian and pave the way for further investigation of the salt depositional environment in the distal domain of the margin.

A Geochemical Mechanism for >10 m Apparent Downward Offsets of Magnetic Reversals Inferred From Comparison of Two Scotia Sea Drill Sites

AbstractWe document an apparent downward displacement of the Matuyama‐Brunhes magnetic reversal by ∼20 m at Scotia Sea International Ocean Discovery Program Site U1538 (Pirie Basin) by comparison with the well‐defined paleomagnetic record at nearby Site U1537 (Dove Basin). Detailed stratigraphic correlation between the two sites is possible due to similar lithologic variations. However, the two sites have distinctly different porewater geochemistry. Notably, Site U1538 indicates a greater demand for electron acceptors to oxidize organic carbon and Fe2+ enrichment below the depth of SO42− depletion. Magnetic parameters indicate enrichment of an authigenic magnetic mineral with strong remanence properties around the depth of SO42− depletion (∼46 m at Site U1538) relative to magnetic parameters at correlative depths at Site U1537. Fe2+ enrichment below the depth of SO42− depletion is not predicted based on the energetically favorable order of electron acceptors for microbial respiration but is documented here and in other depositional settings. This indicates Fe2+ production exceeds the production of H2S by SO42− reduction, providing a geochemical environment that favors the production and preservation of ferrimagnetic remanence‐bearing iron sulfides over paramagnetic pyrite and, thus, a mechanism for deep chemical remanent magnetization acquisition at depths of tens of meters. The influence of authigenic ferrimagnetic iron sulfides on paleomagnetic signals can be difficult to demonstrate with magnetic properties alone; therefore, this finding has implications for evaluating the fidelity of magnetostratigraphic records with complementary geochemical data. Such situations should be considered in other depositional environments with similar porewater Fe2+ accumulation below the SO42− reduction depth.

Facies Analysis Using Attribute Seismic With Simply Rock Typing in Field “X” Kutai Basin

Abstract Field X is one of the hydrocarbons producing fields in the Kutai Basin which is one of the largest and deepest tertiary basins producing the second largest hydrocarbon production in Indonesia with 2.47MMBO oil reserves and 28.1 TCF gas. Based on qualitative analysis, the target zone in the four wells is estimated to be within the range of 1050 – 1400 ms. Sensitivity analysis was employed to determine the most sensitive parameters capable of distinguishing lithology within the formation. To ascertain the optimal number of attributes to be used, linear regression was employed, resulting in the selection of 12 attributes with a correlation coefficient of 0.919. Concurrently, log data was utilized for electro facies analysis. Electro facies analysis involved interpreting the shape of the gamma-ray curve, which indicated that the deposition setting in the four wells resembled a delta system. Additionally, rock typing was employed to classify rocks based on wet clay shale parameters, yielding information about the direction and more detailed settings of deposition. The results from the multi-attribute method, rock typing, and electro facies analysis, it was determined that Field X was deposited in a tidal delta to delta front environment, with a deposition direction from North to South.

Tectono-thermal Evolution of the Proterozoic Mahakoshal Belt along the Northern Margin of Central Indian Tectonic Zone: Constraints from Geochemistry, Phase Equilibria Modelling and in-situ Monazite Dating from a Metapelite Sequence around Jabalpur

ABSTRACT In this contribution, geochemical signatures and petrological evolution of a folded sequence of mica schist from the Jabalpur area of the Mahakoshal Belt in the northern Central Indian Tectonic Zone (CITZ) are described. Multiple tectonic discrimination diagrams incorporating both major and trace element concentrations have constrained back-arc settings for deposition of the sedimentary units. S1 foliation formation marks the beginning of the M1 metamorphic stage. In contrast, the later M2 metamorphic event resulted in garnet and andalusite formation during heating at 2-3 kbar, followed by staurolite-sillimanite, garnet-staurolite assemblages at peak P-T conditions of 5.4 kbar, 550-600°C. Textural evidence suggests that M2 is pre-syn tectonic to D2 deformation, resulting in formation of the NE-SW trending S2 axial planar foliation. From the isopleth thermobarometry, a clockwise P-T path with near-isothermal decompression has been determined for M2. Similar results have been obtained from conventional thermo-barometry performed on multiple samples. Monazite dating (EPMA) of one of the samples suggests 1.9 Ga, age for D1-M1 event. M2 is correlated with the younger age population of monazites yielding 1.5 Ga. A clockwise P-T path constrained for M2 suggests collisional tectonics along the northern margin of the CITZ, as recorded from its southern margin.

PALYNOLOGICAL, PALEOENVIRONMENTAL AND PALEOCLIMATE ANALYSES OF EMI-5-WELL, OFFSHORE NIGER DELTA, NIGERIA

Palynological, Paleoenvironmental and Paleoclimatic investigations of Emi-5 well Offshore, Niger Delta, Nigeria was conducted to established palynozones, determine age, reconstruct paleoenvironmental conditions and paleoclimatic conditions to permit stratigraphic correlation, provide insights into the depositional settings and changes overtime within the basin. Fifty one (51) shaly ditch cutting samples were subjected to standard palynological method involving sample maceration with 10 % HCl and 40 % HF acids to remove carbonates and siliceous components, neutralization with distilled water and sieving through 5-micron sieve, followed by separation of palynomorphs from residue prior to mounting of the grains on glass slides for further analysis. The palynological composition is marked by dorminance of pollen and spores over dinoflagellate cysts. The intercalation of the sandy and shaly intervals across the section evidenced from lithologic, textural, and wire line log data suggested that the entire studied interval belong to the Agbada Formation. Three interval range zones were delineated using the international stratigraphic guide for the establishment of biozones, they are Stereisporites sp – Verrutricolporites rotundiporus; Verrutricolporites rotundiporus – Gemmemonoporites sp. and Retistephanocolpites gracilis – Pachydermites diederixi Zones and based on the aforementioned zones the interval is assigned middle Miocene to late Miocene. Coastal-deltaic environment of deposition is inferred for the studied interval on the basis of occurrence and dominance of diagnostic forms such as Zonocostites ramonae,Rhizophora, Psilatricoloporites crasssus and Acrostichum aureum while good representation of these forms and the identified palyno-ecological groups placed the interval on a wet climatic zone.

Ordovician-Silurian trace fossils from the Takche Formation, Parahio Valley, Spiti Himalaya, India: biostratigraphic and paleoenvironmental significance

ABSTRACT The Ordovician-Silurian trace fossils from the Takche Formation exposed near the Gechang locality, Parahio Valley, Spiti region, Himalaya are described. Following trace fossils were found in the Takche Formation: Arenicolites isp. Aulichnites parkerensis, Archaeonassa fossulata, Catenichnus isp. Cochlichnus isp. Cruziana furcifera, Cruziana goldfussi, Cruziana semiplicata, Chondrites targoni, Chondrites isp. A, Chondrites isp B, Helminthodoichnites isp. Lockeia siliquaria, Laevicyclus mongraensis, Planolites isp. Phycodes circinatus, Phycodes rarus, Phycodes isp. Treptichnus pedum, Thalassinoides isp. and Zoophycos isp. The lowermost part of the Takche Formation yielded significant traces of Cruziana affinity i.e. Cruziana furcifera, Cruziana goldfussi, and Cruziana semiplicata suggesting Tremadocian-Darriwilian age (Lower to Middle Ordovician). The middle part of the Takche Formation is dominated by Zoophycos isp. Chondrites targoni, Chondrites isp. A, and Chondrites isp. B suggests a deeper water offshore setting. The uppermost part yielded Archaeonassa fossulata, Aulichnites parkerensis, Laevicyclus mongraensis, Planolites isp. Helminthoidichnites isp. Phycodes rarus, Phycodes isp. and scratch marks. Overall, the trace fossil assemblages indicate a shallow marine shoreface to offshore depositional setting.

Constraining the age of the Pleistocene sedimentary infill of Cueva Mayor (Atapuerca, N Spain) through a multi-technique dating approach

Establishing chronologies of archaeological sites by using a single dating method may not always reliably constrain the age of a deposit or a fossil, as potential biases may naturally arise, particularly in complex sedimentary settings such as caves. In this study, we performed a multi-technique dating approach that targets different materials in two caves from the Cueva Mayor-Cueva del Silo karst system, Sierra de Atapuerca, Spain. Two new fossil teeth from lithostratigraphic unit LU4 (GE-I pit) at Galería de las Estatuas have been dated by combined U-series/ESR and Amino Acid Racemisation (AAR) methods. The former provides ages of 117 ± 13 and 87 ± 9 ka that agree with previous dating studies of this Neandertal site, confirming the assignment of level LU4 to Marine Isotope Stage (MIS) 5. In contrast, AAR analyses produce very scattered ages, pointing to significant post-burial chemical modifications in dentine. A first set of ages is presented for the Cata Litario pit at Sala de los Cíclopes, a palaeontological site formed exclusively of cave bear fossils belonging to the Middle Pleistocene species Ursus deningeri. We provide U-series/ESR and AAR ages for two bear teeth from lithostratigraphic unit LU5, and extended-range luminescence ages for the sediment deposits that host, and immediately underlie and overlie, the bear accumulation. Depositional ages of the sediment were obtained using single-grain thermally-transferred optically stimulated luminescence (TT-OSL) dating of individual quartz grains and post-infrared-infrared stimulated luminescence (pIR-IR) dating of K-feldspars. Several speleothems, both in situ and reworked fragments embedded in the different lithostratigraphic units, have additionally been dated by U-series to constrain the timing of the sequence formation. The results indicate that the sediment originally entered the cave between MIS-12 and MIS-10 (444 ± 28 ka to 367 ± 32 ka). Nevertheless, the AAR and U-series/ESR fossil ages, as well as some of the speleothem fragment ages from LUs 5 and 6 are significantly younger. Leaving aside methodologically questionable results for one of the bear tooth samples, the most reliable AAR and U-Series/ESR ages range from 287 ± 23 ka to 256 ± 24, while the speleothem fragments indicate several formation episodes between 444 ± 19 and 284 ± 8 ka. The apparent difference between the U-Series/ESR ages on the fossils and the luminescence ages on the host sediment are not likely to be wholly explained by uncertainties on uranium uptake modelling or gamma dose rate evaluation, and thus is most likely related to taphonomic processes or a complex site formation history.The presence of fossils and eroded speleothem clasts of younger age within these levels is consistent with either an event of prior erosion, transport and resedimentation of LUs 4, 5 and 6, and/or a complex post-depositional mixing history for the stratigraphic sequence at Cata Litario. This study highlights the importance of using different dating methods to reconstruct reliable chronological frameworks, and to understand the geological factors that can affect the dated materials. This includes the accumulation and taphonomic histories of fossils, post-depositional mixing or reworking processes affecting sediments during the residence time within the karst system.

Salt tectonics synchronous with salt deposition in the Santos Basin (Ariri Formation, Brazil)

Halokinetic deformations synchronous with salt deposition are processes already suggested in several salt giants including the Lower Cretaceous salt deposits of the Santos Basin in Brazil. However, the dynamics of syn-depositional deformation has never been studied in a coherent depositional and structural framework. This study investigates well data and high-resolution 3D seismic images in the evaporitic Ariri Formation of the Santos Basin to understand the intra-salt deformations, estimate their timing and establish a link with the depositional setting of the evaporites. The seismic data shows numerous examples of intra-salt onlaps, erosive surfaces and sedimentary wedges highlighting the occurrence of deformations during salt deposition. Two dominant processes have initiated salt creep: the density contrasts between a dense upper anhydrite-rich unit and a less dense lower halite-dominated unit and the gravitational system promoted by the late-rift subsidence. Updip, extension prevailed during salt deposition and favored a boudinage of the Ariri Formation. Downdip, early contraction favored the development of well-developed intra-salt minibasins. Deformation occurring during salt deposition and induced topographic relief likely influenced the hydrological conditions. It probably led to intra-salt dissolution/reprecipitation processes and hyper-saline conditions favoring the precipitation of tachyhydrite deposits in localized intra-salt depocenters. Eventually, post-salt minibasins systematically developed above syn-salt depocenters, indicating that syn-salt deposition deformation had a strong impact on post-salt evolution. This new understanding of the intra-salt deformation in the Santos Basin paves the way for new interpretations of halokinetic deformations reported elsewhere in the South Atlantic or in other salt-giant provinces.

Heterogeneous Reservoir Petrophysical Property and Controlling Factors in Semi-Restricted Depositional Setting: A Case Study of Yamama Formation, X Oilfield, Middle East

The Early Cretaceous Yamama Formation of X oilfield, deposited in a semi-restricted setting, holds considerable oil reserves. However, the reservoir is extremely heterogeneous and is poorly studied. Integrating outcrops, cores, cast thin sections, regular or special core analysis, wireline logging data from six wells, and seismic data, this study provides an improved understanding of reservoir petrophysical characteristics and geological controlling factors including sedimentation, diagenesis, and sequence. The results showed that eight lithologies are developed in the Yamama Formation, of which packstone and wackstone are dominant. The physical properties span a wide range, with porosity mainly distributed between 10% and 25%, and the permeability mainly distributed between 0.1 mD and 1 mD. Nine types of pores are developed, with moldic pores, micropores, and skeletal pores being the most developed. The reservoir has six types of microstructures, of which the poorly sorted with mega-throat represent the best reservoir. The Yamama Formation was mainly deposited in a lagoon, along with five other facies, such as supratidal flat, patchy reef, back shoal, shoal, and open shelf. Six types of diageneses are developed, with dissolution during the penecontemporaneous stage being the most beneficial to the reservoir and cementation being the most destructive. Three sequences were recognized in the Yamama Formation. It concluded that the hydrodynamics in semi-restricted depositional setting is weak overall and does not have the potential to develop large-scale high-quality reservoirs. A wide range of bioclasts were selectively dissolved to form a large number of secondary pores. Sediments rich in Algae, Bacinella, and peloids tend to form moldic pores, skeletal pores, and intergranular pores, respectively, which are prone to be favorable reservoirs. Controlled by the coupling of sedimentation and diagenesis driven by sequence, the reservoir is extremely heterogenous.

Evolution of a low-energy carbonate ramp, Lower Mississippian Pekisko Formation, northwestern Alberta, Canada

The lithofacies architecture and depositional evolution of the Lower Mississippian (Tournaisian) Pekisko Formation in the subsurface of the Hawk Hills area in northwestern Alberta have been established by integrating detailed core work and well log data. The formation is composed of skeletal-peloidal limestones and argillaceous limestones that were deposited along the northern flank of the Peace River Embayment, a semi-restricted and tectonically active oceanic re-entrant located along the western margin of Laurasia at low paleolatitude. Lithofacies associations recognized in the study area include the outer ramp to slope (LA 1), outer ramp (LA 2) and mid ramp (LA 3), which are stacked into three decameter-scale, deepening-upward and aggradational cycles that are of regional extent and have meter-scale deepening and shallowing-upward trends. A previously unrecognized paleosol horizon at the top of decameter-scale cycle 2, indicating widespread subaerial exposure of the ramp, is interpreted as a sequence boundary that divides the Pekisko and Shunda formations in the study area (and possibly elsewhere in the Peace River Embayment) into two third-order sequences, each consisting of transgressive and highstand systems tracts. The Pekisko Formation in the study area is interpreted to represent a low-energy, temperature-stratified ramp that was mainly homoclinal, but with transient, distal steepening occurring in the southern part of the study area. Ramp deposition was strongly affected by basement-fault reactivation causing differential subsidence and uplift in the Peace River Embayment. The paleogeography and paleoceanographic conditions of the embayment favored upwelling currents and development of a temperature-stratified ramp, as well as the formation of heterozoan carbonate deposits and mid ramp facies of predominantly packstones and wackestones. This depositional scenario is atypical, as most other documented examples of the Pekisko Formation and other Lower Mississippian ramp successions in western North America and western Europe are characterized by moderate to high-energy, mid to inner ramp facies deposited in open-ocean conditions. The results of this study contribute to an improved understanding of the range of depositional settings along the western margin of Laurasia during the Early Mississippian and demonstrate the applicability of the thermocline-stratified ramp model, with some modification, to ramps in semi-restricted embayments and other low energy settings.