Distal Shelf Research Articles

Depositional Architecture of a Neoproterozoic Clastic Shelf: Clue from the Owk Shale Formation, Kurnool Group, India

Abstract Process-based facies and facies association analysis from the Neoproterozoic Owk Shale Formation, Kurnool Group allowed identification of seven facies types, grouped under four facies associations viz. (i) Greenish grey shale without sand interbed, (ii) Grey shale with thin fine sandstone/siltstone interbeds (iii) Grey shale interbedded with HCS-bearing sheet sandstone (iv) Coarsegrained sandstone interbedded with reddish brown shale. A shallow-marine setting ranging from outer shoreface to distal shelf set-up is interpreted. From facies succession analysis, a steady progradation is documented in course of Owk shale depositional history whereby distal shelf shale is successively overlain by the mid, inner shelf shale and finally by outer shoreface succession. Stacked decimeter-thick progradational depositional cycles (demarcated by mid to inner shelf transitions) constitute the progradational depositional motif. The Owk shelf was storm influenced, though from restriction of storm features within the inner shelf deposit, it is argued that no large-scale coastal set-up occurred at the Owk coastline in course of storm events. Instead, a low to moderate offshore-ward pressure gradient is inferred with coriolis effect that possibly hindered active sediment transport in the distal part of the Owk shelf.

Palynofacies analysis and depositional environment of the upper Triassic Kurra Chine Formation: integrated by samples from sarta and Tawke oil fields and outcrop samples from sirwan valley in the kurdistan region, northern Iraq

Palynofacies (PF) analysis was conducted on 34 crushed samples of the Upper Triassic Kurra Chine Formation. These samples were collected from one outcrop sample and two production wells in northern Iraq. A 10μm nylon mesh sieve was used for the concentration of palynomorphs. Four palynofacies types were found in this study. PF1 was characterized primarily by amorphous organic matter (AOM), suggesting a distal suboxic-anoxic depositional environment. PF2 comprised of AOM, along with some Phytoclasts and palynomorphs, indicating a distal dysoxic-oxic shelf depositional environment. PF3 displayed an increasing volume of palynomorphs, signifying a distal dysoxic-anoxic shelf depositional environment. PF4 predominantly consisted of palynomorphs and Phytoclasts, representing a mud-dominated oxic shelf (distal shelf) depositional environment. Kerogen types were identified as type II, which is oil-prone in three palynofacies, and type III which is gas-prone in one palynofacies.

Authigenic clay mineral constraints on spatiotemporal evolution of restricted, evaporitic conditions during deposition of the Ediacaran Doushantuo Formation

The carbonate-rich shelf facies of the Ediacaran Doushantuo Formation (South China) have produced a broad array of microfossil and isotope proxy records which underpin much of our understanding of environmental change and biospheric evolution during this key time period. Recent reports of locally abundant authigenic, Mg-rich saponite clay in the Yangtze Gorges Area hint at potentially widespread evaporitic conditions in a lagoonal setting. Despite the implications for interpretation of proxy records and the environmental setting of early metazoans, the spatiotemporal extent of restricted, evaporitic conditions across the Yangtze Block remains largely unexplored. Here we use mineralogical and petrographic techniques to document the spatial and stratigraphic distribution of authigenic clay minerals across seven sites, representing a transect from shallow proximal shelf to deep basinal settings. Our results demonstrate the widespread and persistent occurrence of authigenic saponite in proximal shelf settings, whereas talc is identified at more distal shelf sites, consistent with Mg-clay authigenesis in an evaporitic lagoon with spatially variable detrital aluminosilicate input. Slope and basinal sites contain no Mg-clays and are instead characterized by abundant authigenic illite, consistent with an open marine setting. Stratigraphically, illite was found in the basal cap carbonate and uppermost Doushantuo Formation, with Mg-clays only present in the interval in-between, suggesting gradual development of an extensive carbonate rimmed lagoon over the Yangtze shelf that restricted seawater exchange, followed by stronger marine influence due to erosion of the carbonate rim and subsequent marine transgression. We conclude that restricted, evaporitic conditions were much more expansive than previously assumed, potentially providing favorable conditions for the Doushantuo Biota. Further, the widespread presence of authigenic clays across the shelf supports the case for enhanced reverse weathering before the advent of biosilicification.

Dominance of Phycosiphon incertum vs Zoophycos in shelf environment: example from the transgressive Palermo Aike black shale deposits of the Austral-Magallanes Basin, Argentina

The Zoophycos ichnofacies and its main components, Zoophycos isp., Phycosiphon incertum and Chondrites isp. have been related to marine environments of low energy and oxygen. This situation is very common in shelf environments of the past, the most important oil source rocks and/or unconventional reservoir. The Early Cretaceous Palermo Aike Formation of the Austral-Magallanes Basin (Southern Argentina) records marine sedimentation, composed mainly by black shales, with subordinate fine-grained sandstones, and redeposited tuff, deposited in a shelf environment. This succession is the main source rock and constitute a latent unconventional reservoir of the basin. In this paper we present the sedimentological and ichnological study of two well cores from the Tierra del Fuego area. These cores record the initial stage of the marine transgression, and the development of a distal shelf environment, with an alternation of black shales and, fine-grained tuff sandstones hyperpycnal deposits. The ichnological content that records this transgression shows three different ichnofacies: i) autochthonous Cruziana ichnofacies represented by Thalassinoides isp., Teichichnus isp., Planolites isp., Chondrites isp. and Rizocorallium isp. This trace fossil association is rapidly replaced by ii) Zoophycos ichnofacies represented by Phycosiphon incertum, Chondrites isp., and small Zoophycos isp. Finally, another allochthonous Cruziana ichofacies is observed related to the low-density turbiditic deposits, composed by Thalassinoides isp., Gyrolithes, Teichichnus isp., Scolicia isp. and Haenzchellinia isp. In the Santa Cruz area, a large Zoophycos isp. dominance is observed in the fine-grained deposits, occasionally interbedded with coarser-grained low-density turbiditic deposits with doomed pioneers. In the study area (Tierra del Fuego), on the contrary, a dominance of Phycosiphon incertum is observed instead of Zoophycos isp. which when is present have very poor development in size. This could be due to the fact that low-density turbiditic flows are very frequent in the study area, which generates a very unstable environment that does not allow the development of large Zoophycos isp. This environmental differences between the outcrops of Santa Cruz in the north of the basin and the subsurface study area in Tierra del Fuego may be triggered due to climatic, volcanic, or tectonic allogenic control factors.

Record of a dense succession of drowning phases in the Alpstein mountains, northeastern Switzerland: Part II—the Lower Cretaceous Schrattenkalk Formation (late Barremian)

The Schrattenkalk Formation represents a complete succession of Lower Cretaceous shallow-water carbonate platform series cropping out in the Alpstein massif of north-eastern Switzerland. The Schrattenkalk Formation is traditionally divided into two sedimentary units, the “Lower” and the “Upper” Schrattenkalk, separated by the more marly Rawil Member. The “Lower” Schrattenkalk is habitually dated to the late Barremian, while the Rawil Member and the “Upper” Schrattenkalk are dated to the early Aptian. New field observations, however, call the lithostratigraphic dichotomy of the Schrattenkalk into question, as the neritic carbonates are disrupted by several key surfaces associated with karstic episodes and/or transgressive sediments, corresponding to ammonite-rich hemipelagic deposits on the distal shelf. A large number of ammonites were collected in the Drusberg Member as well as rare ammonites from the Schrattenkalk Formation. These ammonites as well as the neritic macrofauna from the Schrattenkalk Formation allow a precise dating of the onset of the Schrattenkalk Formation across the Alpstein massif and its successive phases of progradation. Three successive carbonate bodies and a fourth sedimentary intermediate rock body at the top of the Schrattenkalk platform are defined, based on new biostratigraphic data and updated interpretations of the sequence stratigraphy and geochemical data. The data shows a progressive onset of the Schrattenkalk carbonate platform along the studied transect, following a SE progradation over time. The oldest deposits refer to the upper Barremian T. vandenheckii Zone and the youngest carbonates to the uppermost Barremian M. sarasini Subzone. The new dating of the discontinuity surfaces and key-beds highlight three successive flooding events. The first drowning phase, which correlates with the "Sartousiana" event, dates from the middle late Barremian (upper T. vandenheckii—lower G. sartousiana Zone). The second phase, represented by the Rawil Member, is an incipient drowning, which seems to coincide with the latest Barremian Taxy event (usually reported to the I. giraudi and lowermost M. sarasini zones) according to rare ammonite discoveries. The final demise of the Schrattenkalk platform, situated close to the Barremian-Aptian boundary, is related to an exposure and consecutive drowning event.

Stratigraphy and Sedimentary Record of the Ordovician System in Poland: a Review

Abstract This report provides insight into the main stratigraphic and sedimentary aspects of the Ordovician System in major tectonic units of Poland: (1) East European Craton, (2) Pomeranian Block, (3) Łysogóry Block, (4) Małopolska Block, (5) Upper Silesian Block, and (6) the Sudetes. The first studies of the Ordovician rocks in Poland were conducted as early as the second half of the nineteenth century in outcrops of the Holy Cross Mountains (SE Poland). Moreover, the Ordovician rocks are exposed in the Sudetes, where they are incorporated into the tectonostratigraphic units accreted during the Variscan orogeny. Significant progress has been made since the 1950s in the recognition of the Ordovician stratigraphic and facies architecture due to numerous drilling cores, which have been mostly made in NE Poland (East European Craton). The temporal and spatial facies pattern in the Polish part of the East European Craton indicate that in the Early and Middle Ordovician this area was a part of large, epicontinental carbonate platform replaced in the Late Ordovician by fine-grained clastics of distal shelf. The Ordovician sedimentary record in the Łysogóry and Małopolska blocks (SE Poland) reveals numerous similarities to the facies architecture in the neighbouring East European Craton, especially its marginal localities. Thus, it is possible to distinguish the same eustatic events in these units corresponding to sea-level changes reported in Baltica, although locally a regional tectonic activity masked their effects. The evolution of the sedimentary basin in NW Poland (Pomeranian Block) was influenced by the collision of Baltica with Avalonia in the Late Ordovician, which resulted in the development of the Caledonian fold-and-thrust belt.

Palynostratigraphy and paleoenvironments of the upper Turonian–lower Campanian Abderaz Formation in the east of Kopet-Dagh Basin (NE Iran)

The palynology and palynofacies of the Abderaz Formation have been studied to interpret the paleoenvironments and sea-level changes in the Upper Cretaceous deposits of the Kopet-Dagh Basin. This formation in the Kalat-e-Naderi region (Gharesu section) is composed of 462 m of gray to cream shales, marlstone layers and three chalky limestone units. Seven dinoflagellate cyst (dinocysts) and four planktonic foraminiferal biozones have been identified in the studied succession, which suggest a late Turonian–early Campanian age for this interval. Four different depositional environments have been recognized in the studied section based on the relative proportion of organic matter (terrestrial organic matter, marine palynomorphs and amorphous organic matter). High abundance of marine palynomorphs in most samples indicates the predominance of an oxygenated distal shelf environment in the upper Turonian–lower Campanian interval. Seven third-order depositional sequences are proposed for the Abderaz Formation. A good correlation of sequence boundaries with the global sea-level fall events suggests that eustasy played an important role during the deposition of the Abderaz Formation.

Correlative conformity or subtle unconformity? The distal expression of a sequence boundary in the Upper Cretaceous Mancos Shale, Henry Mountains Region, Utah, U.S.A.

ABSTRACT In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity—the seaward extension of a subaerial unconformity. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. An integrated sedimentologic, petrographic, and sequence stratigraphic analysis was conducted to characterize the sequence boundary that separates the Tununk from the Ferron depositional system (hereafter referred to as the T-F sequence boundary) and its lateral along-depositional-strike variability. Although manifest as a mudstone-on-mudstone contact, the T-F sequence boundary in all three measured sections is a subtle unconformity, characterized by erosional truncation below and onlap above, and marks a distinct basinward shift in facies association. The T-F sequence boundary also marks the change from the Tununk offshore mud-belt system to the Ferron Notom delta system, and therefore represents a surface that divides two genetically different depositional systems. Based on two distinct marker beds that bracket the T-F sequence boundary, the T-F sequence boundary can be traced across the study area with confidence. The lateral variability in the characteristics of the T-F sequence boundary along depositional strike indicates that it was produced by an allogenic base-level fall. Offshore shelfal mudstone strata may contain a significantly higher incidence of subtle unconformities analogous to the T-F sequence boundary than currently appreciated. Careful sedimentologic and petrographic analyses, combined with lateral correlations constrained by reliable chronostratigraphic marker beds, are essential for identifying subtle unconformities in shelf mudstone successions. The accurate recognition of subtle unconformities in mudstone strata is critical to apply the sequence stratigraphic approach appropriately to distal shelf environments, as well as to better constrain the timing and cause (allogenic vs. autogenic) of relative changes of sea level recorded in these rocks.

The Chengjiang Biota inhabited a deltaic environment

The Chengjiang Biota is the earliest Phanerozoic soft-bodied fossil assemblage offering the most complete snapshot of Earth’s initial diversification, the Cambrian Explosion. Although palaeobiologic aspects of this biota are well understood, the precise sedimentary environment inhabited by this biota remains debated. Herein, we examine a non-weathered core from the Yu’anshan Formation including the interval preserving the Chengjiang Biota. Our data indicate that the succession was deposited as part of a delta influenced by storm floods (i.e., produced by upstream river floods resulting from ocean storms). Most Chengjiang animals lived in an oxygen and nutrient-rich delta front environment in which unstable salinity and high sedimentation rates were the main stressors. This unexpected finding allows for sophisticated ecological comparisons with other Burgess Shale-type deposits and emphasizes that the long-held view of Burgess Shale-type faunas as snapshots of stable distal shelf and slope communities needs to be revised based on recent sedimentologic advances.

Sedimentology and lithofacies of organic-rich Namurian Shale, Namur Synclinorium and Campine Basin (Belgium and S-Netherlands)

Organic-rich Namurian A mudstone is the main potential shale gas play in Belgium and neighboring countries. In order to reveal the regional depositional setting and its influence on lithofacies and origin of organic matter, this study investigates lithofacies, depositional processes and mechanisms controlling organic matter accumulation based on analysis of sedimentary structures, grain size, mineralogy, optical petrography, pyrite framboid diameter, total organic carbon and gamma log in five wells from Namurian A mudstone in Belgium and the southern part of the Netherlands. These mudstones, located in the Namur Synclinorium and Campine Basin, consist of an organic-rich mudstone succession that was deposited in the epicontinental basin within the Variscan foredeep within a moderate deep water environment below storm wave base in a distal shelf to basin floor setting under mainly euxinic and anoxic conditions with occasional dysoxic and oxic conditions. These sediments were deposited through various processes, including turbidity currents, debris flows, pelagic suspension settling with minor contribution of bottom currents, slumping processes and storm events. Lithofacies stacks generate repeated regressive and transgressive units. In the Campine Basin and the western Namur Synclinorium, 3 regressive units are characterized by basal lithofacies derived from pelagic suspension settling changing upward to lithofacies derived from mainly turbidity currents and debris flows while 2 transgressive units are characterized by basal lithofacies derived from mainly turbidity currents and debris flows changing up to lithofacies from pelagic suspension settling. In contrast in the eastern Namur Synclinorium which reflects a more proximal depositional setting, only 2 regressive units were identified. They are characterized by basal lithofacies derived from turbidity currents and debris flows changing upward to lithofacies from debris flows, bottom currents and slumping processes. 2 transgressive units were also identified. The first unit is characterized by basal breccia from karstification while in the second unit, lithofacies evolve from debris flows and slumping processes changing upward to lithofacies deposited from turbidity currents, debris flows and bottom currents. Overall, carbonate and silica dominated lithofacies from pelagic suspension settling with minor input from debris flows were deposited during high relative sea-level under euxinic to anoxic conditions. Quartz-rich lithofacies from debris flows and turbidity currents with minor input from pelagic suspension settling and bottom currents were deposited during intermediate relative sea-level under anoxic conditions. In contrast, clay, silt and carbonate-rich lithofacies from turbidity currents, debris flows, bottom currents, slumping processes and storm events were deposited during low relative sea-level under anoxic to dysoxic conditions with occasional oxic conditions near the basin margin. The highest total organic carbon observed in lithofacies from intermediate relative sea-level is caused by relative high primary marine productivity with dominant marine organic matter, low siliciclastic dilution and excellent preservation under anoxic conditions. Organic matter accumulation is low in lithofacies deposited during high and low relative sea-level. During high relative sea-level, it was controlled by clay mineral content, organic matter consumption and carbonate dilution, while during low relative sea-level, it was controlled by water redox conditions and siliciclastic dilution.

Palynofacies Analysis of Selected Organic-rich Shales from Libyan Sahara: Implication to Palaeoenvironment and Hydrocarbon Source Rock Potential

Palynofacies analysis was carried out on three (3) packages of shale-cutting samples derived from a well drilled within Gadames Basin. These included: the upper package (Carboniferous at 5,530 ft), the middle package (Carboniferous/Devonian at 5,600 ft and 5,780 ft), and the lower package (Devonian at 6,700 ft). Two palynofacies were identified: "P-1", involving the upper and middle packages (Carboniferous/Devonian), and "P-2", involving the lower package (Devonian). P-1 and P-2 were considered to form one "palynofacies association" located at a mud-dominated oxic shelf (distal shelf) and are analogous to other palynofacies from other countries that are characterized by the dominance of terrestrial upon marine palynomorphs and moderate to low phytoclasts and AOM respectively that typically deposited within a shoreline zone (probably fluvial-deltaic source or marginal marine environment). Spore color index "SCI" evaluation for source rock and thermal maturation, in turn, indicates early mature oil/gas prone (kerogen type II>III) to post mature kerogen type III>IV "gas prone" for the studied cutting sample packages.

Paleoenvironmental and paleoclimatic changes during the Late Cretaceous and Cretaceous–Paleogene (K/Pg) boundary transition in Tattofte, External Rif, northwestern Morocco: implications from dinoflagellate cysts and palynofacies

This paper presents a quantitative study of dinoflagellate cysts (dinocysts) and palynofacies of the Campanian–Danian marly succession at the village of Tattofte (western External Rif, northwestern Morocco). The paleoenvironmental and paleoclimatic interpretations, inferred from this palynologic analysis, are compared to coeval sequences of other areas in the Northern Hemisphere. Changes in the relative abundances of dinocyst taxa, which are paleoenvironmental indicators, throughout the section suggest a deposition in a marine inner to outer neritic setting. The upper Campanian dinocyst assemblage is characterized by the presence of outer neritic-oceanic and low productivity indicator taxa (e.g., Spiniferites spp., Odontochitina spp.) and cold-water taxa (e.g., Manumiella spp., Chantangiella spp., Laciniadinium spp.), whereas, the lower Maastrichtian assemblage is characterized by inner neritic, high productivity and warm-water indicator taxa (e.g., Palaeocystodinium spp., Andalusiella spp.). The upper Maastrichtian dinocyst assemblage displays a return to an outer neritic environment under a transgressive regime, but with a cooling pulse and a moderate productivity. However, the Cretaceous–Paleogene (K/Pg) boundary interval records remarkable changes in the relative abundances of dinocyst taxa, indicating an inner neritic (coastal) setting, which is the most proximal in the study section; such changes reflect instable paleoenvironmental conditions which may be related to global cooling periods, likely caused by the Deccan volcanism in India and/or the Chicxulub asteroid impact in Mexico at the K/Pg boundary. In the Danian, the dinocyst relative abundances indicate a gradual return to stable environmental conditions.A quantitative analysis of the kerogen plots (palynomorphs, phytoclasts and amorphous organic matter (AOM)) reveals five types of palynofacies, generally indicating oxic to suboxic marine environments. The upper Campanian and upper Maastrichtian (lowermost part) strata are characterized by a playnofacies (V), indicating a distal shelf, while the lower Maastrichtian and lower Danian (uppermost part) strata record a playnofacies (III), reflecting an intermediate inner-outer neritic environment. However, the K/Pg boundary transition is characterized by playnofacies types (I) and (II), indicating a proximal (coastal) environment.

Provenance of northern Gondwana Lower Ordovician sandstones (Khabour Formation, northern Iraq) revealed by detrital zircon using LA‐ICP‐MS dating

We present new zircon U/Pb provenance data using laser ablation induced coupled plasma mass spectroscopy (LA‐ICP‐MS) for eight samples of mature quartz arenites and subarkosic sandstones of the Lower Ordovician Khabour Formation, northern Iraq, to establish their depositional age and provenance. The samples were collected from thin‐ to medium‐bedded sandstones along profiles of the Chalky Nasara and Ora sections. Most analysed detrital zircon analyses are concordant within 100 ± 10% and include a prevalence of Proterozoic over Archean ages. Collectively, the data can be divided into four main age groups: (a) Neoproterozoic ages, 570–1,000 Ma (58%); (b) Mesoproterozoic ages, 1,000–1,500 Ma (10%); (c) Palaeoproterozoic ages, 1,620–2,480 Ma (21%); and (d) Archean ages, 2,500–3,300 Ma (11%). The predominance of Cryogenian–Ediacaran ages (850–550 Ma) reflects major magmatic events associated with the assembly of northern Gondwana. The major age group at ~600 Ma likely represents the unroofing of late‐stage granitic rocks from the Arabian‐Nubian Shield. The lack of zircons younger than 490 Ma suggests that Lower Ordovician sandstones in northern Iraq were probably deposited on the distal shelf of the northern Gondwana passive margin and were not influenced by rifting and magmatism in northern Gondwana caused by the opening of Palaeo‐Tethys. Diagenetic xenotime overgrowths are common on detrital zircon grains, but radiometric dating of these was not possible.

Effects of lithofacies on pore system evolution of storm-wave silt-rich fine-grained sediments. Early Silurian Qusaiba Member (Qaliba Formation), NW Saudi Arabia

Storm-wave sediments are considered a potential unconventional resource in the Middle East region, i.e., the early Silurian Qusaiba Member of the Qaliba Formation in Saudi Arabia. Extremely few studies have examined the controls of lithofacies heterogeneity on pore types, sizes, distribution, and connectivity. Therefore, this study intended to illustrate lithofacies controls on pore systems using various integrated approaches. Based on lithofacies analysis, seven lithofacies were identified and interpreted as storm-wave-dominated distal, medial, and proximal shelf deposits. The results of this study reveal two main types of pore systems: interparticle and intraparticle pores, which have been thoroughly recongized across all recognized lithofacies. Nuclear magnetic resonance (NMR) data showed positively and negatively skewed bimodal porosity distribution curves in the proximal and distal shelf lithofacies, respectively; however, the NMR data of medial shelf lithofacies were found to be normally distributed with unimodal distribution curves. Phyllosilicate minerals associated with interparticle pores (≤1 μm) were observed in the distal lithofacies; however, larger-scale interparticle pores were observed in both medial and proximal lithofacies. Such variation was also reflected in the NMR curves, resulting in negatively and positively skewed distribution curves of T2. Diagenetic facies (i.e., calcite, phosphate, pyrite, iron oxides, and clay minerals cementations) occurred during syndepositional, early, and intermediate burial stages before oil generation window. This study facilitates the understanding of the impacts of lithofacies and diagenetic facies on the pore system, and, the petrophysical characteristics of mudrocks in similar depostional settings.

Depositional environment and source rock quality of the Woodbine and Eagle Ford Groups, southern East Texas (Brazos) Basin: An integrated geochemical, sequence stratigraphic, and petrographic approach

X-ray fluorescence chemostratigraphy of the Cenomanian–Turonian Woodbine and Eagle Ford Groups in the southern parts of the East Texas Basin highlights significant mudstone chemical heterogeneities that commonly are difficult to observe or quantify at the macroscale. Several key elements, namely, Ca, Si, Mo, Mn, and Ni, were correlated to depositional conditions and used in a hierarchical cluster analysis to characterize five chemical facies (i.e., “chemofacies”) across 10 cored intervals of the Woodbine and Eagle Ford Groups: (1) argillaceous, organic-matter poor; (2) transitional, organic-matter poor; (3) transitional, organic-matter moderate; (4) calcareous, organic-matter rich; and (5) calcareous, organic-matter moderate. Characterizations of organic matter richness, mineralogy, and environmental conditions of deposition were established by correlating between key element abundances, total organic carbon measurements, x-ray diffraction measurements, and petrographic observations of lithologic composition, bioturbation, and sedimentary textures. Combined analysis of elemental geochemistry, stratigraphy, and petrographically observed sedimentary textures indicates that all chemofacies were deposited in an intrashelf basin above storm wave base. The most organic-rich chemofacies was deposited on a dominantly dysoxic distal shelf. Mudstone organic matter enrichment is driven dominantly by the minimization of siliciclastic dilution and secondarily enhanced by oxygen restriction. Regional correlations of chemofacies within a sequence stratigraphic framework developed from previous outcrop and subsurface work indicate a clear relationship between interpreted stratigraphy and chemofacies deposition. Generally, the highstand sequence sets of the Woodbine Group and upper Eagle Ford formation are dominated by mineralogically clay-rich, organic matter–lean, siliciclastic sedimentation and contain poor-quality source rock. Conversely, the transgressive sequence set of the lower Eagle Ford formation is dominated by organic matter–rich pelagic carbonate accumulation and contains excellent-quality source rock.

Palynostratigraphic and palynofacies analysis of x and y wells, offshore Niger Delta, Nigeria

A detailed palynostratigraphic and palynofacies study was carried out on two shallow offshore wells, codenamed Well X and Well Y, from the Niger Delta Basin, Nigeria. The study is aimed at establishing the age and palaeoenvironment of the sedimentary sequences penetrated by the wells. Standard palynological/palynofacies laboratory procedures were used to free the palynomorphs and palynomacerals from the embedding matrices.Prepared palynological slides were studied using the Leitz Ortholux II transmitted light microscope. The palynofloral assemblages recorded from the two well sequences include well-preserved palynomorphs such as Zonocostites ramonae, Monoporites annulatus, Retitricolporites irregularis, Cyperaceaepollis sp., Echitricolporites spinosus and Nymphaeapollis clarus among others. The recognition of diagnostic pollen of Cyperaceaepollis sp. and Nymphaeapollis clarus as well as the associated palynomorph assemblage aided the delineation of the well sequences into the P830, P840-P850 and P860 subzones of the P800 palynological zone in the late Miocene. The relative abundance of particulate organic matter (Amorphous Organic Matter (AOM), Phytoclasts and Palynomorphs) revealed the delineation of three palynofacies assemblages, PF-A, PF-B and PF-C, suggesting three distinct paleoenvironments of deposition. These paleoenvironments range from distal shelf through marginal to proximal with varying conditions from oxic, suboxic to anoxic, thus suggesting marginal marine to coastal deltaic environments of deposition for the sedimentary sequences penetrated by the two wells.
 Keywords: Late Miocene, Niger Delta, Paleoenvironment, Palynostratigraphy, Palynofacies.

A fine detail physico-chemical depositional model for Devonian organic-rich mudstones: A petrographic study of the Hare Indian and Canol Formations, Central Mackenzie Valley, Northwest Territories

The Hare Indian and Canol Formations, which are part of the Horn River Group in the Northwest Territories, Canada, primarily consist of organic-rich mudstones deposited during the Middle to Late Devonian. The formations were previously considered to represent marine basin fill accumulated in an oxygen-starved distal shelf setting, evidenced by the organic-rich character, pyrite content, and lack of macro-scale bioturbation. The depositional model, paleo‑oxygenation interpretations, and methods of organic carbon preservation presented in this study are in contrast to previous assumptions of the Horn River Group mudstones. Detailed petrographic, sedimentological, and ichnological analyses were carried out on thin sections taken from several cored Horn River Group intervals. These organic-rich mudstone units contain eight distinct microfacies, representing four main sedimentation processes acting on an interpreted distal shelf setting: (1) pelagic suspension settling, (2) plug-like sediment-gravity flows, (3) surge and surge-like low-density turbidity currents, and (4) debrites. Pelagic suspension-settling dominated in distal, quiet waters out of the reach of persistent storm influence. Debrites, plug-like flows, and low-density turbidite processes represent a continuum, wherein storm influence was the dominant driver in sediment delivery. Several morphologically distinct, microscopic biogenic-sedimentary structures (i.e. ichnofossils) have been identified throughout the mudstone intervals, indicative of sediment pore waters that were at least periodically partially oxygenated. Evaluation of total organic carbon content against bioturbation and microfacies interpretation suggest that persistent anoxia was not the dominant factor in organic carbon preservation, but rather a result of a combination of heightened sedimentation and burial rates and possible amplified rates of primary productivity. The results of this study may be applied to evaluations of other organic rich mudstones to enhance paleo-depositional interpretations.

Shales of Palaeo-Mesoproterozoic Vindhyan Basin, central India: insight into sedimentation dynamics of Proterozoic shelf

AbstractThe Vindhyan Supergroup represents the largest Proterozoic sedimentary basin fill in the Indian shield. In addition to some significant palaeobiological discoveries, the sedimentary sequence of the Vindhyan, particularly its argillaceous intervals, holds crucial information for our understanding of sedimentation dynamics in Proterozoic clastic shelves. Here we attempt an extensive, although not exhaustive, review of the physical characteristics of six argillaceous (shale) intervals (Arangi, Koldaha, Rampur, Bijaygarh, Rewa and Sirbu shale) from the Son valley sector, Vindhyan Basin, augmented with new observations to unravel the status of current understanding in terms of palaeo-flow dynamics, shelf sedimentation processes and dispersal pattern, depositional cyclicity and basinal tectonics. The sedimentary attributes of Vindhyan shales reveal their deposition largely in relative bathymetry fluctuating from distal shoreface or inner shelf (near to fair-weather wave base) to distal shelf below storm wave base. More often than not, the Vindhyan shelf was storm-infested and the operation of both storm-generated return flow and Coriolis-force-guided geostrophic currents are documented from different stratigraphic intervals of argillaceous successions. The thick arenaceous intervals interrupting the deposits of the Koldaha, Rewa and Sirbu shales at multiple stratigraphic levels indicate the presence of a fan delta and braided fluvial system during intermittent regressive stands of sea level or event deposition during a sea-level highstand, respectively. Based on facies pattern and flow vectors, a rift-related half-graben model is inferred for Arangi and Koldaha shale and a low-gradient stable-shelf model with well-defined energy gradient is proposed for successions from Rampur shale onwards.

Evidence of recent flood deposits within a distal shelf depocenter and implications for terrestrial carbon preservation in non-deltaic shelf settings

Extreme episodic events have the capacity to transport large amounts of terrestrial material to the coastal ocean. While estuaries and deltas are typically thought to trap most of this material, some escapes these coastal features and is transported to distal depocenters along the continental shelf. Distal shelf depocenters can act as the ultimate sink for event deposits, yet event sedimentation and the impact of terrestrial carbon burial within these shelf deposits remains understudied. Therefore, this study investigated event sedimentation within the Texas Mud Blanket (TMB), a distal shelf depocenter of the Brazos River located in the northwestern Gulf of Mexico. Using a core that was collected from the TMB during a months-long elevated discharge event in late 2015 to early 2016 we applied a multi-proxy approach to delineate the event deposit. We utilized physical sediment properties (fabric, texture, and bulk density), radioisotopes (7Be and 210Pb), and organic geochemical properties (ẟ13C, C/N, and lignin-phenols) to identify a 10 cm event layer at the surface of the core. Within this 10 cm layer, we also distinguished two sub-layers associated with separate extreme discharge periods in the winter and spring, respectively. Using these event layer characteristics, we identified three other earlier potential event deposits in the core that we have attributed to events in 2010, 2007, and 2000–2001. From these results, this study has demonstrated that the TMB may be an archive for event deposits over multi-decadal time periods, in particular the transition from periods of prolonged drought to extreme wetness associated with the Brazos River. This event-dominated sedimentation regime can enhance long-term carbon burial, potentially increasing current estimates of carbon burial significantly in the TMB and other similar distal shelf depocenters. Ultimately this study highlights the potential for distal shelf depocenters as organic carbon burial “hotspots,” and postulates that this role may expand in the future as climate change increases the intensity and frequency of extreme events.

New dentognathic fossils of Noropithecus bulukensis (Primates, Victoriapithecidae) from the late Early Miocene of Buluk, Kenya

The late Early Miocene site of Buluk, Kenya, has yielded fossil remains of several catarrhine primates, including 16 dentognathic specimens of the stem cercopithecoid Noropithecus bulukensis. With the exception of the large sample of Victoriapithecus macinnesi from the middle Miocene of Maboko Island, Kenya, the majority of stem cercopithecoid taxa are represented by small sample sizes. We describe and analyze 91 new cercopithecoid fossils collected from Buluk between 2004 and 2018, including several previously undescribed tooth positions for N. bulukensis, and provide the first evaluation of dental metric and morphological variation in this sample. The results show that the expanded Buluk sample exhibits high levels of dental variation in the postcanine tooth row, similar to V. macinnesi at Maboko, but this variation is consistent with a single-species hypothesis. Subtle differences in the shape of the I1, breadth of the C1 and P3, relative breadth of M1, upper and lower molar distal shelf lengths, the degree of M2 basal flare, and a less-developed lower molar distal lophid differentiate the dentition of N. bulukensis from V. macinnesi. Although differences exist between the N. bulukensis and V. macinnesi dental samples, the high degree of variation within each sample complicates the identification of many individual specimens. New partial maxillae and mandibles allow reassessment of previously described diagnostic differences between N. bulukensis and V. macinnesi, negating upper molar arcade shape as a diagnostic feature and confirming the existence of differences in mandibular symphyseal morphology. Overall, new fossils from Buluk provide new evidence of the dentognathic anatomy of a medium-sized cercopithecoid that coexisted with a diverse group of noncercopithecoid catarrhines at the end of the early Miocene.