ABSTRACT Presence of an A horizon in paleosols is important for taxonomic classification, interpreting pedogenic processes, and reconstructing paleoenvironments. However, identifying the A horizon in buried contexts is challenging because of potential erosion during burial and post-burial diagenesis. Thus, 745 master horizons from 299 training pedons were analyzed to develop indicators for identifying buried A horizons and non-A horizons. Indicators of A horizons include < 12% organic carbon or < 24% LOI, ped diameters ≤ 1 cm, platy peds, or > 2% root traces in sandy substrates. Non-A horizon indicators represent properties of the O, E, transitional (e.g., AB), B, or C horizons. Together, the indicators yield the highest proportion of true positives for classification of known A horizons and true negatives (TN) for classification of known non-A horizons in the database (∼ 80 to 90% success rate). Confusion-matrix statistics show that the classification performance of the indicators are optimized when considering sensitivity, specificity, precision, and accuracy. In addition, there were no significant differences (z-test) in the performance of the indicators between the training pedons and 30 validation pedons. Testing the indicators on buried Quaternary paleosol sequences demonstrates that most A horizons survive the burial process in alluvial settings, giving confidence in their potential preservation in lithified paleosols of deeply buried contexts. Applying the indicators to selected Quaternary and lithified Miocene through Permian buried paleosols with previously assigned master horizons altered some pedogenic and climate interpretations. Examples are given of problematic buried paleosols for further testing of the indicators.
Abstract Stromatolite and thrombolite are two major categories of microbial carbonate distinguished by macrofabric: laminated in stromatolite and clotted in thrombolite. Since these categories are defined at hand-specimen scale, their distinction is inherently resolution-dependent. Restudy of Aitken’s (1967) typical thrombolite, from the Cambrian of Canada, shows that—in thin section—the individual microbial carbonate clots are delicately laminated. Cambrian thrombolites with similar indistinctly laminated clots exist in China and the USA. Similar, but generally larger and distinctly laminated, ministromatolites (small stromatolites, ≤ 20 mm wide) are common in the Proterozoic. They appear to be generally scarce in the Phanerozoic but occur locally in Holocene freshwater streams and lakes. Clots in present-day near-sea domes and columns at Lake Clifton, Australia, widely regarded as thrombolite analogs and formed by calcified cyanobacterial colonies, closely resemble Aitken’s Cambrian example; their initial lamination can be obliterated during early diagenesis. These observations emphasize that the distinction between thrombolite and stromatolite is based on human perception at hand-specimen scale. We propose that fluctuating decline in marine microbial calcification during the Proterozoic–Phanerozoic transformed distinctly laminated ministromatolites into smaller forms whose lamination is too delicate to be seen by the naked eye—ministromatolite “ghosts”—and that these form the clots in Aitken’s typical Cambrian thrombolite. Proterozoic ministromatolites, Cambrian thrombolites, and their present-day nonmarine analogs, reflect an ∼ 2.5 Gyr history of fluctuating marine calcification in microbial, most likely cyanobacterial, colonies.
ABSTRACT Huguangyan Maar Lake (HML) has long been recognized as a critical repository for reconstructing paleoenvironmental changes in southern China. However, interpretations regarding the dynamics of the Asian monsoon system derived from this site have been diverse and, at times, contradictory. To better understand the sediment source-to-sink processes and associated magnetic characteristics, this study investigates grain-size and rock-magnetic properties based on 56 lake-surface sediments from various water depths, as well as 23 topsoil and 4 volcanic rock samples from the HML catchment. Our findings reveal that the lake sediments are sourced predominantly from the surrounding catchment and transported into the lake via surface runoff. The lake sediment consists mainly of silt and sand, with its distribution decreasing toward the lake center and being strongly influenced by water depth. However, the relationship between grain size and water depth is complex, with a threshold depth of about five meters marking the effective limit of wave-induced sediment reworking. Lake sediment magnetic minerals consist mainly of stable single-domain and superparamagnetic magnetite, like those in local topsoil and volcanic rocks, but different from winter monsoon-transported eolian dust (Chinese loess). The concentration of magnetic minerals is largely source-dependent, with higher values observed near magnetite-rich coastal zones. Additionally, water depth exerts a notable influence on it, likely due to greater transport distance from source areas and dissolution under anoxic conditions. The magnetic record in HML sediments likely reflects intensity variations in the Asian summer monsoon, which controls weathered material and organic-matter supply, as well as water depth.
ABSTRACT Understanding the subaerial dispersal of desert sediments along a downwind aridity gradient is challenging but crucial for promising sustainability in habitable semiarid boundary regions. The Thar Desert of Rajasthan (India) emits copious fine dust towards the Himalayas before the onset of the Indian Summer Monsoon every year. However, the climatic influence (especially of eolian processes and aridity conditions) on the provenance characteristics, downwind dispersal, and postdepositional alteration of sandy sediments across the desert is poorly understood. This knowledge is essential for reliable provenance tracing in arid and semiarid environments. For this purpose, 52 surface sediment samples, spanning the hyperarid southwestern area to the semiarid eastern and northeastern fringes of Rajasthan, have been characterized for geochemical, isotope, mineralogical, and grain-size data. The measured properties reveal a consistent upwind provenance of sandy sediments across the desert, as well as the variable impacts of wind forcing, elevation, and aridity conditions. The Thar sediment signatures from upwind Jaisalmer and Barmer districts can be reliably traced at downwind locations in the Jhunjhunu, Churu, and Jaipur districts. The dune-profile sediments from Jhunjhunu and Rajgarh (Churu), located in the northeastern semiarid fringes, are more weathered chemically and show higher Zr abundances and lower εNd than the other samples. Notably, higher 87Sr/86Sr and quartz enrichment are found in fine sand, especially from the Aravalli highlands. The plausible reason for the grain-size variability of Sr isotopes seems to be K-feldspar or mica preferentially retained in fine sand delivered to highlands under stormy conditions. This study highlights the widespread eolian transport of sandy Thar sediments to downwind locations at distances of ∼ 400–500 km. The widespread Thar sediment dispersal is also accompanied by spatially variable grain size, mineral sorting, and postdepositional chemical weathering under favorable conditions. The modification in geochemical and isotope compositions of widespread desert sediments owing to spatially varying sedimentary processes needs to be accounted for in future provenance studies of downwind eolian sediments.
ABSTRACT This study aims to reconstruct the Triassic paleogeographic setting and refine the chronostratigraphic framework of the Permo-Triassic of the Northern North Sea (NNS) basin though time, using new palynological data, wireline-logs, and core data. Four age-constrained mudstone units and five sandstone units are defined in the Triassic succession. Sedimentological analysis indicates that these units are composed of three facies associations: channel-fill (CFA), splay (SFA), and floodplain (FFA). Using petrophysical signatures from cores calibrated to wireline logs, proportions of facies association are estimated for each age-constrained unit. During sandstone-unit development, the Horda Platform and parts of the Tampen Spur area, have high (> 75%) CFA percentages with single-story channel bodies > 12 m in thickness with an average grain size of medium sand. During deposition of the mudstone units, the entire basin is dominated by high floodplain (> 75%) and splay facies proportions (15–20%), while channel deposits are very limited (< 10%). Variabilities in CFA content, channel thickness, and grain size suggest that the Triassic paleogeographic setting represents a 160-km-wide, distributive fluvial system (DFS) that is soured from the Norwegian mainland. Comparisons between our refined Triassic NNS framework with the UK Central North Sea chronostratigraphic framework may set the foundation for a unified Triassic North Sea stratigraphic scheme, facilitating regional cross-border correlations. A comprehensive Triassic paleographic reconstruction of the NNS provides information on the spatial and temporal distribution of sedimentary facies and enhances predictions of future reservoir distribution.
Abstract This study presents the first radioisotope ages of tephra deposits interbedded with the carbonate succession of the Cabo de Gata Range in SE Spain. These carbonate rocks are a reference section of the Miocene sedimentary record for the western Mediterranean, which is marked by the widespread development of coral reefs dominated by the genus Porites. Although Porites reefs at Cabo de Gata, and across the western Mediterranean, have been assigned to the late Tortonian–Messinian, two new 40Ar/39Ar ages (11.71 ± 0.11 Ma just below the base and 11.64 ± 0.18 Ma in the Porites interval) constrain their formation in the type section of La Molatilla–La Molata to the terminal Serravallian–earliest Tortonian. The radioisotope dating of an additional tephra layer interbedded with coastal oyster- and bryozoan-bearing calcirudites, which overlie the volcanic basement in the Los Escullos area and were previously assigned to the late Tortonian, yields an earliest Tortonian age of 11.29 ± 0.03 Ma. The Los Escullos tephra lies c. 4 m above sea level and about 100 m below terminal Serravallian tephra at La Molatilla–La Molata, suggesting a major sea-level drop of tens of meters during the earliest Tortonian. These findings highlight a more complex Miocene chronostratigraphy in the Cabo de Gata than previously recognized, and indicates that lithostratigraphic units with similar facies and comparable stratigraphic positions, such as skeletal calcarenites and calcirudites overlying the volcanic basement or Porites reefs, may vary significantly in age here and across the western Mediterranean.
ABSTRACT Spaced stratification is a sedimentary structure consisting of multiple inversely graded bands, traditionally interpreted as traction-carpet deposits formed under high-concentration sediment gravity flows. Although widely reported from coarse-grained sandstones deposited by deep-sea turbidity currents, pyroclastic density currents, and hyperconcentrated flows, its formation mechanism remains controversial. Competing hypotheses—ranging from freezing of traction carpets to bedform migration and turbulent sweep–fallout cycles—have lacked quantitative validation from natural deposits. To address this, we conducted a microtextural analysis of spaced stratification in ancient turbidites to clarify its internal variability and depositional implications. We analyzed coarse-grained sandstone samples from the Upper Cretaceous Izumi Group (southwest Japan) using a convolutional-neural-network (CNN)-based semantic segmentation model. The model automatically distinguished grains from the matrix in cross-sectional images with 78.7% accuracy at a resolution of 5 µm per pixel, enabling high-throughput quantification of grain size and orientation. Multivariate analyses of the measured grain data reveal two distinct types of inversely graded bands: 1) Type A, characterized by coarser mean grain size, bimodal distributions of imbrication angles, and pronounced internal variability and 2) Type B, composed of finer grains with unimodal, upstream-dipping imbrication and more uniform textures. These results demonstrate that deposits previously grouped under spaced stratification actually encompass at least two microstructurally distinct types that are likely formed through different depositional dynamics. The CNN-based image analysis provides a new methodological framework for objective, quantitative characterization of sedimentary microtextures at high spatial resolution. Further experimental and numerical studies replicating these two types of banded structures under controlled flow conditions will be essential to constrain their formation processes and refine paleoenvironmental interpretations of high-concentration deposits of sediment gravity flows.
ABSTRACT The temperature of oil expulsion from source rocks is one of the central questions of petroleum geology. Many differing opinions regarding what this temperature or temperature range may be are presented in the literature, and a consensus has not been reached. The present study addresses the topic through detailed petrographic investigation of oil inclusions trapped in carbonate cements, in the dust rims beneath quartz overgrowths, and in albitized plagioclase in the quartzose reservoir sandstones of the Norwegian sector of the North Sea. Previous studies show that the temperature of albitization in the area is approximately 88°C, and that quartz overgrowths start forming at around 80°C. Moreover, pervasive carbonate cement enclosing quartz grains without quartz overgrowths must have formed at temperatures less than the temperature of initial quartz cementation. Regardless of burial depth, most of the studied reservoirs currently filled by oil or wet gas contain oil inclusions in one, two, or all three of the mentioned diagenetic cements where formation temperature is high enough for such cements to be present. This clearly suggests that the temperature of oil filling in North Sea reservoirs is normally less than 80–90°C. Reservoir temperatures of less than 80–90°C were commonly reached in Cretaceous times when the study area’s Upper Jurassic source rocks were not deeply buried, implying temperatures of oil expulsion less than 100°C, possibly considerably less. Also, some of the studied oil-filled sandstones are encased in the Upper Jurassic source rocks, indicating little difference between temperatures of oil expulsion and temperatures of oil filling. The presence of oil inclusions in pre-quartz overgrowth calcite and dolomite cements or in the dust rims beneath quartz overgrowths in almost all these intra-source-rock reservoirs therefore suggests oil expulsion at not more than 80°C. The oil-inclusion data from the North Sea thus point to oil expulsion from source rocks being essentially complete at temperatures of 80–100°C, and the available evidence does not exclude lower expulsion temperatures. This further suggests that oil expulsion has taken place earlier and over a much larger area than normally assumed, and that the need for long-distance oil migration has been exaggerated. It seems reasonable to infer that this will also apply to many other basins.
The middle-upper Eocene to Lower Miocene sedimentary successions of the southwestern Colombian Caribbean comprise coarse- and fine-grained deposits formed within fluvio-deltaic systems developed in a forearc basin. Integrated sedimentological, ichnological, well-log, and micropaleontological analysis of outcrops and well cores from the San Jacinto Fold Belt allowed recognition of twelve facies associations and reconstruction of the spatiotemporal evolution of deltaic bodies and their stacking patterns. Middle-late Eocene deposits record coarse-grained systems dominated by hyperconcentrated and hyperpycnal flows sourced from nearby continental uplifts and reaching the coastline. Amalgamated mouth-bar successions indicate high-energy, fluvial-dominated deltas formed under limited accommodation space and poorly developed delta plains. Ichnological and sedimentological indicators, including Ophiomorpha, fluid muds, and mud drapes, document episodic marine influence linked to short-lived tidal or wave reworking during phases of reduced sediment discharge or distributary avulsion. During the earliest Oligocene, fine-grained transgressive deposits locally overlie Eocene successions despite global sea-level fall, suggesting accommodation generation driven primarily by localized tectonic subsidence within an actively deforming forearc basin. The appearance of meandering fluvial deposits, rhizolith-bearing mudrocks, fungal remains, and abundant morichal palm pollen indicates the establishment of water-logged interdistributary bays and gallery flood forests, marking the development of an expanded lower delta plain during the Eocene–Oligocene transition. Vertical changes in depositional style occurred during this interval of significant climatic and eustatic fluctuations, although local tectonic activity likely exerted the dominant control on accommodation. From the Oligocene to Early Miocene, sedimentation was characterized by repetitive coarsening-upward successions interpreted as progradational deltaic cycles dominated by fluvial processes under relatively high accommodation-to-supply conditions. Deltaic architecture reflects hyperpycnite coalescence and distributary-mouth-bar migration, with ichnological indicators (e.g., tubular tidalites and burrow size) and sedimentological features (fluid muds and mud drapes) recording short-lived tidal or wave modulation controlled by basin morphology. Increased accommodation space is interpreted to relate to fault-controlled subsidence associated with regional tectono-stratigraphic reorganization of the margin. Overall, the study demonstrates that tropical forearc deltas of the Colombian Caribbean were primarily controlled by tectonically driven accommodation and episodic hyperpycnal sediment delivery, whereas marine processes exerted only intermittent influence. These results suggest that tropical fluvio-deltaic systems in northern South America may behave more similarly to tectonically forced high- and mid-latitude systems than to monsoon-dominated equatorial deltas, emphasizing the dominant role of tectonics and orography over climate alone in shaping deltaic evolution.
ABSTRACT Eolian successions are promising targets for geologic carbon storage, often comprising thick, laterally extensive sandstone bodies with high porosity and permeability. However, their effective use as subsurface reservoirs requires a thorough understanding of their stratigraphic complexity and the controls it exerts on reservoir heterogeneity. This study presents a detailed evaluation of the sedimentology, stratigraphy, and reservoir character of the wet eolian Jurassic Entrada Sandstone. Seven stratigraphic sections were measured and correlated in the vicinity of Dinosaur National Monument in northeastern Utah, USA, including 6 sections across a 3.4 km transect. A drone-based photogrammetric model near the seventh section enabled high-resolution visualization and quantification of architectural elements over a horizontal distance of roughly 160 meters. Although the study area does not encompass the full spatial extent of a typical CO2 storage reservoir, the stratigraphic relationships documented here provide insight into how wet eolian heterogeneity may influence subsurface flow. Four sandstone architectural elements were identified: large compound dunes (4–13 m thick), small compound dunes (0.6–4 m), simple dunes (0.3–1.4 m), and damp sand flats (0.2–5.5 m). These elements are organized into alternating dune and interdune–flat packages that are laterally continuous, vary in thickness, and show more complex facies distributions than previously recognized. Porosity and permeability data were collected for each element using thin sections, minipermeametry, and core-plug analyses. Despite significant variability according to methodology, porosity and permeability values suggest higher values for dunes compared to damp sand-flat deposits. Should these trends hold across scales, dune elements may represent primary flow pathways, while interdune sand flats are likely to serve as baffles to flow, resulting in a horizontally layered reservoir architecture with restricted vertical connectivity. Reservoir layering is further enhanced by lower-permeability bounding surfaces in cross-bed sets and grain-size variability at the bed and lamina scale. These sources of heterogeneity may promote reservoir compartmentalization and have important implications for subsurface modeling in wet eolian systems. This study highlights the sedimentological controls on reservoir heterogeneity in the Entrada Sandstone and provides an analogue for evaluating eolian reservoirs for carbon storage applications.
ABSTRACT The sedimentary record of late Paleozoic glaciation is composed mainly of glacially influenced subaqueous deposits, including ice-rafted debris (IRD). IRD is particularly abundant in the Campo do Tenente Formation, the basal to middle part of the Itararé Group (Paraná Basin, Brazil), and appears predominantly in the form of pellets. Despite the importance of IRD as a significant component of the geological record of the Late Paleozoic Ice Age (LPIA) and its potential as a paleoclimatic proxy, evidence supporting its origin from seasonal sea and/or lake ice versus icebergs remains limited. This study aims to address this problem by characterizing the micromorphology of 257 pellets derived from three distinct facies of the Campo do Tenente Formation in order to investigate formative mechanisms that could help to define the rafting agent. Microscopic examination has shown circular arrangements of grains (turbates), plasma and skelsepic fabrics, and clay reorientations associated with shearing. SEM analysis of quartz surfaces also suggests shearing, indicating that the diamicton from which the pellets were derived was subjected to stresses typical of subglacial environments. The presence of skelsepic fabrics and turbated structures supports the interpretation that the pellets are derived from subglacial meltwater movement. Therefore, the Campo do Tenente pellets likely originated from poorly consolidated subglacial sediment beneath wet-based glaciers, which were later incorporated into icebergs and released during melting events.
The Miocene Tekman–Karayazı Sub-basin (TKSB) in Eastern Anatolia records a key stage in the transition from marine to continental conditions during the final closure of the Neotethys. This study reveals a previously unrecognized evaporite system composed of clastic sulphates, primary selenite crystals, and fluidized sulphate domes, formed under the combined influence of tectonically active deformation and high energy surface processes. Field and petrographic data show that evaporitic activity began earlier than previously thought. The replacement of large benthic foraminifera such as Nummulites within older carbonate units, together with gypsum filled porosity, points to long lived sulphate circulation and repeated diagenetic overprinting. Occasional halite pseudomorphs further reflect episodic hypersaline conditions and early mineral transformation. A major outcome of this study is the evidence for vertical recycling of older marine derived brines into Miocene deposits. We link this process to tectonically driven fluid pumping, likely enhanced by lithospheric scale processes such as lithospheric dripping and lithospheric delamination. These mechanisms promoted uplift, increased permeability, and sustained fluid migration through the basin. Sedimentary structures such as hummocky and swaley cross stratification indicate repeated storm wave influence in a high energy shoreface setting. At the same time, deformation features including liquefaction structures, syn depositional faulting, and gypsum injections point to strong seismic control. While storm and tidal processes contributed to sediment instability, seismic events appear to have been the dominant trigger. Overall, the TKSB represents a dynamic evaporite system where tectonics, seismicity, and surface processes operated together to shape sedimentation, deformation, and fluid flow.
The Navarro Formation produces oil from thin sandstones encased in thick marine shales. Nearby outcrop faunal analysis places these mudstones as being deposited in a middle-shelf depositional environment at 100 km or greater from time-equivalent shorelines based on regional correlation. This study integrates core, drill cuttings, and well logs to identify ten lithofacies grouped into four facies associations (FA-1 to FA-4), representing a dynamic mud-dominated depocenter, with characteristics of both subaqueous deltas and shallow-water contourites. High fluvial inputs from the northwest fed an energetic shelf with storm waves and strong geostrophic currents, transporting fine sediment far across the shelf before deposition as elongate, muddy clinoforms that prograded perpendicular to the currents. Additional accommodation along the Mexia-Talco fault promoted more aggradational parasequence stacking and thicker clinoforms locally. Toesets comprise muddy laminated deposits (FA-1) that grade upward across topsets into bioturbated, silty contourite facies (FA-2, FA-3), forming coarsening-upward parasequences. The clinoform rollover lay near storm-wave base, where storm-ebb flows transitioned into along-shelf geostrophic currents, depositing sandy tempestites (FA-4) thick enough to outpace bioturbation and preserve primary structures, yielding sandstone fairways aligned with rollover zones. This study presents a predictive model for isolated sandstone bodies on mud-dominated highstand shelves, informing unconventional reservoir characterization and hydrocarbon migration pathways.
The upper Miocene succession of the Romagna Apennines includes a variety of carbonate deposits that have been included in the "Calcare di Base" lithostratigraphic unit and usually interpreted as primary evaporites recording the onset of the Messinian salinity crisis (MSC). We carried out an, integrated multidisciplinary (stratigraphic, sedimentological, and geochemical) study on a Messinian carbonate, deposit cropping out in the Romagna Apennines, whose origin and stratigraphic position are still not well defined and for this reason here labeled as incertae sedis carbonates (IS-C). In this study we compare the IS-C with other well-known carbonate deposits of the Northern Apennines that differ in stratigraphic position, age, and origin: i) the PRE-C, interbedded within the euxinic shale unit underlying the primary evaporites (pre-MSC); ii) the PLG-C, hosted in the Primary Lower Gypsum (PLG) deposits of the Vena del Gesso Basin (MSC stage 1); and iii) the RLG-C, associated with the Resedimented Lower Gypsum (RLG) deposits of the Gabicca Riviera in the outer Adriatic foredeep (MSC stage 2). The IS-C carbonates, which are stratigraphically sandwiched between upper Tortonian-lower Messinian deepwater deposits below and upperpost Messinian Lago-mare deposits above, are qharacterized by a brecciated texture, and by the presence of both rigid and deformed carbonate clasts, similar to those described for the Calcare di Base of Sicily (CdB type 3), pointing to a clastic origin and emplacement from subaqueous gravity-flow deposits. Petrographic analysis enabled the recognition of eight main lithofacies. No carbonate extraclasts and primary gypsum facies have been found, but locally some secondary nodular gypsum beds occur lateral to the carbonate deposits. The stable-isotope compositions are facies dependent, as reflected by the peloidal grainstone clasts showing more positive 80 values and more negative 8C values, suggesting deposition under strong to moderate evaporative conditions with minor diluted water inputs, similar to that found in the PLG deposits. Conversely, the matrix shows negative values for both isotopes that could probably be related to moderate diagenetic processes. With respect to PRE-C, PLG-C, and RLG-C types, the IS-C carbonates are characterized by a higher content of fecal pellets whose morphological apalysis suggests that they could have been produced by assemblages of opportunistic planktonic metazoans adapted to schizohaline conditions, likely including Artemia salina. No clear in situ peloidal carbonate deposits have been found, suggesting the complete penecontemporaneous dismantlement during MSC stage 2 of carbonate factories developed on the culmination of anticlines related toractive thrust faults.
Improved understanding of mixed-process deltaic systems and their stratigraphic architectures is important for predicting reservoir heterogeneity and coastal geomorphic changes. This study reveals that the lower part of the upper Eocene Maadi Formation in the north Eastern Desert (Egypt) includes stacked deposits of a mixed-energy, sand-dominated deltaic system. The succession consists of six main architectural elements: i) offshore muds, ii) prodelta, iii) delta front, iv) delta plain, v) tide-dominated estuary, and vi) transgressive storm beds. These elements accumulated in two high-frequency transgressive-regressive sequences; each consists of regressive prograding-delta-lobe deposits truncated by transgressive storm deposits. The prograding-delta lobes show mixed storm and tide influences. Storm action prevailed in the distal delta front and waned in the proximal delta front, in contrast to the tidal action that prevailed in the proximal-delta-front elements and the delta-plain elements. The tidal impact increased successively through the studied succession, probably due to tectonically derived tidal amplification. The drowning of the deltaic system with development of an estuary is attributed to fluvial avulsion or autogenic delta-lobe switching. The study introduces a model for variability of the interaction between river, tide, and storm processes and their products in a mixed-energy delta, and the evolution of the mixed storm- and tide-influenced deltaic system. It improves our understanding of the sedimentological parameters and stratigraphic architecture of mixed-energy coastal reservoirs and their heterogeneities.
Rivers that transport fine-grained sandy sediment, and their associated deposits, remain understudied compared to coarser-grained alluvial channels, particularly in the ancient sedimentary record. This study details the sedimentology of a large (mean and maximum bankfull depth of c. 10 and 20 m, respectively), low-slope, finegrained sandy river with low variance in grain size from the Carboniferous (Upper Mississippian) Cypress Formation in the Illinois Basin, USA, at the outcrop and regional scale. A combination of newly collected and existing subsurface (geophysical logs and core) and outcrop data was used to detail the subtle variability in the sedimentology of this fine-grained river, which is shown to be significant for interpreting the depositional setting, paleohydraulics, and reservoir heterogeneity of the Cypress Formation. Thick (up to similar to 50 m) sandstones of the Cypress Formation are very fineto fine-grained and dominated by six lithofacies associated with fining-upward fluvial channel fills deposited during a significant sea-level lowstand. Up to three stories of stacked channel-fill deposits are present, with the upper-most stories being most complete (similar to 14 to 20 m thick) and capped by a basin-wide paleosol. Small (< 0.27 m thick) and low-angle (< 15 degrees) cross-beds, along with planar bedding and attenuated longitudinal bars (similar to 4 m thick) with low-angle (< 10 degrees) master surfaces, dominate the channel fills and are interpreted to be characteristic of fluvial deposition under conditions of high suspended-load transport. As a result, the scaling of cross-set thicknesses to mean bankfull depths provides shallower depth estimates (similar to 4 m) compared with those derived from measured channel-story thicknesses (7-11 m). The erosional bases of channel stories, although subtle, correspond to abrupt grain-size coarsening and up to fourfold increases in permeability, revealing significant heterogeneity that is otherwise obscured and must be incorporated in reservoir and aquifer models to accurately represent flow behavior. Additionally, the well-sorted and texturally homogeneous sandstones comprising channel fills are interpreted to be the result of high rates of suspended-load transport and subsequent deposition, which can mask preserved stratification, and in some cases renders facies as completely massive. Therefore, seemingly massive, well-sorted, very fineto fine-grained sandy facies require additional scrutiny, because their stratified sediments can be easily mistaken as structureless.
Late Paleozoic strata are well preserved in numerous intracratonic basins on the Indian subcontinent, which was once part of East Gondwana. The sources and transport pathways of late Paleozoic sediments in these Gondwanan basins are poorly understood, owing primarily to the lack of geochronological data on detrital sediments. To reconstruct the regional detrital and tectonic history of East Gondwana, we have incorporated multiproxy provenance analyses that includes detrital 40Ar/39Ar geochronology, sandstone petrofacies, and heavy-mineral analyses of late Paleozoic strata of the Barapukuria, Dighipara, and Khalaspir basins of northwestern Bangladesh and the Jharia Basin of east-central India. We present the first detrital thermochronology data from the Jharia, Barapukuria, and Dighipara basins. Sandstone petrographic analysis indicates temporal and spatial variations in the compositions among adjacent Gondwanan basins. Heavy-mineral assemblages suggest amphibolite-and granulite-facies source terranes. Detrital 40Ar/39Ar data from the Barapukuria and Dighupara basins yields mainly Cambrian-Ordovician (524-474 Ma) ages. In contrast, the Barakar Formation from the Jharia Basin shows more scattered ages (1885-460 Ma) with multiple significant peaks in the Neoproterozoic and the dominant peak at Cambro-Ordovician, while the Talchir Formation has a contrasting Neoproterozoic age population of 895 Ma to 810 Ma, with the principal mode at 860 Ma indicating changes in the source terranes. Taken together, data suggest that the Eastern Ghats Mobile Belt of the Kuunga orogenic belt is the primary contributor to the younger early Permian strata, along with minor contributions from the Prydz Bay Belt and northern Prince Charles Mountains of the East Antarctic Craton. On the contrary, the Chotanagpur Granite Gneiss Complex of the Indian Craton and the Prydz Bay Belt on the East Antarctic Craton was the main source of the older Permo-Carboniferous Talchir Formation.
Multiple glaciations characterize the Neoproterozoic icehouse Earth period. The Ediacaran Gaskiers Glaciation corresponds to a localized glaciation limited to > 30 degrees paleo-latitude and relatively short duration (i.e., < 1 Myr) compared to the Cryogenian glaciations, but was significantly followed by rapid Ediacaran biota evolution. The Gaskiers Glaciation (ca. 580 Ma) is best characterized in sedimentary successions exposed on the Avalon Peninsula of southeast Newfoundland, where the Gaskiers Formation records deepwater glaciomarine sedimentation overlain and underlain by deep-marine turbidites. Here, we present a sedimentological and stratigraphic analysis of the correlative shallow glaciomarine Mercantile and Trinity diamictites in the Bonavista Peninsula and associated shallow-marine to fluviatile strata of the Rocky Harbour Group. The glacial Mercantile and Trinity diamictites consist of a clast-rich diamictite with abundant faceted, flat-iron, stoss-and-lee, or bullet-shaped clasts, and rare striated clasts (< 1-2%), overlain by a clast-poor diamictite with dropstones, dump, and grounding structures. The proximal glaciomarine Monk Bay Member underlies the Trinity Diamictite and consists of stacked 5-to 15-m-thick upward-coarsening, shallow-marine clastic parasequences with rare dropstones, each capped by a gravel layer up to 2 m thick. The overlying postglacial Kings Cove North Member comprises a distinctive pistachio silicified tuff, a mudstone succession > 50-m-thick, and an uppermost stratal unit of thick-bedded turbidites. Parasequences of the Monk Bay Member are interpreted as deposits of marine-terminating glacial advance and retreat cycles controlled by climate-driven forcing. The gravel-rich transgressive layer capping each parasequence records wave reworking of likely ice-rafted material. These cycles culminated in the deposition of the Trinity diamictite, marked initially by a continuous rainout, followed by finer-grained fallout of ice-rafted debris. Overlying this, the Kings Cove North Member records glacio-eustatic flooding of the shelf, driven by a glacial retreat that marks the end of the Gaskiers Glaciation on the Bonavista Peninsula and coincident glacioeustatic sea-level rise, which outpaced glacial-isostatic rebound.
Sand injectites form when poorly consolidated sand is fluidized and forcibly intruded into a host sediment. Macroscopic features of injectite complexes record their formative processes, but relatively little is known about the differences in microscopic characteristics of injected versus parent sandstones. Microfracturing of grains in sand injectites has been recognized, and is related to grain-to-grain collisions during transport in turbulent flow. This study documents four injectite complexes and uses optical microscopy and point-counting, together with back-scatter and scanning electron microscopy (SEM) to show that grain fracturing occurred in depositional, partially remobilized, and fully injected sands in the four studied injectite networks. Point-count results show an increase in the proportion of microfractured versus non-fractured quartz grains in injected sandstones (between 0 to 60% more) when compared with samples of depositional or partially remobilized sandstones. SEM data show that the microfractures are commonly oriented across multiple adjacent grains and radiating out from grain point contacts. This geometry and the location of microfractures result from postemplacement mechanisms of grain impingement, dissipation of pore-fluid pressure, and the effect of overburden pressure, rather than being products of flow processes during emplacement. We propose that the effects of overburden pressure are greater on grains in injected sandstones due to more rapid dissipation of pore fluid leading to more abrupt pressure changes than in parent sandstones. Furthermore, the dominance of grain-impingement fracturing in injectites demonstrates that grain-to-grain collision in a relatively lowconcentration turbulent flow during emplacement is unlikely. This supports core and outcrop observations, such as layering, preservation of delicate features on injectite margins, and clasts concentrated at the upper and lower margins of the injectites, suggesting that flow during injectite emplacement is dominantly high concentration and laminar.
