Fringe or background: Characterizing deep-water mudstones beyond the basin-floor fan sandstone pinchout

Mud dominates volumetrically the fraction of sediment delivered and deposited in deep-water environments, and mudstone is a major component of basin-floor successions. However, studies of basin-floor deposits have mainly focused on their proximal sandstone-prone part. A consequent bias therefore remains in the understanding of depositional processes and stratigraphic architecture in mudstone-prone distal settings beyond the sandstone pinchouts of basin-floor fans. This study uses macroscopic and microscopic descriptions of over 500 m of continuous cores from research boreholes from the Permian Skoorsteenberg Formation of the Karoo Basin, South Africa, to document the sedimentology, stratigraphy, and ichnology of a distal mudstone-prone basin-floor succession. Very thin- to thin-bedded mudstones, deposited by low-density turbidity currents, stack to form bedsets bounded by thin packages ( 0.7 m thick) background mudstones. Stratigraphic correlation between cores suggests that bedsets represent the distal fringes of submarine fan lobe elements and/or lobes, and bedset packages represent the distal fringes of lobe complexes and/or lobe complex sets. The internal stacking pattern of bedsets and bedset packages is highly variable vertically and laterally, which records dominantly autogenic processes (e.g., compensational stacking, avulsion of feeder channels). The background mudstones are characterized by remnant tractional structures and outsize particles, and are interpreted as deposited from low-density turbidity currents and debris flows before intense biogenic reworking. These observations challenge the idea that mud accumulates only from hemipelagic suspension fallout in distal basin-floor environments. Thin background mudstones separating bedsets ( 0.7 m thick) are interpreted to dominantly mark allogenically driven regional decrease of sand supply to the basin floor. The recognition of sandstone-prone basin-floor fans passing into genetically linked distal fringe mudstones suggests that submarine lobes are at least ∼ 20 km longer than previously estimated. This study provides sedimentological, stratigraphic, and ichnological criteria to differentiate mudstones deposited in different sub-environments in distal deep-water basin-floor settings, with implications for the accurate characterization of basin-floor fan architecture, and their use as archives of paleoenvironmental change

Mud-dominated basin margin progradation: processes and implications

The accretion of coarse-grained material at the shelf-edge rollover has been emphasized in studies of basin margin progradation, despite fine grained sediment (clay and silt) representing a volumetrically more significant component of subaqueous clinothems. The timing and processes of fine-grained sediment transport across the shelf and onto the slope remains an understudied facet of sedimentary basin stratigraphy. Three exhumed basin margin-scale clinothems of the Permian Waterford Formation, in the Karoo Basin, South Africa, offer outcrop examples of margin development through the accretion of mud during flooded shelf conditions. The progradation of wave/storm-influenced sandy shelf topset deposits over a thick mudstone succession and beyond a previously established sand-rich shelf-edge rollover suggests that some periods of basin margin progradation took place exclusively via dilute mud-rich gravity flows. Detailed outcrop and core study of offshore mudstones reveals a high content of organic debris and mica. Individual beds show normal and inverse grading, internal erosion surfaces and moderate to low bioturbation, reflecting relatively stressed conditions in frequently supplied outer shelf to upper slope regions. The estimated low gradient (<0.7º) of the Karoo Basin margin and prevailing wave/storm conditions facilitated prolonged suspension of fluid mud and transport across the shelf and beyond the shelf-edge rollover in sediment gravity flows. This study represents a rare example of mudstone-dominated shelf-edge rollover deposits documented at outcrop and core, and demonstrates how fine-grained sediment accretion can play a significant role in basin margin progradation. Conventional depositional models do not adequately account for progradation of basin margins in the absence of sand supply, which implies potential risks in the identification of shelf edge rollover positions and application of trajectory analysis in strongly progradational margins

Transport and deposition of mud in deep‐water environments: Processes and stratigraphic implications

Deep-water mudstones are often considered as background sediments, deposited by vertical suspension fallout, and the range of transport and depositional processes are poorly understood compared to their shallow-marine counterparts. Here, we present a dataset from a 538.50 m-thick cored succession through the Permian muddy lower Ecca Group of the Tanqua depocentre (southwest Karoo Basin, South Africa). This study aims to characterize the range of mudstone facies, transport and depositional processes, and stacking patterns recorded in deep-water environments prior to deposition of the Tanqua Karoo sandy basin-floor fans. A combination of macroscopic and microscopic description techniques and ichnological analysis has defined nine sedimentary facies that stack in a repeated pattern to produce 2–26-m-thick depositional units. The lower part of each unit is characterized by bedded mudstone deposited by dilute, low-density turbidity currents with evidence for hyperpycnal-flow processes and sediment remobilization. The upper part of each unit is dominated by more organic-rich 27 bedded mudstone with common mudstone intraclasts, deposited by debris flows and transitional flows, with scarce indicators of suspension fallout. The intensity of bioturbation and burrow size increases upward through each depositional unit, consistent with a decrease in physicochemically stressed conditions, linked to a lower sediment accumulation rate. This vertical facies transition in the single well dataset can be interpreted to represent relative sea level variations, where the hyperpycnal stressed conditions in the lower part of the units were driven by sea level fall and the more bioturbated upper part of the unit represents backstepping, related to sea level rise. Alternatively, this facies transition may represent autogenic compensational stacking. The prevalence of sediment density flow deposits, even in positions distal or lateral to the sediment entry point, challenges the idea that deep-water mudstones are primarily the deposits of passive rainout along continental margins

Architecture and controls of thick, intensely bioturbated, storm-influenced shallow-marine successions

Thick (100 m), highly bioturbated storm-influenced shallow-marine deposits are not common in the stratigraphic record, but some examples have been described in aggradational to retrogradational successions. In these, individual event beds have typically low preservation potential, yet shoreface-offshore depositional settings are still largely characterized based on the inferred frequency or magnitude of storms. Here we present a sedimentological study of a thick, bioturbated exhumed succession deposited during the early post-rift phase of the Neuquen Basin (Argentina). We characterize it and compare its stratigraphic record with examples elsewhere, in order to discuss the potential factors controlling the total overprint of storm-event beds during several million years.Fil: Poyatos Moré, Miquel. University of Oslo; NoruegaFil: Schwarz, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Boya, Salvador. Universidad de Barcelona; EspañaFil: Gomis Cartesio, Luz Elema. Equinor S. A.; NoruegaFil: Midtkandal, Ivar. University of Oslo; Noruega59th British Sedimentological Research Group Annual General MeetingReino UnidoBritish Sedimentological Reseach GroupUniversity of Liverpoo

Agerinia marandati sp. nov., a new early Eocene primate from the Iberian Peninsula, sheds new light on the evolution of the genus Agerinia

Background. The Eocene was the warmest epoch of the Cenozoic and recorded the appearance of several orders of modern mammals, including the first occurrence of Euprimates. During the Eocene, Euprimates were mainly represented by two groups, adapiforms and omomyiforms, which reached great abundance and diversity in the Northern Hemisphere. Despite this relative abundance, the record of early Eocene primates from the European continent is still scarce and poorly known, preventing the observation of clear morphological trends in the evolution of the group and the establishment of phylogenetic relationships among different lineages. However, knowledge about the early Eocene primates from the Iberian Peninsula has been recently increased through the description of new material of the genus Agerinia from several fossil sites from Northeastern Spain. - Methods. Here we present the first detailed study of the euprimate material from the locality of Masia de l'Hereuet (early Eocene, NE Spain). The described remains consist of one fragment of mandible and 15 isolated teeth. This work provides detailed descriptions, accurate measurements, high-resolution figures and thorough comparisons with other species of Agerinia as well with other Eurasian notharctids. Furthermore, the position of the different species of Agerinia has been tested with two phylogenetic analyses. - Results. The new material from Masia de l'Hereuet shows several traits that were previously unknown for the genus Agerinia, such as the morphology of the upper and lower fourth deciduous premolars and the P2, and the unfused mandible. Moreover, this material clearly differs from the other described species of Agerinia, A. roselli and A. smithorum, thus allowing the erection of the new species Agerinia marandati. The phylogenetic analyses place the three species of Agerinia in a single clade, in which A. smithorum is the most primitive species of this genus. - Discussion. The morphology of the upper molars reinforces the distinction of Agerinia from other notharctids like Periconodon. The analysis of the three described species of the genus, A. smithorum, A. marandati and A. roselli, reveals a progressive change in several morphological traits such as the number of roots and the position of the P1 and P2, the molarization of the P4, the reduction of the paraconid on the lower molars and the displacement of the mental foramina. These gradual modifications allow for the interpretation that these three species, described from the early Eocene of the Iberian Peninsula, are part of a single evolutionary lineage. The stratigraphical position of Masia de l'Hereuet and Casa Retjo-1 (type locality of A. smithorum) and the phylogenetic analyses developed in this work support this hypothesis

Deep-ocean channel-wall collapse order of magnitude larger than any other documented

Submarine channels are the largest conveyors of sediment on Earth, yet little is known about their stability in the deep-ocean. Here, 3D seismic data from the deep-ocean Hikurangi channel-levee system, offshore New Zealand, reveal the largest channel-wall failure yet documented. Collapse of both channel-walls along a 68 km stretch created a mass-transport deposit of 19 km3, containing 4 km long blocks. Channel-walls typically collapse piecemeal, but here synchronous failure of both channel-walls and landslide erosion of the seafloor is documented, requiring a new process model for channel-wall failure. Mass-failure on this scale poses an under-appreciated risk to seafloor infrastructure both within channels and over regions extending twice the channel width into their overbank. Hitherto, channel-wall failures of this size are unrecognised in abyssal plains; its scale changes our understanding of how channel-levee systems are constructed and how they conduct sediment, carbon and pollutants into the deep-ocean

Filter Or Conveyor? Establishing Relationships Between Clinoform Rollover Trajectory, Sedimentary Process Regime, and Grain Character Within Intrashelf Clinothems, Offshore New Jersey, U.S.A.

Clinoform geometries and trajectories are widely used to predict the spatial and temporal evolution of sand distribution, but most analytical approaches underplay the significance of topset and shelf process regime in determining how and when sediment is conveyed downdip or stored on the continental shelf. We present an integrated study of clinoform rollover trajectory and detailed grain character analysis to assess the role of topset process regime in determining sand distribution and sediment character across clinothems. This study targets the topset, foreset, and bottomset deposits of four successive Miocene intrashelf clinothem sequences, which represent deposition under either river-dominated or wave-dominated conditions. Seismic reflection data was combined with core analysis and grain character data derived from 664 samples collected from three cored research boreholes. In river-dominated clinothems, the transfer of coarse-grained sediment occurs under both rising and flat-to-falling clinoform rollover trajectories, suggesting that process regime is more important in determining sediment delivery than clinoform trajectory; river-dominated systems are effective conveyors of sediment into deeper water. Wave-dominated clinothems deposited exclusively under rising clinoform rollover trajectories largely retain sand within topset and foreset deposits; wave-dominated systems are effective sediment filters. Notably, deposition under either river- or wave-dominated topset process regimes results in quantifiable differences in grain character attributes along clinoform profiles. Sediments in river-dominated systems are coarser, less well-rounded, and more poorly sorted, and show greater intersequence and intrasequence variability than those in wave-dominated systems; prediction of sediment character is more challenging in river-dominated systems. This study highlights the need for caution when attempting to predict downdip sand distribution from clinoform trajectory alone, and provides a novel perspective into downdip grain character profiles under end-member topset process regime conditions. The results of this study can be used to better constrain sediment grain-size and grain-shape distributions in process-based forward models, and have widespread applications in prediction of reservoir quality in both frontier and mature hydrocarbon basins

Sharp-based, mixed carbonate–siliciclastic shallow-marine deposits (upper Miocene, Betic Cordillera, Spain): The record of ancient transgressive shelf ridges?

Isolated sharp-based sedimentary bodies in shelf settings can develop via the reworking of regressive deposits during transgressions. An example of these are shelf ridges, formed under a wide range of processes, and widely studied due to their high reservoir potential. However, there is still a lack of examples in mixed (carbonate–siliciclastic) successions. This study presents an outcrop example from the Upper Miocene of the Betic Cordillera (Spain), with the aim to propose a model for the development of transgressive sharp-based mixed carbonate–siliciclastic deposits, and to provide criteria to differentiate these from their regressive counterparts. The studied succession is ca. 300 m-thick, and shows a cyclic alternation of coarse and fine-grained mixed deposits. Depositional cycles start with siliciclastic-dominated offshore to offshore transition deposits, progressively replaced by lower shoreface deposits. These are abruptly truncated by sharp erosive contacts bioturbated by passively-infilled large burrows; their ichnological features allow assignation to the Glossifungites ichnofacies. These contacts are interpreted as ravinement surfaces. They are overlain by mixed carbonate–siliciclastic barforms, rich in skeletal fragments and extraclasts, and displaying large-scale cross bedding. These form several m-thick and hundreds of m-long depositional elements interpreted as mixed shelf ridges. These ridges formed in a fine-grained, shallow-water shelf, which occasionally received coarse siliciclastic sediment supply via gravity flows, but had a coeval offshore carbonate factory, which provided the skeletal fragments. The sharp-based, coarser-grained nature and lithological break at the base of these mixed carbonate–clastic deposits could lead to their misinterpretation as forced-regressive wedges. However, the nature of their lower contact, combined with the reworked offshore skeletal fragments, and their stacking pattern are consistent with these mixed units forming during transgression. Other studies in relatively time-equivalent deposits have demonstrated the existence of coeval regressive, coarser siliciclastic-dominated shoreline systems in relatively close localities. These evidence a complex basin configuration in the area during the upper Miocene, with the development of local depocentres and relatively narrow corridors or seaways in the Mediterranean–Atlantic connection, which could have favoured shelf reworking processes, but also promoted the development of diverse stacking patterns, reflecting the differential interaction between active tectonics and sedimentation across the region.FEDER AndalucíaSecretaría de Estado de I+D+IAker BPUniversitetet i OsloUniversidad de Granada PID2019-104625RB-100, PID2020-114381GB-100European Regional Development Fund CGL2017-89618-RJunta de Andalucía B-RNM-072-UGR18, P18-RT-4074Agencia Estatal de Investigació

Controls on the architectural evolution of deep-water channel overbank sediment wave fields: insights from the Hikurangi Channel, offshore New Zealand

Deep-water channels can be bound by overbank deposits, resulting from overspilling flows, which are often ornamented with sediment waves. Here, high-resolution bathymetry, backscatter, and 2D and 3D seismic data are integrated to discern the controls on flow processes on the overbank areas of the Hikurangi Channel. Qualitative seismic interpretation and quantitative analyses of sediment wave morphologies and distributions are conducted through the shallowest 600 m of stratigraphy up to the seafloor. Four outer-bend wave fields are present throughout the studied stratigraphy on the landward margin (left margin looking down-channel) only. Originally closely spaced or combined, these fields evolved to become spatially separated; two of the separate wave fields became further subdivided into distinct outer- and inner-bend fields, whose constituent waves developed distinct differences in morphology and distribution. Sediment wave character is used to interpret the direction and strength of overbank flow. Nine controls on such flow and associated deposition are identified: flow versus conduit size, overbank gradient, flow tuning, Coriolis forcing, contour current activity, flow reflection, centrifugal forcing, interaction with externally derived flows, and interaction of overspill from different locations. Their relative importance may vary across parts of overbank areas, both spatially and temporally, controlling wave field development such that: (1) outer-bend wave fields only develop on the landward margin; (2) the influence of centrifugal force on outer-bend overbanks has increased through time, accompanying a general increase in channel sinuosity; (3) inner-bend wave fields on the landward margin form by the interaction of Coriolis-enhanced inner-bend overbank flow, and outer-bank flow from up-channel bends; (4) inner-bend fields on the oceanward margin form by the interaction of axial flow through wave troughs, and a transverse, toward-channel flow component. This work has implications for interpreting overbank flow from seafloor and seismic data, and for palaeogeographic reconstructions from outcrop data