Basin Flanks Research Articles

Subsurface basement structures of Usangu basin, East African rift system, with implications for basin structural configuration and hydrocarbon potential

The Usangu basin is a rift basin developed along the Eastern arm of the East African rift system trending in the NE-SW direction. Although the general structures of the basin have been well studied, the structural configuration of the basin and the spatial and depth variations of sediment thickness are still not well known. This study investigates the structures in relation to basement configuration and the variation of sediment thickness within the basin using the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM), aeromagnetic and gravity data. Results from DEM data indicate a few lineaments on the basin flanks representing the Usangu and Chimala border faults with no structures in the central part of the basin. The aeromagnetic and gravity data highlight three sets of normal and strike-slip faults, most of which trend NE-SW and others NNE-SSW, while a few trend WNW-ESE or NW-SE. Structures on the southwest of the basin reveal complex patterns attributed to the concentration of important tectonic and seismic activities in the study area. The Euler deconvolution and gravity models used to calculate the depth to the basement show that the basement is shallow in the north and south to southwest, and the basin deepens in the northeastern, northwestern and western parts. The findings also reveal that the basin has two grabens, troughs, depression and intrabasinal basement trending in the same direction as the basin configuration. The general thickness of sediments filling the basin ranges from 3 to 4.5 km, with the maximum accumulation of sediments reaching up to 4.8 km in the two depocenters at the south and southwest of the basin. The depth range of the sediments obtained implies that the basin has potential for hydrocarbon exploration with the possibility of natural gas occurrence.

A Comprehensive Assessment of Submarine Landslides and Mass Wasting Processes Offshore Southern California

AbstractIt is critical to characterize submarine landslide hazards near dense coastal populations, especially in areas with active faults, which can trigger slope failure, subsequent tsunamis, and damage seabed infrastructure during earthquake shaking. Offshore southern California, numerous marine geophysical surveys have been conducted over the past decade, and high‐resolution bathymetric and subsurface data now cover about 60 percent of the total region between Point Conception and the United States‐Mexico border from the California coast out to the base of Patton Escarpment ∼200 km offshore. In a comprehensive compilation and interpretive mapping effort, we find evidence of seafloor failure throughout offshore southern California with nearly 1,500 submarine landslide‐related features, including 63 discrete slide deposits with debris and >1,400 slide‐related scarps. In our analysis, we highlight new mapping of submarine landslides in Catalina Basin, the Del Mar slide, the San Gabriel slide complex, and the 232 km2 San Nicolas slide, the largest area of any known submarine landslide mass offshore southern California. Analysis of the spatial distribution of submarine landslide features suggests that most mapped slide features are located relatively near coastal sediment sources, particularly during sea‐level lowstand conditions, which underscores the importance of sediment supply and sediment accumulation on low‐gradient slopes as failure preconditioning processes. Tectonically driven uplift at shelf edges and along basin flanks is another key preconditioning factor, and our results also suggest that earthquakes along active faults trigger mass wasting, especially for repeated, small‐scale failures on tectonically steepened slopes.

Deposition in the northwest Porcupine Basin during the Cretaceous

Abstract: The paper details the sedimentary infill of the northwest part of the Porcupine Basin, offshore west Ireland, during the Cretaceous. This is a critical area for understanding the sediment input and incremental infilling of the basin, particularly during the early Cretaceous. Late Jurassic rifting and early Berriasian erosion produced a northward narrowing valley on the Base Cretaceous unconformity surface, which provided an entry point for sediment into the basin and also constrained the lateral extent of initial sedimentation. Following Berriasian marine transgression, the first deposits in the Porcupine Basin were turbidites located in localised depressions on the unconformity surface (named here informally as Basal Deposits 1–4). As the unconformity surface became smoother, the succeeding layers were more widespread (Sequences 1–3, the top being of probable Barremian age). Several features, interpreted as mounded fans sourced from the basin flanks, are developed in the Valanginian-Barremian section. Where drilled, the Berriasian to Barremian section is predominantly fine-grained. The rifted marginal horsts were covered in late Barremian time, and Aptian and later sediments onlapped the flanks of the Porcupine High to the west. The 'Near Base Aptian' is a well-defined prograding surface and the 'Near Top Aptian' horizon is a marked unconformity. The Aptian section is comprised predominantly of deep-water mudrocks and siltstones, although a distinctive, possibly shallower, erosive Intra-Aptian event was also mapped. A renewed sediment influx, comprising of sand prone deltaics, occurred at the northern margin of the basin during the Albian. Sand content decreases southwards into the basin. Sedimentation in the late Cretaceous was dominated by the basinwide deposition of the Chalk Group (Cenomanian-Selandian) deposited in a stable thermal subsidence setting. At the northern basin margin Upper Cretaceous deposits are thin and lithologically variable. The well-defined 'Top Chalk' reflector, which shows mounded features, is overlain unconformably by Paleocene clastics.

Deposition in the Northwest Porcupine Basin During the Cretaceous

The paper details the sedimentary infill of the northwest part of the Porcupine Basin, offshore west Ireland, during the Cretaceous. This is a critical area for understanding the sediment input and incremental infilling of the basin, particularly during the early Cretaceous. Late Jurassic rifting and early Berriasian erosion produced a northward narrowing valley on the Base Cretaceous unconformity surface, which provided an entry point for sediment into the basin and also constrained the lateral extent of initial sedimentation. Following Berriasian marine transgression, the first deposits in the Porcupine Basin were turbidites located in localised depressions on the unconformity surface (named here informally as Basal Deposits 1–4). As the unconformity surface became smoother, the succeeding layers were more widespread (Sequences 1–3, the top being of probable Barremian age). Several features, interpreted as mounded fans sourced from the basin flanks, are developed in the Valanginian-Barremian section. Where drilled, the Berriasian to Barremian section is predominantly fine-grained. The rifted marginal horsts were covered in late Barremian time, and Aptian and later sediments onlapped the flanks of the Porcupine High to the west. The ‘Near Base Aptian’ is a well-defined prograding surface and the ‘Near Top Aptian’ horizon is a marked unconformity. The Aptian section is comprised predominantly of deep-water mudrocks and siltstones, although a distinctive, possibly shallower, erosive Intra-Aptian event was also mapped. A renewed sediment influx, comprising of sand prone deltaics, occurred at the northern margin of the basin during the Albian. Sand content decreases southwards into the basin. Sedimentation in the late Cretaceous was dominated by the basinwide deposition of the Chalk Group (Cenomanian-Selandian) deposited in a stable thermal subsidence setting. At the northern basin margin Upper Cretaceous deposits are thin and lithologically variable. The well-defined ‘Top Chalk’ reflector, which shows mounded features, is overlain unconformably by Paleocene clastics.

Combining spectral analysis and geochemical tracers to investigate surface water–groundwater interactions: A case study in an intensive agricultural setting (southern Guatemala)

An increase in the frequency of severe hydrological events has highlighted the importance of sustainable water management in intensive agricultural regions. In a warming climate, improved understanding and stewardship of water resources are needed to guarantee water supply, ensure food security, and build resilience against extreme events. In this study, we evaluate a framework that combines spectral analysis and geochemical tracers as a potential tool for (1) gaining valuable insights into surface water (SW)–groundwater (GW) interactions, and (2) providing guidance for improved water management in an intensive agricultural basin in southern Guatemala. The framework proves to be useful in revealing important water dynamics, exposing key feedback mechanisms for water availability and quality. With the use of power density functions and hydrochemistry (T, pH, EC, and major ions), two specific interaction regimes (influent and effluent) were identified and delimited for the main watercourse. These segments are estimated to interact at high rates with the shallow aquifer in the river channel proximities and would lose influence towards the basin flanks. Furthermore, the δ2H and δ18O values indicate that regional groundwater flow systems play an essential role in the basin groundwater recharge. Lastly, we established three influence zones that depict the spatial extent of the SW-GW interactions within the basin. With these zones, we provide recommendations that will allow for further investigation and application into better water management strategies regulating groundwater development and land use activities within the agricultural context of the area.

Palynofloras, palaeoenvironmental change and the inception of the Paleocene Eocene Thermal Maximum; the record of the Forties Fan, Sele Formation, North Sea Basin

The Paleocene–Eocene Thermal Maximum (PETM) has previously been recorded in marine density flow sedimentary rocks of the Forties Member, Sele Formation, North Sea Basin. Two cored well sections with existing δ 13 C records were analysed for their total palynofloras. Using statistically defined microplankton groups and diversity data, the Sele Formation Unit S1 depositional sequence and its maximum flooding surface was identified in both wells. This highlighted that deposition of the Forties Fan complex began significantly earlier than the inception of the PETM in the proximal fan area. Palynofloras from the Central and West Axial fairways of the Forties Fan have previously been used to indicate increased rainfall, erosion and onset of Forties Fan deposition at the PETM inception. However, winterwet floodplain communities typify the coeval East Axial Fairway, whereas the dominance of swamp-derived communities in the West and Central Axial Fairway sediments reflects source palaeogeography overriding the regional winterwet climate signal. Palynofloras reflecting regional increased seasonality and lower moisture availability increase immediately before the PETM CIE onset, but reflect climate warming over the hyperthermal. However, there is no correlation between Forties Fan initiation and the PETM. NE Atlantic margin basin flank uplift in response to synrift volcanism is indicated as the driving mechanism behind Forties Fan inception. Supplementary material : Raw data, DCA graphs for microplankton, pollen and spore data and δ 13 C data for well 22/6-1 are available at https://doi.org/10.6084/m9.figshare.c.6080873

Facies variations in the Upper Jurassic source rocks of the Norwegian North Sea; from micro- to macro scale

In the northern North Sea, the Upper Jurassic Kimmeridge Clay (KC) and its equivalents in the Norwegian sector (i.e., Mandal, Tau, and Draupne) are the most prolific source rocks. These source rock units are known to record both lateral and vertical heterogeneity in organofacies with increasing terrigenous organic inputs near highs and basin flanks. Existing interpretations rely predominantly on Rock-Eval data, but only a few authors validate their geochemical interpretations with maceral assessments. In this study, Upper Jurassic source rocks from fourteen wells in the northern North Sea were analysed for both organic petrography and geochemistry. A potential correlation between the maceral compositions and Rock-Eval, total organic carbon (TOC), and stable carbon isotopes (saturate and aromatic fractions) of source rock extracts was established. This correlation suggests that carbon isotopes mirror the organofacies, and inferred redox conditions during sedimentation, described for the studied source rocks.Stable carbon isotopes of the saturate and aromatic fractions for 152 oils distributed across the Norwegian North Sea were compared to biomarker compounds sensitive to precursor organic material and oxygen abundance in the sedimentary environment. A correlation consistent with that observed between stable carbon isotopes and organofacies documented for the investigated source rocks is also observable in the oil dataset. Hence, the stable carbon isotope composition of the oils in the North Sea dataset appears to carry information related to the bulk organofacies and redox conditions prevailing during the sedimentation of their source. Oils often reflect the compositional attributes of their parental sources more efficiently than a single, or a set of rock extracts. Therefore, the stable carbon isotopes of migrated oils provide a means of inferring gross organofacies and redox conditions within different source rock kitchens in the study area.

Crustal structure of the Nogal basin, northern Somalia

Gravity, seismic and well data have been used to determine the crustal structure of the poorly-known Nogal basin, northern Somalia. Seismic data delineate a WNW-ESE trending graben infilled mainly by up to 6–7 km of Upper Cretaceous to Oligocene-Miocene sediments. Backstripping of well data show that the basin developed by thinning and rifting of continental crust during the Late Jurassic, Late Cretaceous and Oligocene. The Oligocene rift event is only observed at the centre of the basin, and correlates with the opening of the Gulf of Aden. The Late Jurassic rift event, which has been recognised in the northern Somalia, is largely absent in the basin.For the western part of the basin, Process Oriented Gravity and flexure Modelling (POGM) suggest a best-fit Te of 5 km, which explains both the amplitude and wavelength of the observed free-air gravity anomaly. However, the modelled anomaly in the eastern part of the basin is 30–40 mGal higher than the observed. Sediment-corrected Bouguer gravity anomalies show that the basin is associated with a relative high, which increases in amplitude to the east, towards the Kalis-1 well. We, therefore, propose that the eastern part of the basin to be locally intruded by dense magmatic material in the lower crust and/or upper mantle, which have elevated the gravity.The amount of crustal thinning inferred at Kalis-1 and Nogal-1 wells from backstripping is consistent with the results of POGM and flexure modelling. The models suggest the crust thins from about 35 km beneath the basin flanks to 32.6 km and 27.2 km at the Kalis-1 and Nogal-1 wells respectively. Moreover, the models suggest that Late Jurassic rifting was limited to the western part of the basin.The comparison of the long wavelength free-air gravity anomaly with the gravity effect of the topography based on different gravity/topography ratios suggest that while the crust is locally thinned beneath the basin it is also regionally domed, possibly by flexure due to magmatic intrusion. The doming predates the Late Cretaceous rifting and coincides with the same pre-Cenomanian uplift event during which substantial thicknesses of Upper Jurassic and Lower Cretaceous sediments were exhumed.

Geological and seismic evidence for the tectonic evolution of the NE Oman continental margin and Gulf of Oman

AbstractLate Cretaceous obduction of the Semail ophiolite and underlying thrust sheets of Neo-Tethyan oceanic sediments onto the submerged continental margin of Oman involved thin-skinned SW-vergent thrusting above a thick Guadalupian–Cenomanian shelf-carbonate sequence. A flexural foreland basin (Muti and Aruma Basin) developed due to the thrust loading. Newly available seismic reflection data, tied to wells in the Gulf of Oman, suggest indirectly that the trailing edge of the Semail Ophiolite is not rooted in the Gulf of Oman crust but is truncated by an ENE-dipping extensional fault parallel to the coastline. This fault is inferred to separate the Semail ophiolite to the SW from in situ oceanic Gulf of Oman crust to the NE. It forms the basin margin to a “hinterland” basin formed atop the Gulf of Oman crust, in which 5 km of Late Cretaceous deep-water mudstones accumulated together with 4 km of Miocene and younger deep-water mudstones and sandstones. Syndepositional folding included Paleocene–Eocene folds on N-S axes, and Paleocene to Oligocene growth faults with roll-over anticlines, along the basin flank. Pliocene compression formed, or tightened, box folds whose axes parallel the modern coast with local south-vergent thrusts and reversal of the growth faults. This Pliocene compression resulted in large-scale buckling of the Cenozoic section, truncated above by an intra-Pliocene unconformity. A spectacular 60-km-long, Eocene(?) to Recent, low-angle, extensional, gravitational fault, down-throws the upper basin fill to the north. The inferred basement of the hinterland basin is in situ Late Cretaceous oceanic lithosphere that is subducting northwards beneath the Makran accretionary prism.

Recent morpho-sedimentary processes in Dove Basin, southern Scotia Sea, Antarctica: A basin-scale case of interaction between bottom currents and mass movements

Multibeam bathymetric imagery and acoustic sub-bottom profiles are used to reveal distribution patterns of sub-surface sedimentation in Dove Basin (Scotia Sea). The goals of the study are to determine the imprint of the inflow of deep Antarctic water masses from the Weddell Sea into the Scotia Sea, to establish the factors driving the styles of contourite deposition and to discern the relative contribution of alongslope versus downslope processes to the construction of the uppermost late Quaternary sedimentary record in the basin.The most significant morpho-sedimentary features in Dove Basin are linked to contouritic processes and to mass movements. Plastered drifts on the flanks of the basin constitute the most common contouritic deposits. Basement-controlled drifts on top of structural elevations are common along the central ridge, the central basin plain and scattered along the basin flanks. Sheeted drifts occur on top of adjacent banks or are restricted to the deep basin. In contrast, mounded drifts are poorly represented in Dove basin. A laterally extensive contouritic channel runs along the central ridge. Contouritic channels are also identified in the upper parts of the lateral banks and slopes. Numerous slide scars along the upper parts of the slopes evolve downslope into semitransparent lens-shaped bodies, with occasional development of across-slope channels. Semitransparent lenses occur intercalated within stratified deposits in the slopes of the basin, in the central ridge and in the deepest basin plain.The spatial arrangement of contouritic morphologies points to the influence of the water column structure and the basin physiography. In the eastern sub-basin, two different fractions (lower and upper) of Weddell Sea Deep Water (WSDW) leave an imprint on contourite deposits owing to the sloping interface between the two fractions. Contouritic influence is more subdued in the western sub-basin, and limited to the imprint of the lower WSDW. The upper parts of the surrounding banks are under the influence of deep-reaching Circumpolar waters (i.e., Lower Circumpolar Deep Water), which develops both depositional and erosional morphologies. The cross-section V-shaped morphology of the basin and the common occurrence of structural highs drive the predominance of plastered and basement-controlled drifts in the sediment record. The frequent alternation between contourites and downslope gravity-flow deposits is likely due to different processes associated with over-steepening in the basin, such as basement-controlled steep slopes, deformed drifts atop basement elevations, and the development of thick contouritic piles. Dove Basin is an example of a basin without mounded, plastered or mixed hybrid drifts in the transition between the lower slope and the deep basin, because the upper boundary of the deepest water mass —the Weddell Sea Deep Water— flows shallower along the middle slope. This fact underlines the relevance of the position and depth of water masses in shaping the morphology of the feet of slopes along continental margins.

Impact of growth faults on mixed siliciclastic‐carbonate‐evaporite deposits during rift climax and reorganisation—Billefjorden Trough, Svalbard, Norway

AbstractFault‐controlled mixed siliciclastic‐carbonate‐evaporite depositional systems exhibit distinct sensitivity to tectonic and eustatic controls that are expressed in the sedimentary architecture. In the Upper Carboniferous Billefjorden Trough (Svalbard, Norway), up to 2,000 m of a warm and arid climate syn‐rift basin fill comprises such depositional systems, documented in this study with traditional field techniques supported by helicopter‐ and ground‐based LIDAR models. The basin evolved from siliciclastics‐dominated red beds and paralic units that filled a symmetrical basin, to a rift climax half‐graben with alluvial fans entering the basin along relay ramps of the master fault zone (Billefjorden fault zone). Faults located in the hanging wall dip‐slope prevented the progradation of coarser material to the eastern part of the basin. Later, structural reorganisation in the dipslope led to the cessation of easternmost faults with deformation focusing along one major lineament (Løvehovden fault zone) antithetic to the master fault zone. The basin subsidence became more symmetrical, with main central depocentre and shallower platforms near the basin flanks. Footwall anticlines from faults displacement gradients were sensitive to periodical exposure and recorded dissolution breccias and footwall synclines preserved evaporites coupled with shallow marine siliciclastic deposits. Concurrently, thick gypsum/anhydrite deposits in the basin centre reflect glacio‐eustatic lowstands, whereas evenly thick carbonate deposition characterises highstands. While most analysis of syn‐rift basin fill is based on siliciclastics deposits, we here demonstrate the complexity of tectonism versus eustatic sea level changes in a mixed carbonate‐evaporite syn‐rift deposits. Tectonic influence is ascribed to the deposition of alluvial fans that prograded from the master fault towards the basin centre. On the dipslope glacio‐eustatic signals outperformed tectonic influence on deposition. Sea level lowstands promoted deposition of red sabkha mudstones and gypsum/anhydrite, salinas evaporites or dissolution breccias, interbedded with highstand carbonate beds.

Stratigraphy of volcanic rock successions of the North Atlantic rifted margin: the offshore record of the Faroe–Shetland and Rockall basins

ABSTRACTThe integration of biostratigraphical, wireline log, geophysical and available geochronological ages has identified two principal periods of volcanism in the Faroe–Shetland and Rockall basins. The first is pre-breakup, upper Danian to lower Thanetian: in the Rockall and Faroe–Shetland basins, isolated volcanic activity from 62 Ma to 58.7 Ma is identified in areas closely linked to the SSW–NNE structural fabric of the continental margin. Volcanic activity was concentrated at basin flank fissures and localised point sources. This rift-flank volcanism led to widespread volcanic ash deposition, localised lava flow fields and the formation of igneous centres. Some of the Hebridean and onshore central complexes (e.g., Rum) were uplifted and rapidly eroded during the later pre-breakup period, while additional accommodation space was developed in the adjacent offshore basins. Onset and termination of pre-breakup volcanism is correlated to intra-plate stress regimes in Europe, following the cessation of convergence of Africa and Europe in the Danian. The second is syn-breakup, upper Thanetian to Ypresian, initiated at ca.57 Ma in the Rockall and Faroe–Shetland basins. Initial high-volume extrusive igneous successions were focussed to the W in the Faroe–Shetland Basin. In the centre and E of the Faroe–Shetland and Rockall basins, separate eruption loci developed along pre-existing lineaments either as fissure or point-sourced lava fields. Short-term cessation of eruption at ~55.8 Ma was followed by resumption of flood basalt eruptions and a shift in focus to the NW. Fluctuations in the syn-breakup eruption tempo are reflected in the formation and subsequent rejuvenation of prominent unconformities, only previously recognised as a single erosive event. The W and northward shift of eruption focus, and the eruption of mid ocean ridge basalt-type lavas in the syn-breakup period reflect the onset of lithospheric thinning in the nascent North Atlantic Rift prior to flooding of the rift and eruption of the widespread lower Ypresian Balder Formation tephras.

From rifting to hyperextension: Upper Jurassic–Lower Cretaceous tectono‐stratigraphy of the Porcupine Basin, Irish Atlantic Margin

AbstractHyperextended basins are increasingly recognized along the outboard parts of continental margins as aborted basins created during continental break‐up. Many of the concepts for understanding and modelling basin evolution and fill were developed for regions that have undergone modest crustal stretching (β < 2) and may not be valid in basins where the crust and upper mantle are heavily modified by extreme stretching. The present study uses extensive 2D and 3D seismic and well data to analyse the Late Jurassic–Cretaceous tectono‐stratigraphic evolution of the Porcupine Basin, bracketing the timing of hyperextension. It is an instructive basin, offshore west of Ireland, preserving low‐magnitude strain in the north, with increasing degrees of hyperextension in the south. Detailed mapping of strain domains (proximal, necking and hyperextended) across the Porcupine Basin reveals five main rift segments, each with a distinctive geometry and strain history. During early low‐strain rifting, inherited crustal structures strongly influenced the rift architecture by controlling the location and geometry of fault‐controlled marine depocentres. The transition from hyperextension to post‐rift subsidence was marked by locally developed, unconformity‐bounded, marine sequences that draped the underlying rift topography. Whilst these ‘transition sequences’ are dated as Tithonian above the necking domain, similar but younger Early Cretaceous transition packages developed in the hyperextended domain, suggesting extension migrated towards the rift axis during hyperextension. Early post‐rift sequences were broadly distributed across the rift centre and basin flanks before strong, thermally‐controlled subsidence of the hyperextended crust, along with hinging of the necking domain, locally to the point of slope failure, gave rise to axially‐focused marine deposition. Hyperextension may have left the basin susceptible to intra‐plate stress changes accounting for several unconformities within the post‐rift fill. This study provides an improved basin‐wide understanding of the tectono‐stratigraphic evolution of hyperextended basins.

Deepwater debrites and linked megaturbidites in confined basins: An example from the Onnuri Basin, East Sea of Korea

ABSTRACTWe analyzed data from seven piston cores, multi-channel seismic-reflection (MCS) and chirp profiles, and multibeam echosounder (MBES) data to study the distribution, emplacement time, sedimentary facies, and depositional processes of sediment-gravity-flow deposits in the Onnuri Basin, a confined basin in the East Sea. These data reveal that debris flows have traveled ca. 30 km downslope, forming a seismic facies consisting of stacked, wedge-shaped, transparent units separated by high-amplitude continuous reflectors. Analysis of piston cores shows three distinct sedimentary units, throughout the basin. The lowest unit, I, is a debrite containing numerous mud clasts of varying size and color distributed in a mud-rich matrix; it is absent over elevated basinal highs or ridges, such as the Onnuri Ridge, suggesting that local topography controls its distribution. The debrite forms a recognizable acoustically transparent layer on subbottom chirp profiles (av. 7 m thick), covers approximately 500 km2, and has an estimated volume of ∼ 3.5 km3.The overlying unit, II, contains normally graded beds composed of massive sand, laminated and cross-laminated sand and silt, and a thick cap of structureless mud. This unit is interpreted to be a megaturbidite deposited from turbidity currents that originated from the flow transformation of debris flows on the upper continental slope. The megaturbidite covers the entire basin (at least 650 km2), and has an average thickness of 2.8 m (maximum thickness of 4.35 m), and comprises a volume of 1.8 km3. Variations in grain size and sedimentary structures suggest that the megaturbidite was deposited by progressively waning flows that reflected off basin flanks and ridges. The thick (up to 3.65 m) structureless mud cap further indicates deposition in a confined basin. The sharp basal contact, together with the lack of hemipelagic sediments between debrite and overlying megaturbidite, suggest that both were deposited during the same flow event, likely to have originated from a single catastrophic slope failure. Collapsing slide material evolved into a debris flow, from which a turbidite formed by dilution of the debris flow. Radiocarbon dates suggest that the slope failure occurred about 13–11 ka, a time when sea level was ca. 50 m lower than at the present day. Hemipelagic sediments in the topmost unit, III-2, above the megaturbidite indicate that the basin has been stable since ca. 11 ka.We provide robust evidence that submarine slope failures evolve downslope into slides, debris flows, and finally, thick megaturbidites. This contribution highlights the importance of seafloor morphology on the distribution and stratigraphy of submarine flows in confined basins.

The Rheological Behavior of CO2 Ice: Application to Glacial Flow on Mars

AbstractVast quantities of solid CO2 reside in topographic basins of the south polar layered deposits (SPLD) on Mars and exhibit morphological features indicative of glacial flow. Previous experimental studies showed that CO2 ice is 1–2 orders of magnitude weaker than water ice under Martian polar conditions. Here we present data from deformation experiments on pure, fine‐grained CO2 ice, over a broader range of temperatures than previously explored (158–213 K). The experiments confirm previous observations of highly nonlinear power law creep at larger stresses, but also show a transition to a previously unseen linear‐viscous creep regime at lower stresses. We examine the viscosity of CO2 within the SPLD and predict that the CO2‐rich deposits are modestly stronger than previously thought. Nevertheless, CO2 ice flows much more readily than H2O ice, particularly on the steep flanks of SPLD topographic basins, allowing the CO2 to pond as observed.

Stratigraphy and facies architecture of the Neoproterozoic syn- and inter-eruptive succession: An example from Gabal El Urf, Northeastern Desert, Egypt

A new integrated stratigraphic, sedimentological and geochemical study was carried out on the Gabal El Urf deposits to clarify their facies architecture, sequence stratigraphic framework, and evolutionary stages. The Gabal El Urf in the Northeastern Desert of Egypt is a well-preserved example of the Neoproterozoic volcano-sedimentary deposits. It comprises inter- and syn-eruptive facies that were generated by diverse depositional processes including debris flows, sheet floods, lacustrine flood flows, and pyroclastic ash falls/density currents. These facies correspond to alluvial fan, aeolian -fluvial, lacustrine, and pyroclastic facies associations that are arranged in four depositional cycles. They reveal variations in accommodation to sediment supply (A/S) ratio. Unconformities and lithofacies assemblages define six stages of volcanism and sedimentation and three unconformity-bounded tectonic sequences that should be allied to tectonic subsidence and basin uplift. These sequences may be subdivided in Low-Accommodation Systems Tracts (LAST) having negative A/S (A/S ratio close to zero) and High-Accommodation Systems Tracts (HAST) with fluctuation in A/S ratio between zero and +1. Sequences 1 and 3 are characterized by clast-supported conglomerates and pyroclastics, overlain by fluvial sand bodies and debris flow deposits, respectively denoting a LAST (positive A/S ratio close to zero). Sequence 2 is marked by pyroclastic ash fall facies at the base, passing to lacustrine deposits that are interlayered with fan-delta conglomerates at the top, defining a HAST (A/S ratio >1). The defined sequences are superimposed by thick fissure-fed ignimbrites which are capped by a co-ignimbritic breccia lag deposits with concentrated pyroclastic fountains. Clastic petrofacies of the El Urf succession indicate an adjacent miscellaneous rock sources involving volcanic, plutonic, and metamorphic terrains during the chief sedimentation processes in rift-related paleotectonic regime. The low to moderate magnitude of the CIA and PIA together with low SiO2/Al2O3 ratio indicate sediment recycling and moderate weathering of the protolith near uplifted basin flank(s) under dominant semi-arid to arid conditions. The development of the El Urf basin is akin to the post-amalgamation Ediacaran volcano-sedimentary basins that are widely distributed in the northern part of the Arabo-Nubian Shield, and is a key tectono-stratigraphic marker for the Cryogenian-Ediacaran shift along a major unconformity surface.

Lower Cambrian facies architecture and sequence stratigraphy, NW France: framework for evaluation of basin-wide processes of sedimentation

AbstractA correlation and interpretation of the sequence stratigraphy of the lower Cambrian strata of NW France is presented and used to characterize basin-wide controls on processes of sedimentation. Deposition occurred in two basins, as two third-order stratigraphic sequences. A northern basin in the Cotentin was dominated by deposition of marine siliciclastic sediments of Sequence 1 (Cambrian Stages 2–3). The primary sediment input was from the NW at La Hague. A middle Normandy basin, SW of Caen, was characterized by deposition of platform carbonates and fine-grained, offshore siliciclastics in Sequence 1 and fluvio-deltaic and marine siliciclastics in Sequence 2 (Cambrian Stages 3–4). Major sediment input points lay to the west in Brittany and to the NE of Caen (in Sequence 2). Lowstand and transgressive systems tract strata dominate both sequences, with local preservation of highstand deposits in basin-centre locations. Fluvial deposits occur at basin margins and display sandstone-dominated facies typical of prevegetation systems. Two styles of fluvially dominated siliciclastic delta are recognized, which show delta-front strata composed of fluvial channels overlying either wave-formed deposits or hyperpycnites. The former formed on shallow shelves, whereas the latter formed where basin bathymetry was steeper and commonly fault controlled. Fossil microbial life forms are conspicuous in Sequence 1, as are intervals of low-diversity bioturbation. The balance of life forms is consistent with the punctuated replacement of algal matgrounds with mixgrounds during early Cambrian time. Super-mature quartz arenites were formed by high-energy wave processes on slowly aggrading basin flanks following marine transgression.

Outer forearc high control in an erosional subduction regime: The case of the central Peruvian forearc (6–10°S)

The forearc of the North-Central Peruvian Andes (FNCPA, 6–10°S) provides an exceptional opportunity to study the long-term processes that affect a convergent plate boundary. First, it shows long-term subsidence, depocenter superimposition and individualization. Second, although being mostly extensional and characterized as a typical erosive margin, the FNCPA shows complex uplifted regions. Older deformation is expressed by basement horst and grabens disposed in a complex geometry which onset may have resulted from strike-slip tectonics. A long-lived episode of regional subsidence affected the forearc and led to the relatively thick and regional deposition of the lower Miocene series coeval with a significant increase of the convergence velocity. This period was followed by an episodic uplift of trench-parallel corridors along the so-called Main Deformation Zone. Uplift ceased through the late Miocene and restarted during Pliocene and Quaternary, generating accommodation space by basin flank uplift for a forearc depocenter characterized by landward tilted strata. Significant along-strike differences in the degree of uplift resulted in either uplifting series producing sharp seaward dipping erosional surfaces or less uplifted areas covered by seawards prograding clinoforms. As a consequence of the shallow-water marine setting, the seismic strata geometry, lateral extent and thickness of the deposits for the Neogene successions in the FNCPA have been also tightly controlled by accommodation changes. Uplift is uneven along-strike independently of fault direction and closely followed the increase of the subsidence of the continental slope produced by subduction-erosion. Therefore, sediment underplating seems the most appropriate mechanism at the origin of uplift; as observed in other parts of the Peruvian and Chilean margins. Although the erosive character of the margin, the effects on basin geometry of the raised zone resemble that of typical outer forearc highs in accretionary margins such as in the Kumano basin in Japan.

Triggers of sand remobilization in deep marine deposits

Abstract Sand injections are overrepresented in deep marine deposits and other settings with slope instabilities. We have examined seismic data from the North Sea to reveal their main trigger mechanisms. This was performed by inspection of local seismic observations in the Oligocene to Miocene sediments above the Utsira High and analyses of regional distribution of mounds at the top Hordaland Group surface in the northern North Sea. We make the observations that (1) the uplift of jack-up folds is in most cases larger than the sand thickness within the mounds, (2) onlap to mounds is present in periods with sand deposition, but no such onlap is seen in mudstones that were deposited in periods with little or no sand input, (3) onlapping to several Oligocene surfaces is seen above the Utsira High, (4) rim synclines are not present adjacent to Oligocene and Miocene mounds and (5) mounds that affect the top Hordaland Group surface are present in basin flanks and at basin-flank transitions but not in basin centres. We suggest that triggering by incipient slab sliding can explain these observations.

Stratigraphic setting of fossil log sites in the Morrison Formation (Upper Jurassic) near Dinosaur National Monument, Uintah County, Utah, USA

The outcrop belt of the Upper Jurassic Morrison Formation in the northeastern Uinta Basin and southeastern flank of the Uinta Mountains is particularly rich in dinosaurian and non-dinosaurian faunas, as well as in fossil plants. The discovery of several well-preserved, relatively intact, fossil logs at several locations in Rainbow Draw and one location in Miners Draw, both near Dinosaur National Monument (Utah), has provided an opportunity to study the local paleobotany, stratigraphy, and sedimentology of the Morrison Formation in northeastern Utah. The Morrison Formation in northeastern Utah consists of four members. In ascending chronostratigraphic order, they are the Windy Hill, Tidwell, Salt Wash, and Brushy Basin Members. The lithology (including the presenceof glauconite grains) and fossil assemblage of the lower two members (Windy Hill and Tidwell) indicate a marine to marginal marine (coastal plain) depositional environment, whereas the lithology, fossil flora and faunaassemblage of the upper two members (Salt Wash and Brushy Basin) indicate a fluvial–lacustrine depositional environment. At least 10 fossil log sites in Rainbow Draw have been documented so far, and geologic mapping indicates that the logs and wood all occur in the same stratigraphic interval within the Salt Wash Member, approximately 17 to 27 m above the base of the member. The unit containing the logs and wood is about 11 m thick and consists of very fine to fine-grained sandstone and siltstone with indistinct bedding and no discernible sedimentary features.The logs are siliceous, some have a coaly exterior, and they range in exposed length from 0.5 to 11 m and reach diameters up to 1.1 m. In the Miners Draw area, a single siliceous log is documented in the upper part of the Salt Wash Member within a silty sandstone unit that is 4 m thick; its exposed length is about 6 m. Although the correlation of the Miners Draw log-bearing interval to the interval in Rainbow Draw is uncertain, both units are lithologically similar and both occur in the upper part of the Salt Wash Member. The logs have been identified as araucariaceous conifers that pertain to the same taxon originally described as Araucarioxylon hoodii Tidwell et Medlyn 1993 from Mt. Ellen in the Henry Mountains of southern Utah. Concurrent systematic work will prompt a nomenclatural transfer of this species to the genus Agathoxylon. Based on the abundance of large fossil logs and wood in the same stratigraphic interval in Rainbow Draw, wehypothesize that the area was covered by stands of moderately large trees of araucariaceous conifers. The sedimentological evidence suggests that the trees were not transported far from their original site of growth before they were deposited in a low-energy floodplain environment.