Outcrop Datasets Research Articles

Aperture size distribution, length, and preferential location of bed-parallel veins in shale

Bed-parallel, calcite-filled veins (BPVs) are common in shale formations, and although they have been widely described in other studies, little is known about their population aperture size distribution. To address this knowledge gap, we analyzed BPV sizes in outcrops and cores from the Vaca Muerta Formation, Neuquén Basin, Argentina; in two cores from the Marcellus Formation, Appalachian Basin, northeast Pennsylvania; and in one core from the Wolfcamp Shale, Delaware Basin, West Texas. Nine out of ten aperture size populations follow a negative exponential distribution, with one following a weak power law. Bed-parallel vein size distribution and intensity vary among formations and within the same shale. We define three groups of distributions: (1) Vaca Muerta outcrops, with the highest BPV intensity and the largest BPVs (cumulative frequency of 4.9 BPVs per meter [BPVs/m] for apertures 0.265 mm to 8.7 cm); (2) Vaca Muerta cores with a similar BPV intensity overall but with no apertures wider than 1.2 cm; and (3) Vaca Muerta, Wolfcamp, and Marcellus cores with the fewest BPVs (cumulative frequency up to 0.63 BPVs/m) and very few wider than 1 cm. Aperture and length in two outcrop data sets are weakly positively correlated and follow power laws with exponents of 0.44 and 0.49. Mechanical interfaces at boundaries between different lithologies exert a strong control on BPV location, with 65–75% of observed interfaces having BPVs along them. Only 25–30% of the BPVs occur at observed material interfaces, however, and unless subtle, unobserved mechanical layering is present, other factors must also control location. BPV intensity and organic richness (TOC) from Vaca Muerta well logs are correlated in some instances but not in others, indicating TOC is not always a good proxy for BPV location or intensity. These findings provide useful information for modeling of hydraulic fracture treatments where BPVs may influence development of the stimulated fracture network, for example by limiting height growth.

Controls of accommodation to sediment-supply ratio on sedimentary architecture of continental fluvial successions

The applicability of sequence stratigraphic models to continental fluvial successions has long been topic for debate. To improve our understanding of how fluvial architectures record responses to changes in the ratio between accommodation rate and sediment-supply rate (A/S), two case studies are analyzed, including a densely drilled subsurface fluvial reservoir imaged with a seismic cube, and an outcropping fluvial succession. The subsurface dataset provides a larger, three-dimensional perspective, whereas the outcrop dataset enables observation at higher resolution. On the basis of both datasets, channel-body density, channel-body stacking patterns and their formative river types are interpreted at different scales, and how these may reflect responses to A/S change (the rate of accommodation creation relative to the rate of sediment supply) are discussed. The results indicate that (i) channel-body stacking patterns undergo four evolutionary stages along with the A/S increase, i.e., multi-story, mixed multi- and two-story, two-story, and isolated patterns; (ii) channel-body density decreases along with the channel-body stacking patterns varying from multi-story to isolated; (iii) formative rivers types are interpreted as evolving from braided planforms to braided-meandering planforms and then to meandering ones, with the increase of A/S.

Recognizing genetically related depositional packages using 3D photogrammetric outcrop models in a fluvially dominated, tidally influenced meander-belt succession

Fluvio-tidal estuary deposits are characterized by abundant channel-associated dunes, compound dunes and point bar deposits. The delineation of boundaries between different point bars to identify genetically unrelated packages is difficult to assess, particularly where similarly dipping bars are superimposed on each other. This study uses an outcrop dataset of the McMurray Formation along the Christina River in NE Alberta, Canada, focuses on bedding relationships and the nature of sedimentary contacts pertaining to the two dominant channel-associated facies: 1) dune-scale, cross-bedded sandstone (CB) representing lower parts of estuary point-bar deposit or thalweg-associated dunes; and 2) inclined heterolithic stratified sandstone and mudstone (IHS) bedsets typically representing the middle and upper part of point bar deposits. This study lays a foundation for documenting and differentiating discrete channel deposits using sedimentological observations at the outcrop, photogrammetry, and surface-orientation analyses of the resulting 3D model. The results reveal the following findings: 1) the IHS units may be underlain by erosional surfaces, which may not be genetically associated with the underlying cross-bedded sandstone intervals; 2) the forward-accreted tidal compound dunes and laterally accreted fluvio-tidal point bars share similar facies characteristics, but are discernible using bedding orientation analysis; 3) these tidal compound dunes may be under-recognized in many tidally influenced estuary deposits; and 4) an inner estuary point-bar setting may be characterized by stacked small-scale point bars, or stacked lateral accretion sets of a single point bar. The result of this study provides a detailed view of facies architecture in a tide-influenced fluvial setting, and feasible solutions to the challenge of assigning genetic units in such environment.

Alkaline sill intrusions in sedimentary basins: emplacement of the Mussentuchit Wash Sill in San Rafael Swell, Utah

Sills are important components of magmatic plumbing systems because of their role in magma storage. Previous studies have indirectly investigated sill propagation and architecture by using laboratory experiments, remote sensing, modelling and theory. However, these studies often struggle to include the complexity of natural systems, which commonly involves strong interplay between hosts and intrusions. To highlight the importance of host rock and magma interaction, we present the results from a study of combined unmanned aerial vehicle and outcrop datasets from a world-class 1.3 km long, 30 m high 3D exposure of a 12 m thick alkaline trachybasalt sill in Mussentuchit Wash, San Rafael Swell, Utah. The sill intruded into Jurassic, dominantly sandy, sedimentary rocks. Results of this study show that the propagation of the Mussentuchit Wash Sill features both fracture-driven and complex non-brittle fluid interaction emplacement, which are strongly influenced by local sedimentology and presence of porewater. Segregated melt emplaced progressively within the sill during emplacement is used to document the evolution of sill inflation. The fracture-driven propagation is initiated along sedimentary discontinuities through hydrofracturing, whereas the non-brittle fluid interaction is caused by the presence of local porewater within the sedimentary host rocks. This suggests that local lithology may exert strong control on the architecture and morphology of sills in sedimentary basins. Supplementary material: Uninterpreted images of the sill are available at https://doi.org/10.6084/m9.figshare.21269265.v1 and are included as DR1.

Geochemical studies of the Green River Formation in the Piceance Basin, Colorado

Hierarchical cluster analysis (HCA) was applied to a geochemical dataset representing the Eocene Green River Formation in the Piceance Basin of Colorado to identify chemofacies in core and outcrop samples from the basin margin and the basin center. The input dataset consisted of inductively coupled plasma optical emission spectroscopy and mass spectrometry and total organic carbon (TOC) content analyses of 186 basin margin outcrop samples and 190 basin center core samples discussed in Part 1 of this study (this volume). TOC values and twenty-five major and trace elements were used as variables to define statistical clusters of samples for the overall dataset, for the two basin center cores, and for each separate core or outcrop dataset by HCA applying Euclidean distance and Ward’s method algorithms. For each dataset, five cluster-defined chemofacies were identified. The chemofacies for each dataset show chemical affinities with five informally defined rock types– mudstone, marlstone, carbonate-rich mudstone, siliciclastic-rich mudstone/siltstone/sandstone, and Na-rich (saline) mudstone, with each showing variations in TOC content and abundance of redox sensitive minor and trace elements. A close relationship between enrichment of redox sensitive elements, particularly As and Mo, and TOC is identified in the basin center. Whereas enrichment factors (relative to average shale) are relatively low for many Period IV (PIV) transition metals, as discussed in Part 1 of this study, their consistent coherence in enrichment or depletion in HCA-defined chemofacies demonstrates the expected relationship between redox state and organic richness. Enrichment in PIV transition metals also shows a correlation to enrichment in elements with affinity for siliciclastic sediment. Enrichment/depletion among several groups of redox indicators is not everywhere consistent, with some chemofacies showing, for example, enrichment of PIV transition metals and depletion of sulfur and arsenic. Early timing of saline conditions in the basin margin is clearly displayed in the chemofacies log display, consistent with observations based on geochemical interpretations of concentrations and elemental ratios discussed in Part 1. Overall, the chemofacies are consistent with major mineralogical units and lake history stages defined in previous work, but provide more detail on the fluctuations in lake chemistry that occurred during deposition of Green River oil shale in the Piceance Basin.

ICE FLOW VELOCITY MAPPING IN EAST ANTARCTICA USING HISTORICAL IMAGES FROM 1960s TO 1980s: RECENT PROGRESS

Abstract. Recent research indicates that the estimated elevation changes and associated mass balance in East Antarctica are of some degree of uncertainty; a light accumulation has occurred in its vast inland regions, while mass loss in Wilkes Land appears significant. It is necessary to study the mass change trend in the context of a long period of the East Antarctic Ice Sheet (EAIS). The input-output method based on surface ice flow velocity and ice thickness is one of the most important ways to estimate the mass balance, which can provide longer-term knowledge of mass balance because of the availability of the early satellites in 1960s. In this study, we briefly describe the method of extracting ice velocity based on the historical optical images from 1960s to 1980s. Based on the draft ice velocity map of the EAIS using this method, we conduct a series of validation experiments, including comparisons with in-situ measurement, existing historical maps and rock outcrop dataset. Finally, we use the input-output method to estimate mass balance in some regions of EAIS using the generated velocity map.

Lateral variability of shelf-edge and basin-floor deposits, Santos Basin, offshore Brazil

ABSTRACTConstruction of continental margins is driven by sediment transported across the shelf to the shelf edge, where it is reworked by wave, tide, and fluvial processes in deltas and flanking clastic shorelines. Stalling of continental-margin progradation often results in degradation of the outer shelf to upper slope, with resedimentation to the lower slope and basin floor via a range of sediment gravity flows and mass-movement processes. Typically, our understanding of how these processes contribute to the long-term development of continental margins has been limited to observations from broadly two-dimensional, subsurface and outcrop datasets. Consequently, the three-dimensional variability in process regime and margin evolution is poorly constrained and often underappreciated. We use a large (90 km by 30 km, parallel to depositional strike and dip, respectively) post-stack time-migrated 3D seismic-reflection dataset to investigate along-strike variations in shelf-margin progradation and outer-shelf to upper-slope collapse in the Santos Basin, offshore SE Brazil. Early Paleogene to Eocene progradation of the shelf margin is recorded by spectacularly imaged, SE-dipping clinoforms. Periodic failure of the outer shelf and upper slope formed ca. 30-km-wide (parallel to shelf-margin strike) slump scars, which resulted in a strongly scalloped upper-slope. Margin collapse caused: 1) the emplacement of slope-attached mass-transport complexes (MTCs) (up to ca. 375 m thick, 12+ km long, 20 km wide) on the proximal basin floor, and 2) accommodation creation on the outer shelf to upper slope. This newly formed accommodation was infilled by shelf-edge-delta clinoforms (up to 685 m thick), that nucleated and prograded basinward from the margin-collapse headwall scarp, downlapping onto the underlying slump scar and/or MTCs. Trajectory analysis of the shelf-edge deltas suggests that slope degradation-created accommodation was generated throughout the sea-level cycle, rather than during base-level fall as would be predicted by conventional sequence-stratigraphic models. Our results highlight the significant along-strike variability in depositional style, geometry, and evolution that can occur on this and other continental margins. Coeval strata, separated by only a few kilometers, display strikingly different stratigraphic architectures; this variability, which could be missed in 2D datasets, is not currently captured in conventional 2D sequence stratigraphic models.

Controls on the deposition and preservation of architectural elements within a fluvial multi-storey sandbody

Architectural elements of fluvial multi-storey sandbodies provide principal controls on the distribution of meso-scale (100–101 m scale) heterogeneity and reservoir quality. Consequently, it is valuable to understand the deposition and preservation of sedimentary architecture in such systems in relation to autogenic (stream capture and avulsion) and allogenic controls (subsidence rates, climate and sediment supply). The aims of this study are to quantify the architectural and erosional nature of a fluvial multi-storey sandbody and to establish the effects of downstream distance and subsidence rates upon the preservation of architectural elements, using the Lower Castlegate Sandstone, Utah, USA, as an example. Quantitative architectural element analysis and palaeodischarge reconstructions were undertaken from eight locations using sedimentary logs and three terrestrial photogrammetric outcrop datasets along a 150 km down-dip profile. These observations were supplemented by burial history analysis of ten wells across the same profile. Results show the Lower Castlegate comprises channel-fill, downstream accretion, lateral accretion, upstream accretion and overbank elements. From these observations, calculations of sinuosity and flow depths along with architectural geometric analysis provide evidence of stream capture contemporaneous with foreland basin subsidence. The preservation of lateral accretion and overbank elements is limited within the distal portion of the multi-storey sandbody, as a result of local avulsion and limited subsidence rates. Results demonstrate that complex sedimentary architecture can form in fluvial multi-storey sandbodies as a product of variable discharge rates, the fluvial graded profile and spatially variable aggradation rates, driven principally by subsidence rates. The use of meso-scale architectural analysis, with analysis of in-channel sinuosity and hydrodynamics, along with erosional bounding surfaces, has helped to complemented basin-scale interpretations of fluvial architecture.

Variable‐discharge‐river macroforms in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, USA

AbstractAn outcrop dataset from the early Eocene Sunnyside Delta Interval of the Green River Formation in the Uinta Basin, Utah, USA, documents alluvial channel lithosomes. The abundance of Froude supercritical‐flow sedimentary structures, together with an abundance of high‐deposition‐rate sedimentary structures, in‐channel bioturbation and pedogenic modification, in‐channel muds and thick soft‐clast conglomerates, identify these lithosomes as deposits of variable‐discharge rivers. These recognition criteria are part of an emerging facies model for variable‐discharge rivers. This facies model, however, yet lacks robust recognition criteria for macro‐scale or bar‐scale stratal patterns of variable‐discharge rivers. This study presents a dataset that corroborates some known stratal patterns and provides examples of hitherto unknown bar‐scale stratal patterns of variable‐discharge rivers, including: (i) low‐angle downstream‐accretion sets that may form as washed‐out sheets in high sediment supply conditions or downstream of hydraulic jumps; (ii) high‐angle upstream‐accretion sets that imply deposition from systematically upstream‐migrating channel‐scale hydraulic jumps (cyclic steps); (iii) concave‐up, upward‐flattening high‐angle downstream‐accretion sets that are consistent with aggradation in channel‐scale hydraulic‐jump scours; (iv) upstream‐accretion and lateral‐accretion sets that may be linked to high‐magnitude flood reworking of point bars; and (v) aggradation or vertical‐accretion sets of ambiguous origin. These unconventional stratal patterns are compared to the established bar strata, such as those formed by point bars and braid bars and a discussion is provided on formative conditions for the here documented unconventional strata. This work highlights a need for further studies on the effect of discharge variability on bar formation and on the link between river morphology and bar types.

Froude supercritical flow processes and sedimentary structures: New insights from experiments with a wide range of grain sizes

AbstractRecognition of Froude supercritical flow deposits in environments that range from rivers to the ocean floor has triggered a surge of interest in their flow processes, bedforms and sedimentary structures. Interpreting these supercritical flow deposits is especially important because they often represent the most powerful flows in the geological record. Insights from experiments are key to reconstruct palaeo‐flow processes from the sedimentary record. So far, all experimentally produced supercritical flow deposits are of a narrow grain‐size range (fine to medium sand), while deposits in the rock record often consist of a much wider grain‐size distribution. This paper presents results of supercritical‐flow experiments with a grain‐size distribution from clay to gravel. These experiments show that cyclic step instabilities can produce more complex and a larger variety of sedimentary structures than the previously suggested backsets and ‘scour and fill’ structures. The sedimentary structures are composed of irregular lenses, mounds and wedges with backsets and foresets, as well as undulating planar to low‐angle upstream and downstream dipping laminae. The experiments also demonstrate that the Froude number is not the only control on the sedimentary structures formed by supercritical‐flow processes. Additional controls include the size and migration rate of the hydraulic jump and the substrate cohesion. This study further demonstrates that Froude supercritical flow promotes suspension transport of all grain sizes, including gravels. Surprisingly, it was observed that all grain sizes were rapidly deposited just downstream of hydraulic jumps, including silt and clay. These results expand the range of dynamic mud deposition into supercritical‐flow conditions, where local transient shear stress reduction rather than overall flow waning conditions allow for deposition of fines. Comparison of the experimental deposits with outcrop datasets composed of conglomerates to mudstones, shows significant similarities and highlights the role of hydraulic jumps, rather than overall flow condition changes, in producing lithologically and geometrically complex stratigraphy.

Shallow‐water deltaic clinoforms and process regime

AbstractUtilizing two outcrop data sets with dip direction exposures of shallow‐water (tens of meters) deltaic clinoforms, this paper quantifies sedimentary facies proportions and clinoform lengths and gradients, and links process regimes to delta clinoform dimensions. Both data sets are from foreland basins, the Cretaceous Chimney Rock Sandstone of the Rock Springs Formation from the US Western Interior, and the Eocene Brogniartfjellet Clinoform Complex 8 of the Battfjellet Formation from the Central Basin of Spitsbergen. Sedimentary facies indicate presence of both river‐ and wave‐dominated clinothems in each data set. Facies characteristics and distribution implies that river‐dominated clinothem progradation was primarily driven by deposition from weak hyperpycnal flow turbidity currents across the clinoforms, and minor slumps. Wave‐dominated clinothems were constructed by wave processes rather than alongshore currents, and are also progradational subaerial clinoforms, with one exception, where the formation of a compound subaqueous clinoform set indicates erosion and sediment bypass above the wave base. Sediment distribution and lithological heterogeneity in the river‐dominated clinothems is controlled by individual hyperpycnal flow events or mouth‐bar collapse events, and thus by self‐organization and minimal reworking that results in a heterogeneity that is difficult to predict (high entropy). The efficient reworking of river‐derived sediments in wave‐dominated clinothems results in predictable lithological sediment partitioning (low entropy). Clinoform dimension analyses show that although of similar sediment caliber, river‐dominated clinoforms in both data sets are on average 3–4 times steeper and 3–4 times shorter than the wave‐dominated clinoforms, with mean gradients of ca 4 degrees and ca 1 degree, respectively, and mean lengths of 150–230 m and 640–760 m. These results require corroboration from additional data sets, but do suggest that river‐ and wave‐dominated delta clinoforms are likely to have distinct downdip extents (lengths) and gradients for given clinoform heights. Clinoform shape can thus be a method for differentiating ancient river‐ vs. wave‐dominated deltaic clinoforms, in addition to their sedimentary facies, biogenic features and sandstone maturity, and helpful when incorporated into reservoir models.

Testing and evaluation of 2D/3D digital image analysis methods and inclusion theory for microporosity and S-wave prediction in carbonates

Abstract Three digital image analysis (DIA) methods are proposed to evaluate the geometric properties of the pore system in 2D/3D and identify the macro- and mesopores: 1) optical microscopy of thin sections computed by Fiji-ImageJ software, 2) μCT data volume decomposition using 2D slices and Fiji-ImageJ, and 3) application of GEODICT, 2015) software and PoreShape GeoLab routines to compute the 3D pores. Microporosity is characterized using assumptions based on the difference between helium gas and the image porosity and the classification system by Anselmetti et al. and rock physics modeling of the differential effective medium theory is applied to estimate the micropore aspect ratio via Vp, supplying a calibrated method to predict Vs for similar rocks. According to the results, the 2D processing methods (#1 and #2) are more realistic and theoretically consistent for estimating 2D DIA parameters. The approximation of the detected 3D pores to 2D in method #3 results in poor representativeness of the pore texture, limiting the potential usefulness of the μCT data, although the same resolution is inherent to the μCT data volume and is applied in methods #2 and #3. The use of μCT data from 2D slices (method #2) was evaluated as the best method for computing the 2D DIA parameters, and the optical thin sections used in method #1 were limited by differences in the resolution and the low representativeness of the micrograph quantity in correlating the texture complexities. This work generates a valuable outcrop dataset and original discussions of DIA at different image scales for input modeling to match ultrasonic and elastic properties with the lithological characteristics of Oman samples.

Quantifying tabularity of turbidite beds and its relationship to the inferred degree of basin confinement

Abstract Tabular beds and sheet-like deposits in deep-water systems have been the subject of much research attention; they can form high quality hydrocarbon reservoirs, owing to their excellent lateral continuity and predictable geometry. Additionally, deposit tabularity is a piece of evidence used to infer flow confinement in ancient systems and thus to evaluate the suitability of outcrop datasets as reservoir analogues. However, the quantification of tabularity is rarely attempted and a consistent definition on how to describe it quantitatively is lacking. For this study, published data from eighteen well-constrained ancient turbidite systems in outcrop were analysed. A simple and novel methodology for the quantitative calculation of tabularity along a transect from log panels and photo panels was devised, based on: a) subdividing beds into two groups based on their thickness, b) calculating the percentage of beds continuous across a fixed window (500 m) and c) calculating the rate of thinning for the continuous beds within the same window. Calculations obtained from multiple locations within individual systems enable the investigation of proximal to distal, and axial to lateral changes in tabularity to be captured, and therefore permits the evaluation of tabularity in three-dimensions. A comparison between tabularity of the considered systems and their inferred degree of basin confinement shows that in the confined systems ≥90% of beds are continuous over 500 m compared to ≤40% for the two unconfined systems studied. In addition, different bed types were compared: hybrid event bed thinning rates are shown to be up to three times those of classical turbidites. This methodology provides a new tool to compare tabularity within and between systems quantitatively. It is hoped that the quantitative determination of tabularity will become a common workflow when describing ancient turbidite systems. It is suggested that this approach will enhance the value of outcrop data to inform models capturing the architecture of systems analogous to subsurface hydrocarbon reservoirs.

From conventional outcrop datasets and digital outcrop models to flow simulation in the Pont de Montanyana point-bar deposits (Ypresian, Southern Pyrenees)

Abstract The Pont de Montanyana outcrop exposes strata of the lower Eocene Montllobat Formation deposited in alluvial plains and in meandering river systems of the Tremp-Graus piggyback basin (Southern Pyrenees, Spain). This article focuses on the characterisation of the outcrop using conventional descriptions (photomosaics, correlated stratigraphic logs and palaeocurrent data) together with 3D interpretations of digital outcrop models. The architectural elements identified include lateral-accretion deposits, channel fills (meandering, chute and distal alluvial-fan types), overbank sheets (wings and crevasse splay deposits) and floodplain fines. The channelised deposits were collectively classified, using the width (W) and thickness (T) ratio, as ribbons (W/T ≤ 15) to narrow sheets (W/T ≤ 100). Variations in sandstone/mudstone ratio (between 20% and more than 90% of sandstone) and in the internal architecture of the meandering channel fills suggest deposition in the abandoned channel at different times of the cut-off process and at different locations in relation to the position of the active channel. A high-resolution 3D facies model of the upper stratigraphic interval was constructed, containing point-bar deposits and an associated channel fill. The model was subsequently populated with petrophysical data representative of point-bar reservoirs. Flow simulations based on a water-injection development strategy were carried out. The flow behaviour and the distribution of the bypassed oil are determined by the high degree of internal heterogeneity in the meandering channelised deposits. This heterogeneity is produced by the facies variability, the inclined bedding of the lateral-accretion deposits, the lithology of the channel fill, the downstream fining along the bend and by the change in the orientation of the lateral-accretion surfaces due to expansion and rotation of the point bar. The moveable oil was effectively drained from the sandstone-dominated channel-fill body. However, more than 27% of the moveable oil remained trapped in the uppermost part of the lateral-accretion deposits.

A complete glacier inventory of the Antarctic Peninsula based on Landsat 7 images from 2000 to 2002 and other preexisting data sets

Abstract. The glaciers on the Antarctic Peninsula (AP) potentially make a large contribution to sea level rise. However, this contribution has been difficult to estimate since no complete glacier inventory (outlines, attributes, separation from the ice sheet) is available. This work fills the gap and presents a new glacier inventory of the AP north of 70° S, based on digitally combining preexisting data sets with geographic information system (GIS) techniques. Rock outcrops have been removed from the glacier basin outlines of Cook et al. (2014) by intersection with the latest layer of the Antarctic Digital Database (Burton-Johnson et al., 2016). Glacier-specific topographic parameters (e.g., mean elevation, slope and aspect) as well as hypsometry have been calculated from the DEM of Cook et al. (2012). We also assigned connectivity levels to all glaciers following the concept by Rastner et al. (2012). Moreover, the bedrock data set of Huss and Farinotti (2014) enabled us to add ice thickness and volume for each glacier. The new inventory is available from the Global Land Ice Measurements from Space (GLIMS) database (doi:10.7265/N5V98602) and consists of 1589 glaciers covering an area of 95 273 km2, slightly more than the 89 720 km2 covered by glaciers surrounding the Greenland Ice Sheet. Hence, compared to the preexisting data set of Cook et al. (2014), this data set covers a smaller area and one glacier less due to the intersection with the rock outcrop data set. The total estimated ice volume is 34 590 km3, of which one-third is below sea level. The hypsometric curve has a bimodal shape due to the unique topography of the AP, which consists mainly of ice caps with outlet glaciers. Most of the glacierized area is located at 200–500 m a.s.l., with a secondary maximum at 1500–1900 m. Approximately 63 % of the area is drained by marine-terminating glaciers, and ice-shelf tributary glaciers cover 35 % of the area. This combination indicates a high sensitivity of the glaciers to climate change for several reasons: (1) only slightly rising equilibrium-line altitudes would expose huge additional areas to ablation, (2) rising ocean temperatures increase melting of marine terminating glaciers, and (3) ice shelves have a buttressing effect on their feeding glaciers and their collapse would alter glacier dynamics and strongly enhance ice loss (Rott et al., 2011). The new inventory should facilitate modeling of the related effects using approaches tailored to glaciers for a more accurate determination of their future evolution and contribution to sea level rise.

An automated methodology for differentiating rock from snow, clouds and sea in Antarctica from Landsat 8 imagery: a new rock outcrop map and area estimation for the entire Antarctic continent

Abstract. As the accuracy and sensitivity of remote-sensing satellites improve, there is an increasing demand for more accurate and updated base datasets for surveying and monitoring. However, differentiating rock outcrop from snow and ice is a particular problem in Antarctica, where extensive cloud cover and widespread shaded regions lead to classification errors. The existing rock outcrop dataset has significant georeferencing issues as well as overestimation and generalisation of rock exposure areas. The most commonly used method for automated rock and snow differentiation, the normalised difference snow index (NDSI), has difficulty differentiating rock and snow in Antarctica due to misclassification of shaded pixels and is not able to differentiate illuminated rock from clouds. This study presents a new method for identifying rock exposures using Landsat 8 data. This is the first automated methodology for snow and rock differentiation that excludes areas of snow (both illuminated and shaded), clouds and liquid water whilst identifying both sunlit and shaded rock, achieving higher and more consistent accuracies than alternative data and methods such as the NDSI. The new methodology has been applied to the whole Antarctic continent (north of 82°40′ S) using Landsat 8 data to produce a new rock outcrop dataset for Antarctica. The new data (merged with existing data where Landsat 8 tiles are unavailable; most extensively south of 82°40′ S) reveal that exposed rock forms 0.18 % (21 745 km2) of the total land area of Antarctica: half of previous estimates.

Stratigraphic evolution of a long-lived submarine channel system in the Late Cretaceous Nanaimo Group, British Columbia, Canada

Submarine canyons and slope channel systems are important conveyers of sediment from uplifted catchments to oceanic sedimentary sinks. Long-lived conduits can be established through deep incision of submarine canyons, with bathymetric relief of hundreds of meters to greater than a kilometer in many instances. Alternatively, a combination of erosion of the continental slope and aggradation of levees can yield a broadly comparable stratigraphic product through evolution of channels with more subdued bathymetric relief. Despite differences in formative geomorphic elements on the paleo-seafloor, differentiating the stratigraphic architecture amongst these systems is challenging, particularly in outcrop datasets. Accurate stratigraphic interpretation has significant implications for understanding the frequency and magnitude of controlling processes such as mountain building and denudation or eustatic sea-level fluctuations.In this study, deep-water channel strata of the Late Cretaceous Nanaimo Group are examined at Hornby and Denman islands, British Columbia, Canada. Evidence for a long-lived submarine conduit records the history of sediment transfer at multiple temporal and spatial scales. The composite submarine channel system deposit is 19.5km wide and 1500m thick, which formed and filled over ~15Ma. Facies scale analyses highlight conglomeratic channel fill juxtaposed against thin-bedded out-of-channel deposits. Erosional surfaces are commonly mantled by mass-transport deposits, which provide evidence for conduit wall reworking and maintenance. At a larger scale, a series of composite, conglomerate-prone channelform bodies are observed to stratigraphically stack in two distinct phases: (1) early persistence of laterally offset (migrated) channels; and (2) later vertically aligned and aggraded channels. This stratigraphic trend is comparable to composite, multi-phase degradational-aggradational submarine channel complexes observed globally. As such, we consider the stratigraphic evolution of the large submarine conduit deposit (1500m thick) to have formed through the protracted processes of geomorphic channels with subdued paleo-bathymetric relief (20–200m). Detailed observations at the scale of facies and stratigraphic architecture provide criteria for recognition and interpretation of long-lived slope channel systems, emphasizing a disparate relationship between stratigraphic and geomorphic surfaces.

Late Devonian Carbonate Margins and Foreslopes of the Lennard Shelf, Canning Basin, Western Australia, Part B: Development During Progradation and Across the Frasnian-Famennian Biotic Crisis

Abstract Many studies document the occurrence of carbonate progradational successions that are driven by overall accommodation-limited conditions and represent periods when substantial volumes of sediment are stored in slope settings. However, few have reported on the detailed internal anatomy and components of prograding clinothems, and furthermore, little is known about how environmental parameters affect the development of carbonate margins and slopes when superimposed on the long-term, low-accommodation signal. Upper Devonian (Frasnian and Famennian) carbonate outcrop exposures along the Lennard Shelf, northeast Canning Basin, Western Australia, offer the examination of reefal margin and foreslope development in a variety of settings, one of which is long-term (> 10 Ma) progradation during the late Frasnian and Famennian. In the early phases of progradation, a global biotic crisis and faunal overturn occurred around the Frasnian–Famennian (F-F) boundary, and indications of global climatic cooling are interpreted at points throughout the Late Devonian. These extrinsic and intrinsic controls provide ecologic context and a stratigraphic framework to assess carbonate margin and foreslope architectures and facies distributions in order to better characterize and predict their complex heterogeneity. The outcrop dataset collected includes measured sections tied to interpreted photomosaics and detailed mapping using aerial photographs from the South Lawford Range, Windjana Gorge, and Dingo Gap areas. The outcrop observations highlight distinctive changes in margin style, foreslope composition, stratigraphic packaging, and strike variability within the contexts of the F-F biotic crisis, long-term highstand (HST) progradation, and Late Devonian climatic changes. Intervals before and after the F-F boundary exhibit notable increases in downslope calcimicrobial boundstone encrustation and evidence for waning foreslope contributions from other, previously flourishing, carbonate factories, reflecting stressed ecological conditions and consequent calcimicrobial opportunism during gradual buildup and recovery periods that bracket the extinction. The Famennian slope (post-effects related to the F-F crisis) can be subdivided into a calcimicrobial boundstone-dominated upper slope with a highly complex downdip transition into a mixed debris- and grain-dominated middle slope, and a silt-dominated lower slope to toe of slope, and exhibits a highly complex lateral heterogeneity associated with multiple contributing sediment factories and temporal partitioning of resedimentation. A repeated stacking pattern is observable in the Famennian middle-slope setting, consisting of upward successions of debris followed by a gradation from silt-dominated deposits into grain-dominated deposits; we here interpret these as the expression of high-frequency sequences superimposed upon the longer-term progradation, and a function of transitional (greenhouse to icehouse) climates. These findings offer predictive relationships that link the internal depositional heterogeneity of prograding slope systems to the interplay of long-term accommodation drivers, ecological conditions affecting carbonate sediment factories, and climatic conditions that control the development of high-frequency sequences.

Laramide Signals and Architecture of A Widespread Fluvial Sand Sheet: Canyon Creek Member, Southern Wyoming, U.S.A.

Abstract The Upper Campanian Canyon Creek Member of the Ericson Formation in the Greater Green River Basin of southern Wyoming, USA, is a regionally extensive sandstone sheet of fluvial origin. A well-log and outcrop dataset allows this ancient fluvial system to be examined basin-wide, covering an area of approximately 45,000 km2. Changes in fluvial style and architecture, and in thickness and net-to-gross occur both down-dip and along strike, across the whole fluvial system, allowing insight on both local and regional structural styles in the basin. From the proximal reaches of the system in the west and northwest to the distal areas in the east and southeast, the Canyon Creek Member (and its equivalents) shows a first-order trend of thickening and of net-to-gross reduction, as well as a change of planform style from braided to meandering. The reason for these trends is an eastward increase of subsidence away from the orogenic belt, probably resulting from reduced activity in the Sevier fold–thrust belt, which decreased the importance of the tectonic loading and flexural subsidence and enhanced the signal of the dynamic subsidence. The Laramide orogeny is typically thought to have been initiated during the Maastrichtian or Paleocene. However, study of the Canyon Creek Member strongly indicates earlier initiation during the Campanian. The above general proximal-to-distal trends in the Canyon Creek are locally modified by early differential movement across the Rock Springs Uplift, the Uinta Mountains, the Rawlins Uplift, and the Sierra Madre Uplift, all of which are Laramide-style structures. This early movement is shown by reduced thickness and increased net-to-gross over the crest of these structures and by the opposite trend down-flank on the structure into the footwall of the adjacent structure. This asymmetric pattern of thickness and net-to-gross development within individual Canyon Creek “subbasins” is the typical and distinctive Laramide basin signature. The Canyon Creek Member of the Ericson Formation is a clear example of how varied a fluvial system can be laterally in response to minute changes in accommodation, slope, and other parameters, and illustrates how important it is to recognize that variability before embarking in the more complicated task of recognizing these changes through time in a sequence stratigraphic framework.

Quantitative Analysis of the Dimensions and Distribution of Channelized Fluvial Sandbodies Within A Large Outcrop Dataset: Upper Cretaceous Blackhawk Formation, Wasatch Plateau, Central Utah, U.S.A

Abstract Conceptual and quantitative models of fluvial stratigraphy typically argue that alluvial architecture is driven by a combination of allogenic processes (e.g., tectonic subsidence, sea level, sediment supply), and autogenic behaviors (e.g., avulsion). We test these models via analysis of a large (c. 200 m thick by 100 km depositional-strike extent) alluvial-to-coastal-plain succession that records shoreline progradation in response to decreasing tectonic subsidence (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, U.S.A.). Oblique aerial photographs and architectural panels of six nearly linear, nearly vertical cliff face “windows” were used to characterize the dimensions (apparent widths, thicknesses) and spatial distributions of channelized fluvial sandbodies. Two statistical measures, raster-based lacunarity and point-based L-function, are used to investigate whether the distribution of channelized sandbodies show significant regularity, randomness, or clustering. Over 490 channelized fluvial sandbodies are identified in the six cliff-face “windows.” From base to top of the Blackhawk Formation, sandbodies broadly increase in width and decrease in overall abundance per unit area. Clustering of sandbodies occurs relatively frequently in lower-coastal-plain strata ( 50 km from the coeval shoreline). A strong negative correlation between lacunarity and stratigraphic position is also observed, such that a wider range and greater variety of spatial gaps occurs between sandbodies in lower-coastal-plain strata relative to upper-coastal-plain and alluvial strata. Comparison with numerical modeling studies of fluvial stratigraphic architecture implies the predominance of an avulsion-generated pattern of sandbody distribution that includes an element of compensational stacking on the upper coastal plain and alluvial plain, for a range of distances from the coeval shoreline (c. 40–130 km) and tectonic subsidence rates (c. 40–700 m/ Myr). On the lower coastal plain (< 50 km from the coeval shoreline) localized clustering of channelized sandbodies is interpreted to have formed by avulsion of deltaic distributary channels downstream of delta-apex avulsion nodes, possibly modulated by low-amplitude (< 30 m) sea-level fluctuations.