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Mixed-Energy Shoreline–Shelf Systems: Processes, Facies and Architecture

Description: The shoreline–shelf setting includes a complex array of sedimentary systems (coastal-deltaic, paralic and shallow marine), which are influenced by dynamic interactions between physiography, sediment supply, biology, and depositional processes, amongst other factors. The shoreline–shelf transect is defined by the location, geometry and nature of critical transition zones, e.g. the fluvial-marine transition zone (FMTZ) and shelf-edge rollover zone (SERZ). Within these transition zones, major changes in sediment transport and depositional processes occur. Across and between these zones, studies have recognised the complexities in the preserved stratigraphic record and the impact of climatic and physiographic controls. However, many challenges remain for understanding shoreline–shelf settings e.g. 1) the distribution of facies, ichnofacies and architecture in mixed-process systems; 2) understanding process interaction and sedimentary preservation across a broader range of physiographic settings; and 3) constraining the along-strike and down-dip variability in resulting stratigraphic architecture. Therefore, in this session we invite contributions from modern, ancient and numerical modelling studies of shoreline–shelfal depositional environments to consolidate and improve our understanding of the complex interaction of numerous factors in this segment of source-to-sink systems.

Convener: Daniel CollinsECSECS | Co-conveners: Miquel Poyatos-Moré, Helena van der VegtECSECS
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Wed, 28 Apr, 13:30–14:15

Chairpersons: Helena van der Vegt, Miquel Poyatos-Moré, Daniel Collins

Anna van Yperen et al.

Mouth bars are fundamental architectural elements of deltaic successions. Understanding their internal architecture and complex interaction with coastal processes (fluvial-, tide- and wave-dominated) is therefore paramount to the interpretation of ancient deltaic successions. This is particularly challenging in low-accommodation systems because they are commonly characterized by a thin, condensed and top-truncated expression. In this study we analyze the exhumed Cenomanian Mesa Rica Sandstone (Dakota Group, Western Interior Seaway, USA), which encompasses a fluvio-deltaic system along a ~450 km depositional dip-parallel profile. The study targets the proximal deltaic expression of the system, using 22 sedimentary logs (total of 390 m) spatially correlated within a ~25 km2 study area at the Tucumcari Basin margin. Analysis of facies distribution, depositional architecture and stratigraphic surfaces mapping reveals a 6–10-m-thick, sharp-based and sand-prone deltaic package, comprising several laterally-extensive (>1.4 km width) mouth bars. Within those, we distinguish four different along-strike sub-environments based on differences in grain size, sedimentary structures, bed thicknesses, and bioturbation indices; these are mouth bar axis, off-axis, lateral fringe to distal lateral fringe deposits, and overall reflect waning depositional energy with increasing distality from the distributary channel mouth. The interpreted mouth-bar components also show internal variability in dominant process regime, with overall river dominance but local preservation of tide influence in the lateral fringe and distal fringe environments. However, mouth-bar deposits amalgamate to form an extensive sand-rich sheet body throughout the study area, in which interflood mudstone to very-fine grained sandstone beds are nearly absent. This indicates a low accommodation/supply (A/S) setting, which promoted recurrent channel avulsion/bifurcation and thus reworking of mouth-bar fringe and distal-fringe sediments, where background coastal processes tend to be better recorded.

Trends in along-strike changes in sedimentary characteristics from axial to lateral environments are also recognized in other wave- and river-dominated deltaic settings as well, where axial components consist of higher energy facies associations resulting from high-density currents, whereas heterolithics become dominant towards the fringes, where there is an alternation of low- and high-density deposits combined with an increased recording of finer-grained facies associations. Complemented with our study, this suggests that internal hierarchy of mouth bars is evident and observed regardless of dominant coastal processes. Consequently, subdivision of mouth bars into different components can reduce complexity of models deriving from a myriad of facies subdivisions, and guide prediction of facies changes and sand distribution in future studies of proximal deltaic settings. Finally, results of this study evidence internal process-regime variability within mouth-bar components. This cautions against relying solely on the preserved deposits at one given location in a system to infer dominant and subordinate coastal processes (e.g. tidal indicators), with a consequent risk of underestimating the true mixed-influence nature of low-accommodation deltaic settings.

How to cite: van Yperen, A., Poyatos-Moré, M., Holbrook, J., and Midtkandal, I.: Internal mouth-bar variability and differential preservation of coastal-process indicators in low-accommodation deltaic settings , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2895, https://doi.org/10.5194/egusphere-egu21-2895, 2021.

Matthew Watkinson et al.

Improved understanding of delta mouth bar morphodynamics, and the resulting stratigraphic architectures, is important for predicting the loci of deposition of different sediment fractions, coastal geomorphic change and heterogeneity in mouth bar reservoirs. Facies and architectural analysis of exceptionally well-exposed shallow water (ca. 5 m depth) mouth bars and associated distributaries, from the Xert Formation (Lower Cretaceous), of the Maestrat Basin (east-central Spain), reveal that they grew via a succession of repeated autogenic cycles. The formation is part of a mixed clastic-carbonate succession deposited during a time of active faulting and incipient salt tectonism, but in an area away from their direct influence and where wave and tidal reworking were minimal.

An initial mouth bar accretion element forms after avulsion of a distributary into shallow standing water. Turbulent expansion of the fluvial jet and high bed friction results in rapid flow deceleration, and deposition of sediment in an aggradational to expansional bar-form. Vertical bar growth causes flattening and acceleration of the jet. The accelerated flow scours channels on the bar top, which focuses further expansion of the mouth bar at individual loci where the channels break through the front of the mouth bar. Here, new mouth bar accretion elements form, downlapping and onlapping against a readily recognizable surface of mouth bar reorganization. Vertical growth of the new mouth bar accretion elements causes flattening and re-acceleration of the jet, leading to channelization, and initiation of the next generation of mouth bar accretion elements. Thus the mouth bar grows, until bed-friction effects cause backwater deceleration and superelevation of flow in the feeding distributary. Within-channel sedimentation, choking and upstream avulsion of the feeding channel, results in mouth bar abandonment. In this study, mouth bars are formed of at least two to three accretion elements, before abandonment happened. The results of this study contrast with the notion that mouth bars form by simple vertical aggradation and radial expansion. However, the architecture and facies distributions of shallow water mouth bars are a predictable product of intrinsic processes that operate to deposit them.

How to cite: Watkinson, M., Cole, G., and Jerrett, R.: The architecture and evolution of shallow water delta mouth bars: examples from the Lower Cretaceous of Spain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5702, https://doi.org/10.5194/egusphere-egu21-5702, 2021.

A. Aulia Valencia et al.

Only a limited amount of data is available to quantify the impact of syn-depositional compaction on delta depositional patterns. In this study, we investigate numerically how different scenarios for compaction rate (0 - 10 mm yr-1) drives morphological variations in mud- and sand-rich fluvial-dominated deltas. To do this, a 1D grain-size dependent compaction model was implemented into the open-source Delft3D. This implementation allows deposited sediment to decrease in thickness over time due to the accumulation of newly deposited sediments above. The resultant sedimentary deposits of a prograding delta are post-processed to highlight the changes in depositional patterns under different compaction scenarios. Deposits are classified into sub-environment (e.g., delta top, delta front, and pro delta). The delta top geometry (e.g., area, shape, and rugosity) and the distribution of sediment between different sub-environments are compared. The modeling results verify that the larger compaction-induced subsidence affects accommodation provision. We show that this results in more significant sediment deposition and more evenly distributed sediment across the delta top. Larger compaction results in a smaller area with a more semi-circular shape and less rugose delta top. The modeling results presented here bridges the knowledge gap on the effects of syn-depositional compaction on delta morphology evolution.

How to cite: Valencia, A. A., Storms, J. E. A. (., Walstra, D.-J., van der Vegt, H., and Jagers, H. R. A. (.: The impact of compaction of clastic sediments on fluvial-dominated delta morphology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13530, https://doi.org/10.5194/egusphere-egu21-13530, 2021.

Jiaguang Li et al.

River deltas grow through episodic channel-jumping events, called avulsions, which have caused some of the deadliest floods in human history. Climate change is threatening to drown river deltas through a global increase in sea level; however, it is unclear how sea-level rise may affect the location of avulsion sites. Theory and experiments indicate that the avulsion sites on lowland deltas emerge within the backwater zone of coastal rivers because of the morphodynamic feedbacks arising from natural flood discharge variability and the nonuniform flows caused by the standing water level in the receiving basin. Under this backwater hypothesis, marine transgression should cause the landward-migration of lobe-scale avulsion locations; however, we currently lack field evidence for this theoretical prediction. Here, we analyze the location of river avulsions on the Sulengguole River that drains into the North Hubusun Lake, Qaidam Basin, China. Using analysis of time-series satellite imagery, we identified 7 lobe-scale avulsions that occurred in the distal portions of the Sulengguole River during the observation period of 1985 to 2010 CE. Satellite imagery revealed that the areal extent of the seasonal water in the lake increased at a rate of 1.89±0.80 km2/yr, likely as a result of increase in extreme precipitation rates. The increase in seasonal lake water areas caused the river mouth of the Sulengguole River to translate landward at a rate of 0.36±0.17 km/yr. We show that the avulsion sites also migrated landward at a commensurate rate of 0.24±0.07 km/yr during this period, consistent with the rate of landward migration of the river mouth. Finally, we show that all 7 avulsions had an avulsion length—streamwise distance of the avulsion site to the river mouth—that scales with the estimated backwater lengthscale (mean of 0.50±0.14 times the backwater lengthscale), consistent with the global compilation of avulsion lengths on large, low-gradient deltas. Our work demonstrates, for the first time, that landward migration of river mouth that would result from relative sea-level rise will cause the avulsion locations to migrate inland in a predictable manner, with implications for the sustainable management of the future of deltas and mitigating flood hazards.

How to cite: Li, J., Ganti, V., Wei, H., and Li, C.: Landward migration of backwater-mediated delta avulsion sites in response to increase in seasonal lake level: Qaidam Basin, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6914, https://doi.org/10.5194/egusphere-egu21-6914, 2021.

Brian Willis and Tao Sun

Emergent structures define organizational patterns that spontaneously develop due to interactions between component properties or behaviors of complex dynamic systems, rather than being a simple compilation of the individual parts observed within the system at any one time. Traditional facies models used to predict subsurface lithic variations focus on defining the distribution of depositional environments on Earth’s surface and relating the hierarchy of preserved bedding units to different scales of surficial bedforms. It is increasingly recognized that such static models fail to predict the geometry and character of many types of preserved lithic bodies and discontinuity surfaces unless these observations are placed within the context of the overall evolving system. Numerical depositional process models are presented to show links between evolving depositional patterns and preserved facies patterns within different settings.

     Channel deposit internal variations tend not to be channel shaped, but rather sweet spots within the deposit resemble a string of beads, each formed as individual channel segments meander. Mouth bar deposits generally do not to have the circular to elliptical shape of a modern channel-mouth bedform, but rather tend to be more elongate fingers cut by a diachronous channel filled as river flows are choked off by loss of gradient during progradation. Although the final channel basal erosion surface appears continuous, timelines cross this surface along the length of the deposit. Deltaic shorelines that look identical at a given time preserve very different deposits when the feeding river avulses at different frequency, a condition that can change within an individual deposit formed alternately during periods of sea level rise and fall. Even major stratigraphic surfaces, like lowstand fluvial incision surfaces and wave-ravined falling stage and transgressive surfaces, are likely to gradually emerge from the migration of localized areas of erosion that were never as extensive at any one time as the preserved surface. Such surfaces may be regionally diachronous, with deposits of the same age locally preserved variably above and below the surface. Understanding emergent lithic bodies and internal heterogeneity patterns are fundamental to understanding how deposition is recorded in the rock record and for facies models used to predict how subsurface fluids move through shallow marine deposits.

How to cite: Willis, B. and Sun, T.: Subsurface heterogeneity patterns that emerge from interacting depositional processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3472, https://doi.org/10.5194/egusphere-egu21-3472, 2021.

Amajida Roslim et al.

The geology of a depositional system can mostly be described by looking at sedimentary structures and sedimentary composition. However, in areas of complex shoreline mixed-process system (influenced by fluvial-tides-marine processes), many factors should be put into consideration. In Brunei, the mixed-sediment types occur extensively. The geology is mainly characterized here by thick Neogene siliciclastic facies ranging from fluvial, tidal and marine sediments deposited during periods of deltaic to shelfal setting, affected also by tectonic events. Due to this, differentiating tide and wave dominated facies is often a major challenge in the region.

In this study, it is emphasized that in order to support interpretations on these transitional facies, specific factors such as ichnofacies and microfossil content can be considered. Pollen and spores are more expected and useful in rather terrestrial systems, whilst in marine environments dinoflagellates, foraminifera and nannofossils could be convenient if preserved. Foraminifera and ichnofossils have the merit to be great indicators of a variety of sub-environments within the complex shallow water system.

The methods involve standard outcrop logging of fluvial, tidal and shallow marine outcrops, identifying lithology and key sedimentary features including trace fossils. Clay-rich samples were checked for microfossil content. Laboratory work involved extracting organic (pollen, spores, dinoflagellates) and calcareous (foraminifera, nannofossils) microfossils and documented them with light microscope (LM), stereo microscope, and Scanning Electron Microscope (SEM).

The results revealed that the most common trace fossil assemblages are the Ophiomorpha, Cruziana and Skolithos ichnofacies, and they refer to proximal marine settings. Among the calcareous microfossils recovered were very few coccolithophorids (Sphenolithus abies and Sphenolithus moriformis), which indicate very rare holomarine conditions, while the following benthic foraminifera genera were identified: Ammonia, Nonion, Elphdium, Elphidiella, Quinqueloculina, Ammobaculites, and Trochammina. Each of these genera have specific environmental requirements concerning hydrodynamics, trophic resources, oxygen content, substrate-type and deltaic influence. Results on pollen and spores, mangrove vegetation is marked by Sonneratia and Rhizophora-types, mixed-dipterocarp by Shorea spp., while peat swamp by Verrucatosporites usmensis and Osmunda sp.. Besides few dinoflagellate cysts (Achomosphaera sp., cf. Exosphaeridium sp., cf. Operculodinium sp., gen indet., Lingulodinium? pycnospinosum and Tuberculodinium vancampoae) and two acrtitarch taxa (Cymatiosphaera sp. and Cymatiosphaera cf. nuda) were found. These findings indicate incomplete sets of parasequences with palaeoenvironments of mixed shallow marine conditions. Mangrove pollen retrieved within tidal sediments indicates mangrove-dominated tide-influenced shoreline, while shoreline with diverse ichnofossils show coastal area connected to wave-dominated upper shoreface/ delta front. The calcareous foraminifera and nannofossil differentiate sediments belonging to lower shoreface to offshore/ prodelta deposits.

How to cite: Roslim, A., Briguglio, A., Kocsis, L., and Goeting, S.: Addressing mixed facies interpretation difficulties by coupling sedimentary data with ichnofacies and microfossil data: an example from several paralic deposits in Brunei Darussalam., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6888, https://doi.org/10.5194/egusphere-egu21-6888, 2021.

Daniel Collins and Howard Johnson

The interaction of river and marine processes in the fluvial to marine transition zone (FMTZ) fundamentally impacts sedimentary dynamics and deposition. Heterolithics are important facies within ancient and modern FMTZs but the preserved signal of river flood, wave and tidal variations in heterolithics remains uncertain. This study integrates facies and ichnofacies characteristics of heterolithics in the Lambir Formation (Baram Delta Province, NW Borneo), with information of larger-scale stratigraphic architecture and modern analogue information, to interpret the preserved record of river flood deposits under the influence of tides and waves in an ancient FMTZ. Within the FMTZ of distributary channels, interpreted proximal–distal sedimentological and stratigraphic trends suggest: (1) a proximal fluvial-dominated, tide-influenced subzone; (2) a distal fluvial- to wave-dominated subzone; and (3) a conspicuously absent tide-dominated subzone. During coupled storm and river floods, fluvial processes dominated the FMTZ along major and minor distributary channels and channel mouths, causing significant overprinting of preceding interflood deposits and deposition of thicker, sandier event beds. Intervening interflood deposits are muddier, with increased bioturbation, and may variably preserve sedimentary indicators of tide and wave processes. Despite interpreted fluvial–tidal channel units and mangrove influence implying tidal processes, there is a paucity of unequivocal tidal indicators (e.g. cyclical heterolithic layering). This suggests that process preservation in the FMTZ preserved in the Lambir Formation primarily records episodic (flashy) river discharge, river flood and storm overprinting of tidal processes, and possible backwater dynamics. 

How to cite: Collins, D. and Johnson, H.: Sedimentary signals of fluvial discharge variability under tide and wave influence: Miocene examples in NW Borneo, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14931, https://doi.org/10.5194/egusphere-egu21-14931, 2021.

Cari Johnson et al.

Existing depositional and facies models for ancient barrier island systems are primarily based on modern observations. This approach overlooks processes tied to geologic time scales, such as multi-directional motion, erosion, and reworking, and their resulting expressions in preserved strata. We have investigated these and other challenges of linking modern and ancient barrier islands through outcrop studies and through data compilation from the rock record compared to modern barrier island dimensions. Results emphasize key depositional and preservation processes, and the dimensional differences between deposits formed over geologic versus modern time scales. For example, when comparing deposits from individual barrier islands, thickness measurement comparisons between modern and ancient examples do not vary systematically, suggesting that local accommodation and reworking dictate barrier island thickness preservation. A complementary outcrop study focusing on paralic strata from the Upper Cretaceous Straight Cliffs Formation in southern Utah (USA) is used to update models for barrier island motion and preservation to include geologic time-scale processes. Barrier island deposits are described using four facies associations (FA): backbarrier fill (FA1), lower and upper shoreface (FA2), proximal upper shoreface (FA3), and tidal channel facies (FA4). Three main architectural elements (barrier island shorefaces, shoreface-dominated inlet fill, and channel-dominated inlet fill) occur independently or in combination to create stacked barrier island deposits. Barrier island shorefaces record progradation, while shoreface-dominated inlet fill records lateral migration, and channel-dominated inlet fill records aggradation within the tidal inlet. Barrier islands are bound by lagoons or estuaries and are distinguished from other shoreface deposits by their internal facies and outcrop geometry, association with backbarrier facies, and position within transgressive successions. Tidal processes, in particular, tidal inlet migration and reworking of the upper shoreface, also distinguish barrier island successions. In sum, these datasets demonstrate that improved depositional and facies models must consider multidirectional island motion, ravinement, erosion, inlet migration, and reworking when describing processes and predicting barrier island dimensions.

How to cite: Johnson, C., Mulhern, J., and Green, A.: How to link modern and ancient barrier island systems: Dimensional comparisons and updated sedimentary facies models , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8546, https://doi.org/10.5194/egusphere-egu21-8546, 2021.

Valentin Zuchuat et al.

The physiography (geometry and bathymetry) of a basin and its latitude are the primary parameters that dictate the tidal dynamics in shoreline–shelf systems. Understanding the impact that changes in physiography have on tides allows researchers to 1) improve interpretations of historical sedimentary processes in shallow-marine basins, and 2) better predict potential variations in tidal dynamics in response to an anthropogenic-driven relative sea level change.

Here, we present an analysis of numerical modelling of tidal propagation in the Upper Jurassic Sundance and Curtis Seas demontrating that basin-scale amplification and dampening of tides occurred in different palaeophysiographic configurations, and more localised amplification relating to tidal harmonics occurred in certain physiographic scenarios. Consequently, palaeophysiography was the primary control on both the magnitude and location of tidal amplification, flow speed, and bed shear stress, whereas secondary controls were initial tidal forcing and bottom drag coefficient.

Simulation results for the palaeophysiography with a 600 m depth at the mouth of the system suggest a distribution of sedimentary facies comparable to those documented in the Upper Jurassic lower Curtis Formation, apart from the innermost Curtis Sea, near to the palaeoshoreline. Sediments potentially supplied by aeolian processes during regression and increased aridity were likely reworked by tides during a subsequent a transgression as the climate became more humid. The palaeophysiography with a 600 m depth at the mouth of the system can therefore be considered a realistic palaeophysiographic configuration for the Sundance and Curtis Seas given the similarities that exist between the predicted distribution of sedimentary facies and their actual distribution in the lower Curtis Formation. In this palaeophysiography, the Sundance Sea and the Curtis Sea would have thus attained a maximum depth of ~240 m and 40-45 m, respectively. In this context, the simulated tidal range in the Curtis Sea would have reached 2.60 m, which would classify the Curtis Sea as a meso-tidal system (2x 1.30 m tidal amplitude).

Finally, using change in palaeophysiographic configuration as a proxy for relative sea-level variations revealed the non-uniqueness (sensu Burgess & Prince, 2015) of sedimentary successions deposited in tide-dominated basin, given that tidal amplification in the system was controlled by palaeophysiographic configuration: one specific succession could be the product of several, equally-valid relative sea-level histories. Reciprocally, the impact of relative sea-level change on different successions is non-unique, since local tidal harmonics and the characteristics of coeval deposition may vary significantly during relative sea level changes.

How to cite: Zuchuat, V., Steel, E., Mulligan, R., Collins, D., and Green, J. A. M.: Variations in physiography, tidal forcing, and bottom shear stress in palaeo-seas: lessons learned from numerical modelling., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9042, https://doi.org/10.5194/egusphere-egu21-9042, 2021.

Josep Maria Puig López et al.

Low-accommodation shallow-marine systems are often challenging to interpret and map in the subsurface due to their amalgamated/condensed nature, which often falls within sub-seismic resolution. Outcrop examples are significantly rarer and less well documented than moderate to high accommodation systems. Detailed studies of such systems are scarce, and their resulting depositional architecture is still poorly understood.

The late Jurassic shallow-marine deposits of the Intra Draupne Formation were deposited during the lattermost stage of Jurassic rifting in the South Viking Graben. They are found in a small N-S graben, flanked by two highs, the Haugaland (west) and the Avaldsness (east). Deposition took place under low-accommodation conditions on top of a markedly erosive regional unconformity of Middle Jurassic age. In this study, we performed a detailed 1:1 sedimentological core description and interpretation of 20 wells with the aim of providing a redefined, basin-wide and high-resolution sequence stratigraphic model to constrain the tectonostratigraphic evolution of the basin.

The Intra Draupne Formation is typically 15-20 m thick, with minimum and maximum values of 5 and 40 m respectively. The deposits are remarkably coarse-grained, dominated by coarse to very coarse sand and granule to pebble rich packages showing different degrees of textural maturity, ranging from well-rounded and very well-sorted to subangular and poorly-sorted deposits. The deposits are bioturbated with ichnofacies mainly represented by Skolithos and Thalassinoides and locally abundant Fugichnia escape traces. The bioturbation index is typically 2-3.  Deposits are otherwise structureless graded/non-graded or planar/through cross stratified. Differences in the degree of textural maturity, grain-size trends and sedimentary structures are the basis for a depositional model which includes gravity-flow dominated fan delta front and prodelta lobe deposits with variable degrees of tidal and wave modification and reworking in the form of compound tidal dunes/sand ridges and barred shorefaces.

Our results suggest the presence of an Early Kimmeridgian shoreline at the easternmost part of the study area, with westerly sourced fan-fan delta systems and separated from the Haugaland High and the basin-bounding Western Boundary Fault (WBF), by an inferred (non-preserved) coastal plain. In the Late Kimmeridgian, tectonic subsidence associated with the WBF created a major change in the basin configuration; the former shoreline and alluvial systems to the east were disconnected from their original source area and a semi-enclosed and elongated embayment was formed to the west, characterized by low energy conditions and restricted water circulation. During the Tithonian, further tectonic movements and relative sea-level rise, lead to the creation of a fully-connected marine seaway which was characterized by strong tidal currents to the west and more wave-dominated conditions to the east. Reduction of tectonic activity and sea-level rise from Late Tithonian to Early Rhyazanian forced backstepping of the delivery systems and flooding of the source areas promoting a basin-wide period of sediment starvation, which ended with deposition of the open marine Draupne shale.

This study provides information on how shallow-marine systems develop in low-accommodation settings, with implications for paleogeographic reconstructions, sandstone distribution and characterization in other areas, with more limited datasets.

How to cite: Puig López, J. M., Poyatos-Moré, M., and Howell, J. A.: Sedimentological and tectonostratigraphic evolution of a restricted to fully-connected, low-accommodation, shallow-marine basin: The late Jurassic Intra Draupne Formation in the Johan Sverdrup field, Southern Utsira High (Norwegian North Sea), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9271, https://doi.org/10.5194/egusphere-egu21-9271, 2021.

Miquel Poyatos-Moré et al.

Thick shallow-marine successions associated with long-term transgressions are less well known than their thin, well-sorted counterparts, widely studied due to their potential to form good reservoirs. In these successions, particularly in storm-dominated examples, bioturbation can obliterate primary sedimentary characteristics, making stacking patterns and sequences difficult to define, and challenging our understanding of the main controls in their resulting depositional architecture. This study presents an example from the Jurassic of the Neuquén Basin (Argentina), with the aim to: a) refine the depositional model of a thick, shallow-marine succession associated with a long-term, early post-rift transgression, b) constrain multi-scale controls on stratigraphic architecture and lateral facies variability, and c) discuss their preservation and response to post-depositional processes. To do this, a <300 m-thick succession has been studied along a >10 km continuous exposure, with mapping, sedimentary logging and correlation of stratigraphic units, integrated with subsurface, biostratigraphic and ichnological data. The succession shows an overall retrogradational-aggradational-retrogradational stacking pattern, with several higher frequency regressive units (parasequences and parasequence sets, PSS). The lower part (PSS I) comprises laterally-discontinuous (10's of m) mouth-bars and distributary channel fills, dominated by several m-thick coarsening- and fining-up sandstone packages and m-scale erosive conglomeratic lenses. Above these, the succession (PSS II-IV) is composed by laterally-continuous (>100's of m) storm-dominated lower-shoreface to upper-offshore deposits, dominated by <1m-thick fine-grained and highly bioturbated tabular muddy sandstones and sandy mudstones, with rarely-preserved HCS and bioclastic-rich limestones; their internal characteristics and bed boundaries are diffuse due to pervasive bioturbation, suggesting overall low sedimentation rates and recurrent periods of colonization. The coarse-grained nature and lithology of the mouth bars and channel fills in the lower succession (PSS I) are consistent with a proximal sediment source, associated with erosion of intra-basinal highs. Its variable thickness, lateral distribution and onlap against underlying syn-rift deposits demonstrates partial infill of localized higher-accommodation areas. The well-sorted and finer-grained nature of the shoreface-offshore strata the middle and upper succession (PSS II-IV) indicates a more mature, distal source, with sediment redistributed by longshore currents, and then intensely bioturbated. These deposits display well-defined parasequences internally composed of laterally-continuous bedsets (<5 m-thick). They extend along the entire study area, but show a significant vertical thickness variability. The integration of outcrop and subsurface data mapping (well and seismic) reveals this variability records the stratigraphic response of transgression over a complex, regional-scale ramp-step and underfilled rift topography, which controlled the location of main thickness and facies changes, and promoted areas of favored biogenic reworking. This study offers new insights in how to interpret thick transgressive successions based on primary depositional mechanisms and postdepositional processes, and provides useful tools to understand and predict the nature and potential preservation of these deposits in limited subsurface datasets.

How to cite: Poyatos-Moré, M., Schwarz, E., Boya, S., Gomis-Cartesio, L. E., and Midtkandal, I.: Multi-scale influence of topography on shallow-marine successions associated with long-term transgressions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16005, https://doi.org/10.5194/egusphere-egu21-16005, 2021.

Déborah Harlet et al.

The Moroccan Anti-Atlas consists of a several kilometers thick sediment pile accumulated on the northern Gondwana platform since the latest Precambrian (Ediacaran). This study focuses on the Ktaoua Group, early Late Ordovician (Mid-Sandbian to Katian) in age, which records a major and multiphase transgressive/regressive cycle above the shallow marine sandstones of the underlying First Bani Group. In the western Central Anti-Atlas, the Ktaoua Group is formed by offshore shales to coastal sandstones organized in regressive parasequences. Here, high-resolution field-based stratigraphy is used to constrain the shelf architecture and clinoforms geometries within the Ktaoua Group.

Whereas the lower part of the Ktaoua Group records parasequences from silty-shale to fine to coarse sandstones with hummocky-cross-stratification (HCS), its upper part oscillates between HCS beds and very coarse sandstones. Ferruginous, condensed horizons usually drape the parasequences. In this study, we investigate the platform geometry through the correlation of the stacking patterns of seventeen stratigraphic logs along an 85 km long, well-exposed cliff. Drone images support the logging and the correlations of the sections by imaging clinoforms geometries.

Several decameters of fine to coarse sandstones can be observed to grade laterally into condensed level(s) within a few kilometers, hence evidencing clinoforms pinching out. The visible orientation of the clinoforms along the cliff exposures show a proximal to distal trend from the south-west to the north-east, in agreement with the overall basin geometry. Three clinoforms with distinct geometries and lateral evolution of facies associations are highlighted. The distal part of a clinoform, 15 m in thickness, pinches out onto the top of the underlying First Bani Group within 7 km. The overlying regressive parasequence, approximatively 50 m thick, remains consistent more than 50 km, and is understood as a prograding clinoform. A third clinoform, capped by a prominent sandstone body constantly thicker than 20 m over ca. 20 km, disappears within its last 3.5 km onto the underlying clinoform. This study offers new details on the progradation and regression geometries along a giant platform within a detailed stratigraphic framework.

We would like to thank the Pacha and the Gendarmerie Royale of Foum-Zguid, the governor of Tata and the different persons who gave their approval and facilitated the use of the drone in the region of Souss-Massa for their precious help.

How to cite: Harlet, D., Douillet, G. A., Ghienne, J.-F., Bouscary, C., Razin, P., Dietrich, P., and Schlunegger, F.: Sedimentology, stratigraphy and clinoform architectures of a siliciclastic shallow-marine platform: insights from the Late Ordovician of the Anti-Atlas (Morocco), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10409, https://doi.org/10.5194/egusphere-egu21-10409, 2021.

Laura Bührig et al.

Shelf-edge deltas constitute important components of source-to-sink (S2S) systems. They distribute sediment to continental slopes and basin floors from rivers that have prograded across shelves, and due to their scale they form significant sediment accumulations at shelf margins. Because of their intimate relationship with regressive conditions, several geological controls govern their evolution, including relative sea-level changes, sediment budgets, river hydrology, and hydrodynamic processes; these factors are themselves influenced by characteristics of terrestrial catchments and continental shelves, and by climate. Despite their important role in sediment dispersal to shallow- and deep-marine environments, shelf-edge deltas are commonly overlooked in models that describe S2S systems, perhaps because of their relative paucity during the present-day highstand conditions. In subsurface and outcrop, their recognition can be difficult in cases where information with which to constrain the physiographic environment is limited, such that the spatial position of a delta relative to the shelf margin cannot be determined unequivocally.

This study aims to improve our understanding of controls on the sedimentary characteristics of shelf-edge deltas. For this purpose, >40 shelf-edge deltas of Late Triassic to late Quaternary age from >30 globally-distributed shelf-margin successions have been investigated, utilising literature-derived seafloor-, subsurface- and outcrop data. Following a database approach, sedimentary records have been quantitatively analysed in terms of geometry (e.g. dimensions, thickness, gradients) and facies characteristics (e.g. lithology, sedimentary structures) of depositional environments (e.g. delta top, delta front) and architectural elements (e.g. delta lobes, distributary mouth bars). Specific consideration has been given to assessment of palaeoenvironmental setting (e.g. hydrodynamic process regime, margin type, bathymetric setting, palaeolatitude). Moreover, scaling relationships between these properties and attributes of the S2S system (e.g. fluvial-system and catchment attributes, shelf configuration, shelf-slope transition) have been evaluated. Accordingly, the relative importance of controls on the sedimentary characteristics of shelf-edge deltas has been assessed.

This analysis demonstrates that environmental factors influence the sedimentary record of shelf-edge deltas via a complex interplay of dynamic processes and physiography of the S2S segments catchment, shelf and slope. Based on these findings, new facies models for shelf-edge delta types are developed, which are placed in the context of S2S linkages. Outcomes of this study aid the identification and classification of shelf-edge deltas and their preserved deposits, as well as the reconstruction of associated environmental conditions from stratigraphic records.

How to cite: Bührig, L., Colombera, L., Mountney, N. P., and McCaffrey, W. D.: Controls on the sedimentary characteristics of shelf-edge deltas in a source-to-sink context, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12287, https://doi.org/10.5194/egusphere-egu21-12287, 2021.

Nasar Khan et al.

Abstract: Reservoir assessment of unconventional reservoirs poses numerous exploration challenges. These challenges relate to their fine-grained and heterogeneous nature, which are ultimately controlled by depositional and diagenetic processes. To illustrate such constraints on shale gas reservoirs, this study focuses on lithofacies analysis, paleo-depositional and diagenetic evolution of the Paleocene Patala Formation at Potwar Basin of Pakistan. Integrated sedimentologic, petrographic, X-ray diffraction and TOC (total organic carbon) analyses showed that the formation contained mostly fine-grained carbonaceous, siliceous, calcareous and argilaceous siliciclastic-lithofacies, whereas carbonate microfacies included mudstone, wackestone and packstone. The silicious and carbonaceous lithofacies are considered a potential shale-gas system. The clastic lithofacies are dominated by detrital and calcareous assemblage including quartz, feldspar, calcite, organic matter and clay minerals with auxiliary pyrites and siderites. Fluctuations in depositional and diagenetic conditions caused  lateral and vertical variability in lithofacies. Superimposed on the depositional heterogeneity are spatially variable diagenetic modifications such as dissolution, compaction, cementation and stylolitization. The δ13C and δ15N stable isotopes elucidated that the formation has been deposited under anoxic conditions, which relatively enhanced the preservation of mixed marine and terrigenous organic matter. Overall, the Patala Formation exemplifies deposition in a shallow marine (shelfal) environment with episodic anoxic conditions.

Keywords: Lithofacies, Organic Matter, Paleocene, Potwar Basin, Shale Gas, Shallow Marine.

How to cite: Khan, N., Swennen, R., Weltje, G. J., and Jan, I. U.: Lithofacies, Depositional Environment and Diagenetic Evolution of the Paleocene Patala Formation, Potwar Basin, Pakistan: Implication for Shale Gas Potential, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12555, https://doi.org/10.5194/egusphere-egu21-12555, 2021.

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