Showing papers in "Journal of Sedimentary Research in 2017"

The maturity myth in Sedimentology and provenance analysis

Abstract: We often resort to mythical thinking in the face of natural phenomena that we hardly understand. Sedimentary petrology has several long-standing myths. One is that sediments increase their “maturity” through time by winnowing and sorting, physical wear, chemical weathering, diagenesis, and recycling, as if their destiny were to reach a final stage of perfection represented by quartz spheres of equal size. Such an armchair representation of supermature purity is embodied in white recycled desert sand, whereas a modern analogue for its opposite, the dark dirty rock improperly called graywacke, is yet to be found. Clay matrix does not occur in sand deposited from tractive currents, which is sorted at the instant of settling. Sand grains get rounded relatively rapidly during transport in air but very slowly when transported in water. The degree of winnowing, sorting, and rounding (“textural maturity”) has little to do with the accumulation of “modifying energy” through time. “Mineralogical maturity” does not increase notably even after thousands of kilometers of transport in high-energy environments, and not even during recycling, a physical process that in a weathering-limited denudation regime tends at most to reproduce the mineralogy of parent clastic rocks. Rather, “mineralogical maturity” decreases drastically where sedimentary rock fragments are eroded in abundance from mudrocks interbedded in the source, and sand recycled entirely from orogenic flysch or molasse may contain less than 10% quartz. If natural sediment factories were quartz distilleries, then quartzose sand should become more common through geological time. Pure quartz sand, however, is relatively rare in the modern world, and largely confined to hyperhumid equatorial latitudes or recycled eolian dunefields. A relative increase in more durable minerals is achieved effectively only by chemical processes such as selective leaching in soils, or especially during burial diagenesis, when the time and temperature available for chemical reactions are much longer and higher than at the Earth9s surface. If recycled sediments are commonly quartz-rich with very poor assemblages dominated by zircon, tourmaline, and rutile, it is because their parent sandstones have undergone diagenetic dissolution prolonged for many millions of years at temperatures of many tens of degrees before final exhumation and erosion.

Influence of Diagenesis On the Quality of Lower Cretaceous Pre-salt Lacustrine Carbonate Reservoirs from Northern Campos Basin, Offshore Brazil

Abstract: The genesis and evolution of lacustrine pre-salt carbonate reservoirs, which contain giant hydrocarbon accumulations along the South Atlantic margins, has attracted major research interest. The huge extension and volume, and unusual textural and compositional features, are key elements for understanding the tectonic, structural, stratigraphic, and sedimentological generation and early evolution of the region, as well as potentially of other lacustrine carbonate systems. A systematic petrographic and petrophysical study has been performed on the Lower Cretaceous lacustrine carbonate reservoirs from northern Campos Basin, Offshore Brazil, in order to unravel the main controls on the origin and evolution of pore systems. The main lithologic types recognized in the rift section are bivalve–gastropod grainstones and rudstones, arenites constituted by ooids of syngenetic magnesian clay minerals (mostly stevensite), and dolostones, while stevensitic claystones with calcite spherulites, fascicular calcite crusts, intraclastic rudstones and grainstones, and dolostones are the main lithologic types in the sag section. The eogenetic evolution of bioclastic reservoirs was controlled by the balance between dissolution and neomorphism of the aragonitic bivalve and gastropod bioclasts, favoring either the generation of poorly connected moldic porosity or the preservation of well-connected interparticle porosity. The stevensitic arenites were strongly affected by meteoric dissolution and replacement by dolomite and silica, related to regional uplift and erosion after the rift phase, which generated highly heterogeneous pore systems with moldic, intercrystalline, vugular, and microcrystalline pores. Stevensitic claystones that are replaced by calcite spherulites and dolomite normally show low porosity, but locally constitute reservoirs, where secondary porosity was generated by stevensite dissolution. The precipitation of crystal shrubs of fascicular-optic calcite in coalescent crusts generated growth-framework primary porosity, which was reduced mostly by dolomite cementation, or enlarged by dissolution, enhancing their permeability. Non-coalescent calcite crusts contain abundant syngenetic magnesian clay minerals. Their porosity is related to dissolution of these clays, which generated poor permeability. Intraclastic grainstones and rudstones are compacted and cemented, or rich in clay matrix (‘‘hybrid packstones’’). Where they display preserved interparticle primary porosity or matrix dissolution, they may have good porosity and permeability. The heterogeneous dolomitization of both the rift and the sag deposits either destroyed their primary or early diagenetic porosity, or generated high porosity and permeability values in the dolostones. Relationships between replacement and compaction indicate that most of the diagenetic processes occurred during eodiagenesis, controlled mostly by the instability of the aragonite in the bioclastic reservoirs and of the stevensite in the claystones, ooidal arenites, and fascicular calcite crusts. This study characterizes the major primary and diagenetic aspects of the pre-salt reservoirs, providing insights on the evolution of their porosity and permeability. This is expected to contribute to the prediction of quality during exploration and to enhance hydrocarbon recovery from producing oilfields, as well as to increase the understanding of the origin, evolution, and quality distribution in analogous lacustrine reservoirs.

Silica Diagenesis, Porosity Evolution, and Mechanical Behavior In Siliceous Mudstones, Mowry Shale (Cretaceous), Rocky Mountains, U.S.A.

Abstract: Quartz in mudstones of the Mowry Shale (Cretaceous, Rocky Mountain region) takes several forms: extrabasinal detrital silt (some with transported overgrowths), replacement of skeletal debris, minor overgrowth on detrital quartz and replaced radiolaria, pore-filling in the intragranular pores of allochems, and authigenic microquartz that is dispersed through the clay-size matrix. The matrix-dispersed microquartz has CL character similar to the overgrowths with which it is intergrown, supporting the interpretation that it is authigenic and pore-filling. Radiolaria are widely observed in these mudrocks but are poorly preserved by partial mineral replacement (quartz, dolomite, calcite, pyrite). We interpret dissolved radiolaria, and possibly, now-cryptic diatoms, as the likely source of silica for the overgrowths and microquartz, through a reaction pathway that entailed the formation of opal-CT lepispheres that subsequently recrystallized to microquartz. A newly devised variation of the IGV (intergranular volume) concept, a parameter IGVm, is applied for partitioning the causes of porosity decline in mudstones. All of the samples examined contain authigenic microquartz, which averages around 40% of the rock volume. The large volume of microquartz, together with the volumes of surviving intergranular pores and pore-filling solid hydrocarbon, constrain the porosity to around 50% at the time of initial lepisphere formation. Despite the exceptional abundance of cement, overall porosity decline is compaction-dominated. The presence of significant volumes of cement has implications for mechanical rock properties in the Mowry Shale, which is expected to manifest brittle behavior where cements are most prominently developed.

Frontal and lateral submarine lobe fringes: Comparing sedimentary facies, architecture and flow processes

Abstract: Submarine lobe fringe deposits form heterolithic successions that may include a high proportion of hybrid beds. The identification of lobe fringe successions aids interpretation of paleogeographic setting and the degree of basin confinement. Here, for the first time, the sedimentological and architectural differences between frontal and lateral lobe fringe deposits are investigated. Extensive outcrop and core data from Fan 4, Skoorsteenberg Formation, Karoo Basin, South Africa, allow the rates and style of facies changes from axis to fringe settings of lobes and lobe complexes in both down-dip (frontal) and across-strike (lateral) directions to be tightly constrained over a 800 km2 study area. Fan 4 comprises three sand-prone divisions that form compensationally stacked lobe complexes, separated by thick packages of thin-bedded siltstone and sandstone intercalated with (muddy) siltstone, interpreted as the fringes of lobe complexes. Lobe-fringe facies associations comprise: i) thick-bedded structureless or planar laminated sandstones that pinch and swell, and are associated with underlying debrites; ii) argillaceous and mudclast-rich hybrid beds; and iii) current ripple-laminated sandstones and siltstones. Typically, frontal fringes contain high proportions of hybrid beds and transition from thick-bedded sandstones over length-scales of 1 to 2 km. In contrast, lateral fringe deposits tend to comprise current ripple-laminated sandstones that transition to thick-bedded sandstones in the lobe axis over several kilometers. Variability of primary flow processes are interpreted to control the documented differences in facies association. Preferential deposition of hybrid beds in frontal fringe positions is related to the dominantly downstream momentum of the high-density core of the flow. In contrast, the ripple-laminated thin beds in lateral fringe positions are interpreted to be deposited by more dilute low-density (parts of the) flows. The development of recognition criteria to distinguish between frontal and lateral lobe fringe successions is critical to improving paleogeographic reconstructions of submarine fans at outcrop and in the subsurface, and will help to reduce uncertainty during hydrocarbon field appraisal and development.

Three-Dimensional Reconstruction of Meander-Belt Evolution, Cretaceous Mcmurray Formation, Alberta Foreland Basin, Canada

Abstract: The three-dimensional reconstruction of meander-belt deposits from ancient strata provides insight into the formative processes of meander-bend evolution and paleogeographic interpretations. A significant challenge to such analyses is limited exposures in outcrop belts and widely spaced or sparse subsurface datasets. An unprecedented dataset consisting of 600 km of 3-D seismic data and over 1000 well penetrations from the Cretaceous McMurray Formation in northeastern Alberta, Canada, provides a unique opportunity to characterize an ancient continentalscale river system. Paleochannels ranged from 475 to 1180 m wide and from 35 to 50 m deep, with meander-belt width-to-thickness ratios between 107:1 and 401:1. The data reveal evidence for intra-point-bar erosion and punctuated rotation, counter-point-bar development, and protracted channel cut-off and meander-loop abandonment. Observations enable interpretation of morphodynamic processes that are commonly observed in modern systems, yet rarely described from the rock record. A 3-D geocellular model and reconstructed paleochannel migration patterns reveal the evolutionary history of seventeen individual meander belt-elements, including point bars, counter-point bars, and their associated abandoned channel fills, which have been mapped using core, FMI logs, and seismic data. Results of the study show that intrapoint-bar erosion surfaces bound accretion packages characterized by unique accretion directions, internal stratigraphic architecture, and lithologic properties. We provide evidence for channel-belt-edge confinement and development of a counter-point bar, as well as the deposition of side bars and preservation of a mid-channel bar during meander-bend abandonment. Analysis of changes in meander-belt morphology over time reveal a decrease in channel-belt width/thickness ratio and sinuosity, which we compare with observations from the lower Mississippi River and attribute to the landward migration of the paleo-backwater limit due to transgression of the Cretaceous Boreal Sea into the Alberta foreland basin.

Clay coated sand grains in petroleum reservoirs: understanding their distribution via a modern analogue

Abstract: Clay-coated grains can inhibit ubiquitous, porosity-occluding quartz cement in deeply buried sandstones and thus lead to anomalously high porosity. A moderate amount of clay that is distributed as grain coats is good for reservoir quality in deeply buried sandstones. Being able to predict the distribution of clay-coated sand grains in petroleum reservoirs is thus important to help find and exploit anomalously good reservoir quality. Here we have adopted a high-resolution, analogue approach, using the Ravenglass Estuary marginal-shallow marine system, in NW England, U.K. Extensive geomorphic mapping, grain-size analysis, and bioturbation-intensity counts were linked to a range of scanning electron microscopy techniques to characterize the distribution and origin of clay-coated sand grains in surface sediment. Our work shows that grain coats are common in this marginal–shallow marine system, but they are heterogeneously distributed as a function of grain size, clay fraction, and depositional facies. The distribution and characteristics of detrital-clay-coated grains can be predicted with knowledge of specific depositional environment, clay fraction percentage, and grain size. The most extensive detrital-clay-coated grains are found in sediment composed of fine-grained sand containing 3.5 to 13.0% clay fraction, associated with inner-estuary tidal-flat facies. Thus, against common convention, the work presented here suggests that, in deeply buried prospects, the best porosity might be found in fine-grained, clay-bearing inner-tidal-flat-facies sands and not in coarse, clean channel-fill and bar facies.

Anatomy of A Shoreline Regression: Implications For the High-Resolution Stratigraphic Architecture of Deltas

Abstract: The regressive and subsequent transgressive transit of a shoreline across a clastic shelf generates the standard reservoir flow unit in most marginal to shallow marine hydrocarbon reservoirs. The stratigraphic unit produced by the shoreline transits is commonly referred to as the high-frequency (104 to 105 years), regressive–transgressive sequence (RT sequence). The unit is usually bounded top and base by marine shales marking flooding surfaces, which can act as barriers to fluid flow. Stratigraphic architecture in the RT-sequence is also known to control the internal flow behavior of many reservoirs. Hence, an ability to consistently characterize reservoirs at a sub RT-sequence scale is critical to enabling prediction of reservoir performance and optimization of resource extraction strategies. This study describes the internal architecture of one shallow-water (< 10 m), low-accommodation regressive shoreline succession from the Campanian of the Alberta Basin, Canada, based on an extensive outcrop and subsurface dataset that has been convolved into a 3D geocellular computer model. The architecture and evolution of the ancient mixed-process (waves, tides, and fluvial processes) regressive deltaic shoreline system is compared and contrasted with a partial Holocene analog from northeastern Australia. The same stratigraphic surfaces and units are identified in both the modern and the ancient regressive systems. The key architectural unit is the element complex set (ECS), which is a multi-kilometer-scale, discontinuity-bounded unit that is the product of the reorganization of the coastline, often caused by autogenic backwater-driven avulsions. Multiple avulsions during a regressive shelf transit episode lead to lateral offsets of ECS units in low-accommodation systems. These systems are termed “avulsion-driven systems.” An increasing component of vertical stacking of ECS units is observed in higher-accommodation regressive systems. The mechanisms for generating accommodation on a regressive-shoreline shelf-transit time frame may be allogenic (tectonic subsidence or eustatic sea-level change), autogenic, or a combination of the two mechanisms. A key, localized, autogenic mechanism is related to the distance of progradation during the transit of a deposystem across the shelf. In proximal shelf positions, ECS units tend to offset laterally due to limited available accommodation, whilst in more distal positions, early differential, load-induced, compactional subsidence of underlying prodelta and shelf muds can promote vertical stacking of ECS units. The critical down depositional-dip distance from the transgressive turnaround point at which ECS units become preferentially vertically stacked is a function of shelf gradient, shoreline trajectory, sandstone fraction, and prodelta and shelf mud rheology and is termed the “critical autogenic ECS stacking distance.” Vertical stacking of ECS units may also occur when ECS units overstep underlying shelf topography, such as the distal termination of an older RT sequence. Recognition criteria and nomenclature for intra-regressive-shoreline surfaces and stratigraphic units, as well as predictive models for the ancient record are detailed across a spectrum of types of deltaic systems.

Evaluating Controls On Crevasse-Splay Size: Implications For Floodplain-Basin Filling

Abstract: Although crevasse splays are often considered important contributors to floodplain-basin filling, evidence from modern and ancient fluvial deposits indicates that they may not play a significant role in all river systems. In order to better understand controls on the abundance and scale of crevasse splays produced in fluvial systems, we used aerial photography to observe crevasse-splay deposits along three rivers and conducted a targeted numerical-modeling study to explore factors which promote widespread crevasse-splay deposition on fluvial floodplains. Results demonstrate that splay size does not always scale directly with channel size or discharge and that hydrodynamic factors—primarily sediment size and floodplain-drainage conditions—play a significant role in determining whether a fluvial system is likely to produce large, floodplain-filling splays. This work suggests that widespread crevasse splays demand both a large supply of relatively coarse suspended sediment and a steep water-surface slope away from the channel across the floodplain. Without both these conditions, the role of crevasse splays in flood-basin filling is limited.

Quantifying Mixed-Process Variability In Shallow-Marine Depositional Systems: What Are Sedimentary Structures Really Telling Us?

Abstract: Interpreting the whole range of fluvial, wave, and tidal interactions recorded in shallow-marine, coastal successions can be challenging. The complexity arises because sedimentary structures produced by all three processes can be fully or partially preserved in the same stratal packages, and many of these structures are not diagnostic of a specific process. We therefore need an improved method of capturing the internal facies complexity that characterizes mixed-process coastal systems. We propose a new methodology that assigns a percentage or probability to the likelihood for a bed or stratal unit to be formed by wave (w), tide (t), and fluvial (f) processes via a library of sedimentary structures and their non-unique generating processes. The library was generated through an intensive literature review of ancient, modern, and physical experiment works; the total frequency of association of each structure to each process (wave, tidal, fluvial) is used to calculate the percentage values. Each bed or bedset can be characterized by a specific structure or multiple structures (taking also into account lateral variations). Percentage values of wave/tide/fluvial processes of various structures can be averaged to create a final compound process probability for each bed. Vertical integration of process probability for individual beds in a rock succession creates probability graphs. This methodology has been tested on a 15-meters thick parasequence of the Jurassic Las Lajas Fm., Argentina, and on sedimentary logs of the Cretaceous lower Sego Sandstone, USA, and it is seen to efficiently couple classical facies analysis and surficial-process studies to quantify process variability in ancient systems. Additionally, we assessed the likelihood of association of sedimentary structures not only to hydrodynamic processes but also to depositional sub-environments, through a collection of published sedimentary logs (in modern and ancient deposits) from various basins worldwide. The methodology presented here better quantifies changing process dominance through time, improves the prediction of depositional environment evolution, and helps future studies that aim at a quantification of process variability.

Modeling Stratigraphic Architecture Using Small Unmanned Aerial Vehicles and Photogrammetry: Examples From the Miocene East Coast Basin, New Zealand

Abstract: The ability to view and characterize outcrops that are difficult to study from the ground is greatly improved by aerial investigation. We describe the application of flying a small, unmanned aerial vehicle (UAV) to collect photographic data for modeling rock outcrops and creating detailed digital terrain models (DTMs), which can be used for multi-scale digital feature mapping in true three-dimensional (3-D) space. Georeferenced digital outcrops can complement field measurements, as well as enhance classroom to field learning experiences for undergraduate education. High-resolution photographs, acquired at various elevations and azimuths by a small UAV, are used to convert stratigraphic features to 3-D digital representations with spatial accuracy approaching 1 cm. Complex topography, recorded in 2-D image sequences, is rendered digitally in 3-D surface models using Structure from Motion (SfM) photogrammetry. The resulting DTMs are populated with the high-resolution photographs for stratigraphic interpretation at the centimeter scale. The method of imaging and modeling outcrops is demonstrated in the Miocene East Coast Basin, New Zealand, where steep, coastal cliff exposures of continental-slope deposits and extensive, wave-cut platform exposures of steeply dipping, deep-water basin deposits offer exceptional opportunities to investigate depositional processes and the resulting stratigraphic architecture. Our approach makes it possible to characterize deposits that are exposed in inaccessible, 150-m-high, vertical sea cliffs and along 2000 m of wave-cut platforms, and to quantify their geometric variability. Results yield a spatial and temporal understanding of two depositional systems at a scale that was previously unattainable by conventional field techniques.

The effect of clay type on the properties of cohesive sediment gravity flows and their deposits

Abstract: The present knowledge of cohesive clay-laden sediment gravity flows (SGFs) and their deposits is limited, despite clay being one of the most abundant sediment types on earth and subaqueous SGFs transporting large volumes of sediment into the ocean. Lock-exchange experiments were conducted to contrast SGFs laden with noncohesive silica flour, weakly cohesive kaolinite, and strongly cohesive bentonite in terms of flow behavior, head velocity, runout distance, and deposit geometry across a wide range of suspended-sediment concentrations. The three sediment types shared similar trends in the types of flows they developed, the maximum head velocity of these flows, and the deposit shape. As suspended sediment concentration was increased, the flow type changed from low-density turbidity current (LDTC) via high-density turbidity current (HDTC) and mud flow to slide. As a function of increasing flow density, the maximum head velocity of LDTCs and relatively dilute HDTCs increased, whereas the maximum head velocity of the mud flows, slides, and relatively dense HDTCs decreased. The increase in maximum head velocity was driven by turbulent support of the suspended sediment and the density difference between the flow and the ambient fluid. The decrease in maximum head velocity comprised attenuation of turbulence by frictional interaction between grains in the silica-flour flows and by pervasive cohesive forces in the kaolinite and bentonite flows. The silica-flour flows changed from turbulence-driven to friction-driven at a volumetric concentration of 47% and a maximum head velocity of 0.75 m s−1; the thresholds between turbulence-driven to cohesion-driven flow for kaolinite and bentonite were 22% and 0.50 m s−1, and 16% and 0.37 m s−1, respectively. The HDTCs produced deposits that were wedge-shaped with a block-shaped downflow extension, the mud flows produced wedge-shaped deposits with partly or fully detached outrunner blocks, and the slides produced wedge-shaped deposits without extension. For the mud flows, slides, and most HDTCs, an increasingly higher concentration was needed to produce similar maximum head velocities and runout distances for flows carrying bentonite, kaolinite, and silica flour, respectively. The strongly cohesive bentonite flows were able to create a stronger network of particle bonds than the weakly cohesive kaolinite flows of similar concentration. The silica-flour flows remained mobile up to an extremely high concentration of 52%, and frictional forces were able to counteract the excess density of the flows and attenuate the turbulence in these flows only at concentrations above 47%. Dimensional analysis of the experimental data shows that the yield stress of the pre-failure suspension can be used to predict the runout distance and the dimensionless head velocity of the SGFs, independent of clay type. Extrapolation to the natural environment suggests that high-density SGFs laden with weakly cohesive clay reach a greater distance from their origin than flows that carry strongly cohesive clay at a similar suspended-sediment concentration, whilst equivalent fine-grained, noncohesive SGFs travel the farthest. The contrasting behavior of fine-grained SGFs laden with different clay minerals may extend to differences in the architecture of large-scale sediment bodies in deep marine systems.

Influence of Seabed Morphology and Substrate Composition On Mass-Transport Flow Processes and Pathways: Insights From the Magdalena Fan, Offshore Colombia

Abstract: Although the effects of interactions between turbidity currents and the seabed have been widely studied, the roles of substrate and bathymetry on the emplacement of mass-transport complexes (MTCs) remain poorly constrained. This study investigates the effect of bathymetric variability and substrate heterogeneity on the distribution, morphology, and internal characteristics of nine MTCs imaged within a 3D seismic volume in the southern Magdalena Fan, offshore Colombia. The MTCs overlie substrate units composed mainly of channel–levee-complex sets, with subsidiary deposits of MTCs. MTC dispersal was influenced by tectonic relief, associated with a thin-skinned, deep-water fold-and-thrust belt, and by depositional relief, associated with the underlying channel–levee-complex sets; it was the former that exerted the first-order control on the location of mass-transport pathways. Channel–levee-complex sets channelized, diverted, or blocked mass flows, with the style of response largely controlled by their orientation with respect to the direction of the incoming flow and by the height of the levees with respect to flow thickness. MTC erosion can be relatively deep above channel-fill deposits, whereas more subtle erosional morphologies are observed above adjacent levee units. In the largest MTC, the distribution of the seismic facies is well imaged, being influenced by the underlying bathymetry, with internal horizontal contraction occurring updip of bathymetric highs, erosion and bypass predominating above higher gradient slopes, and increased disaggregation characterizing the margins. Hence, bathymetric irregularities and substrate heterogeneity together influence the pathways, geometries, and internal characteristics of MTCs, which could in turn influence flow rheology, runout distances, the presence and continuity of underlying reservoirs, and the capacity of MTCs to act as either hydrocarbon seals or reservoirs.

Linking Hydraulic Properties In Supercritical Submarine Distributary Channels To Depositional-Lobe Geometry

Abstract: This study investigates the connection between hydraulic and sediment transport processes and channel and lobe geometry in supercritical submarine fans. The geometry of lobate deposits is typically explored as an interrelationship between different geometric measures, e.g., length versus width or thickness versus area, or based on confinement. However, the applicability of these relationships can be unclear due to their multi-scale nature and a lack of understanding of what processes control fan organization at different scales. Here we use a set of laboratory experiments and high-resolution seismic data from the Golo fan to distinguish three different hierarchical scales of nested lobate bodies with different characteristics and stacking (i.e., lobe element, lobe, and lobe complex) and compare characteristics of these mesoscale elements to equations describing 1D hydraulics along supercritical distributary channels. In particular, we are interested in defining the synoptic process and element in steep bedload-dominated supercritical fans that can most accurately be described as a function of hydraulic variables in the distributary channels. Based on the importance of hydraulic jumps in lobe-element formation in the experiments, we propose that the densimetric Froude number of the distributary channel exerts control on the geometry of the lobe element deposits associated with an individual avulsion cycle. Specifically, we hypothesize that higher Froude numbers will lead to lobate deposits that have larger ratios of lobe-element thickness to channel depth compared to those produced at lower Froude numbers. Both the laboratory and field data support this hypothesis. However, the conclusions are dependent on the correct hierarchical linkage between elements in the field and in experiments. In the analysis, the maximum lobe-element thickness was found to be well approximated by the sequent depth of the supercritical flow in the distributary channel. Therefore, this approximation yields a prediction of the lobe-element thickness based only on the hydraulic properties of the distributary channel without the need for any calibration or regression coefficients in the prediction.

The Role of Mass Wasting In the Progressive Development Of Submarine Channels (Espírito Santo Basin, Se Brazil)

Abstract: A Pliocene–Quaternary submarine channel system, influenced by localized mass wasting, is investigated using high-resolution 3D seismic data from offshore Espirito Santo Basin, SE Brazil. Three abandoned channels, a channel belt, and a mass-transport deposit (MTD) are recognized in the channel system in a confluence region confined by salt diapirs. In this confluence region, the cross-sectional area (CSA) of the channel system can be up to 1.2 km2, i.e., 4 to 10 times larger than other parts of the study area. These significant changes in the architecture and morphology of the channel system resulted from the interaction between mass-wasting processes and turbidity flows. We postulate that a basal erosional scar created by mass-wasting processes was later filled with an MTD. This basal scar was then used as a preferential pathway for turbidity flows, which were captured by its headwall and lateral margins. The interpreted data show that the captured turbidity flows greatly widened the basal scar but caused only small modifications in scar height. This predominance of widening processes over channel incision occurred because part of the MTD in the basal scar was removed downslope by turbidity flows and replaced by channel-fill deposits. This paper shows that important flow-capture processes can predominate in channel-confluence regions of continental slopes. Basal scars can capture turbidity flows and facilitate flow channelization, which are key processes for submarine-channel initiation. Importantly, the replacement of MTDs by channel-fill deposits has profound implications for reservoir volumes and net-to-gross ratios in channel systems and partly depends on the properties of the turbidity flows, such as their erosive ability and frequency. The more erosive and frequent flows are captured by the basal scar, the larger is the accommodation space created for subsequent sand-prone turbidites.

Oligocene and Miocene carbonate platform development in the Browse Basin, Australian Northwest Shelf

Abstract: Extensive 2D and 3D seismic-reflection data calibrated by wells provide new insights into the lateral extent, morphology, internal architecture, and age of carbonate platforms in the subsurface of the Browse Basin, Australian Northwest Shelf (NWS). The oldest carbonate build-ups are interpreted as a giant bryozoan build-up complex of Oligocene age (34.03–27.8 Ma). In the late Burdigalian, tropical reef growth began, and numerous reef-rimmed carbonate platforms progressively coalesced into an extensive barrier reef. From the mid-Langhian to the early Tortonian, the Browse Basin contained an elongate (along-margin) barrier-reef system over 500 km long. This barrier reef possibly extended in the southwest into the Northern Carnarvon Basin. After the early Tortonian, reefs on top or landward of this major barrier-reef system were smaller and less connected. This significant reduction in area covered by reef-rimmed platforms likely resulted from cooling following the Mid-Miocene Climate Optimum and relatively high subsidence rates. The reef-rimmed carbonate platforms that formed between the late Burdigalian and the early Tortonian are very similar throughout the Browse Basin. Platform development after the early Tortonian was regionally diverse. High subsidence rates in the northern Browse Basin are interpreted to have promoted the development of thick, aggrading platforms. Lower subsidence in the south led to thinner, more widespread platforms, recurrently shifting location in response to eustatic change. The final phase of reef decline around 6 Ma coincides with an increase in current-driven drift sedimentation. Current-related winnowing of sediments from the lagoons and slopes of the remaining carbonate platforms likely contributed to their final drowning.

Tracing Transcontinental Sand Transport: from Anatolia–zagros To the Rub' Al Khali Sand Sea

Abstract: We used petrographic, heavy-mineral, and geochronological signatures of sand-sized grains to document an exceptional case of long-distance sediment transport dominated by eolian processes in a hyperarid climate. Feldspatho-quartzo-lithic orogenic detritus shed by the Anatolia Plateau and Zagros Mountains-including carbonate, chert, volcanic, metabasite, and ultramafic lithic grains with a rich epidote-amphibole-pyroxene-garnet heavy-mineral suite-was carried to the Arabian-Gulf foreland basin via the Euphrates-Tigris-Karun fluvial system and other rivers draining the Zagros, and blown inland by dominant Shamal winds to reach well into the Arabian foreland. Sediment dispersal over a cumulative distance of up to 4000 km took place in multiple steps, involving extensive eolian reworking of older deposits during lowstand stages of the Pleistocene before final accumulation in the Rub' al Khali sand sea. The siliciclastic fraction of Gulf beaches changes southeastwards from litho-quartzose carbonaticlastic and quartzose north of Qatar to quartzo-lithic carbonaticlastic along the Trucial Coast, but invariably contains chert, volcanic, and metabasite lithics, together with epidote, pyroxene, amphibole, and garnet. Dune sand inland is progressively enriched in quartz until composition becomes feldspatho-quartzose, whereas the heavy-mineral assemblage remains virtually unchanged. Beach and dune sands of the Gulf and northeastern Rub' al Khali were derived from Arabia, Anatolia, and the Zagros in varying proportions, with only local contribution from ophiolites of the northern Oman Mountains as revealed by cellular serpentinite and enstatite grains. In all samples detrital zircons yielded mostly Cambrian to Neoproterozoic ages reflecting "Pan-African" crustal growth and amalgamation of the Arabian shield, but several upper Paleozoic, Mesozoic, and Cenozoic zircons with ages as young as 5 Ma in northeastern Rub' al Khali dunes document ultimate provenance from the Anatolia-Zagros orogen. Quartzose dune sand of the southwestern Rub' al Khali, containing a moderately poor, amphibole-rich heavy-mineral assemblage and very few young zircons, is dominantly Arabian-derived. Relatively soft carbonate grains are typically concentrated in finer sand classes, which is ascribed to both mixing with coarser quartz recycled from Arabian siliciclastic covers and selective mechanical wear during multicyclic long-distance transport in high-energy eolian environments. Understanding the complex transfer of huge detrital masses on the Earth's surface, and mixing of sediments derived from different sources along successive tracts of a composite routing system that may cover cumulative distances of thousands of kilometers across climatic and tectonic boundaries over time periods of millions of years, is essential to enhance the resolution of source-to-sink studies and avoid gross oversimplifications in paleogeographic reconstructions.

Island-Encapsulating Eolian Sedimentary Systems of the Canary and Cape Verde Archipelagos

Abstract: Eolian dunes are generally absent or poorly developed on oceanic islands. Yet, large-scale eolian sedimentary systems characterize the oceanic islands of the Canary and Cape Verde archipelagos. These island-encapsulating sedimentary systems extend around or across entire islands and comprise upwind source areas, eolian transport corridors, and downwind sediment depocenters, each of which is characterized by distinctive dune forms. Upwind beaches are denuded of sand, while downwind locations exhibit progressive shoreline accretion. Cross-island transport corridors developed in topographic lows on the island surface are characterized by a variety of landforms including sandsheets, barchanoid dunes, and transverse dunefields, depending on topography and local sediment volume and supply. Circum-island transport corridors develop when the island topography is high and sediment transport takes place on the island margins, alternating between headland-bypass dunes and longshore transport in the littoral zone in the intervening embayments. Depocenters comprise extensive eolian dunefields, prograding beaches, or beach ridges depending on local topography. Recognition of the interconnected nature of the components of these contemporary systems has important management implications. The presence of these sedimentary systems in the Canary and Cape Verde chains can be attributed to a particular combination of geological and geographical factors. The thick lithosphere in which the island chains occur slows subsidence rates and creates long-lived oceanic islands that are exposed to weathering and erosion for several million years during which terrestrial denudation and biogenic sediment production creates a sufficient volume of littoral sediment. From a geographical perspective, these islands are in arid or semiarid environments with unidirectional or strongly asymmetrical transport-capable winds (i.e., trade winds).

Siltstones Across the Daptocephalus (Dicynodon) and Lystrosaurus Assemblage Zones, Karoo Basin, South Africa, Show No Evidence For Aridification

Abstract: Changes in the record of vertebrate-assemblage zones in the Karoo Basin, South Africa, serve as a model for the response of terrestrial ecosystems to the end-Permian biotic crisis. Vertebrate turnover from the Daptocephalus to Lystrosaurus Assemblage Zone is believed to have begun with the appearance of subtle siltstone-color variance and is coeval with a unique, laminated lithofacies, both of which are interpreted by other workers to represent evidence for an aridification trend. The laminated facies consists of interbedded greenish-gray and dark reddish-gray siltstone, with coloration of the latter reported as a function of an increasing eolian component. The current study combines stratigraphy, petrography, geochemistry, and rock magnetic properties of an interlaminated succession and lateral equivalents at Old Lootsberg Pass in the Eastern Cape Province to determine whether such subtle color variance is a function of aridification and, hence, climate change. Siltstones were evaluated from intervals both below and above a pedogenic nodule-bearing feldspathic wacke that is physically correlated over a > 0.5 km distance. The lower succession consists of interbedded greenish-gray ( sensu latu ) and reddish-gray ( sensu latu ) siltstone in which undisturbed and minimally disturbed primary sedimentary, along with few secondary biogenic structures, are interpreted to represent deposition in an abandoned channel-fill complex. Massive greenish-gray and dark reddish-gray siltstone above the channel complex demonstrate that coloration is variable across laterally equivalent strata. Both lithologies were assessed using X-ray fluorescence, X-ray diffraction, Mossbauer spectroscopy, scanning electron microscopy with EDS, and bulk magnetic susceptibility, stepwise acquisition of an isothermal remanent magnetization (IRM), and backfield direct field demagnetization. Our data indicate that there is no statistically significant difference in major-element concentrations between lithologies and that subtle color difference is a function of the presence of fine ( Lystrosaurus Assemblage Zone was not arid. In contrast, the landscape was wet, that allowed for the continued growth of a Permian-age flora under conditions of high water table that experienced temporal fluxes in the hydrological regime promoting spatial color modification of sediment.

Sandstone Diagenesis in Sediment–lava Sequences: Exceptional Examples of Volcanically Driven Diagenetic Compartmentalization in Dune Valley, Huab Outliers, Nw Namibia

Abstract: At the base of many flood basalt sequences and along volcanic rifted margins, volcanism can compete with the existing sedimentary environments, resulting in interbedded sequences of volcanic rocks and sediments. Here we report on sediment interlayers that are found in the lowermost volcanic units of the Etendeka flood basalts in NW Namibia (Twyfelfontein and Awahab formations), part of the much larger Parana–Etendeka Igneous Province. The sandstone bodies, predominantly eolian dunes, are isolated in a sequence of Lower Cretaceous (∼ 134 Ma) lava flows. The uppermost part or where sediment deposition and lava emplacement is observed to interact is characterized by barchanoid dunes, which were actively migrating during the emplacement of the lava flows. The fossil (isolated by lava) barchan dunes studied in Dune Valley show three characteristically different diagenetic styles. In Dune Valley, each dune body is completely encapsulated by lava, with additional igneous intrusions cutting through some bodies. We recognize three distinct styles of diagenesis: Type 1: fossilised dunes that are red in color and lack major authigenic mineralization, with grain compaction and subsequent porosity loss being the dominant diagenetic process. Type 2: dunes that have been bleached white, which have undergone a more complex diagenetic pathway. Type 2 dunes have abundant calcite, kaolinite, and bohmite as authigenic phases and lack hematite grain coatings. Detrital plagioclase is absent in white dunes (XRD analysis), with pseudomorphs of kaolinite common. This diagenetic assemblage results in the white dunes having lower porosity and permeability compared to the red dunes. The observations are probably due to a flux of carbon dioxide (CO2), hydrogen sulfide (H2S) and/or hydrogen (H2)–rich hydrothermal groundwater derived from igneous intrusions below. Type 3: “hot contact” effects at lava-flow contacts, where the unconsolidated dunes were rapidly indurated during lava emplacement (volcano-eogenesis). Type 3 diagenesis is restricted to << 1 m depth below lava contacts and common in dunes displaying both Type 1 and Type 2 diagenesis. The distribution of diagenetic Types 1 and 2 is dune specific, and throughout Dune Valley approximately half of the dunes have been bleached (e.g., Type 2 diagenesis), whereas diagenetic Type 3 is a hot contact phenomenon and is therefore found along all basal lava and dike contacts. This work has relevance to understanding the development of sediment–lava systems, to hydrocarbon exploration and development in preserved sediment–lava sequences, and the hydrothermal process described provides an example of natural CO2 sequestration.

Estimation of Source-To-Sink Mass Balance By A Fulcrum Approach Using Channel Paleohydrologic Parameters of the Cretaceous Dunvegan Formation, Canada

Abstract: Trunk rivers transport the bulk of the sediment in a source-to-sink (S2S) system, and total mass passing through any cross section (i.e., fulcrum) of a trunk-river over geologic time should allow matching of source-area sediment delivery budgets to downstream sediment volumes deposited in the basin. We analyze the paleohydrology of ancient trunk channels and link downstream deltaic strata of Allomember E of the Cretaceous Dunvegan Alloformation in the Western Canadian Sedimentary Basin to test the total mass-balance fulcrum approach. Bankfull channel depth and width, grain size, paleoslope, velocity, and discharge are derived from outcrop, core, and well logs. Some parameter estimates use multiple methods, providing a range of values and serve as a cross check of independent methods. Estimates of annual flood frequency and paleodischarge, associated with long-term geologic-time estimates, are derived from chronostratigraphic analysis and allow calculation of cumulative sediment discharge. Isopach maps are used to measure sink-area sediment volumes. The results indicate that the trunk river of Allomember E was 10–15 m deep and 150–250 m wide, carried fine- to medium-grained (< 200 microns) sand, and flowed over a low-gradient paleoslope of 4.1–6.1 × 10–5. Annual total sediment discharge is estimated to have ranged from 5.4 to 12 × 106 m3. Within 25,000 years, the river is estimated to have transported 135–307 km3 of sediment into the basin. This is consistent with the 130 km3 of sediment mapped in the study area. However, the upper-range estimate of sediment delivered into the sink is 2.5 times the measured sediment volume in the map area, which, if accurate, suggests significant sediment escape. This supports the hypothesis that in Dunvegan time, mud was widely dispersed southward, along the Alberta Foreland Basin by geostrophic currents associated with storm processes and counterclockwise oceanic gyres in the Cretaceous Seaway.

Depositional Cycles and Sequences In An Organic-Rich Lake Basin: Eocene Green River Formation, Lake Uinta, Colorado and Utah, U.S.A.

Abstract: Early to middle Eocene Green River Formation lacustrine deposits in the eastern portion of Lake Uinta formed in two subbasins, the Piceance basin and the Uinta basin, and represent mixed siliciclastic–carbonate and organic-rich lake deposits formed during the time of the Eocene climate optimum. The formation consists of organic-rich and organic-poor mudstone i.e., oil shale, siliciclastic, and carbonate, formed in a shallow to deep (tens of meters), stratified lake environment. Lacustrine strata are characterized by three types of (decimeter to meter) depositional cycles: (1) type 1 depositional cycle formed in marginal areas. Cycle type 1 starts with siliciclastic-rich deposits, passes upward into alternating carbonate shoal and microbial carbonate, and is covered by mud- to silt-size sublittoral deposits. In the deeper, profundal zone of the lake, two types of depositional cycles occur: (2) type 2 depositional cycles start with lean oil shale and pass upwards into siliciclastic turbidites, and are sharply followed by rich-oil-shale deposits. (3) type 3 depositional cycles begin with evaporites and lean oil shale with evaporites that are sharply overlain by rich oil shale. Stacked depositional cycles form depositional sequences meters to tens of meters thick. Altogether, ten upward-deepening depositional sequences have been defined and are correlated over the Douglas Creek Arch from the Piceance basin into the Uinta basin. Depositional sequences characterize significant changes in lake regime and are divided into periods of low, rising, and high lake that are separated by sequence boundaries, transgressive surfaces, and main flooding surfaces, respectively. These depositional sequences indicate that the two basins were most probably connected during most of the first part of the Green River Formation. In the middle of the Green River Formation the arch acted as a partial sill between the two basins, when thick marginal deposits formed at the arch. In the later part of the Green River Formation, profundal deposits covered the arch. It is suggested here that the development of depositional cycles and depositional sequences in these lacustrine basins is strongly affected by climate changes and respective inflow, i.e., during times of low inflow siliciclastic and nutrient input into the lake decreased. In contrast, the highest input of siliciclastics and nutrients into the lake occurred during increased and high inflow. Low runoff resulted in low lake level and is marked by thin marginal deposits and lean oil shale in the profundal area. At times, evaporite deposition could occur in the deeper part of the basin. In the change to a wetter period, increased runoff is marked along basin margins by sharp-based sandstones. In the profundal area rich oil shale sharply overlies lean oil shale. The evolution of Lake Uinta went through uniform regional and climate changes characterized by, (1) forming of highly cyclic units in the middle of the Green River Formation, at the time when the Douglas Creek Arch acted as a partial sill between the two lakes, (2) an overall deepening-upward trend, and (3) filling of basins with siliciclastic deposits at the end of Green River Formation time. Depositional trends that occur both in the Piceance basin and in the Uinta basin also occur in the Greater Green River basin in Wyoming. Therefore, the regional correlation model for all three Green River Formation lacustrine basins has been suggested here in order to show how different basins with similar tectonic and climate background changes could be correlated with each other.

Distinguishing Depositional Setting For Sandy Deposits In Coastal Landscapes Using Grain Shape

Abstract: Several methods exist that use sediment properties to characterize depositional setting and related mechanisms of transport, including analysis of grain-size distributions, sediment petrology, micromorphology, and grain structure. Techniques that rely on electron or optical microscopy produce results with varying degrees of success and applicability. Here, a new method is presented and used to differentiate between littoral and eolian sands that were extracted from recently formed landforms, as well as landforms that are from mid to late Holocene in age. The method utilizes a standard optical microscope with a mounted digital camera, paired with freely available software (ImageJ) to characterize grain shape parameters. The method was tested on nearly 6000 sand grains from samples with varied transport histories, and it was found that grain solidity was the most distinguishable characteristic between eolian and littoral samples, differentiating them 86% of the time for calibration samples. The method was used to correctly identify the mechanism of transport for 76% of the samples. Patterns in the results indicated that this method could be extended to link potential sediment sources to various depositional basins, and future work includes testing the method in areas with a different mineralogy and/or landscape history.

Controls On the Geometry And Evolution of Humid-Tropical Fluvial Systems: Insights From 3D Seismic Geomorphological Analysis of the Malay Basin, Sunda Shelf, Southeast Asia

Abstract: High-resolution three-dimensional (3D) seismic reflection data from the Pleistocene-to-Recent succession of the Malay Basin (Sunda Shelf, SE Asia) have been used to evaluate the geometry, dimensions, distribution, and evolution of humid-tropical fluvial channel systems. Based on their seismic expression and variations in sinuosity and depth, we identify six fluvial channel types, which occur in eight, seismically defined, 18–145-m-thick depositional units. Each unit is characterized by a consistent vertical change in channel-body geometry and dimensions as follows (bottom to top): (1) relatively large (300–3000 m wide and 15–45 m deep), straight to low-sinuosity channels and/or large incised valleys at their bases, and (2) smaller (75–250 m wide and 8–23 m deep), highly sinuous channels at their tops. This cyclical stratigraphic architecture is interpreted as a mainly climatically driven change in fluvial discharge and sediment supply, rather than by eustatically or tectonically driven changes in relative sea level based on (i) the study area was located far upstream (ca. 700 km) of the coeval shoreline, and (ii) tectonically quiescent nature of the basin during the Pleistocene to Recent. Although the upstream controls were likely the main controls on the stratigraphic organization (Units 2–4 and 6), the large incised valleys (Units 5 and 7) are interpreted to be formed due a relative sea-level fall during the Last Glacial Maximum (LGM). This study demonstrates how an improved understanding of the temporal and spatial variability of fluvial channel systems can be obtained through detailed analysis of regionally extensive, high-resolution 3D seismic reflection data. In particular, regional constraints on channel geometry, scale, and orientation can be more confidently determined than is typically permitted by deeper, lower-resolution subsurface data. Furthermore, we have provided quantitative constraints on the dimensions, geometries, and paleohydrology of humid-tropical fluvial systems, which form significant, but generally less well-imaged, hydrocarbon-bearing reservoirs, in the more deeply buried parts of the Malay Basin and in many other basins around the world.

A Discontinuous Ephemeral Stream Transporting Mud Aggregates In A Continental Rift Basin: The Late Triassic Maleri Formation, India

Abstract: Finer-grained components of fluvial sediments are usually deposited in overbank environments, and their preservation in the rock record is generally subordinate to that of the coarser-grained channel fills. However, though the fossil record and lithological characteristics of the Late Triassic Maleri Formation, Gondwana Supergroup of the Pranhita–Godavari continental rift basin, India, indicate that it is a fluvial deposit, the formation is characterized by thin sandstone bodies vertically separated by thicker fine-grained sediments. With the help of petrographic and sedimentological features, this study characterizes the fluvial system that produced a deposit dominated by fines. These syn-rift sediments provide valuable clues towards understanding the spectrum of fluvial processes operating in continental rift-basins. We observe that the formation is dominated by stratified siltstone (mudrocks) rather than massive mudstones. These mudrocks contain a large quantity of silt to fine-sand-size pedogenic mud aggregates, and the preserved primary structures indicate transportation by traction currents. The internal organization of the mudrock units reveals that an admixture of pedogenic mud aggregates along with other sand-grade siliciclastic grains were transported through the channelized and unconfined reaches of a network of discontinuous ephemeral streams constituting the axial drainage of the rift basin. However, evidence for accumulation of fines in swamp-like environments is preserved in a few pedogenically modified mudstones that occur in between much thicker intervals of stratified mudrocks. On the other hand, the small bodies of cross-bedded carbonate grainstones and laminated marls indicate that the conditions suitable for precipitation of freshwater carbonates prevailed in some of these swampy areas. The sedimentary features of the thinner sheet-sandstone bodies associated with the mudrocks suggest that though the overall character of the fluvial system remained unchanged the sandstones were deposited during the phases of higher discharge. We suggest that the basin was persistently fed with a sediment load rich in mud aggregate produced in the vertic soils forming on the shales and limestones in the source area under a warm climate and seasonal rainfall.

Highly Seasonal and Perennial Fluvial Facies: Implications For Climatic Control On the Douglas Creek and Parachute Creek Members, Green River Formation, Southeastern Uinta Basin, Utah, U.S.A.

Abstract: The early to middle Eocene Green River Formation consists of continental strata deposited in Laramide ponded basins in Utah, Colorado, and Wyoming. This study (1) documents fluvial and lacustrine strata from the Douglas Creek and Parachute Creek members of the middle Green River Formation, southeastern Uinta Basin, Utah, and (2) uses new interpretations of the link between climate and fluvial sedimentary expression to interpret the terrestrial evolution of early Eocene climate. The stratigraphy was analyzed via outcrops along a 10 km transect in Main Canyon on the Tavaputs Plateau, and is divided into three distinct, stratigraphically separated depositional settings: (1) the lowermost Interval 1 is dominated by amalgamated sandstone channels that contain 70–100% upper-flow-regime sedimentary structures. The channels are interpreted to represent fluvial deposits controlled by a highly seasonal climate, where most deposition was limited to seasonal flooding events. (2) Interval 2 is dominated by alternating siliciclastic and carbonate lacustrine deposits, interpreted as local pulsed fluvial siliciclastic input into shallow Lake Uinta, and periods of fluvial quiescence represented by littoral carbonate deposition. (3) The uppermost Interval 3 is dominated by erosionally-based, trough cross bedded sandstone channels interbedded with littoral lacustrine and deltaic deposits. The Interval 3 sandstone channels are interpreted as perennial fluvial deposits with relatively little variation in annual discharge, akin to modern humid-temperate fluvial systems. The stratigraphic transition from seasonally controlled (Interval 1) to perennial (Interval 3) fluvial deposits is interpreted to represent a fundamental shift in Eocene climate, from the peak hyperthermal regime of the Early Eocene Climatic Optimum (EECO) to a more stable post-EECO climate.

Response of A Coal-Bearing Coastal-Plain Succession To Marine Transgression: Campanian Neslen Formation, Utah, U.S.A.

Abstract: The process regime of low-gradient coastal plains, delta plains and shorelines can change during transgression. In ancient successions, accurate assessment of the nature of marine influence is needed to produce detailed paleogeographic reconstructions, and to better predict lithological heterogeneity in hydrocarbon reservoirs. The Campanian lower Neslen Formation represents a fluvial-dominated and tide- and wave-influenced coastal-plain and delta-plain succession that accumulated along the margins of the Western Interior Seaway, USA. The succession records the interactions of multiple coeval sedimentary environments that accumulated during a period of relative sea-level rise. A high-resolution data set based on closely spaced study sites employs vertical sedimentary graphical logs and stratigraphic panels for the recognition and correlation of a series of stratal packages. Each package represents the deposits of different paleoenvironments and process regimes within the context of an established regional sequence stratigraphic framework. Down-dip variations in the occurrence of architectural elements within each package demonstrate increasing marine influence as part of the fluvial-to-marine- transition zone. Three marine-influenced packages are recognized. These exhibit evidence for an increase in the intensity of marine processes upwards as part of an overall transgression through the lower Neslen Formation. These marine-influenced packages likely correlate down-dip to flooding surfaces within the time-equivalent Iles Formation. The stratigraphic arrangement of these packages is attributed to minor rises in sea level, the effects of which were initially buffered by the presence of raised peat mires. Post-depositional auto-compaction of these mires resulted in marine incursion over broad areas of the coastal plain. Results demonstrate that autogenic processes modified the process response to overall rise in relative sea level through time. Understanding the complicated interplay of processes in low-gradient, coal-bearing, paralic settings requires analysis of high-resolution stratigraphic data to discern the relative role of autogenic and allogenic controls.

Source-To-Sink Sediment Budget Analysis of the Cretaceous Ferron Sandstone, Utah, U.S.A., Using the Fulcrum Approach

Abstract: This paper matches sediment fluxes, estimated to have passed through an ancient river system, with mapped downstream sediment volumes in a deltaic sink, providing a test of the recently developed fulcrum approach to source-to-sink analysis. This paper uses field measurements (such as channel depth, width, and grain size) to estimate paleodischarge in an ancient trunk channel of the Cretaceous Ferron Sandstone in central Utah. The estimates of instantaneous discharge are then integrated over the geological duration of the river to estimate the total sediment volume delivered to downstream deltaic sinks in an attempt to balance the estimated sediment flux with the mapped deltaic deposits in the sink. The bankfull channel depths, calculated using the scaling relationship between the flow depth and the mean dune height, vary from 3.3 to 5.5 m with an average depth of 4.4 m. The corresponding bankfull channel width estimates vary from 50 to 80 m, with an average value of 65 m, calculated using scaling relationship between channel width and the width of accretion surfaces. Water discharge calculated for these bankfull dimensions vary from 2.7 × 10 2 m 3 /s to 8.6 × 10 2 m 3 /s, also indicating that these rivers were routinely capable of generating hyperpycnal flows. The instantaneous sediment discharge reaching the fulcrum was calculated using established sediment transport equations. These instantaneous discharge values were first converted to mean annual sediment volume using the bankfull event durations, recurrence intervals, and a factor for the proportion of the total annual sediment load transported during the bankfull period, based on empirical relationships from modern climate analogs, and then projected over the average time duration of individual parasequences in the Ferron Notom clastic wedge, which is approximately 14 kyr. The mass balance across the fulcrum reveals that the average bedload sediment volume derived from the source (about 3 km 3 ) matches with that deposited in the sink within a factor of two. However, underestimation of the bedload volume in the sink suggests sediment escape beyond the limits of currently mapped sink area. Previous models for the Ferron indicate significant SE deflection of sediment due to wave reworking, which may account for the missing sandy sediment. It is also possible that there is an overestimation of time duration for individual valleys, resulting in higher sink-volume estimation and larger source-to-sink mass imbalance. Monte Carlo simulations, based on probabilistic estimation, were used to test the sensitivity of key parameters used in converting bankfull discharge to mean annual discharge. The P10, P50 (median), and P90 values for the average annual bedload volume ( Q mas ) are 9.1 × 10 4 m 3 , 1.7 × 10 5 m 3 , and 3.7 × 10 5 m 3 , respectively. A Q mas value between P50 and P90 yields a source-to-sink balance for bedload volume. The current study establishes a mass balance across the fulcrum with a reduced range of uncertainty for the various parameters used. Uncertainty associated with bankfull channel dimensions has been reduced through inclusion of detailed outcrop data. The uncertainty in estimating average annual sediment volume ( Q mas ) from bankfull events is less than a factor of three. This uncertainty can be further reduced by incorporating a more robust global-discharge dataset from modern analog river systems. Despite many assumptions and uncertainties, our study shows that the fulcrum method appears to be capable of balancing sediment budgets to within at least an order of magnitude in deep-time sedimentary systems.

Plan-view Paleochannel Reconstruction of Amalgamated Meander Belts, Cretaceous Ferron Sandstone, Notom Delta, South-central Utah, U.s.a.

Abstract: Few studies reconstruct paleohydraulic parameters, such as meander amplitude and sinuosity, of ancient rivers based on plan-view outcrop exposures, but rather rely on interpolation from cliff exposures. In this study, extensive plan-view exposures of amalgamated ancient meander belts in the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, Utah, USA, allow paleohydraulic reconstruction, evaluation of grain-size variability in meander belts, and their component unit and compound bars by incorporating the variability of paleocurrents, facies architectural-element analysis, and bar migration patterns. Three channel belts were identified, marked by scour surfaces, an increase in grain size, and abrupt changes in paleocurrent orientations. The youngest channel was about 2.1 m deep and 118 m wide, and had a meander wavelength of 1 km and a sinuosity of 1.1–2.9. The middle channel was about 3.4 m deep, with a meander wavelength of 2.6 km, and a sinuosity of 1.2. The oldest channel was insufficiently exposed to document its plan-view style. Grain-size trends showed systematic upwards fining in each channel story. In plan view, coarsening occurs towards the bend apex along the bend axis, and fining occurs downstream within some meander scrolls. Plan-form grain-size trends were not obvious, partially due to grain-size variation within unit bars, perhaps also reflecting the fact that the exposures capture variable vertical position in each channel belt. Paleocurrents showed systematically varying trends within individual channel belts (from NE, to SW, to E), and abrupt changes between belts. Grain size and vertical facies associations vary as a function of the style of bar migration, as well as position within a bar (upstream vs. downstream). The outcrop shows a dominance of a meandering-river style, with significant lateral amalgamation of successive point bars within each belt. Compound braid bars, built by overlapping unit bars, constitute the youngest channel deposits, and are probably associated with channel abandonment. Independent measurements of meander wavelength, based on plan-view exposures, match results estimated from empirical equations. The Ferron rivers are small to medium in scale according to calculated paleohydraulic parameters (Qw = 1.6 ∼ 2.8 × 102 m3/s). Architectural-element analysis shows that the last stage, and the most sinuous channel belt (A1), filled largely with downstream accreting unit and compound braid bars, similar to those seen in high-sinuosity meander loops of the modern Red River at the Texas–Oklahoma border. This suggests that both braid bars and point bars may occur within a single meander channel and that braid bars may evolve into compound point bars as they migrate and attach to a channel margin.