Showing papers in "Journal of Sedimentary Research in 2018"

Active Ooid Growth Driven By Sediment Transport in a High-Energy Shoal, Little Ambergris Cay, Turks and Caicos Islands

Abstract: Ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies, if the mechanisms that control their formation and growth can be understood quantitatively. There are a number of hypotheses about the controls on ooid growth, each offering different ideas on where and how ooids accrete and what role, if any, sediment transport and abrasion might play. These hypotheses have not been well tested in the field, largely due to the inherent challenges of tracking individual grains over long timescales. This study presents a detailed field test of ooid-growth hypotheses on Little Ambergris Cay in the Turks and Caicos Islands, British Overseas Territories. This field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds. This configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time. Ooid size, shape, and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading. Ooid surface textures, petrographic fabrics, stable-isotope compositions (δ^(13)C, δ^(18)O, and δ^(34)S), lipid geochemistry, and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic (e.g., abiotic precipitation, physical abrasion) and biologically influenced processes (e.g., biologically mediated precipitation, fabric destruction through microbial microboring and micritization) to grain size and character. A convergence of evidence shows that active ooid growth occurs along the transport path in a high-energy shoal environment characterized by frequent suspended-load transport: median ooid size increases by more than 100 μm and bulk radiocarbon ages decrease by 360 yr westward along the ∼ 20 km length of the shoal crest. Lipid and 16S rRNA data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth. Stable-isotope compositions are indistinguishable among samples, and are consistent with abiotic precipitation of aragonite from seawater. Westward increases in ooid sphericity and the abundance of well-polished ooids illustrate that ooids experience subequal amounts of growth and abrasion—in favor of net growth—as they are transported along the shoal crest. Overall, these results demonstrate that, in the Ambergris system, the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode. Microbes play a largely destructive, rather than constructive, role in ooid size and fabric.

Detrital Clay Coats, Clay Minerals, and Pyrite: A Modern Shallow-Core Analogue For Ancient and Deeply Buried Estuarine Sandstones

Abstract: The spatial distribution of clay minerals and authigenic-clay-coated sand grains in ancient and deeply buried petroleum reservoirs, which can enhance or degrade reservoir quality, is poorly understood. Authigenic clay coats are reported to originate from the thermally driven recrystallization of detrital clay coats or through in situ growth from the authigenic alteration of precursor and early-diagenetic minerals during burial diagenesis. To help predict the spatial distribution of authigenic clay coats and clay minerals in estuarine sandstones, this study provides the first modern-analogue study, using the Ravenglass Estuary, UK, which integrates the distribution patterns of lithofacies, Fe-sulfide, and precursor detrital-clay-coats and clay-minerals. X-ray-diffraction-determined mineralogy and the extent of detrital clay-coat coverage of sediment in twenty-three one-meter cores was established, at an unprecedented high resolution. The output from this study shows that detrital clay mineral distribution patterns are controlled principally by the physical sorting of clay minerals by grain size. Chlorite is most abundant in coarsergrained sediment (e.g., low-amplitude dunes), whereas illite is most abundant in finer-grained sub-environments (e.g., mud flats). Kaolinite abundance is relatively homogeneous, whereas smectite abundance is negligible in the Ravenglass Estuary. This study has shown that distribution patterns of detrital-clay-coats and clay-minerals are controlled by processes active during deposition and bio-sediment interaction in the top few millimeters in the primary deposition environment. In the Ravenglass Estuary, distribution patterns of detrital-clay-coats and clay-minerals have not been overprinted by the postdepositional processes of sediment bioturbation or mechanical infiltration. Optimum detritalclay-coat coverage and clay mineralogy, which might serve as a precursor to porosity-preserving authigenic clay coats in deeply buried sandstone reservoirs, is likely to occur in low-amplitude dunes in the inner estuary and central basin. Furthermore, bioturbation in low-amplitude dunes has reduced Fe-sulfide growth due to oxidization, meaning that iron remains available for the formation of authigenic Fe-bearing clay minerals, such as chlorite, that can lead to enhanced reservoir quality in deeply buried sandstones.

Mixed-energy Process Interactions Read from a Compound-clinoform Delta (paleo–orinoco Delta, Trinidad): Preservation of River and Tide Signals By Mud-induced Wave Damping

Abstract: A segment of the Pliocene Orinoco Delta on Trinidad preserved a progradational compound clinoform that reveals detailed interaction of river, wave, and tide processes with fluid mud at a scale of tens of meters to centimeters in the deposits. Based on sedimentary structures, grain size, bioturbation, and organic-matter content, each measured bed or bed set was assigned a probability percentage of having been dominated by a particular process and process probabilistic histograms were generated. Characteristic river signals in these deposits are rare, and are represented by outsized grain sizes, normally to inversely graded beds, and abundant organic matter, though organic matter can also be derived from vegetated supratidal areas without fluvial influence. Wave and storm-wave signals are recorded by symmetrical ripples, hummocky–swaly cross stratification, and a diverse ichnofauna. Tidal signals are represented by stacked, orderly cross-bedded sandstones with mud drapes, abundant mudstone layers, bidirectional ripple laminae, spring–neap tidal bundles, and a restricted ichnofauna. The study gave the following results: (i) at the scale of regressive parasequences (30–50 m), the intensity of preserved wave signals increased from the prodelta towards the outer delta-front platform and decreased to the inner delta-front platform, whereas there was a reciprocal trend in the intensity of tidal signals; the fluvial signals are generally not strong, but sometimes they become irregularly stronger towards the upper levels of a parasequence, (ii) at the scale of individual spring–neap tidal bundles (centimeter to decimeter), thick tidal rippled sandstones with thin to partially eroded fluid-mud layers were deposited during spring tides, and thin tidal rippled sandstones or silt laminae with thick fluid-mud layers were deposited during neap tides on the middle-inner delta-front platform; storm waves reworked the deposits mainly on the outer delta-front platform. The observations suggest that the abundant fluid mud (transported by the wave-driven littoral Guyana Current from the Amazon river mouth) on the delta-front platform caused wave damping and the preferred preservation of river and tidal signals on the middle-inner delta-front platform and distal subaerial delta. INTRODUCTION AND OBJECTIVES Modern delta systems commonly exhibit compound clinoforms that consist of a subaerial clinoform and subaqueous clinoform with topset– foreset–bottomset morphology (tens of meters in relief) (Fig. 1A) (e.g., Pirmez et al. 1998; Driscoll and Karner 1999; Kuehl et al. 2005; Swenson et al. 2005). The subaerial and subaqueous delta clinoforms are genetically and morphologically linked, and they are separated by a broad subaqueous platform as their bottomset and topset, respectively (e.g., Cattaneo et al. 2003; Swenson et al. 2005; Helland-Hansen and Hampson 2009; Walsh and Nittrouer 2009; Patruno et al. 2015a). The subaerial delta clinoforms comprise delta plain, delta-front slope, and delta-front platform (or subaqueous clinoform), and the subaqueous delta clinoforms consist of delta-front platform, prodelta slope, and prodelta bottomset contiguous with the shelf (Fig. 1A). The topset–foreset rollover of the subaerial clinoforms is in proximity to the shoreline (e.g., Ta et al. 2002a; Correggiari et al. 2005) occurring at water depths of less than 5 m, whereas the subaqueous clinoform rollover lies at a variable distance (kilometers to tens of kilometers) offshore from the shoreline (Pirmez et al. 1998; Swenson et al. 2005) at water depths of up to 60 m (Patruno et al. 2015a). The interplay of fluvial input with shallow marine hydrodynamics (i.e., tides, waves, and currents) controls the sediment dispersal and spatial separation of the subaerial and subaqueous clinoforms (Swenson et al. 2005; Walsh and Nittrouer 2009). Subaerial clinoforms are mainly affected by fluvial discharge, whereas subaqueous clinoforms are formed by combined basinal processes (tides, waves, and currents) (Pirmez et al. 1998; Driscoll and Karner 1999; Swenson et al. 2005; Rossi et al. 2016). Some deltas exhibit only a single clinoform when they discharge in lowenergy marine (e.g., the Mississippi River delta) or lake settings. The compound clinoforms are well developed in modern muddy deltas situated in energetic marine environments, such as the Amazon (Kuehl et al. 1986; Nittrouer et al. 1986), the Ganges–Brahmaputra (Kuehl et al. 1997; Michels et al. 1998; Kuehl et al. 2005), the Yangtze (Hori et al. 2001; Liu et al. 2006; Liu et al. 2007), and the Fly River Delta (Walsh et al. 2004). The modern Orinoco delta, as one of these examples, also displays compound clinoform geometry with the subaqueous clinoform rollover at water depths of 10 m and 10–50 km seaward from the coast (Fig. 1B) Published Online: January 2018 Copyright 2018, SEPM (Society for Sedimentary Geology) 1527-1404/18/088-075/$03.00 (Warne et al. 2002), and it is principally affected by waves, tides, and the northwest-directed Guyana Littoral Current. The Guyana Current transports suspended sediments derived from the Amazon River by fluid-mud banks migrating in the littoral zone (5 m to 20 m water depth or less) along the northeast coast of South America (Allison et al. 2000). The mud banks migrate alongshore by constant reworking of their trailing edges and transport of mud towards their leading edges by waves, during which process large volumes of fluid mud are held in suspension (Allison and Lee 2004; Anthony et al. 2014). The Orinoco delta receives at least as much muddy sediments from the Amazon as comes from the Orinoco River itself (Eisma et al. 1978; Meade 1994), and this additional mud promotes shoreline progradation during the Holocene rising to highstand of sea level (Aslan et al. 2003). Although examples of modern subaqueous delta clinoforms or compound clinoforms have been increasingly documented, ancient examples have been documented only rarely (e.g., Hampson 2010; Vakarelov et al. 2012; Patruno et al. 2015b; Rossi and Steel 2016; Hampson and Premwichein 2017), as the complexity of this process mixing is still understudied. In this study, a muddy compound clinoform of a tide-dominated, wave-influenced delta is described in outcrops where there occurs the distal part of a subaerial delta and the proximal to distal subaqueous delta in a clean, well-exposed, continuous stratigraphic segment of the Pliocene Orinoco Delta within the lower Telemaque Sandstone Member of the Manzanilla Formation on Trinidad. In addition, the outcrop demonstrates high-resolution and high-frequency variations of tide-, wave-, and river-generated facies at both large (i.e., tens of meters), medium (meter), and small (decimeter to centimeter) scales. The mixing of interacting river, wave, and tide processes in this succession is documented using a new quantitative methodology for highlighting process mixing to better understand how the processes interacted on a compound clinoform of the Orinoco Delta. Additionally, the role of the abundant fluid-mud deposits in the study succession is evaluated in terms of how the mud, FIG. 1.—A) Schematic cross section of a deltaic compound clinoform comprising subaerial and subaqueous clinoforms (modified after Helland-Hansen and Hampson 2009; Patruno et al. 2015a) associated with deltaic subenvironments. B) Map view of the modern Orinoco delta and shelf with bathymetry and sediment distribution (modified after Warne et al. 2002). PENG ET AL. 76 J S R

Calcium Carbonate Precipitation in Diatom-rich Microbial Mats: The Laguna Negra Hypersaline Lake, Catamarca, Argentina

Abstract: Carbonate microbialites provide a window to understand microbe–mineral interactions in modern environments and in the geological record. Unraveling microbial versus physicochemical controls and biogeochemical signatures is not always straightforward. Environmental and laboratory studies have shown that microbial activity can play a central role in calcium carbonate precipitation. Most studies have focused on the effects of Bacteria and Archaea activity on carbonate precipitation processes (e.g., cyanobacteria, sulfate-reducing bacteria, sulfide-oxidizing bacteria and iron-reducing bacteria). The influence of eukaryotic activity, such as diatoms and other microalgae, on carbonate precipitation and microbialite formation has been the focus of less attention. This study explores carbonate mineralization in active diatom-rich microbial mats developed in a high-altitude groundwater-fed hypersaline lake in the Puna region of Catamarca, Argentina. Previous work has focused on the texture, mineralogy, and stable-isotope chemistry of the subfossil oncoidal and laminar microbialites. Here, using 16S rDNA Bacteria diversity analysis, confocal scanning laser microscopy, scanning electron microscopy, and transmitted-light microscopy, we explore the Bacteria and diatom diversity in the microbial mats and the related carbonates textures and make comparisons with similar textures in the associated oncoidal microbialites. Diatom-rich subspherical aggregates, which have not been previously described in the literature, show a diverse microbial community with abundant exopolymeric substances (EPSs) where carbonate precipitation takes place. These aggregates are a main component of the Laguna Negra microbial mats and show anhedral micritic calcite in the EPS matrix as the main mineral component. Similar calcite micrite textures are also preserved as one lamina type in the associated oncoids. On the other hand, where EPS are absent, carbonate precipitation, related to pennate diatom blooms, is represented by euhedral aragonite needles suggesting different mechanisms and controls. Changes in the microbial communities are recorded in the oncoids as different lamina types, providing a link between the currently active mats and the subfossil oncoidal structures. This is a first survey of these previously unexplored diatom-rich microbial mats developed under extreme environmental conditions in the Laguna Negra. Understanding the effect that the interaction between diatoms and prokaryotic communities has on carbonate precipitation may provide some insight on the evolution of microbialite textures and fabrics, and on the change from prokaryote-dominated systems to mixed eukaryotic–prokaryotic systems.

Topographic controls on the development of contemporaneous but contrasting basin-floor depositional architectures

Abstract: Sediment-laden gravity-driven-flow deposits on the basin floor are typically considered to form either discrete lobes that stack compensationally, or packages of laterally extensive beds, commonly termed “sheets.” These end-member stacking patterns are documented in several basinfills. However, whether they can coexist in a single basin, or there are intermediate or transitional stacking patterns, is poorly understood. An analysis of depositional architecture and stacking patterns along a 70 km dip-oriented transect in the Upper Broto Turbidite System (Jaca Basin, south-central Pyrenees, Spain), which displays disparate stacking patterns in contemporaneous strata, is presented. Proximal and medial deposits are characterized by discrete packages of clean sandstones with sharp bed tops which exhibit predictable lateral and longitudinal facies changes, and are interpreted as lobes. Distal deposits comprise both relatively clean sandstones and hybrid beds that do not stack to form lobes. Instead, localized relatively thick hybrid beds are inferred to have inhibited the development of lobes. Hybrid beds developed under flows which were deflected and entrained carbonate mud substrate off a carbonate slope that bounded the basin to the south; evidence for this interpretation includes: 1) divergent paleoflow indicators and hummock-like features in individual beds; 2) a decrease in hybrid-bed thickness and abundance away from the lateral confining slope; 3) a carbonate-rich upper division, not seen in more proximal turbidites. The study demonstrates the co-occurrence of different styles of basin-floor stacking patterns in the same stratigraphic interval, and suggests that characterization of deep-water systems as either lobes or sheets is a false dichotomy.

River Morphodynamic Evolution Under Dam-Induced Backwater : An Example from the Po River (Italy)

Abstract: River systems evolve in response to the construction of dams and artificial reservoirs, offering the possibility to investigate the short-term effects of base level oscillations on fluvial architecture. A major effort has been dedicated to the understanding of river response downstream of large dams, where deep channel incisions occur in response to the removal of sediment that is sequestered in the upstream reservoir. Integrating field observations and numerical-modeling results, this work quantifies the sedimentary and morphological changes of the Po River (Italy) upstream of the Isola Serafini dam to investigate the impact of dam-induced backwater on river morphodynamics. The construction of a reservoir generates a new base level that forces an upstream shift of alluvial lithofacies and a change in the planform geometry of the river. The lateral migration rate of the channel is up to 45 m/yr upstream of the influence of backwater flow and ca. 10 m/yr at the transition from normal to backwater flow conditions (30 km from the dam). Within this reach, a reduction of the bed shear stress promotes deposition of coarse-grained sediment and the emergence of the gravel–sand transition of the river. The lateral migration of the channel continuously decreases over time, and rates < 5 m/yr can be observed in the reservoir backwater zone. This trend is accompanied by the drowning of channel bars, the reduction of river competence, and an increase in bedform spacing. Oscillatory backwater and drawdown surface water profiles can be observed closer to the dam, which are associated with varying low-discharge and high-discharge events, respectively. While low-flow conditions, persisting for much of the year, allow the deposition of fine-grained sediment, high-discharge events promote not only the resuspension and transport of fine material but also the progressive erosion of channel bars and the overall deepening of the thalweg. This study provides a clear picture of the river evolution in response to the construction of a hydropower dam that may be of help in predicting how other fluvial systems will respond to future human interventions. Moreover, the result of how changes in base level and oscillations in water surface profile (backwater and drawdown) control river hydro-morphodynamics and sediment transport may provide new insights when reconstructing ancient fluvial and deltaic sequences.

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

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

Control of basin water depth on channel morphology and autogenic timescales in deltaic systems

Abstract: River channel geometry is often controlled by upstream boundary conditions, including fluvial discharge and sediment properties. At the coast, downstream boundary conditions (e.g., tides, waves, water depth) also strongly influence channel formation and evolution. We conducted a set of experiments to determine the effects of basin water depth (i.e., a downstream boundary condition) on the evolution and geometry of fluviodeltaic channels and lobes. Internal dynamics (autogenic processes) in the fluviodeltaic system drive channel avulsion through cycles of sediment storage and release. Experimental results indicate an increase in the timescale of autogenic storage and release with increasing basin water depth. Deeper basin water requires a larger volume to be filled within the delta front, thus more time to complete one autogenic storage and release cycle for a given sediment discharge. While a relationship between delta-front volume and autogenic storage and release timescales is expected, we show that autogenically generated morphological changes in the delta topset and distributary channels also exert control on timescales of storage and release. Deltas building into deeper basins develop steeper topsets, and deeper distributary channels that cause high-magnitude topset slope fluctuations, which contribute to the long autogenic timescales. Deposits in shallow basins exhibit both shallower topset slopes and shallower channels. Channel bed slopes are similar (~ 0.06) across all experiments, but lateral channel migration rates varied with basin depth. Deltas building into shallow basins had rapid lateral channel migration, such that channels quickly reworked the delta topset. For deep basins, channel migration rates were much slower, so the topset was reworked less often, allowing the topset to build to steeper slopes before being reworked. These experiments indicate an intimate relationship between lateral channel migration and topset aggradation. In addition, the deeper and more stable channels in deeper basins generally developed a wider range of channel widths, some of which produced elongate lobes. We conclude that the downstream boundary has a strong control on fluviodeltaic morphology, which can result in a striking difference in the autogenic timescale.

U-PB Zircon Ages and Provenance of Upper Cenozoic Sediments from the Da Lat Zone, SE Vietnam: Implications For an Intra-Miocene Unconformity and Paleo-Drainage of the Proto–Mekong River

Abstract: The present-day Mekong River is the twelfth longest river in the world. It drains the Tibetan Plateau and forms a large delta in south Vietnam. Remnants of upper Cenozoic fluvial to marginal marine proto-Mekong sediments are exposed in the Da Lat Zone in southeast Vietnam and are likely the on-land equivalents of large sediment packages offshore in the Cuu Long and Nam Con Son Basins. Provenance studies are used here to identify sources that allow reconstruction of sediment pathways. The Oligo-Miocene Di Linh Formation has a main Cretaceous zircon age population and subordinate Paleoproterozoic (c. 1.8-1.9 Ga) zircons, sourced mainly from Cretaceous plutons. In contrast, the early Pliocene to Pleistocene Song Luy Formation includes additional Permian–Triassic and Ordovician–Silurian age populations which are interpreted to be sourced from basement rocks in central and northern Vietnam. There is a significant change in provenance between these formations, interpreted as an intra-Miocene unconformity. The late Pliocene to Pleistocene Ba Mieu Formation shows a much greater abundance of Precambrian zircons, including a small age peak at c. 2.5 Ga, interpreted to be derived from old basement exposed in South China. Abundant Cretaceous zircons and a lack of Jurassic zircons in all three formations contrast with the present-day Mekong River delta. Therefore, the data are proposed to show a development of the proto–Mekong River progressing northward from the Da Lat Zone, to the Kontum Massif and northern Vietnam, to capturing of reversed rivers and/or reworking of sediments from old cratons in South China.

Clay Cutans and the Origin of Illite Rim Cement: An Example from the Siliciclastic Rotliegend Sandstone in the Dutch Southern Permian Basin

Abstract: In Permian Rotliegend sandstones in the Dutch part of the Southern Permian Basin illite can be a practical annoyance because it strongly influences reservoir properties. Despite its practical importance, the origin and in particular the prediction of the distribution of authigenic illite in siliciclastic sandstone is still under discussion. Three main types of authigenic illite exist, each of which result from different diagenetic processes at different stages during burial. The earliest illite is tangential illite, parallel to the grain surfaces. It has formed by infiltration of suspended clay-size particles into loose surficial sands and can be interpreted as cutanic clay. The clay cutans were occasionally formed in situ but are mostly inherited with grains having been reworked from interdune and sabkha areas by fluvial and aeolian action. The second illite type is authigenic and occurs on top of the clay cutans. Commonly two or three illite generations with different crystal habit are present, ranging from platy to hairy. Clay cutans were nucleation sites and generated the mass for the illite rim cement. Different amounts and habits of illite rim cement are linked to differences in the thickness and completeness of the clay cutans. The more iron oxide in the cutans, the less authigenic illite formed. The illite rim cement precipitated after mechanical compaction, and is probably related to illitization of smectitic clay in the cutans. Illite rim cement is thus related to the local paleogeography of the basin, including aridisol development and erosion of such soils. The third type of authigenic illite occurs in secondary pores after dissolution of detrital K-feldspar or rock fragments containing K-feldspar. The latter illite is not the product of kaolinite illitization but it precipitated as a primary mineral without a kaolinite precursor. The illite in secondary pores after feldspar dissolution thus depends on the detrital composition. Only the illite rim cement reduces intergranular porosity to microporosity, thereby increasing the specific surface area. This results in a high irreducible water content. In consequence the permeability for a given porosity can be reduced by up to two orders of magnitude. The other two types of illite have no significant influence on reservoir properties.

Sedimentological and Ichnological Characterization of the Middle To Upper Devonian Horn River Group, British Columbia, Canada: Insights Into Mudstone Depositional Conditions and Processes Below Storm Wave Base

Abstract: The Middle and Upper Devonian Horn River Group in northeastern British Columbia, Canada, consists predominantly of organic-rich mudstones that are typically described as black shales. This stratigraphic unit has seen substantial exploration and development for natural gas during the last decade. Although black shales such as the Horn River Group have historically been interpreted as deposited in anoxic deep-water basins, detailed sedimentological and ichnological analyses of eleven Horn River Group cores indicate that the depositional conditions varied significantly with respect to paleoenvironments, redox conditions, and physical processes, resulting in distinctive sedimentary facies. This study integrates data on lithology and physical and biogenic sedimentary structures from detailed core descriptions combined with total-organic-carbon content to better understand depositional processes and conditions. Ten mudstone lithofacies and three lithofacies associations are identified. Massive mudstones and pyrite-rich mudstones display very rare planar lamination and graded beds and scarce bioturbation (BI 0–1), and are interpreted to represent anoxic waters, below storm wave base (SWB), with conditions of relatively low to moderate energy. Heterolithic, laminated, and more bioturbated lithofacies are interpreted to represent oxygenated waters below SWB with relatively high-energy conditions. Overall bioturbation intensity varies between moderate to intense (BI 3–6). Where these lithofacies are not intensely bioturbated, oxygenated lithofacies show combinations of well-preserved physical sedimentary structures, including horizontal parallel lamination, soft-sediment deformation, double mud drapes, graded beds, and current ripples. A third lithofacies association represents conditions intermediate between anoxic and oxygenated lithofacies associations, mainly based on local changes in the ichnological characteristics. These lithofacies show either intense or sparse bioturbation (BI 4–6 or BI 0–1, respectively) depending on the core location, possibly indicating local changes in the depositional conditions and processes (e.g., variations in the oxygen content and/or sedimentation rate). They can also be found in thick anoxic and oxygenated deposits. These lithofacies display intervals of faint lamination, amalgamated current ripples, and normally graded bedding. The cores collectively display significant spatial and stratigraphic heterogeneity in lithology, sedimentary structures, and bioturbation that are related to relative water depth, oxygen content, sediment input, and deep-water currents. We interpreted bioturbation intensity to vary mainly as a function of oxygen content related to proximity to the basin margin and sea–level fluctuations, although secondary stress conditions (e.g., nutrient content and sedimentation rate) may also be important. Unbioturbated and massive fabrics represent anoxic to dysoxic conditions. Current-generated structures are present, mainly in the central and distal parts of the basin, and are interpreted to represent the presence of contour currents. In the proximal parts of the basin, laminated and heterolithic units are more common and more bioturbated than in the central and distal basin. This study thus can provide significant insights into other shale basins that show similar lateral and vertical variations. Integration of multidisciplinary studies, particularly sedimentology, ichnology, and total organic carbon aids in understanding and modeling the complexity of these systems.

A 600-Million-Year Carbonate Clumped-Isotope Record from the Sultanate of Oman

Abstract: Carbonate clumped-isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks. This study utilizes a 600-million-year record of marine carbonate rocks from the subsurface and surface of the Sultanate of Oman to explore how burial and exhumation affected the carbonate clumped-isotope thermometer. Samples span 6 km of burial depth, and include calcite and dolomite mineralogies and a range of carbonate rock textures. We find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial. The first group of carbonates yield water δ^(18)O_(VSMOW) compositions slightly enriched or equal to an expected “ice-free” seawater composition of –1.2‰ and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment–water interface. Temperatures from the second group sit on the present-day geotherm, yield highly enriched water δ^(18)O_(VSMOW) compositions, and display fair to poor textural preservation. We find no evidence for solid-state reordering in paired analyses of calcites and dolomites. Our results contribute to a growing body of work that indicates that the seawater δ^(18)O_(VSMOW) composition has not changed significantly over 600 Myr and was not –6‰ in the Ediacaran.

Back To Basics of Sequence Stratigraphy: Early Miocene and Mid-cretaceous Examples from the New Jersey Paleoshelf

Abstract: Many sequence stratigraphic approaches have used relative sea-level curves that are dependent on models or preconceived notions to recognize depositional sequences, key stratal surfaces, and systems tracts, leading to contradictory interpretations. Here, we urge following basic sequence stratigraphic principles independent of sea-level curves using seismic terminations, facies successions and stacking patterns from well logs and sections, and chronostratigraphic data to recognize sequence boundaries, other stratal surfaces, parasequences, and systems tracts. We provide examples from the New Jersey siliciclastic paleoshelf from the: 1) early Miocene using academic-based chronostratigraphic, seismic, core, downhole, and core log data, and 2) mid-Cretaceous using commercial well-log, seismic, and biostratigraphic data. We use classic criteria to identify sequence boundaries on seismic profiles by reflection terminations (onlap, downlap, erosional truncation, and toplap), in cores by surfaces of erosion associated with hiatuses detected using biostratigraphy and Sr-isotope stratigraphy and changes in stacking patterns, and in logs by changes in stacking patterns. Maximum flooding surfaces (MFSs) are major seismic downlap surfaces associated with changes from retrogradational to progradational parasequence stacking patterns. Systems tracts are identified by their bounding surfaces and fining- (generally deepening) and coarsening- (generally shallowing) upward trends in cores and well-log stacking patterns. Our Miocene examples of sequences m5.4 (17.7–16.1 Ma) and m5.8 (20.1–19.2 Ma) illustrate how basic sequence stratigraphic techniques reveal higher-order sequences within Myr scale composite sequences. Our mid-Cretaceous examples from the New Jersey shelf provide a paleoshelf transect spanning the Great Stone Dome to the outer continental shelf to identify parasequences, sequences, and systems tracts. This sequence stratigraphic framework provides insights into Myr scale coeval depositional environments across the paleoshelf and reservoir continuity, and highlights the application of basic sequence stratigraphic criteria to reservoir-scale evaluation, not only for oil and gas resources, but also for carbon storage.

Sediment Volume and Grain-Size Partitioning Between Submarine Channel−Levee Systems and Lobes: An Experimental Study

Abstract: The width and depth of submarine channels change progressively as the channels evolve. This is inferred to act as an important control on the rate of sediment loss due overbank and in-channel deposition. Understanding the downstream extraction of sediment from turbidity currents is important for the prediction of grain-size trends and volume distribution in the stratigraphy. However, the partitioning of sediment by individual turbidity currents as a function of channel dimensions has not been investigated previously. We present a series of physical experiments studying the link between channel dimensions and the resulting partitioning of sediment volume and grain size between sub-environments. The experimental set-up consists of a slope (11°) with a straight pre-formed channel and a horizontal basin floor. An identical flow was released repeatedly into channels with different dimensions, resulting in various styles of overspill, erosion, and deposition under varying degrees of channel confinement. The fraction of sediment that was bypassed through the channel to the basin floor varied between 67% and 89%, depending on the amount of levee and in-channel deposition. The volume of levee deposition correlates well with channel depth. A large channel depth relative to flow thickness limits the amount of overspill. The amount of in-channel deposition correlates well with channel width/depth (W/D) ratio, where low-W/D-ratio channels have less deposition. We compare the experiments to natural system to show that the same patterns of volume and grain-size partitioning are present at different scales. The experiments provide snapshots of different phases of evolution of natural submarine channels. Natural submarine channels in an early evolution phase are inferred to be shallow and the experiments demonstrate that this results in significant sediment loss to levee deposition along the channel. The process of levee deposition preferentially extracts the fine-grained sediment fraction, which overspills from the channel. Therefore, we predict that the initial sediment pulse that reaches the basin floor is coarse grained and volumetrically small. As the channel matures and deepens, it will bypass more sediment with a mix of grain sizes to the basin floor.

Displacive Widening of Calcite Veins in Shale: Insights Into the Force of Crystallization

Abstract: The geometry, microtextures, and c-axis fabrics of calcite “beef” veins in the Lower Jurassic black shales (Wessex Basin, UK) were characterized to investigate the mechanism responsible for widening following fracture propagation. Isolated beef veins exhibit planar tapering tips, whereas closely spaced veins are characterized by blunt tips. Vein surfaces are generally smooth and flat; however, circular ridges appear on vein surfaces that protrude into the host clays where there are solid inclusions below or above the ridges. Fossils with well-preserved morphologies, which are separated by subvertical calcite fibers, are observed on both the lower and upper surfaces of single veins. The shale laminations around beef veins are folded and parallel to vein margins. The beef veins commonly contain blocky zones of small, equant calcite crystals, pyrite, and organic matter. The fibers exhibit a preferred subvertical c-axis orientation, whilst crystals in the median zones and blocky zones have random c-axis orientations. The different crystal sizes, morphologies, and c-axis orientations of the fibers from the blocky crystals suggest that the fibers grew without competition with each other under a nonhydrostatic stress field. The displacive widening of calcite beef veins, which is evident from vein interactions and deformation of individual fossil skeletons, demonstrates that fibers grew incrementally because the crystallization pressure of calcite exceeded the overburden load. The force of crystallization is suggested to be responsible for the c-axis orientations of calcite fibers, whereby crystals with free surfaces normal to c-axis orientations grew preferentially. The present study suggests that the fibrous widening of calcite veins in shale postdates their initiation and may result from displacive crystallization rather than fluid overpressure.

Acid-saline-lake Systems of the Triassic Mercia Mudstone Group, County Antrim, Northern Ireland

Abstract: The Triassic Mercia Mudstone Group of County Antrim, Northern Ireland, is dominated by red beds and evaporites. After only limited study, both marine and continental environments have been proposed previously. Here, we describe these rocks for the first time from core and petrographic observations to interpret depositional environments and conditions, such as water depth, salinity, and aridity. Bedded halite, bedded gypsum, displacive halite, and red siliciclastic mudstone lithologies comprise most (∼ 80%) of the 591.6-m-thick Mercia Mudstone Group in the Gaelectric Carnduff-2 core. Bedded halite consists of chevron and cornet crystals, indicating bottom-growth from shallow surface brines. Bedded-gypsum lithology is composed of halite-replaced pseudomorphs after swallowtail, bottom-growth gypsum crystals. Both bedded halite and bedded gypsum contain dissolution features and are commonly overlain by mud drapes. Bedded-halite and bedded-gypsum lithologies are interpreted to have formed in shallow saline lakes influenced by flooding and evapoconcentration. The displacive-halite lithology is composed of sub-cubic halite crystals, randomly oriented in mudstone, and represents deposition in a saline mudflat. Some mudstones contain ripple cross-lamination, dewatering structures, mudcracks, and rip-up clasts, suggesting shallow surface water and desiccation. Other mudstones are massive (structureless) and may have been deposited by wind. Both of these mudstone units were likely deposited in dry mudflats. Other red mudstones contain soil slickensides, blocky peds, and circumgranular cracks and are interpreted as paleosols. Our observations indicate that the Triassic Mercia Mudstone Group was formed by shallow perennial saline lakes and associated continental environments in an arid climate. Furthermore, lack of carbonates, lack of fossils, and paucity of organic matter suggest acid saline lakes and groundwaters. The Triassic Mercia Mudstone Group is similar to some other Pangean red beds and evaporites. Therefore, we hypothesize that the supercontinent was an arid barren landscape hosting acid-saline lakes.

Composite Particles in Mudstones: Examples from the Late Cretaceous Tununk Shale Member of the Mancos Shale Formation

Abstract: Despite recent advances in understanding the complex dynamics of mud deposition, it remains a challenging task to characterize the grain size, origin of different components, and sedimentary textures of mudstones through detailed petrographic analysis. In this study, the Tununk Shale in Utah has been examined by optical and scanning electron microscopy (SEM) to determine how variations in petrographic characteristics (e.g., composition, texture) of this shelf mudstone succession reflect changing depositional environments. In the context of the general depositional setting, detailed petrographic studies indicate that most mud in the Tununk system were transported in bedload as siltto sand-size mud-dominated composite particles (MCPs), rather than specific components (e.g., clay minerals, silt grains, fossil fragments) of smaller size (micrometers to tens of micrometers). Three types of MCPs in the Tununk Shale can be identified and distinguished from each other. These include fecal pellets, altered volcanic rock fragments, and shale lithics. Two other types of MCPs, namely floccules and soft mud rip-up clasts, likely contributed significantly to the formation of the precursor mud matrix of the Tununk Shale. Due to their water-rich nature, however, floccules and mud rip-up clasts suffer significant compaction. Except in fortunate circumstances, they are therefore no longer discernible in the rock record. MCPs and their role in the formation of fine-grained sedimentary successions has largely gone unnoticed in previous studies. The recognition criteria, as well as petrographic characteristics of each type of MCP in different depositional environments of the Tununk Shale, are summarized here, with the intent that they may benefit future studies of other mudstone successions. The complex variability in the characteristics of different types of MCPs illustrated in this case study, however, highlights the need for additional systematic petrographic studies (integrating both optical and SEM) in order to develop and refine the current recognition criteria of MCPs in fine-grained sedimentary rocks. Detailed petrographic examination of mudstones, though labor intensive, can yield critical information regarding their provenance and depositional setting, as well as provide general insights into the underlying causes for mudstone heterogeneity.

Sediment Storage Partitioning in Alluvial Stratigraphy: The Influence of Discharge Variability

Abstract: Numerical models of formation of alluvial stratigraphy often specify, either explicitly or implicitly, the proportion of channel and overbank sediments that are deposited during a given interval of time. However, little is known about the factors that affect the partitioning of sediment between channels and the overbank environment over long time intervals, and the fidelity with which that partition is preserved in the stratigraphic record. Here we use physical experiments to investigate the role that discharge variability plays in this partitioning in fluvial stratigraphy. We find that channels formed under constant flow conditions have low lateral mobility and act mostly as conduits for sediments to reach the shoreline. The bulk of the aggradation in this case is derived from sediment-laden flow that escapes the main channels. By contrast, including floods increases channel lateral mobility, and this change is recorded in stratigraphy as an increased proportion of channel deposits relative to overbank deposits. When variable flow is included as an input condition a large volume of in-channel deposition occurs, rendering the channels substantial contributors to stratigraphic volume on their own. The increase in channel deposit volume is driven mainly by a threefold increase in the average time that a location is subject to in-channel aggradation. Other factors include a slight increase of in-channel aggradation rates, and an increase in erosion of the overbank environment that results from energetic overbank flows. Our study shows that the character of a river's hydrograph exerts a significant influence on the proportion of channel to overbank sediment bodies in alluvial successions, which is an unexamined source of uncertainty in common stratigraphic models.

U-Pb Detrital-zircon Geochronology of the Middle Bloyd Sandstone (morrowan) of Northern Arkansas (U.S.A.): Implications For Early Pennsylvanian Sediment Dispersal in the Laurentian Foreland

Abstract: Early Pennsylvanian sediment dispersal across the Laurentian supercontinent reflects the growth of the Appalachian–Ouachita Orogen that was formed by its collision with Gondwana. Occupying a key position within the Laurentian foreland of the Appalachian–Ouachita Orogen, the middle Bloyd sandstone member of the Bloyd Formation (Early Pennsylvanian) preserves evidence of Morrowan-age fluvio-deltaic depocenters in the southern Laurentian continental margin. The middle Bloyd sandstone provides direct constraints on provenance and dispersal paths of detritus shed from the Appalachian–Ouachita Orogen and other sediment sources on the North American craton during the Early Pennsylvanian. Eight outcrop samples of the middle Bloyd sandstone were collected and analyzed by U-Pb laser ablation ICP-MS detrital-zircon geochronology, producing a total of 850 concordant individual detrital-zircon ages with age ranging from 3.15 Ga to 374 Ma. Detrital-zircon age spectra of the eight middle Bloyd sandstone samples are characterized by a prominent Grenvillian age group (∼ 1350–900 Ma) with two discrete age peaks at ∼ 1070 and ∼ 1169 Ma. The remaining zircon fractions are interpreted as being sourced from the Archean Superior Province (> ∼ 2500 Ma), the Penokean and Trans-Hudson Proterozoic terranes (∼ 1900–1800 Ma), the Yavapai–Mazatzal terrane (∼ 1800–1600 Ma), the Granite–Rhyolite Province (∼ 1600–1350 Ma), and a minor Paleozoic age population (∼ 500–350 Ma) that likely reflects mixed Appalachian and Gondwanan sources. Overall, detrital-zircon age populations of the middle Bloyd sandstone share great similarity with Lower Pennsylvanian sandstones from the Michigan Basin and the central and southern Appalachian foreland, except that the Bloyd sandstone shows greater enrichment in the Yavapai–Mazatzal age population. These new age data provide additional constraints on multiple dispersal paths for fundamentally Laurentian-derived sediment that are ultimately directed southward toward the Ouachita foreland shelf during the Early Pennsylvanian. The Early Pennsylvanian (Morrowan) sediment dispersal in this part of Laurentia was accomplished by a regional-scale drainage system that integrated intracratonic and Appalachian–Ouachita foreland basin depocenters, all recording a recycled orogenic provenance. The new detrital-zircon data indicate that the middle Bloyd sandstone also incorporated significant contributions from Precambrian-age basement uplifts, possibly Ancestral Rocky Mountain uplifts to the west, indicating a complex geography of the Laurentian supercontinent at this time.