Studies of mud transportation and deposition in modern shelf settings indicate that fine-grained siliciclastic sediment is deposited from suspension in various ways. However, contemporary interpretations of ancient analogs (e.g., epeiric seaways such as North America’s Cretaceous Western Interior Seaway, or Devonian Appalachian Basin) focus almost entirely on mud deposition from energetic bottom currents. If suspension deposits are not present in the rock record, but modern studies indicate those deposits should be present, it is appropriate to ask where all the suspended sediment went in the geologic past. In this paper I argue that mud dispersal from a river mouth to the shelf is likely to be a multi-stage process that includes transportation in suspension and in bottom currents. Most or much silt and clay discharged at a river mouth is deposited on the shelf within a few tens of kilometers from hypopycnal plumes. Data from modern rivers suggests that hypopycnal flows are relatively rare events. Plume deposits on the inner shelf can be reworked downslope by wave-enhanced sediment gravity flows. Other types of currents ensure that most mud deposited on a shelf is reworked and transported further offshore and along depositional strike by bottom currents. The sedimentary record preserves evidence only of the last phase of sediment transport, i.e. via bottom currents. Undisturbed muddy suspension deposits might be preserved in very proximal settings (associated with high sedimentation rates), interbedded with sandy storm deposits (as “fair-weather shales”) or in very distal settings as claystones or in marine snow.
Each bed in a stratigraphic succession encodes the set of geomorphic conditions that generated it as well as the time over which those processes occurred. However, recovering that information from the rock record is nontrivial due to the incomplete nature of stratigraphy. Herein, we evaluate the utility of machine learning approaches to recover this information using a data-rich, fluvio-deltaic experiment with precise tracking of its geomorphic surface, resultant stratigraphy, and spatiotemporal distribution of sub-environments (e.g., channels, lobes, wet overbanks, dry overbanks). Surprisingly, we find that relatively simple random forest models, with just a few predictor attributes, can reliably reconstruct the amount of sediment lost to erosion from a bed and the time missing from a bed with mean absolute errors of ~18% and ~6%, respectively. We show that even binary, categorical attributes can significantly improve model outputs. These findings are encouraging for the application of machine learning approaches to field-scale systems, if the correct non-dimensional framework can be constructed. Our initial efforts suggest it may be possible to recover the distribution of missing sediment on spatial-scales less than a meter and time-scales less than 10 4 years.
Carbonate turbidity currents play a crucial but underemphasized role in the global carbon cycle through the transport and burial of carbon in the marine environment. Carbonate reef-slope systems transport inorganic carbon (IC) stored in carbonate minerals and organic carbon (OC) from terrestrial and marine sources into the deep sea. Since carbonate sediments and rocks comprise a significant portion of the modern systems and the sedimentary record, understanding IC and carbonate-associated OC transport into the deep sea is vital for quantifying carbon burial in carbonate systems and its effects on climate change, ocean acidification, and deoxygenation. This study presents the first experiments on carbonate turbidity currents that focus on flow dynamics and carbon burial. Using natural carbonate lagoon sediments (sands and muds), four experiments with increasing sediment concentrations (1.1–11%) demonstrate concentration-dependent flow structure, sediment transport, and carbon partitioning. Experimental results show variable distal OC transport, with enrichment of approximately ~45% to 100% in distal location relative to proximal locations, alongside systematic IC depletion of ~15% to 30% from proximal to distal regions across all experiments. Natural records from the western longitudinal transects of the Neogene Great Bahama Bank slope show variable and sequence-dependent OC trends and a consistent proximal-to-distal decrease in IC, indicating high spatial and temporal variability in OC transport. We estimate carbon-burial rates for the Great Bahama Bank over the last 25 million years to be between 0.0059 × 10⁻ 5 to 6.2 × 10⁻ 5 PgC/year (0.0059 × 10⁻ 2 to 6.2 × 10⁻ 2 MtC/year) for OC and 0.09 × 10⁻ 4 to 1.9 × 10⁻ 4 PgC/year (0.09 × 10⁻ 1 to 1.9 × 10⁻ 1 MtC/year) for IC, suggesting that turbidity currents are an important vehicle for carbon redistribution and preservation in deep-sea sediments, which reduces the likelihood of carbonate dissolution and CO₂ release. These findings contribute to our understanding of sediment routing and carbon burial in submarine carbonate environments and highlight the need to reassess the role of carbonate turbidity currents in the long-term carbon cycle.
Studies of the Silurian sequence on Gotland have significantly advanced our understanding of Silurian climate dynamics, with much of the research focusing on small outcrops and short cores. Gotland has an extensive network of abandoned oil and gas wells. Most of these include gamma-ray well logs, which have not yet been fully utilised for their stratigraphic value. While the Ordovician succession has been successfully correlated using these well logs, the correlation for the Silurian succession has yet to be fully realised. The present study addresses some of the limitations of correlations performed using Dynamical Time Warping (DTW) by combining DTW with Barycenter Averaging (DBA). This enables a semi-automated correlation of the Silurian well-logs over a ~60 km-long transect and the subdivision of gamma-ray log logs into parts that can be linked with their lithostratigraphic surface counterparts. Additionally, we tracked changes in gamma-ray logs associated with these subsurface units, providing new insights into biogeochemical events (e.g., δ¹³C excursions) previously mapped only at the surface of Gotland. Our results demonstrate the application of a DTW/DBA-based workflow for stratigraphic correlation and highlight the potential to integrate subsurface well-log data to refine Silurian stratigraphy on Gotland.
A remarkably preserved dolomite oolite from the onset of the Ediacaran Shuram excursion—a critical, yet enigmatic, interval in Earth’s history—offers new insights into pathways of dolomite formation during the Neoproterozoic era. We examine ooids from the Khufai Formation using electron backscatter diffraction (EBSD) and polarization-dependent imaging contrast (PIC) mapping to characterize crystal preservation and orientation at the nanoscale. For comparison, we analyze two other well-preserved Ediacaran dolomite fabrics from the same stratigraphic succession: spherulitic dolomite from conical stromatolites in the Buah Formation, and fibrous dolomite cements from a supratidal pisolite in the Birba Formation. The Khufai ooids exhibit distinctive characteristics: fibrous radial crystals organized into plumose (feather-like) bundles, with the c-axis consistently oriented perpendicular to the elongation direction (length-slow), concentric banding with evidence of abrasion, and microfabrics indicating competitive growth dynamics. This combination of features is less consistent with typical replacement processes, including mimetic or synsedimentary dolomitization, than with primary precipitation of crystals with a dolomite lattice, likely imperfectly ordered at nucleation. Primary dolomite precipitation in ooid shoals may reflect a rare and still unknown set of environmental conditions during the onset of the Shuram excursion.
Major League Baseball (MLB) prepares a minimum of 156 baseballs for each game by rubbing them with mud, sourced from the same company since the 1950’s (Baseball Rubbing Mud, n.d.). This mud is intended to add grip and color to the baseball in a way that ensures competitive consistency across the league. Despite its long-standing use, the geologic characteristics of this mud and its precise effects on the baseball are still poorly understood. In this study, we analyze the mineralogy and physical properties of the baseball rubbing mud in the context of its depositional environment in a tributary of the Delaware River near Philadelphia. Samples of game-used and new baseballs were analyzed with Scanning Electron Microscopy (SEM), along with SEM analysis of the un-applied mud. These data were supplemented with X-Ray Diffraction (XRD), Particle Size Distribution (PSD), and Rock-Eval of the mud itself. Results show that the mud is composed primarily of non-swelling clays (Kaolinite, Chlorite, and Illite/Mica) and quartz, with minor amounts of other components. SEM images show that the clays primarily accumulate in the pores, adding the desired color to the baseball, while the coarser quartz grains serve as a scouring material, inducing scratches, micro-cracks, patches of erosion, and occasional flaking of the leather. This provides minor grip enhancement to the ball without significantly altering ball aerodynamics. The lack of swelling clays also prevent overly slippery or inconsistent grip in the presence of moisture. While muds could be sourced from different locations, changing the location of mud collection would result in variations in composition, grain size, and color that could alter the grip and color uniformity in ways that could provide competitive inconsistencies.
The Washington County Quartzite (WCQ) in southeastern Iowa is the southernmost occurrence of the Geon 17 “Baraboo Interval” quartzites in the Laurentian midcontinent region. Three drill holes encountered poorly sorted quartzite and phyllite, likely deposited in a braided fluvial or deltaic environment near the Laurentian continental margin on the Columbia supercontinent. 100 new LA-ICPMS detrital zircon U-Pb ages from the WCQ show a prominent 1.78 age peak, representing local Yavapai-aged basement, a secondary peak at 1.8-1.9 Ga representing a distal Penokean source, and a minor <2.5 Ga peak derived from distal sources in the Superior Province. Multidimensional scaling of other Baraboo Interval quartzites and potential sources show that the WCQ is indistinguishable from the lower interval of the Baraboo Quartzite. Cumulative distribution plots also reflect principal source areas derived by erosion of underlying Yavapai-aged crust and distally derived Penokean and older sources from the southern Superior Province. New whole rock geochemical data from the WCQ and potential granitic sources from northwestern Iowa are compared and show high (>80) chemical index of alteration (CIA) values, indicating similar weathering intensities to other Baraboo Interval localities. The WCQ likely serves as the down slope equivalent during initial Baraboo deposition.
The Bear Valley Formation (Fm.) is a distinctive eolian sandstone interbedded with thick volcanic rocks of the Marysvale volcanic field of southwest Utah, the southern part of which failed during eruptive activity along three mega-scale gravity slides. The formation is as thick as 300 m and extends over an area of >2,500 km 2 in the Black Mountains and Markagunt Plateau. The Bear Valley Fm. is composed of tuffaceous sandstone interbedded with tuff, conglomerate, and polymict volcanic mudflow breccias. The sandstone beds are lithic arenite and lithic wacke that occur as massive beds with large-scale cross bedding. The Bear Valley Fm. occurs in the upper plate of the Markagunt gravity slide and is in both the upper and lower plates of the Black Mountains gravity slide. We used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to acquire U/Pb dates of detrital zircons (N = 3, n = 346) from the autochthonous Bear Valley Fm. at Kane Spring and Jako Wash in the Black Mountains and the allochthonous Bear Valley at Sandy Wash in the central Markagunt Plateau. All samples are dominated by Oligocene zircons with maximum likelihood ages for deposition ranging from 23.6 to 24.0 Ma. The western-most sample from Jako Wash also preserves a slightly older group of zircons, indicating derivation from either the underlying Wah Wah Springs Fm. or another unit erupted from the Indian Peak caldera complex to the west. Thus, the upper Bear Valley Fm. was deposited within ~400 kyr before the emplacement of the Markagunt gravity slide at 23 Ma, reflecting accelerated uplift of the northern Marysvale complex that ultimately resulted in collapse and slide emplacement.
As the mass of human-made materials now surpasses that of Earth’s total dry biomass, there is a critical need for sedimentologists to account for anthropogenic materials when analyzing depositional environments. To address this, a classification scheme is presented that extends traditional sedimentological models to encompass the diversity of modern sediments and their complex dynamics. Through relying on established sedimentological principles, the framework incorporates bed-scale, grain-scale, and sedimentary structure descriptors, as well as a methodology for describing the composition of a deposit and flexible nomenclature. It is designed to be integrative and adaptable, permitting the incorporation of additional descriptions as necessary, and is applicable across depositional settings that are dominated by either natural or anthropogenic processes. The scheme offers a practical, systematic approach to categorize and analyze the textures and structures of anthropogenic sediments, facilitating the reconstruction of modern and geologically recent environments. This unifying starting point enables more detailed predictions of material behavior and their environmental impacts, with implications for recycling, reuse, and management strategies. Furthermore, standardized nomenclature will enhance the capacity for data comparability across field sites, facilitating further understanding of our environment. Applications of this classification scheme include many interdisciplinary possibilities, overlapping with archeology, environmental monitoring, and engineering. By adopting this classification, sedimentologists can forge a deeper understanding of the stratigraphic record of the Anthropocene, contribute to developing more comprehensive strategies to manage Earth’s changing landscapes, and better understand our future geological record.
Perceived order in submarine fans is traditionally used to support simple sequence stratigraphic interpretation of allogenic forcing but evidence is typically limited to qualitative evaluation of the geological dataset. Quantitative analysis of the well-studied ‘Unit A’ basin-floor fan from the Laingsburg Formation, Karoo Basin, South Africa suggests a more complex interpretation. We use Markov facies analysis to test for order in the facies successions, and runs tests and power spectral analyses to assess order in the succession of bed thicknesses. Some bed-by-bed order is detectable in the facies successions, but thickness successions are more complex; no order is detectable as simple consistent bed-by-bed runs of thickening or thinning-upwards. Spectral analysis does detect some significant periodicities, but the periods are different in each vertical section analysed, ranging from 35-45 beds, 50-55 beds and from 60-70 beds in respective vertical sections. This perhaps indicates that complex autogenic compensational stacking processes control the details of how patterns emerge at specific locations, even when larger-scale trends do occur in regional stacking of lobes and lobe complexes and there is no evidence for a single, dominant allogenic control the Karoo submarine fan strata. More comprehensively, this analysis suggests that sequence stratigraphic interpretations of outcrop and core deep-water strata will be more robust with integrated quantitative bed thickness and facies trend analysis as a standard approach. Sequence stratigraphic interpretations of deep-water strata should avoid a model-driven single dominant control approach, and instead use the quantitative analysis to more fully explore the typical complexity of these strata.
Channel mouth bifurcation angles on modern river deltas are remarkably consistent with a theoretical prediction of 72°. However, the persistence of this angle through channel evolution and preservation into the stratigraphic record remains untested. Ancient channel mouth bifurcations were measured using stratal slices from 3D seismic volumes as well as outcropping delta deposits in Mars orbital imagery. We find that channel mouth bifurcations interpreted from terrestrial strata exhibit a mean angle of 71.9° ± 3.8° (95% confidence interval), consistent with modern deltas as well as theoretical and numerical predictions. Angles from martian strata preserved as inverted topography exhibit a mean angle of 80.1° ± 4.8°. A larger angle on Mars may be biased by measurements on eroding outcrops, or possibly the signature of altered sediment transport processes on Mars. Expanding channel network analysis into the stratigraphic archive for the first time provides a new mechanism interpreting paleohydraulics on Earth and Mars.
Sediment accumulation rates are a powerful tool for interpreting the rock record, offering insight into the depositional environment of a given locality and the (in)completeness of a given stratigraphic record. Classic approaches to sediment accumulation rate characterization required large data compilations, but the advent of high-resolution age models enables the use of individual sections and regional composite records to generate a large enough sample size of accumulation rates to estimate stratigraphic completeness. Because of these prior limitations, it is unclear how precisely accumulation rates are known for many stratigraphic sections, with rare and discrete horizon-horizon accumulation rates potentially biasing results. Here, I explore how to leverage high-resolution age-depth models to reveal accumulation rate-duration trends for individual sections and to better understand depositional histories by calculating accumulation rates for every horizon in a given section. First, I demonstrate these analyses on three synthetic age-depth models. I then examine regional composite carbon isotope records from the Ediacaran to test the effectiveness of my method on real data. Based on the accumulation rate–duration relationship, I estimate that the Ediacaran Oman and China stratigraphies are ~22% and ~37% complete at a 1-Myr interval, respectively. I find that accumulation rates drop following the Gaskiers glaciation and are relatively low during the Shuram carbon isotope excursion. Furthermore, the Oman carbon isotope stratigraphy demonstrates increasing accumulation rates across the Ediacaran, peaking near the Ediacaran–Cambrian boundary. Using an iterative technique, I estimate mean accumulation rates and durations, with uncertainty, and demonstrate how iterative-style Sadler plots can be used to interrogate depositional histories. In an effort to facilitate this approach and further quantitative developments across the stratigraphy community, I provide an open-source function that generates the plots herein for any stratigraphic record with an accompanying age-depth model.
Slim Buttes is a 30 km long by 10 km wide set of buttes containing Paleogene strata in northwest South Dakota. At Reva Gap in northern Slim Buttes, Eocene-Oligocene terrestrial strata of Chadron and Brule Formations of the White River Group unconformably overlie the Paleocene Fort Union Formation. An angular unconformity separates the White River Group from overlying Oligocene and Miocene strata of the Arikaree Group. Using detrital zircon U-Pb ages, we determine the provenance of these rocks as part of a broader synthesis of post-Laramide sedimentation in the Rocky Mountains and western Great Plains. The Chadron Formation age spectrum is dominated by Cretaceous and Proterozoic grains that are interpreted to be locally recycled from the underlying Cretaceous and Paleocene strata. The Brule Formation has a maximum depositional age of ~34 Ma; Paleogene zircons dominate the age spectrum, and a wide variety of older zircons are also present. The Oligocene zircons are interpreted to have been sourced from volcanic systems in the Great Basin to the southwest, while the subsequent proportions of the zircons were derived from a variety of source areas in the Nevadaplano and Rocky Mountain areas to the southwest. Sparse amounts of Archean zircons are thought to represent the burial of Laramide uplifts throughout Wyoming at the time of Brule deposition, making for a regional paleotopography with little relief across the western interior of the United States. The Miocene-age Arikaree Group sand has a maximum depositional age of ~26 Ma and a multimodal detrital zircon age spectrum. The Arikaree Group provenance likely represents continued sourcing in the Great Basin volcanic systems and Nevadaplano, the beginnings of the re-exhumation of Laramide basement uplifts, and subsequent sediment evacuation out of the western interior and into the Gulf of Mexico to the southeast. Our findings indicate that the transport process and detrital zircon provenance signatures of these strata are decoupled, and each have their own independent evolution. The volcanic signature is primarily transported via aeolian processes (i.e. volcanic ash), and the recycled detrital zircon signature is primarily transported via fluvial processes.
Alluvial systems affected by peat layer compaction of a variable degree are studied all over the world, especially in areas of large river deltas and coastal plains. Ordinarily studied Holocene peat accumulations do not reach thicknesses of more than 6–10 m. This paper documents four ancient alluvial systems affected by peat compaction during the Lower Miocene in large exposures of open-cast lignite mines within the Most Basin, Czech Republic. Field observations and the study of 3000 borehole sections showed a clear dependence of the thickness and composition of the underlying peat on the scale of the accommodation space for loading by younger alluvial sediments. If the underlying partly compacted peat was tens to hundreds of meters in thickness, significant deformations of the peat appeared during its loading, leading up to the formation of growth faults. In such cases, missing parts of the coal seams were also observed below the alluvial systems. Especially borehole data showed that a linear correlation exists between the thickness of the alluvial sediments and the maximum instantaneous available compaction capacity of the underlying peat.
The Cambrian (Furongian) Potosi Dolomite (100-183 m) in Illinois is part of the Cambro-Ordovician Knox Group. It is a uniformly dolomitized unit with very low intercrystalline porosity but contains very permeable vug, fracture/cavern porosity intervals. Here, we interpret the characteristics of the widespread porous zones in the Potosi as paleokarst features formed by rising hypogenic basinal/hydrothermal fluids. The conformity bounded Potosi Dolomite is characterized by massive dolomitization, overdolomitization and occlusion of previously generated intercrystalline porosity, void filling mineralization, and extensive dissolution and formation of cavity-conduit systems. The pore spaces are typically lined with drusy quartz or are characterized by partial to complete infilling with chalcedonic silica and/or dolomite cements. Clay minerals may partially fill pore spaces; physical properties and thorium-potassium crossplot suggest chlorite as the main clay mineral present. Dolomite crystals typically are planar-s or nonplanar with open-space filling, inclusion rich saddle dolomite displaying curved and zigzag crystal faces. Void filling cement does not exhibit sign of pressure solution and in places vug porosity is developed along bedding parallel stylolite indicating post burial origin of these features. Cavern reservoirs in the Potosi are laterally extensive and often stacked with intervening very low porosity dolomite; very low bulk density, excursion of caliper log signature from the baseline, and loss of fluid circulation during drilling in these intervals signify anomalously high porosity and permeability interpreted as being the result of cavern forming multiple paleokarst events. Post burial origin of cavities and void filling cements, association of saddle dolomite and chlorite, and occurrence of Mississippi Valley-type (MVT) ore deposits in Missouri suggest karstification by hypogenic warm basinal/hydrothermal fluids. Dissolution and mineralization likely occurred by flow of deep basinal formation waters and hydrothermal fluids (sourced from the crystalline basement underlying the Reelfoot Rift and the Illinois Basin) along numerous basement-rooted normal, reverse, and strike-slip faults, and the associated fold and fractures. Expansion and contraction because of fault-related seismicity likely developed fracture porosity in brittle host dolomite and possibly ruptured any underling impermeable units to enable large-scale upward and outward fluid movement. The Potosi fracture/cavern porosity intervals are confined by thick very low porosity dolomite intervals that could serve as effective seal. There is no report of any show of oil in the Potosi Dolomite, but the unit has an excellent potential to serve as a combined reservoir and seal for storing anthropogenic CO2 and waste material.
Ancient river deposits are important archives of past landscape conditions on planetary surfaces. On Earth, they host valuable groundwater, energy resources, and carbon-storage potential. Reconstructing details of paleochannel forms and movements refines our understanding of the controls on river behavior under different climate, landcover, and tectonic conditions, and improves predictions and models of subsurface reservoirs. While studies have shown detailed connections between channel kinematics and bar-deposit architecture in meandering river systems, similar connections between braided river movements and preserved braided river deposits have not been established. Here we explore the potential for connecting braided river deposits to paleochannel movements, form, and flow conditions, and we evaluate the controls on bar preservation using synthetic stratigraphy generated with a numerical morphodynamic model. We investigate how attributes of channel morphodynamics, like channel widening or braiding intensity, impact bar deposits’ preservation, scale, geometry, and architecture. We then assess how the scale, preservation, and facies composition of bar deposits reflect formative flow conditions of the channel. Our results demonstrate that no diagnostic signature of braided channel morphodynamics is recorded in bar-deposit geometry, facies, or preservation patterns. Rather, the unique local history of thread movements combines stochastically to preserve or rework bar deposits, and the timing of channel avulsion is the dominant control on bar preservation. Our results also show that representative paleochannel flow conditions will likely be accurately reflected in aggregate observations of braid bar deposits within channel-belt sandbodies at a regional or member/formation scale. These results demonstrate the need for broad sampling and statistical approaches to subsurface prediction and paleo-flow reconstruction in ancient, braided river deposits.
Morphological characteristics in river systems, including channel dimensions and river gradients, scale to drainage basin area, which provides the means for such elements to be predicted, measured and modeled. Moreover, recent studies interpret downstream changes in channel morphological and sedimentary characteristics to be the product of changing flow hydraulics as rivers transit from the normal flow to the backwater reach and approach the coastal ocean. This paper quantifies how large modern rivers undergo morphological and sedimentary transformations in response to normal flow to backwater transition. Morphologies adapting to such backwater hydraulic conditions is a potential for further investigation. With applications in modeling of modern river systems, this also provides the means for paleoenvironment reconstructions based on changing morphological characteristics since such quantitative framework is grounded by similar depositional processes. Building on previous studies, we construct river-long profiles, estimate backwater lengths, measure the ratio between channel-belt and channel widths (BChB/BCh), and measure the ratio between sand-rich to mud-dominated environments of deposition (S/M ratio) in five large modern river systems. We use results from >55,000 measurements of morphological and lithological characteristics from ~3,850 valley cross-sections over ~5,500 river kilometers to show that: (a) channel gradients decrease by ~30-50% as the channel goes through the normal flow to backwater transition, whereas (b) BChB/BCh decreases by >~60% and (c) S/M ratios decrease by ~35-90% within the upper backwater reach. These values further decrease in the lower backwater reach and approach unity (BChB/BCh = 1; S/M = 0) as the gradient reaches zero (sea level). Such systematic transformations in morphologic and sedimentary characteristics indicate they are both inherent and predictable, and can be used to interpret normal flow vs. backwater hydraulics in ancient fluvial deposits.
Levees in modern deep-marine systems have been shown to sequester significant amounts of organic carbon due largely to their expanse and high rates of sedimentation. However, relatively few studies have examined organic carbon sequestration in ancient deep-marine leveed slope channel systems. Physical and geochemical analyses of well-exposed levee deposits in the Neoproterozoic Windermere Supergroup in B.C., Canada have shown that intervals of organic-rich (up to 4% TOC) strata correlate with conditions of elevated sea level and primary productivity on the shelf. Organic matter (OM) occurs primarily as micro- to nano-scale carbon adsorbed onto the surface of clay grains and notably occurs mostly in anomalously thick, mud-rich sandstone beds that are interspersed within successions of thin-bedded, comparatively organic-poor turbidites. The concentration of organic carbon in thick beds suggests that even when primary productivity is high it only becomes mobilized in significant quantities into the deep sea by uncommon, outsized turbidity currents. Although markedly more common in organic-rich intervals, thick, organic-rich beds occur also in organic-poor levee deposits, suggesting that the occurrence and frequency of outsized flows may be linked to primary productivity on the shelf. High rates of OM production and fallout would result in rapid accumulation of OM on the seafloor that then binds and provides mechanical strength to the accumulating sediment. Later this overthickened, organic-rich sediment pile becomes gravitationally unstable and ultimately remobilized downslope. These failure events create large, surge-like flows that are considerably thicker than the depth of the slope channels through which they travel. Accordingly, continuous overspill over the channel margins results in the deposition of an anomalously thick, sand- and organic-rich bed. These episodic events not only deplete the outer continental shelf of OM, but apparently also reduces the gradient slope of the local seabed, which then results in the more typical smaller, channel-confined organic-poor turbidity currents. Additionally, the abrupt and single-bed occurrence of OM-rich strata suggests that the buildup of organic-rich strata and seafloor stabilization was rapid but only of limited duration. Significantly, this study suggests that outsized turbidity currents that originate on the outer continental shelf are the primary mechanism for organic matter delivery to the deep sea, at least in pre-vegetation times, and that flow size and frequency, in addition to primary productivity, exerts an important control on the distribution of organic carbon in deep-sea sediments.
Sediment routing systems transport sediment and environmental signals inefficiently. The storage and recycling of sediment buffers the responses of sedimentary systems to tectonic, climatic, and geomorphic changes. Long-term ( >106 a.) storage may occur in megafans—large, low-relief, hemiconical fluvial deposystems—but the behavior of these systems over such timescales is unclear. We examine late Neogene megafan deposits on the High Plains, USA that are stored in the catchment of the continental-scale Rocky Mountain-Gulf of Mexico source-to-sink system. Using high-resolution elevation data, we map numerous fluvial ridges (inverted channel relics) that can be differentiated into five, chronologically distinct groups. The oldest four groups comprise radial arrays with multiple channel divergences (avulsion nodes). The inferred fan apices and longitudinal intersection points are offset successively downgradient, indicating that fanhead entrenchment and fan-lobe deposition were approximately contemporaneous and strongly suggesting a telescopic morphology. The youngest group of channels, also the lowest in elevation, is confined to terraces along the modern valley of the South Platte River. We interpret this group as direct evidence for the abandonment of the megafan and the incision of the present valley. Uplift was the chief driving mechanism for early telescoping, but channel widening and uniform downcutting in the younger groups suggest that change in stream power was the primary driver of later telescoping and incision. The storage of sediment in the megafan effectively decoupled sources from downstream sinks. Some of this sediment was recycled during entrenchment, but much of it remains in storage as the Ogallala Group and Broadwater Formation. Emplacement of telescopic megafans should be considered as a long-term ( ≥106 years) buffer in other modern and ancient sediment routing systems.
