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.

Deep‐marine levées are areally extensive features that experience high rates of sedimentation, making them ideal sites for significant carbon burial and preservation. Although modern deep‐marine levées have been shown to sequester a large proportion of the world's total buried organic carbon, few studies have attempted to assess carbon deposition and preservation in ancient deep‐marine levée deposits. Observations of well‐exposed levée deposits of the Neoproterozoic Windermere Supergroup in British Columbia, Canada, have shown that depositional processes in levées can result in the concentration and enrichment of sedimentary marine organic matter. In contrast to many previous studies where organic‐rich strata occur as black shales formed in anoxic conditions, organic matter in this study occurs mostly in banded, mud‐rich sandstones deposited under oxic conditions. Scanning electron microscopy shows that this organic carbon occurs primarily as nano‐scale coatings on clay particles, but also as uncommon sand‐sized organomineralic aggregates and discrete sand‐sized amorphous grains. As flows overspilled the channel margins, the rapid collapse of the turbulent suspension resulted in elevated rates of sediment fallout that promoted aggregation of organic matter and clay particles, increasing cohesive forces in the flow, and ultimately en masse deposition. Rapid burial plus association with clay mineral surfaces prevented organic matter degradation, thereby effectively sequestering significant amounts of carbon in the sediment and therefore a net sink for atmospheric CO2. However, the mechanisms and efficiency by which organic matter is buried and preserved on geological timescales is influenced by many factors, including glacial, sea‐level and tectonic cycles. This work elucidates the fundamental physical and chemical processes that control organic matter accumulation and preservation in deep‐marine levées, and how these processes have evolved throughout geological time.

ABSTRACT Deep-water channel and levee deposits are common depositional elements on modern and ancient continental slopes. Unlike their channel counterparts, the spatial and temporal evolution of levee stratigraphy is much less well understood, in part because of the typically more recessive nature of levee deposits in the ancient sedimentary record, and sparse, widely spaced core control or seismic images of insufficient resolution in the modern. Moreover, it is generally inferred that levee development, at least in part, precedes the main phase of channel filling, the reasons for which remain largely unknown. In the Isaac Formation of the Windermere Supergroup (Neoproterozoic) of east-central British Columbia, Canada, well-exposed levee deposits are divided vertically into packages, each consisting of a sand-rich lower part overlain sharply by a mud-rich upper part. The lower part (3–10 m thick) consists mostly of medium- to thick-bedded, upper medium- to coarse-grained, lower-division turbidites intercalated with thin-bedded, fine-grained, upper-division turbidites. Along depositional strike away from channel-fill margins, the thickness of lower-division turbidites exhibit a distinctive thickening and then thinning over a few hundreds of meters that results in a similar thickening and thinning of the entire lower part of a package. The upper part (3–16 m thick) consists mostly of thin-bedded, fine-grained, upper-division turbidites intercalated with uncommon medium- to thick-bedded, medium-grained, lower-division turbidites. Significantly, the thickness of very thin- and thin-bedded turbidites in the upper part generally decreases stratigraphically upward whereas the thickness of intercalated medium- and thick-bedded turbidites changes little. The lateral and vertical changes in these deposits suggest that channelized flows were initially coarse grained and moderately well-sorted, causing them to exhibit negligible density stratification, and therefore high flow efficiency. We interpret that the velocity maximum occurred above the height of the incipient channel margins, thereby allowing the lower, coarse-grained, dense part of flows to easily overspill and deposit thick-bedded, coarse-grained turbidites in the lower part of each package. The sharp contact with the upper part of each package marks the point when relief from channel floor to levee crest exceeded the height of the velocity maximum in average throughgoing turbidity currents. Above this height, density of the flow decreased abruptly and consisted of significantly finer-grained sediment that overspilled to form the upper, finer-grained part of each package. Later the makeup of the sediment supply changed to a more polydispersed grain-size distribution, which caused the throughgoing currents to be more density stratified. This enhanced near-bed stratification and concentration effects, which in addition to intense interfacial mixing, resulted in rapid kinetic energy loss, and promoted deposition in the channel.

Modern deep-marine levees have been shown to be an important reservoir of the world's total buried organic carbon; however, few studies have attempted to assess this in ancient systems. Deep-marine levees are extensive features that experience high rates of sedimentation, making them ideal sites for significant carbon burial, and therefore examining the distribution of organic material in ancient levee deposits could provide insight into paleoenvironmental conditions and the evolution of ancient ocean and climate systems. In this study, a 350-mthick succession of mudstone-dominated levee deposits of the Windermere Supergroup in British Columbia, Canada, was stratigraphically logged in centimeter-scale detail and samples taken with 4-10 m spacing. X-ray fluorescence (XRF) and total organic carbon (TOC), were conducted to evaluate the elemental composition and distribution of organic carbon to evaluate trends in paleoenvironmental conditions such as primary productivity, ocean redox, weathering intensity, and detrital flux. TOC ranges from <0.2% to 15.55% (corrected for the effects of metamorphism). Organic-rich strata, taken to be >= 1% TOC, are principally confined to a single 60 m-thick stratigraphic interval, and typically occur as anomalously thick, mud-rich sandstone turbidites, with organic matter occurring mostly as micro-scale carbon sorbed onto the surface of clay grains. In this same interval, trends in XRF data indicate an increase in primary productivity, weathering intensity, and detrital influx, and a decrease in ocean oxygenation levels. These data suggest that intense continental weathering, high terrigenous input, elevated sea level, and relatively low oxygenation conditions all act to enhance organic matter production, accumulation, and preservation. However, although all these components contributed to increased organic production, accumulation, and preservation on their own, the results of this study suggest that it is the temporal coincidence of all of them in a "perfect storm" that is required for significant organic carbon enrichment.

An allostratigraphic approach was used to investigate the stratigraphy of the Late Albian Joli Fou and Pelican formations across 77 000 km(2) of east-central Alberta. Although the Pelican Formation is known to be broadly equivalent to the Viking Formation to the south, its internal stratigraphy and precise allostratigraphic relationship to the Viking and Paddy alloformations to the south and west have not been established. The Joli Fou alloformation is bounded below by transgressive surface JE0 and above by the basal surface VE0 of the Viking alloformation. Within the study area, the Joli Fou comprises marine mudstone that thins from approximately 20 m in the SE to a pinch-out in the NW where it onlaps against a NE-SW-trending topographic high dubbed the 'Smoky River Ridge'. The Pelican alloformation comprises four regionally mappable, upward shoaling allomembers PeA to PeD, each bounded by a marine flooding surface. Collectively, allomembers PeA and PeB are bounded by Viking surfaces VE0 and VE1 and hence are temporally equivalent to Viking allomember VA. PeC and PeD are bounded by VE1 and VE3 and are equivalent to Viking VB. To the west, allomembers PeA and PeB merge laterally with the upper Paddy alloformation, but allomembers PeC and PeD toplap against VE3, with the margin of PeD offset to the SE relative to PeC. Pelican allomembers form sandier-upward successions, typically 5-10 m thick, and can show marked lateral changes in thickness. Where exposed on the Athabasca River, Pelican sandstones are dominated by dm-scale, uni-or bi-directional cross-bedding, with cross-sets capped by wave ripples and thick mudstone drapes; evidence for storm wave processes (e.g. hummocky and swaley cross-stratification) appears to be scarce. The successions are interpreted to represent river-and/ or tide-dominated delta systems. Pelican sandstones are distinctive quartz arenites that collectively thicken to the NE, strongly suggesting provenance from the Canadian Shield. In the vicinity of Peace River town, the Paddy alloformation comprises chert-rich litharenites of Cordilleran provenance interstratified with quartz arenites. This observation implies that rivers from the Shield had constructed a delta system across the entire 400-500 km width of the Joli Fou Sea, in order to deliver sediment to the Paddy depocentre to the west of the Smoky River Ridge. Sandstone bodies in each Pelican allomember form south-facing lobes, interpreted to have formed on the south side of the delta complex where they were protected from storm waves from the north. In contrast, the north-facing margin of the Paddy depocentre in the west forms a linear strandplain, reflecting strong wave influence from the open sea to the north. The southern entrance to the Joli Fou Seaway, formed by a topographic ridge in Kansas, is known to have closed in late Joli Fou-Skull Creek time. Closure of the northern entrance by the Pelican delta system probably led to a slow decrease in salinity of the landlocked sea, and may explain the absence of fully marine macrofauna from the Viking, Bow Island, Muddy Sandstone and equivalent formations throughout the length of the Seaway.