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The application of sequence stratigraphy to deep-water mud-dominated systems is more challenging than in shallow water siliciclastic settings, largely because mudstones display subtle contrasts in grain size and lithology that make identification of facies and recognition of major stratigraphic surfaces difficult. However, the integration of sedimentology, ichnology and geochemistry and an extensive suite of cores from the Middle to Upper Devonian Horn River Group shales allow us to better understand how deep-water mud-dominated systems respond to fluctuations in the relative sea level, manifested in sequence stratigraphic relationships. The Horn River Group consists of, in ascending order, the Evie Member and Otter Park Member of the Horn River Formation and the Muskwa Formation. Four systems tracts, including highstand, transgressive, lowstand and falling stage systems tracts (HST, TST, LST, FSST, respectively) are identified and mapped. Highstand and transgressive systems tracts are both represented by dominantly massive mudstone lithofacies, sparse bioturbation, high Mo/Al ratios and high TOC values (avg. 3.1 and 3.4 wt%, respectively), as well as low Al 2 O 3 concentrations. They can be distinguished by upward increasing gamma ray values for transgressive systems tracts and upward decreasing values for highstand systems tracts. The falling stage and low stand systems tracts display more heterolithic and laminated lithofacies, sparse to intense bioturbation, lower Mo/Al ratio and lower TOC values (avg. 1.2 and 1.7 wt%, respectively) and high Al 2 O 3 , concentrations and relatively low gamma ray readings. Sequence boundaries are represented by a transition from unbioturbated units to moderately or intensely bioturbated units, suggesting increased bottom water oxygen levels, which we infer to represent shallower waters. They are gradational and clearly conformable contacts in distal parts of the basin. They are sharp in more proximal settings, but no erosional surfaces were identified that would indicate unconformable contacts. Maximum flooding surfaces are characterized by the highest TOC contents and an increased gamma ray values. Our results suggest that the Otter Park Member belongs to a long-period sea level lowstand (e.g., 2nd order cycle), whereas the Evie Member and Muskwa Formation belong to long-period sea level highstands. [ABSTRACT FROM AUTHOR]

Abstract: Holothurians (sea cucumbers) constitute one of the largest and most abundant infaunal groups inhabiting the Fraser River delta front (0 to 150m water depth) and prodelta (greater than 150m water depth). The spatial distribution and density of animals reveal their preferred substrate grain sizes, and their tolerance or sensitivity to various stress factors, including salinity fluctuations, available food resources, sedimentation rate, and pollution. In the delta front, below 25m water depth (below storm-wave base), holothurians are asymmetrically distributed with respect to the main distributary channel. Population densities primarily reflect asymmetry in grain-size distributions across the delta front. Holothurians are absent or occur in low density in the sand-dominated southern delta front (SDF) whereas the mud-dominated northern delta front (NDF) is more heavily colonized. Sedimentation rate and nutrient content are equally important in holothurian distribution. Population densities of certain holothurians show significant increase where there are moderate sedimentation rates (2–4cmyr−1), and river-derived nutrient supply. Several subsurface traces and their surface expressions are attributed to holothurians. Subsurface traces include diminutive- and robust-Artichnus, Thalassinoides, and U-shaped bio-deformation structures. On the sediment surface, bio-topographic structures such as mounds and funnel-shaped depressions are produced by the activity of sea cucumbers. Of the holothurian-generated traces, only Artichnus can conclusively be attributed to sea cucumbers. Based on the distributions of the trace makers of Artichnus, we hypothesize that in the rock record this trace fossil can be used as evidence of stable euhaline salinity conditions, and sediment deposition and colonization below storm-wave base. Artichnus occurs mainly in mud-dominated (>55% mud) substrates. [Copyright &y& Elsevier]

Abstract: Three anomalous events (AEs) were recorded off the Fraser River, Canada during the freshet of 2008, and are ascribed to quasi-continuous gravity flows. These flows transport warm, low-salinity Fraser River waters to the upper delta front of the Fraser delta to at least 50m water depth. All flows occurred during the freshet and spring ebbing tide, and were characterized by deposition of fine-grained sand and silt beds with likely high-water contents. These sediments were susceptible to resuspension by the subsequent flood-tide current, producing very high near bed suspended sediment concentrations (SSC) with enough excess density to generate hyper-concentrated flows. Our observations show that these episodic events result from the combination of high river discharge, high SSC, and strong tides, and therefore, are likely to occur on the delta fronts of other large tide-dominated and tide-influenced rivers. In shallow-water basins (< 50m water depth), these tide-supported gravity flows can affect sediment deposition on both the delta front and prodelta. [Copyright &y& Elsevier]

The integration of geochemistry and sequence stratigraphic models in the study of shale formations is critical to the development of robust stratigraphic correlations and paleoenvironmental interpretations. The Middle and Upper Devonian Horn River Group is a prominent organic-rich shale sequence in northeastern British Columbia, Canada, and it also hosts major natural gas reserves. The availability of high-resolution geochemical data sets on five continuous cores and an independent core-based sequence stratigraphic framework provide new insights into the effects of relative sea-level changes on redox conditions, productivity, detrital flux, and organic matter enrichment patterns and their geographic variation. Sequence stratigraphic analysis documents three third-order transgressive-regressive cycles within the Horn River Group. Organic carbon content is typically enriched in transgressive systems tracts and depleted in regressive systems tracts. Correlations among total organic carbon (TOC) content and multiple geochemical proxies indicate that organic matter accumulation was controlled by redox conditions, bioproductivity, and detrital dilution, all of which were directly affected by relative sea-level fluctuations. Redox proxies are strongly correlated to TOC content, suggesting that redox conditions exerted a major control over organic carbon accumulation. Redox-sensitive traceelemental ratios and Corg-Fe-S relationships suggest that bottom water was less oxygenated during transgressions than regressions and in distal areas than in proximal parts of the basin. Productivity may have been linked to anoxia via mineralization of organic matter during periods of high productivity, resulting in water-column oxygen depletion. However, biogenic silica concentrations demonstrate enhanced productivity during transgressions, perhaps related to enhanced recycling of nutrients under anoxic conditions. Relative detrital sediment flux to the basin, as measured by aluminum and titanium concentrations, varied systematically as a function of relative sea level and paleogeographic position. Specifically, elevated detrital flux accompanied regressions and was greatest in proximal areas of the basin, resulting in dilution of organic matter concentration at these times in these regions of the basin. Our study demonstrates that relative sea-level fluctuations exerted substantial control on patterns of organic carbon enrichment. However, geochemical signatures of sea-level cycles vary with respect to paleogeographic location within the basin and are most obvious at intermediate locations and water depths, where sea-level falls brought chemoclines to the seabed and forced clastic sediments to the basin floor. [ABSTRACT FROM AUTHOR]