The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post-depositional consolidation and soft-sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain-support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non-cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle-support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain-to-grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle-support mechanism depends upon flow conditions, particle concentration, grain-size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse-grain tail (or dense-grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large-scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensuBagnold, 1962), in which fluid turbulence is the main particle-support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge-like flows and quasi-steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge-like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi-steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening-up units capped by fining-up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near-steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co-exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.

The physical character of subaqueous sedimentary density flows and their deposits

The ever-changing distribution of continents and ocean basins on Earth is fundamental to the environment of the planet. Recent ideas regarding pre-Pangea geography and tectonics offer fresh opportunities to examine possible causative relations between tectonics and environmental and biologic changes during the Neoproterozoic and Paleozoic eras. The starting point is an appreciation that Laurentia, the rift-bounded Precambrian core of North America, could have been juxtaposed with the cratonic cores of some present-day southern continents. This has led to reconstructions of Rodinia and Pannotia, supercontinents that may have existed in early and latest Neoproterozoic time, respectively, before and after the opening of the Pacific Ocean basin. Recognition that the Precordillera of northwest Argentina constitutes a terrane derived from Laurentia may provide critical longitudinal control on the relations of that craton to Gondwana during the Precambrian-Cambrian boundary transition, and in the early Paleozoic. The Precordillera was most likely derived from the general area of the Ouachita embayment, and may have been part of a hypothetical promontory of Laurentia, the “Texas plateau,” which was detached from the Cape of Good Hope embayment within Gondwana between the present-day Falkland-Malvinas Plateau and Transantarctic Mountains margins. Thus the American continents may represent geometric “twins” detached from the Pannotian and Pangean supercontinents in Early Cambrian and Early Cretaceous time, respectively—the new mid-ocean ridge crests of those times initiating the two environmental supercycles of Phanerozoic history 400 m.y. apart. In this scenario, the extremity of the Texas plateau was detached from Laurentia during the Caradocian Epoch, in a rift event ca. 455 Ma that followed Middle Ordovician collision with the proto-Andean margin of Gondwana as part of the complex Indonesian-style Taconic-Famatinian orogeny, which involved several island arc-continent collisions between the two major continental entities. Laurentia then continued its clockwise relative motion around the proto-Andean margin, colliding with other arc terranes, Avalonia, and Baltica en route to the Ouachita-Alleghanian-Hercynian-Uralian collision that completed the amalgamation of Pangea. The important change in single-celled organisms at the Mesoproterozoic-Neoproterozoic boundary (1000 Ma) accompanied assembly of Rodinia along Grenvillian sutures. Possible divergence of metazoan phyla, the appearance and disappearance of the Ediacaran fauna (ca. 650–545 Ma), and the Cambrian “explosion” of skeletalized metazoans (ca. 545–500 Ma) also appear to have taken place within the framework of tectonic change of truly global proportions. These are the opening of the Pacific Ocean basin; uplift and erosion of orogens within the newly assembled Gondwana portion of Pannotia, including a collisional mountain range extending ≈7500 km from Arabia to the Pacific margin of Antarctica; the development of a Pannotia-splitting oceanic spreading ridge system nearly 10 000 km long as Laurentia broke away from Gondwana, Baltica, and Siberia; and initiation of subduction zones along thousands of kilometres of the South American and Antarctic-Australian continental margins. The Middle Ordovician sea-level changes and biologic radiation broadly coincided with initiation of the Appalachian-Andean mountain system along >7000 km of the Taconic and Famatinian belts. These correlations, based on testable paleogeographic reconstructions, invite further speculation about possible causative relations between the internally driven long-term tectonic evolution of the planet, its surface environment, and life.

OVERVIEW: Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation

http://basin.earth.ncu.edu.tw/download/courses/seminar_MSc/2012/1018_1_1.pdf

Marine hyperpycnal flows: initiation, behavior and related deposits. A review

Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (TE-3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE-2) and finely laminated mud (TE-1) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (TD) and ripple cross-laminated (TC) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (TB-1), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (TB-2). More widely spaced planar lamination (TB-3) occurs beneath massive clean sand (TA), and is also formed by high-density turbidity currents. High-density turbidite deposits (TA, TB-2 and TB-3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (DM-2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM-1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.

Subaqueous sediment density flows: Depositional processes and deposit types

Preface 1. Introduction to rock physics 2. Rock physics interpretation of texture, lithology and compaction 3. Statistical rock physics: combining rock physics, information theory, and statistics to reduce uncertainty 4. Common techniques for quantitative seismic interpretation 5. Case studies: lithology and pore-fluid prediction from seismic data 6. Workflows and guide lines 7. Hands-on References Index.

Quantitative Seismic Interpretation: Applying Rock Physics Tools to Reduce Interpretation Risk

Contrary to the conventional belief that humic coal generates primarily gas, 3 billion bbl of recoverable oil has been discovered in the humic coaly succession of the fluviodeltaic Latrobe Group (Upper Cretaceous-Tertiary) that serves as both the reservoir and the source for hydrocarbons in the offshore Gippsland basin of southeastern Australia. Evidence for generation of liquid hydrocarbons from the coaly succession includes: (1) similarity of n-alkane distribution in the oil and in the coal extracts; (2) high wax content of oil (up to 27% by weight); (3) high ratio of pristane/phytane in oil (5-6); and (4) dominance of C29 steranes in the oil. In the Gippsland basin, coniferous rain forests dominated by kauri vegetation flourished in a raised bog setting. Present temperate climate and kauri vegetation of New Zealand are considered to be the modern analog to the Gippsland basin. The coniferous vegetation provided large quantities of hydrogen-rich exinite macerals, such as cutinite and resinite, with potential to generate oil. High rainfall, raised ground-water level, low oxygen, high acidity, and low-nutrient conditions of a raised bog setting were suitable for preserving organic matter. A comparison of gas chromatograms of oils in the Gippsland basin with gas chromatograms of oils generated by hydrous pyrolysis in the laboratory from the immature source rocks suggests that the paraffinic fraction of the oil was derived from coal, and the naphthenic fraction was derived chiefly from resin.

Significance of Coniferous Rain Forests and Related Organic Matter in Generating Commercial Quantities of Oil, Gippsland Basin, Australia

50 years of the turbidite paradigm (1950s—1990s): deep-water processes and facies models—a critical perspective

Two types of coarse-grained deltas are recognized: fan-deltas and braid deltas. Fan-deltas are gravel-rich deltas formed where an alluvial fan is deposited directly into a standing body of water from an adjacent highland. They occupy a space between the highland (usually a fault-bounded margin) and the standing body of water. In contrast, braid deltas (here introduced) are gravel-rich deltas that form where a braided fluvial system progrades into a standing body of water. Braid deltas have no necessary relationship with alluvial fans, as exemplified by fluvioglacial braid deltas. Braid deltas have previously been classified as fan-deltas even though the geomorphic and sedimentologic settings of the two systems can be vastly different. Braid deltas are a common present-day geomorphic feature and are abundant in the geological record. Fan-deltas and braid deltas can be distinguished in the rock record by distinctive subaerial components of these depositional systems; the shoreline and subaqueous components of both are similar. Fan-delta sequences have a subaerial component that is an alluvial-fan facies comprising interbedded sheetflood, debris-flow, and braided-channel deposits. Fan-deltas produce small (a few tens of square kilometres), wedge-shaped bodies of sediment, commonly displaying high variability in paleocurrent patterns and abrupt changes in facies. The deposits are generally very coarse grained (with large out-sized clasts), very poorly sorted, matrix-rich, polymictic, heterolithic, partially cemented by penecontemporaneous carbonate, and have low porosity and permeability. Braid-deltas, in contrast, have a subaerial component consisting entirely of braided-river or braidplain facies. Their deposits display better sorting, roundness, and clast orientation than do fan-delta sediments; they lack a muddy matrix; they display size grading and bar migration; they commonly have a sheet geometry with high lateral continuity (tens to hundreds of square kilometres); and they exhibit moderate to high porosity and permeability. Valuable paleogeographic and tectonic information concerning the proximity of highlands and major fault zones may be misinterpreted or lost if these two coarse-grained deltaic systems are not differentiated.

Fan-deltas and braid deltas: Varieties of coarse-grained deltas

Sequence of structures in fine-grained turbidites: Comparison of recent deep-sea and ancient flysch sediments

Conventionally, turbidity currents are considered as fluidal flows in which sediment is supported by fluid turbulence, whereas debris flows are plastic flows in which sediment is supported by matrix strength, dispersive pressure, and buoyant lift. The concept of high-density turbidity current refers to high-concentration, commonly nonturbulent, flows of fluids in which sediment is supported mainly by matrix strength, dispersive pressure, and buoyant lift. The conventional wisdom that traction carpets with entrained turbulent clouds on top represent high-density turbidity currents is a misnomer because traction carpets are neither fluidal nor turbulent. Debris flows may also have entrained turbulent clouds on top. The traction carpet/debris flow and the overriding turbulent clouds are two separate entities in terms of flow rheology and sediment-support mechanism. In experimental and theoretical studies, which has linked massive sands and floating clasts to high-density turbidity currents, the term "high-density turbidity current" has actually been used for laminar flows. In alleviating this conceptual problem, sandy debris flow is suggested as a substitute for high-density turbidity current. Sandy debris flows represent a continuous spectrum of processes between cohesive and cohesionless debris flows. Commonly they are rheologically plastic. They may occur with or without entrained turbulent clouds on top. Their sediment-support mechanisms include matrix strength, dispersive pressure, and buoyant lift. They are characterized by laminar flow conditions, a moderate to high grain concentration, and a low to moderate mud content. Although flows evolve and transform during the course of transport in density-stratified flows, the preserved features in a deposit are useful to decipher only the final stages of deposition. At present, there are no established criteria to decipher transport mechanisms from the depositional record.

Ganapathy Shanmugam

Papers

Significance of Coniferous Rain Forests and Related Organic Matter in Generating Commercial Quantities of Oil, Gippsland Basin, Australia

50 years of the turbidite paradigm (1950s—1990s): deep-water processes and facies models—a critical perspective

Fan-deltas and braid deltas: Varieties of coarse-grained deltas

Sequence of structures in fine-grained turbidites: Comparison of recent deep-sea and ancient flysch sediments

High-density turbidity currents; are they sandy debris flows?