Conference Paper

Types of Hyperpycnal Flows and Deposits in Lacustrine and Marine Basins

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A hyperpycnal flow forms when a land derived dense flow enters a marine or lacustrine water reservoir. As a consequence of its excess in density, the flow plunges in coastal areas generating a highly dynamic and often long lived dense underflow. Depending on the characteristics of the parent flow (flow duration and flow type) and basin salinity the resulting deposits (hyperpycnites) can be very variable. According to flow duration, hyperpycnal flows can be classified into short lived (SLHF) or long lived (LLHF) hyperpycnal flows. SLHF lasts for minutes or hours, and are mostly related to small mountainous river discharges, alluvial fans, collapse of natural dams, landslides, volcanic eruptions, jökulhlaups, etc. LLHF last for days, weeks or even months, and are mostly associated to medium to large size river discharges. Concerning the characteristics of the incoming flow, hyperpycnal flows can be initiated by non-Newtonian (cohesive debris flows), Newtonian supercritical (lahars, hyperconcentrated flows, and concentrated flows) or Newtonian subcritical flows (bedload, sandy or muddy dominated fully turbulent flows). Once plunged, non-Newtonian and Newtonian supercritical flows require steep slopes to accelerate, allow the incorporation of ambient water and develop flow transformations to evolve into a turbidity current and travel farter basinward. Their resulting deposits are difficult to differentiate from those related to intrabasinal turbidites, except for the occurrence of extrabasinal elements like wood, charcoal or plant remnants. On the contrary, Newtonian subcritical hyperpycnal flows (NSHF) are capable of transfer huge volumes of sediment, freshwater and organic matter far from the coast with gentle or flat slopes. In marine settings, the buoyant effect of interstitial freshwater in bedload and sandy hyperpycnal flows can result in lofting due to density reversal. Since the excess of density in muddy hyperpycnal flows is provided by silt-clay sediments in turbulent suspension, lofting is not possible even in marine basins. NSHF can also erode the basin bottom during its travel basinward, allowing the incorporation and transfer of intrabasinal organic matter and sediments. Long lived NSHF deposits exhibit typical characteristics that allow a clear differentiation respect to those related to intrabasinal turbidites. Main features include (1) complex beds with gradual and recurrent changes in sediment grain size and sedimentary structures, (2) mixture of extrabasinal & intrabasinal components, (3) internal and discontinuous erosional surfaces and (4) lofting rhythmites in marine settings.

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... The vertical stacking of channel and levee deposits further indicates the coupling of bed-load-dominated and suspension-load-dominated hyperpycnal flows as well as the migration of gravity-flow channels. Therefore, the stratified flows, with bed load dominant in the lower part and suspension load dominant in the upper part, are an ideal situation to reach a supercritical condition (Cartigny, 2012;Hamilton et al., 2017;Zavala, 2018). Drag of the upper, suspension-load-dominated part may be the formation mechanism for the crude stratification in the lower bed-load-dominated part (Hiscott, 1994;Sohn, 1997;Cartigny et al., 2013;Zavala and Arcuri, 2016) (Figure 14B, Time 2). ...
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"Gravity flows may be triggered by different initiation processes in both marine and lacustrine basins. Recognizing the different initiation processes of gravity flow based on their deposits is vital to accurately establish gravity-flow sandstone distribution, which is important for defining paleogeography and for efficient oil and gas exploration. Gravity-flow deposits in the Dongying sag were analyzed using three-dimensional seismic, well-log, grain size, and porosity and permeability data, along with core descriptions. Eleven lithofacies, nine bed types, and six bed-type associations were recognized in the gravity-flow deposits in the Dongying sag. Gravity-flow deposits around well Niu-110 were caused by delta-fed sediment failure. These deposits are characterized by medium to very fine-grained sandstone, abundant liquefaction and soft-sediment deformation structures, and thick laminae rich in plant debris. They formed massive sandstones accompanied by normally graded sandstone and lenticular- shaped sandbodies and are composed of chaotic deposits and tongue lobes. The above features collectively are indicative of typical collapsed-sediment transport to deep water by slumping and poorly cohesive debris flow to low-density turbidity current. Gravity-flow deposits around well Shi-100 are interpreted to have been caused by flooding river-fed hyperpycnal flows. These deposits are characterized by gravel to very fine-grained sand, abundant erosional structures and climbing ripples, and thin laminae rich in plant debris. They formed massive sandstone with some space stratification accompanied by inverse- then-normal grading sandstone and elongate or fan-shaped sand- bodies and are composed of channel-levee systems and lobes. Stratified hyperpycnal flow is prone to form a hydraulic jump at "the slope break. After the hydraulic jump, coarse-grained sediments were transported to the basin under the drag and shear of the upper part of the suspension flow. Gravity-flow deposits caused by flooding river-fed hyperpycnal flow are better reservoirs than those caused by delta-fed sediment failure under the same conditions. This study offers insight into the recognition criteria and flow processes of gravity flows caused by the different initiation processes in a lacustrine basin."
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