matrix of fine-grained clay. Because the muddy matrix has cohesive strength, unusually large clasts may be able to float on top of the muddy material making up the flow matrix, and thereby end up preserved on the upper bed boundary of the resulting deposit. Low-density turbidity current deposits (turbidites) are characterized by a succession of sedimentary structures referred to as the Bouma sequence, which result from decreasing energy within the flow (i.e., waning flow), as the turbidity current moves downslope. High-density turbidity current deposits are characterized by much coarser grain size than in low-density turbidites, with the basal portions of the deposits often characterized by features that result from the close proximity of the grains to each other. Thus, indications of grain-to-grain interactions (i.e., grain flow processes), and interaction of grains with the substratum (i.e., traction) are generally present in the lower portions of these deposits. Complete Bouma sequences are rare, and generally only the Bouma A and B layers are evident. Hybrid event beds (HEB) transitional between mud flows and turbidity currents are characterized by features indicative of both cohesionless (turbulence-supported) and cohesive (mud-supported) flow with no separating bed boundary between the two. In most cases, they are represented by grain-supported textures that grade upward within the bed into mud-supported textures. It is not uncommon for debris flows and mud flows to evolve downslope into turbidity currents, and vice versa. Also, flows internally may transition upward from one flow process to another. Modern and ancient examples Modern and ancient (outcrop) examples of deposits resulting from different types of sediment gravity flows. Significance Sediment gravity flows, primarily turbidity currents, but to a lesser extent debris flows and mud flows, are thought to be the primary processes responsible for depositing sand on the deep ocean floor. Because anoxic conditions at depth in the deep oceans are conducive to the preservation of organic matter, which with deep burial and subsequent maturation through the absorption of heat can generate oil and gas, the deposition of sand in deep ocean settings can ultimately juxtapose petroleum reservoirs and source rocks. In fact, a significant portion of the oil and gas produced in the world today is found in deposits (reservoirs) originating from sediment gravity flows. See also Bouma sequence High-density turbidity current (Lowe sequence) Pyroclastic flow Turbidite Turbidity current References Sedimentology