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“The floor of the ocean”--what a misleading phrase that is. Floors are flat and featureless, but the land beneath the sea has more ups and downs than Colorado. Beginning at the edge of dry land, a man walking out into the sea would first traverse the gently sloping continental shelf. Then he would come to the steep slope that tilts suddenly downward into the deep sea, to the abyssal plain onto which sediments have been deposited for thousands of years. But our submarine explorer would find smooth plains the exception. Everywhere he would see individual mountain peaks and the enormous mountain ranges that extend the entire length of oceans. He would peer down into submarine canyon systems looking like the branched river systems on the continents and in some cases connecting with river valleys on the land. And perhaps most astonishing of all, he would come upon deep ocean trenches gaping miles below him.
C. P. Idyll (The Abyss, 1976)


Continental margins, whether “active” or “passive” in the tectonic sense, usually are very dynamic environments of deposition. Deep-water slope environments in the marine realm, taken in the broad sense to include the outer continental shelf, continental slope and continental rise, exhibit considerable variation in sedimentary facies (Fig. 18-1). Deposition may be by sliding (e.g., glide or slump), gravity transport (e.g., grain flow or debris flow) or gravity-induced currents (e.g., turbidity currents). Sedimentation is spasmodic, with long periods of non-deposition punctuated by short bursts of rapid sediment influx.

Turbidites, or flysch deposits in general, are spasmodically deposited slope sediments (Bouma, 1962; Mutti and Ricci Lucchi, 1972; Middleton and Hampton, 1973; Walker and Mutti, 1973; Walker, 1979b). They may occur along oceanic continental margins, within isolated marine basins or even in large lakes, although it is only the marine deposits that contain well-known ichnofaunas. In the strict sense, turbidites are packages of sediment deposited by gravity-driven, fluidized turbidity currents. Typically, but certainly not invariably, such deposition occurs in fairly deep water (i.e., bathyal and abyssal depths).

Turbidite packages usually exhibit a characteristic cyclicity of microfacies, known widely as the “Bouma sequence” (Fig. 18-2). An ideal Bouma sequence contains the following succession: Ta, massive or graded sand layer; Tb, planar parallel-laminated sand layer; Tc, ripple cross-laminated or convolute-laminated fine sand or silt layer; Td, planar parallel-laminated silt layer; Te, hemipelagic mud layer, which may be finefy laminated, bioturbated or apparently structureless. Many turbidites do not contain such an idealized sequence at all, and

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