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Abstract

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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