What is a Turbidite?
1999. "What is a Turbidite?", An Introduction to the Analysis of Ancient Turbidite Basins from an Outcrop Perspective, Emiliano Mutti, Roberto Tinterri, Eduard Remacha, Nicola Mavilla, Stefano Angella, Luca Fava
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To most geologists a turbidite is a graded bed consisting of a sandstone/mudstone couplet which has been deposited by a turbidity current and is commonly overlain by a hemipelagic mudstone containing deep-water fossil assemblages. The Bouma sequence typifies this ideal kind of deposit (Fig. 2a).
Bouma (1962) developed his classical model essentially from observations made in upper Paleogene and Neogene turbidite strata of the western Alps (Annot Sandstone) and the northern Apennines (Macigno and Marnoso-arenacea formations). Although commonly restricted to the internal sequence of depositional divisions observed within an individual turbidite bed, the Bouma model actually includes the fundamental concept of “depositional cone” (Fig. 2b), implying the depletive character of turbidity currents, i.e. their deceleration in space away from their origin. On the basis of this concept, Parea (1965) and Walker (1967) developed their models of proximal vs. distal turbidite deposition — an approach which in many respects still has considerable potential for future research.
Bouma (1962) interpreted his sequence as the deposit of a typical turbidity current without discussing hydrodynamic processes in detail; Harms and Fahnestock (1965) and Walker (1967) reinterpreted the sequence in terms of laboratory flow regime postulating an upper flow regime for the basal “a” division (Fig. 3). In a very influential paper, Middleton and Hampton (1973) suggested that the entire Bouma sequence was the deposit of a fully turbulent turbidity current and envisaged the “a” division as the result of very rapid deposition from suspension preventing the formation of traction structures.
Subsequent work has shown that the Bouma
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Basin Studies Field Methods and Studies Sandstones and Clastics Sequence Stratigraphy. Turbidite sedimentation of ancient orogenic belts are considered as closely related to that of marginal flood-dominated fluvio-deltaic systems. Many conclusions are new and far from conventional. These notes represent a need for fresh air from the area of outcrop studies.