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Sunday, 22 January 2012

Turbidite Sequences

Turbidite Sequences

   Turbidites are syntectonic sedimentary deposits orginated by deep submarine turbidity currents, especially in foreland basins, in tectonic setting of convergent margin (active). To most part of geologists, turbidite is a sequence of layers that consist of a grano-classified set of sandstone strata/pelitic sediments, generally in fining-upward that were deposited by turbidity currents and is commonly covered by hemipelagic pelites containing assemblages of  deep-water fossils.

   The deposits are formed by concentrated hyperpicnal flow, building a succession of  sandy and hemipelagic sediments very thick set of sedimentary strata (layes), deposited in a short period of geologic time. They are linked to the rising of orogenic chains, exposing the crystalline basement, intense denudation through small rivers that lead quickly (in terms of geological time) with large amounts of sediment deposition in a confined environment, in the context of deep water, forming typical sedimentary structures. The composition of turbidite sequences may be siliciclastic (sands, pelites), carbonates, lithic fragments, salt, volcaniclastics and others.

   The turbidites are characterized by layers (bed) with great lateral continuity, bedding regularly and generally gradational with thinning of the grains to the top of each stratum (layer), ripple marks, association of hemipelagic sediments, base-layer structures as sole marks, flutecasts, marks objects (toolmarks, grooves). Each stratum of turbidite (bed) is deposited in a single one event (flow). The partition of energy between dense and turbulent flow during a turbidity event gives the typical features of these deposits.  In Bi-partite flows dense and fast deposition commonly form massive sandstones while turbulent flow will deposit fine sediments (pelites).The deceleration of the turbulent flow may form ripple marks before decanting the less dense materials and the finer particles, such as clays and silts.

The characterization of some facies and processes associated with siliciclastic turbidites comes primarily from the observation of the structures formed in the ignimbrite flows, which are volcaniclastic rocks.

Spetacular turbidite outcrop. Gorgoglione Flysch, Miocene, South Italy

Classical turbidite deposits with high efficiency flux occur in the European foreland basins, with the best expositions in the Apennines of Italy, Spanish  Pyrenees, Western Alps in France. In Brazil, State of Santa Catarina, typical turbidites occur in Early Paleozoic Itajai Basin and in some Proterozoic basins.

   The turbidite sequences are highly sought by petroleum geologists because they can constitute good reservoirs for hydrocarbons (oil and natural gas).

   Among the scholars of the turbidite sequences are Carlo Migliorini, Kuenen, Arnold Bouma, Emiliano Mutti, Franco Ricci-Lucchi, Shanmugan, Mulder, Pickering, Normark, Lowe, Middleton, P. Heller, Kneller, R. Tinterri and many others.

   But what would be the unconventional ideas on the issue of use of the term turbidite? The issue is the problem caused by the confusion that is made to designate and generalize as turbidites all sedimentary sequences deposited in the context of deep water, especially those that occur in divergent margins (passive margins). Deep-water deposits in divergent margin could not be designated as turbidites because they are related to flows of low efficiency (energy), mainly linked to the response of sea level oscillation, frequently in Lowstand System Tract. The big rivers carry sediments during flood events, but if the sea level remains stable, these sediments are not transported beyond the slope and remain, whether in bars or as mouth-bars of the rivers, in deltaic deposits, dominated by rivers, waves or tides. Part of these sediments can reach regions offshore platform, but the transfer of large amounts of sediment can only be transported by catastrophic event with significant fall of sea level. In this context, strong erosion occur in the onshore basin drainage (watershed) and forming incised valleys in environments of near-shore and platform (shelf), developing of deep cut canyons on the slope that will facilitate transport beyond the slope of those materials present on the platform, and its final deposition at basin floor. In some areas, deep-water bottom-currents may rework slightly the top of turbidite sediments and form contourite deposits. However, even all these process does not have high efficiency to form the typical features of classic turbidites present in foreland basins. Thus it seems desirable to use the term turbidite only for deposits with typical sedimentary facies and its association such as those occurring in the foreland basins, with deposition associated to deep confined environments, source and transport of sediments due high rising of mountain chain and small dirty-rivers. In any case, much remains to be understood about the processes that form turbidite sequences.

Zumaia is a small village along the coast of Guipúzcoa (Basque Country). Famous in this area is known as the geological formation Flysch Zumaia. These impressive outcrops, often with verticalized layers and extraordinary continuity, ranging from Late Cretaceous to Paleogene. This alternation of calcareous clay and marl are a classic example of Flysch, i.e. classical turbidite sediments.

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