Sistema De Turbiditas

Marine and Petroleum Geology 17 (2000) 137±143

Coarse-grained and ®ne-grained turbidite systems as end member models: applicability and dangers
Arnold H. Bouma*
Department of Geology & Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA Received 2 December 1998; received in revised form 5 May 1999; accepted 5 May 1999

Abstract Submarine fan/turbidite systems can haveany net-to-gross percentage. Prior to developing a model that best represents the deposits under investigation, the earth scientist should have an adequate understanding of the major external parameters that in¯uence the transport and deposition of sediment. Tectonics in¯uence the sediment source area and its overall distance to the shoreline, shelf width and basin morphology. The sand/clay ratioand size range dictate how far sediment can be transported. Climate in¯uences erosion, transport and diagenesis. Relative sea-level ¯uctuations impact the transport from coast to basin. Using those parameters and two end-membersÐcoarse-grained and ®ne-grainedÐan applicable model can be developed that best characterizes the fan under consideration. # 2000 Elsevier Science Ltd. All rights reserved.Keywords: Turbidite systems; End members; Models

1. Introduction The increasing interest in deepwater sands (submarine fans or turbidite systems), coupled with high costs when operating o€shore in deep water, requires an integrated approach. Teamwork of several disciplines in earth sciences, engineering, and other ®elds has demonstrated that to be very e€ective. From the earth sciences sideit is well known that these potentially major oil and gas reservoirs lack the level of knowledge that is available from other depositional environments. Seismic studies on subsurface deposits, especially when using 3-D, produce images that can reveal the architecture of a turbidite system, especially when channels are present. Research on modern fan systems is rather sporadic because of high costs.Highresolution seismic imaging of the upper sediment column may only show the upper part of a depositional

* Tel.: +1-225-388-6186; fax: +1-225-388-2302. E-mail address: bouma@geol.lsu.edu (A.H. Bouma).

unit which may not be representative as it reveals only the waning stage. Side-scan sonar interpretations, combined with high-resolution seismics and standard piston and gravity coring, haverevealed important information (e.g., Twichell, Kenyon, Parson & McGregor, 1991). However, interpretations typically are based on 2-D data from each instrument with shallow penetration, while outcrops lack the third dimension in horizontal direction. Hence, we have to improve interdisciplinary exchange of information. Normally outcrops comprise tilted formations, limiting lateral continuity. Itis evident, therefore, that outcrops can provide valuable information on deepwater developments (Chapin, 1998). The terms `poorly ecient' and `ecient' were introduced by Mutti (1979) to indicate the eciency of density currents to transport sand over short or long distances, respectively. The terms submarine fans and turbidity system are often used synonymously, with `submarine fan' typicallyrepresenting a modern fan and `turbidity system' referring to subsurface occurrences and outcrops. A succession of turbidite systems,

0264-8172/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 8 1 7 2 ( 9 9 ) 0 0 0 2 0 - 3

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A.H. Bouma / Marine and Petroleum Geology 17 (2000) 137±143

including the shale series that separates them, is known as acomplex (turbidite complex or submarine fan complex; Mutti & Normark, 1987, 1991). Models can be very dangerous when not applied selectively and critically. As long as it is understood that no two deposits are mirror images but may have several characteristics in common, a proper use of models can be very advantageous. The question arises which of the large number of available models should be...