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SEPM Society for Sedimentary Geology SEPM Society for Sedimentary Geology 4111 S Darlington Phone: 918-610-3361 Suite 100 Fax: 918-621-1685 Tulsa, Oklahoma 74135 www.sepm.org USA This PDF Content is made available by SEPM—Society for Sedi- mentary Geology for non-commercial use. This file does contain security features to prevent changing, copying items or printing the document. Additional restrictions and information can be found below. ———————–———————————————————- Connect to other SEPM publications below. www.sepm.org to learn more about the Society, membership, conferences and other publications www.sepm.org/bookstore/storehome.htm for purchase other SEPM Book Publications. www.sepmonline.org to access both Book and Journals online. ————————————————–——————————- Copyright not claimed on content prepared by wholly by U.S. government employees within scope of their employment. Individual scientists are granted permission, without fees or fur- ther requests to SEPM, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works. To make unlimited copies of items in SEPM publications for noncommercial use in classrooms to further education and science without fees please contact SEPM. This file may not be posted to any other Web site. SEPM provides this and other forums for the presentation for the of diverse opinions and positions by scientists globally. Ideas in this publications do not necessarily reflect the official position of the Society. Journal of Sedimentary Research, 2006, v. 76, 718–730 Perspectives DOI: 10.2110/jsr.2006.073 THE TSUNAMITE PROBLEM G. SHANMUGAM1 1Department of Earth and Environmental Sciences, The University of Texas at Arlington, P.O. Box 19049, Arlington, Texas 76019-0049, U.S.A. e-mail: [email protected] ABSTRACT: The genetic term tsunamite is used for a potpourri of deposits formed from a wide range of processes (overwash surges, backwash flows, oscillatory flows, combined flows, soft-sediment deformation, slides, slumps, debris flows, and turbidity currents) related to tsunamis in lacustrine, coastal, shallow-marine, and deep-marine environments. Tsunamites exhibit enormous variability of features (e.g., normally graded sand, floating mudstone clasts, hummocky cross stratification, etc.). These sedimentary features may also be interpreted as deposits of turbidity currents (turbidites), debris flows (debrites), or storms (tempestites). However, sedimentary features play a passive role when these same deposits are reinterpreted as tsunamites on the basis of historical evidence for tsunamis and their triggering mechanisms (e.g., earthquakes, volcanic explosions, landslides, and meteorite impacts). This bipartite (sedimentological vs. historical) approach, which allows here classification of the same deposit as both turbidite and tsunamite, has blurred the distinction between shallow-marine and deep- marine facies. A solution to this problem is to classify deposits solely by a descriptive sedimentological approach. The notion that tsunami waves can directly deposit sediment in the deep sea is unrealistic because tsunami waves represent transfer of energy and they are sediment starved. During tsunamis and major storms, submarine canyons serve as the physical link between shallow-water and deep-water environments for sediment transport. Tsunami-related deposition involves four progressive steps: (1) triggering stage (offshore), (2) tsunami stage (incoming waves), (3) transformation stage (near the coast), and (4) depositional stage (outgoing sediment flows). In this progression, deep-water deposition can commence only after the demise of incoming tsunami waves due to their transformation into outgoing sediment flows. Deposits of these sediment flows already have established names (e.g., debrite and turbidite). Therefore, the term tsunamite for these deposits is obsolete. INTRODUCTION problem is an amalgamation of issues at several levels. They are difficult to define precisely, but they can be described broadly. The term ‘‘tsunamite’’ has been formally adopted in thematic special volumes (Shiki et al. 2000), printed journal articles (Michalik 1997; Pratt 2002; Simms 2003), online Encyclopedia articles (Rodolfo 2003), and Overlap in Nomenclature other publications (Aschoff et al. 2001; and Barnett and Ettensohn 2003). The crux of the problem is concerned with reinterpretation of deep- A ‘‘GeoRef ’’ Database search has yielded Gong (1988) as the earliest water turbidites (deposits of turbidity currents) and debrites (deposits of reference for the usage of the term ‘‘tsunamite.’’ Yamazaki et al. (1989) debris flows) as tsunamites. Unlike turbidites, which are interpreted on used the term ‘‘tsunamite’’ for tractive-current-reworked conglomerates the basis of sedimentary features, tsunamites are interpreted on the basis by tsunamis. Shiki and Yamazaki (1996, p. 177) defined tsunamites as of historical evidence. In interpreting deep-water muddy deposits as follows: ‘‘We use the term tsunamites not only for sediments transported products of tsunamis using historical evidence, Cita and Aloisi (2000, by the tsunami wave itself, but also for tsunami-induced current deposits. p. 181) acknowledged that, ‘‘No sedimentological characteristics peculiar This usage is much the same as that of the term tempestites, which is used to tsunamites are observed in the deep-sea homogenite of the eastern for storm-induced sediments.’’ Like the term ‘‘turbidite’’ for deposit of Mediterranean.’’ The problem is that the term ‘‘homogenite’’ is used here a turbidity current, it makes sense to label deposit of a tsunami as as a synonym for ‘‘tsunamite.’’ The other problem is that the term ‘‘tsunamite.’’ Unfortunately, this is where the simple logic ends on this ‘‘homogenite’’ actually represents ‘‘turbidite’’ (Cita and Aloisi 2000, their matter. fig. 12). In other words: homogenite 5 tsunamite 5 turbidite. Similarly, The term tsunamite is not a self-defining expression of a single debrite beds were reclassified as tsunamites (Barnett and Ettensohn 2003). depositional process. Although the term appears to be a simple word with The reason for this nomenclatural overlap is that tsunamis are a straightforward meaning, a closer examination reveals a highly complex oceanographic phenomena of local to global magnitude. As a phenom- word with multiple and convoluted meanings. Furthermore, the meaning enon (extraordinary event), a tsunami can trigger a variety of processes, of the word changes drastically from one case to the next. The tsunamite including turbidity currents and debris flows. Without realizing this, Copyright E 2006, SEPM (Society for Sedimentary Geology) 1527-1404/06/076-718/$03.00 JSR THE TSUNAMITE PROBLEM 719 many turbidites and debrites have been reinterpreted as ‘‘tsunamites,’’ have served as the impetus for the proliferation of genetic nomenclature, causing nomenclatural congestion in the geologic literature. turbidites themselves have become the subject of controversy (Shanmu- gam 2002a). Nevertheless, flow behaviors of turbidity currents and debris Depositional Process versus Triggering Event flows have been reasonably well established (Sanders 1965; Hampton 1972). When a deposit is interpreted as a turbidite, the emphasis is focused In order for a genetic term to succeed: (1) it must be based on sound solely on the process of deposition (turbidity current). On the other hand, fluid dynamic principles; and (2) its usage must be accurate (relying on when the same deposit is reinterpreted as a tsunamite, the focus shifts sedimentological description), precise (referring to a single process), and from a depositional process (turbidity current) to a triggering event consistent (requiring a steady and a uniform application in time and (tsunami). As a result, useful information on depositional process is being space). Natural amalgamated deposits, however, are often the result of replaced by ambiguous information on tsunami. The ambiguity comes multiple processes. They exhibit a complex array of features. To maintain from a plethora of processes (overwash surges, backwash flows, the integrity of a genetic term, researchers are often forced to de- oscillatory flows, combined flows, slides, slumps, debris flows, and emphasize features that are too ‘‘complex’’ to meet the requirements of turbidity currents) that can be generated by tsunamis in a range of a particular process-based genetic term. In discouraging the application environments (lacustrine, inland sea, coastal, bay, fjord, shallow-marine, of such rigid schemes (e.g., fluvial facies scheme) to the complex rock and deep-marine). As a result, virtually any sedimentary deposit in the record, Leeder (1997, p. 374) cautioned, ‘‘The main philosophical reason geologic record could be reinterpreted as a tsunamite, so long as there is is that it, and other schemes like it, are lazy intellectually and deny the historical evidence. Such a reinterpretation undermines sedimentological great potential richness of the sedimentary record, full of possible progress that has been made over the past fifty years to distinguish variation not adequately tapped by rigid classification.’’ If one chooses to specific depositional facies (e.g., turbidites vs. debrites). Thus the basic classify a deposit using a genetic term, no matter how complex the deposit premise behind reinterpreting a deposit as a tsunamite is flawed. may be, the basic tenet (i.e., the built-in process interpretation) of the genetic term must be maintained. A prudent approach to classify complex Deep-Water Deposition
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