Sunday, June 2, 2013

13. Defective turbidite paradigm


Deep-water Processes by G. Shanmugam


Defective turbidite paradigm

 The turbidite exuberance that dominated the deep-water domain for nearly a century is waning as a consequence of emerging new concepts of sandy mass-transport deposits (SMTD). In fact, numerous authors have interpreted or reinterpreted sandy and gravelly deep-water deposits as SMTDs in the ancient stratigraphic record (Shanmugam, 2012, his Table 1.2). . The turbidite paradigm is fundamentally defective because it is built on facies models, such as the “Bouma Sequence” for classic turbidites deposited by low-density turbidity currents (Bouma, 1962) and the “Lowe Sequence” for coarse-grained turbidites deposited by high-density turbidity currents (Lowe, 1982). These vertical facies models were derived solely from the study of ancient rock record using outcrops, without any empirical data of “sandy turbidity currents” from modern oceans. The primary attraction to the vertical facies model of high-density turbidity currents (HDTC) and their deposits, composed of R1, R2, R3, S1, S2, and S3 divisions in ascending order (Lowe, 1982), is that it allows one to interpret ancient deep-water coarse sandstone and conglomerate deposits as turbidites (Mutti, 1992; Mulder, 2011). But turbidite facies models are fatally flawed for the following key reasons.

  1. Turbidity currents are inherently low in sediment concentration or low in flow density (Figure 1B), and hence, true HDTC cannot exist in nature (Shanmugam, 1996, 2000, 2012).
  2. No one has ever documented empirical data on active ‘gravelly or sandy turbidity currents’ in modern oceans using vertical sediment concentration profiles and grain-size measurements. All claims of modern sandy turbidity currents are dubious (Shanmugam, 2012).
  3. No one has ever documented the vertical facies model showing the R1, R2, R3, S1, S2, and S3 divisions in ascending order in modern deep-sea sediments (Shanmugam, 2000, 2012).
  4. No one has ever replicated turbulent turbidity currents that could carry coarse sand and gravel in suspension in laboratory flume experiments that could produce the R1, R2, R3, S1, S2, and S3 divisions in ascending order (Shanmugam, 2012).
  5. The complete “Bouma Sequence” (with Ta, Tb, Tc, Td, and Te divisions) has never been documented in modern deep-sea sediments. Nor has it been reproduced in flume experiments. Furthermore, this model suffers from a lack of sound theoretical basis (Sanders, 1965; Shanmugam, 1997; Hsü, 2004; Leclair and Arnott, 2005). Leclair and Arnott (2005, p. 4) state that “. . . the debate on the upward change from massive (Ta) to parallel laminated (Tb) sand in a Bouma-type turbidite remains unresolved.”
Figure 1. A. Schematic diagram showing four common types of gravity-driven downslope processes (slides, slumps, debris flows, and turbidity currents) that transport sediment into deep-marine environments.  After Shanmugam et al. (1994). B. Sediment concentration (% by volume) in gravity-driven processes. Note that turbidity currents are low in sediment concentration (i.e., low-density flows). After Shanmugam (2000). C. Based on mechanical behavior of gravity-driven downslope processes, mass-transport processes are considered to include slide, slump, and debris flow, but not turbidity currents (Dott, 1963).  D. The prefix “sandy” is used for mass-transport deposits (SMTDs) that have grain (>0.06 mm: sand and gravel) concentration value equal to or above 20% by volume. The 20% value is adopted from the original field classification of sedimentary rocks by Krynine (1948). Figure after Shanmugam (2012).

Despite the lack of vital validation from modern sediments and the absence of experimental corroboration, the turbidite paradigm grew in popularity due to ten popular myths (Shanmugam, 2002). In stark contrast to elusive HDTC in modern oceans, sandy mass-transport processes and their deposits (SMTD) have been documented extensively by direct observations, underwater photographs, and remote sensing techniques in modern submarine canyons (Shepard and Dill, 1966), on modern submarine fan lobes (Gardner et al., 1996), and on modern continental rise (USGS, 1994). Given the fact that the very existence of sandy and gravelly turbidity currents has never been documented in modern oceans, the outcrop-based turbidite facies models (Bouma, 1962; Lowe, 1982) and their more recent  derivatives with an alternative ensemble of nomenclature (Talling et al., 2012) and explanations (Postma et al., 2014) are unnecessary distractions for interpreting ancient rock record objectively. The turbidite facies models, which are nothing more than a "groupthink", have suppressed scientific curiosity by averting  novel observations and by preventing innovative process interpretations during the past 50  years. Because turbidite facies models tend to promote complacency and intellectual laziness, which undermine one's ability to practice  pragmatic process sedimentology, it is imperative to discard these flawed facies models altogether (Shanmugam, 2012, 2013, and 2014a, b).

References

Bouma, A. H. 1962, Sedimentology of some flysch deposits: A graphic approach to facies interpretation: Amsterdam, Elsevier, 168 p. 
Gardner, J.V., R.G. Bohannon, M.E. Field, and D.G. Masson, 1996, The morphology, processes, and evolution of Monterey Fan: A revisit, in  J.V. Gardner, M.E. Field, and D.C. Twichell, eds., Geology of the United States’ Sea Floor: The View from GLORIA: New York,  Cambridge University Press, p. 193–220.
Hsü, K. J., 2004, Physics of sedimentology. 2nd Edition: Berlin, Springer, 240 p.
Krynine, P.D., 1948, The megascopic study and field classification of sedimentary rocks: The Journal of Geology, v. 56, p. 130-165.
Leclair, S., and R. W. C. Arnott, 2005, Parallel lamination formed by high-density turbidity currents: Journal of Sedimentary Research, v. 75, p. 1– 5.
Lowe, D.R., 1982, Sediment gravity flows:  II.  Depositional models with special reference to the deposits of high-density turbidity currents:  Journal of Sedimentary Petrology, v. 52, p. 279-297.
Mulder, T., 2011, Gravity processes and deposits on continental slope, rise and abyssal plains, in H. Hüneke, and T. Mulder, eds., Deep-Sea Sediments: Amsterdam, Elsevier, Developments in Sedimentology 63. p. 25-148. Chapter 2.
Mutti, E., 1992, Turbidite Sandstones: Milan, Italy, Agip Special publication, 275 p.
Postma, G.,  Kleverlaan, K., Cartigny, M.J.B., 2014. Recognition of cyclic steps in sandy and gravelly turbidite sequences, and consequences for the Bouma facies model. Sedimentology, doi: 10.1111/sed.12135
Sanders, J.E., 1965, Primary sedimentary structures formed by turbidity currents and related resedimentation mechanisms, in G.V. Middleton, ed., Primary Sedimentary Structures and Their Hydrodynamic Interpretation: SEPM, Special Publication 12, p. 192-219.
Shanmugam, G., 1996, High-density turbidity currents: Are they sandy debris flows? Journal of Sedimentary Research, v. 66, p. 210.
Shanmugam, G., 1997, The Bouma sequence and the turbidite mind set: Earth-Science Reviews, v. 42, p. 201–229.
Shanmugam, G., 2000, 50 years of the turbidite paradigm (1950s1990s): Deep-water processes and facies modelsA critical perspective: Marine and Petroleum Geology, v. 17, p. 285342.
Shanmugam, G., 2012, New perspectives on deep-water sandstones: Origin, recognition, initiation, and reservoir quality: Amsterdam, Elsevier, Handbook of petroleum exploration and production, v. 9, 524 p.
Shanmugam, G. 2013, Slides, Slumps, Debris Flows, and Turbidity Currents. In: Earth Systems and Environmental Sciences Reference Module. Elsevier (online), in press.
Shanmugam, G., 2014a. Review of research in internal-wave and internal-tide deposits of China:         Discussion.  Journal of Palaeogeography, v. 3 (October, No. 4), p. 332-350.
             To download PDF, click or copy and paste URL
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Shanmugam, G., 2014b. Modern internal waves and internal tides along oceanic pycnoclines: Challenges and implications for ancient deep-marine baroclinic sands: Reply. AAPG Bulletin 98, 858-879. To download PDF, click or copy and paste URL
https://drive.google.com/file/d/0B7yqaSoBdSEVZDJlNUljOC1Fanc/edit?usp=sharing
Shanmugam, G., L. R. Lehtonen, T. Straume, S. E. Syversten,  R. J. Hodgkinson, and M. Skibeli, 1994, Slump and debris flow dominated upper slope facies in the Cretaceous of the Norwegian and Northern North Seas (61º–67º N): implications for sand distribution: AAPG Bulletin, v. 78, p. 910–937.
 Shepard, F. P., and R. F.  Dill, 1966, Submarine Canyons and Other Sea Valleys: Chicago, Rand McNally & Co., 381 p.
Talling, P.J., D.G. Masson, F.J. Sumner, and G. Malgesini, 2012, Subaqueous sediment density flows: Depositional processes and deposit types: Sedimentology, v. 59, p. 1937–2003.

USGS (U.S. Geological Survey), 1994, Geologic features of the sea bottom around a municipal sludge dumpsite near 39°N, 73°W, Offshore New Jersey and New York: U.S. Geological Survey Open-file Report 94-152. http://pubs.usgs.gov/of/1994/of94-152/description.html (Accessed June 2, 2013)