Deep Marine Systems: Processes, Deposits, Environments, Download Pdf
Deep-Water Processes
Bottom currents
G. Shanmugam , in Mass Transport, Gravity Flows, and Bottom Currents, 2021
8.1 Introduction
In order to provide a complete story on deep-water processes, the objective of this section is to discuss deep-water bottom currents and their deposits from oceanographic and sedimentological viewpoints. The domain of bottom currents has a long history of contributions on both physical oceanography and process sedimentology ( Wüst, 1933; Stommel, 1958; Heezen and Hollister 1964; Hubert, 1964; Hsü, 1964; Heezen et al., 1966; Klein, 1966; Hollister, 1967; Hollister and Heezen, 1972; Pequegnat, 1972; Bouma and Hollister, 1973; Shepard et al., 1979; Stow and Lovell, 1979; Shanmugam et al., 1993a, b Shanmugam et al., 1993a Shanmugam et al., 1993b ; Apel et al., 2006; Hernández-Molina et al., 2006; Zenk, 2008; St. Laurent et al., 2012; Talley, 2013; Rebesco et al., 2014; Shanmugam, 2018a; Eberli and Betzler, 2019; Mulder et al., 2019; de Castro et al., 2020; Fonnesu et al., 2020; Fuhrmann et al., 2020, among others). The influence of the European research community on contourite research is evident in three Geological Society of London publications (Stow and Piper, 1984; Stow et al., 2002; Viana and Rebesco, 2007). The European influence is even more striking in a thematic volume "Contourites" edited by Rebesco and Camerlenghi (2008). Of the 25 chapters in the volume, 22 (88%) are from the European research community (Table 8.1).
Table 8.1. Contourite research contributions by country for the 25 chapters in the edited volume "Contourites" (Rebesco and Camerlenghi, 2008).
| Chapter | Contribution (chapter title) | Authorship | First author's affiliated institution or residence by country |
|---|---|---|---|
| 1 | Contourite research: a field in full development | M. Rebesco, A. Camerlenghi, and A.J. Van Loon | Italy |
| 2 | Personal reminiscences on the history of contourites | K.J. Hsü | United Kingdom |
| 3 | Methods for contourite research | J.A. Howe | United Kingdom |
| 4 | Abyssal and contour currents | W. Zenk | Germany |
| 5 | Deep-water bottom currents and their deposits | G. Shanmugam | United States |
| 6 | Dynamics of the bottom boundary layer | S. Salon, A. Crise, and A.J. Van Loon | Italy |
| 7 | Sediment entrainment | Y. He, T. Duan, and Z. Gao | China |
| 8 | Size sorting during transport and deposition of fine sediments: sortable silt and flow speed | I.N. McCave | United Kingdom |
| 9 | The nature of contourite deposition | D.A.V. Stow, S. Hunter, D. Wilkinson, and F.J. Hernández-Molina | United Kingdom |
| 10 | Traction structures in contourites | J. Martín-Chivelet, M.A. Fregenal-Martínez, and B. Chacón | Spain |
| 11 | Bioturbation and biogenic sedimentary structures in contourites | A. Wetzel, F. Werner, and D.A.V. Stow | Switzerland |
| 12 | Some aspects of diagenesis in contourites | P. Giresse | France |
| 13 | Contourite facies and the facies model | D.A.V. Stow and J.-C. Faugères | United Kingdom |
| 14 | Contourite drifts: nature, evolution and controls | J.-C. Faugères and D.A.V. Stow | France |
| 15 | Sediment waves and bedforms | R.B. Wynn and D.G. Masson | United Kingdom |
| 16 | Seismic expression of contourite depositional systems | T. Nielsen, P.C. Knutz, and A. Kuijpers | Denmark |
| 17 | Identification of ancient contourites: problems and palaeoceanographic significance | H. Hüneke and D.A.V. Stow | Germany |
| 18 | Abyssal plain contourites | F.J. Hernández-Molina, A. Maldonado, and D.A.V. Stow | Spain |
| 19 | Continental slope contourites | F.J. Hernández-Molina, E. Llave, and D.A.V. Stow | Spain |
| 20 | Shallow-water contourites | G. Verdicchio and F. Trincardi | Italy |
| 21 | Mixed turbidite–contourite systems | T. Mulder, J.-C. Faugères and E. Gonthier | France |
| 22 | High-latitude contourites | T. van Weering, M. Stoker, and M. Rebesco | The Netherlands |
| 23 | Economic relevance of contourites | A.R. Viana | Brazil |
| 24 | Palaeoceanographic significance of contourite drifts | P.C. Knutz | Denmark |
| 25 | The significance of contourites for submarine slope stability | J.S. Laberg and A. Camerlenghi | Norway |
Note that 22 chapters (88%) represent contributions from the European Research Community, and three chapters (5, 7, and 23) are from non-European countries (United States, China, and Brazil).
The U.S. Atlantic Margin is characterized by both downslope mass-transport deposit (MTD) and alongslope contour currents (Fig. 8.1). However, both processes are originally triggered by downslope gravity-driven mechanisms. These complexities need to be explained in developing a clear understanding of deep-water processes, which is one of the objectives of this volume. In achieving this objective, datasets from 35 case studies worldwide have been used (Fig. 8.2, Table 8.2).
Figure 8.1. Comparison of downslope mass lows and their deposits (i.e., debrites, left map) (Embley, 1980; with alongslope contour currents and their deposits (i.e., contourites, right map) (Flood and Hollister, 1974) on the U.S. Atlantic Margin.
Source: Shanmugam, G., 2017b. Contourites: physical oceanography, process sedimentology, and petroleum geology. Pet. Explor. Dev. 44 (2), 183–216. Elsevier. 
Figure 8.2. Map showing the locations of case studies used In this review, which include 22 critical case studies by other researchers (locations A–V), and locations of studies by other researchers that resulted in recent debates on deep-water processes (locations G, K, and N). Note 35 locations of core and outcrop descriptions of deep-water sandstones with traction structures that were interpreted by the present author as products of bottom-current reworking (Table 9.2).
Source: Blank world map credit: http://upload.wikimedia.org/wikipedia/commons/8/83/Equirectangular_projection_SW.jpg (accessed 24.01.16.). Table 8.2. Summary of 21 locations of published case studies on deep-marine bottom currents by other researchers that are used in this chapter (locations: A–U shown by filled squares, see Fig. 8.2).
| Location symbol and number in Fig. 8.2 | Case studies | Data: thickness of core and outcrop described by the author (not applicable to studies by other researchers) a | Comment (this chapter) |
|---|---|---|---|
| A. Case study: Blake Plateau and Blake-Bahama Outer Ridge (Heezen et al., 1966) | Modern contour currents | Echo sounding, bottom photographs, sediment cores | Introduction of basic concept of contour currents |
| B. Case study: Straits of Florida (Mullins et al., 1980) | Modern sandy carbonate contourites | Seismic profiles, cores, rocks recovered by dredging and in situ sampling | Porosity and permeability data (Table 8.4) |
| B. Case study: Straits of Florida (Lüdmann et al., 2016 ) | Bottom currents | Hydroacoustic data, high-resolution multichannel seismic reflection data, conductivity, temperature, and depth (CTD) casts and sampling | Modern carbonate mounds |
| C. Case study: Argentine Basin (Klaus and Ledbetter, 1988) | Modern muddy contourites | High-resolution seismic records (3.5 kHz echograms) | Sheet-like sediment waves |
| D. Case study: Eirik Drift (Stanford et al., 2011) | Deep western boundary current (DWBC) |
|
|
| E. Case study: Weddell Sea (Michels et al., 2002) |
| Current-meter records | Velocity: 24 cm s−1 |
| F. Case study: Gulf of Cadiz (Faugères et al., 1984; Gonthier et al., 1984; Stow and Faugères, 2008) |
| 3.5 kHz seismic profiles, sediment cores | Discussion of problematic contourite facies model (discussed in this chapter) |
| F. Case study: Gulf of Cadiz (Hernández-Molina et al., 2006; Garcia et al., 2009) |
|
| Discussion of complex origin of erosional features (discussed in this chapter) |
| F. Case study: Gulf of Cadiz (Mulder et al., 2013) |
| Sediment cores, grain-size analysis, thin-section studies | Discussion of problematic contourite facies model in terms of velocity (discussed in this chapter) |
| F. Case study: Gulf of Cadiz (Stow et al., 2013) |
| 2 gravity cores and over 3000 submarine photographs (Stow et al., 2013) | Discussion of problematic origin contourite sands (discussed in this chapter) |
| G. Case study: NE Spain (Pomar et al., 2012) | Ricla Section, Upper Jurassic | 1 outcrop section (Bádenas et al., 2012; Pomar et al., 2012) | Discussion of problematic internal-wave and internal-tide deposits ( Shanmugam, 2013a, b Shanmugam, 2013a Shanmugam, 2013b ) |
| H. Case study: Southern Adriatic Sea (Chiggiato et al., 2016) | Dense water | Various measurements and sampling | Velocity of bottom currents: 40–50 cm s−1 |
| I. Case study: Israel (Bein and Weiler, 1976) | Ancient sandy carbonate contourite (Cretaceous Talme Yafe Formation) | Outcrop and core | Sediment prism |
| J. Case study: Southeast of South Africa (Bryden et al., 2005) | Agulhas Current | Mooring deployment off Fort Edward from coast out to 203 km offshore; maximum depth of measurement: 2200 m |
|
| K. Case study: China (He et al., 2011) | Ningxia, Middle Ordovician | Several outcrop sections (He et al., 2011) | Discussion of problematic internal-wave and internal-tide deposits ( Shanmugam, 2012b, 2014b Shanmugam, 2012b Shanmugam, 2014b ) |
| L. Case study: South China Sea (Yu et al., 2014) | Modern contourites | Bottom simulating reflectors | Gas hydrates |
| M. Case study: West Philippine Sea (Lien et al., 2015) | Kuroshio and Luzon Undercurrent | Field experiment | The annual Kuroshio transport is 16±4 Sv |
| N. Case study: Makassar Strait (Saller et al., 2006) | Kutei Basin, Miocene | 2 wells? ( Saller et al., 2006, 2008a Saller et al., 2006 Saller et al., 2008a ,b) | Discussion of deep tidal currents (Shanmugam, 2008c) |
| N. Case study: Makassar Strait (Dunham and Saller, 2014) | Kutei Basin, Miocene | 2 wells ( Saller et al., 2006, 2008a Saller et al., 2006 Saller et al., 2008a , b) | Reply to a discussion on the reservoir quality of bottom-current reworked sands (Shanmugam, 2014a) |
| O. Case study: Off Fraser Island, SE Australia (Boyd et al., 2008) | Deep-marine tidal bottom currents | Regional bathymetry and multibeam echo sounding | Highstand transport of coastal sand to the deep ocean |
| P. Case study: Canterbury Drifts, SW Pacific Ocean (Carter, 2007) |
| ODP 1119 | Planar-bedded units up to several meters thick |
| Q. Case study: Offshore of the Pennell Coast, Antarctica (Rodriguez and Anderson, 2004) | Modern sandy volcaniclastic contourites |
| Sheet contourites |
| R. Case study: Meiji Drift Emperor seamount chain (Kerr et al., 2005) | Thermohaline circulation (THC) | Seismic data | Dimensions:
|
| S. Case study: Horizon Guyot, Mid-Pacific Mountains (Lonsdale et al., 1972) | Baroclinic currents reworking sediments on flat tops of towering guyot terraces | Narrow-beam echo-sounding system, a pair of side-looking sonars, a 3.5-kHz seismic profiler and a proton magnetometer, and deep-sea cameras | Asymmetrical dunes and ripples Bathymetry of bedforms: 1630–32 m |
| T. Case study: Equatorial Pacific (Dubois and Mitchell, 2012) | Large-scale sediment redistribution by bottom currents |
| Bottom-current induced resedimentation |
| U. Case study, Monterey Canyon, U.S. Pacific Margin (Shepard et al., 1979) | Deep-marine tidal bottom currents | Current meter | Velocity of bottom currents: 30 cm s−1 (both upcanyon and downcanyon) |
| V. Santos Basin, SW Atlantic Ocean (Duarte and Viana, 2007) | Miocene erosional channels | 3D seismic reflection profiles | Erosional channel geometry |
| 1. Gulf of Mexico, United States (Shanmugam et al., 1988b) |
|
| Modern submarine fan |
| 1. Gulf of Mexico, United States ( Shanmugam et al., 1993a, b Shanmugam et al., 1993a Shanmugam et al., 1993b ; 1995b; Shanmugam and Zimbrick, 1996) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common |
| 2. California (Shanmugam and Clayton, 1989; Shanmugam, 2006a, 2012a Shanmugam, 2006a Shanmugam, 2012a ) |
|
| Sandy mass-transport deposits and bottom-current reworked sands |
| 3. Ouachita Mountains, Arkansas and Oklahoma, United States (Shanmugam and Moiola, 1995) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common |
| 4. Southern Appalachians, Tennessee, United States (Shanmugam, 1978; Shanmugam and Benedict, 1978; Shanmugam and Walker, 1978, 1980 Shanmugam and Walker, 1978 Shanmugam and Walker, 1980 ) |
|
| Ancient submarine fan |
| 5. Brazil ( Shanmugam, 2006a, 2012a Shanmugam, 2006a Shanmugam, 2012a ) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common |
| 6. North Sea (Shanmugam et al., 1995a) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common |
| 7. U.K. Atlantic Margin (Shanmugam et al., 1995a) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common; contourites have been reported (Damuth and Olson, 2001) |
|
| ||
| 8. Norwegian Sea and vicinity (Shanmugam et al., 1994) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common |
| 9. French Maritime Alps, Southeastern France ( Shanmugam, 2002a, 2003a Shanmugam, 2002a Shanmugam, 2003a ) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common (deep tidal currents) |
| 10. Nigeria (Shanmugam, 1997b; Shanmugam, 2006a, 2012a Shanmugam, 2006a Shanmugam, 2012a ) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common (deep tidal currents) |
| 11. Equatorial Guinea (Famakinwa et al., 1996; Shanmugam, 2006a, 2012a Shanmugam, 2006a Shanmugam, 2012a ) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common |
| 12. Gabon ( Shanmugam, 2006a, 2012a Shanmugam, 2006a Shanmugam, 2012a ) |
|
| Sandy mass-transport deposits and bottom-current reworked sands common |
| 13. Bay of Bengal, India (Shanmugam, et al., 2009) |
|
| Sandy debrites and tidalites common |
| Total thickness of rocks described by the author | 11,463 m | ||
Note conventional core and outcrop description carried out by the present author worldwide (locations: 1–13, filled circles, see Fig. 8.2). Traction structures of bottom-current origin are common in all 35 case studies carried out by the author.
- a
- The rock description of 35 case studies of deep-water systems comprises 32 petroleum-producing massive sands worldwide. Description of core and outcrop was carried out at a scale of 1:20–1:50, totaling 11,463 m, during 1974–2011, by G. Shanmugam as a Ph.D. student (1974–78), as an employee of Mobil Oil Corporation (1978–2000), and as a consultant (2000–11). Global studies of cores and outcrops include a total of 7832 m of conventional cores from 123 wells, representing 32 petroleum fields worldwide (Shanmugam, 2015a). These modern and ancient deep-water systems include both marine and lacustrine settings.
- b
- The Peira Cava outcrop section was originally described by Bouma (1962), and later by Pickering and Hilton (1988, their Fig. 62), among others.
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Introduction
G. Shanmugam , in Mass Transport, Gravity Flows, and Bottom Currents, 2021
1.8 Synopsis
This book is a one-stop knowledge source on deep-water processes and their deposits. It is a compilation of empirical data on gravity-induced sediment movements in both downslope and alongslope environments. A total of 540 case studies are used. Although the primary focus is on deep-water settings, other environments covering terrestrial, shallow-water, lacustrine, and extraterrestrial are considered. This book does not promote genetic facies models because available data suggest that most processes are complex transitional and hybrid kinds rather than end-member types. There are no shortcut means (i.e., facies models) to interpreting deep-water processes. I am hopeful that this universal case study–based approach has the potential to minimize confusion and to enhance clarity on gravity-driven processes.
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New Perspectives on Deep-water Sandstones
Dr. G. Shanmugam , in Handbook of Petroleum Exploration and Production, 2012
3.7 Problems with Interpretation of Seismic Facies
In a sequence-stratigraphic framework, seismic facies and geometries have been used to classify deep-water systems into basin-floor fans and slope fans (Vail et al., 1991). In turn, these models have been used to predict turbidite reservoirs. It should be realized that the term "turbidite" has a precise meaning in terms of its origin by a turbidity current with Newtonian rheology and turbulent state. Evidence for Newtonian rheology and flow turbulence cannot be established directly from seismic-reflection profiles; rather, these properties can only be ascertained from direct examination of the rocks. Although sequence-stratigraphic methodology relies on seismic geometries for interpreting submarine fans and related turbidite systems (Posmentier and Erskine, 1991), interpretation of depositional facies (processes) using seismic data encounters fundamental problems.
First, no one has systematically developed objective seismic criteria for interpreting the physics and hydrodynamics of 11 depositional processes of sand and 15 depositional processes of mud in deep-water environments (Section 6.9). For example, Butman et al. (2006) mapped MTD in the Hudson Canyon region, offshore New York and New Jersey, using seismic data (Figure 3.78). However, whether these MTDs represent slide, slump, or debrite cannot be distinguished from seismic data alone; such a distinction can be made only with the ground-truth.
Figure 3.78. Map showing distribution of MTDs in deep-water environments of the Hudson Canyon region, offshore New York and New Jersey. Map is based on backscatter intensity and subbottom characteristics revealed in widely spaced 3.5-kHz-subbottom profiles.
After Butman et al. (2006), U.S. Geological Survey.Second, each depositional process develops a unique unit with characteristic bedding contacts and delicate internal features for SMTD (Figure 3.76). To interpret sandy debrites, for example, depositional features of sandy debrites (e.g., sharp upper contact, inverse grading, floating mudstone clasts, and planar clast fabric) should be recognized. But these features cannot be resolved on seismic data. This is because vertical seismic resolution of a sand bed is controlled by the velocity of the formation, frequency content of the wave traveling through the media, and wavelength of the wave. If a sand bed is buried in 3,000 m of sediment in deep water, for example, then in order to be resolvable and observed on normal stacked seismic section, the sand bed must be in the range of 18–34 m in thickness. This thickness range depends upon the composition of the sand bed, its acoustic property, nature of seismic data acquisition, and seismic data processing parameters. At present, no technology is available to resolve centimeter-thick sand beds and their intricate internal features on industry-standard seismic data.
Third, seismically resolvable thick packages (~25–30 m) usually contain multiple beds of varying origins (Shanmugam et al., 1995a, their Figures 15 and 16). The notion that a specific depositional process can be interpreted from a seismic geometry is ill-founded because seismic geometries cannot be proxies for all different deep water sandy processes.
Fourth, calibration of cored intervals with seismic-reflection profiles ( Shanmugam et al., 1995a, 1996 Shanmugam et al., 1995a Shanmugam et al., 1996 ; Shanmugam and Zimbrick, 1996) suggests that seismic geometries are unreliable indicators of individual depositional facies in a system's tract approach. The reason being that sandy slump/debris flow facies exhibit a variety of external (e.g., mounded, sheet, and lateral pinch out) and internal (e.g., bidirectional downlap, chaotic, parallel-continuous, and irregular-discontinuous) seismic facies (Figure 3.79). Our core-seismic calibration studies also reveal that a single seismic facies can represent more than one depositional facies. Further, kilometer-scale seismic hummocks (i.e., mounds) have been interpreted as fluidization structures of a deep-water channel on the Niger Delta (Davies, 2003). At present, our understanding of the sedimentary facies that form different seismic facies and geometries is poor because of the insufficient core studies of these features using systematic calibrations of long conventional cores (i.e., the ground-truth) with seismic data.
Figure 3.79. Schematic diagram showing that a single depositional facies of sandy debrites can generate multiple seismic facies and geometries (e.g., mounded, sheet, and pinch out). See text for details.
After Shanmugam (2000), reprinted by permission of Elsevier.Although the methodology of using seismic faces for interpreting deep water processes is popular in marine geology and seismic stratigraphy (e.g., Gee et al., 2006), these methods still fall short without core calibration. Selected examples are as follows:
- 1.
-
In their interpretation of seismic data of modern deep-water deposits from the Makassar Strait (Indonesia), Posamentier and Kolla (2003) recognized debris-flow channels based on (a) low-amplitude reflections, (b) chaotic reflections, (c) reflection-free (transparent) intervals, and (d) uneven-upper contacts. On the other hand, these same seismic criteria are equally applicable to deposits of submarine slides (reflection-free intervals and uneven-upper contacts), synsedimentary slumps (chaotic reflections and uneven-upper contacts), and postdepositional clastic injections (reflection-free intervals and uneven-upper contacts). In the tectonically active Makassar Strait region, with earthquakes and active volcanoes, slides, slumps, debris flows, and clastic injections are all viable possibilities. Disappointingly, Posamentier and Kolla (2003) did not present any conventional core data to validate the physics of debris flows in terms of plastic rheology, laminar flow state, and en masse deposition in the Makassar Strait.
- 2.
-
Some of the best studied seismic examples of MTDs are in the area of the Storegga Slide on the mid-Norwegian continental margin (Solheim et al., 2005b). Even in these cases, the authors acknowledged the practical difficulties in distinguishing slides from debrites on seismic profiles. This is because both slides and debrites exhibit homogeneous (i.e., transparent) to chaotic reflections (Figure 3.80, see color plate). In distinguishing slides from debrites, Solheim et al. (2005b) used additional criteria, such as the existence of a headwall as well as sidewalls. These real-world examples reveal the limitations of relying on seismic facies for distinguishing specific types of deep-water MTDs or SMTDs.
Figure 3.80. Seismic profile showing transparent (homogeneous) to chaotic internal reflections of slide deposits (SD). Note continuous and parallel internal reflections of contourite deposits (CD) (see color plate).
Profile courtesy of A. Solheim. After Solheim et al. (2005b), with permission from Elsevier - 3.
-
Figure 3.81B (see color plate) shows lobate and sinuous planform geometries on root mean square (RMS) amplitude maps in the Pliocene interval of the deep-water Krishna–Godavari (KG) Basin, Bay of Bengal (see Chapter 7). Without core calibration, one could interpret these geometries to represent base-of-slope submarine fans with turbidite lobes and channels. But the calibrated core and the modern KG Basin setting prove that these seismic geometries are composed of sandy debrite on an upper-slope canyon setting (Shanmugam et al., 2009).
Figure 3.81. (A) Sedimentological log showing an amalgamated massive sand interval with floating quartz granules and floating mudstone clasts indicating deposition from sandy debris flows. This sandy interval corresponds to the lobate form 1 (Figure 3.81B), which is bright red in seismic amplitude map. Hence, bright red amplitude areas are inferred to be gas-charged sand in our study area. (B) RMS seismic amplitude map (25 ms time window) showing sinuous planform geometries and canyon-mouth lobate forms. SF 3, a well-developed sinuous form, with 90° deflections (deflected arrow), is at least 22 km long along its thalweg. Continuous bright red amplitude in sinuous forms suggests continuous distribution of sand along the entire length of the sinuous canyon. The lobate form 1, which is 3 km long and 2.5 km wide, corresponds to the cored interval of amalgamated sandy debrites (more than 10 m thick) (see color plate).
After Shanmugam et al. (2009), with permission from SEPM - 4.
-
Different researchers tend to interpret the transparent echo character differently: (a) as deposits of debris flows (Wynn et al., 2000, their Table 1), (b) as slides (Laberg et al., 2000), (c) as contourites (Nielsen et al., 2008), and (d) as distal prodelta deposits (Kim et al., 2008). In addition, Loncke et al. (2002) introduced "transparent bedded" echo type to represent a combination of mass flow, turbidity current, and hemipelagic deposits. By definition, the "transparent" echo type should be free of internal reflections. So, this hybrid "transparent bedded" type not only violates the original definition but also defeats the purpose of interpreting a single depositional process based on a unique echo character.
- 5.
-
Wynn et al. (2000, their Table 1) used echo character showing "Rough seabed with abundant large hyperbolae" for interpreting debris avalanches. Wynn et al. (2000, their Figure 2d) also interpreted a TOBI (Towed Ocean Bottom Instrument) side-scan sonar image showing scattered "blocks" lying on the seafloor as evidence for debris avalanche. But these authors did not explain how these side-scan sonar images reveal anything about the velocity of a fast-moving debris avalanche (Varnes, 1978; Cruden and Varnes, 1996). In contrast, Gardner et al. (1996, their Figure 12.17) interpreted similar TOBI images showing fields of boulders on the seafloor of the Monterey Fan (California) simply as mass-transport deposits, without any velocity connotation. Rapp (1963) recommended that the term "debris avalanche" should be abandoned.
- 6.
-
Leat et al. (2010) interpreted hummocky seafloor topography, seen on multibeam bathymetric sonar images, as debris avalanches. But they did not explain the reason behind interpreting hummocky topography as evidence for high-velocity debris avalanches. Furthermore, hummocky topography is not unique to debris avalanches. High-resolution seismic profiles show hummocky topography of the seafloor associated with sediment waves of complex origins (e.g., Nielsen et al., 2008, their Figure 16.5; Heiniö and Davies, 2009, their Figure 4).
- 7.
-
Milia et al. (2003) interpreted debris avalanches on high-resolution seismic profiles based on chaotic seismic facies and hummocky surfaces. It is unclear as to how the chaotic seismic facies suggest the velocity of a fast-moving mass. Other authors have interpreted chaotic seismic facies to represent slumps (e.g., Yu and Huang, 2006), mass-transport complexes (Moscardelli et al., 2006), and "landslides" (Gee et al., 2007).
The above examples illustrate that seismic criteria for recognizing a particular type of MTD are in a perpetual state of flux. No one has ever developed an objective set of seismic criteria for discriminating one process from the other. The current practice of interpreting a specific depositional process and implying transport velocity from seismic criteria is untenable. Direct examination of the rocks must always precede seismic interpretations (Mutti, 2009). Fifty years of progress made on process sedimentology, since the publication of the pioneering article by Sanders (1963), has been reverted by the casual and even careless application of sedimentological principles and nomenclature in interpreting seismic data.
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New Perspectives on Deep-water Sandstones
Dr. G. Shanmugam , in Handbook of Petroleum Exploration and Production, 2012
2.8 Subaqueous Processes Based on Process Continuum
Stow (1985, his Figure 2) proposed a "process-continuum" classification for deep-water processes by combining (1) resedimentation processes, (2) normal bottom currents, and (3) surface currents. In this case, only the resedimentation processes refer to mass-transport processes that comprise rockfall, creep, slide, slump, debris flow, grain flow, fluidized flow, liquefied flow, and turbidity current. The confusion with this scheme is threefold. First, it uses velocity terms, such as creep ( Section 2.9). Second, it considers turbidity currents in the same category as slide, slump, and debris flow. Third, it creates unnecessary nomenclatural problems by distinguishing resedimentation processes from bottom-current processes. For example, gravity-driven resedimentation processes essentially erode transport, and deposit previously emplaced material. The same is true for reworking processes by thermohaline-driven, tide-driven, and wind-driven bottom currents (Shanmugam, 2008c). In other words, bottom-current reworked sands are indeed resedimented deposits. The difference is that the term "mass transport" refers strictly to downslope processes under the pull of gravity, whereas bottom currents are more complex in their origin (Chapter 4). Gravity-driven downslope processes are totally unrelated to the origin of bottom currents; and therefore, these two family of processes cannot be in continuum.
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New Perspectives on Deep-water Sandstones
Dr. G. Shanmugam , in Handbook of Petroleum Exploration and Production, 2012
5.2.4 Tsunami Waves
Because of the current complacent attitude towards sea-level lowstand model, the real-world factors that control deep-water sediment failures are often ignored. For example, when I emphasized the importance of tsunami waves in my book on deep-water processes and facies models ( Shanmugam, 2006a), Mulder (2006) in his review of the book states that 'Shanmugam surfs on the tsunami wave, and considerable importance (15 pages) is given to this process which is not of primary importance in deep-sea environments.' Given the robust empirical data sets that are available in support of the constructive functions of tsunami waves and tropical cyclones on deep-water sedimentation (Shanmugam, 2006b; 2008a), Mulder's cursory dismissal of tsunamis is troubling.
Tsunamis are oceanographic phenomena that represent a water wave or series of waves, with long wavelengths and long periods, caused by an impulsive vertical displacement of the body of water by earthquakes, landslides, volcanic explosions or extraterrestrial (meteorite) impacts. Earthquakes commonly generate tsunamis through the transfer of large-scale elastic deformation associated with rupture to potential energy within the water column (Geist 2005). The two prominent tsunamis of the 21st century were triggered by M=9 earthquakes in (1) off West coast of Sumatra on December 26, 2004 (Figure 5.6) and (2) offshore Honshu, Japan on March 11, 2011 (USGS, 2011). Any skeptic of tsunamis as a real geologic force in controlling sediment transport and deposition must watch You Tube videos of the 2011 Honshu tsunami waters that destroy huge buildings in a matter of minutes and carry large buildings, trucks, and cars in suspension into the Pacific Ocean.
Figure 5.6. Plot of earthquakes in the Indian Ocean region during 1995–2004. Note the 2004 Indian Ocean Earthquake (M=9.0) that triggered the 2004 tsunami (arrow). United Nations Environment Programme (UNEP). http://na.unep.net/atlas/onePlanetManyPeople/images/chapters/Atlas_Chapter4_Screen.pdf Accessed July 8, 2011.
Wave heights of the 2004 Indian Ocean Tsunami reached up to 15 m. The coastline of Sumatra, near the fault boundary, received waves over 10 m tall, while those of Sri Lanka and Thailand received waves over 4 m (NOAA, 2005). On the other side of the Indian Ocean, Somalia and the Seychelles were struck by waves approaching 4 m in height. Wave height measured from space, 2 hours after the earthquake, reached 60 cm near the east coast of India (Figure 5.7).
Figure 5.7. (A) Map showing propagating tsunami waves away from the epicentre (solid dot) of the 2004 Indian Ocean Earthquake on December 26, 2004. The epicentre was located 3.307°N 95.947°E off the west coast of Sumatra. Measurements of sea level were made from space using the Satellite (Jason-1) 2 h after the earthquake. (B) Plot of relative sea level along the transect X–X΄ (see Figure 5.7A for location).
(Modified after NOAA (2005) .)A tsunami wave can trigger a number of transportational processes, such as overwash surge, backwash flow, debris flow, turbidity current, bottom current, etc. These processes, in turn, will emplace sediment from a variety of depositional mechanisms, namely sudden freezing, settling from suspension and bed load or traction (Shanmugam, 2011c). The transport of tsunami-induced sediment into the deep sea (Kastens et al., 1981), which includes mass transport, was discussed by Shanmugam (2006b).
Tsunami-related deposition in deep-water environments may be explained in four progressive steps (Figure 5.8): (1) triggering stage, (2) tsunami stage, (3) transformation stage, and (4) depositional stage. During the triggering stage, earthquakes, volcanic explosions, undersea landslides, and meteorite impacts can trigger displacement of a large quantity of water either up or down, causing tsunami waves. During the tsunami stage, tsunami waves carry energy traveling through the water, but these waves do not move the water, nor do they transport sediment. During the transformation stage, the tsunami waves erode and incorporate sediment into the incoming wave. The enormous tsunami waves are important triggering mechanisms of sediment failures. The advancing wave front from a tsunami is capable of generating large hydrodynamic pressures on the sea floor that would produce soil movements and slope instabilities (Wright and Rathje, 2003). The transformation stage is evident in sediment-rich backwash flows during the 2004 Indian Ocean tsunami at Kalutara Beach, southwestern Sri Lanka (Figure 5.9, see color plate). The incoming ocean waters are clearly blue in color (implying sediment free), but these waters transform into brown in color near the coast because of their incorporation of sediment. The transformation to brown color is the result of the wave breaking, and the wave will break in different water depths according to its wavelength and sea-floor irregularities. Frohlich et al. (2009) documented huge exotic boulders from the Tongatapu Island, south-west Pacific where the largest boulder has dimensions of 15 m×9 m×11 m (Figure 5.10). Frohlich et al. (2009) estimated masses of boulders to be in the range of 70 to 1,600 metric tonnes. Such boulder emplacement could be attributed to the transformation stage and related sediment emplacement. These tsunami-induced backwash flows should not be confused with hyperpycnal flows introduced by river waters (Bates, 1953).
Figure 5.8. Depositional model showing the link between tsunamis and deep-water deposition. (A) 1. Triggering stage in which earthquakes trigger tsunami waves. 2. Tsunami stage in which an incoming (up-run) tsunami wave increases in wave height as it approaches the coast. 3. Transformation stage in which an incoming tsunami wave erodes and incorporates sediment, and transforms into sediment flows. (B) 4. Depositional stage in which outgoing (backwash) sediment flows (i.e., debris flows and turbidity currents) deposit sediment in deep-water environments. Suspended mud created by tsunami-related events would be deposited via hemipelagic settling. After Shanmugam (2006b), with permission from SEPM.
Figure 5.9. An aerial image showing sediment-rich backwash flows during the 2004 Indian Ocean tsunami. Note that position of shoreline has retreated seaward nearly 300 meters (arrow) during the tsunami. Also note the development of backwash fans along the position of former shoreline, Kalutara Beach (south of Colombo), southwestern Sri Lanka. Image collected on 26 December 2004.
Image credit: Courtesy of DigitalGlobe. After Shanmugam (2006b), with permission from SEPM. 
Figure 5.10. Photograph showing an exotic limestone boulder, interpreted to be emplaced by tsunami-related processes, in Tongatapu Island, south-west Pacific. Dimensions of the boulder are 15 m×11 m×9 m.
Photo courtesy of C. Frohlich. See Frohlich et al. (2009) for more details.During the final depositional stage; the outgoing sediment could generate slides, slumps, debris flows, and turbidity currents (Figure 5.8). Holocene deposits of tsunami-related processes from these environments exhibit a multitude of physical, biological and geochemical features. These features include basal erosional surfaces, anomalously coarse sand layers, imbricated boulders, chaotic bedding, rip-up mud clasts, normal grading, inverse grading, landward-fining trend, horizontal planar laminae, cross-stratification, hummocky cross-stratification, massive sand rich in marine fossils, sand with high K, Mg and Na elemental concentrations and sand injections (Figure 5.11). These sedimentological features imply extreme variability in processes that include erosion, bed load (traction), lower flow regime currents, upper-flow regime currents, oscillatory flows, combined flows, bidirectional currents, mass emplacement, freezing en masse, settling from suspension and sand injection. The notion that a 'tsunami' event represents a single (unique) depositional process is a myth. Although many sedimentary features are considered to be reliable criteria for recognizing potential paleo-tsunami deposits, similar features are also common in cyclone-induced deposits. At present, paleo-tsunami deposits cannot be distinguished from paleo-cyclone deposits using sedimentological features alone, without historical information (Shanmugam, 2011c).
Figure 5.11. Published sedimentological features claim to be associated with tsunami-related deposits by other authors. These features are also associated with cyclone-related deposits.
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The Contourite Problem
G. Shanmugam , in Sediment Provenance, 2017
4.7.5 Multiple Interactive Processes
The muddy contourite facies model was based on the notion that a single process, namely deposition from contour currents, was solely responsible for the deposit (Faugères et al., 1984). But Stow et al. (2013) have demonstrated that multiple interactive processes are operating in the Gulf of Cadiz. In 1984, prior to detailed velocity measurements of MOW (Price et al., 1993) and numerous other investigations of internal waves and internal tides in the Gulf of Cadiz, it was reasonable for Faugères et al. (1984) to propose a contourite facies model at a time when we were grappling with complex deep-water processes, without much data. But today, a great wealth of empirical data (see references in Stow et al., 2013) is available. The Gulf of Cadiz is an extremely complex setting in terms of physical oceanography with multiple processes (e.g., MOW, internal waves, and internal tides) and bottom topography with channels, ridges, and sills. The physical, chemical, and sedimentological aspects of the MOW are equally complex (Ambar et al., 2002; Criado-Aldeanueva et al., 2006). Rebesco et al. (2014, p. 139) acknowledge that "Regardless, the previous research on this issue holds two important lessons: firstly, that there is no unique facies sequence for contourites; and secondly, that traction sedimentary structures are also common within contourites…" Deep-water depositional processes are variable in time and space. Furthermore, extensive bioturbation caused by influx of prolific oxygen in deep-sea currents obliterates physical structures. From a practical viewpoint of interpreting ancient deposits as contourites on land, there is no way of knowing the contours of the paleo-seafloor (Stow et al., 1998). In summary, the global applicability of the contourite facies model is dubious.
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New Perspectives on Deep-water Sandstones
Dr. G. Shanmugam , in Handbook of Petroleum Exploration and Production, 2012
4.8.6 Sedimentological Criteria
The potential significance of shoaling internal waves for causing sediment movement on continental shelves and slopes has been discussed by Cacchione and Southard (1974). Laboratory experiments confirmed that shoaling interfacial waves could generate ripples and larger bedforms in artificial sediment (Southard and Cacchione, 1972). Stride and Tucker (1960) attributed the development of modern sand waves near the shelf edge to internal waves. Karl et al. (1986), using sparker profiles, documented sand waves in the heads of submarine canyons of the Bering Sea. In this case, a surface sediment sample (C1) was composed of 19% gravel, 76% sand, and 5% mud. The modal class of this sample was fine sand. However, no sedimentary structures were described from the cores of these sand waves. Karl et al. (1986) speculated that internal waves were responsible for the origin of sand waves. They also suggested that delivery of large volumes of freshwater and large quantities of sediment directly to the heads of submarine canyons during periods of low sea level might have enhanced the propagation of high-frequency internal waves.
Gao et al. (1998) interpreted ancient strata with bidirectional cross-bedding (see also Gao and Eriksson, 1991), flaser bedding, wavy bedding, and lenticular bedding as deposits of internal tides based on associated deep-water turbidite and slump facies. The supreme evidence for interpreting deposits of "internal tides" or baroclinic currents in the rock record is the proof for tidal currents in a stratified deep ocean. Without that evidence for density stratification (pycnocline), there is no difference between a tidal deposit formed by surface (barotropic) tide in a shallow-marine shelf and a tidal deposit formed by internal (baroclinic) tide in a deep-marine slope or canyon environment. Furthermore, not all tidal bottom currents in submarine canyons are related to density stratification.
Unlike barotropic tidal currents that flow along the axis of the canyon, baroclinic currents flow across the canyon and in the direction parallel to the shelf break (Allen and Durrieu de Madron, 2009). If so, these two different types of currents may not generate the same kind of deposits. For example, the origin of bidirectional cross-bedding by internal tides in submarine channels (Gao and Eriksson, 1991) would be difficult to explain if baroclinic currents were to flow across the channel, instead of up- and down- the channel.
He et al. (2011, their Figure 11) recently proposed the first facies model (i.e., ideal vertical sequence) for internal tide-related deposits. This sequence is composed of a basal sandy turbidite division (Layer I), a middle sandy division with traction structures formed by reworking by internal waves and internal tides (Layer II), and an upper hemipelagic muddy division (Layer III). This facies model is untenable for the following reasons (Shanmugam 2011e).
- 1.
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The ideal sequence, in a bizarre way, mimics Ta, Tc, and Te divisions of the "Bouma Sequence". But the authors do not make any reference to the original turbidite facies model of Bouma (1962).
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The basal massive sandstone layer has been interpreted as turbidites. But the origin of massive sands can be explained by numerous alternative deep-water processes: (a) contour currents ( Rodriguez and Anderson, 2004), (b) bed-load deposition (Sanders, 1965), (c) grain flows (Stauffer, 1967), (d) pseudoplastic quick bed (Middleton, 1967), (e) density-modified grain flows (Lowe, 1976), and (f) sandy debris flows (Shanmugam, 1996a).
- 3.
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He et al. (2011) reasoned that bidirectional cross-bedding cannot be explained by either turbidity currents or by contour currents, and therefore it must be formed by internal tides. But bidirectional cross-laminae have been documented in deep-water traction deposits associated with wind-driven Loop Current in the Gulf of Mexico ( Shanmugam et al., 1993a, 1993b, 1995c Shanmugam et al., 1993a Shanmugam et al., 1993b Shanmugam et al., 1995c ). The Loop Current generates eddies that can create complex current orientations.
- 4.
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He et al. (2011, their Figure 2) proposed that their study area is located in an open "abyssal basin" environment. But internal waves and tides are commonly associated with shelf-edge settings (Inman et al., 1976, see their Figure 4). Although it is conceivable that weak internal tides may be generated near seamounts in the open ocean, seamounts are ineffective barriers for generating major internal tides (Holloway and Merrifield, 1999). Importantly, there is no evidence of seamount near the study area in Figure 2 of He et al. (2011).
Finally, internal tides are akin to tsunamis in terms of our ignorance. Analogous to tsunamis (Shanmugam, 2011c), we know nothing about the flow behavior of internal tides. The implication here is that the less the knowledge we possess on a given oceanographic phenomenon, the more the nonsense we propagate on its purported deposits. In order to interpret a sedimentary feature as the product of internal tides, one needs to establish its association with paleo pycnoclines. Thus far, sedimentologists who study the phenomenon of internal tides have realized that it is impossible to provide physical evidence for (1) density-stratified waters and (2) for the timing and depth at which the discussed sedimentary features were developed. Until we develop objective and reliable sedimentological criteria for recognizing deposits of baroclinic currents (internal tidalites) of stratified water bodies, it is preferred to classify deep-water deposits with tidal signatures as products of "deep-water tidal bottom currents" rather than that of "baroclinic currents" (i.e., internal tidalites). This is a topic for future research on deep-water tidal sedimentation.
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SEDIMENTARY PROCESSES | Post-Depositional Sedimentary Structures
J. Collinson , in Encyclopedia of Geology, 2005
Soft-Sediment Deformation Structures
This section deals with the descriptive aspects of soft-sediment deformation and relates the structures to the deforming processes. The structures themselves do not fall naturally into well-defined classes, but Figure 1 shows a scheme which relates all soft-sediment structures to processes. This diagram illustrates the possible interactions between different causes of a loss of strength and deforming forces, and shows that processes are gradational with tectonic deformation and with resedimentation. It also shows the wide range of scales across which deformation occurs, from microloading to growth faults at the scale of continental slopes, and highlights the differences in timing and duration of deformation. Structures formed during deposition, such as overturned cross-bedding, contrast with structures that require significant later burial, such as growth faults. Commonly occurring deformation structures are described below, in order of increasing scale, but moderated by considerations of process.
Figure 1. The relationship of various post-depositional structures to the nature of the loss of strength and the type of deforming force. Slumps and slides, which involve lateral resedimentation of material, are dealt with in detail in (see SEDIMENTARY PROCESSES | Particle-Driven Subaqueous Gravity Processes). Modified from Collinson JD (1994) Sedimentary deformational structures. In: Maltman A (ed.) The Geological Deformation of Sediments, pp. 95–125. London: Chapman & Hall; and Collinson JD (2003) Deformation structures and growth faults. In: Middleton GV (ed.) Encyclopedia of Sediments and Sedimentary Rocks, pp. 193–195. Dordrecht, Kluwer: Academic Publishers.
Load Casts and Pseudonodules
These usually occur at the interface between a mud layer and an overlying sand layer. Downward-facing bulbous load casts of sand are separated by upward-pointing flame structures of mud (Figure 2). Some occur on otherwise flat bedding surfaces, whilst others accentuate erosional or depositional irregularities. In extreme cases, sand may be detached from its source bed and occur as isolated load balls, or pseudonodules, within highly disturbed muds or silts, a features sometimes referred to as a 'ball and pillow' structure. Internal lamination within both sand and mud is highly contorted. Loading is caused by gravity acting on unstable density stratification where the sand layer is denser than the muds and where both layers are weakened by excess pore fluid.
Figure 2. Small-scale load casts within thin-bedded turbidites. The downward-sinking sand lobes are separated by upward-pointing flames of mud. Carboniferous, North Cornwall, England.
Convolute Lamination
This commonly occurs within sands or silts and involves the distortion of depositional lamination by folding of variable intensity (Figure 3). In some cases, the folding is chaotic, whilst in others upward movement of escaping water has dragged lamination into upright folds. Cuspate folds with sharp anticlines and more rounded synclines are typical. In many cases, depositional lamination can still be identified, commonly cross-bedding or cross-lamination. The good sorting and high porosities of aeolian sands make them particularly susceptible to liquefaction when the water table rises rapidly through them, giving convolute lamination on a large scale. Convolute lamination is common in turbidite sandstones, often associated with the ripple laminated (Bouma C) interval (see SEDIMENTARY PROCESSES | Depositional Sedimentary Structures SEDIMENTARY PROCESSES | Deep Water Processes and Deposits).
Figure 3. Convolute bedding developed in coarse cross-bedded fluvial sandstones as a result of the sediment experiencing a short-lived period of liquefaction shortly after deposition. Namurian, Yorkshire, England.
All examples reflect liquefaction of the sand, which can happen for a variety of reasons, including seismic shock, rapid sedimentation, breaking waves, or a shift in water table. Where seismic shock caused liquefaction, deformed layers may be very widespread and have stratigraphical significance. Upward escape of excess pore water, internal density inversions, and down-slope components of gravity may all contribute to deformation. Dewatering leads to reconsolidation of the sediment from the bottom up, 'freezing' the deforming laminae. Protracted liquefaction may lead to total homogenization.
Overturned Cross-Bedding
This is a special case of convolute lamination in which liquefaction occurred in an actively migrating bedform (Figure 4). Current shear on the sediment–water interface dragged the liquefied sand in a down-current direction in an essentially laminar style. An upward-migrating front of reconsolidation allowed higher parts of the sediment to be sheared longer, giving down-current-facing recumbent folds.
Figure 4. Overturned cross-bedding in two small sets within a shallow marine sandstone. This was caused by ongoing bed shear by a current during a short-lived loss of strength. The deformed sand was progressively 'frozen' as a front of reconsolidation moved upwards through the sediment. Late Precambrian, north-east Greenland.
Dish and Pillar Structures
These are most common in thick, otherwise massive sands and result from dewatering following rapid deposition. Dish and pillar structures are direct products of dewatering and are not distortions of pre-existing lamination. Dish structures are thin concave-upward, subhorizontal zones of slight clay enrichment produced by local filtering out of the elutriated fines (Figure 5). Their upturned edges merge with pillar structures, which are vertical conduits of water escape and record fluidized or near-fluidized conditions.
Figure 5. A thick bed of sandstone in a turbidite setting showing intense development of dish structures due to rapid escape of pore water shortly after rapid deposition. Eocene, Fuenterrabia, Spain.
Sand Injection Structures
Where a sand body is encased in mud and progressively buried, compaction and dewatering of the muds can lead to overpressure in the sands. If the fluid pressure exceeds the tensile strength of the overlying mud, rupturing occurs and complex networks of dykes and sills of liquefied sand are intruded into the muds. This appears to be quite common in subsurface channel sand bodies in deep-water and slope settings, although outcrop examples are rare.
Small-scale injection dykes occur in mudstones of interbedded sandstone and mudstone successions. These may be folded ptygmatically because of differential compaction. Sand within dykes is typically structureless, although some examples show foliation parallel with dyke margins. Remobilized sands are increasingly being recognized as important petroleum reservoirs in the North Sea and elsewhere in the world.
Sand Volcanoes and Extruded Sheets
Sometimes upward-intruding liquefied sand, both dykes and pipes, penetrates to the free sediment surface where the sand is extruded. Where the escape conduits are pipes, sand volcanoes form, at scales from a few centimetres to a few metres (Figure 6). If the intrusions are dykes, fissure extrusion occurs and liquefied sand may flow as sheets before dewatering and 'freezing'. Where sand dykes penetrate muddy debris flows, extruded sand may be interfolded with the upper levels of the debris flow.
Figure 6. Sand volcanoes on top of a slump sheet. Dewatering of the slump led to rapid upwards injection of a sand slurry which was extruded at a point vent to give the volcano. Namurian, County Clare, Ireland.
Mud Diapirs
Overpressure develops where muds are buried quite rapidly, as in a prograding delta. The resultant loss of strength and the significant overburden lead to both vertical and horizontal flowage of the muds. Where the overlying sediment is denser than the mud, as in an upward-coarsening deltaic succession, mud may rise vertically as a diapir or mudlump, pushing aside or penetrating mouth bar sands. In the Mississippi delta, mudlumps rise to sea-level and create short-lived islands offshore from distributary mouths. Mud diapirism may lead to great variability in the thickness of mouth bar sands, which thicken into withdrawal synclines between diapirs (Figure 7).
Figure 7. Mud diapir penetrating delta front mouth bar sands. Overpressuring of the buried muds led to a loss of strength which, combined with a density inversion, led to the upwards flow of mud. Namurian, County Clare, Ireland.
Horizontal flowage of overpressured muds leads to the extrusion of muds at the base of the prograding slope as imbricate thrusts and folds. The same motion is partly responsible for extensional stresses that drive extensional sedimentary growth faults in deltaic successions. Mud diapirs are widespread in Tertiary deltas around the world and may help to trap petroleum.
Slumps and Slides
Gravitational body forces acting on sediments lying on a slope lead to slumps and slides when cohesive (plastic) and tensile (brittle) strengths are exceeded. In both types, shearing is concentrated on discrete basal surfaces. They differ principally in their internal deformation. Slides are largely undeformed, but may have internal faults, whilst slumps show plastic folding. The two styles may coexist in the same deformed unit. The fold orientation may indicate palaeoslope direction, but analysis must be carried out with caution. Gravitational mass movements are dealt with more fully in (see SEDIMENTARY PROCESSES | Particle-Driven Subaqueous Gravity Processes).
Sedimentary Growth Faults
These occur in deltaic successions. They are large-scale features and require exceptional exposures to be identified at outcrop (Figure 8). In small exposures, they may be confused with later tectonic faults. They are caused by shear failure at discrete dislocations within the sedimentary pile, and they commonly occur as listric surfaces that sole out on a basal 'décollement'. In small examples, within a single progradational unit, the basal surface may be within particularly fine-grained mudstone directly above a basal flooding surface. In larger examples, penetrating several progradational units, the décollement may be within a thick interval of overpressured muds, whose lateral flowage towards the free surface slope combines with gravitational forces on the hanging wall block to drive that block down-slope. Growth faults are characterized by slow and continual movement and are aided by progressive sediment loading. Their hanging wall areas act as local depocentres in which thickened delta-front sediments are deposited and preserved. Continued progradation of individual deltas or of the continental slope causes the active faulting to shift progressively basinwards. Large growth faults are important traps for petroleum in deltas such as the Mississippi and the Niger.
Figure 8. Small, synsedimentary growth fault within deltaic sediment. Note the thickening of sands into the hanging wall of the fault and the roll-over within these sands. The synsedimentary nature of the fault is shown by the fact that the uppermost beds pass over the fault without disturbance. Namurian, County Clare, Ireland.
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50th Anniversary Special Issue
Michele Rebesco , ... Anna Wåhlin , in Marine Geology, 2014
5.4 Deep-sea storms
One intermittent deep-water process that is closely related to eddy formation is the generation of deep-sea storms (also called benthic storms or abyssal storms), which remains poorly understood. These storms involve the periodic intensification of normal bottom-current flow alongslope or following the isobaths (Fig. 13), where their mean flow velocity typically increases by two to five times, especially close to boundaries of strong surface currents. The HEBBLE project was the first to document the occurrence of benthic storm events and demonstrated their importance in the winnowing, transport and redistribution of sediments (Hollister et al., 1974; Nowell and Hollister, 1985; Hollister, 1993). Once ripped up by the erosional effects of increased bottom shear, sediments can be transported by bottom currents and deposited in quiet regions downstream (Hollister and McCave, 1984; Flood and Shor, 1988; Von Lom-Keil et al., 2002). In some cases, the flow has a velocity exceeding 20 cm/s, a very high concentration of suspended matter (up to 5 g L− 1), and strong erosional capability. Although benthic storms typically last from 2 to 20 days (most often, 3 to 5 days), they can have much longer-lasting effects on the suspension of bottom sediment, production of plankton blooms, and supply of considerable amounts of organic matter to the drifts (Richardson et al., 1993; Von Lom-Keil et al., 2002). The regions subjected to particularly intense deep-sea storms can also exhibit significant erosion in their continental slopes and produce large submarine slides (Pickering et al., 1989; Stow et al., 1996; Gao et al., 1998; Einsele, 2000).
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Deep Marine Systems: Processes, Deposits, Environments, Download Pdf
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