Deep-water Processes by G.
Shanmugam
Triggering mechanisms of sediment failures in deep water
The triggering of sediment failures is
controlled by the pull of gravity, the weight of material, the slope, and the
planes of weaknesses. Masson et al. (2006) and Feeley (2007) provide a general
review of triggering mechanisms of MTDs. There are at least 21 triggering
mechanisms of sediment failures that can initiate mass-transport deposits in
deep water (Table 1). In many continental margins, several triggering
mechanisms work concurrently, such as earthquakes and tsunamis.
Table 1. Triggering mechanisms of sediment
failures. Modified after Shanmugam (2012a, b).
Triggering Mechanisms
|
Environment of MTD
Emplacement
|
Duration
|
(Heezen and Ewing, 1952)
(Claeys et al. 2002) (Figure 1)
(Tilling et al., 1990)
(Shanmugam, 2006a, b)
(Figure 2)
(Dysthe et al., 2008)
(Boyd et al., 2008)
(Petley, 2012)
(Brönnimann, 2011)
(Cannon et al., 2001)
|
Subaerial & submarine
Subaerial & submarine
Subaerial & submarine
Subaerial & submarine
Submarine
Subaerial & submarine
Submarine
Submarine
Subaerial
Subaerial & submarine
Subaerial
Subaerial & submarine
|
Short-term events:
a few minutes to several hours, days or
months
|
|
Subaerial & submarine
Submarine
Submarine
Submarine
Submarine
Submarine
Submarine
Submarine
|
Intermediate-term events: hundreds to
thousands of years
|
|
Submarine
|
Long-term events:
thousands to millions of years
|
* Although human activity is considered to be
the second most common triggering mechanism (next to earthquakes) for known
historic submarine mass (Mosher et al., 2010), it is Irrelevant for
interpreting ancient rock record.
** Some tectonic events may extend over
millions of years
The
triggering mechanisms are grouped into three major types based on their
duration: (1) short-term triggering events that last for only a few minutes to
several hours or days, (2) intermediate-term triggering events that last for
hundreds to thousands of years, and (3) long-term triggering events that last
for thousands to millions of years (Shanmugam, 2012a). Conceivably, some intermediate-term
events may last for a longer duration. The importance here is that short-term
events and long-term events are markedly different in their duration.
Furthermore, these triggering mechanisms should not be confused with
depositional processes, such as debris flows. Also, more than one triggering
mechanism can cause debris flows at a given time in one site. However, there
are no objective criteria to infer either the triggering mechanism or the
transportational process from the depositional record.
Figure 1. Map showing the site of Chicxulub meteorite impact at the K-T boundary in Yucatan, Mexico. Stars represent approximate locations of mass-transport deposits and tsunami-related deposits associated with the Chicxulub impact at the K-T boundary boundary. Generalized outline of Lower Tertiary Wilcox Trend is from several sources. After Shanmugam (2012b).
Figure 2. 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 (2006a).
Figure 3. A. Highstand sedimentological model showing calm shelf waters and
limited
extent of
sediment transport in the shoreface zone (short green arrow) during
fair-weather periods. Shoreface bottom-current velocities during fair weather
are in the range of 10-20 cm s-1. The shelf edge at 200 m water
depth separates shallow- water (shelf) from deep-water (slope) environments.
(B) Highstand sedimentological model showing sediment transport on the open
shelf, over the shelf edge, and in submarine canyons during periods of tropical
cyclones (storm weather) into deep water (long red arrow). Mass-transport
processes are commonly induced by intense cyclones (e.g., Hurricane Katrina,
2005). Modified after Shanmugam (2008a).
The obsolescence of sea-level lowstand model
In
the petroleum industry, the sea-level lowstand model is the perceived norm for
explaining the timing of deep-water sands (Figure 4A).
Figure 4. A. Conventional sea-level
model showing the popular belief that deep-water deposition of sand occurs only
during periods of lowstand and deposition of mud occurs during periods of
highstand. The present highstand is estimated to represent a period of 20,000
years. BP=before present. B. 200,000
cyclones are estimated to occur during the present highstand in the Indian
Ocean (Bay of Bengal) and the Atlantic Ocean. C. 140,000 tsunamis are estimated to occur during the present
highstand in the Pacific Ocean. The implication is that sands can be deposited
during periods of sea-level highstands, thus making the conventional model
obsolete. After Shanmugam (2008a).
Saller et al. (2006), for example, attributed
the timing of reservoir sands in the Kutei Basin in the Makassar Strait
(Indonesian Seas) to a lowstand of sea-level. Nevertheless, the location of the
Kutei Basin is frequently affected by earthquakes, volcanoes, tsunamis,
tropical cyclones, monsoon floods, the Indonesian throughflow, and M2
baroclinic tides (Shanmugam, 2008b). These daily activities of the solar system
(e.g., earthquakes, meteorite impacts, tsunamis, cyclonic waves, etc.) do not
come to a halt during sea-level lowstands. In tectonically and
oceanographically tumultuous locations,
such as the Indonesian Seas, the short-term events are the primary triggering
mechanisms of deepwater sediment failures and they occur in a matter of hours
or days during long periods of both highstands and lowstands (Shanmugam, 2008a).
Deep-water
Paleocene sand (100 m thick) of the Lower Tertiary Wilcox trend, which occurs
above the K-T boundary in the BAHA #2 wildcat test well, has been interpreted
as “lowstand” turbidite fan in the northern Gulf of Mexico (Meyer et al., 2007).
However, because of the opportune location of the Lower Tertiary Wilcox trend
and the stratigraphic position and age, the drilled Paleocene sand could
alternatively be attributed to the Chicxulub impact and related seismic shocks
and tsunamis (Figure 1). Tsunami-related deposition on continental margins has
been discussed by Shanmugam (2006a) (Figure 2). Such alternative real-world
possibilities are often overlooked because of the prevailing mindset of the
sea-level lowstand model.
Shanmugam
(2008a) discussed the importance of tropical cyclones in understanding
deep-water sand deposition during periods of sea-level highstands (Figure 4).
The Hurricane Hugo (Hubbard, 1992), which passed over St. Croix in the U.S.
Virgin Islands on 17 September 1989, had generated winds in excess of 110 knots
(204 km hr-1, Category 3 in the Saffir-Simpson Scale) and waves 6–7
m in height. In the Salt River submarine canyon (>100 m deep), offshore St.
Croix, a current meter measured net down-canyon currents reaching velocities of
2 m s-1 and oscillatory flows up to 4 m s-1. Hurricane
Hugo had caused erosion of 2 m of sand in the Salt River Canyon at a depth of
about 30 m. A minimum of 2 million kg of sediment were flushed down the Salt
River Canyon into deep water (Hubbard, 1992). The transport rate associated
with Hurricane Hugo was 11 orders of magnitude greater than that measured
during fair-weather period. In the Salt River Canyon, much of the soft reef
cover (e.g., sponges) had been eroded away by the power of the hurricane.
Debris composed of palm fronds, trash, and pieces of boats found in the canyon
were the evidence for storm-generated debris flows. Storm-induced sediment
flows have also been reported in a submarine canyon off Bangladesh (Kudrass et
al., 1998), in the Capbreton Canyon, Bay of Biscay in SW France (Mulder et al.,
2001), in the Cap de Creus Canyon in the Gulf of Lions (Palanques et al., 2006),
and in the Eel Canyon, California (Puig et al., 2003), among others. In short,
sediment transport in modern submarine canyons is accelerated by tropical
cyclones and tsunamis in the world’s oceans during the present sea-level
highstand (Shanmugam, 2008a). In fact, empirical data clearly show that
tropical cyclones and tsunamis are the two most important phenomena in
transporting sediment into the deep sea during the present sea-level highstand
(Figure 4). Therefore, the sea-level lowstand model is obsolete for explaining
the triggering of deep-water SMTDs worldwide (Shanmugam, 2007, 2008a, 2013c).
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