104
4 TRANSPORTATION AND SEDIMENTATION
Subsequently Heezen and Ewing (1952) attributed these breakages to a turbidity current. They postulated that the earthquake triggered slumps on the continental slope
of the Grand Banks. These liquefied as they fell and mixed with the seawater until they
acquired the physical properties of a turbidity flow. According to the sequence and timing of cable breaks, this flow moved out onto the ocean floor at speeds ofup to 100 km/h,
ultimately covering an area of some 280,000 km 2. Subsequent coring has revealed an extensive, sharp based, clean graded silt bed over this region.
Many other studies have been published attributing deep-sea sands to turbidity current deposition. These have been described from the Californian Coast (e.g., Hand and
Emery, 1964) and the Gulf of Mexico (Conolly and Ewing, 1967) to the Mediterranean
(Van Straaten, 1964) and the Antarctic (Payne et al., 1972). Modern bioclastic deep-sea
sands have been attributed to turbidite transportation from adjacent carbonate shelves
by Rusnak and Nesteroff (1964), Bornhold and Pilkey (1971), and Mullins et al. (1984).
These modern turbidite sands show a variety of features. They have abrupt, often
erosional bases, but none of the characteristic bottom structures found under ancient
turbidites. This may be due to the problems of collecting small cores of unconsolidated
sediment. Upward size grading is sometimes but not always present. The sands are frequently clean; an interstitial clay matrix is generally absent. Internally the sands are
massive, laminated or cross-laminated. A shallow-water fauna is sometimes present especially in the bioclastic sands; this contrasts markedly with the pelagic fauna of the intervening muds. By analogy with modern experimental and lacustrine turbidites, it can
be convincingly argued that deep-sea sands such as these were transported from the
continental shelves by turbidity currents. These moved down submarine canyons cut
into continental margins and out onto the ocean floors. The decrease in gradient would
cause the flow to lose velocity and deposit its load in a graded bed, the coarsest particles
settling out first.
This attractive mechanism has been criticized for a number of reasons. First, it has
been pointed out that many deep-sea sands do not conform to the ideal turbidite model.
Some are clean and well sorted, and are internally cross-laminated. These features could
indicate that the sand was deposited by a traction current. Studies of modern continental rises show the existence of currents flowing perpendicular to the slope. These are
termed geostrophic or contour currents. Photographs and cores show that these currents deposit cross-laminated clean sand. Seismic data show the existence of stacked
megaripples, tens of meters high, whose axes parallel the slope (Heezen and Hollister,
1971; Hollister and Heezen, 1972; Bouma, 1972). An additional argument against the
turbidity current mechanism for deep-sea sand transport is to be found in the submarine canyons down which they are believed to flow. Attempts to trigger turbidites by
explosions in canyon heads have been unsuccessful (Dill, 1964). The sediments of the
canyons themselves often suggest transportation by normal traction currents aided by
some slumping and grain flow (Shepard and Dill, 1966; Shepard et al., 1969). Other critiques of the turbidity current mechanism have been made by Hubert (1964), Van der
Lingen (1969) and Simpson (1982).
Turn now from the problems of deciding the role played by turbidity currents in modern deep-sea sands to their ancient analogs. There is a particular sedimentary facies
that used to be termed flysch, which is described in Chapter 6. Many geologists use this
term interchangeably with turbidite, implying that these rocks were deposited from tur-
4 TRANSPORTATION AND SEDIMENTATION
Subsequently Heezen and Ewing (1952) attributed these breakages to a turbidity current. They postulated that the earthquake triggered slumps on the continental slope
of the Grand Banks. These liquefied as they fell and mixed with the seawater until they
acquired the physical properties of a turbidity flow. According to the sequence and timing of cable breaks, this flow moved out onto the ocean floor at speeds ofup to 100 km/h,
ultimately covering an area of some 280,000 km 2. Subsequent coring has revealed an extensive, sharp based, clean graded silt bed over this region.
Many other studies have been published attributing deep-sea sands to turbidity current deposition. These have been described from the Californian Coast (e.g., Hand and
Emery, 1964) and the Gulf of Mexico (Conolly and Ewing, 1967) to the Mediterranean
(Van Straaten, 1964) and the Antarctic (Payne et al., 1972). Modern bioclastic deep-sea
sands have been attributed to turbidite transportation from adjacent carbonate shelves
by Rusnak and Nesteroff (1964), Bornhold and Pilkey (1971), and Mullins et al. (1984).
These modern turbidite sands show a variety of features. They have abrupt, often
erosional bases, but none of the characteristic bottom structures found under ancient
turbidites. This may be due to the problems of collecting small cores of unconsolidated
sediment. Upward size grading is sometimes but not always present. The sands are frequently clean; an interstitial clay matrix is generally absent. Internally the sands are
massive, laminated or cross-laminated. A shallow-water fauna is sometimes present especially in the bioclastic sands; this contrasts markedly with the pelagic fauna of the intervening muds. By analogy with modern experimental and lacustrine turbidites, it can
be convincingly argued that deep-sea sands such as these were transported from the
continental shelves by turbidity currents. These moved down submarine canyons cut
into continental margins and out onto the ocean floors. The decrease in gradient would
cause the flow to lose velocity and deposit its load in a graded bed, the coarsest particles
settling out first.
This attractive mechanism has been criticized for a number of reasons. First, it has
been pointed out that many deep-sea sands do not conform to the ideal turbidite model.
Some are clean and well sorted, and are internally cross-laminated. These features could
indicate that the sand was deposited by a traction current. Studies of modern continental rises show the existence of currents flowing perpendicular to the slope. These are
termed geostrophic or contour currents. Photographs and cores show that these currents deposit cross-laminated clean sand. Seismic data show the existence of stacked
megaripples, tens of meters high, whose axes parallel the slope (Heezen and Hollister,
1971; Hollister and Heezen, 1972; Bouma, 1972). An additional argument against the
turbidity current mechanism for deep-sea sand transport is to be found in the submarine canyons down which they are believed to flow. Attempts to trigger turbidites by
explosions in canyon heads have been unsuccessful (Dill, 1964). The sediments of the
canyons themselves often suggest transportation by normal traction currents aided by
some slumping and grain flow (Shepard and Dill, 1966; Shepard et al., 1969). Other critiques of the turbidity current mechanism have been made by Hubert (1964), Van der
Lingen (1969) and Simpson (1982).
Turn now from the problems of deciding the role played by turbidity currents in modern deep-sea sands to their ancient analogs. There is a particular sedimentary facies
that used to be termed flysch, which is described in Chapter 6. Many geologists use this
term interchangeably with turbidite, implying that these rocks were deposited from tur-