Turbidity Currents
65
Fig. 2.17. Build up of deep-sea fans. 1 Canyon cutting through shelf and upper slope, traps and
funnels sediment to the fan; 2 upper fan valley, walls with slump features (U), bottom with debris
flows (V) and mostly graded coarse grained beds, forming conglomerates in fossil examples. Levees
with thin-bedded turbidities (X). Levees can be breached (note dead channels); 3 active suprafan
with distributary channel, filled with pebbly or massive sands (Y); 4 outer fan with classical turbidites (2); 5 abyssal hill region beyond fan. Valleys between hills may have distal fan material.
[Based on a sketch by W. R. Normark, 1970, Am Assoc Pet Geol Bull 54: 2170 and R. Walker,
1978, Am. Assoc. Petrol Geol. Bull. 62: 932.
about 2 mis, or 5 miles per hour.) While their calculations were not generally accepted, their studies did raise the possibility that such currents exist.
There are but few direct observtions of turbidity currents in the marine realm.
These currents are rare events, and when they do occur, instruments for measung
velocities or suspended matter are in much danger of being lost. We do not know,
therefore, what a "typical" turbidity current might look like. In Bute Inlet, in British
Columbia, where direct observations were made in the 1980s, floods from two rivers
entering the inlet produce muddy flows with velocities of about 3 m/s. The thickness
of the currents is more than 30 m, and coarse sand occurs at least up to 7 m above the
sea floor. Fine sand is transported far offshore; it was seen out to 50 km, and at a
depth of 620 m. The transport channel has bottom slopes of generally less than 1 0
(D. P. Prior et aI., Science 237, 1330, 1987).
Back to the submarine canyons! Normally, nothing much is happening in them
except for the gentle back and forth motion of the tides and internal waves. But from
65
Fig. 2.17. Build up of deep-sea fans. 1 Canyon cutting through shelf and upper slope, traps and
funnels sediment to the fan; 2 upper fan valley, walls with slump features (U), bottom with debris
flows (V) and mostly graded coarse grained beds, forming conglomerates in fossil examples. Levees
with thin-bedded turbidities (X). Levees can be breached (note dead channels); 3 active suprafan
with distributary channel, filled with pebbly or massive sands (Y); 4 outer fan with classical turbidites (2); 5 abyssal hill region beyond fan. Valleys between hills may have distal fan material.
[Based on a sketch by W. R. Normark, 1970, Am Assoc Pet Geol Bull 54: 2170 and R. Walker,
1978, Am. Assoc. Petrol Geol. Bull. 62: 932.
about 2 mis, or 5 miles per hour.) While their calculations were not generally accepted, their studies did raise the possibility that such currents exist.
There are but few direct observtions of turbidity currents in the marine realm.
These currents are rare events, and when they do occur, instruments for measung
velocities or suspended matter are in much danger of being lost. We do not know,
therefore, what a "typical" turbidity current might look like. In Bute Inlet, in British
Columbia, where direct observations were made in the 1980s, floods from two rivers
entering the inlet produce muddy flows with velocities of about 3 m/s. The thickness
of the currents is more than 30 m, and coarse sand occurs at least up to 7 m above the
sea floor. Fine sand is transported far offshore; it was seen out to 50 km, and at a
depth of 620 m. The transport channel has bottom slopes of generally less than 1 0
(D. P. Prior et aI., Science 237, 1330, 1987).
Back to the submarine canyons! Normally, nothing much is happening in them
except for the gentle back and forth motion of the tides and internal waves. But from