Turbidites are sedimentary rocks formations that result from turbidity currents. Turbidity currents are essentially underwater avalanches or landslides of sand or mud. The debris deposited as a result of these subaqueous flows typically forms a graded bed of sediment built up at the base of a submarine slope. They are sea-floor deposit frequently formed by massive slope failures where rivers have deposited large deltas. These slopes fail in response to earthquake shaking or excessive sedimentation load. The temporal correlation of turbidite occurrence for some deltas of the Pacific Northwest suggests that these deposits have been formed by earthquakes.
Turbidity currents are thought responsible for distributing vast amounts of clastic sediment. Clastics are rocks or sediment made mainly from broken fragments of other rocks or minerals and found far from their place of origin. The commonest clastics are sandstone and shale. Distal turbidites are fine grained clastics formed farthest from the source area. Proximal turbidites are coarser and formed nearest the source area.
Turbidity currents are rapid, underwater avalanches or landslides of sand or mud. What is important is that because of the stirred up sediment that the current contains, its density is greater than water and so it flows down slopes and spreads out on horizontal surfaces at the bottom of the slope. Turbidity currents can originate in many ways, storm waves, tsunamis, earthquakes, and rivers in spate carrying much sediment.
Dougherty Gap, Georgia
Carl Froede and Jack Cowart investigated the outcrop of sandstone and shale found at Dougherty Gap in Walker County in the northwest corner of Georgia. They argue that turbidity currents are a better model to describe how these sediments were laid down than the uniformitarian pro-grading delta model.
Tiny animals left burrows that were fossilized as casts, and it is argued that this "bioturbation" or mixing from biological activity, is found in sediment deposited in turbidity currents as well as in deltas. Large spans of sandstone without these fossils is difficult to explain if the sand was laid down a few layers at a time because the animals would have plenty of time to look for food and construct vertical escape structures. The lack of fossil traces would seem to imply large amounts of deposition at one time. Burrows made in salt water are often lined with clay, but clay lined burrows were not found. A large number of animals should have destroyed the ripple marks at the top of each sand layer, but traces were only found at the bottom of the layer as if the animals survived for a time at the bottom but did not have time to penetrate and destroy the ripple patterns. Incoming sand may have preserved traces found at the top of previous layer.
- Dougherty Gap: Evidence for a Turbidity Current Paleoenvironment by Carl R. Froede, Jr. and Jack H. Cowart, Creation Research Society Quarterly 32(4), March 1996, 32:202-214.