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NCGT/SEAS + SEA FLOORS
« on: January 29, 2017, 10:47:21 pm »
Volume 3, Number 1, March 2015. ISSN 2202-0039. Editor: Dong R. CHOI (editor@ncgt.org). www.ncgt.org

MASSIVE CHANGES IN CLIMATE & SEA LEVEL
(Excerpt #1, abridged from an unpublished monograph,
EXTINCTIONS: the Pattern of Global Cataclysms)
Peter M. JAMES
Dunalley, Tasmania 7177, Australia
petermjames35@gmail.com

5 Deep Sea Drilling Results
Much of the DSDP program has been aimed at supporting plate tectonics predictions so that information relevant to sea level change is largely fortuitous. Nonetheless, boreholes drilled in the deep ocean, hundreds of kilometres from land, have recovered evapourites, coarse sediments, terriginous materials, wood and even leaves. To date, all these items – except for the evaporites - have typically been labelled the result of turbidity current activity, despite the fact that this has typically meant stretching the known principles of hydraulics past breaking point. Selected boreholes are quoted below.

80 NCGT Journal, V. 3, No. 1, March 2015. www.ncgt.org
Borehole 156 (Galapagos area). Basalt met at a depth of 2.5 km below the surface of the ocean was found to be oxidized, indicating exposure to air, either by sea level change or massive subsidence of the land in this locality. Or perhaps some new way of producing oxidation of rock under deep water? Incidentally, the exploration program associated with this borehole revealed that the sea floor in this equatorial region is deeply dissected and eroded in an east-west direction.
Borehole 240, recovered land detritus and reef material within sand deposits in the upper stratigraphic units. This was drilled in the Indian Ocean, some 500 km from the equatorial African coast, in water of some 5 km depth.
Borehole 518 recorded an erosional unconformity at the Miocene/Pliocene boundary, revealing that the region was then either dry or at least a shallow water domain. It is now at some 4 km depth and the unconformity is overlain by deep water sediments.
Borehole 217, drilled in deep water on the 90º E Ridge, recovered Cretaceous Age sediments containing dried out mud cracks.
Borehole 661, drilled in the Atlantic off Africa’s north west coastline, encountered a deposit of Cretaceous anhydrite. Evaporites are indicative of a shallow, enclosed, tropical basin and such deposits also occur in the Mediterranean which is known to have been dry on a couple of occasions. Such deposits have also been recorded the Red Sea. Now, they have been found in the ocean depths.
6 Submarine Valleys
Underwater canyons and valleys are present in all the world’s seas and oceans and almost ninety percent of them can be traced back to existing drainage systems on land, although sometimes the linkage is disturbed or lost where the former drainage system crosses the continental shelf. Normally, however, it can be picked up once more on the continental slope, from where a majority of submarine valleys continue on down to the abyssal plains. Here, in water depths that can range up to four kilometres or more, large alluvial-type fans have been deposited.
In their systems, submarine valleys exhibit most of the major characteristics of terrestrial drainage systems: gorges cut in the hard rock of the continental slopes; tributaries; distinct bedding; incised drainage patterns in the surfaces of the alluvial fans. All these features would normally be seen as the result of gravitational forces and hydraulic gradients that are in operation only above sea level. Indeed, according to Shepard and Dill in their classic tome on Submarine Valleys and Other Sea Valleys (1966), the most logical explanation to fit all the submarine valley features would be a drowned river origin: that is to say, valleys formed in the manner of normal terrestrial rivers and then subsequently submerged. However, they jibbed at the idea of such massive drops in sea level.
Many oceanographers also jib at the idea of massive sea level changes and look for alternative explanations such as turbidity currents, despite the fact that no one has ever successfully demonstrated how an intermittent and superficial turbidity current, acting under water without the power of hydraulic gradients, is able to erode a massive canyon in hard rock. There is another problem with the turbidity current premise. Turbidity currents are currents supercharged with sediments, which sediments they tend to drop on the run, as it were, as their velocity reduces after leaving the continental slope. This process produces graded deposits: initially gravels or gravelly sands, grading out into sands and then into silts as one progresses out from the base of a continental slope. However, sediments deposited in the abyssal fans typically exhibit defined bedding planes, as found in terrestrial streams.
Examples of submarine valleys are given below to illustrate the above arguments, starting with the submarine valleys of the Mediterranean Sea, which is known to have been dry on a couple of occasions, the last time being dated at around five million years ago.5 The Mediterranean therefore provides no problem with regard to a drowned river origin. Canyons in the Mediterranean are also quite frequent, with some significant ones being extensions of the Rhone. Another occurs beneath the mouth of the Nile, running from
5 Although Greek mythology does speak of a more recent occasion when Hyperion, the sun god, was persuaded to let his incompetent nephew drive the sun chariot across the sky. The unruly steeds became uncontrollable and the chariot crashed to earth, causing the Mediterranean to boil dry and the Ethiopians to turn black.

the ground surface near Memphis and deepening down to the base of the Mediterranean at some distance out to sea. This canyon is now infilled to form the Nile Delta.
Precipitous canyons are present around the island of Corsica, beginning not far above present sea level as little more than notches in the present-day rocky coastline. That is, there is no potential here for any turbidity current activity. Below sea level, however, the notches develop rapidly into canyons in the hard rock and, in this form, continue down to the base of the sea at several kilometres depth. The sediment loads of shallow water materials, such as sea grass, have been spilt out onto the sea floor as a small fan deposits.
The morphology of the drowned Mediterranean canyons can now be compared with other submarine canyons present in the major oceans, where the removal of the much larger bodies of water is less easy to explain.
The east coast of Sri Lanka has several canyons, the largest being the Trincomalee Canyon extending off the country’s largest river, the Mahaweli. This canyon runs a twisting, precipitous course in a V-shaped valley that has cut its way down through hard pre-Cambrian granites and quartzites to a final oceanic depth of around 4-5 km, some 60 km out from the land. Now, the Mahaweli ("Big Sand") River has the potential to carry a reasonable sediment load and hence an origin related to turbidity currents has sometimes been proffered to explain its impressive gorge in hard rock. But the Trincomalee Canyon is not alone on the east coast of Sri Lanka. There are several more canyons to the south, each of similar magnitude and each eroded into hard rock. But, in these instances, there is no major river at the head of the canyons and no potential for any large sediment load to call on, if one were considering a turbidity current origin. The logical solution is to accept that, at some stage in the geological history of the region, the sea level in this part of the Indian Ocean was four kilometres lower than it is today. This is not as absurd as it first sounds.
Travelling east into the Bay of Bengal, supporting evidence for the above interpretation is to be found in the Bengal submarine system. This voluminous system extends out from the mouth of the Ganges River, firstly as discrete canyons in the rock of the continental slope, then as a meandering and braided network of valleys incised in a huge sediment fan, which stretches south for a distance of 2,500 km from the Ganges mouth, Figure 6.
Figure 6. The submarine valley system of the Bay of Bengal. Elongate shaded areas represent incised channels in the sediment fan.
The presence of coarse layers within the predominant silts of the fan indicates that there have been four major pulses of sedimentation, ranging in age from the Cretaceous, though the Miocene and Pliocene, to the Quaternary. The youngest deposit, of Pleistocene Age, is overlain by deep sea ooze. This, in itself, is a prime example of changes in the relative elevations of land and sea.
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