DIR Return Create A Forum - Home
---------------------------------------------------------
FUNDAY
HTML https://funday.createaforum.com
---------------------------------------------------------
*****************************************************
DIR Return to: LK4 Continental Drift & Orogeny
*****************************************************
#Post#: 89--------------------------------------------------
NCGT/SEAS + SEA FLOORS
By: Admin Date: January 29, 2017, 10:47 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(Excerpt #1, abridged from an unpublished monograph,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.
*****************************************************