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Submarine Fans and Seismic Facies
Submarine fans form at the continental
slope. The basic model used, called the channel-feeding lobe
model, and describes fans in three basic parts:
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a single leveed valley on the upper fan.
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a mid-fan build up of suprafan depositional lobes
at the ends of channels with these lobes switching positions periodically.
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a topographically smooth lower fan without channels.
Nomark proposed this basic model in 1978, which used
information taken from the Navy Fan, which includes much information taken
from side-scan and seismic profiling, as well as core samples. In
1985 Mutti proposed that there are three types of fans rather than three
major descriptors of fans. The theoretical types of fans are based
on sea-level rise and fall and are as follows:
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Type I. Large sandy flows move a way from the channel
forming lobes far away from the feeder channel.
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Type II. Is much like Nomark's model.
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Type III, muddy channel levees form when, sea
level rises.
Walker and James (1992) say that neither model
can be supported by today's findings.
To study submarine fans seismic profiling used
to observe the seismic facies and from this submarine fans can be described. The following table helps to summarize these characteristics of seismic
facies.
This table describes four large-scale facies associations,
which are:
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channel-levee systems.
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continuous unchannelized sheet-like deposits (overbank
or basin plain.
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mass transport complexes.
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slumps and debris flows associated with levee failure.
Channel-levee systems are characterized by High Amplitude
Discontinuous reflectors, that could indicate the interbedding of coarse
sands and gravels with muddier horizons (Walker and James 1992). If these areas are acoustically transparent, there is no acoustic response,
then the lithology is probably uniform. This could be channel filling
with mud, or sand.
The channel-filling characterizes a channel that
is in disuse where the coarser grained lithology characterizes thalweg
channels.
Levees are defined by LAD and LAC (Low Amplitude
Discontinuous and Low Amplitude Continuous) divergent reflectors within
wedge-shaped external forms (1992).Walker and James (1992) suggest that
amplitudes of reflectors might be higher and become more continuous at
levees which suggests a slower deposition . Levees can be as
steep as 3.5° at the crest and are prone to slumping. The
thickness of channel-levee systems is 150-500 m and width is 30-250 km.
Continuous sheet like deposits occur in the
lower fan and they also under lie channel-levee systems. They
are also found I levees and overbank areas. The reflector
tend to be LAC and HAC and represent the deposits of widespread
unchannelized turbidity currents. Because sheet-like feature
have been found to represent turbidites, by the study of cores,
the seismic profiles show that turbidites are laterally extensive
and unchannelized.
Mass transport complexes are probably poorly sorted
and stratified. Not much is known of these facies and everything
that is known about them is summed up in the above chart. It describes
how they look in the seismic profile so they can be identified.
The slump deposits are described as acoustically
transparent and again the information is summed up in the above
chart.
Reference
Walker, R. G., James, N. P.,
Facies Models. Geological Association of Canada, 1992 |
