204
Gerardo M.E. Perillo . M. Cintia Piccolo
velocities and some kind of geomorphologic "trap" that allows for sand deposition.
The most common traps are either relatively closed basins and ebb or flood sinus
(Ludwick 1974).
Due to both, areal distribution and bedform characteristics, the most important
of these fields is the one located on the Principal Channel between Puerto Rosales and
the mouth of EI Toro Channel (Fig. 9.5). This field covers an elliptical area of about
5.1 km2 with its main axis (6.5 km) parallel to EI Cuchillo shoal and width of over 1.2 km.
The basin where the field has developed is about 22 m in depth and it is bordered by
the EI Cuchillo shoal on the south and areas oflesser depth (18-20 m to the north and
west and 14 m to the east).
Even though depth variations are important, there may be other reasons that limit
the extent of the field: sediment composition and sand thickness appear as determinant. Both to the north and west of the field as well as we move up EI Cuchillo shoal
flank, the percentage of silt and clay in the sediment increases from less that j-5% to
10-15%. On the other hand, from some few (and non conclusive) low penetration seismic data, it appears that the depth of the top of the Chasico Fm is larger within the
field than on the rest of the channel. Aliotta (1988) gives a figure in which bedforms
develop atop of a hard rock terrace in a nearby, less important sand wave field. These
bedforms become progressively larger with distance from the terrace border. Although
with the presently available data no conclusion can be reached, it is possible to speculate that another issue associated with the bedform size increase may be related to
the development of a boundary layer on top of the terrace as shown by the classical
example of vertical velocity distribution over a flat plate (i.e., Streeter 1948).
Bedforms within this field have heights up to 6 m and wavelengths between 80 and
600 m. The most relevant characteristics are:
1. in all cases these bedforms have their lee side directed to the ebb;
2. lee inclinations have an average of n°, with maximum values of the order of 31°;
3. on the through and flat areas associated to type of bedforms described for the first
time where found: megaripple fans (Aliotta and Perillo 1987, 1994);
4. estimated migration rates for the bedforms vary from 5-90 m yrl with an average
of 33myrl.
The fact that lee sides are steeper than 10° and the formation of the megaripple fans
are evidences that flow separation occur on the crest of the 3D dunes. Normally, flow
separation in marine environments only has been observed in very shallow depths
and intertidal areas normally related to sma1l2D and 3D dunes (ripples). There is no
previous mention in the literature of lee steepness so high in such large number of
subtidal deep bedforms. Both elements make this sand wave field unique in the world.
An immediate question is why these forms appear here and nowhere else? First of
all, we consider that if they are observed here, that does not prevent that other sand
wave field with similar characteristics may occur in other places, they have not been
described yet. However, the possible reasons for these characteristics may be associated to the lack of sediment input into the field. One interesting feature of the larger
bedforms located closer to the center of the Principal Channel is that they are solitary bedforms in the sense of Perillo and Ludwick (1984). This means that any evolution of the bedform is independent of the neighbour ones.
Gerardo M.E. Perillo . M. Cintia Piccolo
velocities and some kind of geomorphologic "trap" that allows for sand deposition.
The most common traps are either relatively closed basins and ebb or flood sinus
(Ludwick 1974).
Due to both, areal distribution and bedform characteristics, the most important
of these fields is the one located on the Principal Channel between Puerto Rosales and
the mouth of EI Toro Channel (Fig. 9.5). This field covers an elliptical area of about
5.1 km2 with its main axis (6.5 km) parallel to EI Cuchillo shoal and width of over 1.2 km.
The basin where the field has developed is about 22 m in depth and it is bordered by
the EI Cuchillo shoal on the south and areas oflesser depth (18-20 m to the north and
west and 14 m to the east).
Even though depth variations are important, there may be other reasons that limit
the extent of the field: sediment composition and sand thickness appear as determinant. Both to the north and west of the field as well as we move up EI Cuchillo shoal
flank, the percentage of silt and clay in the sediment increases from less that j-5% to
10-15%. On the other hand, from some few (and non conclusive) low penetration seismic data, it appears that the depth of the top of the Chasico Fm is larger within the
field than on the rest of the channel. Aliotta (1988) gives a figure in which bedforms
develop atop of a hard rock terrace in a nearby, less important sand wave field. These
bedforms become progressively larger with distance from the terrace border. Although
with the presently available data no conclusion can be reached, it is possible to speculate that another issue associated with the bedform size increase may be related to
the development of a boundary layer on top of the terrace as shown by the classical
example of vertical velocity distribution over a flat plate (i.e., Streeter 1948).
Bedforms within this field have heights up to 6 m and wavelengths between 80 and
600 m. The most relevant characteristics are:
1. in all cases these bedforms have their lee side directed to the ebb;
2. lee inclinations have an average of n°, with maximum values of the order of 31°;
3. on the through and flat areas associated to type of bedforms described for the first
time where found: megaripple fans (Aliotta and Perillo 1987, 1994);
4. estimated migration rates for the bedforms vary from 5-90 m yrl with an average
of 33myrl.
The fact that lee sides are steeper than 10° and the formation of the megaripple fans
are evidences that flow separation occur on the crest of the 3D dunes. Normally, flow
separation in marine environments only has been observed in very shallow depths
and intertidal areas normally related to sma1l2D and 3D dunes (ripples). There is no
previous mention in the literature of lee steepness so high in such large number of
subtidal deep bedforms. Both elements make this sand wave field unique in the world.
An immediate question is why these forms appear here and nowhere else? First of
all, we consider that if they are observed here, that does not prevent that other sand
wave field with similar characteristics may occur in other places, they have not been
described yet. However, the possible reasons for these characteristics may be associated to the lack of sediment input into the field. One interesting feature of the larger
bedforms located closer to the center of the Principal Channel is that they are solitary bedforms in the sense of Perillo and Ludwick (1984). This means that any evolution of the bedform is independent of the neighbour ones.