|Abstract or Summary
- The Cocos Gap is a deeper portion, or saddle, of the Cocos
Ridge and forms part of the western boundary of the Panama Basing
It is probably typical of saddles within most submarine ridges, In
order to determine the mechanisms controlling sediment dispersal,
the nature and sources of the sediments at 23 core locations were
defined by hydrodynamic size separation (> 63, 2- 63, <2 micron) and
microscopic or X-ray diffraction analysis of the individual fractions.
In addition, calcium carbonate, organic carbon, opal and quartz
determinations were made for the total sediment.
The silt sized fraction was resolved into eight textural modes,
The coarse modes reflect the progressive breakage and winnowing of
the corase fraction (foraminifera) under the influence of bottom
currents and gravity. Above 2000 m mechanical breakdown, winnowing
and relocation by bottom currents mask the effects of depth related
dissolution of the carbonate fraction, Intermediate modes in general
represent a transitional facies with both biogenic and terrigenous influences,
while the finest modes characterize a distal regime of clay
deposition, The clay fraction is amorphous material with very low
percentages of well crystallized clays. Three main sources and
transport paths were recognized, including one associated with the
circulation of the Panama Basin.
Sedimentation within the Gap is controlled by local processes,
predominantly the interaction between tidally induced intensification
of bottom water flow and directional (thermohaline) flow. The steep'
ness of the sea floor slope is a major factor controlling the efficiency
of winnowing of the sediment away from certain higher elevations
(biogenic source areas) to the sheltered parts and flanks of the ridge.
Superimposed upon this sediment dispersal is the influx of terrigenous
material carried by directional bottom currents that operate as
postulated upper and lower contour currents along the flanks of the
The crest of the Cocos Gap acts as a catchment area for the
biogenic components, while the adjacent more sloping region, the
sub-plateau, acts as a source area. The extreme breakage of the
foraminifera is most likely a function of the tidally induced intensification
of the bottom water flow, characteristic of many shallow ridges,
and is probably most significant in the subplateau. Hydrographic
data indicates that there is no significant transport of bottom water
across the Cocos Gap into the Panama Basin, but downslope transport
of carbonate and siliceous fragments and minerals from the Gap into
the basin is associated with cyclical tidal bottom water flow.