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ments are also present in some samples, probably as horizontal laminations produced by bottom cur-
resulting from the erosion of outcrops on the shelf rents. These concentrations can be interpreted in
during the eustatic low stands of sea level. terms of hydraulic equivalents inasmuch as the
large flaky grains are more easily maintained in
suspension than the smaller round ones. The rela-
SAND-SILT SEDIMENT TYPE
tively denser and more spherical particles, such as
In the sand-silt type we consider ali textural heavy minerals, tend to be concentrated in the
types ranging from sand to silty sand to sandy silt lower turbidite divisions although they are smaller
to silt. In the coarse sand-silt type, the sand con- in size than the majority of grains with which they
tent is above 20% of the entire fraction, and the are associated (Rupke and Stanley, 1974).
sand fraction generally comprises more than 50%
calcareous biogenic components. However, in shal-
VoLcANic AsH TYPE
low platform samples (i.e., AS 6-Sb) the calcareous
content is lower due to a masking effect produced The volcanic ash layers, as in the above-described
by the addition of detrital materia!. sediment type, also comprise a textural mix of
The sand-silt sediments collected in the shallow sand- and silt-size grades. However, composition-
environments bave a biogenic content usually well ally this type is quite characteristic and is thus dis-
represented by shells, shell fragments, benthonic cussed as a separate entity. Ash layers usually
foraminifera, and other calcareous biogenic com- contain a high sand fraction (above 30%), much of
ponents similar to those of the coarse and bio- which is constituted by fragments of volcanic ori-
clastic sands. Planktonic components are also gin; there are many textural varieties ranging from
present. sand to mud. Two distinct types are recognized.
The composition of the organic fraction in the The first is characterized by its calcareous biogenic
majority of the sand-silt samples collected in the content in the sand fraction, which is composed
deep environments is more varied: planktonic and mostly of foraminifera, pteropods, and minor sub-
benthonic foraminifera, echinoderm spicules and ordinate amounts of other biogenic components.
fragments, bryozoa, calcareous algae, sponge spic- The composition of this calcareous fraction is simi-
ules, mollusc shell and shell fragments, plants, etc. lar to the one displayed in the hemipelagic mud
Radiolarian fragments, pteropods, and diatoms type, defined later. The second type of volcanic
also occur. In generai, this type of ''mixed" bio- ash 1ayer presents a more variable calcareous frac-
clastic assemblage is interpreted as a thanatocoeno- tion, represented by mixed biogenic assemblage
sis containing biogenic remains derived from from different environments (i.e., similar to that
various environments (Parker, 1958). Thus, it is of the sand-silt sediment type). The first type re-
interpreted as a resedimented deposit. sults from pelagic settling of volcanic air-borne
Quartz and feldspar account for much of the ash, which may eventually be winnowed and modi-
inorganic fraction except in volcanic ash layers. fied by bottom currents (cf. tephra layers described
Heavy minerals, glauconite, and mica also occur. by Ninkovich and Heezen, 1965; Keller et al.,
The composition of the sand fraction of this 1974). The second type of volcanic ash 1ayer is re-
sediment type provides some information as to (l) lated with turbiditic sedimentation (Sarnthein and
sediment provenance and depositional environ- Bartolini, 1973).
ment, and (2) hydrodynamic processes. The first
factor is interpreted mainly on the basis of the
SHALLOW WATER Mun TYPE
biogenic components and, to a lesser extent, the
inorganic (mostly authigenic minerals) fraction; The shallow water mud type is highly variable
insight as to the processes is provided by the physi- in both sand-size content (4%-20%) and biogenic
cal character of the grains, particularly density and content (30%-90%). There is a graduai transition
particle shape. As an example, the mica and flaky between this type and the sand-silt type from shal-
particles, including shell fragments, tend to be con- low water environments described earlier. The
centrated in parallel laminae, i.e., either as part of sand fraction in the mud indudes benthonic fora-
the d-division of the Bouma turbidite sequence or minifera an d a relatively low ( < 30%) planktonic