Page 38 - Maldonado_Stanley_1976

34 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

preserved and broken tests of foraminifera and and Cyclococcolithus leptoporus account for most
calcareous tunicate spicules are also commonly of the nannoplankton remains, but a detailed
associated with this sediment type (Figure 18n). analysis of the coccoliths has not been made.
The presence of calcareous ascidian ( tunicates, t) The coarse silt fraction of the hemipelagic mud
spicules (Figure 18n-o) are of primary interest type is dominated by biogenic calcareous and ag-
because they characterize the turbidite sand-silt gregate grains (Figure l9E). In the fine fraction
sediment type. Factors controlling the distribution two subtypes are distinguished: (l) sediments rich
of living ascidians are the bathymetric range and in coccoliths, and (2) sediments rich in poorly
the nature of the substrate. The didemnids, a com- defined fine silt- to clay-size crystals, needles, plates,
mon type of ascidian, extend from the intertidal and irregularly shaped grains, with a relatively low
zone (very abundant) to about 500 m; they prefer coccolith content (Figure l9E). The generai aspect
a hard substrate on which to build their colonies of this second type resembles the magnesian calcite
(P. Mather, pers. comm.; Hekel, 1973: 10). Thus, sediments of the eastern Mediterranean (Milliman
the presence of a high concentration of ascidian and Miiller, 1973).
spicules in samples collected in deep water may The silt fraction of the shallow water mud shows
indicate an originai shallow water origin and sub- a high percent of needles, plates, and irregularly
sequent transport downslope. sha ped grains, an d, in generai, a lower nanno-
In the fine silt and day fraction of the sand-silt plankton content (Figure l9F) than in the other
sediment type (Figures l8E,F, l9A-c) the more mud types sampled. However, coccolith-rich mud
common components are calcareous nannoplank- has been reported elsewhere from shallow water
ton, especially coccoliths, aggregate grains (Figure environments (Scholle and Kling, 1972).
19n, a), terrigenous clay-size particles, and non- The coarse fraction of the shallow water mud is
identified calcareous grains of biogenic origin. dominated by calcareous biogenic grains. The bet-
Coccoliths may represent up to 50% of this fraction. ter state of preservation of these grains facilitates
Their characteristic is a poor preservation and a their identification on the basis of grain surface
mixed thanatocoenosis, which may include re- texture (cf., Stieglitz, 1972). Calcareous algae, fora-
worked Tertiary species (e.g., discoaster, Figure miniferal, and molluscan fragments are among the
l9n, d). However, ali the samples studied belong to most characteristic calcareous components.
the Emiliania huxleyi zone of Upper Quaternary Volcanic ash is well distinguished with the aid of
age. the SEM (Heiken, 1974). Two main types of ash
The terrigenous grains usually show an angular particles have been identified: (a) broken droplets
to round outline and well-developed facets (Figure with abundant vesicles, and (b) angular glass
18A, f). In generai, the surface texture characteris- shard, where inclusions are rare (Figure 20A).
tics of the siliceous grains (regardless of their min- Furthermore, the carbonate-poor tephra air-borne
eralogy) in this very fine fraction cannot be cor- layers are characterized by a decrease in calcareous
related with similar structures observed on coarser (usually nannoplankton) components, while ash
grains (Krinsley and Doormkamp, 1973; Krinsley layers reworked by turbidity currents, slumps and
et al., 1973; Margolis and Krinsley, 1974; Whalley other subaqueous gravity mechanisms contain a
and Krinsley, 1974). For instance most of the fine significant calcareous fraction derived from several
silt-size grains investigated show very high relief shallow to deepwater environments. The first type
conchoidal breakage pattern, imbricated breakage represents an essentially primary ash layer deposit,
blocks, and other grain surface features which have while the second type is identified as a downslope
been related to glacial action by these authors. reworked unit.
This interpretation is probably not applicable to The organic oozes are characterized by a great
these Meditertanean sediments. abundance and variety of nannoplankton species
Turbiditic mud (Figure l9o) displays many of (Figure 20c,o). Coccolithus pelagicus (Wallich)
the same characteristics as the sand-silt sediment Schiller, and Scyphosphaera apsteini Lohmann
type. However, fine turbidites are better sorted to- are among the abundant species in these oozes;
ward the fine fraction and there is a noticeable they are reported from cold water (Mclntyre and
scarcity of intact fossi! remains. Emiliania huxleyi Bé, 1956; Miiller, 1973). The other major com-