Page 34 - Maldonado_Stanley_1976

30 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

foraminiferal content. In these samples the rela- are contrasted as follows: one has a very Iow sand
tion of planktonic to benthonic tests can be used fraction (=== l%) which is mostly bioclastic (~ 70%),
as an indication of depth (Boltouskoy, 1965). How- and the other is a more sandy terrigenous turbi-
ever, use of this ratio is valid only if the sediment ditic mud (= 5% sand fraction of which 50 to 80%
accumulates in situ; interpretation of these ratios is bioclastic). In the first type the calcareous frac-
is difficult when sediments are transported to tion includes mostly p1anktonic foraminifera, while
deeper environments (for example in the case of the second type has a higher content of benthonic
turbiditic muds). Some samples (cf., LY II-4a), foraminifera and shell fragments. The terrigenous
located in intermediate to deep water, are inter- components are ana1ogous to those of hemipelagic
preted as a sediment type transitional between muds, although the burrows and tubes are rare.
shallow water mud and turbiditic mud. Such sam- The composition of the sand fraction in the
ples have a relatively high benthonic foraminifera turbiditic and hemipelagic muds differs from that
and shell fragment content and the degree of abra- in the Algéro-Balearic Basin described by Rupke
sion observed indicates that they have been re- and Stan1ey (1972). This difference reflects (I) a
worked to some extent prior to final deposition. higher terrigenous influx in the Strait of Sicily, as
Compositional plots of samples from this type of a result of proximity to land (i.e., important
mud are shown in Figure 17 A. Here the samples clastic influence) and (2) more extensive vertical
tend to be distributed on the 1eft side of the tri- bioturbation of the sediments. In some cases, it is
angu1ar diagrams. Figure 17s revea1s a re1ative1y difficult to distinguish between hemipelagic and
high biogenic fraction but a 1ow p1anktonic fora- turbiditic mud on the basis of composition because
minifera content in the sand fraction. of reworking by currents (see Table 3).
The turbiditic mud samples occupy a random
distribution and are not distinctly concentrated on
HEMIPELAGIC M UD TYPE the triangular representation in Figure 17A • How-
2
ever, this sediment type is better delineated in the
The hemipe1agic muds have a sand content of
triangular diagrams A 1 and B 1 in Figure 17; here
2.5% to 6.5%. This sand fraction is 80% to 99%
turbiditic mud occupies an intermediate position
biogenic and usually comprises more than 40% of
between hemipelagic and the shallow water mud.
planktonic foraminifera 3 (Figure 17, Tab1e 3).
The three types of mud appear transitional on the
Other components of the biogenic fraction include
basis of composition.
pteropods, mollusks, radio1aria, diatoms, sponge
spicules, ostracoda, fecal pellets, and benthonic
foraminifera. The inorganic clastic fraction con- SAPROPEL TYPE
sists of quartz, mica, pyrite, heavy minerals, glau-
conite, and volcanic ash. Fragments of pyritized The sand content in the sapropel type ranges
burrow tubes also are common in some samp1es. from 5% to l 0%, an d is mostly biogeni c (95%-
The compositional plots of this type of mud 1 00%). The inorganic fraction consists of authigenic
tend to be distributed in the lower right sector of minerals (pyrite and gypsum) and, to a lesser extent,
the triangular representation (Figure 17s). The mica. The calcareous sand fraction is dominated by
position occupied by this type of mud is distinct planktonic foraminifera (generally ~ 70%) and also
from that of the shallow water mud type in tri- includes plant fragments, pteropods, radiolaria,
angu1ar diagrams A 1 an d B 1 in Figure 17. and diatoms. A high proportion of foraminiferal
tests are pyritized. Ali these components indicate
a low detrital input during deposition, although
TuRBIDITic Mun TYPE norma! turbidites can be intercalated in the sapro-
pel deposits (van Straaten, 1972; Nesteroff, 1973).
The two end members of the turbiditic mud type
The characteristic composition of sapropel sam-
ples is shown in triangular representations s and
3 A list of planktonic foraminifera identified in cores KS 33, 1
78, and 105 and an evaluation of their vertical distribution are B2 in Figure I 7. Sapropel deposits are concentrated
presented by Blanc-Vernet et al. (1975). in the lower right corner of both types. of triangle.