Page 28 - Maldonado_Stanley_1976

24 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

well-defined sedimentary structures including cross- Muns.-The mud type is by far the most abun-
lamination and oblique lamination, graded bed- dant deposit in the cores studied. Neritic-bathyal
ding, and diverse types of parallel or ripple lamina- environment cores consist almost exclusively of
tion. The base of this type of layer usually is sharp mud. Genetically there are three main types of
and erosional, and locally displays scour and fili mud which can be recognized on the basis of sedi-
structures. mentary structures and composition. These are:
The fine silt deposits of this sediment type show shallow water mud, hemipelagic mud, and turbi-
structural and lithologic continuity with some ditic mud. In many cases these are transitional,
coarser deposits, i.e., they usually fine upward tex- and not clearly distinguishable.
turally (graded bedding) from silt to mud. The Shallow water muds collected at neritic depths
most common structures of the silts are parallel are structurally homogeneous and the only types of
and low-angle oblique lamination. Their sand con- sedimentary structures commonly recognized are
tent is low, usually less than 5% and frequently biogenic ones. This mud type may contain a rela-
only about l%· tively high (to 10%-15%) percentage of sand
The composition of this type is variable. There fraction, including either biogenic (mostly well-
are bioclastic and foraminiferal sands, where the preserved neritic molluscs and benthic and plank-
coarse fraction is dominated by biogenic carbon- tonic foraminifera) or clastic grains. The hemi-
ates andjor planktonic foraminifera; in other in- pelagic muds are homogeneous calcareous oozes
stances the sand consists almost entirely of volcanic which also lack well-defined primary sedimentary
ash. Volcanic ash layers also could be included in structures, including Iamination, and generaiiy are
this group, but their characteristic composition characterized by vertical bioturbation. The sand
and origin warrant their assignment in another fraction content, lower than in the shallow water
facies category. Sand composed in part of detrital mud, consists largely of calcareous planktonic com-
feldspathic grains is Iess common. ponents. The turbiditic muds are characterized by
VoLCANIC AsH.-Volcanic tephra layers are ob- a delicate basai lamination or small-scale biotur-
served in the deep basin cores, especially those bation visible in X-radiographs, and a smooth
from Linosa Trough. Although texturally analo- uniform aspect in split cores. They occasionally
gous to the former type, they stand out by their show continuity and gradation with the sand-silt
characteristic composition. These deposits do not sediment type in terms of structure and gross lith-
represent a major sediment type in Strait cores in ology (Rupke and Stanley, 1974).
terms of total thickness. SAPROPEL AND 0RGANIC OozE.-Sapropels are dis-
Two different types of layers containing vol- tinctive dark gray to black deposits which have
canic ash an d dust are distinguished: (l) air-borne been extensively studied in the eastern Mediter-
tephra layers, derived directly from ash flows, mud ranean (Olausson, 1961; Ryan, 1972; van Straaten,
flows, or base surges consisting predominantly of 1972; Nesteroff, 1973; Maldonado and Stanley,
volcanic vitric ash and variable amounts of mud; l 975; and others). Recently described sequences
and (2) layers of volcanic ash particles or turbidite from the Black Sea appear similar to Mediterra-
layers, which include a significant benthonic nean sapropels (Ross and Degens, l 974, uni t 2).
and planktonic calcareous bioclastic sand-size frac- This sediment type is commonly encountered in
tion. These two types can be differentiated on the the eastern Mediterranean basin from cores col-
basis of petrographic characteristics and primary lected a t depths exceeding 700 to l 000 m. Sapropels
sedimentary structures (compare the ash layers in are retrieved on the slope east of the Strait (LY
cores KS 69, KS 118, KS 120; cf. Figures 27, 31). II-3, 2432 m). However, cores in the Strait of
The carbonate-free tephra layers display a vertical Sicily proper (including the deep Pantelleria,
grain-size gradation (fining or coarsening upward), Malta, or Linosa troughs) and in the western Med-
parallel lamination, or in some instances they are iterranean have not recovered this type of deposit.
structureless. The mixed volcanic-bioclastic layers The typical sapropel is formed by an alternating
show well-developed structures and may be similar sequence of delicate horizontal laminae of white
to the sand-silt type sediments described in the pre- (coccolith rich) and black (largely mud) layers.
vious section. The sand-size fraction content is about l 0%, most of