Page 50 - Maldonado_Stanley_1976

46 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

The sediment types within most sequences are ward increase in sand and coarser fractions (largely
gradational and transitional with each other; in bioclastic), including shells (largely molluscan).
some cases, however, there is a sharp distinction The basai mud of this sequence (Figure 24E,F,G)
between the sediment types. An example of the contains a neritic fauna! assemblage, including
latter case are the sapropel deposits that record benthic foraminifera. The transitional facies show
marked changes in environmental factors. A se- a graduai change between the extreme textural
quence can be related to a plexus of specific sedi- types. The total thickness of the sequence can ex-
mentary processes and thus it represents a natural ceed 5m (Figure 34, cores AS 6-7, 6-8). The
grouping of sediments (cf., Visher, 1965). Each upward-fining sequence shows an inverse develop-
sequence comprises several lithosomes ("a body of ment of texture and sedimentary structures (cf.,
sediment deposited under uniform physicochemi- Figure 34, upper part of core AS 6-7.).
cal conditions," American Geologica! Institute, Of the four major sequences, these show the
1972:413). most gradational transition between the different
Four major sequences are distinguished (Figure sediment types. In some cases, however, the bio-
29): (I) upward-coarsening or upward-fining se- clastic sand layer displays an irregular basai con-
quence; (2) uniform sequence; (3) turbiditic se- tact as a result of erosion or nondeposition. Where
quence; (4) sapropel sequence. Probably the most the base is sharply defined, evidence of biogenic
characteristic of these in the Strait of Sicily is the activity is preserved in the form of sand burrows
uniform sequence. The upward-fining and upward- in the underlying mud just below the coarse cal-
coarsening sequences are also well represented, careous sand Iayer.
particularly on the shallow platform and banks. Cores in the Strait Narrows are interesting in
that, although recovered from greater depths, they
display sequences similar to those described as typi-
U PWARD-FINING AND U PWARD-COARSENING
cal of shallow platform sedimentation. Core 140,
SEQUENCES
Verna 14, at 166 m (Figure 33), shows a marked
The upward-lìning and upward-coarsening se- upward-fining sequence, like those in other shal-
quences (Figure 29A) are best developed in the low depth bank environments, while core KS 12 at
shallow platfÒrm environment (8); they comprise 956 m (Figure 35) shows distinct alternations of
bioclastic sands (Figure 24A,B) and shallow water thick (to 90 m) coarse sands and bioturbated muds.
muds (Figure 24E,F,G). The transition between the The sands have been introduced periodically by
different terms of these sequences is often graduai turbidity currents (graded sand to mud units) and
and the limits are not well defined (Figure 24c,D). mass flow mechanisms (texturally clean grain flow
The upward-coarsening sequence shows an up- to muddy debris flow units). One core, 7, San Pablo

8 c D
~ (~
~~ --,..--
~ «
r( 1 l

-_"l-:_ _ Proto-Sapropel
~~
-,--
~
\~~ Hemipelagic mud
UPWARD-COARSENING ANO
UNIFORM SEQUENCE COMPLETE TURBIDITE SEQUENCE SAPROPEL SEQUENCE
UPWARD-FINING SEQUENCES
FIGURE 29.-Schematic representation of the major sediment sequences discussed in text.
(Symbols here are used in core logs (Figures 33-35) and on X-radiographs (Figure 24-32);
explanation in text.)