Page 4 - TonarelliTurgutcan1992

most heterogeneous; Local depositional controls
would seem to be important through al least the
history of deposition seen in these uppermost
sediments. Calcium carbonate varies between
91.0% in grab 19/76 and 98.2% of grab 17/76.
j. - .. ··-ì ·- ---.. .. ....... .. -- " "~. ! ~.. .. _ ...l
· Core l Position Date , rather than storms. Lack of continuity at these
l ; ·133. 'i . 35°19.4''N -·:· 24;8-71 " ·
!ml
65
't34 ·-'~. 12°40 4' E --:;~· 25·8·71 .. 65
"35°18:0, N
; 12•35.0· E !
141 . . 35•t4.7' N 1 01-3-73 89
· 12•51.2' E ,
185 , . 35°20.7' 'N ; 03·8·82
. 12"35.2' E . 63
64
186 : · 35•22·:o N . .. '07-8-82 125
12°41.9' E !
187 35°22.0' N 1 08·8·82 ..
12°50.7' E •
252 35°15.3' N ' 20-4-91 62
12°46.0' E
253 35°21.3' N 02-5-91 24
12°34.8' E
Table 5.. Lampedusa Banl<, Core samples
SEDIMENT SIZE RANGES ANO
COMPOSITION
The shallow water sediments of the
Adventure Banl< and the Lampedusa Banl< are
very similar, but not the same (Table 7) . Samples
from each area form two closely related groups.
The sediments are mainly composed of gravely
sand and sand. Lampedusa Banl< appears to have
a slightly higher percentage of sand fraction (at
about 10%), but otherwise the sed iment size
ranges in both areas are almost indistinguishable.
In addition, percentages of CaC03 are virtually the
same in the two areas. There may be more grave!
horizons present in the uppermost sediments on
the Adventure Bank. There is the same
relationship between mean size (phi) and
carbonate conteni of the sediment in both areas.
Where the average sediment particle size is low
(high phi), for instance in a clay-rich sample, the
quantity of carbonate is low. Conversely, where
particle size is larger, carbonate comprises a larger
part of the total specimen. This relationship holds
for both sand and grave! fractions where many of
the grains can be identified as being shell,
phytoplanl<ton, or calcareous algae (i .e .
Lithothamnia) debris.
Coarse layers in ceres from both the
Adventure and Lampedusa Banks presented the
usual problems for particle size analyses in that
much of the detritus is composed of strongly
angular shell fragments, which largely retain their
own sectional shape, having not yet been
reduced to relatively equi-dimensionai clasts. This
is typical for areas where shelly bieta are strongly
active and sediment supply from other sources is
minima!. Although these fragments are referred to
as grave! in analyses, simply because of their
ma~imum size: their irregularity in fact confers a
s~d1ment fabnc complexity that is not seen in
e1ther the west-central Malta Plateau or the
dee~er waters where pelagic sedimentation
dommates.
Sedimentary structures, such as
convolution, bed bottom compaction structures,
or bed flow features, are absent from our ceres.
Bedding appears to be essentially planar, ~ith
hints of minor erosion below more coarse-gramed
horizons that could be interpreted as storm
activity. However, currents in the area are known
to be high (Colantoni, et al., this vol.) and these
erosive horizons and their overlying coarse-
grained beds may be due to local current activity,
!ml ~ high energy sedimentary horizons between our
ceres, suggests that current, rather than storm
1.70 . ·~ activity was responsible for their formation. Storm
activity could be expected to form more
widespread deposits . Persistence of s~ell
' fragments in individuai ceres (187) and more fme
1.30 -·; grained materia! in other ceres (252) suggests that
suggests that there has been some stability in the
, location of active shell banl<s and locallow-current
sites in al least the uppermost sediment sampled
2.50 ··;.
by us.
In strong centrasi to the Adventure and
.!1.40
Lampedusa Banks, the uppermost sediment of
1.80... ; the centrai Malta Plateau is composed mainly of
clayey silt with consistently higher mean phi values
(smaller particle size) . Sediments in the centrai
Malta Plateau also have between one-half and
one-third as much carbonate as on the shallower
banl<s where shelly carbonate materia! is actively
1.13 being produced. The generai relationship
between mean particle size and carbonate
composition on the Ad•;enture and Lampedusa
2 .3 0 Banks, however, is maintaired on the Malta
Plateau , where the carbonate·rich sands and
gravels are not present. Where particle size
fractions are smaller on the shallov1er banl<s,
carbonate conteni is lower .
Bioturbation is common in the ceres from
the west-central Malta Plateau. Branching filled
tubes containing more dense and sl ightly more
fine grained materia! are common, and whole areas
locally lack fine·grained parallel lamination while
displaying an almost concretionary pattern.
Reduction spots may indicate local fecal deposits
that have been oxidized. Some compaction
structures occur al the base of coarser beds, but
have noi developed relief of more than 0.5 cm.
These loading structures appear to have no
directional or slope orientation, which is not
surprising considering their gentle manner of
deposition. the indicated lack of strong currents,
and the extremely gentle slope.
The long core in the Pantelleria Basin is
dominated by clay with minor silt-rich horizons.
Carbonate conteni of the sediment is similar lo the
sediments of the centrai Malta Plateau, confirming
the relationship between mean particle size and
carbonate conteni seen in the shallower water
areas. Local sources of coarse carbonate materia!
in the silt beds, which is especially common on thé
Adventure and Lampedusa Banks, would appear
to be the provenance for the more coarse-grained
beds, even in the basin centre.
126