Page 22 - Maldonado_Stanley_1976

18 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

parent uniformity and lack of stratification (i.e., in seismic profiles (Figure 5); they have been in-
core KS 53). However, it is possible to individu- terpreted as grabens related to postorogenic fault-
alize some vertically graded mud turbidite se- ing (Zarudzki, 1972; Finetti and Morelli, 1972a).
quences on the basis of X-radiography. The reflec- These deep basins have trapped a thick sequence
tors on the 3.5 kHz records are more apparent in of unconsolidated sediments; approximately 1000
this environment than on the flatter platform. As m of sediments (to about 1.2 seconds penetration)
in the case of environment 4 these reflectors do not are measured in the Malta Trough (Finetti and
appear to correlate with prominent Iayers in the Morelli, 1972a). The bottom is a smooth, flat sur-
core s. face resulting from sediment accretion. The strata
BAsiN (Environments 6, 7).-This zone includes pinch out sharply against the walls of the depres-
the major elongate, troughlike basins of Malta, sion, and no prominent rise is dev.eloped at the
Pantelleria, and Linosa, as well as small basins oc- foot of the slope (Figure 5, a; Figure 13n 2 '~ c).
curring either as deep depressions on the conti- Cores collected in the deep basins show distinct
nental borderland and on the slopes of the three stratification and the most diverse assemblage of
major deep troughs. Two subdivisions are made, sediment types observed in the Strait. The most
based on the depth and aerial importance of these characteristic types are turbidite sequences, but
basins. terphra (ash) Iayers are also important locally.
Intermediate Depth Basin (Environment 6): Bioclastic sand layers are present, usually at the
The intermediate depth basins are small depres- base of the turbidite sequences. Evidence of slump-
sions, partially enclosed and characterized by a ing is also noted in some basin cores (i.e., core
considerable thickness of sediment. Figure 13 (A~ 139, Verna 14). Orderly layering and latera! con-
km 480-495) shows one of these basins located on tinuity of strata are apparent on the 3.5 kHz rec-
the slope of the Pantelleria Trough. ords (Figure 13).
Visual examination of split cores collected in SHALLOW PLATFORM (Environment 8).-About
this environmen.t (i.e., cores LY II-5, KS 100, KS 45% of the surface of the Strait is shallower than
104) reveals sedimentary sequences almost as 200 m. The seismic profiles show that the shallow
uniform as .those in environments 4 and 5. platform, for the most part, is covered by a con-
X-radiographs, however, show enhanced stratifica- siderably reduced unconsolidated sediment cover.
tion attributed to turbidite sequences and a de- The unconsolidated strata are gently tilted, tectoni-
crease in the degree of bioturbation, particularly cally offset, and truncated (Figure 14, arrow a).
in the deeper basins. Adventure Bank (km 630-718, Figure 14) is es-
Core KS 12, located in one intermediate depth sentially a horst structure consisting of Tertiary
basin in the Strait Narrows, is exceptional because and Mesozoic deposits. Well-defined terraces are
it shows well-marked stratification and includes cut at about 107 (± 3) m (arrow b) and at 140
severa! coarse layers of bioclastic sand. These char- ( ± l O) m ( arrow c) o n some bank margins. The
acteristics, attributed to the particular geographic terrace at 140 m forms a gently dipping seaward
position of this core, are discussed later. slope, which may represent a foreshore surface.
Deep Basin (Environment 7): Three narrow, Most of the bank surfaces are characterized by a
deep, elongate basins, accounting for somewhat Iess gentle slope, interrupted by small mounts and
than three percent of the Strait area, occur in the gentle depressions. This topography is largely the
.center of the Strait and ali three parallel its result of alternating erosion and deposition related
northwest-southeast trending axis: (l) Malta, 150 to the Quaternary oscillations of sea level; recent
km long and 30 km wide; (2) Pantelleria, 90 km structural activity, including diapirism and vol-
long and 30 km wide (Figure 13c); (3) Linosa, canism, also has affected this zone. In this respect,
75 km long and 17 km wide. Respective depths are submarine mounts on the northern Adventure
about 1700, 1300, and 1600 m. Other basins, such Bank have been interpreted as diapiric structures
as the one west of Marettimo Island (Figure 13B ') (Zarudzki, 1972), and the southeast extension of
2
also have been sampled (cf. core LY II-6). this bank is interpreted as the most active volcanic
The three deep, troughlike basins stand out by area in the Strait (Finetti and Morelli, 1972a;
their straight, fault-bounded steep walls apparent Zarudzki, 1972).