Page 52 - Maldonado_Stanley_1976

48 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

ding can be ascribed to dwelling structures; the 1974), have been suggested as a major factor in
single cylindrical type may be feeding andjor deepsea sediment deposition although it is ques-
dwelling structures, while the spirai pattern bur- tionab1e that they wou1d result in graded deposits.
row may represent travel traces (Seilacher, 1953; Other proposed mechanisms are low-density and
Frey, 1973). gravity-assisted flows carrying fine-grained sedi-
The processes that result in homogeneous se- ments to deep marine environments (Moore, 1969;
quences are either of primary or secondary origin. Stan1ey et al., 1970, 1971; Huang and Stanley,
Homogeneous sediments result from regular, uni- 1972; van Straaten, 1972).
form deposition or very high sedimentation rates The base of turbidites in Strait basins sometimes
( = primary type, cf. Moore an d Scruto n, 1957). includes calcareous sand, but more often the turb~­
Homogeneity of deposits can also result by a total dites consist almost completely of silt and day.
destruction of minor internai structures by bur- Compositionally, the bioclastic sand fraction in
rowing organisms (= secondary origin); this process turbidites is comparable to sands on the shallow
has been described in the Gulf of Mexico (Moore and neritic-bathya1 p1atform above the basins but
and Scruton, 1957). The presence of poorly pre- contains a higher percentage of planktonic
served turbiditic layers intercalated with uniform foram in i fera.
sequences and a rate of sedimentation which is not In some cores, particu1arly those in the Linosa
particularly high suggest that the latter origin is of Trough, tephra (ash) layers are important (Fig-
greater importance in the Strait of Sicily. ure 27, 31); a number of these sand-size deposits
also present typical turbidite structures including
graded bedding (Figure 31A). Although some vol-
TURBIDITIC SEQUENCE
canic sands have been transported into the basins
The most distinctive sediment types in basin by turbidity currents, we suggest that graded vol-
cores (environments 6 and 7) are the turbiditic canic 1ayers a1so can form by latera! wind and
units alternating, with hemipelagic mud. The tur- water transport of ash and subsequent settling of
bidites include classic sand and silt turbidites (Fig- these particles through the water column.
ures 30, 31) gisplaying various terms defined by Some layers composed largely of ash particles
Bouma (1962) and others, as well as mud turbi- show the typical sequence of turbidite structures
dites, T e (t>, as described in detail by Rupke and (Figure 31A, 108-30 cm; 31B, 530-508 cm; Fig-
Stanley (1974). Only rarely is the complete Bouma ure 35). Sarnthein and Bartolini (1973) discussed
sequence (Figure 29c) observed; one example of a this type of sedimentation in the Tyrrhenian Sea.
sequence of Ta-b-c-d units is shown in Figure 30. In contrast, air-borne tephra layers lack the verti-
Most of the turbidites in Strait basins are base-cut cal sequence of primary sedimentary structures
units of the Bouma division, i.e., Tb-c-d, Td associated with the typical turbidites although
(Figure 25). they may display reversed or norma! grading
The mud turbidite sequences are important in (either coarsening upward, Figure 27s, 345-328
this area. These display ali of the characteristics of cm or fining upward, Figure 27A, 194-187 cm and
sand turbidites including fine lamination (analo- Figure 31A, 115-111 cm) or horizontal lamination
gous to the d-division of sand-silt turbidites), (Figure 27c, 150-143 cm). These two structures
vertical sorting of the components, and graded bed- reflect different types of explosive activity.
ding. Evidence for this type of fine-grained tur- The adjacent is1ands of Linosa (di Paola, 1973)
bidite has been provided during the past few years and Pantelleria (Villari, 1969) shed some informa-
by severa! authors (van Straaten, 1970; Piper, tion on this matter. Pantelleria, for examp1e, is
1973; Rupke and Stanley, 1974; etc.). Neverthe- composed of a large number of interbedded ignim-
less, it should be noted that other depositional brites, and both we1ded and unwelded pumice fall
mechanisms have been proposed for vertically deposits an d some well-stratified fine ash (R.S.J.
graded pelitic sediments, in addition to pelagic Sparks, pers. comm.). Gas-b1ast eruptions of the
settling and turbidity currents. For instance, nor- P1inian or sub-P1inian type are known to generally
ma! bottom currents, and in particu1ar geostrophic produce poorly stratified and often reversely graded
currents (Heezen et al., 1966; Flood and Hollister, deposits (Sparks et al., 1973; Walker, 1973). Well-