Page 46 - Maldonado_Stanley_1976

42 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

serves to define sediment sequences. The X- in the mud are represented by black spots dis-
radiographs illustrated in this study (Figures 24- seminated in a Iight matrix (Figure 27B).
32) in most cases are negatives (identica! to the Tephra layers also may display horizontal lami-
originai radiograph, where silt and sand layers ap- nation (Figure 2'7c). However, the other types of
pear as light-toned bands); in a few cases we have structures recorded in the sand-silt sediments, par-
used positives (Gesite cores in Figures 27, 31, 32). ticularly cross and ripple lamination, are uncom-
The coarse calcareous sand type can easily be mon in these deposits. The type of tephra layer
recognized in both split cores and X-radiographs. structures are related to some degree with the dis-
It does not show any primary stratification or grad- tance from the volcano and the type of explosive
ing (Figure 24A,B), and well-preserved shell, small activity.
shell fragments, and detrital grains are well mixed Turbiditic ash layers, on the contrary, show the
texturally. Different degrees of abrasion are dis- same type of s tructures as reported from the sand-
played by the coarse calcareous debris of relict and silt type sediment (Figure 32B) and are difficult to
residua! origin. These characteristics are the result differentiate from typical terrigenous turbidites on
of intense vertical bioturbation (cf., Sarnthein, the basis of structures alone.
1972; Reineck, 1973; Kulm et al., 1975). Burrow- The shallow water, hemipelagic, and turbiditic
ing activity revealed in cores includes thick bur- mud types are differentiated in X-radiographs on
rows and pods filled with coarse calcareous the basis of the degree of bioturbation and the cal-
materials. careous biogenic fraction disseminateci in the mud
The sand-silt sediment type generally presents matrix. Shallow water mud is highly bioturbated
an excellent set of structures: parallel, ripple, and and contains abundant mollusc shells (gastropods,
cross lamination are the most common. Sets of l pelecypods) floating in the mud matrix (Figure
mm or less in thickness prevail, while the cosets 2'4E-F, arrows g, p). This mud types does not show
range from a few millimeters to severa! centi- any kind of lamination or other type of primary
meters (Figures 25B,E,F, 32B). The basai contact sedimentary structure. Biogenic structures are
of the sand an d •sii t layers records erosion, as indi- abundant and include various types of burrows
cated by scour-and-fill structures (Figure 25E). (mycellia in Figure 24c, arrow m; coils and spiral-
However, some of the finest grained silt deposits do shaped burrows; single cylindrical burrows; etc.).
not display an erosional basai contact (cf., Balearic Shallow water mud is usually gradually transi-
Basin plain deposits discussed by Rupke and Stan- tional with the sand-silt type of shallow water
ley, 1974). Vertical graded bedding is usually visi- deposits (Figure 24c,n); both types display similar
ble in the lower member of the Iayers; a generai features in the X-radiographs.
upward decrease in grain size within the sand to The hemipelagic mud type is characterized by
silt grades is also present. Parallel lamination of diverse types of biogenic structures (see Uniform
fine silt and mud (darker zones in the X- Sequences, next section), Iight specks dispersed in
radiograph negative prints) .are in continuity with the mud matrix and pteropod shells "floating" in
the basai silt or sandy Iayer (Figure 25F). Biogenic the fine matrix (Figures 25, 26.) The speckled as-
structures are rare in these deposits, except for pect is produced by Iarge numbers of foraminiferal
some so-called "escape" traces. tests and small fragments of pteropods (cf., Rupke
The two basic types of ash Iayers differentiated and Stanley, 1974).
earlier are also well marked in the X-radiographs. The turbiditic mud type displays a generally
The pelagic settling type of volcanic air-borne ash smooth, uniform aspect in the X-radiographs; its
is in most cases nonlaminated (Figure 27A,B); how- basai part, however, may be fìnely laminated (Fig-
ever, an upward coarsening or fining (i.e., an in- ure 25B,D). This mud type is generally not bio-
crease or a decrease in the amo un t an d size of ash turbated, but may show threadlike vertical tubes
particles in the mud) is often apparent (Figure (Figure 25c). Mycellia, although uncommon in
27 A,B). This graduai vertical change in the this type of mud, is sometimes observed (Figure
amount of ash particles in the mud is reflected in 25B).
X-radiographs by a graduai change in tonality, Sapropel Iayers are characterized by fine, hori-
from gray to black. Individuai coarse grains of ash zontal Iaminae and no biogenic structures (Figure