Plate 4

Bedding in sediment cores

Stratification is often ill-defined in sediments cored from the sea bottom. Only the superficial, still unconsolidated deposits are represented in cores. Moreover, corers do not usually penetrate thick sand layers. The difficulty in recognizing beds and layers in cores stems, then, from two reasons: the slight lithologic contrasts and the soft, prediagenetic state of materials (diagenesis causes hardening, and hardening is differential and enhances physical contrasts).

Cores are opened and photographed as a routine procedure in marine geoscience or, if X-ray equipment is available, they are radiographed before opening. The picture shown here is magnified as compared with that of figure 4 (see introductory section): the length of each segment is about 35 cm. The white borders are sawed plastic liners.

The sediment consists almost entirely of mud, except for some silty sand with laminations in the middle-upper part of the left segment. The sand is graded, and fades upward into mud; it lies upon a dark, sharp-based level rich in volcanic fragments. The overall graded unit is a turbidite layer, and its sharp base is probably erosional. Mud beds are distinguished by differences in color or tone, with darker hues indicating concentrations of organic matter.

Cores, of course, do not give us any information about the geometry of bedding; one can only appreciate vertical contacts and local thickness. In principle, a cored bed could be recognized and traced in a seismic section, but the bottom reflections tend to mask and "amalgamate" reflectors of the immediate subbottom, which are rarely resolved.

Coring site: deep sea plain of the Tyrrhenian Sea.

Photo: Institute for Marine Geology, C.N.R. 1970.