Plate 9

Verticalized parallel beds

When sedimentary beds are verticalized like this by tectonic stresses, good opportunities are created by differential erosion for observing structures on bed surfaces (see plates 82 to 103, in particular 88, 91, 98, 99, and 103), both in place and in detached slabs. The close alternation of softer and harder rock types is especially favorable in this respect.

This spectacular thin-bedded succession is made of: turbiditic sandstone beds (intermediate color and resistance to weathering), turbiditic mudstone beds (darker color, lesser resistance) and hemipelagic, marly limestone beds (whitish color, maximum resistance). Their mutual vertical relationships are those already described in plate 7, indicating the same depositional motives although in a different formation (Tertiary Flysch of Guipuzcoa, Spain). In spite of their modest thickness, the beds are remarkably continuous and sheetlike, as the pages of a book. They accumulated on a smooth, originally horizontal surface, similar to that of Modern deep sea plains; turbidity currents exhaust their energy there, abandoning their residual load of fine particles.

How did these horizontal planes become tilted like this? In particular, did they rotate clockwise or anti clockwise around their strike? The presence of interfacial structures allow us to give the right answer to this question. Look at up facing surfaces of bedding: if you find ripple marks on them, for example, they represent bed tops (compare with plates 3 and 8). If, on the other hand, you see sole markings, you are looking at original bottoms. Sole marks, in fact, are preserved at the base of beds, and belong for this reason to basal structures  (see figure 5 in the introduction). In conclusion, if the strata shown here are still upright in spite of their high inclination, they become younger to the left and have rotated anti clockwise. If they are upside down, the rotation occurred in the opposite sense.

Zumaya, near San Sebastian, NE Spain.