Plate 16

Primary and secondary inclination of bedding compared

Plate 16 should clarify the distinction between clinostratification and tectonic dip of bedding, while the next one will insist on problems of interpretation of primary inclination. The outcrop shows fluvial and lacustrine deposits of late Miocene age in Tuscany.

All beds dip to the left but with different angles. Three bedsets are distinguishable with angular contacts between them. In the lower and upper set, sandstones and clays alternate in plane-parallel beds; the intermediate set is made of sandstones only. What happened? Were all sets deposited on horizontal planes and tilted afterwards at different times? Or were some inclined from the start, and, in this case, which ones?

Let us rotate the book clockwise until the lower set of the photo becomes horizontal. The situation then appears like that of the previous plate, with a set of inclined beds sandwiched between two horizontal sets. The depositional dip of the intermediate set is toward the right. Therefore, the paleo-dip is opposite to the present one, which has been determined by tectonic forces and is, therefore, secondary. Now, the depositional setting can be interpreted with one of the models discussed before: frontal versus lateral accretion of a sedimentary body. If tectonic rotation had occurred in the opposite sense, i.e., clockwise, the primary dip of beds would have changed in steepness (increasing it) but not in direction.

When discussing vertical stratification (plate 9), I pointed out that tectonic tilting can occur clockwise or counterclockwise, and way-up criteria in beds can be applied to decide which the right way was. Here, the problem is compounded by that of recognizing a paleo-horizontal datum  among various possible candidates. One can proceed by trial and error, manipulating a photograph until it assumes a convincing geometric setting. Convincing means plausible  in terms of known sedimentological and stratigraphical models, such as those discussed so far. By the principle of parsimony, scientists give preference to the simpler explanation with respect to the more complicated one that comes to mind.