What Are Stratigraphic Surfaces?

Abstract:

The surfaces we find preserved in strata can represent everything from fully preserved, intact geomorphic surfaces to highly composite surfaces that bear little resemblance to the geomorphic surfaces that created them. The former tell us a great deal about morphology but little about kinematics, while the latter record kinematics but tell us relatively little about morphology. We present examples of both extremes as well as intermediate cases in which the stratal surfaces resemble geomorphic surfaces even though they are highly composite. Only fully intact geomorphic surfaces represent time horizons. We use a ‘coherent migration distance’ to measure the distance over which a particular geomorphic feature migrates before losing its identity and show that the greater the distance of coherent migration, the worse the correspondence between geomorphic and stratigraphic surfaces. Thus in a sense, well preserved geomorphic surfaces imply spatially incoherent kinematics. We also review measures for the degree of time transgression of surfaces and relate these to migration coherence, and show how serious errors of reconstruction can result from assuming that stratal surface represent geomorphology when they do not.