Concepts of Scale

Abstract

It has become a geological truism that many sedi­mentary units accumulate as a result of short inter­vals of rapid sedimentation separated by long intervals of time when little or no sediment is depo­sited (Ager 1981, 1993). It is also now widely realized that rates of sedimentation measured in modern depositional environments or the ancient record vary in proportion to the time scale over which they are measured. Sadler (1981) documented this in de­tail, and showed that measured sedimentation rates vary by 11 orders of magnitude, from 10⁻⁴ to 10⁷ m/ka. This wide variation reflects the increasing number and length of intervals of nondeposition or erosion factored into the measurements as the length of the measured stratigraphic record in­creases. Breaks in the record include such events as the nondeposition or erosion that takes place in front of an advancing bedform (a few seconds to minutes), the nondeposition due to drying out at ebb tide (a few hours), up to the major regional un-conformity generated by orogeny (millions of years). The variation in sedimentation rate also re­flects the variation in actual rates of continuous accumulation, from the rapid sand flow or grain-fall accumulation of a cross-bed foreset lamina (time measured in seconds), and the dumping of graded beds from a turbidity current (time measured in hours to days), to the slow pelagic fill of an oceanic abyssal plain (undisturbed in places for hundreds or thousands of years, or more). There clearly exists a wide variety of time scales of sedimentary processes (Fig. 3.1).