Relict Sediments on Continental Shelves: A Reconsideration

Abstract

Relict sediments on shelves, originally defined as "remnant from a different earlier environment," are recognized by petrographic criteria (grain size, iron staining, etc.), fauna, and topography. Recent studies have revealed a second set of attributes which indicate that these deposits, although originating in an earlier environment, are dynamic systems which are undergoing modification in response to their present environment, especially the hydraulic regime, and are approaching a state of equilibrium with this environment. The modification may be simulated by means of a stochastic process model. A spectrum of modern shelf regimes and the resulting deposits is considered. The high-energy, tide-dominated shelf seas of western Europe have extensively reworked their Pleistocene and Holocene transgressive substrates, producing a constructional topography and regional textural gradients. Similar topography and textural gradients are reported from the tide-swept shoals and banks off northeastern North America and from farther south in the Middle Atlantic Bight, a wave-dominated shelf. Reworking in lower-energy environments such as the Gulf of Mexico may result only in textural mixing of the products of deposition of different periods of time and different sources. The reworked portions of relict sediments are thus a facies in transition, physically induced analogues of the chemically induced soil profiles of subaerial surfaces. While "relict sediment" is a valuable genetic name for the unreworked sediment type, "palimpsest sediment" is a convenient operational descriptive term for the reworked parts. A palimpsest sediment is one which exhibits petrographic attributes of an earlier depositional environment and, in addition, petrographic attributes of a later environment. All intermediate stages are possible from pure relict, through palimpsest sediments, to "modern" autochthonous sediments in which all of the earlier petrographic and physiographic attributes have been changed.