Miogeoclines (Miogeosynclines) in Space and Time

Abstract

Many continental margins are capped by wedge-shaped prisms of Cretaceous to Recent shallow-water marine strata. These were deposited on downflexing continental margins, presumably subsiding because of regional isostatic compensation caused by the growth of adjacent continental-rise prisms. We equate these continental terrace wedges with miogeosynclines of the past, which are wedge-shaped as now preserved but which probably never were synclinal in form; hence the shortened term "miogeocline." Modern miogeoclines thicken toward the ocean and terminate by "thickening out" against water at the continental slope; we presume that ancient ones did also. Ancient miogeoclines thicken toward, and abut, a deformed eugeosynclinal lithofacies. These we interpret to be collapsed continental rises, deposited synchronously with their adjacent miogeocline and later accreted to the continent. Miogeoclines probably have been formed by marginal sedimentation throughout geologic history, their outer limits marking former continental boundaries before the accretion of new fold belts. The Appalachian miogeocline may be one Paleozoic example, and the Millard miogeocline of the western United States may be another. More speculatively, Precambrian examples of miogeoclines may be provided by the Belt Series of Montana, the Huronian metasedimentary sequence abutting the Grenville fold belt in Canada, and the Witwatersrand Series in South Africa. A model for continental accretion results with the positions of earlier continental margins delineated by the thickened-out edges of ancient miogeoclines.