EARLY CAMBRIAN PROGRADATIONAL AND TRANSGRESSIVE SEDIMENTATION PATTERNS IN VIRGINIA: AN EXAMPLE OF THE EARLY HISTORY OF A PASSIVE MARGIN 1

Abstract

Occurrence of both the Hampton and Erwin Formations within a number of thrust sheets provides an oblique cross section of the paleoslope permitting an across-strike reconstruction of the Early Cambrian passive margin. Five environmental settings are reflected in facies comprising the Hampton and Erwin Formations. Sedimentation on the outer-shelf was dominated by geostrophic flows and suspension settling of mud under fair-weather conditions. Distal inner-shelf sediments resulted from storm-wave reworking below fair-weather wave base. Proximal inner-shelf facies consist of detritus transported from the nearshore and reworked in situ by the complex interaction of unidirectional and oscillatory currents generated by storms. Shoreface sediments display evidence of reworking by shore-parallel currents generated by shoaling fair-weather or waning storm waves. Tidal-flat sedimentation was dominated by flood-currents with late-stage ebb runoff. Shelf and shoreface facies are associated in five, 65 to 250 m-thick parasequences that coarsen and thicken upwards and contain thin fining- and thickening-upward intervals at their bases. Parasequenees within northwestern thrust sheets consist of outer-shelf, inner-shelf and shoreface facies, whereas outer-shelf and distal inner-shelf facies with condensed sections and glauconitic horizons predominate within southeasterly thrust sheets. The five stacked parasequences represent an overall ofllap package and comprise a highstand systems tract. The uppermost Erwin Formation is a fining- and thinning-upward parasequence that represents part of a transgressive systems tract. Tidal-flat and shoreface facies within northwestern thrust sheets unconformably overlie the highstand systems tract, whereas shelf facies within southeastern thrust sheets are conformable on the highstand systems tract. Stratigraphic evolution of the Hampton and Erwin Formations is related to different orders of relative sea-level change. Initial transgression is reflected in alluvial and tidal facies in the underlying upper Unicoi Formation and records the late-Precambrian to early Cambrian, second-order sea-level rise; a condensed section at the base of the Hampton Formation reflects maximum flooding. The Hampton Formation and most of the Erwin Formation document outbuilding of the passive-margin prism associated with a decrease in rate of rise of relative sea level resulting from a third-order eustatic fall superimposed on the second-order rise. Parasequences in the Hampton and Erwin Formations record lower-order eustatic oscillations superimposed on the third-order sea-level fall.