Recognition and Hydrologic Significance of Passive-Margin Updip Sequences: An Example from Eocene Coastal-Plain Deposits, U.S.A.

Abstract

ABSTRACT Sequence-stratigraphic interpretations are extended to a passive-margin updip setting using sedimentological observations from cores and well-to-well correlations in Eocene coastal-plain strata of South Carolina. In the succession studied, formation contacts tend to follow sequence boundaries even as lithologies vary within individual sequences. Sequence boundaries at the base of the Congaree Formation and near the top of the Tinker Formation formed during relative sea-level fall associated with second-order global cycles. These boundaries can be recognized clearly in both downdip and updip areas of the passive margin. Three other sequence boundaries, which occur at the base and top of the Warley Hill Formation and within the Tinker Formation, are interpreted as formed during relative sea-level fall associated with third-order global cycles. Hydrostratigraphic units in the strata studied conform to depositional patterns that were controlled by changes in sea level and sediment influx. Sand of the Gordon aquifer comprises a thick aggradational succession deposited predominantly in the nearshore-shelf environment as accommodation increased during relative rise in sea level. Because of high rates of sedimentation in the proximal setting, aggradational stacking of nearshore sands may be common to other updip marine strata in accommodation-dominated regimes. The overlying Gordon confining unit consists of marine-shelf clay and carbonate sediment deposited in response to maximum transgression. During subsequent relative sea-level fall, accumulation of shoreface deposits produced a progradational succession that is represented by the lower part of the Upper Three Runs aquifer. Unlike formation contacts, boundaries between the clastic aquifers and confining units do not follow the third-order sequence boundaries because of lithologic variations within the sequences. An increasing degree of breaching through the Gordon confining unit, which results in downward contaminant migration into the Gordon aquifer, occurs in the updip direction because of nearly continuous sand deposition. Near the base of the interval studied, contaminants migrate locally through the Crouch Branch confining unit because of erosional breaches formed by scour during sea-level lowstand. In this example and in other strata deposited in an updip passive-margin setting, the extent to which confining units are breached at sequence boundaries decreases in the downdip direction, resulting in better protection of the adjacent aquifer.