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A synthesis of drill-hole data from wave- and tide-dominated coastal settings suggests that 40% or more of clastic shoreline deposits are tidal-inlet related. Inlet channels rework adjacent barrier islands, replacing "classic" coarsening-upward shoreface sequences with fining-upward tidal-inlet deposits.
An antipathetic relationship between wave height and tidal range along the South Carolina to North Carolina coast results in distinct tide-dominated versus wave-dominated inlet sequences. In addition to hydrographic regime, pre-Holocene topography and sediment supply modify tidal-inlet sequences, geometries, and lithologies. Ephemeral, rapidly migrating wave-dominated inlets, filled by landward and longshore sediment transport, deposit a fining-upward sequence of: (1) inlet floor of coarse shell and pebble lag; (2) channel deposits of planar and trough cross-bedded sand and shell; and (3) a spit platform, composed of planar cross-bedded and horizontally laminated fine-grained sand. Channel migration and abandonment results in preservation of isolated shore-parallel (strike) wedge to l nticular-shaped inlet-fill sand bodies occurring randomly along the shoreline. Measurements of cross-sectional profiles from 12 wave-dominated relict inlet sand bodies reveals a consistent (1:125, 250,500...) thickness to width ratio.
Lower migration rates and bar-bypassing at tide-dominated inlet mouths concentrate inlet deposits in the updrift portion of a barrier island. Abandoned inlet channels exhibit symmetrical, U-shaped strike geometries and crescentic, concave-upward, dip geometries. The most seaward, tide-dominated inlet sequences become fine upward from basal trough and planar cross-bedded active inlet channel sand and shell to a trough cross-bedded and rippled ebb-tidal delta sand. Coarsening-upward foreshore sand and shell overlie abandoned inlet deposits. Landward, the overlying ebb-tidal delta and foreshore sands interfinger with wavy to lenticular-bedded silts and clays of the abandoned inlet fill and bioturbated salt marsh which form an impermeable updip seal over the inlet channel. Isolation of th se wave or tidally influenced inlets by paleotopography confines inlet deposition to a small area, causing vertical stacking of abandoned inlet channels.
Modern tidal inlets compose 5% of North Carolina's wave-dominated coast and 20% of the tide-dominated South Carolina coast; however, Holocene barriers in North Carolina contain 35% inlet-fill whereas South Carolina barriers average 15% inlet fill. Extensive inlet formation, migration, and abandonment account for this 1:7 ratio of modern wave-dominated inlets to relict inlet deposits. Less extensive migration of more stable tide-dominated inlets accounts for an approximate 1:1 ratio of modern inlets to preserved inlet sequences. With a higher preservation potential for inlet deposits over shoreface or foreshore deposits, fining-upward sand and/or mud-rich inlet sequences will dominate ancient clastic shorelines. Porous, fining-upward, quartz-rich inlet sand bodies are the most preserva le facies in barrier shoreline sequences and exhibit consistent thickness to width ratios and lateral geometries. These insights will serve as a valuable stratigraphic tool in the exploration of interdeltaic clastic reservoirs.
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