Relict Inlets: Preservation and Occurrence in the Holocene Stratigraphy of Southern Core Banks, North Carolina

ABSTRACT A complex depositional history, dominated by barrier retreat and inlet formation and migration, is found within the Holocene section of southern Core Banks, a transgressive, microtidal barrier island. The common occurrence of barrier sands overlying backbarrier silty-sands and salt marsh peats indicate that oceanic overwash and consequent landward migration have been active processes in this island's evolution. However, in five isolated sections, the Holocene section is completely reworked by the action of a migrating inlet environment. Drill hole transects located seaward of arcuate, relict flood-tidal deltas associated with historically recorded inlets show five inlet-fill bodies within the Holocene stratigraphy. Each inlet-fill body has a distinct stratigraphic sequence typically represented by three depositional facies: (1) inlet floor, a coarse shell and pebble gravel lag; (2) channel, medium- to coarse-grained sand and abraded shell material; and (3) inlet margin (spit platform), a clean, very fine- to medium-grained sand capping the sequence. The inlet sequences vary in thickness from 2.8 to 16.8 m and in maximum width from 0.72 to 2.1 km, making them readily comparable to the dimensions of modern active inlets along the Outer Banks and in other microtidal environment. This study documents the high preservation potential of inlet-fill bodies and the importance of the migrating inlet environment within barrier island stratigraphy. This depositional environment represents 14-16% of the Holocene sediments beneath Core Banks and may well represent a significant percentage of the material preserved within barrier systems in the rock record.