Cape Lookout Cuspate Foreland Depositional Systems
Thomas F. Moslow, S. Duncan Heron, Jr., 1986. "Cape Lookout Cuspate Foreland Depositional Systems", Southeastern United States: Third Annual Midyear Meeting, 1986, Raleigh, North Carolina, Daniel A. Textoris
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The sedimentary record of 130 km of microtidal (0.9 m tidal range) high wave energy (1.5 m average wave height) barrier island shoreline of the Cape Lookout cuspate foreland has been evaluated through examination of 3136 m of subsurface samples from closely spaced drill holes. Holocene sedimentation and coastal evolution has been a function of five major depositional processes: (1) eustatic sea-level rise and barrier-shoreline transgression; (2) lateral tidal inlet migration and reworking of barrier island deposits; (3) shoreface sedimentation and local barrier progradation; (4) storm washover deposition with infilling of shallow lagoons; and (5) floodtidal delta sedimentation in back-barrier environments.
Twenty-five radiocarbon dates of subsurface peat and shell material from the Cape Lookout area are the basis for a late Holocene sea- level curve. From 9000 to 4000 B.P. eustatic sea level rose rapidly, resulting in landward migration of both barrier limbs of the cuspate foreland. A decline in the rate of sea-level rise since 4000 B.P. resulted in relative shoreline stabilization and deposition of contrasting coastal sedimentary sequences. The higher energy, northeast barrier li.h (Core and Portsmouth Banks) has migrated landward producing a transgressive sequence of coarse-grained, horizontally bedded washover sands overlying burrowed to laminated backbarrier and lagoonal silty sands. Locally, ephemeral tidal inlets have reworked the transgressive barrier sequence depositing fining-upward spit platform and channel-fill sequences of cross-bedded, pebble gravel to fine sand and shell. Shoreface sedimentation along a portion of the lower energy, northwest barrier limb (Bogue Banks) has resulted in shoreline progradation and deposition of a coarsening-up sequence of burrowed to cross-bedded and laminated, fine-grained shoreface and foreshore sands. In contrast, the adjacent barrier island (Shackleford Banks) consists almost totally of inlet-fill sediments deposited by lateral tidal inlet migration. Holocene sediments in the shallow lagoons behind the barriers are 5-8 m thick fining-up sequences of interbedded burrowed, rooted and laminated flood- tidal delta, salt marsh, and washover sands, silts and clays.
While barrier island sequences are 10 m in thickness, inlet-fill sequences may be as much as 25 m thick and comprise an average of 35% of the Holocene sedimentary deposits. Tidal inlet-fill, back- barrier (including flood-tidal delta) and shoreface deposits are the most highly preservable facies in the wave-dominated barrier-shoreline setting. In the Cape Lookout cuspate foreland, these three facies account for over 80% of the sedimentary deposits preserved beneath the barriers. Foreshore, spit platform and overwash facies account for the remaining 20%.
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Southeastern United States–Guidebook is comprised of twelve field trips that were organized for the Third Annual Midyear Meeting held in Raleigh, North Carolina in 1986. The spectrum of geologic time represented in the Upper Precambrian, Lower Paleozoic, Mesozoic, Cenozoic and the very Recent. The geologic provinces are the Valley and Ridge, Blue Ridge Mountains, Piedmont, and the Coastal Plain. Besides North Carolina, trips include Virginia, South Carolina and West Virginia.