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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Author(s): Donald
J. Colquhoun
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The relation between coastal-plain terraces and their underlying formations has long been obscure. Atlantic coastal-plain terraces are underlain by "cyclic formations" in the sense proposed by Stephenson (1928). They consist of continental and marine contemporaneous cyclic sequences. The continental sequence consists of stream, fresh-water, marsh, lacustrine, estuarine, and deltaic sedimentary facies. The marine cycle consists of littoral, sublittoral, bar, barrier island-lagoon, and barrier-island tidal-marsh sedimentary facies. The two sequences are gradational within estuarine facies and disconformable along former strandlines.
The cyclic formations overlie an unconformity that has been cut into older stratigraphic units. Landward the unconformity surface consists of stream valleys and divides over which the continental sequence was deposited during a rise in sea-level. Seaward the unconformity has been modified by marine scour. The marine sedimentary sequence occurs on this scoured surface. Initial marine erosion proceeds landward during a rise in sea-level until estuaries are filled and sediments supplied to the ocean balance sediments being eroded. From this stage onward, during slow transgression through subsequent regression, coastal-plain accretion takes place seaward with construction of one or more barrier-island and tidal-marsh stages and seaward growth of deltas.
The terminal surface of the cyclic formation is the terrace which contains both continental and marine land forms representing the last processes operative in the area during regression. Thus geomorphology and pedology reflect the terminal nature of the underlying litho-, bio-, and environmental facies. The underlying stratigraphic facies illustrate the cyclic sequence of environmental stages necessary to develop the terminal land form.
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