Criteria for Recognizing Ancient Barrier Coastlines: ABSTRACT

Abstract

Worldwide modern barrier coastlines constitute a minor part of the total coastlines of all the continents. The aggregate length of present barrier coastlines in the world is approximately 3,530 mi, distributed as follows: North America, 2,000 mi; Europe, 500 mi; South America, 350 mi; Africa, 300 mi; Australia, 200 mi; and Asia, 200 mi. Barrier islands commonly border coastal plains adjacent to broad continental shelves. They form in areas of abundant sand accumulation where longshore currents are prominent. Sandstone lenses which represent ancient barrier islands would be expected in thick wedges of interfingering terrestrial and marine sandstone, siltstone, and mudstone. Barrier islands of Pleistocene age have been recognized inshore from present End_Page 706------------------------------ coastlines, and drowned Holocene barrier coastline features have been described on the continental shelves. Pre-Holocene linear sandstone bodies resembling barrier islands have been described in ancient rocks of Pennsylvanian, Cretaceous, and Tertiary ages. Probable barrier island sandstone bodies in ancient rocks have been described by previous investigators on the basis of comparison with features of modern analogs: geometry, sedimentary structures within the sand lens, physical properties of the sand, and the nature of associated environments. Recognition criteria used in this report are based partly on previous work and partly on recent studies along the Texas and North Carolina coasts. Barrier islands are linear, have a length to width ratio generally greater than 10:1 and commonly are less than 60 ft thick. Padre Island, Texas, consists of four morphological units that have characteristic sedimentary structures: beach, foredune, barrier flats, and wind tidal flats--though the development of the foredunes and wind tidal flats changes considerably from north to south. Along the North Carolina coast, wind tidal flats are absent, but accretionary beach ridges are locally prominent. Superimposed on the islands of both coasts are storm washovers of hurricane origin that breach the foredunes and channel inlets that cross the island and connect the sea with the lagoon behind the islands. Beaches contain laminae of different thicknesses that dip principally seaward; the san is locally shelly and fine laminae of heavy minerals may be prominent. The foredunes are markedly cross-bedded in an oriented pattern that reflects strongly the predominant wind direction. Barrier flats are underlain by sand which ranges from structureless to highly laminated; vegetal remains are common. Wind tidal-flat sediments that border the lagoon are an interlayered mixture of sand beds containing some fine shell fragments, and laminae of clay and algal remains. Sand is fine grained throughout. However, shell fragments, locally abundant, exhibit greater variability in size, shape, and sorting. Sand which refills channel inlets ranges from horizontally bedded to structureless; this contrasts sharply with the cut-and-fill cross-bedded sand common in stream-channel deposits. The associated lagoon sediments are organic and calcareous mud which interfingers with barrier-island sand; the fauna is less diverse than that of the open sea and unbroken shells are abundant. Tongues of sand--washover deltas and fans which are built by storm flood tides--are prominent local features of the lagoons. Marshes overlying peat are characteristic of the inshore side of the bays along the North Carolina coast. The geometry and alignment of the barrier islands and the close association of the sand in the barrier island with the organic mud of the lagoon are the key factors for the recognition of a barrier coastline. Attendant washover deltas and fans, cross-cutting inlet fill, and associated biota are important supplementary aids. End_of_Article - Last_Page 707------------