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Publication authorized by the Director, U.S. Geological Survey

Abstract

Recognition of ancient barrier coastlines is largely dependent on criteria derived from the study of modern barrier coastlines, especially where interpretations are based on limited data.

Ancient barrier coastlines are of special interest to petroleum geologists, because potential reservoir rocks, source beds, and cap rock are found in close proximity.

Barrier islands are formed on broad, gently sloping coastlines which have an abundant supply of sand. The formation of the islands and their diverse character are controlled by the action of the wind and the sea, together with tectonism and sediment supply. Barrier islands are long and narrow and nearly straight or gently curved in outline. They generally are lenticular in cross section unless altered, after deposition.

Shoreface sediments which form the seaward margin of barrier islands are composed mainly of shelly and clayey sand that lacks lamination because of the activity of burrowing organisms. Beach sediments are generally composed of clean, well-sorted, gently dipping bedded sand. Sand dunes, if preserved, are important criteria, but their recognition is difficult because most of their sedimentary structures are not unique and are easily destroyed by erosion and vegetation. Tidal flats, which border the lagoon or bay margins, contain partly laminated sediments consisting mostly of clayey, silty sand. Algal mats, shells, and a variety of sedimentary structures, including ripple marks, slump structures, and mud cracks, are present in tidal flat sediments. Sediments deposited in tidal inlets and tidal deltas are diverse because these areas receive sediments from the lagoon, bay or salt marsh, and the open sea. Various current structures are common in the inlet sediments. Washover fans and flats may contain uniquely associated beds consisting of shelly sand on the bottom, pond deposits in the middle, and eolian sand on top. Sediments of eolian flats, which are generally covered with vegetation, consist mostly of structureless, fine-grained sand with some mottling caused by burrows and decaying plant roots. Soil horizons are occasionally found in these deposits. Lagoon, bay, and sound sediments are diverse in character, but generally are a mixture of sand, silt, and clay. They are finer near the center of the depositional basin, and they may contain peat, oyster reefs, abundant shells, and evaporite deposits.

Sedimentary environments of the modern barrier coastline may be distinguished by their biota. Similar interpretations are possible in ancient deposits if the fossils are preserved.

The internal structure of a barrier-island sand body depends on climate, sediment supply, tidal range, and tectonism; and the interplay of these factors produces islands of three basically different types: (1) prograding, (2) stationary, and (3) migrating landward.

Ancient barrier islands or related features have been recognized largely by their characteristic geometry and relation to strand lines. More recent attempts have utilized to a greater degree paleontologic and sedimentary analyses.

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