6.3 SEDIMENTARY MODELS
233
parts of the north German and Polish coasts and the Younghusband peninsula of South
Australia. These coasts do show, nevertheless, considerable input of sediment from rivers draining their hinterlands. Coastlines have been classified according to their tidal
range. Microtidal coasts have a tidal range of less than 2 m. Mesotidal coasts have tidal
ranges between 2 and 4 m. Macrotidal coasts have tidal ranges in excess of 4 m (Davies,
1973). Barrier islands are generally best developed where tidal range is relatively low.
6.3.2.6.1 Modern barrier coasts
Recent barrier islands and the processes that form them have been intensely studied and
debated (Davis, 1978; Swift and Palmer, 1978; Carter,1995). There is considerable controversy over the genesis of modern barrier islands. The two major mechanisms proposed are the progressive up-building of offshore bars and the submergence of coastal
beaches and dune belts. One of the problems of studying modern bars is that we live in
an ice age, albeit an interglacial, and eustatic shoreline changes have occurred several
times in the last million years. Modern shorelines show both raised and drowned beach
features and many modern barrier islands contain cores of older sediments. This is,
therefore, not the best moment in geological time to study barrier island formation. It
seems reasonable to postulate a polygenetic origin for barrier islands (Schwarz, 1971);
additional data and discussions are found in Hoyt (1967), Guilcher (1970), Steers (1971),
and King (1972).
Some of the best studied of modern barrier island complexes include the Gulf Coast
of west Texas in general, and Padre and Galveston islands in particular (Shepard, 1960;
Bernard et aL, 1962). Of the barrier coastlines of the eastern coast of North America,
Sapelo Island is one of the most studied (Hoyt et aL, 1964; Hoyt and Henry, 1967). Outside North America, the Dutch and German barrier coasts are some of the best known
(Horn, 1965; Van Straaten, 1965).
Coupling studies of these modern coasts with their ancient analogs, a well-defined
barrier island sedimentary model has been put together by Visher (1965), Potter (1967),
Shelton (1967), Davies et aL (1971), Davis (1978), McCubbin (1982), and Reading and
Collinson (1996). Essentially, a barrier island is a linear sand body exposed at high tide
that runs parallel to the coast, separating the open sea from sheltered bays, lagoons, and
tidal fiats (Fig. 6.42). In most modern barrier coasts, two high-energy environments alternate laterally with two low-energy environments. On the landward side is a fluvial
coastal plain of sands, silts, clay, and peats. This grades seaward, generally through salt
marsh deposits, into quiet-water tidal flats and lagoons (Black et al., 1998). The sediments of these environments consist of laminated, cross-laminated, and flaser-bedded
fine sands, silts, and clays. This zone is characterized by intense bioturbation, by shell
beds, often of oysters and mussels, and by upward-fining tidal creek sequences (e.g.,
Evans, 1965; Ginsburg, 1975).
The barrier island itself consists of a number of distinctive physiographic units. On
its landward side there may be a complex of wash-over fans and barrier flats formed
from sand washed over the barrier island during storms. The crest of the barrier island is often marked by eolian sand dunes, sometimes stabilized by soil horizons and
vegetation. This crestal area passes seaward through a beach zone to the open sea.
233
parts of the north German and Polish coasts and the Younghusband peninsula of South
Australia. These coasts do show, nevertheless, considerable input of sediment from rivers draining their hinterlands. Coastlines have been classified according to their tidal
range. Microtidal coasts have a tidal range of less than 2 m. Mesotidal coasts have tidal
ranges between 2 and 4 m. Macrotidal coasts have tidal ranges in excess of 4 m (Davies,
1973). Barrier islands are generally best developed where tidal range is relatively low.
6.3.2.6.1 Modern barrier coasts
Recent barrier islands and the processes that form them have been intensely studied and
debated (Davis, 1978; Swift and Palmer, 1978; Carter,1995). There is considerable controversy over the genesis of modern barrier islands. The two major mechanisms proposed are the progressive up-building of offshore bars and the submergence of coastal
beaches and dune belts. One of the problems of studying modern bars is that we live in
an ice age, albeit an interglacial, and eustatic shoreline changes have occurred several
times in the last million years. Modern shorelines show both raised and drowned beach
features and many modern barrier islands contain cores of older sediments. This is,
therefore, not the best moment in geological time to study barrier island formation. It
seems reasonable to postulate a polygenetic origin for barrier islands (Schwarz, 1971);
additional data and discussions are found in Hoyt (1967), Guilcher (1970), Steers (1971),
and King (1972).
Some of the best studied of modern barrier island complexes include the Gulf Coast
of west Texas in general, and Padre and Galveston islands in particular (Shepard, 1960;
Bernard et aL, 1962). Of the barrier coastlines of the eastern coast of North America,
Sapelo Island is one of the most studied (Hoyt et aL, 1964; Hoyt and Henry, 1967). Outside North America, the Dutch and German barrier coasts are some of the best known
(Horn, 1965; Van Straaten, 1965).
Coupling studies of these modern coasts with their ancient analogs, a well-defined
barrier island sedimentary model has been put together by Visher (1965), Potter (1967),
Shelton (1967), Davies et aL (1971), Davis (1978), McCubbin (1982), and Reading and
Collinson (1996). Essentially, a barrier island is a linear sand body exposed at high tide
that runs parallel to the coast, separating the open sea from sheltered bays, lagoons, and
tidal fiats (Fig. 6.42). In most modern barrier coasts, two high-energy environments alternate laterally with two low-energy environments. On the landward side is a fluvial
coastal plain of sands, silts, clay, and peats. This grades seaward, generally through salt
marsh deposits, into quiet-water tidal flats and lagoons (Black et al., 1998). The sediments of these environments consist of laminated, cross-laminated, and flaser-bedded
fine sands, silts, and clays. This zone is characterized by intense bioturbation, by shell
beds, often of oysters and mussels, and by upward-fining tidal creek sequences (e.g.,
Evans, 1965; Ginsburg, 1975).
The barrier island itself consists of a number of distinctive physiographic units. On
its landward side there may be a complex of wash-over fans and barrier flats formed
from sand washed over the barrier island during storms. The crest of the barrier island is often marked by eolian sand dunes, sometimes stabilized by soil horizons and
vegetation. This crestal area passes seaward through a beach zone to the open sea.