6.3 SEDIMENTARY MODELS
243
tively rock reefs and subaqueous sand shoals may provide protection. The high-energy
shelf environment may be one of erosion, where a marine terrace is cut across bedrock;
it may be an environment of equilibrium, where sand shoals migrate to and fro by tidal
scour; rarely is it an environment of deposition (Stride, 1982). Carbonate and terrigenous continental shelves are described separately.
6.3.2.7.2 Terrigenous shelves
Modern terrigenous shelves have been intensively studied (Swift et al., 1972, 1973, 1992;
Stanley and Swift, 1976; Bouma et al., 1982; McCave, 1985). Continental shelves can be
classified into tide, ocean, storm, and wave-dominated models (Johnson and Baldwin,
1986). The continental shelf of northwestern Europe is an example of a tidally dominated shelf. The southeast African coast is an example of a shelf dominated by oceanic
currents. The Oregon-Washington coast is an example of a storm-dominated shelf, and
the Baltic Sea is an example of a wave-dominated shelf.
The tide-dominated shelf of northwest Europe is particularly well known (e.g., Stride,
1963, 1982; Kenyon and Stride, 1970; Belderson et al., 1971; Banner et al., 1979). Much
of this shelf is floored by Pleistocene glacial and fluvioglacial deposits. Three major
zones can be recognized, one predominantly gravel floored, the second sand, and the
third mud (Fig. 6.49). The gravel floored parts of the shelf are those subjected to the
strongest tidal currents. They are essentially areas of erosion from which sand and mud
have been winnowed to leave a lag gravel deposit. These gravel seabeds are traversed
by ephemeral sand ribbons up to 2.5 km long and 100 m wide, which are aligned parallel to the axis of tidal flow.
The sand floored parts of the shelf are essentially environments of equilibrium. Much
sediment is moved to and fro, but there is little net sand deposition. The dominant bed
form of these areas is sand waves (Fig. 6.50). These are large underwater dunes with
heights of up to 20 m and wavelengths of up to 1 km. The surface of these sand bodies
is modified by smaller dunes and ripples. Acoustic and sparker surveys show low-angle
bedding within these sand bodies. Coring reveals cross-bedded sets of shelly sand.
Detailed studies of these sand bodies show a complex relationship between external
morphology and internal structure (Houbolt, 1968). There is no correlation between
dune height and water depth (Stride, 1970). The complex morphology and structure reflect the response of the sand waves to the ever-changing tidal flow regime.
Studies of analogous smaller scale inshore sand bodies have been carried out where
they are exposed at low tide. Bipolar cross-bedding has been observed dipping in the
two opposing tidal current directions (Hulsemann, 1955; Reineck, 1963, 1971). Ancient
analogs of these tidal sand bodies have been recognized by Narayan (1970), De Raaf
and Boersma (1971), Reineck (1971), and Swett et aL (1971).
Mud is the third sediment type to be deposited on the modern shelf of western Europe. This takes place in two settings. Tidal mudflats deposit upward-fining sequences
in which subtidal sands grade up through flaser-bedded and bioturbated muds into salt
marsh peats (Fig. 6.51). These intertidal deposits have been described from the Wadden
Zee of Holland, and from the Wash embayment of England by Van Straaten (1965) and
Evans (1965), respectively. Finally, mud settles out on the continental shelf below the
243
tively rock reefs and subaqueous sand shoals may provide protection. The high-energy
shelf environment may be one of erosion, where a marine terrace is cut across bedrock;
it may be an environment of equilibrium, where sand shoals migrate to and fro by tidal
scour; rarely is it an environment of deposition (Stride, 1982). Carbonate and terrigenous continental shelves are described separately.
6.3.2.7.2 Terrigenous shelves
Modern terrigenous shelves have been intensively studied (Swift et al., 1972, 1973, 1992;
Stanley and Swift, 1976; Bouma et al., 1982; McCave, 1985). Continental shelves can be
classified into tide, ocean, storm, and wave-dominated models (Johnson and Baldwin,
1986). The continental shelf of northwestern Europe is an example of a tidally dominated shelf. The southeast African coast is an example of a shelf dominated by oceanic
currents. The Oregon-Washington coast is an example of a storm-dominated shelf, and
the Baltic Sea is an example of a wave-dominated shelf.
The tide-dominated shelf of northwest Europe is particularly well known (e.g., Stride,
1963, 1982; Kenyon and Stride, 1970; Belderson et al., 1971; Banner et al., 1979). Much
of this shelf is floored by Pleistocene glacial and fluvioglacial deposits. Three major
zones can be recognized, one predominantly gravel floored, the second sand, and the
third mud (Fig. 6.49). The gravel floored parts of the shelf are those subjected to the
strongest tidal currents. They are essentially areas of erosion from which sand and mud
have been winnowed to leave a lag gravel deposit. These gravel seabeds are traversed
by ephemeral sand ribbons up to 2.5 km long and 100 m wide, which are aligned parallel to the axis of tidal flow.
The sand floored parts of the shelf are essentially environments of equilibrium. Much
sediment is moved to and fro, but there is little net sand deposition. The dominant bed
form of these areas is sand waves (Fig. 6.50). These are large underwater dunes with
heights of up to 20 m and wavelengths of up to 1 km. The surface of these sand bodies
is modified by smaller dunes and ripples. Acoustic and sparker surveys show low-angle
bedding within these sand bodies. Coring reveals cross-bedded sets of shelly sand.
Detailed studies of these sand bodies show a complex relationship between external
morphology and internal structure (Houbolt, 1968). There is no correlation between
dune height and water depth (Stride, 1970). The complex morphology and structure reflect the response of the sand waves to the ever-changing tidal flow regime.
Studies of analogous smaller scale inshore sand bodies have been carried out where
they are exposed at low tide. Bipolar cross-bedding has been observed dipping in the
two opposing tidal current directions (Hulsemann, 1955; Reineck, 1963, 1971). Ancient
analogs of these tidal sand bodies have been recognized by Narayan (1970), De Raaf
and Boersma (1971), Reineck (1971), and Swett et aL (1971).
Mud is the third sediment type to be deposited on the modern shelf of western Europe. This takes place in two settings. Tidal mudflats deposit upward-fining sequences
in which subtidal sands grade up through flaser-bedded and bioturbated muds into salt
marsh peats (Fig. 6.51). These intertidal deposits have been described from the Wadden
Zee of Holland, and from the Wash embayment of England by Van Straaten (1965) and
Evans (1965), respectively. Finally, mud settles out on the continental shelf below the