3 Coastal and Shallow Sea Sediments
3.3 Sediments of Shallow Seas
(Siliciclastics)
3.3.1 Introduction
3.3.2 Factors Controlling Shallow-Sea Sediments
3.3.3 Facies Models for Shallow Seas
High Terrigenous Input
Low Terrigenous Input
3.3.4 Modifications ofthe Facies Models
Relative Sea-Level Change and Other Factors
Sediment-Starved Shelves
3.3.5 Large-Scale Shallow-Marine Sand bodies
3.3.6 Summary (Siliciclastic ShelfSediments)
3.3.1 Introduction
One can distinguish between two types of shallow
seas (Fig. 3.19; cf. Sect. 1.2):
- Epeiric or epicontinental seas which develop on
continental crust and are bordered on several sides by
land areas.
- Marginal or pericontinental seas (shelf seas) 10cated on top of transitional crust along the boundary
between continents and deep oceans. This type corresponds with the present-day shelves, particularly
those on passive continental margins.
The water depth of both types of shallow seas ranges
from 10 to 20 m near the coast (apart from the
nearshore areas) to some 100 m. The shelf edge or
shelf break of modem continental shelves is commonly 150-200 m deep. Some subbasins on shelves
and in epicontinental seas can attain greater depths.
Both types of seas are fully marine, and deviations
from the mean salt content of the open sea are limited
(cf. Sect. 5.2). However, basins in a transitional stage
between semi-enclosed epicontinental seas and adjacent seas (cf. Chap. 4) tend to become brackish or
hypersaline depending on the climate of the region.
The physical processes briefly described above
(Sects. 3.1 and 3.2) also largely control the sediment
dispersal and sedimentary structures of shallow-marine environments. Wind-generated waves and currents as well as tidal currents can affect the deposition and reworking of sediment even in relatively
deep water. In addition, storm-generated density currents and oceanic currents of different origin playa
role (cf. Sect. 5.2). Nevertheless, the hydrodynamic
regimes of the two types of shallow seas may show
significant differences:
- Continental shelves are normally fully exposed to
the high-energy conditions of large neighboring
oeeans. They may be affected by distant storm centers and long-distant oceanie eurrents, whereas
epieontinental seas are often better protected from
these influences (Fig. 3.19). If epicontinental seas are
partly enelosed, their areal extent is not sufficiently
large for the initiation of long, high-energy waves or
significant tidal waves. Although, for example, the
North Sea and the Bering Sea are situated on continental crust, they are not typical epieontinental seas
in this sense.
- Shelf seas often lose a great part of their sediments
(sand and finer grained material) into deeper water
by traction eurrents, suspension currents, and gravity
mass movements. This occurs less in typical, widely
extended epicontinental seas, although even these
basins collect thicker sediments in topographic deeps
than in shallower regions.
- Shelf seas normally exhibit stronger subsidence,
partieularly during their early stages of basin development, beeause they evolve on thinning transitional
crnst. They therefore have the potential for aecumulating thieker and possibly other sedimentary sequenees than epicontinental seas (cf. Sect. 12.2).
3.3.2 Factors Controlling Shallow-Sea Sediments
Sediments of shallow seas oceur in all geologieal
epoehs all over the world. They probably constitute
the most important group of all sedimentary rocks,
but they vary signifieantly from loeation to location
for the following reasons:
- Marked differenees in the input of terrigenous material from neighboring land areas due to the presence or absence of large river deltas, the size, nature and climate in the source areas relevant to the
loeation studied.
- Variations in biogenie produetion.
- Different hydraulic regimes (low versus high-energy; tide- versus wave-dominated, and various
combinations of these factors).
3.3 Sediments of Shallow Seas
(Siliciclastics)
3.3.1 Introduction
3.3.2 Factors Controlling Shallow-Sea Sediments
3.3.3 Facies Models for Shallow Seas
High Terrigenous Input
Low Terrigenous Input
3.3.4 Modifications ofthe Facies Models
Relative Sea-Level Change and Other Factors
Sediment-Starved Shelves
3.3.5 Large-Scale Shallow-Marine Sand bodies
3.3.6 Summary (Siliciclastic ShelfSediments)
3.3.1 Introduction
One can distinguish between two types of shallow
seas (Fig. 3.19; cf. Sect. 1.2):
- Epeiric or epicontinental seas which develop on
continental crust and are bordered on several sides by
land areas.
- Marginal or pericontinental seas (shelf seas) 10cated on top of transitional crust along the boundary
between continents and deep oceans. This type corresponds with the present-day shelves, particularly
those on passive continental margins.
The water depth of both types of shallow seas ranges
from 10 to 20 m near the coast (apart from the
nearshore areas) to some 100 m. The shelf edge or
shelf break of modem continental shelves is commonly 150-200 m deep. Some subbasins on shelves
and in epicontinental seas can attain greater depths.
Both types of seas are fully marine, and deviations
from the mean salt content of the open sea are limited
(cf. Sect. 5.2). However, basins in a transitional stage
between semi-enclosed epicontinental seas and adjacent seas (cf. Chap. 4) tend to become brackish or
hypersaline depending on the climate of the region.
The physical processes briefly described above
(Sects. 3.1 and 3.2) also largely control the sediment
dispersal and sedimentary structures of shallow-marine environments. Wind-generated waves and currents as well as tidal currents can affect the deposition and reworking of sediment even in relatively
deep water. In addition, storm-generated density currents and oceanic currents of different origin playa
role (cf. Sect. 5.2). Nevertheless, the hydrodynamic
regimes of the two types of shallow seas may show
significant differences:
- Continental shelves are normally fully exposed to
the high-energy conditions of large neighboring
oeeans. They may be affected by distant storm centers and long-distant oceanie eurrents, whereas
epieontinental seas are often better protected from
these influences (Fig. 3.19). If epicontinental seas are
partly enelosed, their areal extent is not sufficiently
large for the initiation of long, high-energy waves or
significant tidal waves. Although, for example, the
North Sea and the Bering Sea are situated on continental crust, they are not typical epieontinental seas
in this sense.
- Shelf seas often lose a great part of their sediments
(sand and finer grained material) into deeper water
by traction eurrents, suspension currents, and gravity
mass movements. This occurs less in typical, widely
extended epicontinental seas, although even these
basins collect thicker sediments in topographic deeps
than in shallower regions.
- Shelf seas normally exhibit stronger subsidence,
partieularly during their early stages of basin development, beeause they evolve on thinning transitional
crnst. They therefore have the potential for aecumulating thieker and possibly other sedimentary sequenees than epicontinental seas (cf. Sect. 12.2).
3.3.2 Factors Controlling Shallow-Sea Sediments
Sediments of shallow seas oceur in all geologieal
epoehs all over the world. They probably constitute
the most important group of all sedimentary rocks,
but they vary signifieantly from loeation to location
for the following reasons:
- Marked differenees in the input of terrigenous material from neighboring land areas due to the presence or absence of large river deltas, the size, nature and climate in the source areas relevant to the
loeation studied.
- Variations in biogenie produetion.
- Different hydraulic regimes (low versus high-energy; tide- versus wave-dominated, and various
combinations of these factors).