242
6 DEPOSITIONAL SYSTEMS
1963; Emery, 1963). The relationship between the anoxic zone and the mudline obviously has important implications for the deposition of organic-rich muds that may become petroleum source beds. The sunlight that planktonic algae require for photosynthesis decreases with depth. The lower limit of the photic zone varies according to the
turbidity of the water, but is normally taken as about 80 m. The relationship between
the oxygenated zone and the photic zone is important because most carbonate sediment
is organically precipitated. The abundance of life is closely related to sunlight and oxygen (Fig. 6.48).
It is therefore appropriate to consider terrigenous and carbonate shelves separately.
In the same way that modern coasts are in some ways unsuitable for studying the evolution of barrier deposits, so are modern shelves not ideal for study as a key to their ancient counterparts. Modern continental shelves include environments of erosion, equilibrium, and deposition. These have inherited features gained when the shelves were
largely exposed due to low sea levels during glacial maxima. The present-day sediments
of the continental shelves were largely laid down in fluvial, periglacial and lagoonal environments. They have subsequently been reworked since the last postglacial rise in sea
level. It has been estimated that some 70% of the continental shelves of the world are
covered by these "relict" sediments (Emery, 1968).
Modern shelf deposits are of three kinds: those in which the Pleistocene sediments
have been reworked so as to be in equilibrium with the present hydrodynamic environment; those which are truly "relict" and have been essentially unaltered by the present
regime; and those which are now being buried beneath a mud blanket (Curray, 1965;
Klein, 1977; Johnson and Baldwin, 1986). Continental shelf deposits in equilibrium with
the present hydrodynamic environment tend to be shallow and inshore, whereas the unmodified relict deposits are largely preserved in the deeper waters near the continental
margins. Burial of relict Pleistocene deposits beneath mud occurs near the mouths of
major rivers in areas of low current velocities. These facies types contrast with the concept of a graded shelf, that is, a shelf whose sedimentary cover is in equilibrium with its
hydrodynamic regime and, ideally, one in which sediment grade fines seaward (Swift,
1969). The Bering Sea has been cited as one example of such a modern graded shelf
(Sharma, 1972). The erratic behavior of sea level in the last million years makes it difficult to use modern continental shelves to study the evolution of deposits through time.
Nevertheless, the processes and structures of individual environments can be studied
and applied to ancient sedimentary facies.
There are three main environments on modern shelves: an inshore zone of sandbars and tidal flats, an open marine high-energy environment, and an open marine lowenergy environment. In the ideal situation of a gentle uniformly dipping shelf, these
three zones would trend parallel to the shore in linear belts. In practice, because of the
complex tectonic and glacial histories of many modern shelves, these zones are erratically arranged. The inshore zone tidal flats deposit an overall upward-fining prograding sequence. The thickness of the individual units of this sequence is controlled by the
tidal range. It has been argued that the paleotidal range of ancient tidal flat sequences
may be measured from them (Klein, 1971; Fleming and Bartholoma, 1995). The relatively sheltered inshore environments of tidal flats are generally protected from the open
sea by a high-energy zone. This may be a barrier island, as already described. Alterna-
6 DEPOSITIONAL SYSTEMS
1963; Emery, 1963). The relationship between the anoxic zone and the mudline obviously has important implications for the deposition of organic-rich muds that may become petroleum source beds. The sunlight that planktonic algae require for photosynthesis decreases with depth. The lower limit of the photic zone varies according to the
turbidity of the water, but is normally taken as about 80 m. The relationship between
the oxygenated zone and the photic zone is important because most carbonate sediment
is organically precipitated. The abundance of life is closely related to sunlight and oxygen (Fig. 6.48).
It is therefore appropriate to consider terrigenous and carbonate shelves separately.
In the same way that modern coasts are in some ways unsuitable for studying the evolution of barrier deposits, so are modern shelves not ideal for study as a key to their ancient counterparts. Modern continental shelves include environments of erosion, equilibrium, and deposition. These have inherited features gained when the shelves were
largely exposed due to low sea levels during glacial maxima. The present-day sediments
of the continental shelves were largely laid down in fluvial, periglacial and lagoonal environments. They have subsequently been reworked since the last postglacial rise in sea
level. It has been estimated that some 70% of the continental shelves of the world are
covered by these "relict" sediments (Emery, 1968).
Modern shelf deposits are of three kinds: those in which the Pleistocene sediments
have been reworked so as to be in equilibrium with the present hydrodynamic environment; those which are truly "relict" and have been essentially unaltered by the present
regime; and those which are now being buried beneath a mud blanket (Curray, 1965;
Klein, 1977; Johnson and Baldwin, 1986). Continental shelf deposits in equilibrium with
the present hydrodynamic environment tend to be shallow and inshore, whereas the unmodified relict deposits are largely preserved in the deeper waters near the continental
margins. Burial of relict Pleistocene deposits beneath mud occurs near the mouths of
major rivers in areas of low current velocities. These facies types contrast with the concept of a graded shelf, that is, a shelf whose sedimentary cover is in equilibrium with its
hydrodynamic regime and, ideally, one in which sediment grade fines seaward (Swift,
1969). The Bering Sea has been cited as one example of such a modern graded shelf
(Sharma, 1972). The erratic behavior of sea level in the last million years makes it difficult to use modern continental shelves to study the evolution of deposits through time.
Nevertheless, the processes and structures of individual environments can be studied
and applied to ancient sedimentary facies.
There are three main environments on modern shelves: an inshore zone of sandbars and tidal flats, an open marine high-energy environment, and an open marine lowenergy environment. In the ideal situation of a gentle uniformly dipping shelf, these
three zones would trend parallel to the shore in linear belts. In practice, because of the
complex tectonic and glacial histories of many modern shelves, these zones are erratically arranged. The inshore zone tidal flats deposit an overall upward-fining prograding sequence. The thickness of the individual units of this sequence is controlled by the
tidal range. It has been argued that the paleotidal range of ancient tidal flat sequences
may be measured from them (Klein, 1971; Fleming and Bartholoma, 1995). The relatively sheltered inshore environments of tidal flats are generally protected from the open
sea by a high-energy zone. This may be a barrier island, as already described. Alterna-