10.2 Sedimentation Rates in Various Environments
mentation rates are normally slower than the potential rates
are delta plains, lagoons, tidal flats, and shallow carbonate
platforrns.
The sedimentation rate data discussed in the following sections include scenarios in which SRa is equal
to SRp (e.g. water-filled deep basins) as well as cases
in which SRa
10.2 Sedimentation Rates in Various
Depositional Environments
(Overview)
10.2.1 Introduction and Overview
Sediments principally consist of two different components (Fig. 10.3):
- Allochthonous clastic or detrital sediments which
are derived from land sources (terrigenous material)
outside of the depositional area. These sediments
mainly consist of siliciclastic material but may include eroded bioclastic components from older rocks.
- Autochthonous, mostly biogenic sediments produced in the depositional area, including biogenic
skeletal material and organic matter. Locally, biochemically induced carbonates and chemically precipitated evaporites playa role.
Biogenic sediments are formed from the skeletal
remains of plants and animals. In addition, some carbonate may be precipitated from supersaturated surface waters, for example as a result of CO 2 uptake by
subaqueous plants. Most of the biogenic sediment is
produced by a few groups of organisms. In shallow
marine waters, benthic organisrns, such as various
reef-builders, algae, sea grass, molluscs, brachiopods, and bryozoa, are most important. Since the Mesozoic, biogenic sediments in deeper basins consist
mainly of the tiny remains of planktonic organisms
including calcareous nannofossils, foraminifera, and
pteropods, or siliceous tests of diatorns and
radiolaria. In their freshly deposited state these deepwater sediments form water-rich oozes with porosities of around 80%. Later, they compact and undergo
chemical-mineralogical diagenesis, leading to chalk
and pelagic limestone, or to porcellanite and chert
(cf. Sect. 5.3).
The sedimentation rates of autochthonous biogenic
sediments are controlled by the production and dissolution of skeletal carbonate and opaline silica. (Phosphatic remains playa minor role in rock-forming proces ses and are therefore not discussed here.) As regards dissolution in the water colurnn and at the sediment surface, calcareous and siliceous skeletal re459
mains behave quite differently. While carbonate skeletons are preserved in shallow and medium deep waters and only dissolve in deep water regions below
the lysocline and CCD, siliceous organism tend to
dissolve in shallow waters which are undersaturated
with respect to silica (cf. Sect. 5.3.5). This difference
in carbonate and silica dissolution contributes to the
general rule that shallow-water carbonates are commonly poor in opaline silica, and that pure siliceous
sediments mostly occur below the carbonate compensation depth in relatively deep water.
The highest biogenic sedimentation rates, apart
from reefs (see below), are observed below zones of
coastal and equatorial upwelling, in estuaries, marginal basins with sufficient nutrient supply by rivers,
and shelf. seas. Such sediments may accumulate at
rates up to 10 cmlka and in special cases up to 50 to
100 cmlka.
Very low biogenic sedimentation rates are found
below central parts of large ocean basins where fresh
supp1y and recycling of nutrients are limited. The
modem Sargasso Sea· in the Atlantic is an example of
this situation. When the CCD is elevated and silica
concentration in near-surface sea water is low, the
preservation of skeletal carbonate and silica may be
markedly reduced, and sediments devoid of carbonate andlor siliceous remains may be deposited. The
actual sedimentation rates of biogenic materials are
mostly equal to the potential rates, except for carbonate deposition in very shallow water arid tidal flats
(see below).
Biogenic and biochemically precipitated carbonate
in fresh-water lakes may form at rates of 10 to 100
cmlka and more. The highest rates are observed in
the littoral zones, due to the assimilating activity of
macrophytes and the abundance of gastropods and
molluscs (Kelts and Hsü 1978). Sedimentation rates
similar to thoseof carbonates can be expected for
lakes rich in diatoms (cf. Sect. 2.5).
10.2.2 Clastie Sediments
The sedimentation rates of clastic (mostly siliciclastic) sediments are controlled mainly by the size
and characteristics of the source area (cf. Sect. 11.2).
Furthermore, the distance of the depositional area
from the site of sediment input plays a role. Lakes,
marine prodelta slopes, and deep-sea fans fed with
sediments from a major point source may accumulate
large amounts of terrigenous material in short time
periods. Similarly, proglacial lowlands, lakes, and
marine basins in front of advancing glaciers and ice
shields frequently display very high sedimentation
rates (cf. Sect. 9.3.2). On the Barents shelf between
northem Norway and Swalbard, glacigenic sediments
accumulated at rates between 10 and 100 cmlka; the
Holocene sedimentation rates on the shelf off south-
mentation rates are normally slower than the potential rates
are delta plains, lagoons, tidal flats, and shallow carbonate
platforrns.
The sedimentation rate data discussed in the following sections include scenarios in which SRa is equal
to SRp (e.g. water-filled deep basins) as well as cases
in which SRa
10.2 Sedimentation Rates in Various
Depositional Environments
(Overview)
10.2.1 Introduction and Overview
Sediments principally consist of two different components (Fig. 10.3):
- Allochthonous clastic or detrital sediments which
are derived from land sources (terrigenous material)
outside of the depositional area. These sediments
mainly consist of siliciclastic material but may include eroded bioclastic components from older rocks.
- Autochthonous, mostly biogenic sediments produced in the depositional area, including biogenic
skeletal material and organic matter. Locally, biochemically induced carbonates and chemically precipitated evaporites playa role.
Biogenic sediments are formed from the skeletal
remains of plants and animals. In addition, some carbonate may be precipitated from supersaturated surface waters, for example as a result of CO 2 uptake by
subaqueous plants. Most of the biogenic sediment is
produced by a few groups of organisms. In shallow
marine waters, benthic organisrns, such as various
reef-builders, algae, sea grass, molluscs, brachiopods, and bryozoa, are most important. Since the Mesozoic, biogenic sediments in deeper basins consist
mainly of the tiny remains of planktonic organisms
including calcareous nannofossils, foraminifera, and
pteropods, or siliceous tests of diatorns and
radiolaria. In their freshly deposited state these deepwater sediments form water-rich oozes with porosities of around 80%. Later, they compact and undergo
chemical-mineralogical diagenesis, leading to chalk
and pelagic limestone, or to porcellanite and chert
(cf. Sect. 5.3).
The sedimentation rates of autochthonous biogenic
sediments are controlled by the production and dissolution of skeletal carbonate and opaline silica. (Phosphatic remains playa minor role in rock-forming proces ses and are therefore not discussed here.) As regards dissolution in the water colurnn and at the sediment surface, calcareous and siliceous skeletal re459
mains behave quite differently. While carbonate skeletons are preserved in shallow and medium deep waters and only dissolve in deep water regions below
the lysocline and CCD, siliceous organism tend to
dissolve in shallow waters which are undersaturated
with respect to silica (cf. Sect. 5.3.5). This difference
in carbonate and silica dissolution contributes to the
general rule that shallow-water carbonates are commonly poor in opaline silica, and that pure siliceous
sediments mostly occur below the carbonate compensation depth in relatively deep water.
The highest biogenic sedimentation rates, apart
from reefs (see below), are observed below zones of
coastal and equatorial upwelling, in estuaries, marginal basins with sufficient nutrient supply by rivers,
and shelf. seas. Such sediments may accumulate at
rates up to 10 cmlka and in special cases up to 50 to
100 cmlka.
Very low biogenic sedimentation rates are found
below central parts of large ocean basins where fresh
supp1y and recycling of nutrients are limited. The
modem Sargasso Sea· in the Atlantic is an example of
this situation. When the CCD is elevated and silica
concentration in near-surface sea water is low, the
preservation of skeletal carbonate and silica may be
markedly reduced, and sediments devoid of carbonate andlor siliceous remains may be deposited. The
actual sedimentation rates of biogenic materials are
mostly equal to the potential rates, except for carbonate deposition in very shallow water arid tidal flats
(see below).
Biogenic and biochemically precipitated carbonate
in fresh-water lakes may form at rates of 10 to 100
cmlka and more. The highest rates are observed in
the littoral zones, due to the assimilating activity of
macrophytes and the abundance of gastropods and
molluscs (Kelts and Hsü 1978). Sedimentation rates
similar to thoseof carbonates can be expected for
lakes rich in diatoms (cf. Sect. 2.5).
10.2.2 Clastie Sediments
The sedimentation rates of clastic (mostly siliciclastic) sediments are controlled mainly by the size
and characteristics of the source area (cf. Sect. 11.2).
Furthermore, the distance of the depositional area
from the site of sediment input plays a role. Lakes,
marine prodelta slopes, and deep-sea fans fed with
sediments from a major point source may accumulate
large amounts of terrigenous material in short time
periods. Similarly, proglacial lowlands, lakes, and
marine basins in front of advancing glaciers and ice
shields frequently display very high sedimentation
rates (cf. Sect. 9.3.2). On the Barents shelf between
northem Norway and Swalbard, glacigenic sediments
accumulated at rates between 10 and 100 cmlka; the
Holocene sedimentation rates on the shelf off south-