184
6 DEPOSITIONAL SYSTEMS
Table 6.2
Example of the Classical Type of Classification of Sedimentary Environments
Continental
Transitional
Marine
Terrestrial
Aqueous
Desert
Glacial
Fluvial
Paludal (swamp)
Lacustrine
Cave (spelean)
Deltaic
Estuarine
Lagoonal
Littoral (intertidal)
Reef
Neritic (between low tide and 200 m)
Bathyal (between 200 and 2000 m)
Abyssal (deeper than 2000 m)
6.1.3 Environments Classified
Since the earliest days of geological studies it has been found convenient to classify sedimentary environments into various groups and subgroups. Such a classification provides
a useful formal framework on which to base more detailed analyses of specific environmental types. Table 6.2 is an example of the classical classification of environments. Variants of this type will be found in many textbooks (e.g., Twenhofel, 1926, p. 784; Pettijohn,
1956, p. 633; Dunbar and Rodgers, 1957; Krumbein and Sloss, 1959, p. 196; Blatt et al.,
1978).
This sort of all-embracing scheme, while excellent for modern environmental studies, is limited in its application to ancient sedimentary rocks. There are two reasons for
this. First, this is a classification of sedimentary environments and, as already shown,
only a limited number of these are quantitatively significant depositional environments.
Thus rocks of spelean, glacial, and abyssal origin are very rare in the geological column.
A second reason why this classification is difficult to apply to ancient sediments is because it is extremely hard to determine the depth of water in which ancient marine deposits originated. It is generally possible to recognize the relative position of a sequence
of facies with respect to a shoreline, but it is often extremely hard to equate these with
absolute depths (Hallam, 1967, p. 330). For this reason a classification of marine environments into depth-defined neritic, bathyal, and abyssal realms is hard to apply to ancient sediments.
In a review of the problems of environmental classifications, Crosby (1972) cites a
version compiled by Shepard and McKee. This is particularly useful because its hierarchical structure allows the inclusions of many minor subenvironments which have been
identified by studies of modern sediments (Table 6.3). Crosby (1972) has also prepared a useful classification of marine environments based on water depth, water circulation, and energy level. These three parameters define the limits of a number of depositional environments that have been commonly recognized in sedimentary rocks.
Finally, Table 6.4 is a classification of major depositional sedimentary environments. It
tabulates only those environments that generate quantitatively significant deposits and
6 DEPOSITIONAL SYSTEMS
Table 6.2
Example of the Classical Type of Classification of Sedimentary Environments
Continental
Transitional
Marine
Terrestrial
Aqueous
Desert
Glacial
Fluvial
Paludal (swamp)
Lacustrine
Cave (spelean)
Deltaic
Estuarine
Lagoonal
Littoral (intertidal)
Reef
Neritic (between low tide and 200 m)
Bathyal (between 200 and 2000 m)
Abyssal (deeper than 2000 m)
6.1.3 Environments Classified
Since the earliest days of geological studies it has been found convenient to classify sedimentary environments into various groups and subgroups. Such a classification provides
a useful formal framework on which to base more detailed analyses of specific environmental types. Table 6.2 is an example of the classical classification of environments. Variants of this type will be found in many textbooks (e.g., Twenhofel, 1926, p. 784; Pettijohn,
1956, p. 633; Dunbar and Rodgers, 1957; Krumbein and Sloss, 1959, p. 196; Blatt et al.,
1978).
This sort of all-embracing scheme, while excellent for modern environmental studies, is limited in its application to ancient sedimentary rocks. There are two reasons for
this. First, this is a classification of sedimentary environments and, as already shown,
only a limited number of these are quantitatively significant depositional environments.
Thus rocks of spelean, glacial, and abyssal origin are very rare in the geological column.
A second reason why this classification is difficult to apply to ancient sediments is because it is extremely hard to determine the depth of water in which ancient marine deposits originated. It is generally possible to recognize the relative position of a sequence
of facies with respect to a shoreline, but it is often extremely hard to equate these with
absolute depths (Hallam, 1967, p. 330). For this reason a classification of marine environments into depth-defined neritic, bathyal, and abyssal realms is hard to apply to ancient sediments.
In a review of the problems of environmental classifications, Crosby (1972) cites a
version compiled by Shepard and McKee. This is particularly useful because its hierarchical structure allows the inclusions of many minor subenvironments which have been
identified by studies of modern sediments (Table 6.3). Crosby (1972) has also prepared a useful classification of marine environments based on water depth, water circulation, and energy level. These three parameters define the limits of a number of depositional environments that have been commonly recognized in sedimentary rocks.
Finally, Table 6.4 is a classification of major depositional sedimentary environments. It
tabulates only those environments that generate quantitatively significant deposits and