190
6 DEPOSITIONAL SYSTEMS
Observation 1: There are on the earth's surface today a finite number of sedimentary environments. (Qualifying remarks: Detailed examination shows no two similar
environments to be identical. Environments show both abrupt and gradational lateral transitions.)
Observation 2: There are a finite number of sedimentary facies which recur in time
and space in the geological record. (Qualifying remarks: Detailed examination shows
no two similar facies to be identical. Facies show both abrupt and gradational lateral
and vertical transitions.)
Interpretation: The parameters of ancient sedimentary facies of unknown origin can
be matched with modern deposits whose environments are known. Thus may the depositional environments of ancient sedimentary facies be discovered.
Conclusion: There are, and always have been, a finite number of sedimentary environments which deposit characteristic sedimentary facies. These can be classified into
various ideal systems or models.
These ideas have been implicit, though seldom so baldly stated, in most, if not all facies analyses. Potter in particular has discussed sedimentary and facies models (apparently as interchangeable terms). Potter and Pettijohn (1977, p. 314) write: "A sedimentary model in essence describes a recurring pattern of sedimentation" and again "the
fundamental assumption of the model concept is that there is a close relationship between the arrangement of major sedimentation in a basin and direction structures in as
much as both are a product of a common dispersal pattern" (p. 228).
Additional discussion of models, sedimentary, and facies has been given by Potter
(1959), Visher (1965), Pettijohn et al. (1972, p. 523), Blatt et al. (1978), Curtis (1978),
Walker (1984a,b), Reading and Levell (1996), and Selley (1996).
The next part of this chapter is devoted to the description of the major sedimentary
models that can be defined. The classification of these models is based on the classification of depositional environments given in Table 6.4. Each major depositional environment can be correlated with a sedimentary model. More detailed accounts of these
are found in books by Laporte (1979), Klein (1980), Reineck and Singh (1980), Scholle
and Spearing (1982), Scholle et al. (1983a,b), Anderton (1985), Reading (1996), and Selley (1996).
6.3.2 Some Models Described
6.3.2.1 Piedmont Fanglomerates
Between mountains and adjacent lowlands it is often possible to define a distinct belt
known as the piedmont zone (literally from the French: mountain foot). At the feet of
mountains, valleys debouch their sediments into the plains. The piedmont zone is characterized by small alluvial cones which are fed by gullies and build out onto pediment
surfaces cut subhorizontally into the bedrock (Fig. 6.2). Larger alluvial fans are fed by
complex valley systems and grade out into the deposits of the alluvial plain (Fig. 6.3).
Depositional gradients in the piedmont zone are steep, up to 30 ~ in marginal screes,
but diminish radially down-fan. This change in gradient correlates with changes in process and sediment type. Alluvial cones and the heads of alluvial valleys are characterized by boulder beds and conglomerates deposited by gravity slides from the adjacent
mountain sides. These grade down the fan into conglomerates and massive or crudely
6 DEPOSITIONAL SYSTEMS
Observation 1: There are on the earth's surface today a finite number of sedimentary environments. (Qualifying remarks: Detailed examination shows no two similar
environments to be identical. Environments show both abrupt and gradational lateral transitions.)
Observation 2: There are a finite number of sedimentary facies which recur in time
and space in the geological record. (Qualifying remarks: Detailed examination shows
no two similar facies to be identical. Facies show both abrupt and gradational lateral
and vertical transitions.)
Interpretation: The parameters of ancient sedimentary facies of unknown origin can
be matched with modern deposits whose environments are known. Thus may the depositional environments of ancient sedimentary facies be discovered.
Conclusion: There are, and always have been, a finite number of sedimentary environments which deposit characteristic sedimentary facies. These can be classified into
various ideal systems or models.
These ideas have been implicit, though seldom so baldly stated, in most, if not all facies analyses. Potter in particular has discussed sedimentary and facies models (apparently as interchangeable terms). Potter and Pettijohn (1977, p. 314) write: "A sedimentary model in essence describes a recurring pattern of sedimentation" and again "the
fundamental assumption of the model concept is that there is a close relationship between the arrangement of major sedimentation in a basin and direction structures in as
much as both are a product of a common dispersal pattern" (p. 228).
Additional discussion of models, sedimentary, and facies has been given by Potter
(1959), Visher (1965), Pettijohn et al. (1972, p. 523), Blatt et al. (1978), Curtis (1978),
Walker (1984a,b), Reading and Levell (1996), and Selley (1996).
The next part of this chapter is devoted to the description of the major sedimentary
models that can be defined. The classification of these models is based on the classification of depositional environments given in Table 6.4. Each major depositional environment can be correlated with a sedimentary model. More detailed accounts of these
are found in books by Laporte (1979), Klein (1980), Reineck and Singh (1980), Scholle
and Spearing (1982), Scholle et al. (1983a,b), Anderton (1985), Reading (1996), and Selley (1996).
6.3.2 Some Models Described
6.3.2.1 Piedmont Fanglomerates
Between mountains and adjacent lowlands it is often possible to define a distinct belt
known as the piedmont zone (literally from the French: mountain foot). At the feet of
mountains, valleys debouch their sediments into the plains. The piedmont zone is characterized by small alluvial cones which are fed by gullies and build out onto pediment
surfaces cut subhorizontally into the bedrock (Fig. 6.2). Larger alluvial fans are fed by
complex valley systems and grade out into the deposits of the alluvial plain (Fig. 6.3).
Depositional gradients in the piedmont zone are steep, up to 30 ~ in marginal screes,
but diminish radially down-fan. This change in gradient correlates with changes in process and sediment type. Alluvial cones and the heads of alluvial valleys are characterized by boulder beds and conglomerates deposited by gravity slides from the adjacent
mountain sides. These grade down the fan into conglomerates and massive or crudely