10.2 SEDIMENTARY BASINS CLASSIFIED AND DESCRIBED
491
type. The Devonian rocks of the North Atlantic environs also provide good examples
of postorogenic sedimentary basins. These continental red beds include both postorogenic molasse deposited in fore-deeps, such as the Pocono and Catskill facies of the Appalachians, and intramontane rift basins such as the Midland Valley of Scotland (Friend,
1969).
The geosynclinal model outlined here has been reviewed and modified in the light
of plate tectonics. The concept has been criticized by Ahmad (1968) and Coney (1970),
largely on the grounds that the time connotations of the geosynclinal cycle with beginning, a middle, and an end, are hard to reconcile with the continuous processes of subduction and sea floor spreading postulated by plate theory. More constructive attempts
to integrate geosynclines and plate tectonics have been made by Mitchell and Reading
(1969, 1978), Schwab (1971), Roberts (1972), Crostella (1977), and Dickinson and Seely
(1979). Mitchell and Reading have shown that most geosynclines, now perceived as zones
of subduction at compressive plate boundaries, fall into three main categories.
The various types of geosynclinal trough are now seen, therefore, to reflect the nature of the plate boundaries. Subduction may occur where two continental masses converge (e.g., the modern Himalayas), where a continental and an oceanic plate converges
(e.g., the Pacific coast of the Americas), or where the two oceanic plates converge (e.g.,
the Japanese island arc) (Dickinson and Seely, 1979). This scheme successfully integrates
the geosynclinal and plate theories and helps to explain the several different types of
ancient troughs and their associated mountain chains.
In summary, this brief review of sedimentation associated with plate convergence
describes the genesis of four different types of trough. Each trough has a characteristic
morphology, plate tectonic setting, time of development, and type of fill. The eugeosynclinal (now fore-arc) basin is composed of preorogenic pelagic sediments overlain
by synrogenic flysch facies. These are generally tectonically deformed and metamorphosed as the fore-arc basin migrates outward over the trench. The main flysch trough
is often host to minor rifted troughs filled with postorogenic continental clastics.
The former miogeosyncline, with its fill of preorogenic shelf sediments and postorogenic molasse is now termed a "back-arc basin." Where this is developed on continental crust, and it is encroached by an active thrust system, it is termed a foreland basin
(Beaumont, 1981; Dorobek and Ross, 1995; Mascle, 1998).
10.2.4 Basins Caused by Plate Divergence
There is an evolutionary series of basins related to crustal separation at boundaries
of plate divergence. This may be termed the rift-drift suite of basins (refer back to
Fig. 10.11).
Rift basins are long fault-bounded troughs which occur in various plate settings and
all show a corresponding diversity of sediment fill. They are of considerable economic
importance as sources of hydrocarbons, evaporites, and metals. Furthermore, rift basins
contain many clues to the understanding of global tectonics. Important publications on
this topic include works by Quennell (1985), Artyushkov (1987), Frostick et al. (1987),
Olsen (1995), and Purser and Bosence (1998). Essentially there are four types of rift basin, each characterized by a particular type of tectonic setting and sedimentary fill.
The postorogenic intramontane rifts were described in the previous section. They are
491
type. The Devonian rocks of the North Atlantic environs also provide good examples
of postorogenic sedimentary basins. These continental red beds include both postorogenic molasse deposited in fore-deeps, such as the Pocono and Catskill facies of the Appalachians, and intramontane rift basins such as the Midland Valley of Scotland (Friend,
1969).
The geosynclinal model outlined here has been reviewed and modified in the light
of plate tectonics. The concept has been criticized by Ahmad (1968) and Coney (1970),
largely on the grounds that the time connotations of the geosynclinal cycle with beginning, a middle, and an end, are hard to reconcile with the continuous processes of subduction and sea floor spreading postulated by plate theory. More constructive attempts
to integrate geosynclines and plate tectonics have been made by Mitchell and Reading
(1969, 1978), Schwab (1971), Roberts (1972), Crostella (1977), and Dickinson and Seely
(1979). Mitchell and Reading have shown that most geosynclines, now perceived as zones
of subduction at compressive plate boundaries, fall into three main categories.
The various types of geosynclinal trough are now seen, therefore, to reflect the nature of the plate boundaries. Subduction may occur where two continental masses converge (e.g., the modern Himalayas), where a continental and an oceanic plate converges
(e.g., the Pacific coast of the Americas), or where the two oceanic plates converge (e.g.,
the Japanese island arc) (Dickinson and Seely, 1979). This scheme successfully integrates
the geosynclinal and plate theories and helps to explain the several different types of
ancient troughs and their associated mountain chains.
In summary, this brief review of sedimentation associated with plate convergence
describes the genesis of four different types of trough. Each trough has a characteristic
morphology, plate tectonic setting, time of development, and type of fill. The eugeosynclinal (now fore-arc) basin is composed of preorogenic pelagic sediments overlain
by synrogenic flysch facies. These are generally tectonically deformed and metamorphosed as the fore-arc basin migrates outward over the trench. The main flysch trough
is often host to minor rifted troughs filled with postorogenic continental clastics.
The former miogeosyncline, with its fill of preorogenic shelf sediments and postorogenic molasse is now termed a "back-arc basin." Where this is developed on continental crust, and it is encroached by an active thrust system, it is termed a foreland basin
(Beaumont, 1981; Dorobek and Ross, 1995; Mascle, 1998).
10.2.4 Basins Caused by Plate Divergence
There is an evolutionary series of basins related to crustal separation at boundaries
of plate divergence. This may be termed the rift-drift suite of basins (refer back to
Fig. 10.11).
Rift basins are long fault-bounded troughs which occur in various plate settings and
all show a corresponding diversity of sediment fill. They are of considerable economic
importance as sources of hydrocarbons, evaporites, and metals. Furthermore, rift basins
contain many clues to the understanding of global tectonics. Important publications on
this topic include works by Quennell (1985), Artyushkov (1987), Frostick et al. (1987),
Olsen (1995), and Purser and Bosence (1998). Essentially there are four types of rift basin, each characterized by a particular type of tectonic setting and sedimentary fill.
The postorogenic intramontane rifts were described in the previous section. They are