490
10 SEDIMENTARY BASINS
This is separated by a positive axis, the miogeanticline, from the eugeosynclinal furrow.
This is the main active and unstable trough. The eugeosyncline is separated from the
open oceanic basin by a second ridge, the eugeanticline. This second positive feature is
an island arc of rising volcanics. The evolution of a geosyncline commences with the development of a tensional sag on the site of the future eugeosyncline. Pelagic limestones
and radiolarian cherts develop in the basin synchronously with carbonates and sands on
the adjacent shelf. The sag deepens to form the eugeosynclinal furrow. Subaqueous igneous activity causes the cherts to be interbedded with pillow lavas, ophiolites, and spilites (e.g., Hynes et al., 1972). This is the Steinmann Trinity discussed in Section 8.5.2.3.
Simultaneously, the miogeosynclinal furrow forms, separated from the eugeosyncline
by the miogeanticline. Sedimentation in the miogeosyncline is still largely of shelf and
shallow marine facies because subsidence is generally sufficiently gradual for deposition
to keep pace. The sediments of the miogeosyncline differ from those of the foreland
in thickness rather than facies. Miogeosynclines are in many ways similar to epicratonic
basins. Continued tectonic activity leads to uplift of the eugeanticline. This becomes a
major source of terrigenous sediment, immature in texture and mineralogy and often
volcaniclastic. These deposits are laid down within the eugeosyncline largely as turbidites to form flysch facies.
The term ttysch has had a lengthy and much abused history in the literature of geology. It has a high preservation potential. Detailed accounts of flysch facies have been
given by Dzulinski and Walton (1965) and Lajoie (1970). Hsu (1970) discussed flysch
semantics and Reading (1972) reviewed flysch facies within the context of plate tectonics. Flysch is generally regarded as the synorogenic deposit of geosynclinal troughs.
Sedimentologically most fiysch show the features of turbidites. Flysch is petrographically diverse but in older Paleozoic and Proterozoic examples it is characteristically of
greywacke type. Carbonate flysch is known, however, and, in the Hellenide geosyncline,
troughs of detrital carbonate turbidites were supplied from the adjacent shelf. The main
orogenic phase of the geosyncline is diverse and beyond the scope of this text. It may
range from the high-angle thrust faulting and tight synclinal folding of the Rocky Mountains, to the recumbent folds and low-angle thrusts of the Alps (DeCelles and Mitra,
1999). Orogenesis also involves regional metamorphism and plutonic activity within the
trough. During mountain building the tectonic front gradually migrates away from the
trough axis. This forces the depositional axis of the flysch facies in the same direction,
spilling sediment into the fore-deep. During this migration the deposits change from
flysch to molasse facies.
The term molasse, like flysch, has had a chequered career since was first used in
the French Alps by Bertrand in 1897. Molasse deposits are generally taken to be "late
orogenic clastic wedges deposited in the linear fore-deep on the flank a craton" (Van
Houten, 1973). Molasse facies are largely coarse terrigenous clastics with abundant conglomerates, few shales, and negligible limestones. These beds are generally laid down
in lacustrine, fluvial, and fanglomerate environments. Grain size decreases and beds
thin away from the mountain front.
The final phase of mountain building is regional epeirogenic uplift. This is typically
accompanied by block faulting within the new mountain chain. Intramontane troughs
are infilled with thick nonmarine clastic sequences, often accompanied by basalt effusion along the faults. The range and basin province of the Rocky Mountains are of this
10 SEDIMENTARY BASINS
This is separated by a positive axis, the miogeanticline, from the eugeosynclinal furrow.
This is the main active and unstable trough. The eugeosyncline is separated from the
open oceanic basin by a second ridge, the eugeanticline. This second positive feature is
an island arc of rising volcanics. The evolution of a geosyncline commences with the development of a tensional sag on the site of the future eugeosyncline. Pelagic limestones
and radiolarian cherts develop in the basin synchronously with carbonates and sands on
the adjacent shelf. The sag deepens to form the eugeosynclinal furrow. Subaqueous igneous activity causes the cherts to be interbedded with pillow lavas, ophiolites, and spilites (e.g., Hynes et al., 1972). This is the Steinmann Trinity discussed in Section 8.5.2.3.
Simultaneously, the miogeosynclinal furrow forms, separated from the eugeosyncline
by the miogeanticline. Sedimentation in the miogeosyncline is still largely of shelf and
shallow marine facies because subsidence is generally sufficiently gradual for deposition
to keep pace. The sediments of the miogeosyncline differ from those of the foreland
in thickness rather than facies. Miogeosynclines are in many ways similar to epicratonic
basins. Continued tectonic activity leads to uplift of the eugeanticline. This becomes a
major source of terrigenous sediment, immature in texture and mineralogy and often
volcaniclastic. These deposits are laid down within the eugeosyncline largely as turbidites to form flysch facies.
The term ttysch has had a lengthy and much abused history in the literature of geology. It has a high preservation potential. Detailed accounts of flysch facies have been
given by Dzulinski and Walton (1965) and Lajoie (1970). Hsu (1970) discussed flysch
semantics and Reading (1972) reviewed flysch facies within the context of plate tectonics. Flysch is generally regarded as the synorogenic deposit of geosynclinal troughs.
Sedimentologically most fiysch show the features of turbidites. Flysch is petrographically diverse but in older Paleozoic and Proterozoic examples it is characteristically of
greywacke type. Carbonate flysch is known, however, and, in the Hellenide geosyncline,
troughs of detrital carbonate turbidites were supplied from the adjacent shelf. The main
orogenic phase of the geosyncline is diverse and beyond the scope of this text. It may
range from the high-angle thrust faulting and tight synclinal folding of the Rocky Mountains, to the recumbent folds and low-angle thrusts of the Alps (DeCelles and Mitra,
1999). Orogenesis also involves regional metamorphism and plutonic activity within the
trough. During mountain building the tectonic front gradually migrates away from the
trough axis. This forces the depositional axis of the flysch facies in the same direction,
spilling sediment into the fore-deep. During this migration the deposits change from
flysch to molasse facies.
The term molasse, like flysch, has had a chequered career since was first used in
the French Alps by Bertrand in 1897. Molasse deposits are generally taken to be "late
orogenic clastic wedges deposited in the linear fore-deep on the flank a craton" (Van
Houten, 1973). Molasse facies are largely coarse terrigenous clastics with abundant conglomerates, few shales, and negligible limestones. These beds are generally laid down
in lacustrine, fluvial, and fanglomerate environments. Grain size decreases and beds
thin away from the mountain front.
The final phase of mountain building is regional epeirogenic uplift. This is typically
accompanied by block faulting within the new mountain chain. Intramontane troughs
are infilled with thick nonmarine clastic sequences, often accompanied by basalt effusion along the faults. The range and basin province of the Rocky Mountains are of this