620
North Alpine foreland. Whereas in Switzerland an early
and late main phase of overthrusting are'. assumed, northward thrusting of the Eastern Alps ceased after the early
active phase in the late Eocene/early Oligocene. Here, the
second transgressive-regressive megacycle is explained by
first decreasing and thereafter again increasing sediment
supply due to intermittent storage and remobilization of
sediment within the mountain belt (Fig. 12.36a; Zweigel
1998). A greater number of tectonically controlled megacycles has been reported from the long-persisting Western
Interior basin ofNorth America (see below).
- Prograding ofindividual clastic wedges (continental) fed by different sediment sourees.
Swiss molasse basin. In the Lower Freshwater Molasse of
Switzerland (-30 to 20 or 15 Ma BP) two to three separate
units were identified which prograded cratonward with
time (Fig. 12.35c) and received their sediments from different source areas. In these, various sedimentary or crystalline rocks either from the internal and outer zones of the
Alps or from overthrust older Marine Molasse were exposed to erosion (Schlunegger et al. 1997a, 1998; Kempf et
al. 1999). The individual sedimentary units show characteristic transitions from proximal to distal facies. These begin
with large alluvial fans (including debris flows, coarsegrained ribbon channel conglomerates, very thick massive
pebble-to-cobble conglomerates) followed by conglomerate
channel belts and overbank fines. The latter grade into
sands tone channel belts, wide flood plains and lacustrine
sediments of cratonward decreasing thicknesses. The main
units are separated by surfaces of erosion or non-deposition
which may partially result from lateral switching of the
depositional systems. Diachronous facies boundaries and a
significant sequence boundary (hiatus) have also been
found in the younger Upper Marine Molasse Group of this
area (Schlunegger et al. 1997b).
- Proximal uncoriformities, change in gravel composition, tecto-sedimentary units. This topic was addressed in the western part (Rioja trough) ofthe Ebro
basin which is a composite foreland basin in northern
Spain, surrounded on all sides by overthrust belts of
mountain ranges (Fig. 12.37a; e.g. Marzo et al.
1998).
Rioja trough, Ebro basin. In the Triassic, the Variscan
basement of the basin formed a topographical high and was
covered by continental deposits followed by Jurassic to
early Tertiary shallow-marine sediments (Fig. 12.37b).
Overthrusting of the southern Pyrenees from the north and
the Iberian ranges from the south started in the Paleocene.
From then on the rapidly subsiding basin was filled along
its margins mainly by alluvial fans and in its central part by
fluvial and lake sediments. The Eocene to Miocene sediments reach thicknesses between 2500 and 5000 m. In the
Miocene, the approaching overthrust fronts had narrowed
the Rioja trough to a small fraction of its original width.
Intraplate thrusts contributed to this shortening. As a result,
the Tertiary strata are folded and thrust near the basin margin.
Cross sections at the southern margin of this basin demonstrate not only the common transition from proximal to
distal facies, but also the interplay between short episodes
Chapter 12 Basin Evolution and Sediments
of overthrusting and times ofreduced tectonic activity (Fig.
12.37c,d; Munoz-Jimenez and Casas-Sainz 1997). Active
overthrusting created prograding and coarsening-upward
facies associations with high sedimentation rates; periods
of low tectonic activity caused retrograding (backstepping)
fining-upward associations with reduced sedimentation
rates. The transitions from fining- to coarsening-upward
trends record the minima of sediment supply and vice
versa. At the basin margin, fining-upward is associated
with onlap, coarsening-upward with offlap structures. Renewed overthrusting of carbonate rocks is partially recorded by high contents of calcareous gravel in the proximal alluvial fans (Fig. 12.37e), whereas siliciclastic components predominate in the low-activity intervals. The
progradation-retrogradation cycles are linked to angular
unconformities along the basin margin and therefore interpreted as being entirely tectonic in origin (tectono-sedimentary units). These can also be identified by breaks in
the vertical sediment buildup toward the basin center.
- Segmentation of foreland basins into subbasins,
intra-plate thrusts, and growth faults. These phenomena are introduced by some further examples.
Western Interior seaway, North America. The huge,
along strike about 5000 km long retro-foreland basin east
of the Rocky Mountains (Fig. 12.38a,b) is an outstanding
example of basin segmentation. The basin was flooded
from the Aptian to the late Cretaceous and formed a broad
seaway, connecting the proto-Gulf of Mexico and the
Circum-Boreal seaway for nearly 35 Ma (e.g. Molenaar
and Rice 1988).
The simplified scheme of Figure 12.38a, describing the
formation of this retro-foreland basin by the successive
emplacement of thrust sheets, can be regarded only as a
first approximation. In reality, the basin evolved in a very
heterogeneous way. Within the large basin, a number of
smaller subbasins formed which display marked differences
in their subsidence histories and sediment accumulation
(up to about 6000 m in thickness). The upper Cretaceous
development of this region is shown in several profiles
across the basin (Fig. 12.38d,e).
The segmentation of the basin (Fig. 12.38b) probably
results from heterogeneities in the basement inherited from
Precambrian rifting (Pang and Nummedal 1995). The differences in the amounts and time periods of maximum subsidence (Fig. 12.38d) are caused by different degrees of
crustal shortening along strike. The sedimentary facies in
the basin include fluvial and deltaic sediments, coal seams,
coastal barrier sandstones, offshore siltstones and shales,
shelf sandstones and limestones, black shales, and even
some turbidites.
A special feature of this basin is its wide eastward extension onto the stable craton beyond the forebulge zone. This
is ascribed to regional epeirogenetic subsidence, the heavy,
thrust belt-derived sediment load deposited farther to the
east, as well as to the unusually high eustatic sea level during the middle Cretaceous.
The basin experienced five to six major tectonically controlled transgressive-regressive cycles, as mentioned above,
and numerous shorter, partially sea-level controlled cycles
(e.g. Van Wagoner and Bertram 1995; Gardner 1995; cf.
Sect. 7.2). The major cycles are characterized by clastic
wedges, separated by erosional unconformities of different
duration (e.g. Leckie and Smith 1992). A stratigraphic
North Alpine foreland. Whereas in Switzerland an early
and late main phase of overthrusting are'. assumed, northward thrusting of the Eastern Alps ceased after the early
active phase in the late Eocene/early Oligocene. Here, the
second transgressive-regressive megacycle is explained by
first decreasing and thereafter again increasing sediment
supply due to intermittent storage and remobilization of
sediment within the mountain belt (Fig. 12.36a; Zweigel
1998). A greater number of tectonically controlled megacycles has been reported from the long-persisting Western
Interior basin ofNorth America (see below).
- Prograding ofindividual clastic wedges (continental) fed by different sediment sourees.
Swiss molasse basin. In the Lower Freshwater Molasse of
Switzerland (-30 to 20 or 15 Ma BP) two to three separate
units were identified which prograded cratonward with
time (Fig. 12.35c) and received their sediments from different source areas. In these, various sedimentary or crystalline rocks either from the internal and outer zones of the
Alps or from overthrust older Marine Molasse were exposed to erosion (Schlunegger et al. 1997a, 1998; Kempf et
al. 1999). The individual sedimentary units show characteristic transitions from proximal to distal facies. These begin
with large alluvial fans (including debris flows, coarsegrained ribbon channel conglomerates, very thick massive
pebble-to-cobble conglomerates) followed by conglomerate
channel belts and overbank fines. The latter grade into
sands tone channel belts, wide flood plains and lacustrine
sediments of cratonward decreasing thicknesses. The main
units are separated by surfaces of erosion or non-deposition
which may partially result from lateral switching of the
depositional systems. Diachronous facies boundaries and a
significant sequence boundary (hiatus) have also been
found in the younger Upper Marine Molasse Group of this
area (Schlunegger et al. 1997b).
- Proximal uncoriformities, change in gravel composition, tecto-sedimentary units. This topic was addressed in the western part (Rioja trough) ofthe Ebro
basin which is a composite foreland basin in northern
Spain, surrounded on all sides by overthrust belts of
mountain ranges (Fig. 12.37a; e.g. Marzo et al.
1998).
Rioja trough, Ebro basin. In the Triassic, the Variscan
basement of the basin formed a topographical high and was
covered by continental deposits followed by Jurassic to
early Tertiary shallow-marine sediments (Fig. 12.37b).
Overthrusting of the southern Pyrenees from the north and
the Iberian ranges from the south started in the Paleocene.
From then on the rapidly subsiding basin was filled along
its margins mainly by alluvial fans and in its central part by
fluvial and lake sediments. The Eocene to Miocene sediments reach thicknesses between 2500 and 5000 m. In the
Miocene, the approaching overthrust fronts had narrowed
the Rioja trough to a small fraction of its original width.
Intraplate thrusts contributed to this shortening. As a result,
the Tertiary strata are folded and thrust near the basin margin.
Cross sections at the southern margin of this basin demonstrate not only the common transition from proximal to
distal facies, but also the interplay between short episodes
Chapter 12 Basin Evolution and Sediments
of overthrusting and times ofreduced tectonic activity (Fig.
12.37c,d; Munoz-Jimenez and Casas-Sainz 1997). Active
overthrusting created prograding and coarsening-upward
facies associations with high sedimentation rates; periods
of low tectonic activity caused retrograding (backstepping)
fining-upward associations with reduced sedimentation
rates. The transitions from fining- to coarsening-upward
trends record the minima of sediment supply and vice
versa. At the basin margin, fining-upward is associated
with onlap, coarsening-upward with offlap structures. Renewed overthrusting of carbonate rocks is partially recorded by high contents of calcareous gravel in the proximal alluvial fans (Fig. 12.37e), whereas siliciclastic components predominate in the low-activity intervals. The
progradation-retrogradation cycles are linked to angular
unconformities along the basin margin and therefore interpreted as being entirely tectonic in origin (tectono-sedimentary units). These can also be identified by breaks in
the vertical sediment buildup toward the basin center.
- Segmentation of foreland basins into subbasins,
intra-plate thrusts, and growth faults. These phenomena are introduced by some further examples.
Western Interior seaway, North America. The huge,
along strike about 5000 km long retro-foreland basin east
of the Rocky Mountains (Fig. 12.38a,b) is an outstanding
example of basin segmentation. The basin was flooded
from the Aptian to the late Cretaceous and formed a broad
seaway, connecting the proto-Gulf of Mexico and the
Circum-Boreal seaway for nearly 35 Ma (e.g. Molenaar
and Rice 1988).
The simplified scheme of Figure 12.38a, describing the
formation of this retro-foreland basin by the successive
emplacement of thrust sheets, can be regarded only as a
first approximation. In reality, the basin evolved in a very
heterogeneous way. Within the large basin, a number of
smaller subbasins formed which display marked differences
in their subsidence histories and sediment accumulation
(up to about 6000 m in thickness). The upper Cretaceous
development of this region is shown in several profiles
across the basin (Fig. 12.38d,e).
The segmentation of the basin (Fig. 12.38b) probably
results from heterogeneities in the basement inherited from
Precambrian rifting (Pang and Nummedal 1995). The differences in the amounts and time periods of maximum subsidence (Fig. 12.38d) are caused by different degrees of
crustal shortening along strike. The sedimentary facies in
the basin include fluvial and deltaic sediments, coal seams,
coastal barrier sandstones, offshore siltstones and shales,
shelf sandstones and limestones, black shales, and even
some turbidites.
A special feature of this basin is its wide eastward extension onto the stable craton beyond the forebulge zone. This
is ascribed to regional epeirogenetic subsidence, the heavy,
thrust belt-derived sediment load deposited farther to the
east, as well as to the unusually high eustatic sea level during the middle Cretaceous.
The basin experienced five to six major tectonically controlled transgressive-regressive cycles, as mentioned above,
and numerous shorter, partially sea-level controlled cycles
(e.g. Van Wagoner and Bertram 1995; Gardner 1995; cf.
Sect. 7.2). The major cycles are characterized by clastic
wedges, separated by erosional unconformities of different
duration (e.g. Leckie and Smith 1992). A stratigraphic