338
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
headward. It can affect the river courses far away from
the highstand coastline (in the last glacial period up to
some 100 km in young alluvium), depending on ~he
stream power, the time available for erosion and the
erodibility of the underlying alluvium or bedrock. Alluvial fans reaching the coastline during HST can be
dissected during LST ("fan valley interval" of Muto
1993). Due to lateral erosion, the fan valley widens
and may later accommodate younger fan sediments.
Incised valleys on land and on continental shelves
are again filled with sediment during the subsequent
sea-level rise. The facies architecture of these valley
fills varies significantly in relation to sediment supply,
the hydrodynamic regime ofthe water-filled basin, the
rate of relative sea-level rise (frequency of sea-level
changes), and other factors. This is demonstrated by
some modem and ancient examples which also inelude
the occurrence of coal seams associated with incised
valleys.
Submarine and subaerial valleys incised during the last glacial period and filled with sediment during the Holocene
have been described from many regions. To the east ofHudson Bay, for example, the wide Atlantic shelf ofNorth America was cut by a valley about 200 km in length. In the Gulf of
Mexico and the coastal region ofTexas several ofthese valleys were investigated in more detail using, among others,
high-resolution seismic methods (Thomas and Anderson
1994; Nichol et al. 1994; Anderson et al. 1996). This region
can serve as an example of a passive continental margin setting with relatively high sediment supply.
The principal results of these studies are discussed
here in a largely generalized way (Fig. 7.26b and c).
Due to the high-frequency Pleistocene sea-level
changes, both the erosional and depositional features
of the emerged and subsequently flooded shelf are
widely preserved (cf. Sect. 7.3.1). Basinward, valley
incision reached the shelfbreak where lowstand deltas
formed and slope failures were triggered. During the
Holocene transgression, shelf deltas and coastal sand
ridges migrated landward. Behind barriers located
within the valleys, tidal and lagoonal sediments accumulated. This facies association stepped backward either continuously or in certain steps (Fig. 7 .26c) until
the sea level had reached its highest position (MFS).
From then on, deltas growing within the valleys or entering a wider embayment ( or lake) started to pro grade
seaward. The same occurred to coastal sands fed by
longshore transport. On land, the rivers were affected
by the late PleistocenelHolocene sea-level history only
in their lowermost reaches.
Along the Colorado River entering the Gulf of Mexico, for
example, valley incision reached the apex of the present-day
subsiding alluvial deltaic plain, approximately 100 km upstream ofthe coastline (Fig. 7.26b and d; Blum 1993). However, this part ofthe Holocene valley fill is purely fluvial and
does not show any direct marine influence. Instead, it reflects
alternating periods of filling and erosion controlled byelirnatic changes in the hinterland. This is also true of the young
history of the river further upstream where short periods of
downcutting alternated with phases of alluvial deposition.
Another detailed study of the Holocene fill of an incised
valley has been carried out for the Gironde estuary downstream Bordeaux, Southwest France (Allen and Posamentier
1993). The valley was cut into the underlying Tertiary carbonate rocks during the last (Würmian) eustatic sea-level
fall; it reached about 50 m in depth at the present coastline
and affected a length ofabout 120 km upstream. Its sedimentary fill consists of a relatively thin fluvial LST (coarse sand
and gravel), a thick wedge oftidal-estuarine sands and muds
(TST) reaching 80 km inland, and a prograding bayhead
delta (HST).
Many examples of incised valleys have been reported
from foreland basins where they mainly occur in a
proximal position, i.e. elose to the front of the
overthrust belt. These valleys run more or less perpendicular away from the mountain front basinward. Under favorable conditions, these valleys contain coal
searns (see below). Some paleo-valleys also formed
parallel to the strike ofthe foreland basin. One ofthese
examples is located in the Oligo-Miocene fill of the
northem Alpine foreland basin (Fig. 7.28e; cf. Sect.
12.6.2).
The paleo-valley is about 300 km in length, 10 to 15 km
wide and runs along the distal basin margin more or less parallel to the strike ofthe basin, opposite to the direction ofthe
present-day Danube river (Fig. 7.28e; based on Buchner et
al. 1996). It was incised more than 50 m deep into freshwater
and marine molasse deposits (FM and MM in Fig. 7.28e,
cross-section elose to the city of Ulm). The position of the
valley seerns to have been controlled by the northward gradient ofthe emerging Upper Marine Molasse as weil as a persisting marine basin in the southwestern part of the foreland
basin.
The valley fill mainly consists of coarse-grained (pebblesize) fluvial deposits ("Graupensand-Rinne", LST) and sediments of a ± brackish water, estuarine environment (representing TST and HST). These are overlain by the Upper
Freshwater Molasse ofthe Middle Miocene. Calcretes on the
flanks and shoulders of the valley indicate an earlier phase of
non-deposition. The age of valley incision and filling was a
matter of lively debate (Reichenbacher et al. 1998). It now
seems to be weil established that the valley is older (Lower
Miocene, about 18 Ma) than the meteorite impact creating
the Ries crater (15 Ma). Later, the depositional environment
of the foreland blisin remained continental and sea-level
changes no longer directly affected the over-filled basin.
How far inland from the lowstand or highstand coastline incised valleys are filled with estuarine and tidal
sediments depends on the sediment supply ofthe entering rivers and the rate ofrelative sea-level rise creating
space for sediment accumulation.
The transition from purely continental processes,
including tectonic structures controlling the river
courses and the initiation of continental basins, to valleys affected by sea-level change is shown in a general
way in Fig. 7.27. Datable terraces and paleosols of
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
headward. It can affect the river courses far away from
the highstand coastline (in the last glacial period up to
some 100 km in young alluvium), depending on ~he
stream power, the time available for erosion and the
erodibility of the underlying alluvium or bedrock. Alluvial fans reaching the coastline during HST can be
dissected during LST ("fan valley interval" of Muto
1993). Due to lateral erosion, the fan valley widens
and may later accommodate younger fan sediments.
Incised valleys on land and on continental shelves
are again filled with sediment during the subsequent
sea-level rise. The facies architecture of these valley
fills varies significantly in relation to sediment supply,
the hydrodynamic regime ofthe water-filled basin, the
rate of relative sea-level rise (frequency of sea-level
changes), and other factors. This is demonstrated by
some modem and ancient examples which also inelude
the occurrence of coal seams associated with incised
valleys.
Submarine and subaerial valleys incised during the last glacial period and filled with sediment during the Holocene
have been described from many regions. To the east ofHudson Bay, for example, the wide Atlantic shelf ofNorth America was cut by a valley about 200 km in length. In the Gulf of
Mexico and the coastal region ofTexas several ofthese valleys were investigated in more detail using, among others,
high-resolution seismic methods (Thomas and Anderson
1994; Nichol et al. 1994; Anderson et al. 1996). This region
can serve as an example of a passive continental margin setting with relatively high sediment supply.
The principal results of these studies are discussed
here in a largely generalized way (Fig. 7.26b and c).
Due to the high-frequency Pleistocene sea-level
changes, both the erosional and depositional features
of the emerged and subsequently flooded shelf are
widely preserved (cf. Sect. 7.3.1). Basinward, valley
incision reached the shelfbreak where lowstand deltas
formed and slope failures were triggered. During the
Holocene transgression, shelf deltas and coastal sand
ridges migrated landward. Behind barriers located
within the valleys, tidal and lagoonal sediments accumulated. This facies association stepped backward either continuously or in certain steps (Fig. 7 .26c) until
the sea level had reached its highest position (MFS).
From then on, deltas growing within the valleys or entering a wider embayment ( or lake) started to pro grade
seaward. The same occurred to coastal sands fed by
longshore transport. On land, the rivers were affected
by the late PleistocenelHolocene sea-level history only
in their lowermost reaches.
Along the Colorado River entering the Gulf of Mexico, for
example, valley incision reached the apex of the present-day
subsiding alluvial deltaic plain, approximately 100 km upstream ofthe coastline (Fig. 7.26b and d; Blum 1993). However, this part ofthe Holocene valley fill is purely fluvial and
does not show any direct marine influence. Instead, it reflects
alternating periods of filling and erosion controlled byelirnatic changes in the hinterland. This is also true of the young
history of the river further upstream where short periods of
downcutting alternated with phases of alluvial deposition.
Another detailed study of the Holocene fill of an incised
valley has been carried out for the Gironde estuary downstream Bordeaux, Southwest France (Allen and Posamentier
1993). The valley was cut into the underlying Tertiary carbonate rocks during the last (Würmian) eustatic sea-level
fall; it reached about 50 m in depth at the present coastline
and affected a length ofabout 120 km upstream. Its sedimentary fill consists of a relatively thin fluvial LST (coarse sand
and gravel), a thick wedge oftidal-estuarine sands and muds
(TST) reaching 80 km inland, and a prograding bayhead
delta (HST).
Many examples of incised valleys have been reported
from foreland basins where they mainly occur in a
proximal position, i.e. elose to the front of the
overthrust belt. These valleys run more or less perpendicular away from the mountain front basinward. Under favorable conditions, these valleys contain coal
searns (see below). Some paleo-valleys also formed
parallel to the strike ofthe foreland basin. One ofthese
examples is located in the Oligo-Miocene fill of the
northem Alpine foreland basin (Fig. 7.28e; cf. Sect.
12.6.2).
The paleo-valley is about 300 km in length, 10 to 15 km
wide and runs along the distal basin margin more or less parallel to the strike ofthe basin, opposite to the direction ofthe
present-day Danube river (Fig. 7.28e; based on Buchner et
al. 1996). It was incised more than 50 m deep into freshwater
and marine molasse deposits (FM and MM in Fig. 7.28e,
cross-section elose to the city of Ulm). The position of the
valley seerns to have been controlled by the northward gradient ofthe emerging Upper Marine Molasse as weil as a persisting marine basin in the southwestern part of the foreland
basin.
The valley fill mainly consists of coarse-grained (pebblesize) fluvial deposits ("Graupensand-Rinne", LST) and sediments of a ± brackish water, estuarine environment (representing TST and HST). These are overlain by the Upper
Freshwater Molasse ofthe Middle Miocene. Calcretes on the
flanks and shoulders of the valley indicate an earlier phase of
non-deposition. The age of valley incision and filling was a
matter of lively debate (Reichenbacher et al. 1998). It now
seems to be weil established that the valley is older (Lower
Miocene, about 18 Ma) than the meteorite impact creating
the Ries crater (15 Ma). Later, the depositional environment
of the foreland blisin remained continental and sea-level
changes no longer directly affected the over-filled basin.
How far inland from the lowstand or highstand coastline incised valleys are filled with estuarine and tidal
sediments depends on the sediment supply ofthe entering rivers and the rate ofrelative sea-level rise creating
space for sediment accumulation.
The transition from purely continental processes,
including tectonic structures controlling the river
courses and the initiation of continental basins, to valleys affected by sea-level change is shown in a general
way in Fig. 7.27. Datable terraces and paleosols of