12.1 Rift Basins
basin. For example, widening and deepening of the
half-graben may lead to a deepening-upward facies
succession (Fig. 12.4a-c) or, with slowing sub sidence, to a shallowing-upward succession. Similarly,
axial through-drainage may be followed by internal
drainage (Fig. 12.4, transition from a to d; cf. Sect.
11.3.3 and Fig. 11.11). Regional subsidence rnay allow the sea to enter and to turn a semi-arid to arid
continental environment into a marine carbonate basin (Fig. 14.4d,e). Uplifted basin margins, as known
from the modem Red Sea and Gulf of Aden, divert
the river drainage pattern away from the basins and
thus prevent significant terrigenous sediment influx.
Other complications are brought about by
basinward migration of the active boundary fault and
minor tectonic structures (e.g. horst-and-graben features) within the rift zone (Fig. 12.4t). Inthe first
case, the relative uplift of early rift deposits (e.g. alluvial fans) can lead to their rapid erosion and
redeposition. Similarly, uplifted and often tilted
blocks within the graben rnay lose part of their early
rift sediments by erosion. Thus, truncated sediment
sections (with unconformities) may occur elose to
complete sections thickened by rift-interior sediment
redistribution. A similar scenario is shown in Figure
12.3d where, in addition, episodic rapid subsidence
caused interrnittent topographie basin elosure and
playa deposition, later followed by through-going
fluvial drainage.
Such phenomena have been described from many rift basins (e.g. Smoot 1991; Frostick and Stee11993; Leeder and
Jackson 1993; Cather et al. 1994; Cohen et al. 1995;
Bosence et al. 1996; Nichols and Watchorn 1998; Ravnas
and Steel 1998). WeH exposed middle Jurassic to early
Cretaceous rift basins of eastern Greenland represent excellent examples of half-graben fiHings consisting of alluvial
fans, fan-deltas, shallow-marine sediments, and deeper
marine fan to basin associations (Surlyk et al. 1981).
V olcanic activity is neglected in the above facies models, but may significantly modify them.
12.1.4 Examples of Young Rift Zones
Many young rift basins all over the world have been
studied in some detail and are still under investigation. Of these, only a few can be briefly described.
The following examples inelude some of the elassical
rift structures, but there are possibly some better
suited cases than those
presented here.
The East African rift zone. The most prominent
and frequently described present-day rift zone, the
4000 km long East African rift, is an example of the
enormous variability of rift basins and their sediment
fills. It lies within a large, elongated dome structure
(the Kenyan and Ethiopian dome) which has risen
549
ab out 1800 m since the middle Tertiary (Fig. 12.3a).
The narrow Western Rift is occupied by lakes arranged en echelon (cf. Fig. 12.2a) and displays a relief of approxirnately 3000 m between the rift shoulders and the floors of the deepest lakes (Lake
Tanganyika and Lake Malawi). Here, volcanic activity is virtually absent. The 50 km wide Eastern Riß
(or Gregory Rift) exhibits lower relief, and is subdivided by trans verse ridges into several srnall, shallow
basins, partially occupied by lakes. The Eastern Rift
in particular was strongly affected by pre-rift and
syn-rift volcanism of basaltic to more felsic and
alkali-rich nature (Fig. 12.3b). The dip direction of
the detachment zone varies from east to west and is
marked by "off-axis" volcansism (Bosworth 1987).
Where the volume of volcanic products exceeded the
space created by rifting, the rift zone was totally
filled by volcanic flows and volcaniclastic sediments.
In the main Ethiopian Rift, six major volcanic episodes
were recognized from the late Oligocene to the Quaternary
(Woldegabriel et al. 1990; see also Uemura and Mizutani
1984; Frostick et al. 1986; Foster et al. 1997). Extruding
carbonatitic magmas are at least partially responsible for
the extreme\y high alkalinity (pR up to 10.3) and chemistry
of sodium carbonate lakes of the Magadi-type in the
Gregory Rift (Eugster 1986). The present-day climatic conditions of the East African rift zone range from a tropical,
high-rainfall belt to desert areas. Open lake systems with
fresh water alternate with closed lake basins where
evaporites accumulate. The sediments of these basins have
been described, e.g.m by Tierce\in and Faure (1978),
Pickford (1986) and many others.
The East African rift basins displaya large variety of
alluvial fan, fluvial, and lake deposits. The latter reflect changing elirnatic conditions through time and
from basin to basin. Diatomites and black shales occur in the more humid regions, while arid zones are
characterized by playa lakes with evaporites, ineluding sodium carbonate and lacustrine chert. The rift
basins of the Eastern Rift in particular contain abundant volcanielastics of varying composition. Repeated ashfalls favored the preservation of footprints,
soft-bodied organisms, and all kinds of organic relics. Sedimentation rates in this rift zone range from
about 10 cmlka up to several 100 cmlka.
The Red Sea and Gulf of Suez. To the north, the
East African rift system continues and forms the Red
Sea basin. The evolution of this basin started 70 Ma
aga and is more advanced than the East African rift
zone. One can distinguish four phases of development (e.g., Guennoc et al. 1988): (1) A Cretaceous to
Eocene pre-rift stage, (2) Oligocene rift formation,
(3) Miocene subsidence (Red Sea depression), and
(4) Pliocene to Recent sea-floor spreading. The sedimentary history of this basin is briefly discussed in
Section 4.3.3. The Miocene evaporites were depos-
basin. For example, widening and deepening of the
half-graben may lead to a deepening-upward facies
succession (Fig. 12.4a-c) or, with slowing sub sidence, to a shallowing-upward succession. Similarly,
axial through-drainage may be followed by internal
drainage (Fig. 12.4, transition from a to d; cf. Sect.
11.3.3 and Fig. 11.11). Regional subsidence rnay allow the sea to enter and to turn a semi-arid to arid
continental environment into a marine carbonate basin (Fig. 14.4d,e). Uplifted basin margins, as known
from the modem Red Sea and Gulf of Aden, divert
the river drainage pattern away from the basins and
thus prevent significant terrigenous sediment influx.
Other complications are brought about by
basinward migration of the active boundary fault and
minor tectonic structures (e.g. horst-and-graben features) within the rift zone (Fig. 12.4t). Inthe first
case, the relative uplift of early rift deposits (e.g. alluvial fans) can lead to their rapid erosion and
redeposition. Similarly, uplifted and often tilted
blocks within the graben rnay lose part of their early
rift sediments by erosion. Thus, truncated sediment
sections (with unconformities) may occur elose to
complete sections thickened by rift-interior sediment
redistribution. A similar scenario is shown in Figure
12.3d where, in addition, episodic rapid subsidence
caused interrnittent topographie basin elosure and
playa deposition, later followed by through-going
fluvial drainage.
Such phenomena have been described from many rift basins (e.g. Smoot 1991; Frostick and Stee11993; Leeder and
Jackson 1993; Cather et al. 1994; Cohen et al. 1995;
Bosence et al. 1996; Nichols and Watchorn 1998; Ravnas
and Steel 1998). WeH exposed middle Jurassic to early
Cretaceous rift basins of eastern Greenland represent excellent examples of half-graben fiHings consisting of alluvial
fans, fan-deltas, shallow-marine sediments, and deeper
marine fan to basin associations (Surlyk et al. 1981).
V olcanic activity is neglected in the above facies models, but may significantly modify them.
12.1.4 Examples of Young Rift Zones
Many young rift basins all over the world have been
studied in some detail and are still under investigation. Of these, only a few can be briefly described.
The following examples inelude some of the elassical
rift structures, but there are possibly some better
suited cases than those
presented here.
The East African rift zone. The most prominent
and frequently described present-day rift zone, the
4000 km long East African rift, is an example of the
enormous variability of rift basins and their sediment
fills. It lies within a large, elongated dome structure
(the Kenyan and Ethiopian dome) which has risen
549
ab out 1800 m since the middle Tertiary (Fig. 12.3a).
The narrow Western Rift is occupied by lakes arranged en echelon (cf. Fig. 12.2a) and displays a relief of approxirnately 3000 m between the rift shoulders and the floors of the deepest lakes (Lake
Tanganyika and Lake Malawi). Here, volcanic activity is virtually absent. The 50 km wide Eastern Riß
(or Gregory Rift) exhibits lower relief, and is subdivided by trans verse ridges into several srnall, shallow
basins, partially occupied by lakes. The Eastern Rift
in particular was strongly affected by pre-rift and
syn-rift volcanism of basaltic to more felsic and
alkali-rich nature (Fig. 12.3b). The dip direction of
the detachment zone varies from east to west and is
marked by "off-axis" volcansism (Bosworth 1987).
Where the volume of volcanic products exceeded the
space created by rifting, the rift zone was totally
filled by volcanic flows and volcaniclastic sediments.
In the main Ethiopian Rift, six major volcanic episodes
were recognized from the late Oligocene to the Quaternary
(Woldegabriel et al. 1990; see also Uemura and Mizutani
1984; Frostick et al. 1986; Foster et al. 1997). Extruding
carbonatitic magmas are at least partially responsible for
the extreme\y high alkalinity (pR up to 10.3) and chemistry
of sodium carbonate lakes of the Magadi-type in the
Gregory Rift (Eugster 1986). The present-day climatic conditions of the East African rift zone range from a tropical,
high-rainfall belt to desert areas. Open lake systems with
fresh water alternate with closed lake basins where
evaporites accumulate. The sediments of these basins have
been described, e.g.m by Tierce\in and Faure (1978),
Pickford (1986) and many others.
The East African rift basins displaya large variety of
alluvial fan, fluvial, and lake deposits. The latter reflect changing elirnatic conditions through time and
from basin to basin. Diatomites and black shales occur in the more humid regions, while arid zones are
characterized by playa lakes with evaporites, ineluding sodium carbonate and lacustrine chert. The rift
basins of the Eastern Rift in particular contain abundant volcanielastics of varying composition. Repeated ashfalls favored the preservation of footprints,
soft-bodied organisms, and all kinds of organic relics. Sedimentation rates in this rift zone range from
about 10 cmlka up to several 100 cmlka.
The Red Sea and Gulf of Suez. To the north, the
East African rift system continues and forms the Red
Sea basin. The evolution of this basin started 70 Ma
aga and is more advanced than the East African rift
zone. One can distinguish four phases of development (e.g., Guennoc et al. 1988): (1) A Cretaceous to
Eocene pre-rift stage, (2) Oligocene rift formation,
(3) Miocene subsidence (Red Sea depression), and
(4) Pliocene to Recent sea-floor spreading. The sedimentary history of this basin is briefly discussed in
Section 4.3.3. The Miocene evaporites were depos-