500
10 SEDIMENTARY BASINS
distribution and effectiveness of seals, and tectonic style. All of these together control
the degree of resistance of the basin to petroleum migration. One of the major controls
of petroleum generation is the heat flow to which the basin is subjected. As described
earlier, McKenzie (1978) produced a mathematical analysis of the relationship between
crustal extension (quantified as the/3 factor), crustal thinning, heat flow, and the rate of
rift basin subsidence. The model begins with a dramatic phase of rapid crustal extension, subsidence, and high heat flow. This is followed by a subsequent phase of slower
subsidence and a return to normal heat flow, as the thinned crust reestablishes its thermal equilibrium.
In the first phase faults extend up to the earth's surface causing the formation of rift
valleys. Subsequent slower subsidence allows sediment to blanket the faults and gentler
basin slopes to prevail. Neugebauer (1987) has reviewed the subsequent evolution of
models of basin formation. As currently perceived the sequence sometimes, but not invariably, begins with a prerift phase of crustal doming (Fischer, 1975). The prebasinal
phase of uplift is attributed to localized heating and expansion in the underlying asthenosphere (Fig. 10.25A). Crustal doming is followed by erosion of the crest and then
by rifting. These domes cause regional negative gravity anomalies, and may be up to
1 km high and 2000 km in diameter (Kazmin, 1987). Neugebauer (1987) cites the Hoggar and Tibesti domes of the Sahara as modern examples. The Tertiary Rhine graben
is an example of a rift developed on a regional crustal dome (Fig. 10.19). An ancient
example is provided by the Sirte embayment of Libya. This resulted from the midCretaceous collapse of a persistent Paleozoic crustal uplift, the Tibesti-Sirte arch (Section 10.2.2.1). Not all basin-forming mechanisms are preceded by a domal phase, however. Kazmin (1987) differentiates rifting associated with pulsating phases of high heat
flow, and rifts related to plate separation. The latter are unassociated with doming, and
their high heat flow is not the cause of rifting, but the result of the associated crustal
thinning.
Whether preceded by crustal doming or not, the resultant rift basins are initially
infilled by continental sediments and are associated with volcanic activity and high heat
flow. The East African rifts illustrate this phase of development (Section 10.2.4). The
early basin fill sediments are laid down in continental environments because the rift
floor is still substantially above sea level. Fluvial sediments along the rift margins pass
into finer grained sediments in the basin center. In humid climates this will be a permanent lake in which organic matter may be preserved to evolve into petroleum source
beds. In arid climates the center of the rift may contain ephemeral lakes and sabkhas,
such as the Quattara depression in the Western Desert of Egypt.
Meteoric fluids move downward and laterally through the sedimentary pile hydrodynamically in response to gravity. These fluids are normally oxidizing and acidic as discussed elsewhere (see Section 8.5.3.3). The volcanic activity that is associated with rifting has an important role to play in metallogeny. It facilitates the emplacement of
numerous minerals within the sedimentary cover. Some of these may remain as primary
ore deposits in the volcanogenic pile, but others may undergo remobilization. The flow
of acid oxidizing groundwater leaches out soluble minerals, transports them in solution,
and reprecipitates them elsewhere during the first or subsequent sedimentary cycles.
Uranium carnotite ores are emplaced in this manner (see Section 6.3.2.2.4). In arid
areas sodium-rich evaporites form in ephemeral desert lakes. Copper ore deposits are
10 SEDIMENTARY BASINS
distribution and effectiveness of seals, and tectonic style. All of these together control
the degree of resistance of the basin to petroleum migration. One of the major controls
of petroleum generation is the heat flow to which the basin is subjected. As described
earlier, McKenzie (1978) produced a mathematical analysis of the relationship between
crustal extension (quantified as the/3 factor), crustal thinning, heat flow, and the rate of
rift basin subsidence. The model begins with a dramatic phase of rapid crustal extension, subsidence, and high heat flow. This is followed by a subsequent phase of slower
subsidence and a return to normal heat flow, as the thinned crust reestablishes its thermal equilibrium.
In the first phase faults extend up to the earth's surface causing the formation of rift
valleys. Subsequent slower subsidence allows sediment to blanket the faults and gentler
basin slopes to prevail. Neugebauer (1987) has reviewed the subsequent evolution of
models of basin formation. As currently perceived the sequence sometimes, but not invariably, begins with a prerift phase of crustal doming (Fischer, 1975). The prebasinal
phase of uplift is attributed to localized heating and expansion in the underlying asthenosphere (Fig. 10.25A). Crustal doming is followed by erosion of the crest and then
by rifting. These domes cause regional negative gravity anomalies, and may be up to
1 km high and 2000 km in diameter (Kazmin, 1987). Neugebauer (1987) cites the Hoggar and Tibesti domes of the Sahara as modern examples. The Tertiary Rhine graben
is an example of a rift developed on a regional crustal dome (Fig. 10.19). An ancient
example is provided by the Sirte embayment of Libya. This resulted from the midCretaceous collapse of a persistent Paleozoic crustal uplift, the Tibesti-Sirte arch (Section 10.2.2.1). Not all basin-forming mechanisms are preceded by a domal phase, however. Kazmin (1987) differentiates rifting associated with pulsating phases of high heat
flow, and rifts related to plate separation. The latter are unassociated with doming, and
their high heat flow is not the cause of rifting, but the result of the associated crustal
thinning.
Whether preceded by crustal doming or not, the resultant rift basins are initially
infilled by continental sediments and are associated with volcanic activity and high heat
flow. The East African rifts illustrate this phase of development (Section 10.2.4). The
early basin fill sediments are laid down in continental environments because the rift
floor is still substantially above sea level. Fluvial sediments along the rift margins pass
into finer grained sediments in the basin center. In humid climates this will be a permanent lake in which organic matter may be preserved to evolve into petroleum source
beds. In arid climates the center of the rift may contain ephemeral lakes and sabkhas,
such as the Quattara depression in the Western Desert of Egypt.
Meteoric fluids move downward and laterally through the sedimentary pile hydrodynamically in response to gravity. These fluids are normally oxidizing and acidic as discussed elsewhere (see Section 8.5.3.3). The volcanic activity that is associated with rifting has an important role to play in metallogeny. It facilitates the emplacement of
numerous minerals within the sedimentary cover. Some of these may remain as primary
ore deposits in the volcanogenic pile, but others may undergo remobilization. The flow
of acid oxidizing groundwater leaches out soluble minerals, transports them in solution,
and reprecipitates them elsewhere during the first or subsequent sedimentary cycles.
Uranium carnotite ores are emplaced in this manner (see Section 6.3.2.2.4). In arid
areas sodium-rich evaporites form in ephemeral desert lakes. Copper ore deposits are