10.3 BASIN EVOLUTION, METALLOGENY, AND THE PETROLEUM SYSTEM
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frequently emplaced in continental red beds during this early phase of basin development (Flint, 1986). Insoluble heavy minerals may weather out of the volcanogenic rocks
and become segregated into placer ores. These may then undergo repeated phases of
erosion and redeposition throughout subsequent sedimentary cycles within the basin
(Fig. 10.25B).
McKenzie's model, supported by field evidence, shows that crustal stretching, basin
subsidence, and heat flow gradually diminish with time. Faulting continues, but the
slower rate of movement permits sediment to blanket them, and they are no longer
manifest on the surface. As the rift loses its surface expression it evolves into a gently
sloping elongate or subcircular basin.
With continuing subsidence the basin floor may descend to sea level. In arid climates
this may permit vast coastal sabkhas to form, from which evaporites may be deposited
with associated limestone reefs (Fig. 10.25C). The modern Gulf of California exemplifies this stage. The Miocene evaporites and reefs of the Red Sea are an ancient analog. In humid climates, however, evaporites will be absent, but the conditions are suitable for the development of marshes and swamps. The resultant peat deposits may be
preserved and evolve into coals during burial.
With continued subsidence the sea will invade the trough permanently. Restricted
access and circulation at the commencement of this phase may favor the development
of thermally or salinity induced density layering in the seawater. This may result in
anoxic bottom conditions that favor the preservation of organic matter. The earlier
continental and evaporitic deposits thus become overlain by potential petroleum
source beds (Fig. 10.25D).
As subsidence continues the basin becomes infilled with open marine sediments. If
the hinterland is of low relief the basin may be infilled by carbonates. Where the hinterland is high, and subsidence active, however, deep-sea sands may be deposited at
the foot of rapidly prograding deltas (Fig. 10.25E). Now all sorts of exciting things begin to happen. If the basin fill included a phase of evaporite deposition, then evaporitederived brines may mobilize, concentrate, and precipitate Mississippi Valley-type leadzinc ore in adjacent carbonate reefs (see Section 9.6.5). The overlying organic-rich
shales may now reach thermal maturity, aided by the thermal blanket of rapidly deposited and often overpressured muds (Section 7.2). Petroleum is generated and migrates up to be trapped in reservoirs around the basin margin (McKenzie, 1981). Simultaneously, the evaporites may undergo halokinesis to form traps for the migrating
petroleum (Fig. 10.25F).
Now the basin gradually enters a new phase. Subsidence declines and stops, heat flow
wanes, and overpressured connate fluids bleed off at the surface. Pore pressures adjust
to normal hydrostatic conditions. Deep within the basin, connate fluids may move in
thermally driven convection cells (Neglia, 1979; Bonham, 1980). If the basin is in isostatic equilibrium, the land surface may be intensely weathered, leading to the formation of gossans over sulfide ore bodies and to diverse residual pedogenic and placer
mineral deposits (Fig. 10.25G).
Finally, and this may happen after many millions of years of equilibrium, some basins
undergo a subsequent phase of uplift. The causes of inversion are several, and include
crustal compression and wrenching. Uplift initiates a new series of processes, the most
obvious of which is the erosion of the upper part of the sedimentary fill. Hydrostatic
501
frequently emplaced in continental red beds during this early phase of basin development (Flint, 1986). Insoluble heavy minerals may weather out of the volcanogenic rocks
and become segregated into placer ores. These may then undergo repeated phases of
erosion and redeposition throughout subsequent sedimentary cycles within the basin
(Fig. 10.25B).
McKenzie's model, supported by field evidence, shows that crustal stretching, basin
subsidence, and heat flow gradually diminish with time. Faulting continues, but the
slower rate of movement permits sediment to blanket them, and they are no longer
manifest on the surface. As the rift loses its surface expression it evolves into a gently
sloping elongate or subcircular basin.
With continuing subsidence the basin floor may descend to sea level. In arid climates
this may permit vast coastal sabkhas to form, from which evaporites may be deposited
with associated limestone reefs (Fig. 10.25C). The modern Gulf of California exemplifies this stage. The Miocene evaporites and reefs of the Red Sea are an ancient analog. In humid climates, however, evaporites will be absent, but the conditions are suitable for the development of marshes and swamps. The resultant peat deposits may be
preserved and evolve into coals during burial.
With continued subsidence the sea will invade the trough permanently. Restricted
access and circulation at the commencement of this phase may favor the development
of thermally or salinity induced density layering in the seawater. This may result in
anoxic bottom conditions that favor the preservation of organic matter. The earlier
continental and evaporitic deposits thus become overlain by potential petroleum
source beds (Fig. 10.25D).
As subsidence continues the basin becomes infilled with open marine sediments. If
the hinterland is of low relief the basin may be infilled by carbonates. Where the hinterland is high, and subsidence active, however, deep-sea sands may be deposited at
the foot of rapidly prograding deltas (Fig. 10.25E). Now all sorts of exciting things begin to happen. If the basin fill included a phase of evaporite deposition, then evaporitederived brines may mobilize, concentrate, and precipitate Mississippi Valley-type leadzinc ore in adjacent carbonate reefs (see Section 9.6.5). The overlying organic-rich
shales may now reach thermal maturity, aided by the thermal blanket of rapidly deposited and often overpressured muds (Section 7.2). Petroleum is generated and migrates up to be trapped in reservoirs around the basin margin (McKenzie, 1981). Simultaneously, the evaporites may undergo halokinesis to form traps for the migrating
petroleum (Fig. 10.25F).
Now the basin gradually enters a new phase. Subsidence declines and stops, heat flow
wanes, and overpressured connate fluids bleed off at the surface. Pore pressures adjust
to normal hydrostatic conditions. Deep within the basin, connate fluids may move in
thermally driven convection cells (Neglia, 1979; Bonham, 1980). If the basin is in isostatic equilibrium, the land surface may be intensely weathered, leading to the formation of gossans over sulfide ore bodies and to diverse residual pedogenic and placer
mineral deposits (Fig. 10.25G).
Finally, and this may happen after many millions of years of equilibrium, some basins
undergo a subsequent phase of uplift. The causes of inversion are several, and include
crustal compression and wrenching. Uplift initiates a new series of processes, the most
obvious of which is the erosion of the upper part of the sedimentary fill. Hydrostatic