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12.1.7 Summary (RiftBasins)
- Rift basins result from crustal stretching, either
by pure shear (old continental crust of normal
thickness, steeply dipping bounding faults) or
simple shear along an intra-crustal detachment
surface (younger, hotter crust).
- Supra-detachment basins with gently dipping
bounding faults are COlnmon on continental
margins passing from a long rifting stage to
breakup and the formation of oceanic crust. Here,
the syn-rift basin tills are buried under
continental margin sediments.
- Rift basins are frequently segmented along strike
by transfer zones acting as preferred sites of
volcanism.
- Syn-rift sediments strongly vary in relation to
their position close to or far away from effective
terrigenous sediment sources in different climatic
zones. During early rifting, many basins are
overfilled with fluvial siliciclastics from proximal
Chapter 12 Basin Evolution and Sediments
and distal sources, but may later become
underfilled (lacustrine or marine). Multi-phase
rifting and tilted crustal blocks lead to erosion
and sediment redistribution within the basins,
unconforrnities, and wedge-like half-graben
tillings.
- Many rift basins favored the establishment of
oxygen-restricted lacustrine ormarine conditions
and thus the formation of hydrocarbon source
rocks of limited lateral extent. In addition,
structural traps (e.g. along tilted fault blocks)
and seals were frequently realized.
- Although some general trends in the evolution of
rift basins can be identified, each basin should
be regarded as an individual case.
- Post-rift sediments often overly syn-rift
sediments with an unconforrnity and tend to be
more uniform and cover wider areas (see below).