Page 394 - Petroleum Geology

361

on the products of organic diagenesis, but that facies impose variations on
these.

RIFT BASINS

Most of the principles discussed above apply equally to rift basins, but
they deserve separate consideration because they provide evidence of a world-
wide crustal process that is still incompletely understood. With little varia-
tion in detail, we find a sequence of geological events that spanned approxi-
mately 200 m.y. recorded in rift basins in the North Sea, and the western
margin of Australia. Very similar events were recorded in the north of Alaska
and in Arctic Canada, in the Atlantic margin of South America and around
Africa. The data acquired in the exploration for petroleum has shown that
the “Cretaceous” transgression is but an episode in a sequence of events of
far greater importance that lasted from the Permian throughout the Mesozoic
and into the Tertiary (Kent, 1977). It will be recalled from the discussion of
stratigraphic traps in Chapter 13 that the main features of these events are
these :
- Normal faulting that was contemporaneous with the accumulation of
sediment during the Triassic and Jurassic.
- Erosion of these sediments on highs as deformation continued during
the Jurassic and into the early Cretaceous, commonly resulting in two uncon-
formities.
- Accumulation of fine-grained, low-energy mudstones, mark and silts,
over the disconformity-unconformity surfaces.
- Faulting died out, but epeirogenic subsidence continued irregularly for
the rest of the Cretaceous and into the Tertiary. Accumulation during the
Tertiary was generally without faulting or folding, except in those areas where
regression took place.
- The throw of the bounding fault or faults of the pre-unconformity se-
quence may be very large, to be measured in kilurnetres.
Just as the Cretaceous transgression has been found to be of rather differ-
ent ages in different parts of the world, so the preceding deformation is also
found to vary in age in different parts of the world - even different parts of
a continent.
Along the western and north-western margins of Australia, numerous sedi-
mentary basins (or sub-basins of a very large sedimentary basin) record the
events listed above. Major structural trends were initiated at the end of the
Permian by block faulting (Powell, 1976) that continued to be active in places
throughout the Mesozoic. The main development of rift basins appears to
have, been late Triassic in the north. The phases were not contemporaneous
over the whole area, but tended to migrate south with time. At the southern
end of the north-west shelf, the main rifting phase, with the formation of