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cations: the North Sea is more of a graben, and Permian salt is diapiric in
places. The main structural features are shown in Fig. 16-12.
Subsidence during the Permian and Triassic caused accumulation of sedi-
ments of various environments, and by the Triassic this subsidence and accu-
mulation of sediment was accompanied by normal faulting that influenced
the rate of subsidence and so the thicknesses accumulated. During the Creta-
ceous, fault movement became less important, and it had virtually ceased by
the end of the Cretaceous. Thereafter, epeirogenic subsidence continued
without faulting throughout the Tertiary, the rate of subsidence being variable
in both space and time, and so controlling thicknesses. The physiography of
the North Sea suggests that this subsidence is continuing.
In broad outline, the Permian period was continental in the North Sea
area to begin with, followed by marine transgression of great extent in north-
west Europe, and the accumulation of a sequence of evaporites that were to
become diapiric in the southern North Sea and in north-west Germany. The
continent persisted in the North Sea area, with conformable accumulation of
cyclic sequences of evaporites (Brennand, 1975), with some marine influence
in middle Triassic times. Marine conditions became more widespread by the
Jurassic, and dominated thereafter as regards sediment accumulation.
But sediment accumulation was not continuous either in space or time.
The North Sea basin consists of several sub-basins, the histories of which
varied and are reflected in the petroleum accumulations. Early Jurassic sedi-
mentary rocks, deformed by contemporaneous faulting, were eroded on highs;
and in the northern North Sea there is sporadic evidence of the local accumu-
lations of some of the products of this erosion, but in general it is not known
where the products were carried. There was an episode of non-accumulation,
followed by the accumulation of a thin sequence of Upper Jurassic shales
and mudstones. Of these, the Kimmeridge Clay Formation appears to result
from a transgression over the whole area with but minor exceptions, and the
consistent facies and generally high organic content make this an important
event in the evolution of the North Sea basin. The stratigraphic relationships
about this late Jurassic non-accumulation are broadly of disconformity, but
with local unconformity on eroded highs.
Still later in the Jurassic and into the early Cretaceous there was another,
perhaps more important period of non-accumulation of sediment, especially
in the north. The nature of this non-accumulation is not easily determined
because the data by which rocks can be dated is only acquired from boreholes
and these tend to be restricted to the highs on which erosion may have been
deeper and transgression subsequently later. In the Danish North Sea, Childs
and Reed (1975) report continuous shale accumulation from Late Jurassic
into Early Cretaceous with but a shallowing of the seas (as indicated by sub-
littoral microfaunas). In the northern North Sea there is a hiatus that lasted
typically from late Kimmeridge or Barremian times to the beginning of the
Coniacian or, on top of the highs, even as late as the beginning of the Maas-