Ja. Clausen et al., Development of intraformational (Oligocene-Miocene) faults in the northernNorth Sea: influence of remote stresses and doming of Fennoscandia, J STRUC GEO, 21(10), 1999, pp. 1457-1475
The post-rift Cretaceous sequence of the Horda Platform (eastern margin of
the Viking Graben, northern North Sea) is overlain by Cenozoic silisiclasti
c sediments. Within the latter sequence (Hordaland Group; claystone and thi
nly layered sands), a system of intraformational faults with a strongly dom
inant NW-SE-trend are seen in a transgressive unit of late Oligocene age. O
ccasionally, the faults are associated with deeper pre-Tertiary structures,
but generally there is no such connection. Still, this indicates that the
Oligocene deformation involved reactivation of Mesozoic or even older fault
s. The base of the sequence is represented by an angular unconformity with
a primary mean slope angle of 1.8 degrees when the post-Oligocene tilting o
f the area is corrected for present. The faults have a dominant strike (NW-
SE) deviating 45 degrees with respect to the N-S-striking slope. The lowerm
ost part of the late Oligocene sequence rests on an unstable unit where inc
ipient clay pillows and diapirs are observed.
An evolutionary model including anomalously high fluid pressure, downslope
gravity sliding, gravity collapse and regional tectonic stresses is suggest
ed to account for the origin of the faulting observed. It is likely that a
high fluid pressure, associated gravity collapse and downslope gravity slid
ing were critical for fault initiation. Still, the orientation homogeneity
and parallelism of the fault system suggests that the deformation was influ
enced by a remote tectonic stress system related to ridge-push, doming of F
ennoscandia and the differential subsidence of the North Sea.
This model contrasts to the deformation style seen further to the southwest
in the North Sea, where complex fault geometries and cellular networks com
prising polygonal prismatic and pyramidal forms are observed. The dominant
deformation mechanism in these areas is believed to be failure due to an an
omalously high fluid pressure, without the influence of remote tectonic str
esses. (C) 1999 Elsevier Science Ltd. All rights reserved.