KINEMATICS OF GRAVITY-DRIVEN NORMAL FAULTING AND IMPLICATIONS
FOR SYN-RIFT STRATAL ARCHITECTURE AND FACIES DISTRIBUTION;
GUDRUN AREA, SOUTH VIKING GRABEN, OFFSHORE NORWAY
Christopher A-L. Jackson
Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial
College, Prince Consort Road, London, SW7 2BP, England, UK
email: c.jackson@imperial.ac.uk
Syn-depositional normal faulting is a key control on the stratigraphic evolution of syn-rift
sedimentary sequences. In this study, 3D seismic and well data are utilised to constrain the
kinematic history of a gravity-driven normal growth fault above an intra-stratal salt detachment
and its impact on syn-rift sedimentation patterns.
Average sediment accumulation rates adjacent to the studied fault (i.e. 0.037 and 0.07
mm a-1 for the footwall and hangingwall of the fault, respectively) are, except from a relatively
brief period, always higher than the average fault throw rate (0.055 mm a-1). Coupled with the
observation that sediment thickness, but not facies, vary across the fault, apart from during one
period, this suggests that the fault was usually blanketed by sediment; as such, syn-rift thickness
variations can be used to document the temporal evolution of the fault. Four discrete periods of
activity are recognised on the fault; (i) a pre-kinematic period, prior to the initiation of activity;
(ii) a relatively protracted period (ca. 8.5 Myr) characterised by low throw rates (average 0.025
mm a-1), punctuated by a period (ca. 3 Myr) when the fault was essentially inactive; (iii) a
relatively short period (ca. 1.8 Myr) characterised by very high throw rates (average 0.182 mm
a-1); and (iv) a final period characterised by waning throw rates and eventual fault death.
Analysis of expansion indices (EI) (sensu Thorsen, 1963) for syn-rift growth strata correlate
with these patterns in fault activity, with high and low EI values characterising periods of high
and low throw rates, respectively, on the fault.
Syn-rift sediment accumulation rate varies through time, with high rates (0.107-0.164
mm a-1), which are associated with deltaic sedimentation during the earliest periods of fault
activity (i.e. period (ii)), resulting in thickness but, importantly, not facies variations across the
fault. In contrast, during the period of high throw rates (i.e. period (iii)) the fault hangingwall
was underfilled and even high (up to 0.128 mm a-1) turbidity current-fed sediment accumulation
rates were not able to fill the developing bathymetric relief. As a result, both bulk thickness and
facies distributions were controlled by the fault, with sand-rich turbidites restricted to the
immediate hangingwall of the fault. Periods of low sediment accumulation rate (0.012-0.075
mm a-1), dominated by mud-prone, shelf-to-slope conditions, correlate with periods of low
throw rate.
The results of this study imply that periods of high sediment supply (during both the riftinitiation and rift-climax) and periods of relatively intense activity on this gravity-driven growth
fault are coupled. Initial activity on the fault appears to correlate with the initiation of activity
on the main half graben-bounding fault, and extensional ‘rafting’ of cover strata above the saltrich, intra-stratal detachment; high sediment accumulation rates associated with a large, riftinitiation deltaic system may have augmented throw rates at this time. Variations in sediment
supply during the rift-climax are speculated to have been driven by the organisation of drainage
systems in updip catchments, after an early syn-rift flooding event. In addition, intra-basin
growth faulting controls the local preservation of syn-rift stratigraphy on the typically lowsubsidence/accommodation hangingwall dipslope, and may control facies distributions if the
fault throw rate is greater than the local sediment accumulation rate.