Heterogeneous exhumation in the Inner Moray Firth, UK North Sea: Constraints from new AFTA and seismic data

Journal of the Geological Society, Nov 2002 by Argent, J D, Stewart, S A, Green, P F, Underhill, J R

Calculation of exhumation magnitude from wells 12/23-1 and 12/24-2

Estimates of the maximum palaeotemperatures from samples taken over a vertical depth range, such as from an exploration well, provide the capability of determining the palaeogeothermal gradient immediately before the onset of cooling. Having constrained the palaeogeothermal gradient, and assuming a value for the surface temperature at the time, the amount of section subsequently removed by exhumation and erosion can be calculated as illustrated in Fig. 6. The total amount of section removed is obtained by dividing the difference between the palaeo-surface temperature (T^sub s^) and the intercept of the temperature profile al the appropriate unconformity (T^sub i^) by the estimated palaeogeothermal gradient. This method relies on the assumption that the temperature profile was linear both throughout the section analysed and through the overlying section that has been removed. Consistency between estimates of eroded section derived in this way and other estimates such as seismic sections suggests that this assumption should be valid (Green et al. 1995).

The calculated Early Tertiary palaeotemperature profile in well 12/23-1 is linear and subparallel to the present-day temperature profile (with a gradient of c. 30 deg C km in each case), supporting the evidence discussed above that heating was mainly due to additional depth of burial (with subsequent cooling as a result of exhumation) rather than elevated, transient heat flow. An estimate of the thickness of section eroded from the sea-bed unconformity can be made by making a linear extrapolation of the present-day and palaeogeothermal gradients to a surface temperature of 6 deg C. This method yields 1.05-1.25 km of missing section from well 12/23-1.

In contrast, results from well 12/24-2 do not require any appreciable amount of eroded section. This indicates a significant variation in the amount of exhumation between these two wells, in spite of their limited spatial separation (5 km). However, the wells lie either side of the Smith Bank Fault, a major normal fault that is seen to have a history of post-Jurassic extensional movement on seismic data. A possible alternative interpretation is that the palaeogeothermal gradient in well 12/24-2 could have been underestimated, such that the section was hotter in the past, therefore allowing some degree of Tertiary exhumation and erosion. However, the amounts of removed section must be fairly small as the Chalk Group is still present in the shallow section of the well and some degree of structural control would still be required to explain the differences in thickness of the preserved section between the two wells.

Integration of AFTA and VR results with seismic imaging of Smith Bank Fault

A seismic line tying wells 12/23-1 and 12/24-2 illustrates the post-Jurassic history of extensional movement on the Smith Bank Fault (Fig. 11). Mapping of the whole dataset (Fig. la) shows that most of this displacement is post-Cretaceous. The depths to Base Jurassic and Base Cretaceous in the wells, tied to the seismic image, indicate that there has been 2350 m of postTriassic displacement on the Smith Bank Fault, of which 750 m is post-Jurassic. Uncertainty in these estimates arises principally from seismic time to depth conversion away from the well control and we estimate that the error in these measurements is