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

The capacity of AFTA to provide an independent determination of the timing at which cooling began is particularly useful in the reconstruction of thermal and burial histories of sedimentary basins, especially if there are unconformities representing large time gaps (Duncan et al. 1998). In the case of the Inner Moray Firth, the Cretaceous and Tertiary sections that are present elsewhere in the North Sea are absent. Their absence is due to either exhumation and erosion of the section or non-deposition. The evidence discussed so far (seismic, VR, sonic velocity) has indicated that exhumation and erosion were responsible. AFTA provides a further, independent, means of testing this and has the potential to yield estimates of timing in addition to the magnitude of basin exhumation.

Combined AFTA and VR in the determination of exhumation magnitudes

The combined interpretation of the thermal history of both AFTA and VR data begins by assessing whether the fission-track age and track length data in each sample (and/or the VR value) could have been produced if the downhole sample had never been hotter than the present-day temperature at any time since its deposition. A burial history derived from progressively removing the overlying preserved sedimentary section is combined with the present-day geothermal gradient to construct a `default thermal history' for each sample. Using this history, default AFTA and VR parameters can be modelled for each sample. If the observed data are consistent with the values predicted from this default history, then the sample is at or close to its maximum post-depositional temperature. If, however, the data show a greater degree of fission-track annealing or VR maturity than expected on the basis of this default history, the sample must have been hotter in the past than it is at the present-day depth of burial. In such cases, AFTA provides an estimate of the time at which cooling began, and both AFTA and VR constrain the magnitude of the maximum palaeotemperature reached by individual samples. Further details on the thermal history response of fission tracks in apatite, the development of AFTA parameters, and the use of AFTA and VR to extract thermal history solutions in sedimentary basins have been given by Green et al. (200 1a, 200 1b, 2002).

Analysis of samples and determination of palaeotemperatures over a range of depths within a vertical well section allows determination of the palaeogeothermal gradient at the onset of cooling. Extrapolation of the palaeogeothermal gradient to an assumed palaeo-surface temperature allows estimation of the amount of missing section (Fig. 6). This provides a measure of the magnitude of exhumation. Combining results from AFTA and VR has two advantages. First, because each technique is calibrated independently, each of the two methods provides independent verification of the results from the other. Second, the joint approach also affords the opportunity to obtain data from various lithologies through the well, to provide palaeotemperature assessment over as wide a depth interval as possible (allowing more precise constraints on palaeogeothermal gradients and removed section).