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

Abstract: Integration of regional seismic interpretation, sonic velocity, vitrinite reflectance and apatite fission-- track analysis (AFTA(R)) studies has demonstrated that the western region of the Moray Firth rift arm (UK North Sea) experienced pronounced exhumation during the Cenozoic. Although this basin is usually considered to have experienced regionally uniform exhumation, interpretation of new seismic data has revealed the presence of a major system of post-Jurassic normal faults, with throws commonly in the range of 10300 rn and locally exceeding 1 km. New, high-quality seismic data are used in combination with AFTA and vitrinite reflectance data to investigate the role of extensional faulting during exhumation of this basin. Results of this interpretation not only confirm the offsets across major faults, but also show that greater exhumation and erosion occurred on their footwalls than on their hanging walls. We conclude that the localized, differential exhumation is the result of superposition of local or short-spatial-wavelength extensional tectonics upon regional, long-spatial-wavelength exhumation. These results suggest that differential exhumation might be characteristic of unroofed rift basins where normal faults subcrop the exhumation-related unconformity and that, in such cases, thermal histories from footwall locations may yield inaccurate predictions of the burial history of hanging-wall depocentres. Inaccurate burial histories will lead to a misrepresentation of the thermal history, with an impact on the estimation of hydrocarbon source rock maturity for petroleum basins.

Keywords: Moray Firth, exhumation, inversion tectonics, AFTA, source rocks.

The Moray Firth rift basin is located on the western arm of the UK North Sea Late Jurassic trilete rift system. Stratigraphic datum points in the western extremity of the Moray Firth basin, the Inner Moray Firth, are c. 500-1500 m shallower than they are in the Viking and Central Graben areas to the east. The shallower depth of Jurassic sediments, and the major sea-bed unconformity in the Inner Moray Firth, is generally regarded as the result of early Tertiary exhumation (Underhill 1991a; Hillis et al. 1994). (In this paper we use the term 'exhumation' to refer to height above maximum burial depth relative to the datum of present-day mean sea level.)

The deep structure of the Inner Moray Firth basin is dominated by numerous extensional tilted fault blocks that were active during the Late Jurassic synrift episode (Underhill 1991a). The basin's overall configuration is defined by three major fault systems; the Wick Fault on the northern margin, the Banff Fault to the south and the Helmsdale Fault to the west (Fig. 1a). Newly acquired seismic data have allowed detailed mapping of the entire basin structure bounded by these fault systems (Fig. 1a). The purpose of this paper is to integrate this new structural interpretation with direct measurements of exhumation from wells in the basin, using new apatite fission-track analysis (AFTA(R)) and vitrinite reflectance (VR) data obtained from deep petroleum exploration wells in the basin interior, to more fully understand the kinematics of post-Jurassic basin evolution. On the basis of these results, specifically from two exploration wells, we present here a new model of partitioned net exhumation for this basin.

Database

The seismic database consists of over 2500 km of regional 2D seismic lines. Line spacing is generally of the order of 1-2 km. Well data consist of 54 penetrations, almost all drilled to depths in excess of 2500 m (Fig. 1a). New AFTA data have been made available to this study from a number of these wells and onshore locations, but we examine in detail two offshore exploration wells, 12/23-1 and 12/24-2 (Fig. 1a). These two wells are located with 5 km separation on either side of the Smith Bank Fault and are conveniently placed to compare the exhumation on either side of this major sub-basin fault (Fig. 1a). These new offshore AFTA data were integrated with new VR measurements to give estimates of the magnitude and timing of exhumation.

Data analysis: evidence for presence, timing and magnitude of basin exhumation

Regional seismic mapping

Tilt of the whole of the western area of the North Sea is apparent from regional studies of seismic and well data (Underhill 1991a; Underhill & Partington 1993; Japsen 1997, 1999). This tilt is responsible for the relative elevation of the Inner Moray Firth rift in comparison with the deeper North Sea sub-basins further east. Sea-bed subcrop mapping within the Inner Moray Firth confirms the easterly dip of the strata (Andrews et al. 1990; Fig. 1a). The stratigraphic column penetrated to date by exploratory wells within the entire Moray Firth rift is illustrated in Fig. lb. The complete section spans the Devonian to Tertiary. However, in the Inner Moray Firth the uppermost section of the Upper Cretaceous Chalk Group and any subsequent Tertiary sequences are absent. East-west-trending seismic lines that pass from the Inner to Outer Moray Firth show that the Upper Cretaceous Chalk Group and lowest Paleocene sediments are truncated at the sea bed, implying that exhumation began after Chalk Group deposition in the Tertiary (see fig. 4 of Argent et al. 2000). Argent et al. (2000) pointed out that the Chalk Group is the cause of many sub-Cretaceous seismic imaging problems in the North Sea, even with recent high-fidelity 3D data. Its absence from the Inner Moray Firth results in improved image quality, allowing correlations across faulted Lower Cretaceous and Jurassic sections to be made with a high degree of confidence. As indicated in Fig. la, most of the wells in the basin were available to this study and were used to constrain the seismic interpretation. Present-day Base Jurassic structure as mapped is shown in Fig. 2. The quality of data on which this mapping is based and the nature of the structures themselves is shown in Fig. 3, which illustrates an Upper Jurassic synrift package growing into the major extensional sub-basinal faults in the middle of the basin (Underhill 1991b). The largest faults in the Inner Moray Firth have up to 2 km offset of the Base Jurassic reflector. However, these large sub-basin bounding faults also offset the Base Cretaceous (Fig. 3) and the Base Tertiary, with throws in the range of 10-300 m. Therefore, although these structures are principally Mesozoic in age, they have a significant post-Jurassic history of reactivation that is essentially absent from the deeper North Sea sub-basins (e.g. Glennie & Underhill 1998). Some minor movement may have occurred during the deposition of the earliest Early Cretaceous, Hauterivian Stage (Fig. 3). This movement is of the order of tens of metres of displacement on the Smith Bank Fault, indicated by the minor sequence thickening towards the fault at this level. Correlation of footwall with hanging-wall reflectors suggests that subsequent Cretaceous deposition was not associated with any further extensional fault movement. The most significant component of extensional reactivation of these structures was post-Cretaceous, indicated by mapped offsets of the Top and Base Cretaceous across the main faults in the basin (Fig. 1a).