Apatite fission track analysis (AFTA) is a method of obtaining thermal history information in sediments. In sections that have been hotter in the past, AFTA provides estimates of maximum palaeotemperatures, and the time at which a sedimentary section began cooling from maximum palaeotemperatures. It is useful for studying the thermal history of sequences containing source rocks, and can provide information critical to the understanding of the timing of oil generation. Integration of AFTA and vitrinite reflectance (VR) data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining AFTA and VR data. This approach allows definition of the major facets of thermal history based on directly measurable parameters, rather than on assumed values of palaeo-heatflow, and provides rigorous estimates of the amount of uplift and erosion. The case study demonstrates the application of this approach to the East Midlands Shelf region. Here AFTA data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 Ma. Combined AFTA and VR data show that this cooling was due to kilometre-scale early Tertiary uplift and erosion under near normal geothermal gradients. Our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion, but was regional in extent. This has implications for the evolution of source rock maturity in this region and in other basins in around the UK where AFTA has indicated early Tertiary heating and where the extent and amount of Tertiary uplift and erosion may have also been underestimated by previous workers.
Apatite fission track analysis has been used to study the thermal and tectonic history of the East Midlands Shelf and surrounding areas of onshore UK. Apatites from outcrops of Palaeozoic crystalline basement give fission track ages between 300 ± 29 and 420 ± 31 Ma, which are the oldest yet reported in the UK region and reflect the relative stability of this cratonic region. Mean track lengths in these samples are between 12 and 13
μm
. Namurian and Triassic sandstones from outcrop give apatite fission track ages between
c
. 100 and
c
. 390 Ma, with mean track lengths generally between 10 and 12.5
μm
. Distributions of single grain ages and track lengths suggest that all samples have been annealed (implying exposure to higher temperatures) since deposition, with maximum palaeotemperatures reached some time in the 50–80 Ma range prior to the onset of cooling.
Samples from five exploration wells show consistent patterns, in which apatite fission track ages decrease sharply from around or just less than the stratigraphic age at near surface, to values in the 50–80 Ma range at depths of
c
. 1–2 km (present temperatures of
c
. 40–70 °C), showing that the samples studied have all been exposed to higher temperatures. Single grain ages and track length data suggest that maximum palaeotemperatures were attained at or before
c
. 60 Ma. Subsequent cooling was protracted and the data are compatible with two phases, in the early and mid-Tertiary. Palaeotemperature analysis suggests that palaeo-geothermal gradients were similar to present values, and that elevated palaeotemperatures were due to greater depth of burial, with cooling due to subsequent uplift and erosion. Estimates of uplift and erosion vary from
c
. 1–1.5 km on the east coast (around the Humber estuary) to
c
. 2.2 km in the East Midlands and southern Pennines.
Compilation of palaeotemperature estimates from all samples analysed from the UK onshore region shows that Mesozoic burial and Tertiary uplift has affected a wide area, and that uplift was due to broad-scale, regional warping, with recognized ‘inversion axes’ being areas where uplift and erosion were particularly pronounced or where younger sediments and/or inverted basinal structures allow the identification of uplift and erosion. The recognition of Late Cretaceous/early Tertiary unconformities across wide areas of northwest Europe, both onshore and offshore, suggests that the regional uplift and erosion may have been far more widespread than previously recognized. This has important implications for hydrocarbon exploration in Northwest Europe.
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