Middle Devonian palynomorphs and carbon isotope stratigraphy of borehole cores in the Dounreay area, Caithness

This preliminary report describes progress in the use of fossil palynomorph assemblages and δ13C org stratigraphy in correlating cores of Devonian strata from the Dounreay Site area. A total of 134 samples from Nirex BH 2, SHIP-2, SHIP-8, SHIP-9, SWB-6, SWB-8 and SWB-9. boreholes have been analysed for palynology, while a further 94 samples from Nirex BH 2 and SHIP-2 boreholes were analysed for bulk δ13C org. A list of samples and sampling intervals is given in Section 13. The objectives of the study were to: 1. determine the age of spore assemblages in the cores, using standard Devonian biozonation schemes; 2. test the presently accepted lithological and geophysical correlations between the boreholes, using standard palynostratigraphical methods; 3. characterise cycles 21 and 26 within the Nirex BH 2 and SHIP-2 boreholes using palynology and δ13C org; 4. correlate the most complete cored reference sequences in the Nirex BH 2 and SHIP-2 boreholes using characterised cycles, and thus test traditional correlation based on the lithological character of cycles and cycle position. Overall, the organic yield of the samples is high. The organic residue is dominated by yellow brown amorphous organic material (AOM), and palynomorphs (land plant spores, fungal spores and ?spores of unknown affinity) are usually rare. A small number of samples, notably within cycles 21 and 26 (Nirex BH 2 and SHIP-2) contain significant quantities of land-derived phytoclasts (resistant particles of plant material) and very poorly preserved spores of vascular plant origin. Spore colour indicates that thermal maturity is low (TAI value 2 to 2+) and comparable to that recorded in other studies from the Dounreay area (Stephenson 2003, Auton 2004). Spores-pergramme yields vary between 0 and approximately 1200. The detailed trends of palynofacies, palynology and yields-per-gramme of spores within the boreholes are shown in Figures 1-7. The scarcity of age-diagnostic palynomorphs makes age determination of these sequences difficult. However, most of the sections contain spores similar to Rhabdosporites langii and zonate spores similar to Grandispora velata, and none contain the distinctive spore Geminospora lemurata. This indicates a probable early Eifelian to earliest Givetian age for the sampled sections (Stephenson 2003, Richardson & McGregor 1986, Marshall 2000). Conventional palynostratigraphic correlation between the boreholes using the ranges of palynomorph taxa is difficult to apply because the assemblages are dominated by long ranging taxa, which are commonly poorly preserved. In addition, the sparse nature of assemblages prevents reliable correlation, because presence or absence of significant palynomorphs is probably as much a matter of chance as evolution and phylogeny. In order to improve the precision of correlation between the cores and to improve knowledge of Dounreay Middle Devonian palaeoecology and palaeoenvironment, a new correlation method has been employed. This involves determining the palynological, palynofacies and δ13C org characteristics of certain groups of strata in order to ‘fingerprint’ them. Using this approach it has been possible to fingerprint lacustrine cycles 21 and 26 in Nirex BH 2, and match these to fingerprints of corresponding cycles previously identified by lithological logging in SHIP-2 (Figures 9 and 10). Thus, the presently accepted correlations (JacobsGIBB 2002) based on down-hole geophysical logs (notably natural gamma) and lithological correlations based on ‘cycle-counting’ between Nirex BH 2 and SHIP-2 are now also supported by palynological, palynofacies and δ13C org evidence. This is crucial independent evidence required for the development of an accurate 3D model of the geology of the Dounreay site area and should provide firm constraints on the estimates of the cumulative throw of faults between the 2 borehole sites, c. 1 km apart. The presence of abundant AOM, which has been shown throughout the Orcadian Basin to be of lacustrine algal origin, indicates that the sediments in the Dounreay site area were deposited in a lake that was occupied by algae. The scarcity of terrestrially-sourced phytoclasts, as well as the low spores-per-gramme values throughout the sections, even in facies indicating a relatively proximal depositional position, could indicate a relatively sparse flora occupied the hinterland of the Dounreay during the mid-Devonian. This may have been due to aridity or to plant assemblages reflecting only early stages of colonisation. The low ratio of phytoclasts to spores, which is unlike that seen in younger sediments (for example of the Carboniferous or Permian), probably indicates the lack of supportive plant tissues in the hinterland plants, which were likely to have been small and herbaceous in character. This joint palynological, palynofacies and δ13C org analysis has the potential to provide a better broad correlation between the cored sequences than palynology alone can achieve and may allow better correlation between units mapped at the surface and those recognised in the cores. Consequently, the next phase of work, which mainly consists of collection and analysis of surface samples from across the area will use a similar methodology to that developed here. The aim of this new work will be to link the surface exposures in the Site Area to the cores, put the Dounreay sequence in its regional context, and also allow (in conjunction with petrological studies) correlation of the Baligill and Achanarras fish bed maker horizons across the district. An understanding will also be gained of how the Dounreay sequence differs from younger Orcadian successions farther east.