Palynological evidence of Ordovician strata in Minco CA003 (Gudhamgill, North Pennines)

This report describes the biostratigraphic results of a set of 21 samples taken from the Minco CA001–003 cores (drilled near Nenthead, Northern Pennines) to date a succession of suspected Ordovician age below Yoredale facies: • three spot samples from Minco CA001 • five spot samples from Minco CA002 • thirteen samples from Minco CA003 including the succession of unknown age All samples from Minco CA001 yield a high abundance and diversity of Carboniferous spores, Preservation is poor, and the organic material is heavily carbonized (high maturity) and opaque despite oxidation during the preparation protocol. The assemblage is dominated by Lycospora, L. pusilla and Camptotriletes? sp. 1 with other long ranging specimens from the genera Densosporites and Schulzospora. The five samples collected from Minco CA002 yielded the best preserved palyno-assemblages including some excellent biostratigraphic markers. The unequivocal presence of Triquitrites distinctus constrains the sampled interval to the TC–VF Biozone (Asbian to earliest Brigantian). A single specimen of Murospora parthenopia combined with Discernisporites micromanifestus and Stenozonotriletes coronatus suggests a possible further refinement to the Asbian NM Zone. This latter assignment is tentative as only a single index taxon for the NM Zone was recovered. The Carboniferous palyno-assemblages from Minco CA003 are dominated by Lycospora pusilla which has its FA in the PU Zone while abundant Densosporites and Schulzospora become abundant in the fossil record in the Asbian TC Zone. The repeated presence of S. coronatus with an LAD in the VF Zone suggests the Carboniferous samples from the Minco CA003 borehole are from the Visean TC–VF interval, most likely from the Asbian to possibly the earliest Brigantian British Substage. The interval 574.72–601.07 m of the Minco CA003 Borehole contains very poorly preserved acritarchs and chitinozoans which are almost exclusively entirely opaque, despite the use of fuming nitric acid to oxidise the organic matter. The opaqueness is a result of thermal alteration as the organic matter was subjected to high temperatures during burial (maturation), which also results in the material becoming brittle and susceptible to breaking (e.g., Booth, 1979). Consequently, acritarchs in this study were mainly identified to genus level and the assemblages are dominated by long ranging genera (e.g., Veryhachium, Polygonium, Peteinosphaeridium). The genus Striatotheca (including S. microrugulata) however, was recognised in several samples and in the UK is known to occur from the Arenig to the Llanvirn Series. Two taxa were retrieved from Minco CA003 which are currently only known from the Caradoc (Sandbian–Lower Katian): M. irregulare and N. ancepsipuncta. The latter taxon was represented by a single, very poorly preserved specimen and it would be imprudent to base a chronostratigraphic correlation on this rare occurrence. The most convincing evidence of a Caradoc age is the repeated occurrence of M. irregulare in HEN214 and HEN216, although the same species has been recorded from the Darriwillian (equivalent to upper Arenig – Llanvirn) strata on the Baltica palaeocontinent (Raevskaya et al., 2006). The “pre-Yoredale” strata yielded palyno-assemblages bearing most resemblance to those described from the Skiddaw Group, but it has been shown that these can be reworked into strata dated as Caradoc. Lithologically, there are comparisons with the nearby Allenheads 1 borehole where the Skiddaw Group was identified overlain by the Robinson Limestone (part of the Melmerby Scar Limestone). To firmly establish this tentative correlation, an expanded sampling strategy combined with scanning electron microscopy of the palynomorphs should be conducted. Such a study should include palynological analyses (including scanning electron microscopy) and, if possible, U-Pb dating of the ash/tuff layers described in the “pre-Yoredale” strata from the Minco and Allenheads 1 boreholes.