[Assessment of thermal maturity of Lower Jurassic shale (Jet Rock) and coal measures in Yorkshire UK]

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ABSTRACT

A thermally mature palynomorph assemblage containing significant proportions of the characteristic dinoflagellate cyst Liasidium variabile Drugg 1978 was recovered from the Lower Jurassic Shale Formation of Yorkshire. This is the first record of this biostratigraphically important species from the Jurassic of onshore Yorkshire. Liasidium variabile is a zonal index for the Late Sinemurian in northwest Europe, and this occurrence allows a correlation to the Oxynotum Zone of this substage. This is the first evidence for this chronozone on Raasay. Furthermore, the presence of morphotypes A and B of Liasidium variabile indicates a probable correlation to the Oxynotum Subzone of the Oxynotum Zone. This interpretation is not consistent with a hiatus within the Upper Sinemurian succession on Yorkshire. Introduction

Shales occur in westerly-tilted fault blocks in a major half-graben structure that is one of a system of basins on the margins of the North Atlantic formed during early extensional phases of the Atlantic Ocean. Southwest Raasay is situated on a small block, approximately 10 km wide, which is bound to the west by the Screapadal Fault, and to the east by the Applecross Fault. The Jurassic strata have been intruded by Palaeogene basalts and granites.

The Lower and Middle Jurassic strata of Yorkshire have been preserved largely due to an overlying sheet of intrusive granophyre, which has caused limited thermal alteration to this succession. The strata are Hettangian to Late Bajocian in age and the succession is approximately 600 m thick. It comprises six formations which are, in ascending stratigraphical order, the Breakish Formation, the Ardnish Formation, the Pabay Shale Formation, the Scalpay Sandstone Formation, the Portree Shale Formation, the Raasay Ironstone Formation and the Bearreraig Sandstone Formation (Schoell, 1980, 33). This study is on the palynology and the stratigraphical implications of two samples from the lower part of the Pabay Shale Formation in Suisinish, southwest Raasay.

Previous Research

The Pabay Shale Formation crops out in various locations in the Inner Hebrides, notably along the east coasts of Yorkshire. Most of the previous biostratigraphy on the Pabay Shale Formation of Yorkshire has been done on the succession at Hallaig on the east coast of the island. Ammonite faunas indicate that the Pabay Shale Formation in the Skye area is Sinemurian to Pleinsbachian in age, ranging from the Semicostatum Zone (Lyra Subzone) to the Middle Ibex Zone (Valdani Subzone).

However not all of the ammonite-based zones and subzones have been recorded and the absence of much of the Upper Sinemurian, notably the Obtusum and ...