The progressive illitization of interstratified illite-smectite from Carboniferous sediments of Northern England and its relationship to organic maturity indicators

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Abstract
Interstratified illite-smectites from Carboniferous sediments from the Alston and Askrigg blocks and the intervening Stainmore trough of the northern Pennines, UK, were examined by XRD. Both blocks are underlain by Caledonian granites. During late Carboniferous times sediments overlying the Alston block were intruded by the Whin Sill. The percentage smectite in illite-smectite (%S in I/S) varies from ∼35% to <5% only homogeneous mudstones give consistent results. In the case of the Askrigg block and Stainmore trough, a good inverse correlation was found between %S in I/S and vitrinite reflectance. Both illitization of illite-smectite and vitrinite reflectance increase towards the centre of the block. This is thought to be related to high heat-flow centred about the granite basement. In the case of the Alston block, there is no direct relationship between clay and vitrinite data, vitrinite reflectance being controlled by the position and thickness of the Whin Sill. Except where close to the contact, the Whin Sill had no apparent effect on %S in I/S. As observed for the Askrigg block, %S in I/S is directly related to the position of the underlying granite. The progressive illitization of illite-smectites of low expandability is very slow when compared to vitrinite alteration in response to rapidly increasing temperature. Consequently, %S in I/S is a potential indicator of thermal maturation in situations where vitrinites, by virtue of their rapid response to increasing temperatures, fail to provide a regional view of heat-flow patterns.