Clay mineral diagenesis in carboniferous sediment of the northern Pennines and its relationship to organic maturity indicators

Smart, Gary (1988) Clay mineral diagenesis in carboniferous sediment of the northern Pennines and its relationship to organic maturity indicators. University of Southampton, Doctoral Thesis.

Abstract

Clay minerals from Carboniferous sediments of the Alston and Askrigg blocks and the intervening Stainmore Trough of the northern Pennines, UK, have been examined by XRD, SEM and BEM. Both blocks are underlain by Caledonian granites. During Late Carboniferous times, sediments overlying the Alston Block were intruded by the Whin sills. The percentage smectite in illite-smectite (% S in I/S) ranges from 35% to < 5%. Only homogeneous mudrocks give consistent results. In the case of the Askrigg Block and the Stainmore Trough a good inverse correlation exists between % S in I/S and vitrinite reflectance. Both illitisation 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 underlying 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 Great Whin Sill. Except where close to the contact, the Whin sills had no apparent effect on the % S in I/S. As observed for the Askrigg Block, % S in I/S is directly related to the position of the underlying granite. Kaolinite is absent from the centre of the Alston block, and its occurrence is also related to the granite basement. Unlike the alteration of vitrinite, both the progressive illitisation of illite-smectites of low expandability and the destruction of kaolinite, occur too slowly to reflect the thermal effects of minor igneous intrusions, except where close to the contact. Consequently, clay minerals are potential indicators 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. Within the contact zones of the Whin sills, metamorphic Fe-rich chlorite is common. Occurrences of rectorite and paragonite-muscovite are also restricted to this region. Temperatures in excess of 250^oC were necessary for their formation. Their occurrence is linked to the presence of abundant detrital plagioclase grains, which are considerably corroded where these phases are present. The chemical composition of rectorite is extremely variable, even within the same pore. Rectorite formation was controlled by very localised variations in the chemical environment during the intrusion of the Whin sills. Corrensite occurs as a retrogressive phase after metamorphic amphibole in the contact metamorphic aureole of the Great Whin Sill. Clay and vitrinite data indicate that several localities along major hinge lines surrounding the Alston and Askrigg blocks were the site of considerable heat flow, possibly related to continual hot fluid migration along fault planes.

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