Clay mineral diagenesis and organic maturity indicators in Devonian lacustrine mudrocks from the Orcadian basin, Scotland

Hillier, Stephen John (1989) Clay mineral diagenesis and organic maturity indicators in Devonian lacustrine mudrocks from the Orcadian basin, Scotland. University of Southampton, Doctoral Thesis.

Abstract

Clay mineral assemblages in mudrocks from the Orcadian Basin consist of illite/smectite (I/S) + illite ± chlorite ± chlorite/smectite (C/S) ± kaolinite ± vermiculite. I/S and illite are the most abundant group, followed by chlorite minerals, kaolinite, and vermiculite, which is only a minor phase. The expandability of I/S varies from 50% to < 5%, the most expandable I/S occurring in the Moray Firth region. In Caithness and Orkney, expandability is < = 10%, `illite crystallinity' varies from 0.9 to 0.18<sup>o</sup> 2θ and polytypes show a parallel change from 1M to 2M. Chlorite minerals are generally abundant in the Lower Old Red Sandstone, absent in the Upper Old Red Sandstone and of variable occurrence in the Middle Old Red Sandstone. Kaolinite has the opposite distribution on chlorite. Both spore colour and vitrinite reflectance are suppressed in Type I, Type II kerogens. Only results from Type III kerogens are used to assess the level of organic maturation, which shows an extreme range. Spore colour varies from yellow-orange to black and mean random vitrinite reflectance from 0.6 to 10.5%. This indicates palaeotemperatures from 100 to 400^oC. The highest maturities are related to the contact metamorphic affects of Devonian plutons and it is postulated that a previously unrecognised pluton underlies much of Caithness. Expandability of I/S correlates with vitrinite reflectance, but the relationship in the Moray Firth area is different to that for the Caithness-Orkney area. Geological evidence favours an I/S origin from original smectite and the different relationships are believed to result from the different relative response of organic maturity indicators and illitization to temperature and reaction time. Lower expandabilities in Caithness and Orkney at a given vitrinite reflectance are attributed to relatively longer burial times. `Illite crystallinity' and polytypes in the Caithness-Orkney area are also correlated with maturity. In areas affected by incipient metamorphism the 1M-2M polytype conversion is complete and illite has been replaced by phengitic mica. The distribution of chlorite minerals with respect to maturity shows that both chlorite and C/S formed during diagenesis at temperatures of 120^oC+ . This occurred by the reaction of illite/smectite and kaolinite with dolomite. Apart from the first appearance of corrensite, C/S diagenesis is not simply related to palaeotemperature. There is only a general palaeotemperature interval (120-260^oC) in which a range of C/S minerals between corrensite and chlorite occur. All C/S minerals are R = 1 ordered suggesting that they are more accurately represented as randomly interstratified (R = 0) chlorite/corrensite. Original detrital kaolinite is common in areas where vitrinite reflectance is below 1.4% but is generally absent in areas of higher maturity due to its destruction during diagenesis. Vermiculite is confined to samples with vitrinite reflectance values > 3% and is interpreted as metamorphic vermiculite. By using organic maturity indicators as a constraint on diagenetic grade the stratigraphic and regional distribution of clay minerals in Orcadian Basin mudrocks is shown to be due to diagenesis, rather than major differences in the original detrital mineralogy. Provided it is calibrated against organic maturity data the smectite-to-illite reaction can be used as an indicator of maturity. Use of both techniques enables different time-temperature histories to be identified and should further understanding of the kinetics of illitization.

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