| Summary: | This study deals with the mineralogy and geochemistry of some Namurian sediments from the Central Pennine Basin, England. The rocks were collected from the Mam Tor area and a borehole at Tansley near Matlock. The Mam Tor rocks (Ric zone) consist of closely associated turbidite sandstones and shales. In the Tansley Borehole (R-E zones) the Ashover Sandstone overlies a condensed sequence of marine and non-marine shales with a volcanic clay at the base. Petrographic and X-ray, diffraction studies, together with major and trace element analyses, have been carried out on whole rock samples and separated size fractions. The Mam Tor sandstones are greywackes, whereas the Ashover sandstones are arkoses. In the sandstones, kaolinite, chlorite and iron oxides are partly of diagenetic origin. The mineralogy of the rocks is comparatively simple; quartz, kaolinite and illite are the principal minerals, with lesser amounts of feldspar, siderite, pyrite, dolomite, calcite and organic matter. Chemical and mineralogical variations in the Mam Tor rocks are due to textural differences, particularly grain size fractionation. Further information on the association of elements with minerals is provided by correlation analysis and t-tests. Chlorite is shown to be rich in iron and illite is the lMd polytype. It is concluded that these rocks have not reached the stage of deep burial diagenesis. Replacement reactions are important in the sandstones, but not in the shales. The volcanic clay is identified as a K-bentonite and originated by reaction between basalt and seawater. The transition from non-marine to marine shales is accompanied by increases in the contentsof illite, pyrite, calcite and organic matter and a decrease in the kaolinite content. Siderite is confined to the non-marine shales in which phosphate and TiO 2 are relatively concentrated. The marine shales are enriched in Ni, V, Zn, Cu, Sr and Pb, whereas the non-marine shales are higher in Zr. The concentration of Cu, Sr and Pb are the most effective discriminators, using discriminant function analysis, between the two groups of shales. The trace element enrichment is attributed mainly to organic matter and pyrite. The variation in Si02/A1203, quartz/combined silica, T102/A1203 and Na20/K20 ratio are related to grain size, the rate of sedimentation and distance from source. The distribution of diagenetic minerals is attributed to changing physico-chemical conditions including salinity. The change from the pyrite to the siderite stability field is considered to reflect decreasing pH and rising Eh. The lateral variation in facies deposited contemporaneously in different sectors reveals the influence of the Derbyshire Block acting as a submarine ridge. It is shown that the textural and geochemical characteristics of the Mam Tor rocks are similar to those of turbidites. The data of the Tansley Borehole sediments indicate their similarity to those from a shallow water, semi-restricted, basin environment onto which a deltaic sedimentation was encroaching.
|
|---|
