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.