Mineral diagenesis and petrology of the Dala Sandstone, central Sweden

Sammanfattning: The Dala Sandstone, of Proterozoic age, is the most extensive sedimentary unit in central Sweden. The rocks are quartz sandstones, lithic sandstones, feldspathic sandstones, arkoses and greywackes with intercalations of shales and siltstones less than 1 m thick. The Öje Basalt divides the sedimentary sequence into two parts. Dolerite dikes and sills of various compositions cut the sedimentary sequence, and in some parts the basalt. The clastic sediments were deposited in a dominantly continental environment. The detrital components of the sandstones are mainly quartz, feldspar and rock fragments. The most common diagenetic minerals are quartz, illite, chlorite, hematite, titanium minerals, calcite and feldspar. Diagenetic illite and chlorite were formed by alteration of detrital feldspars and biotite by authigenesis, and by aggradation crystallization of the fine-crystalline interstitial clay minerals. The sources of iron and titanium for hematite and titanium minerals, respectively, are mainly detrital biotite and ilmenite. The origin of authigenic quartz is mainly the silica produced from alteration of detrital grains, pressure solution of quartz, and replacement of detrital grains by calcite. Calcite and epidote are common as cement in sandstones close to dolerite. The diagenetic chlorite and most of the illite are Fe-rich and generally well crystallized. Their chemical composition and the crystallinity index of illite, as well as the general absence of diagenetic mixed-layer clay minerals (e.g. illite—smectite) and kaolinite, suggest their formation in the temperature range between 150 and 200°C and at maximum pressure of 1.5 kb.