ABSTRACT

Detailed petrographic examination, X-ray diffraction and microprobe analysis of the clay minerals, and iron-content analysis suggest a diagenetic origin of color for the clastic sedimentary rocks of the Visingso Group (Upper Proterozoic, southern Sweden). The drab colors (grey, green, and black) were developed under reducing conditions by reduction of ferric oxide and/or ferric oxide hydrate. The resulting ferrous iron was incorporated in the authigenesis of iron-rich chlorite and pyrite. In the red, yellow, and orange rocks the clay minerals are predominantly mixed-layer illite-montmorillonite, illite, and kaolinite. In the red sandstones the chlorite is less rich in iron whereas the illite is frequently richer in iron compared to those in the drab sandstones. The red rocks are chara terized by higher ferric iron content due to the presence of pigmental and euhedral hexagonal hematite. The hematite most commonly occurs as particle coating, on altered detrital ferromagnesian silicates, and with interstitial clay minerals. Iron was essentially derived from the diagenetic alteration of detrital iron-beating minerals.