Thermal history of the Phanerozoic sedimentary succession of Skåne, southern Sweden, and implications for applied geology

Abstract: Post-depositional thermal conditions have been investigated throughout the Phanerozoic sedimentary succession in Skåne. Clay mineralogy and illite crystallinity have been the main tools for the Lower Palaeozoic strata. Within the Mesozoic sequence thermal influence was examined using RockEval pyrolysis and vitrinite reflectance analysis. Clay mineralogy of the Lower Palaeozoic shales and mudstones is generally very uniform with illite and chlorite present. Illite crystallinity values (Kübler index) for these rocks range between 0.41–1.2 (delta degrees two theta) (air-dried), indicating diagenetic to high-diagenetic/anchimetamorphic conditions. In SW Skåne a trend with improved illite crystallinity in the more deeply buried and older strata is observed. This is to be expected in a sedimentary basin subjected to burial controlled diagenetic/thermal impact only. However, in the Sorgenfrei-Tornquist Zone (STZ), where illite crystallinity data show no obvious relationship with stratigraphical position, a more complex thermal history is implied. Most striking in the illite crystallinity data from the STZ is perhaps the improved illite crystallinity recorded for the Upper Silurian Colonus Shale and Öved-Ramsåsa Group, i.e., the youngest Palaeozoic strata in Skåne. High-grade illites there indicate a Post-Silurian thermal event, probably caused by the intrusion of numerous dolerite dykes during the Permian and Carboniferous. Besides thermal effects close to intersecting dykes, the subsequent superficial effusives have constituted more extensive thermal impact on the uppermost part of the sedimentary column. In the proximity of intruding dykes anomalous clay mineralogy has been observed, in contact-metamorphosed shale. For example kaolinite has been found at Åkarpsmölla, where a 40 m wide dolerite dyke intrudes Upper Cambrian Alum Shale. Swelling clay minerals, exotic to the Colonus Shale in Skåne, has been identified in the dolerite quarry at Rönnarp. Here a mixed-layer chlorite/smectite mineral was present in the innermost contact-zone, whereas illite was lacking. Mesozoic thermal conditions generally indicate low to moderate palaeotemperatures in the range of c. 40–90 degrees (C). These data are in agreement with burial estimates and suggest that burial imposed the main diagenetic impact. A deviation from this picture of limited thermal influence has been observed from the Höör Sandstone in Central Skåne and is also indicated in SW Skåne close to the Svedala Fault. Hot fluids derived from tectonic and volcanic activity are believed to have affected these strata causing their anomalous thermal maturity.