Geological study of the sedimentary sequence lithology, depositional history and geothermal alteration at Sog in Trölladyngja area, SW-Iceland

Abstract

Findings from surface geological and geothermal investigations carried out at Sog in the Trölladyngja area in SW-Iceland are presented. Good geological information on the stratigraphy of the Holocene sedimentary sequence in Sog area has been lacking, which called for a detailed geological study to unravel the stratigraphical succession and the history of its hydrothermal activity. The findings will contribute to the ongoing geological effort to understand the entire geological/ geothermal system of the Krýsuvík-Trölladyngja-Sandfell area in SW-Iceland. The stratigraphic succession of the Sog field dates back from the present to probably the Saalian glacial period. The bottom of the profile is clearly marked by the underlying hyaloclastite rocks whereas the top is marked by very recent sediments (brownish fine sandy soil, 0.3 m thick) overlying a thin layer of black volcanic ash from 1226 AD, the Stampar eruption at Reykjanes. The Holocene profile consists of about sixty alternating layers of sand, silt, clay and peat deposited in an ancient subglacial basin between two hyaloclastite ridges from which the sediments derive by erosion. The field is hydrothermally altered from no or very slight alteration at the top of the stratigraphic succession, to very intense alteration of the lower parts, similar to the adjacent hyaloclastite ridges. The hyaloclastites have, in places, been completely altered into bluish smectite and mixed-layer smectite / chlorite clays, in an assemblage with other hydrothermal minerals like gypsum, travertine calcite and kaolinite clay, observed in veins and extinct fumaroles. The hydrothermal activity ceased in late Saalian or Weichselian time and has in Holocene times mostly been restricted to fumarolic activity along some of the faults.