Geological and geothermal mapping in Trölladyngja - Sog area SW-Iceland

Abstract

Geological and hydrothermal alteration mapping was carried out in the Sog area within the Trölladyngja geothermal field on the Reykjanes Peninsula, SW-Iceland. The bedrock consists mainly of hyaloclastites, erupted subglacially in upper Pleistocene time. They were divided into 5 eruptive units on the basis of different petrography, minor supraglacial basaltic lavas and intercalated sediment. Holocene lava flows cover the east side and the west side of the study area. The hyaloclastites form bulky NE-SW trending ridges, 100-200 m high, and about a kilometre broad leaving sediment traps between them. The Sog valley is one such, about 1.5 km in length. It is filled with clayey sediment, most of it lacustrine, which is overlain by about 10 m of clay interlayered with peat, clearly of Holocene age. A gorge formed in the Holocene drains the Sog-valley towards west with its tributaries. Erosion has cut a valley transversally across the ridge complex and exposed different grades of alteration down into the hyaloclastite rock. The alteration grades reach from an uppermost pale brown rock with palagonite, down into dark brown or blackish rock with smectite as the dominant clay and below it to a greyish green mixed-layer facies of smectite-chlorite. The transition from smectite to mixed layer clays corresponds to at least 180-200°C and a hydrostatic pressure of at least 17 bar at the time of alteration. The hydrothermal alteration zones imply that the alteration took place subsurface. Despite some erosion an ice thickness of at least 100 m would be needed to explain the chlorite-smectite alteration. A late near surface alteration to plastic smectite clay characterizes the filling of the Sog valley ...