The Mesoproterozoic sub-Heddersvatnet unconformity, Telemark, South Norway

• Main Author: K. Laajoki
• Format: Article
• Language: English
• Published: Geological Society of Finland 2005-06-01
• Series: Bulletin of the Geological Society of Finland
• Subjects: metasedimentary rocks; metavolcanic rocks; lithostratigraphy; unconformities; Mesoproterozoic; Rjukan; Vindeggen; Telemark; Norway
• Online Access: http://www.geologinenseura.fi/bulletin/Volume77/Laajoki.pdf

The 1.51 Ga Rjukan group,Telemark, S Norway, is divided into felsic volcanic rocks of the Tuddal formation and mafic rocks of the Vemork formation. It is overlain by the sedimentary rocks of the Vindeggen group (1.50-1.17 Ga), starting with the arkosic Heddersvatnet formation.The contact between the Rjukan and Vindeggen groups has been variably interpreted in the literature. New field data indicate that the contact corresponds to an unconformity, corroborating Wyckoff's early observations in 1934.The contact is referred to as the sub-Heddersvatnet unconformity. The nature of the contact varies. Around Lake Heddersvatnet, it most likely represents an angular unconformity with a sharp, erosional surface, whereas near Lake Skjesvatnet a thin in situ palaeoweathering crust developed on a massive Tuddal porphyry defines it, These observations indicate that the sub-Heddersvatnet unconformity represents a deeply weathered land surface cutting diverse folded Tuddal units. How big is the time gap it represents and the nature of the pre-Vindeggen deformation are open questions as the sedimentation age of the Heddersvatnet formation is unknown and the structure of the Rjukan group has not been studied on a regional scale.