Geological investigations by drill core logging for the Rogfast tunnel project

Abstract

In the framework of Rogfast undersea tunnel project, the Norwegian road authorities, western region, have cored four locations along the tunnel trace. The report contains the results of the geological logging of several of these cores from drilling at four locations. The first core comes from Alstein, an island between Randaberg and Kvitsøy, and displays the typical intrusive mainly gabbroic rocks of the Karmøy Ophiolite unit. A shear zone at the bottom of the core may indicate the vicinity of the sole detachment of the Hardangerfjord Nappe. The three other cores are from Kvitsøy. They display the typica lmeta-volcanic and volcano-clastic greenstones outcropping on the island but also a large proportion of graphite-rich blackshales which was not previously reported in the bedrock suite. The inspection of the cores drilled northward from Sauholmen, an island north of Kvitsøy, confirms the presence of a tectonic fault zone along the Boknafjord. The other cores were drilled toward the east from Krågøy and Hesteholmen, two islands east of Kvitsøy. Along these cores, quartz-rich units and cherty schists are widely present and interlayered with the black shales and greenstones. The cores show an impressive amount of large fault rock intervals and damaged host rocks toward the expected location of steep regional meridian fault zones.