The geometry and structural development of Caledonian folds and faults, inner Oslofjord

Abstract

The main goal of this thesis is to make a detailed lithostratigraphic map of Gressholmen, Rambergøya, Heggholmen and Bleikøya, using an improved topographical database.

Upper Ordovician to Lower Silurian sediments are present within the study area, and are involved in a macroscopic anticline, called the Gressholmen anticline. This anticline is bordered to the NW by the Skipsløpet syncline, with another anticline, the Hovedøya anticline, is located still further to the NW. Eighteen different lithostratigraphic units and three types of intrusions have been mapped out in the study area.

The structural geology of the study area reflects deformation of a heterogeneous material undergoing progressive deformation. To understand the kinematic evolution of the Gressholmen anticline it is important to understand the various structures observed within the study area, and what has happened to the heterogeneous material of variable orientations during different stages in the progressive deformations.

Four deformation stages connected to the study area are identified: Firstly early contractional faults with flat-ramp geometry, secondly active buckling, thirdly cleavage development followed by the Upper Paleozoic Oslo Rift.