Cauldron subsidences, ring-structures, and major faults in the Skien district, Norway

Permian cauldrons and ring-structures new to the Skien district of the Oslo Region are described. The Vealøs and Skrehelle cauldrons are mostly composed of monzonitic (larvikite) and syenitic rocks (nordmarkites) with xenoliths of volcanic rocks (B1, RP1, etc.). The Mykle ring-structure is composed of monzonite (larvikite) with granite (ekerite) as the central pluton. All three have ring-dikes of syenitic composition. Total field aeromagnetic maps show the well known cauldrons of the Oslo Region as characteristic curved magnetic anomalies. Corresponding anomalies in the Skien district confirm the cauldron hypothesis concerning this district. Major faults active in Precambrian and Permian times may have determined the location of the cauldrons in the Oslo Region.