Mixed siliciclastic-carbonatesedimentation in the Late Ordovician shallow marine epicontinental sea, Oslo Region

Abstract

The Upper Ordovician (Upper Katian to Middle Hirnantian) of the Skogerholmen Formation, Husbergøya Formation and the Langøyene Formation has been studied on the islands Hovedøya, Rambergøya and Langøyene in the inner part of Oslofjorden. During the Upper Katian to Middle Hirnantian there was a shift from the dominance of calcareous shale and limestone to an increase in coarser siliciclastics with deposition of mainly siltstone-and sandstone beds. Transport and sedimentation have mainly taken place through storm-generated currents and suspensions, with water depths varying from beneath storm-wave base to proximal settings of a prograding shoreface. The sedimentary logs display an upwards shallowing in the uppermost part of the Skogerholmen Formation by the occurrence of a thin limestone conglomerate. This was followed by a flooding event and a subsequent slow upwards shallowing in the Husbergøya Formation. Rhythmic variations in thickness of storm-deposited beds are observed throughout the Husbergøya Formation and indicate sea-level fluctuations. There is a marked increase in very fine sand fraction at the top of the studied succession. This indicates a rapid fall in sea-level which was most likely controlled by the Late Ordovician glaciation in Gondwana land. A forced regression with progradation of the shoreline is proposed as mechanism for the increasing sand content in the uppermost part of the studied succession. Five sedimentary logs on three localities were created during the fieldwork. Each log represents approximately 30 meters of succession. Three of the sections are logged in the scale of 1:10 and show measured thickness, lithology, sedimentary structures and fossil content of each individual observed bed. Both siliciclastic and carbonate samples were collected and studied in thin sections and polished slabs.