Depositional history of Late Ordovician – earliest Silurian storm dominated shelf, incised valley and open marine settings, inner Oslofjorden islands (Oslo Region)

Abstract

The uppermost Ordovician (Hirnantian) of the Oslo Region has been studied at three selected outcrops in the Husbergøya, Langøyene and Solvik formations on the islands Hovedøya, Rambergøya and Langøyene in the inner Oslofjord. The outcrops have been logged in detail, from the uppermost part of the Husbergøya Formation, trough Langøyene Formation up to the first meters of the Silurian Solvik Formation. Representative samples have been collected from all three sections, and various laboratory and microscope techniques have been performed. The uppermost very fine-grained sandstone beds with brown colour in weathered outcrops in the Husbergøya Formation are capped by clean sand layers at the base of the Langøyene Formation. The Langøyene Formation is subdivided in a lower and upper part along a major unconformity. The lower part consists of mudstone and thin limestone layers interbedded with sand layers, forming a generally upwards coarsening succession. The erosional unconformity that cuts the lower succession reveals a relief of at least 35 m and is draped by a matrix-supported conglomerate with intrabasinal clasts up to boulder size. Imbricated clasts indicate a transport direction to the southeast. The infill succession above the lower erosional boundary contains two additional erosional unconformities with associated infill successions. Sedimentary structures and lithologies indicate a shallow, high energy, marine epicontinental sea during deposition of the lower part of the Langøyene Formation, terminated by the first major erosional surface as a subaerial, fluvial unconformity. The upper part of the Langøyene Formation is interpreted as deposited in three superimposed incised valleys, each defined by a lower subaerial unconformity and overlying sand-rich fluvial to estuarine marine high-energy infill succession. The uppermost unconformity and infill succession is interpreted laterally equivalent to a karstic surface on top of an oolithic Langøyene arenaceous limestone capped by a flooding surface beneath brown mudstone, and shale and nodular limestone of the Silurian Solvik Formation. The study favours that three events of fall in sea level took place during the Hirnantian; these are most likely connected to eustacy as a result of the glaciation of the Gondwana continent.