High-energy ephemeral stream deltas; an example from the Upper Silurian Holmestrand Formation of the Oslo Region, Norway

The late orogenic deposits of the Upper Silurian Holmestrand Formation of the Oslo Region, Norway, comprises sharply alternating ephemeral stream and sandy beach face deposits. The formation is divided into three units reflecting the down-slope change on an extensive ephemeral stream delta system. The lower unit of the formation comprises ephemeral stream sediments as stacked sheet-sandstone bodies, deposited on a broad coastal floodplain. Each sheet is bounded by a lower intraclast strewn scoured surface overlain by massive, horizontally stratified, low-angle and trough cross-bedded sandstones. In the medial and upper units of the formation the ephemeral stream sediments alternate with beach face deposits, forming ephemeral stream deltas. The ephemeral stream deltas are marked by intraclast strewn scoured surfaces, and dominated by trough cross-bedded sandstones. They may contain escapement burrows, marine bioturbation and wave-working of the delta surfaces. The beach face deposits are composed of alternating irregular, horizontally stratified sandstones with current and wave ripple cross-lamination and tabular cross-bedded sandstones. Deposition took place in a coastal, non-to micro-tidal, shallow-water environment, influenced by longshore currents and low palaeo-wave energy. Seven ichnotaxa are recognized and support the sedimentary record of a transitional environment between a broad coastal floodplain and a shallow marine environment. The presence of calcretes indicate an arid to semi-arid palaeo-climate.