TECTONIC CONTROL ON TURBIDITE DEPOSITION – BALTICA'S WESTERN MARGIN DURING THE MIDDLE TO UPPER ORDOVICIAN

The shale-dominated Elnes Formation in southern Norway was deposited on Baltica's deep shelf in the Late Middle Ordovician (late Darriwilian). During this time interval, two episodes of turbidite deposition occurred that brought in a mix of carbonate and siliciclastic silt- and sand from the west and south. These deposits represent the erosional debris of uplifted basement blocks which formed the western rim of the continental portion of the Baltica plate.

The intimate mix of carbonate and quartz grains in the turbidites reflects the erosion of basement as well as the formation of near shore facies around these uplifted blocks which consisted partially of carbonate sand. Both carbonate and siliciclastic grains were suspended into sediment clouds during storms and carried offshore where they evolved into true turbidites. Due to their lesser density the carbonate particles were carried out further into the basin and preferentially accumulated in distal turbidites.

The onset of turbidite deposition reflects the uplift of the basement at the western Baltica margin, whereas the ceasing of turbidite influx represents the flooding of these elevated zones and their completed denudation. Intervals of turbidite deposition do not match base level lowstands reconstructed from carbonate input from the shallow Swedish shelf to the east. The uplift and subsidence of the Norwegian elevated zones was therefore likely controlled by tectonic stress related to movements along the plate boundary with Iapetus Ocean terranes and arcs.

The Elnes Formation shales also reflect the decreasing width of the Iapetus Ocean. During early Elnes times, deposition was characterized by pure shales. From the Pseudamplexograptus distichus Biozone on, however, sedimentation occurred in fining-upward “shale cycles” with silt at the base grading into pure shales towards the top. The silt originated either from a larger terrane or more likely from the approaching North American continent itself.