UPPER PROTEROZOIC AND ORDOVICIAN SUBMARINE OUTLIERS IN THE ARCHIPELAGO NORTHEAST OF STOCKHOLM, SWEDEN

Geophysical measurements, a submarine excavation, and frequency counts of erratics indicate previously unknown submarine in situ occurrences of Middle Riphean-Ordovician sedimentary rock sequences in the Oregru nd archipelago, along the western coast of the Aland Sea, and in the e astern part of the Stockholm archipelago. The oldest is Middle Riphean sandstone, preserved as a fissure filling, succeeded by poorly consol idated sandstones of Late Riphean-Vendian age, bearing witness of a on ce more extensive distribution of Upper Proterozoic sedimentary rocks than hitherto known. Younger submarine outcrops consist of Ordovician limestones. These occurrences are situated between continuous sheets o f Ordovician bedrock in the Bothnian Sea and in the Baltic Sea proper. The thickness of the Ordovician successions indicates that a substant ial part of this system is represented; the upper Kundan Stage is demo nstrated by inarticulate brachiopods. The submarine excavation and sam pling confirmed the specific seismic signature of the Ordovician limes tones. The sequences are mostly present in fault zones but also pre se rved due to block movements of the underlying crystalline bedrock. The general absence of Cambrian strata suggests erosion and subsequent ex posure of the crystalline basement in certain areas prior to the sedim entation during the Ordovician.