Deep boring at Gotska Sandön

A preliminary report on a boring at Hamnudden on the island of Gotska Sandön (Fig. 1).The boring was started on a shore terrace c. 2 m above sea-level, and pierced the following sequence: loose deposits (Quaternary) 73 m, bedded limestones (Ordovician) 86,5 m, sandstones, clayey sandstones, and clays (Cambrian) 72,5 m, and hard greyish, occasionally reddish sandstone (pre-Cambrian, possibly Jotnian) 9 m. A diagrammatic section of the rock sequence is given in Fig. 2, the figures indicating the depth below the terrace surface. The Cambrian-Ordovician boundary separates Lower Cambrian strata from Lower Ordovician arenaceous limestone belonging to the Lower Llanvirnian (Raniceps limestone or Gigas limestone according to the Swedish stratigraphical column). There are several surfaces of discontinuity (D) within the Lower and Middle Ordovician beds, and the Middle Ordovician Ludibundus beds contain also a few layers of bentonite (B). Fig. 3 demonstrates the correlation made possible by the bentonite layers between sequences in Västergötland (Kinnek., Bill., Möss.), Östergötland (Ö.G.), Gotland (Gotl.), and Gotska Sandön (G.S.).