Shipborne geophysical study of an Ordovician-Silurian carbonate platform, Fårö - Hiiumaa area, northeastern Baltic Sea

Abstract: The lithological-stratigraphical subdivision, the internal structure and the present relief of the Lower Paleozoic bedrock sequence was investigated by means of high resolution shallow marine seismic reflection profiling in the northeastern Baltic Sea.The Cambrian, as well as the lowermost Silurian layers, show rapidly changing lithologies and thicknesses and provide no trans-Baltic seismic reflectors. The laterally stable Ordovician sequence, yields two trans-Baltic reflectors, namely at its contact with the underlaying Cambrian sequence and at the strongly clayey Vormsi-Fjäcka level in the Upper Ordovician.The trans-Baltic Ordovician-Silurian boundary correlation is complicated due to the highly anomalous bedding configuration of the Upper Ordovician layers northeast of Fårö, brought about by the rapidly varying carbonate buildup environment and the Late Ordovician erosional features. The carbonate buildups are shown to decrease in size and numbers downslope. Their seismic signatures point towards an increase in reef building organisms and enhanced rigidity and stability upslope. The 2-4 km wide, elongated zone right southeast of Gotska Sandön, which obviously is made up of several coalescent patch reefs, existed obviously as some sort of linear reef of fringing or barrier type. There appears an easily recognisable downslope change in facies across it.Late Ordovician erosional features appear at two different levels. Firstly as a regularly placed channel or system of channels, obviously ancient river valley(s) beneath the Vormsi-Fjäcka sequence and secondly as a widespread latest Ordovician erosional surface, evidently induced by the Gondwana glaciation.The submarine bedrock layers are some 0.1-0.3 degrees tilted towards south-southeast. As on the Estonian mainland, this homoclinal structure is intersected by numerous, 1-4 km wide and several tens of kilometres long, tectonically heavily disturbed areas, that are referred to as linear zones of disturbances. These zones as a whole induce considerable height changes in the structural level, which is mainly caused by large flexure-like bending of the layers.The bedrock relief shows two differently orientated relief systems superposed up to region during the Cenozoic uplift and the Pleistocene glaciation. The first, older one appears as the cuesta-like alternation of sublatitudinally directed escarpments and plains. It is intersected by younger, submeridionally orientated exarational valleys and troughs of the Pleistocene glaciation. The geological, as well as the bedrock relief features show up a regular increase in erosional activity towards west-southwest during the Cenozoic, which was probably caused by the subsided Baltic-Bothnian mobile zone being as the main Cenozoic drainage system forming factor in the northern Baltic area.