Pre-Devonian landscape of the Baltic Oil-Shale Basin, NW of the Russian Platform

Abstract

The erosional relief of Ordovician and Silurian deposits in Estonia was developed during the continental period in late Silurian and early middle Devonian times. The uplift of the area and marine regression were induced by compressional tectonics in the continental interior related to the closure of the Iapetus and Tornqvist Oceans. In the northern part of the Baltic sedimentary basin (Estonia), on the gentle southerly dipping slope between the Fennoscandian Shield (Finland) and Baltic Syneclise (Latvia), a pre-Devonian, slightly rugged erosional relief with few cuesta was developed. The preDevonian erosional landforms hills, depressions and escarpments reaching 150m in height were probably buried under the Devonian deposits and then partly re-exposed by pre-Quaternary erosion. These landforms are described in detail using data from several thousands of cores drilled in the course of oil-shale and phosphorite exploration and mining. Erosional relief on Ordovician and Silurian deposits in the northwest of the East European Craton was developed during the continental period of exposure in the late Silurian and early-middle Devonian. Uplift of the territory and marine regression were induced by compressional tectonics in the continental interior due to the closure of the external Iapetus Ocean to the northwest (Phillips et al. 1976), and the Tornqvist Ocean to the southwest (Ziegler 1990; Schwan 1997). At the beginning of pre-Devonian erosion, the land surface was of generally low relief due to Ordovician and Silurian near-shore marine sedimentation. Structurally induced local landforms were almost non-existent before the late Caledonian phase of compression. Before the late Silurian-early Devonian phase of deformation, the whole East Baltic area was a shallow, intracontinental seabed or near-shore continental flat with few structural complications in the early Palaeozoic sedimentary cover. The compressional tectonics along the craton margins caused uplift of the region and deformation of the crystalline basement and sedimentary cover in the western corner of the Russian Platform. East of the present Baltic Sea, at least two areas of different types of structure were formed. In the central part of the Baltic sedimentary basin, i.e. in Latvia, tectonic deformation was most extensive, and fault throws reached 700m (Suveizdis 1979). After early Devonian erosion, the faultand dome-related residual escarpments and hills had altitudes of up to 150m (Polivko & Ulst 1969). In the northern part of the sedimentary basin (Estonia), within the gentle southerly dipping slope between the Fennoscandian Shield (Finland) and Baltic Syneclise (Latvia), a flatter hilly relief developed. The influence of faults with throws of up to 50 m on the formation of the pre-Devonian topography was negligible. These oldest erosional landforms are now partly buried under Devonian deposits and partly re-exposed during late-Phanerozoic erosion. Following the previous preliminary interpretations published in Estonian and Russian (Puura 1974; Puura et al. 1987; Tuuling 1988), this paper discusses sedimentary bedrock structure-topography relationships and reconstruction of the pre-Devonian landscape. Geological and geomorphological setting The study area is situated in the northwestern part of the Russian Platform of the East European Craton south of the Fennoscandian Shield (Fig. 1). Oil-shale deposits of the Baltic Oil-Shale Basin occur in northeastern Estonia and adjacent areas of northwestern Russia extending from 25~ 59~ to 29~ 59~ (Fig. 2). It is the best known portion of the basin because of extensive exploration for oil shale and phosphorite (over 15000 boreholes in 5000 km2). The oil-shale basin is located in the sub-surface of a slightly hilly plain south of the coastal cliffs of the Gulf of Finland. The highest point of the plain is 166 m above sea level on the Pandivere Heights (PH; Fig. 2). The Devonian is distributed around the periphery of the bedrock hills south of the Pandivere Heights and between the Ahtme Elevation (AE) and Izhora Heights (IH). From: SMITH, B. J., WHALLEY, W. B. & WARKE, P. A. (eds) 1999. Uplift, Erosion and Stability: Perspectives on Longterm Landscape Development. Geological Society, London, Special Publications, 162, 75-83. 1-86239-030-4/99/ $15.00 9 The Geological Society of London 1999. by guest on January 6, 2014 http://sp.lyellcollection.org/ Downloaded from