Igor Tuuling

A Silurian reefal succession in the Gotland area, Baltic Sea

Abstract Seismic reflection profiling east of Gotland has given information on a succession of four reefal units in the lower Ludlow stratal sequence ranging from the Klinteberg Formation to the Hemse Group in the Gotland sequence. In this paper they are named the Klinte Reef (Klinteberg Formation unit f), the Hammarudden Reef (Hemse Group ‘younger’ units a-c), the Östergarn Reef (Hemse Group unit d) and the Millklint Reef (Hemse Group unit e). The reef units form a mainly regressive succession with younger reefs resting on the seaward slopes of older reefs. The reef succession is exposed on Gotland and in the Baltic Sea east of Gotland, whereas towards the East Baltic coast it is overlain by Pridoli sedimentary bedrock. Each reef unit corresponds to a well-defined seismic unit with distinct facies zonality ranging from lagoon via reef barrier and biohermal slope to basin facies. The ‘fore-reef’ facies is in this paper termed biohermal slope to express its characteristic reefal features. Reconstruction of the post-depositional tilting of the sequence indicates lagoonal depths of up to 10 m. The reef barriers developed at depths of up to 20 m and the biohermal slopes at between 10 and 55 m. The reef barrier and the biohermal slope are generally separated by a debris fan of waste products from the reef, 1-2 km wide. The reef barriers are generally 1-3 km wide, which is about the same width as their lagoons. The biohermal slopes become successively wider towards the upper part of reef succession, ranging from 5-9 km at the Klinte Reef to 15-18 km at the Millklint Reef. Comparisons with the Gotland sequence show that the reef barriers are biostromal stromatoporoid reefs in a matrix of crinoid debris. The biostromes are of the Kuppen type. The bioherms are of the Axelro and Hoburgen types. The bioherms occur randomly on the seaward slope of the barriers.

Late Ordovician carbonate buildups and erosional features northeast of Gotland, northern Baltic Sea

Abstract The dimensions and form, structure and distribution of the Upper Ordovician carbonate buildups in the Baltic Sea, northeast of the Island of Gotland, have been determined by means of shallow-marine seismic reflection profiling. They are shown to decrease in size and number downslope, towards the deeper part of the Palaeobaltic Basin. The seismic signature variations, reflecting the structural and compositional changes of the carbonate buildups point to an increase in reef skeleton forming organisms and enhanced rigidity and stability of the buildups upslope. Supported by the tectonic setting, a local shallow-water break was formed on the slightly sloping carbonate ramp of the Late Ordovician sea, closely east of the Isle of Gotska Sandön. This depth development promoted the formation of a major linear reef of fringing or barrier character, separating a patch reef complex and reefs in the offshore belt adjacent to deep shelf muds from a beach or tidal flat. In the offshore area northeast of northern Gotland, the Late Ordovician sedimentary environment was entirely different from that farther east, offshore the Island of Hiiumaa, Estonia. The great diversity of the sediments, and the rapidly changing thicknesses of the layers, laid down on the fore-reef slope, were to a large extent governed by the numerous mud mounds and reef structures. Syn- and post-depositional compaction effects induced relative movements of the masses inside the deep-water carbonate buildups which led to their lateral spalling and collapse. The locally discordant reflectors from these rapidly changeable lithological units and strongly anomalous, chaotic reflector configuration around the collapsed carbonate buildups are intertwined with reflectors of erosional origin. Channel-like depressions, which probably form a unique set of ancient riverbeds, appear in the Late Ordovician sequence about midway between northeastern Gotland and Hiiumaa. They were formed before the formation of the Vormsi-Fjäcka layers, i.e., before the establishment of the Upper Ordovician carbonate buildup environment. The latest Ordovician erosional surface, most likely related to the Gondwana glaciation, is distinctly imprinted in the bedrock of the area.

The structure and relief of the bedrock sequence in the Gotland-Hiiumaa area, northern Baltic Sea

Abstract Based on high resolution seismic reflection profiling, structural and relief maps of the sedimentary bedrock between Gotland and Hiiumaa in the Baltic Sea have been composed and analysed. The general structure and relief of the submarine Lower Palaeozoic succession reveal a westward extension of the homoclinal structure distinguished in the Estonian mainland. The main bedrock structures offshore are 1-4 km wide, and several tens of kilometres long, linear zones of disturbances. On the structural map, these disturbances appear as submeridional zones of contour changes, up to several tens of metres in offset. The seismic profiles usually reveal a faint flexure-like bending of the layers through the zone. Locally, this flexure can be intersected by small faults. These bedrock structures are ascribed to fault movements in the crystalline basement. Two different bedrock relief systems were superposed on the region during the Cenozoic uplift and the Pleistocene glaciations. The first event resulted in the formation of a subparallel cuesta-like system of alternating erosional scarps and plains. Glacial erosion created submeridional valleys and troughs. Today three large bedrock forms, namely the Baltic and the Silurian clints and the Ordovician plateau, characterize the area. The outlines of the cuesta relief, and the amount of eroded sediments, advocate a regional increase in erosional activity from the St. Petersburg district to the area of the Baltic-Bothnian mobile zone northeast of Gotland. This zone existed as a subsided meridional lower ground during the Cenozoic, accommodating a main river that collected water both from the craton margins and the inner platform areas.

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 pre-Devonian erosional landforms — hills, depressions and escarpments reaching 150 m 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.

Silurian reefs off Saaremaa and their extension towards Gotland, central Baltic Sea

The Silurian reefs off Saaremaa in the Baltic Sea were studied by means of high-resolution seismic reflection profiling. The abundance and diversity of the reefs increases off Saaremaa concomitantly with the deepening of the Baltic Silurian Basin towards Gotland. The peak of the reefs around Saaremaa occurred during the Middle Wenlock. The reef facies retreated further off Saaremaa during the Late Wenlock, became episodically restored around the island during the early Ludlow and finally ceased during the late Ludlow. A similar SW–NE–SW migration of reefs reflects an alternating transgressive–regressive pattern in the nearshore shallow shelf environment, where already minor sea-level fluctuations strongly influenced the conditions for reef growth. During the early Wenlock, a bathymetric break with a large barrier-reef-like structure divided the shallow shelf offshore Saaremaa facially into back-reef and southerly sloping fore-reef areas. The width of this SW-migrating barrier ( c. 8 km) and the extent of reef bodies within it ( c. 4 km) are the largest known in the Baltic region. During the Silurian, the reefs between Saaremaa and Gotland were flourishing to a much larger extent than is visible today. The primary Silurian reef pattern was increasingly destroyed towards Gotland by later erosion. This has resulted in a large area void of Wenlock reefs off northern Gotland. Off southern Gotland, the narrow conical low-energy deeper-water reefs prevail, as the wave-agitated shallow-water facies with larger and lenticular Wenlock and Ludlow reefs are largely eroded around the Gotland Deep.