Paleozoic rocks structure versus Cenozoic cuesta relief along the Baltic Shield–East European Platform transect

Based on structural maps, the bedrock structure on the southern slope of the Baltic Shield is analysed. Gently southerly dipping (0.1–0.2°) Paleozoic layers from Estonia to the Swedish east coast form the Baltic Homocline (BH). Numerous monoclinal folds, forced by basement faulting, induce slight variations in bedrock attitude across the BH. Studies on faults exposed in Southern Finland suggest that many monoclinal folds inducing basement faults within the BH are of Precambrian origin and have been active in pulses. The present monoclines were shaped by the Caledonian Orogeny. Submeridional tectonic hinge-lines induce minor regional-scale changes in the structural setting and control trends/styles of the forced folds. From the west, the BH bounds with the Baltic–Bothnian mobile zone that has been intermittently active since Mesoproterozoic times. The evolving Baltic Syneclise in the Silurian rearranged the structural setting typical for the Ordovician Baltic Basin. The Cenozoic uplift of Scandinavia created a SE-to-E-dipping bedrock sequence around the Gotland–Öland area. As the latter area had a different attitude than the BH, two Cenozoic cuesta-sets started to evolve around the northern Baltic Proper. They both had an outlet to the N–S-flowing Eridanos River around the Gotska Sandön area, creating thus two independent sections of the Baltic Klint

The Ordovician-Silurian boundary beds between Saaremaa and Gotland, Baltic Sea, based on high resolution seismic data

New seismic profiles have been used to revise earlier interpretations of the Ordovician-Silurian bound ary beds be tween Saaremaa and Gotland. Atrans-Baltic reflector with erosional fea ures (S2) above the erosional Ordo vician-Silurian boundary reflector (S1) correlates with the bound ary between the Raikküla and Adavere stages. The sporadic reflector or2 below the S1 reflector off shore from Gotland represents the erosional boundary between the Pirgu and Porkuni stages. Three stratigraphic gaps occur in the Ordovician-Silurian boundary beds off shore from Gotland. The amount of eroded rocks between the Pirgu and Porkuni stages, the Ordovician and Silurian systems and the Raikküla and Adavere stages can change rapidly. Consequently, the thick ness and stratigraphy of the Ordovician-Silurian boundary beds around Gotland can change considerably across short distances. The O4-5-S1 unit off shore from Gotland, including carbonate build ups and erosional incisions infilled with Porkuni strata, belongs facially to the transi ional belt between the Estonian Shelf and the Livonian Tongue. The thick ness changes in the S1-S2 unit (Juuru and Raikküla stages) indicate an extensive subma rine erosional channel, streching from north of Estonia across the Baltic Sea and central Gotland, which developed in the Baltic Basin along a shelf to deep-basin transect dur ng Llandovery time

Structure and development of the Valmiera-Lokno Uplift – a highly elevated basement block with a strongly deformed and eroded platform cover in the East European Craton interior around the Estonian-Latvian-Russian borderland

Based on drillings, a number of geological cross-sections, and structure contour and isopach maps were composed to describe/analyse the structure and development of the Valmiera-Lokno Uplift (VLU), a basement block elevated up to 700 m with a heavily deformed and eroded platform cover in the East European Craton interior, along the regional Liepaja-Riga-Pskov Fault Zone (LRPFZ). Five isolated basement-cored anticlines (BCA), the Lokno, Haanja, Mõniste, Valmiera and Smiltene uplifts, arise in the platform cover on the VLU, whereas the downthrown LRPFZ side defines a complex monoclinal fold. The anticlines, straddling or occurring near the monocline, merge with it and thus have highly asymmetrical shapes. Thickness changes of stratigraphic units across the VLU reveal its complex history, reflecting regional tectonic activation pulses that varied noticeably even between neighbouring BCAs. In all, the latest Precambrian-earliest Ordovician initiation epoch of the VLU was followed by modest tectonic activity or a standstill period in the Middle Ordovician-Early Silurian. Intensifying tectonic movements culminated again in the prime of the Caledonian Orogeny in latest Silurian-earliest Devonian time, and faded thereafter towards the end of Early Devonian. The VLU has been reactivated occasionally since the latest Devonian and emerges as a crustal weakness in the recent movement and seismicity patterns. To decipher the origin of the VLU, hitherto factually undiscussed topics, a more detailed study of the LRPFZ, analysis of its fault pattern and kinematics alongside the regional tectonic setting/history is needed. A cursory look hints to a substantial Early Paleozoic sinistral strike-slip along the LRPFZ, allowing interpreting the VLU as a possible restraining bend structure

Streamlined topographical features in and around the Gulf of Riga as evidence of Late Weichselian glacial dynamics

Based on various cartographic sources, a digital terrain model and acoustic profiling data, linear relief features of glacial origin have been distin guished and analysed in the Gulf of Riga and adjacent mainland areas in order to reconstruct the dynamics and pathways of former ice streams. North-east–south-west oriented features in the till topography prevail in the central part of the gulf and along the southern coast of the island of Saaremaa, which corroborate the previously known south/south-east di rection of the main Riga ice stream. North-east to south-west directed features dominate in the Pärnu Bay and around the Irbe Strait. Similar deviations from the Riga ice stream are most likely due to ice divide zones, namely the Sakala Upland in Southern Estonia and Kurzeme in north western Latvia, which locally changed the course of the main ice flow. The influence of the Kurzeme ice divide is traceable at the bottom of the gulf up to the southern coast of Saaremaa. There is no evidence of an ice-marginal zone cross ing the central part of the Gulf of Riga as was supposed earlier. The Pandivere-Neva and Palivere ice-marginal zones, which merge on the Sorve Peninsula, probably continue offshore into the Irbe Strait. As the age of the glacier relieffeatures is poorly contained, the chronologi reconstruction of the ice dynamics is tentative

Upper Ordovician carbonate mounds on Gotland, central Baltic Sea: Distribution, composition and reservoir characteristics

We present a multidisciplinary description of the Upper Ordovician carbonate mounds which are found throughout the central Baltic Sea and which were studied in detail on the island of Gotland. These mounds were the subject of intense exploration between 1974 and 1992 and a total of 323 shallow wells were completed in more than 100 mounds on Gotland. Many of these were put into production and a total of 100, 000 cu. m of high quality oil was produced. In this paper, we discuss the mounds' occurrence, lithological characteristics, age, faunal composition, petroleum chemistry and reservoir properties. The study is based on analyses of core material from wells in the northern part of Gotland. Upper Ordovician mound reservoirs here contain oil which has a similar geochemical signature to that in Lithuania and in the Kaliningrad district (Russia). The oil was probably derived from marine anoxic shales and migrated up-dip from more central parts of the Baltic Syneclise where oil-prone shales are known to occur. An intraformational origin for the oil is unlikely. The mounds contain large numbers of vugs and moulds which communicate mainly through dissolution fractures and surfaces and probably reflect a marine regression. Various lithofacies were recognized from petrographic studies including sub-mound, intra-mound, cap and flank, and supra-mound facies. Algae and stromatolites dominate the intra-mound facies, providing an organic framework for the entire structure. Consequently, the mounds are not merely poorly defined build-ups of mud and micrite, as has previously been assumed. Biostratigraphic data indicates a late Caradoc to Ashgill age for the mounds and their associated lithologies