Seismic correlation of Palaeozoic rocks across the northern Baltic Proper – Swedish–Estonian project since 1990, a review

Abstract

After a short historical review of the correlation of Palaeozoic rocks between Estonia and Sweden, this paper focuses on the results of marine seismic studies, achieved during the cooperative Swedish-Estonian project since 1990. The most recent seismic correlation scheme of the Cambrian, Ordovician, and Silurian strata and their distribution at the seafloor across the northern Baltic Proper are presented. Thickness changes and trends, as well as the sedimentary structures, reef bodies, and erosional features of different seismic units, are treated in connection with structural and facies changes in the Palaeobaltic Basin. The immediate background of this project is outlined and the locations of the seismic lines shot during the joint expeditions to the Baltic Sea are shown.

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

“…To ob tain the best re li abil ity be tween the on shore and off shore data and to maxi mise structure con tour ing data points off shore, the Cam brian-Or do vi cian bound ary was cho sen as the most in for ma tive struc tural surface. This sur face, co in cid ing largely with the con tact of terrigenous and cal car e ous rocks (Tuuling and Flodén, 2009a;Niel sen and Schovsbo, 2011), is eas ily rec og niz able as a strong seis mic re flec tor (O 1 ) just be low the rim of the sub ma rine Bal tic Klint (Tuuling and Flodén, 2016), and can be firmly fixed even in the bore holes when no core was taken. Fur ther more, this level en ables the struc tural tran si tion and changes from the cen tral Bal tic Sea area to be fol lowed di rectly to wards the Swed ish east coast; its emer gence above the sea can be precisely lo cated on the geo log i cal map of Öland (Figs.…”

“…As a re sult, the Or do vi cian se quence with N-S deep en ing fa cies/rock units de scribed in Es to nia ex tends to the Swed ish east coast. It can be eas ily fol lowed in the seis mic lines across the cen tral Bal tic Sea (Flodén et al, 1994;Tuuling and Flodén, 2009a) and is as cer tained be tween the Swed ish is lands of Gotska Sandön and Öland (Grahn and Nõlvak, 2010). However, the more clayey and com plete Si lu rian suc ces sion on Gotland rep re sents a sig nif i cantly deeper basinal sec tion compared to that on Saaremaa.…”

“…The seis mic cor re la tion of the Si lu rian se quence across the Bal tic Sea, how ever, is highly ten ta tive. Be sides the vari able and rapidly chang ing li thol ogy, the con ti nu ity of the seafloor-form ing Silu rian rocks is dis mem bered by nu mer ous smaller or larger erosional cuts (Tuuling and Flodén, 2009a, b, 2011, 2016 Seis mic stud ies have proved that the gen eral tec tonic setting along side the plat form Pre cam brian base ment and Pa leozoic ve neer re la tions de scribed in Es to nia (Puura and Vaher, 1997) con tin ues be low the cen tral Bal tic Sea area . Thus, at a gen tle an gle (6-13') un der the platform cover, the south ern slope of the Bal tic Shield forms an exten sive pe ne plain that is eroded into Pre cam brian crys tal line rocks (Winterhalter et al, 1981;Puura et al, 1996Puura et al, , 1999.…”

“…1). Al though the dis tri bu tion of co eval rocks across the cen tral Bal tic Sea has al ready been re solved in the mid-1800s (Murchi son, 1844; Schmidt, 1859; see Tuuling and Flodén, 2009a), their ex act geo log i cal/struc tural re la tions have re mained largely un clear for about a cen tury. Only the bore holes drilled in the 1940-1950s in the Bal tic coun tries, Rus sia, Po land and Swe den (e.g., Martinsson, 1958;Männil, 1966;Kaljo, 1970) ce mented the idea of a unique Palaeobaltic Or do vi cian-Si lu rian ba sin.…”

“…Ex ten sive seis mic pro fil ing since the early 1960s has resulted in a huge amount of data about the dis tri bu tion, thick -nesses, and thus struc ture of dif fer ent Pa leo zoic lay ers beneath the cen tral Bal tic Sea (Flodén, 1975(Flodén, , 1980Flodén et al, 1994Flodén et al, , 2001Tuuling et al, 1995Tuuling et al, , 1997Tuuling and Flodén 2000a, 2009a, b, 2011. Fus ing this in forma tion with the sur round ing main land knowl edge, this pa per aims to gen er al ise the struc ture of the plat form sed i men tary cover on the south ern slope of the Bal tic Shield, and to analyse its changes/trends from Es to nia to the Swed ish east coast.…”