Rozwój osadów ordowiku na Pomorzu i przyległym akwenie Bałtyku

Abstract

Przedstawiono niektóre wyniki badań stratygraficznych osadów ordowiku w otworach wiertniczych wykonanych przez WOPN Petrobaltic w polskiej strefie Bałtyku, w nawiązaniu do rezultatów z obszaru lądowego zachodniej części syneklizy perybałtyckiej. Rozpoznano podstawowe prawidłowości rozwoju tych osadów oraz wykreślono szkice rozmieszczenia litofacji i miąższości na Południowym Bałtyku i przyległych obszarach lądowych.

DEVELOPMENT OF ORDOVICIAN SEDIMENTS IN POMERANIA AND ADJACENT BALTIC

BASIN

The paper presents stratigraphic and facial-thickness description of Ordovician sediments in Pomerania and adjacent Baltic Basin. The Ordovician of the inland area was described in previous publications, the main attention was therefore paid to new data from boreholes in the sea, done by the enterprise"Petrobaltic". Core samples from Ordovician sediments in these boreholes were only occasionally collected. The Ordovician was subdivided mainly on the basis of geophysic correlation with fully-cored sections from the Gdansk Pomerania. A stratigraphy prepared in this way is quite unquestionable and only in the sections further to the west, between Łeba and Bornholm, it is hypothetical due to different sediments of the Ordovician than in the far-distant key sections.

The Lowest Ordovician (Tremadoc) occurs in marine sections in stratigraphic and sedimentary continuity with the Upper Cambrian and is represented by black claystones. Age of these sediments is documented by the fauna of Dictyonema flabelliforme flabelliforme (Eichwald), D. f norvegicum (Kjerulf), D. f rossicum Obut, Clonograptus tenellus (Linnarsson), Bryograptus kjerulfi Lapworth and others.

Thickness of the Tremadoc sequence in marine sections is equal 0-8.5 m. Sediments of the Arenigian start with a thin transgressive bed that overlies various members of the Middle Cambrian, Upper Cambrian and Tremadoc. Above, the Lower Arenigian is represented by grey and grey-green claystones laminated with black ones with graptolithic fauna. Marine sections contain among others Phyllograptus angustifolius Hall, P. a. tenuis Monsen, P. anna (Hall).

The Upper Arenigian is composed of grey and brown-cherryred limestones and marly limestones.

Only in the section to the northwest-west of Łeba there are clayey sediments. Total thickness of the Arenigian sediments in sections from the Baltic reservoir is equal from 13.5 to about 45 m.

The Llanvirnian is also predominated by limestone lithofacies. The latter is composed of grey and brown-red limestones with trilobites of the genus Asaphus. To the west from the line Łeba-Bornholm

limestones are replaced by claystones that constitute the equivalent of the so-called "upper didymograptus slates" of Scania.

Washed surface of the Llanvirnian sediments are covered by considerably reduced in thickness sediments of the Llandeilian. In the southwestern part of the area they are represented by claystones and marls, and in the northeast by marly and organodetritic limestones with Botrioides ? sp., Lonchodomas rostratus (Sars) and others. Thickness of the Llandeilian in marine sections is equal 1.5-10 m.

The Caradoc in the southwest is represented by dark grey and black claystones with inserts of bentonites. These sediments contain a graptolithic fauna that documents a presence of horizons from Nemagraptus gracilis to Climacograptus styloideus. In the northeastern part of the area the Lowest Caradoc is composed of limestones and marls with trilobites of Neoasaphus ludibundus Tornquist, Ogmasaphus sp. and others. A thickness of the Caradoc in marine sections is equal 30-60 m.

The Lower Ashgillian in southern part of the area is represented by silty claystones with inserts of marls that pass northwards into marls and limestones. Age of sediments is defined by trilobite fauna with Panderia megalaphthalma (Linnarsson), Tretaspis sp. and others. The Upper Ashgillian is composed of marls and limy claystones with key trilobites of the genus Mucranaspis. A thickness of the Ashgillian sequence in marine sections varies from 4 to 19.5 m.

Distribution of thickness and lithofacies of sediments of the Ordovician is illustrated by sketches in Figs. 1 and 2. In the eastern part of the area there is a distinct conformity of facial and thickness changes, expressed by increased content of limestones in areas with small thicknesses i.e. at paleotectonic elevations, and their lower content in areas with larger thicknesses i.e. paleotectonic elevations.

In the west in the Kościerzyna-Smołdzino region and in Bornholm the Ordovician sediments are strongly reduced and their thickness is bellow 50 m. These areas have not been probably the paleotectonic elevations but the ones, in which subsidence has not been compensated by sedimentation.