Transgresja późnodewońska w profilu Unisławia

Abstract

Urozmaicony profil najwyższego żywetu i niższego franu w otworze Unisław IG 1 (rdzeniowany w pełni na odcinku 224 m) został zinterpretowany jako ciągła sekwencja facji, od równi aluwialnej, przez proksymalny i dystalny pas laguny, aż do strefy budowli węglanowych, zapewne z rafami gniazdowymi, oddzielających lagunę od otwartego morza. Dolna część analizowanego odcinka ujawnia silną zmienność warunków sedymentacji, wyższa zaś - ich względną stabilizację. Profit wskazuje na bliskość obszaru lądowego (tzw. półwyspu mazurskiego) w pierwszej fazie transgresji.

LATE DEVONIAN TRANSGRESSION IN THE UNISŁAW SEQUENCE, CENTRAL POLAND

The Unisław IG 1 borehole, north of Toruń (Fig. 1) has revealed the Devonian section, the lower part of which, 224 m thick, has been continuously cored with an excellent core recovery (Figs. 2 and 3). This interval belongs mainly to the lower part of the Koczała Formation (uppermost Givetian – Lower Frasnian) and records well the course of the Late Devonian transgresion.

Six major facies have been recognized, their individual beds being denoted by different symbols (Figs. 4 - 7). They are defined and interpreted as follows, the interpretation (Fig. 9) being inspired by the general models by J.L. Wilson (fide B.W. Sellwood, 1978), M.L. Irwin (1965) and P.H. Heckel (1972).

PP facies (beds A-L) - quartz arenites, sometimes wackes, fine-grained, non-calcareous, light grey, with lithoclasts and plant detritus (Table I, Figs. 10-12 and 15).

IH facies (beds a-i) - non-calcareous mudstones, shales and heteroliths, grey or grey-green, with vertical or sub-vertical burrowing traces, plant detritus, occasionally with fish teeth and scales (Table I, Figs. 12-14).

These both facies, interfingering with each other, arranged mostly in fining upward cycles and abounding in erosional surfaces, are thought to be deposited in the fluvio-deltaic environments. However, some of sandstones of the PP facies - situated higher in the sequence (beds F, K and L) – which are better sorted and more mature, contain coral fragments and pass upwards into the WK facies limestones, may have originated in the offshore sandy shoals.

PW facies (bells I - VIII) - quartz arenites or wackes, subordinately siltstones, calcareous, dark grey and brown, in places strongly bioturbated and containing unidentifiable bioclasts (Table I, Fig. 17; Table II, Fig. 18).

IW facies (beds (a) - (1)) - dark grey mudstones and shales, predominantly marly, with poor assemblage of bivalves, inarticulate brachiopods and ostracods (Table III, Fig. 32).

WL facies (beds (1)-(16)) micritic or biomicritic, marly limestones, sometitpes silty or pelletal, dark grey, with common crinoid plates and articulate brachiopods, subordinately with inarticulate brachiopods, bivalves, ostracods, algae, and fish scales and teeth (Table II, Figs. 19 and 20).

WK facies, prevailing in the sequence (beds 1 - 20) - packed biomicrites, subordinately biosparites, pelmicrites, biopelmicrites and dismicrites, frequently marly, containing various forms of stromatoporoids and tabulate corals (massive, tabular, branching), and sparse solitary rugose corals, articulate brachiopods, ostracods, gastropods and foraminifers (Table II, Figs. 21 - 26; Table III, Figs. 27 - 32).

Facies PW/IW - WL-WK represent a natural sequence arranged basinwards, from the proximal, brackish part of the lagoon, through its distal part of normal salinity, to the belt of carbonate build-ups, fringing the lagoon. Siltstones and sandstones of the PW facies are regarded as a long-shore equivalent of the IW facies shales: the latter may have been deposited in quiet waters, the former - in the waters of higher energy. Relatively rich faunal assemblage in the WL facies points to the free communication between lagoon and open sea, and suggests that in the zone of build-ups only patch-reefs may have occurred. Fossils in the WK facies are often immersed in the marly matrix. Some of their massive and tabular forms are preserved in growth position. However, the majority of branching forms are broken, not rounded, sometimes sorted and aligned in current direction, and evidently transported. Some of the WK limestone beds (1 - 3), composed entirely of such fragments, and passing into rocks of the PW facies, may have originated in the proximal zone of the lagoon. Basinal marls and nodular limestones were deposited in front of carbonate platform (Fig. 9 - Człuchów complex), in the deeper part of the shelf. They replaced later, during the Frasnian, the carbonates of the Koczała Formation.

The lower part of the sequence is characterized by frequent changes of the sedimentary environment (Fig. 8) - the transgression bore an oscillatory character. The main regressive episode (beds h - J, Figs. 5 and 6) may have been controlled by the eustatic sea-level changes, while the earlier minor pulsations may have resulted from the local processes of progradation of the near-shore facies on the lagoonal deposits and on carbonate platform in the periods of sea-level still-stand. The upper part of the profile evidences the stability of conditions in the zone of carbonate build-ups.

The Unisław sequence was influenced by the neighbouring land area, situated probably to the north- east (Mazury Peninsula). The presumed connection between the Central European Basin and the Peribaltic and Moscow Devonian basins run north-west of it. The traces of this linkage were completely destroyed by the Late Paleozoic erosion.