Zarys petrografii kambru górnego wschodniej części obniżenia pery bałtyckiego

Bronisław Szymański





The intense drilling works carried out in the eastern part of the Peribaltic Depression in the years 1960–1972 provided core material. Successive elaboration of that material made it possible to differentiate Upper Cambrian deposists with paleontological record, hitherto unknown from that region (B. Szymanski, in press).

Upper Cambrian deposits occurring in situ were found in several borehole profiles (see Fig. 1). Redeposited Upper Cambrian rocks mostly limestone debris and pebbles, have also been found in several places in the last few years, mostly in detrital material of basal conglomerates of the Lower Tremadocian and Lower Arenigian.

The Upper Cambrian of the eastern part of the Peribaltic Depression is represented by originally tripartite series of clastic-carbonate rocks; the lower part of the series is formed of calcareous quartz sandstones, the middle – of quartz sandstones with sandy limestone intercalations, and the upper, that is the carbonate part – of sandy and crystalline limestones (Fig. 2). The Upper Cambrian sandstone- carbonate series, uniform in development, represents a separate, transgressive sedimentary cycle (B. Szymanski, 1974).

Upper Cambrian strata discordantly overlay the Middle Carnbrian sandstone-siltstone-clayey series along an uneven erodional boundary, and are overlayed by either transgressive conglomeratic deposits of the Lower Tremadocian (Pakerort) or glauconitic conglomerates of the Lower Arenigian (Latorp). The Upper Cambrian sandstone-carbonate series is 0.7–1.1 m thick.

The Upper Cambrian of the eastern part of the Peribaltic Depression comprises rocks representing two lithofacies: sandstone lithofacies (oligomictic, medium- and various grained calcareous quartz sandstones) and carbonate lithofacies (sandy and crystalline limestones).

The Upper Cambrian sandstone rocks form a series uniform in development, comprising light-gray, compact, variously and medium-grained quartz sandstones; their mineral composition is fairly uniform; roundness index of clastic material ranges from good to very good. Texture is random, compact (Table I, Figs. 4, 5; Table II, Figs. 6, 7), unclearly horizontally bedded in some places or, very rarely, concretional (variously grained sandstones). Structure of these sandstones is usually psammitic, medium – (Table II, Figs. 6, 7) and variously grained (Table I, Figs. 4, 5).

The sandstones consist mainly of detrital quartz granis, granis and debrit of phosphates, clay matter and accumulation of partly recrystallized silica. Glauconite, pyrite, weathered feldspars, debris of brachiopod shells, pebbles and debris of rocks fine muscovite plates as well as secondary minerals (phosphates, iron hydroxides and oxides and siderite) occur in subordinate amounts. Heavy minerals are represented by zircon, tourmaline, rutile, apatite, magnetite and ilmenite? Matrix is fairly abundant and uniformly distributed; it is mostly of the basic (Table I, Figs. 4, 5; Table II, Figs, 6, 7) or basic-infilling type; it primarily consists of carbonates. This is a carbonate matrix, changing in some places to carbonate-clay or, rarely, to clay-carbonate matrix rich in phosphate.

The carbonate rocks form a natural separate rock series, represented in the lower part of the Upper Cambrian by gray sandy limestones, and in the upper part – by gray and light-gray crystalline limestones. They are heavily diagenesed, compact, hard, with uneven irregular fracture. The mineral composition is poorly differentiated or monotonous. The macrotexture of sandy and crystalline limestones is poorly visible, usually represented by parallel horizontal bedding; the micro texture is random, massive (Table III, Fig. 8; Table IV, Figs. 10, 11; Table VI, Fig. 14; Table VII, Fig. 17; Table VIII, Fig. 18), sometimes bandy or lenticular with poorly accentuated, oriented arrangement of clay matter and detrital quartz (Table III, Fig. 9). The structure is grained (sparry), medium- to coars-crystalline.

The limestones are formed of medium- and coarse-crystalline carbonates, clastic quartz of the aleurite and psammite fractions, clay matter (illite, chlorite, sericite), glauconite and pebbles and debris of older rocks. Pyrite, phosphate, silica, muscovite, feldspars, fine bioclasts, carbonaceous-bitumen matter and secondary minerals occur in subordinate amounts. Heavy mineral, also occurring in subordinate amounts are represented by: zircon, tourmaline, rutile, apatite and opaque minerals.

The profile of the borehole Pasłęk IG-1 is here proposed as the lithostratotype of the Upper Cambrian of the eastern part of the Peribaltic Depression as in this profile typically developed sandstone-carbonate rocks of the Late Cambrian age were for the first time differentiated in sedimentary cover of the Precambrian Platform of NE Poland.

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