Facje neokomu między Mogilnem a Kołem

Abstract

Wyróżniono 8 głównych litofacji i 5 subfacji skał klastycznych, ułożonych przeważnie w cykle odwrócone. Sekwencję tę zinterpretowano jako wynik kilkakrotnej progradacji przybrzeża płytszego na głębsze w zbiorniku bezpływowym z możliwością występowania pływów w wyższej części profilu. Wyraźniejsze transgresje występują w późnym riazaniu i wczesnym apcie. Zwrócono uwagę na niejednoznaczności podziału litostratygraficznego w tego typu sekwencjach.

NEOCOMIAN FACIFS BETWEEN MOGILNO AND KOLO (CENTRAL POLAND)

In the fully cored borehole sequences from the lowermost Cretaceous interval (Figs. 1-6) eight facies and five sub-facies have been distinguished. They are, as follows:

A - unequigranular sandstones, badly sorted, predominantly immature, structureless or parallel-bedded (Table I, Figs. 8 - 12); sub-facies Al contains chamosite and goethite oolites and Fe-minerals in the matrix.

B - very fine-grained and fine-grained quartz arenites, moderately and well sorted, white and light grey, structureless or parallel-bedded (Table I, Fig. 13); B1 sub-facies contains mud clasts, clay laminae and scarce burrows (Table I, Figs. 14, 15).

C - sandstones as above with more common clay laminae, passing into sandy heteroliths with flaser bedding (Table I, Fig. 16); Cl sub-facies is composed of two kinds of sandstones: with and without clayey matrix (Table II, Figs. 17 - 19); C2 sub-facies comprises the strongly bioturbated rocks of the CI sub-facies (Table ll, Figs. 20, 21).

D - highly bioturbated sandy and equicomponent (= with roughly equal proportion between sand and clay) heteroliths, with relics of primary bedding, grey and brown (Table II, Figs. 22 - 24); D1 sub-facies (Table II, Fig. 25) contains abundant glauconite.

E - subordinate facies of sandstones and sandy heteroliths with small-scale cross-stratification (Table III, Figs. 28 - 32).

F - sandy and equicomponent heteroliths, grey and dark grey, with flaser and wavy bedding. in sandstone layers with small-scale cross-stratification, containing small and not abundant, horizontal and subhorizontaJ burrows (Table Ill, Figs. 33, 34; Table IV, Figs. 35, 36).

G - equicomponent and shaly heteroliths, dark grey, with wavy and lenticular bedding, abundant burrows as above, and very scarce impressions of bivalves (Table IV, Figs. 37 - 42),

H - dark grey and black shales with siderite nodules and pyrite aggregates, containing bivalves and plant remains (Table III, Fig. 26, upper part). The ideal sequence of facies is arranged predominantly in the reversed cycles: H - G - F - C - (D) – B1 - B (Fig. 7), 2 - 8 m thick, beginning with a sharp boundary between the H facies shales and under-lying B facies arenites of the earlier cycle. Other boundaries are gradational except for the A facies sand-stones which are intercalated within various facies of the above sequence. These sandstones are thought to be the result of the periodical supply of coarser clastics and Fe - minerals by rivers and longshore currents.

In spite of the occurrence of a succession of the lenticular-wavy-flaser bedding (characteristic of the tidal flats) in the G - F - C facies, other features of tidal sedimentation, such as frequent erosional surfaces, bimodal cross-bedding and low regularity in the vertical sequence, have not been recognized. Therefore, the whole sequence has been tentatively interpreted rather in terms of progradation of the sandy foreshore on the more and more muddy shoreface and offshore zone of deeper shelf in a non-tidal basin. The periods of progradation were succeeded by relatively rapid transgressions, first of all in the Late Ryazanian and Early Aptian. However, in view of vague indications of the existence of mud flats, in younger units, tidal sedimentation is not excluded in these parts of the sequence when the conditions were more favorable in a widening basin, more freely connected with the Atlantic ocean.