Jura i kreda w rejonie Bydgoszczy

Abstract

Przedstawiono wyniki badań nad litologią i stratygrafią portlandu górnego, beriasu, walanżynu dolnego, barremu, aptu, albu środkowego i górnego, cenomanu i turonu Bydgoszczy. Opracowanie oparto na analizach próbek z wierceń; ustalenia stratygraficzne uzasadniono przede wszystkim zespołami mikrofauny. JURASSIC AND CRETACEOUS OF THE BYDGOSZCZ REGION Bydgoszcz is situated on limb of the Pomeranian Swell. Quaternary deposits highly varying in thickness usually overlay here Tertiary or, sometimes, Cretaceous or even Jurassic rocks. Older rocks evidenced by numerous drillings made in this region include Upper Portlandian, Berriasian, Lower Walanginian, Barremian–Aptian, Middle and Upper Albian, Cenomanian and Turonian.Upper Portlandian is represented by claystone-marly rocks of the ostracod zone C, differentiated in the Polish Lowlands (W. Bielecka, J. Sztejn, 1966; S. Marek, J. Sztejn, W. Bielecka, 1969). The stratigraphic position of these rocks is shown by the presence of bivalves. Cyrenidae as well as numerous ostracodes such as Cypridea dunkeri J o n e s, Cypridea binodosa M a r t i n, Cypridea sp., Klieana alata M a r t i n, Rhinocypris jurassica jurassica (M a r t I n), Darwinulla leguminella(F o r be s), Scabriculocypris trapezoides A n d e r s o n.Marine Berriasian deposits are divided into two lithological complexes: lower, comprising markedly sandy, calcareous silts and siltstones, and upper, comprising dark-grey, calcareous and very slightly sandy clay siltstones. The stratigraphic position of these rocks is evidenced by microfaunal assemblage shown in Table 1. The stratigraphic gap between the Jurassic and Cretaceous in the Bydgoszcz. region comprises the uppermost zones of the Portlandian and lower zones of the Berriasian. Lower Walanginian of the Bydgoszcz region is subdivided into three lithological complexes also differing in microfaunal assemblage. It mainly comprises grey-coloured clay, calcareous siltstones. Besides microfauna shown in Table 1, there also occur shells of small bivalves and gastropods which are especially common in lower parts of the profile. Exogyra sinuata predominates here.Barremian and Aptian are represented by a series of noncalcareous fine-grained quartz sands without macro- or microfauna. Middle Albian comprises medium- to coarse-grained sands with admixture of fine quartz gravel and without microfauna, occurring below the base of Upper Albian with microfaunistic record.Upper Albian comprises series of calcareous sands with phosphatic layer at the top and overlain by white marls. Lower part of these marls is assigned to Albian, and upper – to Cenomanian on the basis of microfaunistic datings. The microfaunal assemblage is shown in Table 2. Marly and claystone Cenomanian deposits are assigned to three microfaunistic zones characterized by assemblages shown here in Table 2. The zones I, II, and III, differentiated here, correspond to those differentiated in other parts of the Polish Lowlands (E. Gawor-Biedowa, 1965, 1972).The Cenomanian-Turonian boundary could not be traced in lithology and it was delineated on the basis of changes in foraminifer assemblages. The differences in foraminifer assemblages made it possible to differentiate Turonian a and b (Table 2).The continuity of sedimentation and sequences of foraminifer assemblages throughout the Albian – Cenomanian – Turonian implicate preservation of complete Cenomanian and Turonian in the Bydgoszcz region. The thickness of the Cenomanian and Turonian is estimated at 52 and 52.5 m, respectively.