PROFIL SEDYMENTOLOGICZNY PRZEDKELOWEJSKICH (JURAJSKICH) UTWORÓW SILIKOKLASTYCZNYCH W OTWORZE BADAWCZYM CIANOWICE 2 (OKOLICE KRAKOWA)

Abstract

SEDIMENTOLOGICAL PROFILE OF THE PRE-CALLOVIAN (JURASSIC) SILICICLASTIC DEPOSITS IN THE CIANOWICE 2 BOREHOLE (NEAR KRAKÓW, POLAND)Abstract. In the Cianowice 2 borehole (located in the vicinity of Kraków), straight on the erosional unconformity on the top of metamorphosed Neo-Proterozoic (Ediacaran) shales and below carbonate deposits of Callovian, 20 meters thick interval of siliciclastic rocks has been encountered. The siliciclastic rocks are composed of conglomerates, sandstones, mudstones and subordinate intercalations of coal, siderite and marls. Stratigraphical position of this interval can be inferred based on poorly-preserved miospore assemblage, spanning relatively long geological time (Jadwiga Ziaja, pers. comm.) – it can represent either Early or Middle Jurassic, or both of those epochs, while the lowermost coarse-grained package can be even of an older, i.e. Triassic age. The interval was subdivided into 5 well-distinguished sedimentary successions, separated by bounding surfaces, mostly of erosional character – only succession 3 starts with sharp lithological contrast between sandy deposits and overlying mudstones, which reflects flooding and rapid retrogradation (either lacustrine or lagoonal). All these bounding surfaces (particularly bottoms of successions 1, 3 and 5) are of regional correlative significance-erosional bounding surfaces can represent sequence boundaries, while bottom of the succession 3 can represent correlative surface of a transgression. In each succession, except for the lowermost one, subordinate fining-upward cycles are dominating. They represent diminishing-upward energy of transport. The succession 1 is composed of breccia and conglomerates with chaotic structure, indicative of mudflow – dominated fans, possibly passing into alluvial fans and back to the mudflow fan again in the top. There is a marked lithological contrast (possibly connected also with considerable hiatus) between the coarse-grained deposits of succession 1 and following successions built of fine-grained sandstones, siltstones, mudstones and claystones. The succession 2 is composed of five fining-upward cycles, indicative of fluvial environment (fluvial plain), with traces of plant vegetation. The succession 3 is entirely composed of mudstones of lacustrine origin, with numerous traces of plant vegetation, siderite concretions and bands and coals at the top. Successions 4 i 5 again contain typical fining-upward fluvial cycles with traces of plant vegetation. The overall profile shows peneplanation of a landscape and continuous diminishing of energy of sedimentary processes up to the top of succession 3 marked with coals. Then, fluvial sedimentation returned. Regional comparison of the Cianowice 2 borehole with borehole Parkoszowice 58 BN, located some 40 km to NW tends to suggest the Early Jurassic (most probably Pliensbachian–Toarcian) age of the profile (the lowermost coarse-grained part can be of a Triassic age), but until more reliable biostratigraphical or chemostratigraphical evidences are obtained, these more detailed interpretations of stratigraphical division remain tentative.