Cyclic sedimentation in the Middle Jurassic of central Poland

Abstract

Nine boreholes were drilled in the late eighties in central Poland to investigate the Middle Jurassic sedimentary successions. The boreholes were arranged in three lines (Ciechocinek, Brześć Kujawski and Wojszyce lines) running across three anticlines underlain by salt pillows. Long intervals of boreholes have been cored, often with 100% core recovery, thus enabling a detailed examination of sedimentary evolution. Sequences are composed of a full range of clastic rocks, from conglomerates through sandstones and heteroliths to shales. They are arranged in sedimentary cycles, predominantly regressive (coarsening upwards). These are interpreted as deposited in a shallow, wave/storm-dominated, shelf environment, each cycle being a result of progradation of fore-shore to near-shore heteroliths and sands over the shales of an open sea. The basin was probably non-tidal or microtidal. These essential (lower order) cycles, equivalents of the IVth order cycles in the world-wide scheme, are assembled in higher order cycles which resemble the IIIrd order cycles of that scheme. The bases of the higher order cycles are good lithostratigraphic markers, three of them being probably equivalents of chronostratigraphic boundaries (bases of the upper Aalenian, upper Bajocian and Bathonian, respectively). Correlation of borehole sections points to limited salt movement of the Ciechocinek and Wojszyce salt pillows during the Middle Jurassic. These are indicated, first of all, by thickness reductions seen in boreholes located above the tops of salt pillows. In the Ciechocinek area, the upward movement of salt occurred during the latest Bajocian/earliest Bathonian while in the Wojszyce area - during the early Bathonian. Coarser clastics were shed into the basin from the south-west, north-west (along the Mid-Polish Trough) and north-east during the Aalenian, and mainly from the north-east (from the East European Craton) in later times