Abstract: Silesian Sequence Stratigraphy, Sandstone Body Development and Prediction, Northwest Europe

Adrian Hartley

Silesian (Upper Carboniferous/Pennsylvanian) sediments of northwest Europe are characterized by the development of numerous coarsening-upward parasequences (cyclothems). Two types of idealised parasequence are recognized: (1) black shales with brackish fauna passing upwards into lacustrine and overbank siltstones to fine-grained sandstones capped by coals, (2) exactly the same succession except the black shales contain open marine fauna, and major (5-40 m thick) erosional-based productive, reservoir sandstones are developed beneath the coals. Parasequences range from 5 to 50 m in thickness; thicker ones are correlatable across and between basins. Numerous studies suggest a fourth-order glacio-eustatic, Milankowitch cyclicity for parasequence development. However, why do type 1 cycles epresent simple parasequences, whilst type 2 cycles record major incision, valley-fill development and major open marine transgression as opposed to brackish type 1 cycles?

Major sandstone bodies and associated marine horizons occur at specific stratigraphic intervals: (1) upper Namurian C-lower Westphalian A, (2) Westphalian A/B boundary, and (3) upper Westphalian B to lower Westphalian C which, based on recent time scales, indicates a recurrence interval of 2 to 3 m.y. (third-order cyclicity). In contrast, parasequence development is estimated to have taken place on a 200,000 to 400,000 year time scale (fourth-order cyclicity). As the type 2 parasequences are developed on the same scale as those of type 1, it is suggested that type 2 parasequences resulted from enhanced sea level falls with related transgressions due to a phase coincidence between third and fourth-order eustatic sea level cycles.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994