Architecture, paleosols and cyclicity of the Middle-Late Pennsylvanian proximal fluvial system (Nyrany Member, Pilsen Basin, Czech Republic)
Abstrakt
The Nyrany Member in the Pilsen Basin records the evolution of a broad paleotropical alluvial plain that preserves amalgamated lenticular to sheet sandstone bodies separated by basin-wide floodplains. Facies and architectural element analysis in conjunction with well-log correlation was employed in order to reconstruct fluvial system stacking patterns and decipher the controls responsible for the preserved fluvial architectures and identify the possible influence of allogenic processes on basin fill architectures.
The main architectural components identified include: 1) sheet-like and lenticular pebbly to cobbly conglomerate thalweg fill representing the lowermost portion of channels that are preferentially preserved; 2) inclined gravel-sand and sandy strata of barforms and rarely preserved (3) mudstone units representing floodplains and abandoned channel fills. These architectural elements combine to form a hierarchy of alluvial cycles represented by channel-bar systems (3-10 m thick), channel-belts (4-15 m thick) and stacked channel-belt complexes (10-35 m thick).
Autogenic processes of lateral migration and local and regional avulsion of braided channels of intermediate sinuosity (1.5) controlled the formation of the smaller scale cycles. While the geometry of stacked channel belt complexes (sandbodies) appear to be driven by variations in subsidence rates, their cyclic alternation with extensive floodplains is considered to be related to allogenic processes of variable seasonality of precipitation, which affected sedimentation rates and water-table level in the basin.
Their maximum periodicity of 140 k.y. indicate that eccentricity paced changes of Earth's orbit may be a driving force controlling climate stability in the Pennsylvanian paleotropics.