Coal-bearing fluvial cycles of the late Paleozoic tropics; astronomical control on sediment supply constrained by high-precision radioisotopic ages, Upper Silesian Basin
Abstrakt
The late Paleozoic sedimentary record is well known for its mixed marine-terrestrial coal-bearing sedimentary sequences, traditionally called cyclothems, and their hypothetical glacioeustatic sea-level control. In contrast, coeval sedimentary cycles deposited in continental, nonmarine settings have received comparatively little attention.
These fluvial and fluvio-lacustrine cycles are common in many late Paleozoic basins across paleoclimatic belts ranging from the tropics to the higher latitudes. Several mechanisms explaining the origins of these sequences have been suggested, including autogenic processes and tectonically driven allocycles.
However, progress in understanding the climatic signal recorded in paleosols together with cyclostratigraphic analysis indicate that these terrestrial cycles record climate-driven upstream changes in sediment supply, possibly linked to astronomical ("Milankovitch") forcing. We evaluate this hypothesis by reviewing the existing models for repetitive fluvial facies and the lateral continuity of coal-bearing fluvial cycles in the Upper Silesian Basin (~3°S paleolatitude).
In this basin, mixed terrestrial-marine short eccentricity-driven coal-bearing cyclothems of Serpukhovian age are replaced in the Early Pennsylvanian by coal-bearing fluvial allocycles of the Saddle and Lower Such ' a Members (Bashkirian). Facies and cyclic pattern analyses suggest that the terrestrial cycles record alternation of contrasting fluvial styles: a low sinuosity avulsive/laterally migrating sand and gravel dominated braided river and fluvial systems with well-developed floodplains and narrow channels transporting minimal sediment load across widespread peat swamps.
Tonsteins in coals indicate that peat swamps were broadly isochronous over a large area. Lateraly persistent coals and fluvial channels indicate clastic deposition not coeval with periods of active peat accretion in extensive and long-lasting swamps.
Cyclostratigraphic analysis together with new high-precision radioisotopic U-Pb zircon ages of three tonsteins suggest that major fluvial cycles were linked to the ~100-thousand year term of orbital eccentricity. Our conceptual model involves an upstream control on vegetation cover and sediment supply through changes in seasonal rainfall related to interactions of the summer-hemisphere monsoon and the Intertropical Convergence Zone.
Furthemore, we develop an integrated time scale for the late Namurian to earliest Westphalian; the Namurian - Westphalian boundary is dated 319.17 +/- 0.41 million years ago (Ma); bases of the Marsdenian and Yeadonian substages are estimated as 320.26 +/- 0.43 Ma and 319.92 +/- 0.43 Ma, respectively.
