RHYTHMIC UPPER PALEOZOIC STRATA OF EURAMERICA: EQUATORIAL INDICATORS OF GONDWANAN GLACIATION?

Rhythmic Upper Paleozoic strata in European and North American basins, formerly situated near the equator, have been attributed to glacioeustasy caused by the Gondwanan glaciation. However, recent studies on Gondwana suggest that some glacial phases were less prominent than previously thought. How reliable an indicator are these rhythmic successions?

The Maritimes Basin of Atlantic Canada (Devonian to Permian) spans the Gondwanan glacial period. Following mid-Devonian collisions, Pangean convergence continued for >100 million years, creating uplands and restricted seaways. Fault-bounded basins and salt tectonics, coupled with high sediment supply, promoted rapid subsidence and accumulation. Although rhythmic strata occur throughout the basinal record, two prominent suites (mid-late Visean Windsor Group and Westphalian D to Cantabrian Morien Group) do not match closely with recently identified Gondwanan glacial periods.

For the Windsor Group, spectral analysis of carbonate/clastic rhythms yields a full complement of Milankovitch orbital frequencies. In the Morien Group, high-frequency sequences in the Milankovitch band show transgressive coals and limestones (sea-level rise) and lowstand calcretes and paleovalley fills (sea-level fall). Significantly, both successions represent thermal subsidence periods, during which glacioeustasy was strongly expressed in the basin fill. Cyclic clastic successions at other levels formed during rapid-subsidence periods, which tended to damp down base-level falls, resulting in fewer valley fills and mature paleosols. For example, the Langsettian Joggins Formation formed during rapid subsidence enhanced by salt withdrawal, and its cycles could be tectonic in origin rather than glacioeustatic.

Spectral analysis and evidence for both rise and fall of base-level in paleo-equatorial rhythms support a glacioeustatic mechanism for the two Maritimes Basin examples. This suggests that some important Gondwanan glacial periods may be incompletely documented, although modest sea-level changes could have caused strong equatorial effects. However, some rhythms may reflect tectonic activity and are not sufficient alone to infer Gondwanan linkages, especially in the convergent tectonic setting that characterized much of the Euramerican record.