ABSTRACT: The Cambrian Eustatic Signal: Not So Grand?
Clinton A. Cowan, Noel P. James
Efforts to decipher potential high-frequency (fourth- and fifth-order) eustatic signals within third-order grand cycles in Cambrian strata of western Newfoundland have yielded a surprising result: the conspicuous, large-scale stratigraphic rhythms, grand cycles, were not principally a bathymetric phenomenon. grand cycles, i.e., the stratigraphic repetition of tens-of-meters-thick lithosomes, which are alternately carbonate and terrigenous clastic rich, are widely cited as evidence for lower Paleozoic third-order eustatic fluctuations. Evidence from Middle to Upper Cambrian platform strata in western Newfoundland indicates that grand cyclicity in this area was not simply a response to sea level change. Instead, the stratigraphic signal of eustasy is marked by the presence f terrigenous clastics in an otherwise carbonate-prone succession.
Detailed facies analysis reveals that both terrigenous and carbonate lithosomes are locally constructed of predictable, meter-scale, coarsening-upward cycles. Lithofacies constituting meter-scale cycles in both lithosomes are bathymetrically indistinguishable with respect to environmental energy, ichnofauna, and exposure index. Meter-scale
cycles in either lithosome are typically capped by the same lithology, precluding contemporaneous generation of carbonate vs. terrigenous cycles along some presumed bathymetric gradient. Obvious lithologic differences between carbonate and terrigenous meter-scale cycles obscure their common origin. Terrigenous cycles are best explained by the incursion of siliciclastic fines into a shallow-water carbonate environment irrespective of sea level change.
If grand cyclicity is not eustatic in nature, then several contentious points of Cambrian sequence stratigraphy may be reconciled: extreme variability in bathymetry of terrigenous lithosomes documented in other regions, ambiguity in pan-continental correlation of grand cycle tops; and apparent lack of correlation with major faunal breaks (trilobite biomeres). Long-term variability in the delivery of terrigenous clastics to lower Paleozoic pericratonic carbonate platforms may be attributable to climate change in pre-land vegetation North America. Work is currently underway to untangle potential eustatic and climatic stratigraphic signals across a spectrum of scale.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990