TSUNAMIS IN A STORMY SEA: MIDDLE CAMBRIAN INNER SHELF LIMESTONES OF WESTERN ARGENTINA

Coarse-grained strata deposited in some tens of meters of water on continental shelves and in epeiric seas are conventionally ascribed simply to storms of varying intensity which interrupted the settling out of suspended mud. Surprisingly, the possibility of tsunamis is virtually never evaluated even though they must have been common in many regions. Portions of the Middle Cambrian La Laja Formation of the Argentine Precordillera formed in this setting. They comprise a suite of three carbonate facies: burrowed mudstone, bioclastic grainstone, and intraclastic conglomerate composed of pieces of mudstone including cemented haloes around burrows. Sporadically developed eocrinoid meadows and trilobite populations are recorded respectively by disarticulated ossicles and usually broken sclerites. This indicates that, generally, exoskeletons became fragmented by repeated, albeit comparatively weak, bottom turbulence. Hence, the mudstones and grainstones show that before stabilization by calcite cementation which began just beneath the sediment-water interface, physical reworking was so frequent that it did not leave a specific signature of individual storms. By contrast, the scattered conglomerates were due to brief episodes of anomalously deep scour and strong oscillatory flow. Because the two coarse facies are lenticular and abruptly interbedded within uniform mudstone, and there are no stratigraphic patterns, allochthonous particles, or unconformities that might indicate sea-level fluctuation, the powerful events that created the conglomerates were more likely tsunamis, not storms. They were presumably unleashed by rifting beyond the shelf-margin to the (present-day) west, but diffracted by marginal shoals and attenuated by the shallowness of the shelf; the study area was not affected by backwash. Recognition of tsunami-lain beds urges a re-examination of the standard tempestite paradigm with the use of coarse-grained strata as a simple measure of storm frequency and intensity.