Abstract

The Lyellian legacy implying dominantly uniform, continuous sedimentation over geological time is being replaced by a more punctuated view. Catchy terms like catastrophic uniformitarianism are abhorrent because of their semantical obfuscation and because episodic deposition is both geologically common and non-miraculous. Our knowledge of modem environments is biased toward the “normal” condition, however, so we must look to the ancient record for a fuller perspective. Episodic sedimentation results from events whose magnitude deviates significantly from the norm. Both positive deviations, such as storms, and negative deviations, such as non-deposition, constitute episodes. Of interest here are events recorded at the scale of cores and outcrops; their recurrence frequencies range mostly from decades to millenia. Recovery time and preservation potential are additional factors critical in assessing the ultimate importance of episodic sedimentation to the rock record. Ancient sandy deposits in epeiric seas illustrate many examples of episodic shelf sedimentation, most of which can be related to modern processes. Clearest positive deviations are recorded by thin conglomerate lenses, coquinas, flat-pebble intraclast layers, scoured surfaces, and hummocky stratification. Sporadic graded sandy or shelly beds, rich glauconite lags, alternating unburrowed and bioturbated intervals also reflect episodicity as do polygonal cracks and other emergence features. Negative deviations are recorded by hardgrounds (yes, even in sandstone) and/or bored surfaces. Scale, if present, commonly occurs as sharply delineated, thin intercalations within sandstones, indicating repeated deviations from the norm. The volumetrically abundant cross-bedded sandstone facies may provide the most subtle evidence of episodic sedimentation. Dune bed forms in medium-to-coarse sand imply mean current speeds of 80–180 cm/s, which is more than double that typical of modern permanent shelf currents, except at topographic constriction. Many cross-bedded shelf sandstones lack clear evidence of tidal currents, therefore it may be that one of the most common sediments was actively transported only during rare, high-energy events.