Abstract

Two types of subaerial exposure surfaces on carbonate rocks are described; 1) those that formed at major unconformities which bound cratonic sequences, and 2) those which cap shallowing-upwards carbonate cycles formed in response to high frequency sea-level fluctuations and autocyclic processes. These differences are a function of the duration of subaerial exposure and the characteristics of the underlying limestones.

Major unconformities at cratonic sequence boundaries form by subaerial exposure events that have durations on the order of millions of years. At the time of subaerial exposure the limestones were well lithified by compaction and burial diagenesis, and may have contained tectonicly produced fractures. The geologic record of this type of subaerial exposure event includes truncation of stratigraphic units, low to high relief on the unconformity surface, cavern systems with collapse breccias, meteoric diagenesis up to 100 meters below the unconformity surface, well developed paleosols and solution features infilled by subsequent transgressive sediments.

The duration of the subaerial exposure events which produce exposure surfaces that cap carbonate shallowing upwards sequences is on the order of tens to hundreds of thousands of years. At the time of exposure the carbonate rocks have not undergone burial diagenesis and compaction, and have a high initial porosity. The characteristics of exposure surfaces capping shallowing-upward carbonate cycles include; multiple exposure surfaces within a stratigraphic unit, exposure surfaces concordant with bedding which reflect the depositional topography, small-scale caves and solution features, horizontal zones of enhanced porosity, poorly developed paleosols, and solution features filled with sediment of the same age and composition as the host.