Exposure surfaces occur on land and under the sea, but in this chapter we are concerned only with subaerial exposure surfaces. More specifically we are concerned with the effects of subaerial exposure on carbonate sequences. Subaerial exposure surfaces are areas where upper bounding surfaces of sediment or rock show the effects of being exposed at the Earth’s surface. In order to recognize fossil subaerial exposure surfaces they need to be exposed long enough to allow subaerial diagenetic processes to modify or obliterate pre-existing fabrics. This will be recorded as a break in the sedimentary sequence. This usually means that significant periods of time have passed before exposed surfaces are buried by new deposits. What we mean by significant periods of time is a relative concept dependent on our limits of resolution and powers of observation. When considering absolutes of time we will offer only abstracts; our intention here is not to quantify absolutes of timing, duration or intensity of processes acting upon exposure surfaces, nor to examine critically the processes themselves. Rather we will document common and characteristic products of subaerial exposure, list criteria which aid in recognition of fossil subaerial exposure surfaces, and point out the significance and economic importance of subaerial exposure surfaces in ancient carbonate sequences.
Adhering to this outlined conceptual framework we can define a subaerial exposure surface as a distinct surface on land which indicates: (1) non-deposition and commonly erosion; and (2) a break in the sedimentary sequence. Regardless of cause or length enough
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Carbonate Depositional Environments
For more than 100 years geologists have been ex amining and describing modern sediments with an eye toward using characteristic features to aid in the interpretation of depositional settings of ancient strata. This field of interest developed particularly during the 1950s and 1960s with the creation of detailed models for modern carbonate deposition in Florida, the Bahamas, Cuba, the Persian Gulf, Belize, Pacific atolls, the Great Barrier Reef and other areas. An understanding of the depositional environments of these modern models, coupled with increased understanding of diagenetic effects, has led to vastly improved interpretations of ancient limestones. Such models also improved the “predictability” of many carbonate reservoir rocks.
In spite of the great strides made in our knowledge about carbonate depositional environments, their characteristic features have never been synthesized in a single work. Although excellent textbooks exist which describe some aspects of the interpretation of both ancient strata and modern sediments, systematic treatment of the entire subject is available only in the primary literature.
This book is an attempt to bring together this widely disseminated literature. The volume is specifically designed for use by the non—specialist-the petroleum geologist or field geologist—who needs to use carbonate depositional environments in facies reconstructions and environmental interpretations. Yet it is hoped that the book will also serve as a valuable reference for the specialist or advanced graduate student.
Toward that purpose, the book is extensively illustrated with color diagrams and photographs of sedimentary structures and facies assemblages. The text focuses on the recognition of depositional environments rather than on the hydrodynamic mechanisms of sediment movement.