Bank Margin Environment
Published:January 01, 1983
Carbonate sand accumulations of reservoir size commonly occur on or near the seaward edge of banks, platforms and shelves. They may also form within the platform interiors or on topographically high areas in regionally deep water, but these occurrences are not as common as those along the margins. Bank-margin sand accumulations may grade landward or seaward within a fraction of a kilometer of other environments and, thus, do not have wide lateral extent in a dip direction. Such accumulations are sufficiently distinct and economically important as carbonate reservoirs to warrant their treatment in some detail. This paper provides an overview of modern and ancient carbonate sand bodies, and refers the reader to detailed work covering many aspects of carbonate sands.
Bank-margin carbonate sands occur repeatedly throughout the geologic record and are a prominent component of carbonate facies models (Fig. 1). Shaw (1964) and Irwin (1965) recognized the persistence of this facies in epeiric sea models as a seaward high-energy zone separating low-energy, deeper-water sediments from low-energy, shallow-water lagoonal deposits. More recent models by Heckel (1972), Lees (1973), and Wilson (1975) emphasize the importance of the zone in which carbonate sands accumulate. In nature, sand accumulations are not distinct and isolated from other facies (as are the chapters of this volume from each other, for example), and there will necessarily be some overlap between this chapter and those concerning reefs, beaches and islands, and lagoons.
Our understanding of carbonate sand deposition is biased toward bank-margin deposits because most modern studies have focused on these.
Figures & Tables
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.