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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 10. (October)

First Page: 1681

Last Page: 1681

Title: Geologic History of Reefs: ABSTRACT

Author(s): Noel P. James

Article Type: Meeting abstract

Abstract:

A reef, whether fossil or modern, is the physical expression of a community of calcium carbonate-secreting organisms which grew in one place for an extended period of time, forming either isolated structures or cores of complex buildups. These sea-floor highs are not only sites of rapid carbonate fixation and accretion but also locales of internal sedimentation, synsedimentary lithification, and active bioerosion--attributes which set them apart from most other sedimentary deposits.

The core facies of large and complex fossil reefs generally illustrate a succession of growth stages, each of which is characterized by specific lithologies and invertibrate taxa. These stages, now recognized in reefs throughout the geologic record, are generally referred to as the pioneer (stabilization), colonization, diversification, and domination (climax) phases of reef growth. They record the transition from shoals of skeletal sand populated by small, rooted invertebrates and/or algae to thickets of branching or lamellar organisms to complexes made up of many different taxa with varied growth forms and life habits to a cap comprising only a few, generally lamellar to encrusting skeletons.

Just as the development of any one reef is dependent upon inherently biological factors so the history of reefs in general reflects the evolution of Phanerozoic marine invertebrates. For a reef illustrating all stages to develop, a prerequisite is the existence of metazoans capable of secreting large skeletons of variable growth form. At those times in geologic history when only small, prone, branching sessile invertebrates occur, buildups called reef mounds are present that illustrate only the first two stages of growth. Thus there are times when no reefs occur, periods when reef mounds are the norm, and periods when complex reefs dominate.

Although the attributes that characterize all reefs are present in the very earliest Cambrian bioherms, against the backdrop of geologic time, there are two general cycles of reef growth. Each begins with reef mounds which are subsequently populated by sponges and then gradually transformed into complex reefs by the appearance of corals and/or stromatoporoids. The first cycle begins in Early Cambrian time and culminates in the Late Devonian (240 m.y.); the second cycle begins in the Mississippian and has continued to the present (340 m.y.). During the early phases of these cycles reef mounds occur on or around shoal-rimmed platforms; in the latter stages reefs commonly form the rim facies, and control platform evolution.

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