Ancient Sediments of Bahaman Type

ABSTRACT Ancient limestones of Bahaman type are considered to be a common rock type. Their recognition and interpretation of the regional facies relationships may lead to the discovery of oil reservoirs in the marginal zones of ancient sediments of Bahaman type. An exaggerated role has been ascribed to the formation of limestones by the breakdown of preexisting limestones. Such derived limestones have commonly been referred to in the literature as clastic limestones and they have been regarded as a major limestone type comparable with skeletal (or bioclastic, of some authors) limestones and precipitated limestones. Precipitated limestones consisting of limestone grains may grade from calcilutites to calcirudites, and the grains may exhibit either oriented (e.g., oölitic) or disorganized (mosaic) micro-structure. Precipitated calcarenites are forming under very shallow marine conditions at present. Similar conditions of deposition are inferred for ancient deposits. Many, in fact probably most, of the extensive granular limestones hitherto commonly referred to as derived are now regarded as limestones of Bahaman type and are thought to have been precipitated. Criteria for recognizing varying types of calcarenite are discussed.