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Bored and encrusted surfaces, generally called "hardgrounds," are common in some ancient limestones. Proper identification of the environment of lithification in ancient hardgrounds is important because it can greatly influence the interpreted depositional history of the associated limestones. In addition, proper identification of the environment can determine the usefulness of hardgrounds as stratigraphic markers.
The presence of bored and encrusted Holocene submarine cemented layers in the Persian Gulf suggests that some ancient hardgrounds could have formed underwater and not by exposure to meteoric water. Holocene hardgrounds covering hundreds of square miles in the Persian Gulf commonly occur as multiples of bored beds (as many as 4 have been observed) interbedded with noncemented carbonate grainstone or mud. Each hardground bed is thoroughly cemented by fibrous aragonite and/or magnesium calcite at the upper surface and generally the degree of cementation decreases downward. Each hardground bed contains pelleted geopetal internal sediment which commonly lies on fibrous aragonite. Similar features, now altered by calcite, may be the key to understanding the origin of many ancient hardground .
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