hardground

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hardground A term first introduced into geologic literature in 1897, drawn from an oceanographic source but used in a narrower sense to describe a specific horizon that had initially been lithified a short distance beneath the sea floor. Lithification resulted from calcite precipitation around local foci, giving rise to concretions. The sediment above and surrounding the concretions remained soft and was consequently bioturbated (see BIOTURBATION). The initial cementation of the hardground may have been triggered by the chemical influence of organisms that dwelt within the sediment. Progressive encroachment of the cemented areas on to the soft sediment gradually resulted in total lithification. ‘Incipient’ hardgrounds represent periods of arrestation in the cementation process, with the result that nodular and softsediment units alternate one with the other. The cemented hardground may itself support a fauna of fixed or cemented organisms and may be bored extensively by bivalves (Bivalvia). Pyrite films may line burrows, which may also be filled by silica.

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