--> Unconformities in Submarine Environments: the Significance of Hardgrounds in Sequence Stratigraphy and their Influence on Porosity Development, by M. Mutti and D. Bernoulli; #90986 (1994).

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Abstract: Unconformities in Submarine Environments: the Significance of Hardgrounds in Sequence Stratigraphy and their Influence on Porosity Development

Maria Mutti, Daniel Bernoulli

Submarine hardgrounds are common features in shelf and upper slope carbonates, and coincide with regional unconformities. Because of extensive cementation, these surfaces are well recognizable in outcrops and cores, and commonly form distinctive seismic horizons. Cementation of laterally extensive layers can occur in limestones that were never exposed to meteoric diagenesis or subaerial environments. Although these surfaces are commonly used in sequence subdivision, the factors controlling their occurrence and the mechanisms of their formation remain to be investigated.

Hardgrounds in Miocene outcrops in the Apennines and from cores drilled in the Bahamas and in the Great Barrier Reef show similar characteristics and suggest that their formation is controlled by environmental factors. The hardgrounds occur in bryozoan-echinoid sands or in plankton-rich slope sediments, and cap coarsening-upward cycles. Micritic cementation is confined to the uppermost 0.1-1 m. The occurrence of phosphates and silica in the cemented intervals suggest high levels of nutrients, while Fe and Mn oxides indicate cementation in a well oxidized environment. Stable isotope profiles in hardground cements show covariance of ^dgr180 and ^dgr13C towards more depleted values, suggesting precipitation from cool, nutrient-rich waters. Well sorted and bedded pla ktonic foraminifera grainstones overlie the hardgrounds, indicating the existence of strong currents and possibly a crisis in platform production.

Facies association, mineralogy, and geochemistry indicate that hardground formation was strongly controlled by high nutrients, changes in water temperature and currents. Changes in environmental factors can dramatically affect carbonate systems, leading to sudden changes in facies associations, geometrical organization and differential lithification.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994