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Of 104 benthic foraminiferal species identified from Miocene strata in Onslow Bay, 27 occur commonly enough to be useful in paleoenvironmental interpretations. Variations in the abundance and distribution of six species (Bolivina paula, Buliminella elegantissima, Valvulineria floridana, Lenticulina americana, Hanzawaia concentrica and Cibicides floridanus) account for about 84% of the total variance in benthic foraminiferal assemblages. B. paula and B. elegantissima thrive under conditions of nutrient enrichment and oxygen depletion. The other species thrive in well-oxygenated bottom waters. Biofacies, which are largely defined by the relative abundance of these six species, indicate that changing water mass properties, particularly with reference to nutrient and dissolved oxygen content, exerted control over Miocene benthic foraminiferal distributions.

Individual biofacies, each defined by cluster analysis, generally coincide with specific lithofacies, indicating that substrate type also influenced benthic faunal distributions. Lithofacies were related to some extent to water mass chemistry, as phosphate accumulated in nutrient-enriched, oxygen-depleted waters associated with coastal upwelling: siliciclastics predominated under well-oxygenated conditions. Hence, water mass properties and substrate type combined to partially control the species content and distribution of Miocene benthic foraminiferal biofacies. Other environmental factors and diagenetic processes obscured details of faunal-sediment relationships such that only generalized, larger-scale patterns remain.

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