Clay-Mineral Suites in Cyclic Miocene Sediments: A Model for Continental-Margin Deposition in a Mixed Siliciclastic-Phosphatic-Dolomitic-Biogenic System

Abstract

ABSTRACT Clay-size mineral assemblages from lithologically diverse Miocene shelf and upper-slope sediments of the North Carolina continental margin show strong environmental zonation. Three distinct clay-mineral suites are recognized in the Miocene sediments of the Pungo River Formation. (1) A detrital kaolinite suite was deposited proximal to siliciclastic point sources in muddy quartz sand lithofacies. This assemblage reflects intense chemical weathering on the adjacent Coastal Plain and Piedmont Provinces. (2) A detrital illite and smectite suite deposited in phosphatic lithofacies is distinguishable from the clay mineralogy of muddy (distal) terrigenous and biogenic-dominated lithofacies only because illite is more highly crystalline. (3) Lithofacies characterized by authigenic dolomite co tain a clay-mineral suite with palygorskite and sepiolite. These Mg-clay minerals formed authigenically in the upper decimeters of sediment, penecontemporaneously with dolomitization, and in association with diatom dissolution, which provided the silica source for neoformation. Clay-mineral assemblages have not been significantly altered by burial or late-stage diagenesis and therefore appear to be original signal depositional indicators of changing environmental conditions through major Miocene fluctuations in sea level. The kaolinite suite is present in a fourth-order cycle immediately preceding the sea-level lowstand. Palygorskite and sepiolite are restricted to organic-rich dolomitic sediments, deposited under highly dysaerobic bottom conditions associated with the highstand and formation of the condensed section.