RESOLVING THE ENIGMA OF THE CONTINENTAL SHELF GEOLOGIC FRAMEWORK: A TRIBUTE TO ORRIN H. PILKEY

O.H. Pilkey pioneered geologic exploration on the SE US Atlantic continental margin using rock dredges and surface grab samplers aboard the R/V Eastward. For example, OHP et al. mapped phosphate distribution in modern surface sediments of Onslow Bay delineating two phosphate-rich 'hotspots'. They interpreted the surficial deposits as weathering residuums being released from slightly phosphatic limestone and concentrated by physical processes. Their work demonstrated the extremely dynamic and complex nature of continental shelf sediment systems and laid the foundation for those who followed with improved technology and tools.

Subsequent work by Riggs et al. during the 1980s and 90s integrated high-resolution seismic and side-scan sonar surveys with 10-m vibracores, submersibles, and SCUBA diving to dissect the Onslow Bay geologic framework. Working on the last R/V Eastward and many R/V Cape Hatteras cruises, this multidisciplinary research focused on the Neogene stratigraphy and modern sediment dynamics of Onslow Bay. An intimate relationship exists between surface sediments and underlying lithofacies as demonstrated by indicator sediments (i.e., phosphate) that act as excellent tracers for stratigraphic sources and subsequent transport paths. The surficial phosphate hotspots occur directly over two Miocene phosphate-rich muddy sand lithofacies that grade upsection into slightly phosphatic dolosilts within the complex Pungo River Fm. Phosphate, as well as other 'new' surface sediment components, are actively reworked from outcropping stratigraphic units into the modern largely through bioerosional processes and are rapidly diluted away from the outcrop zone by physical transport processes and mixing with other sediments. Thus, the complex geologic framework largely determines surface sediment composition and distribution patterns and storm dynamics generally result in sediment dilution and removal from the shallow shelf system.