However, it is believed that a high dominance of typical marginal marine 

 mollusks, such as Mulirvta lateralis Say or the distinctive foraminifer 

 Elphidiicn guntevi Cole, in shelf floor sediments probably indicates 

 deposition in the shallower transgressing sea. Even though such species 

 may occur at existing depths, they are not likely to be a dominant faunal 

 element in modem shelf floor deposits. More restricted marginal marine 

 species, such as Cvassostrea virginioa Gmelin and Gemma gemma Totten, 

 locally provide a more reliable indication of shallow-water origin in 

 shelf surface deposits. Lithologically, these shallow-water transgressive 

 deposits tend to be muddy and much more heterogeneous in composition and 

 size-distribution characteristics than the modem shelf, shoal, and shore- 

 face sediments. 



6. Natural Tracers. 



Unique constituents of a sediment body are often of value in engineer- 

 ing site investigations and sedimentological studies as indicators of 

 vertical mixing, sediment source, and sediment transport routes. Heavy 

 mineral suites are often used for this purpose; however, many other common 

 elements of sediment bodies can be used. Some of the more easily identi- 

 fied constituents are listed in Table D-1. Many faunal elements are of 

 potential value for natural tracers . 



Latemauer and Pilkey (1967) used phosphorite grains in North Carolina 

 shelf to investigate the sediment source for adjacent beaches and to de- 

 lineate shelf sediment transport patterns. They concluded that the 

 phosphorite grains were of detrital origin and not formed in place as 

 authigenic minerals. They also found that sediments of river and estuary 

 bottoms opening on the shelf are presently devoid of phosphorite; thus, 

 phosphorite grains are not being contributed to shelf sediments from 

 nearby land areas. Their data showed that areas of high phosphorite 

 concentration occurred on the shelf floor, particularly in a locale 

 centered about 31.5 kilometers (17 nautical miles) southeast of Cape Fear 

 which they believed to be an outcrop of Tertiary phosphatic sediment and 

 the likely source of phosphorite grains distributed throughout Onslow Bay. 

 The presence of phosphorite on adjacent beaches led them also to conclude 

 that the shelf was an important source of the beach sediment. From their 

 studies Laternauer and Pilkey found that phosphorite grains were useful 

 in tracing sediment provenance and transport in Onslow Bay and were an 

 indicator of sediment interchange with adjacent Long and Raleigh Bays. 



Cores from the study area show that units closely underlying the 

 inner shelf floor of both Onslow and Long Bays contain phosphorite which 

 is contributed to the surficial sediments by reworking and lateral trans- 

 port from outcrops. None of these deposits are as high in phosphorite 

 grain content as the 40 percent content found by Laternauer and Pilkey 

 (1967). The Miocene deposit at core 95, where the phosphorite content 

 by grain count was nearly 25 percent, is the closest. Substantial amounts 

 of phosphorite also occur locally in the Cretaceous and Oligocene deposits 

 underlying Long Bay and Onslow Bay, and are usually present in more modest 

 amounts elsewhere (Table D-1) . This widespread occurrence of phosphorite 



