ABSTRACT

In order to provide petrographic criteria for interpreting ancient mixed clastic-carbonate shelf deposits, 300 thin sections were studied from the Carolina continental shelf in an area extending from Raleigh Bay, North Carolina, to Cape Romain, South Carolina.

As shown by previous investigators, percentage of carbonate grains in this area is chiefly a function of dilution from adjacent terrigenous sources. Skeletal components, the dominant carbonate grain type, form 10 to 60 percent of the total sediment and are mainly pelecypods, gastropods, and coralline algae. Foraminifers, bryozoans, echinoderms, barnacles, ostracodes, corals, and worm tubes are minor constituents. Non-skeletal constituents, which usually comprise less than 10 percent of the total sediment, consist of peloids, ooids, lumps, and assorted carbonate lithoclasts (oomicrites, biomicrites, and biosparites). Terrigenous constituents are chiefly quartz, with subordinate amounts of feldspar, rock fragments, and heavy minerals. Glauconite and phosphorite are also present.

Generally, there is a systematic, but irregular increase in total carbonate components in a seaward direction. Molluscan assemblages characterize most inner and middle shelf sands, while coralline algae ar prevalent in outer shelf sands. Other carbonate components show no trends in an offshore direction.

A number of subsea and subaerial diagenetic processes have affected most of the carbonate grains and lithoclasts. Principal diagenetic features of carbonate grains area: 1) discrete macro-and microborings with or without fillings of orange-brown cryptocrystalline carbonate, 2) partial or complete micritization of skeletal grains with obliteration of skeletal architecture, 3) homogenization of ooids by micritization to cryptocrystalline carbonate, and 4) recrystallization of pelecypod fragments to coarse fibrous spar. Carbonate lithoclasts and rocks display a variety of diagenetic features that include chiefly: aggrading recrystallization of micrite to form pseudospar; development of moldic porosity with or without calcite spar fillings; recrystallization of high-Mg allochems to low-Mg calcite; and growth of primary void-filling calcite spar.