| Summary |
Benthic foraniniferal assemblages of the Pungo River Formation from the central North Carolina Coastal Plain indicate deposition in nearshore inner continental shelf to middle shelf environments. In Beaufort and Pamlico Counties, part of the Aurora Embayment, faunas from the lower part of the formation (Units A and B) are dominated by Buliminella elegantissima, Caucasina gracilis, and Elphidium excavatum. Florilus pizzarensis, Cibicides lobatulus, Elphidium limatulum, Nonionella miocenica, Hanzawaia cončntrica, and H. nipponica occur sporadically and are of secondary importance when present. High faunal dominance (averaging greater than 50%), low diversity values (Shannon-Wiener Information Function averaging 1.4), and the rarity of planktic foraminifera suggest that these units were deposited in nearshore, shallow water environments. With the exception of E. limatulum, dominant species from the upper part of the formation (Units C, D, and DD) are those of secondary importance in the lower units. Faunal dominance averages 33%, diversity values are moderate (averaging 2.3), and planktic specimens are more abundant. These units were deposited in more open water marine environments. In Craven County, along the northern flank of a pre-Miocene topographic high, the dominant species throughout the formation are Cassidulina laevigata and Uvigerina calvertensis, forms suggesting an t. T. outer shelf environment. However, high faunal dominance, moderate diversity values (2.0 to 2.7), and the rarity of planktic specimens are inconsistent with an outer shelf setting. Perhaps typical offshore species migrated shoreward in response to favorable conditions related to the adjacent high. Assemblages of the unit associated with this high (Unit CC) grade laterally into the units farther north. In Carteret County, south of this high and along the northern edge of Onslow Bay, faunas are similar to those from Units C, D, and DD in the Aurora Embayment. Several species present here are absent in the Aurora Embayment, their migration perhaps prevented by environments associated with the intervening topographic high. Based on planktic foraminifera, an age range from the latter part of Zone N. 7 to the latter part of Zone N. 8 (late Early Miocene) is tentatively proposed for the entire Pungo River section in the study area. Benthic foraminiferal assemblages from the Yorktown Formation indicate deposition during an initial marine transgression (the lower Yorktown) followed by a general marine regression (the upper Yorktown). The lower Yorktown is a fining upward sequence of terrigenous sediments dominated by Buliminella elegantissima. Caucasina gracilis, and Florilus pizzarensis. Species of secondary importance include Cibicides lobatulus, Nonionella miocenica, Fursenkoina fusiformis, and Elphldium excavatum. Species diversity values are generally high (averaging 2.7) and faunal dominance is low (averaging 34%). The upper Yorktown is a coarsening upward sequence of terrigenous sediments in which planktic specimens become progressively scarcer. It is dominated by Cibicides lobatulus, Elphidlum excavatum. Hanzawala cončntrica, Florllus pizzarensls, and Bollvlna lowmanl. Species of secondary Importance Include Cancrls sagra, Globocassldullna crassa, Bollvlna paula, and Uvlgerlna subperlgrlna. Species diversity values are generally lower than those for the lower Yorktown (averaging 2.4). Faunal dominance averages 35%, ranging as high as 47% In one sample. There Is a general shift upward through the unit from characteristic midshelf assemblages to faunas indicative of more nearshore environments. Water depths were probably no greater than 50 to 60 meters during maximum transgression, and were [less-than-or-equal-to] 30 or 40 meters during that portion of the regressive cycle that has been preserved. Planktic foraminifera indicate an age of late Early to early Late Pliocene (middle of Zone N. 18 to the lower-middle of Zone N. 20) for Yorktown and equivalent strata (Duplin Formation). |
