|Description||Up to 30 m of phosphatic sediments of early and middle Miocene Pungo River Formation were deposited in the north-south-trending Aurora Embayment of North Carolina. These sediments thin to approximately 10 to 15 m over the Cape Lookout High, a pre-Miocene feature which forms the southern boundary of the Aurora Embayment. The western and updip limit of the formation parallels a regional north-south structural hingeline or White Oak Lineament. The formation thins to a feather-edge at this lineament and thickens rapidly to the east and southeast. Deposition of the Pungo River Formation extended some unknown distance to the west of the White Oak Lineament, the present updip erosional limit.
The Pungo River Formation consists of the four major sediment sequences in the Aurora Area (units A, B, C, and D as described by Riggs and others, 1982b) and three lateral facies (units BB, CC, and DD). Phosphate sedimentation was concentrated in units A, B, and C which are laterally correlative throughout most of the study area. However, the muddy phosphorite quartz sands of unit B and possibly the phosphorite quartz sands and carbonate sediments of unit C grade downdip to the southeast into an 11-m-thick diatomaceous facies (unit BB). Units A, B, and C grade into a slightly phosphatic, calcareous, quartz sand facies (unit CC) to the south, in the area of the Cape Lookout High, which probably represents a shoaling environment. Dolomitic unit D, of the northern and eastern portions of the Aurora Embayment, grades laterally into calcareous unit DD in the central portion of the embayment.
Allochemical phosphate grains of the intraclastic variety dominate all sediment units in the formation. However, unit A contains abundant pelletal phosphate in the fine to very fine sand-size fraction. The highest phosphate concentrations occur along the upper shelflike basin margin in the west-central portion of the Aurora Embayment. Updip to the west, the phosphate concentration decreases within each unit which also thins due to subsequent erosion. Major facies changes within the sediment units have resulted in decreased phosphate contents downdip to the east and south within the Aurora Embayment.
Within the Aurora Area, units A through C of the Pungo River Formation are generally characterized by the cyclic deposition consisting of decreasing terrigenous and increasing phosphate sedimentation upward through the units; the deposition of each unit culminated with the formation of a carbonate cap-rock. The depositional pattern of these regionally persistent and cyclical lithologies suggests that units A through C were deposited during a major transgression. The overlying unit D was deposited during the early stages of a subsequent regressive phase. Truncation of the units by erosion took place prior to the deposition of the Pliocene Yorktown Formation. Thus, this extensive erosion has produced an apparent offlap configuration of the Pungo River units that actually represents a major transgressive or onlap sediment sequence and an early stage regressive sequence.||en_US