Cyclic Deposition of Neogene Phosphorites in the Aurora Area, North Carolina, and their Possible Relationship to Global Sea-Level Fluctuations

Abstract

The Neogene phosphorites in the Aurora Area occur within the Miocene Pungo River Formation (units A, B, C, and D) and the Pliocene Yorktown Formation (lower and upper units). These units are characterized by the following patterns of sedimentation. 1) Three major erosional unconformities and four minor unconformable surfaces or hiatuses mark the boundaries between consecutive units and the under- and overlying formations. 2) Indurated carbonate sediments, which usually contain either a weathered fossil assemblage or are completely moldic, cap each unit. 3) Phosphate sedimentation began in unit A and increased to a maximum through unit C, was negligible in unit D, was reinitiated in the lower Yorktown, and was nonexistent in the upper Yorktown. 4) Phosphate concentration generally increases upward within each unit until carbonate sediments become important, then the phosphate decreases. 5) The dominant carbonate within each unit is as follows: units A and B, dolosilt; unit C, calcitic micrite; unit D, dolosilt with abundant calcite shell material; and both Yorktown units, calcitic micrite with abundant calcite shells. This sequence of upper Tertiary sediment units suggests a cyclical pattern controlled by global eustatic sea-level fluctuations. Each depositional unit, its carbonate cap, and the associated unconformable surfaces may correlate with established third-order sea-level cycles of Vail and others (1977 and 1979). Units A, B, and C appear to represent the maximum transgressive portion of the second-order Miocene supercycle. Phosphate sedimentation was coincident with each of the third-order transgressions, which culminated in carbonate sedimentation at the apex of each transgressive cycle. The magnitude of phosphate deposition in the Aurora Area increased with each third-order cycle to a maximum during the transgression forming the apex of the second-order supercycle. Unit D was deposited only over the eastern portion of the area as a regressive facies of the Miocene supercycle. The Pliocene Yorktown sediments were deposited during the next supercycle. The lower Yorktown phosphorites coincided with the transgression while the nonphosphatic upper Yorktown was deposited during the subsequent regressive phase.