The upper Tertiary 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 features of sedimentation. (1) Three major erosional unconformities and five diastemic surfaces mark the boundaries between consecutive units. (2) Indurated carbonate sediments, which usually contain either a weathered fossil assemblage or are completely moldic, cap each unit. The carbonate surfaces locally contain a rock-boring infauna and are commonly phosphatized. (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 uppe 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: unit 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 diastemic surfaces correlate with established third order sea level cycles. Units A, B, and C appear to represent the maximum transgressive part of the second order Miocene supercycle. Phosphate sedimentation was coincident with the transgression; the maximum deposition occurred during the highest level of the sea. Unit D was deposited only over the eastern area as a regressive facies of the supercycle. The Pliocene Yorktown sediments were deposited during the next supercycle. The lower Yorktown phosphorites coincided with the maximum transgression while the nonphosphatic upper Yorktown was deposi ed during the subsequent regressive phase.

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