Transgressive pulses of relatively short duration inundated the middle Atlantic Coastal Plain during Miocene time, resulting in a discontinuous depositional pattern. The spottiness of the strata is more pronounced in the Albemarle Embayment of North Carolina than in the Salisbury Embayment which extended from New Jersey to Maryland. The locus of deposition also changed, which further fragmented the continuity of the record. In the Salisbury Embayment, the depocenter generally moved southward during the Miocene, but a northward movement of the locus is indicated for the Albemarle Embayment.
Biogenic deposition of highly diatomaceous clay took place in two pulses in the Salisbury Embayment during the early and early middle Miocene; three pulses of phosphatic clay and sand and intercalated diatomaceous clay characterize the Albemarle Embayment during the early and middle Miocene. Depositional environments are interpreted largely on the basis of faunal data combined with sedimentologic characteristics. Miocene strata preserved onshore generally range from marginal-marine to shallow open-marine environments; environments approaching middle-shelf water depths are present in the Albemarle Embayment.
The southward building of deltas from southern New Jersey into the northern part of the eastern shore of Maryland brought significant amounts of clastic sediment into the Salisbury Embayment during the early middle Miocene, and deltaic sedimentation continued throughout the rest of the Miocene. Uplift of the Appalachian source area, beginning in the northern part of this area in the middle Miocene, brought widespread clastic sedimentation to both embayments during the late Miocene.