This paper describes unconformable stratigraphic relations within basin-margin deposits from an outcrop rather than seismic perspective, emphasizing the physical characteristics, stratigraphic context, age, origin, and usefulness of A.dojunconformities in stratigraphic subdivision and correlation. Stratigraphic relations within the outcropping Miocene of the Salisbury Embayment, middle Atlantic Coastal Plain, are more complex than previously described. The Calvert (Plum Point Member) and Choptank formations are subdivided by a series of erosional surfaces into six depositional sequences traceable over a 9,000-sq-km (3,475-sq-mi) study area in Maryland and Virginia. The disconformities take the form of burrowed firmgrounds in outcrop, but exhibit up to 14 m (46 ft) of topographic relief locally and represent transgressive and regressive ravinement surfaces. One of the surfaces records sediment starvation in a distal marine environment. Internal facies relations within the depositional sequences are complex but basically cyclic, consisting usually of a basal condensed shell or bone deposit formed under prolonged conditions of reduced net sedimentation and grading upward into less fossiliferous and siltier facies in regressive sequence. Each sequence consists of two or more of the original lithologic zones of Shattuck (1904).

Diatom biostratigraphic data indicate that the disconformities are not measurably diachronous within the study area and represent less than one biozone except where erosion has enlarged the vacuity; they thus provide a basis for fine-scale chronostratigraphic subdivision and correlation within the Miocene outcrop belt. Diatom data published elsewhere identify a major, 2.5 m.y.-long hiatus within the Fairhaven Member of the Calvert Formation that encompasses the Burdigalian-Langhian stage boundary, where Vail and Hardenbol (1979) have identified a minor interregional unconformity in seismic profiles (16.5 m.y. event). The disconformity at the base of the Plum Point Member of the Calvert Formation corresponds in age to another minor reflector at the Langhian-Serravallian boundary (15.5 .y. event), and a disconformity between the Choptank and St. Marys formations may correspond to Vail and Hardenbol's (1979) mid-Serravallian reflector (13.0 m.y. event). Other disconformities within the studied section, including the disconformable contact of the Calvert and Choptank formations, are expressions of stratigraphic complexity relating to transgressive-regressive migrations of marginal marine and open marine strandlines within the basin margin record.