Upper Tertiary beds of the Chesapeake Group deposited in the Salisbury embayment preserve a detailed record of periodic sedimentation that is punctuated by unconformities and diastems. Correlation of these Miocene and Pliocene units is possible by the use of various microfossil and macrofossil groups. The unconformities that separate the beds are easily recognized and regionally traceable. Detailed analysis of the beds indicates that most formations are the product of one or more marine transgressions.

One transgression is represented by the Old Church Formation (upper Oligocene/lower Miocene), six by the Calvert Formation (lower and lower middle Miocene), two by the Choptank Formation (middle middle Miocene), three by the St. Marys Formation (upper middle Miocene), two by the Eastover Formation (upper Miocene), three by the Yorktown Formation (middle and upper Pliocene), and one by the Chowan River Formation (upper Pliocene).

In this sequence, on the basis of mollusk data, a temperature chart can be constructed that reflects oscillation within the cool-temperate to subtropical range. The relative stability of the temperatures and the continuing submergence of the Salisbury embayment allowed a richly fossiliferous, diverse, essentially temperate molluscan assemblage to form. This nearly endemic, perched fauna was decimated during the period of low sea level after the Yorktown transgression and before the Chowan River transgression. The result was a large-scale extinction at the generic as well as specific level because of low temperatures and the loss of habitat due to the low sea level stand.

Thus, the beds of the Chesapeake Group record a detailed history of regional and local tectonism, and global and local sea level fluctuations.

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