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The Yorktown Formation of Virginia and North Carolina is divided into three ostracode assemblage zones based on the occurrence of 230 species in 43 samples. The samples were compared in Q-mode using the Dice binary similarity coefficient and cluster analysis. From oldest to youngest, the three zones are the Pterygocythereis inexpectata, Orionina vaughani, and Puriana mesacostalis Zones. The first two are late Miocene in age and the last, which is poorly represented in Virginia, is considered to be early Pliocene in age. The level of association (R-mode study using the Dice coefficient) between 80 of the more commonly occurring species is shown by means of a dendrogram; values for the biostratigraphic fidelity and constancy of each of these species for each of the zones are given. A general trend through time from late Miocene to early Pliocene for the Yorktown transgression and regression is suggested, based on the occurrence of brackish-water ostracodes and bivalves, particularly Cyprideis and Corbicula. It is suggested that rapid transgression of the sea occurred during early Yorktown time to a maximum during the time represented by the middle Orionina vaughani Zone. A more or less steady regression of the sea followed during the remainder of Yorktown time. INTRODUCTION The Yorktown Formation (upper Tertiary) of North Carolina and Virginia is a very fossiliferous, lithologically heterogeneous unit with an exposed thickness (natural exposures or open-pit mines) of about 70-120 feet. The sandy clays and clayey sands assigned to the Yorktown crop out discontinuously in a 16,800-squaremile area between the Rappahannock River in Virginia and the Neuse River in North Carolina (fig. 1). South of the Neuse River, deposits of the same general age as the Yorktown are usually mapped as Duplin Marl. No microfossil zonation exists for the Yorktown. This report presents the results of a reconnaissance collecting program in conjunction with examination of existing museum collections, the object being to provide a regional ostracode zonation. Localities (fig. 1; p. 10) were chosen on the basis of geographic and stratigraphic spread. More detailed collecting will be done in locally complex areas such as the York-James peninsula where the distribution of numerous lithofacies within the Yorktown is being studied (Johnson, 1969; Coch, 1968; Bick and Coch, 1969). 78 0 25 50 75 100 KILOMETERS i i i i 11 i I I FIGURE 1. Location of collections; numbers correspond to those in locality list (p. 10). Dashed line encloses area in which Yorktown Formation crops out in natural exposures.

Ostracode biostratigraphy of the Yorktown Formation (upper Miocene and lower Pliocene) of Virginia and North Carolina