OSTRACODES AND CHRONOSTRATIGRAPHIC POSITION OF THE UPPER CRETACEOUS ARKADELPHIA FORMATION OF ARKANSAS

The ostracode assemblage of the Arkadelphia Formation, upper Maastrich tian, of southwestern Arkansas is characterized by a well-preserved, r elatively diverse, and abundant fauna representing 36 podocopid specie s and an undetermined number of species of the platycopid genus Cyther ella. Thirteen of these account for 85 percent of the fauna. The domin ant forms are Cytherella spp., Brachycythere rhomboidalis (Berry, 1925 ), Haplocytheridea renfroensis Crane, 1965, Haplocytheridea bruceclark i (Israelsky, 1929), and Brachycythere ovata (Berry, 1925), which acco unt for about 57 percent of the specimens found. Other species that ar e less common, but are characteristic of the Arkadelphia, are Antibyth ocypris macropora (Alexander, 1929), Ascetoleberis hazardi (Israelsky, 1929), Aversovalva fossata (Skinner, 1956), Brachycythere ledaforma ( Israelsky, 1929), Curfsina communis (Israelsky, 1929), Cytheromorpha a rbenzi (Skinner, 1956), Escharacytheridea micropunctata (Alexander, 19 29), and Veenia arachoides (Berry, 1925). The ostracode assemblage ind icates that the Arkadelphia was deposited in the inner sublittoral zon e. The species Veenia parallelopora (Alexander, 1929) and Brachycyther e foraminosa Alexander, 1934, are restricted to the Arkadelphia Format ion and allow correlation with other Gulf Coastal Plain Upper Cretaceo us units. A new ostracode interval zone, the Veenia parallelopora Zone , is proposed. This zone divides the Platycosta lixula Zone. Calcareou s planktic microfossils reported from the Arkadelphia and its correlat ives in the Veenia parallelopora Zone indicate that these deposits arc late Maastrichtian in age, not middle Maastrichtian as some authors h ave thought.