Outcrop samples of the Castle Hayne Limestone and the Castle Hayne and New Bern Formations and equivalent subsurface rocks yielded 188 species of Foraminifera. Planktonic Foraminifera indicate that these carbonate units are of middle Eocene age and were deposited during the time represented by the upper Globigerinatheka subconglobata (P11) and Morozovella lehneri (P12) Zones (44 to 46.5 m.y.B.P.). This section is bounded above and below by depositional hiatuses; it is overlain by Oligocene and younger rocks and underlain by Paleocene and older rocks. Previous studies have included some of these bounding units as part of the Castle Hayne Limestone. The section studied: (1) correlates with the lower part of the Claiborne Group of the Gulf region; (2) is defined as an inform l time-stratigraphic unit; (3) is herein designated the lower Claibornian unit to distinguish it from Castle Hayne s. l.; (4) does not contain internal hiatuses of significant duration.

Cluster analysis of samples of the lower Claibornian unit indicate benthic foraminiferal biofacies representing inner, middle, and outer shelf depositional environments. Paleobathymetric reconstructions utilizing these interpretations indicate that this unit was deposited on an actively block-faulted sea floor.

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