A refinement of the nannofossil biostratigraphy is presented in Table T2 and an age-depth plot based on the nannofossil datums of this study plus shipboard foraminiferal datums is shown in Figure F2.
There are significant numbers of nannofossils in Cores 194-1193A-31X and 32X (Table T1), and these two cores are the only ones that recovered significant sediment in lithologic Unit III (Fig. F2). These nannofossils suggest that the carbonate platform had not prograded over the site or that the carbonate platform retreated from the site when the top of lithologic Subunit IIIB was deposited.
Since Sphenolithus heteromorphus has been found as high as Sample 194-1193A-31X-2, 25 cm (Table T2), it indicates an age older than 13.6 Ma. The sample immediately above is barren of nannofossils and the next 25 cores uphole did not yield any sediment samples suitable for this study. With the new age constraint (>13.6 Ma) from Sample 194-1193A-31X-2, 25 cm, together with those age constraints in the lower Miocene (Fig. F2), it is roughly estimated that the top of lithologic Unit III, or the termination of the carbonate platform and the large sea level fall, is middle Miocene, assuming sedimentation rates did not decline an order of magnitude in the middle Miocene compared to the late early Miocene and sea level fall did not remove >100 m of the platform sediment (Fig. F2).