DEEP-SEA BIOSTRATIGRAPHY OF PROGRADING PLATFORM MARGINS (NEOGENE, BAHAMAS) - KEY EVIDENCE LINKED TO DEPOSITIONAL RHYTHM

New foraminiferal evidence from two boreholes on the paleoshelf and sl ope of western Great Bahama Bank has wide-ranging implications for und erstanding formation and evolution of carbonate-platform margins. The new data, abundant well-preserved planktic foraminifera, were obtained by disaggregating samples from intercalated pelagic layers and select ed parts of thick hemipelagic limestone. Earlier efforts to obtain bio stratigraphic ages identified six biostratigraphic units in each boreh ole, provided biozonal age alternatives for both holes, and resulted i n different Pliocene biozones between them. The new data define six un its in one hole and seven in the other, bracket the biozones present a nd their ages, indicate different sedimentation rates, and show that w ithin the limits of biostratigraphic resolution the biozones are corre lative between the holes. Most importantly, the revised ages show that the paleoshelf borehole probably penetrated the late Miocene rather t han middle Miocene. The oldest unit is on the paleoshelf and the young est (uppermost Pliocene) is on the slope. Between the holes, the strat igraphic interval spans the temporal interval from an inferred maximum of similar to 10.2 Ma to a minimum of similar to 1.6 Ma. Although the biozones range sequentially from the Neogloboquadrina acostaensis (N1 6) Zone to the basal part of the Globorotalia truncatulinoides truncat ulinoides (N22) Zone (Globorotalia crassaformis viola Subzone), absenc e of key species indicates that deposition was discontinuous. Numerous periods of erosion and/or nondeposition are inferred, the largest of which is a condensed section/ hiatus (similar to 1.2 Myr) above the pa leoslope Miocene/Pliocene boundary. In addition, the late Pliocene Glo borotalia tosaensis tosaensis (N21) Zone is not recognized on the slop e. Its absence is consistent with a widespread regional unconformity. Sedimentation rates and depths of series boundaries vary widely in bot h holes. The paleoslope Miocene/Pliocene boundary lies at similar to 5 40 m below top of the hole. The lower/upper Pliocene boundary is place d at or near 444 m. Position of the Pliocene/ Pleistocene boundary is less certain but is within the top 382 m of the hole. Its placement an ywhere within this interval is a reasonable assessment considering an exceptionally high rate of sedimentation ( similar to 562 m/Myr; 168.6 m interval, based on topmost foraminiferal sample; 1.9-1.6 Ma). As ex pected, the lowest sedimentation rate occurs in the condensed section overlying the Miocene/Pliocene boundary ( similar to 5 m/Myr, 9.5 m, 5 .3-4.1 Ma). The paleoshelf Miocene/Pliocene boundary lies below a hiat al condensed section (295-278 m below top of the hole) that has a grea ter sedimentation rate ( similar to 89 m/Myr, 17.7 m, 5.5-5.3 Ma) than that at the slope. The lower/upper Pliocene boundary is placed at or near a depth of 236 m, and the Pliocene/Pleistocene boundary lies with in the top 113 m of the hole. Sedimentation rates on the shelf range f rom similar to 15 m/Myr above the condensed section (22.9 m, 5.3-3.8 M a) to a late Pliocene high of similar to 183 m/Myr (54.9 m interval, b ased on the point at which the age-depth line crosses the 1.9 Myr mark between the topmost two fossiliferous samples; 2.2-1.9 Ma).