Deep-sea biostratigraphy of prograding platform margins (Neogene, Bahamas): key evidence linked to depositional rhythm

Abstract

New foraminiferal evidence from two boreholes on the paleoshelf and slope of western Great Bahama Bank has wide-ranging implications for understanding 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 selected parts of thick hemipelagic limestone. Earlier efforts to obtain biostratigraphic ages identified six biostratigraphic units in each borehole, provided biozonal age alternatives for both holes, and resulted in different Pliocene biozones between them. The new data define six units in one hole and seven in the other, bracket the biozones present and their ages, indicate different sedimentation rates, and show that within the limits of biostratigraphic resolution the biozones are correlative between the holes. Most importantly, the revised ages show that the paleoshelf borehole probably penetrated the late Miocene rather than middle Miocene. The oldest unit is on the paleoshelf and the youngest (uppermost Pliocene) is on the slope. Between the holes, the stratigraphic interval spans the temporal interval from an inferred maximum of ~ 10.2 Ma to a minimum of ~ 1.6 Ma. Although the biozones range sequentially from the Neogloboquadrina acostaensis (N16) Zone to the basal part of the Globorotalia truncatulinoides truncatulinoides (N22) Zone ( Globorotalia crassaformis viola Subzone), absence 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 ( ~ 1.2 Myr) above the paleoslope Miocene/Pliocene boundary. In addition, the late Pliocene Globorotalia tosaensis tosaensis (N21) Zone is not recognized on the slope. Its absence is consistent with a widespread regional unconformity. Sedimentation rates and depths of series boundaries vary widely in both holes. The paleoslope Miocene/Pliocene boundary lies at ~540 m below top of the hole. The lower/upper Pliocene boundary is placed 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 anywhere within this interval is a reasonable assessment considering an exceptionally high rate of sedimentation ( ~562 m/Myr; 168.6 m interval, based on topmost foraminiferal sample; 1.9–1.6 Ma). As expected, the lowest sedimentation rate occurs in the condensed section overlying the Miocene/Pliocene boundary ( ~ 5 m/Myr, 9.5 m, 5.3–4.1 Ma). The paleoshelf Miocene/Pliocene boundary lies below a hiatal condensed section (295–278 m below top of the hole) that has a greater sedimentation rate ( ~ 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 within the top 113 m of the hole. Sedimentation rates on the shelf range from ~ 15 m/Myr above the condensed section (22.9 m, 5.3–3.8 Ma) to a late Pliocene high of ~ 183 m/Myr (54.9 m interval, based 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).