Abstract: The Sr-isotope composition of pore fluids and co-occurring sediments (carbonates, phosphorites, and dolomites) have been measured from two cores retrieved from western margin of the Great Bahama Bank. The Sr-isotope compositions of the pore fluids, corrected for possible contamination by surface waters using tritium, indicate that the formation fluids have 87 Sr/ 86 Sr ratios less radiogenic than modern sea water, but in all instances considerably more radiogenic than the co-occurring sediments and rocks. Assuming that similar conditions of fluid movement have been prevalent since deposition, the Sr-isotopic composition of the sediments has been used to constrain the age of deposition and diagenesis. The 87 Sr/ 86 Sr ratios of the solid components can be treated in two ways. First, if it is assumed that the chronostratigraphy determined from biostratigraphic and magnetostratigraphic methods is correct, then the Sr-isotopic composition can be used to place constraints on the timing of diagenesis. Second, if the diagenetic alteration occurred relatively soon after deposition, then the Sr-isotope composition can constrain the age of deposition. The data from the sediments show that the 87 Sr/ 86 Sr ratios are in most instances close to the 87 Sr/ 86 Sr ratio calculated from the chronostratigraphic age. The only substantial disagreement between the Sr-isotope age and the biostratigraphic/magnetostratigraphic age occurs between 300 and 500 mbsf in Clino. This could indicate either recrystalli-zation after deposition by more radiogenic solutions or slight uncertainties in the chronostratigraphy in this interval.

Abstract: Two cores drilled on the western margin of Great Bahama Banks penetrated a total of 1131 m of Quaternary to Late Neogene platform and periplatform carbonate sediments. These carbonates show a wide range of diagenetic textures ranging from largely unaltered primary carbonates to texturally mature limestones and dolostones. Although some of these carbonates have been diagenetically altered under freshwater conditions and show classic geochemical and petrographic indicators of such, other rocks exhibit similar petrographic features yet have never experienced meteoric influences. These features include micrite envelopes, moldic porosity, blocky spar cementation, and aragonite neomorphism The core Unda (453.8 m) includes three successions of shallow-water platform to reef sediments that alternate with deeper shelf deposits of silt to coarse sand. Clino, the more distal core, penetrated 677.3 m. It contains an upper reef to platform (21.6-140 m) overlying a thick package of deep forereef to upper and lower slope sediments (140-677.3 m). Three diagenetic zones were identified on the basis of similar diagenetic fabrics. Diagenetic zone 1 (Unda 0-108.08 m; Clino 0-152.71 m) is characterized by minor early marine diagenesis overprinted by numerous episodes of subaerial exposure with caliches, large-scale dissolution, and blocky spar cementation. Diagenetic zone II includes the lower reef and platform in Unda (292.82360.28 m) and has the most complex alteration in these cores. Minor early marine diagenesis was followed by several episodes of meteoric diagenesis, Most of the early fabrics, however, were destroyed during pervasive dolomitization in the burial environment. Diagenetic zone III includes deeper shelf to slope facies in both cores (Unda 108.08-292.82 m; 360.28-452.94 m; Clino 152.71-677.27 m) and shows only marine to marine-burial diagenesis. The exact fabrics within these intervals varies by lithology. Peloidal packstones to grainstones show minimal alteration (primarily compaction, minor dolomitization, and some recrystallization). Skeletal grainstones show two different styles of modification. Most commonly the grainstones are characterized by nearly complete dissolution of aragonitic components, minor cementation, and 10% to 20% early burial dolomite. Grainstones interbedded with the tight peloidal sediments, on the other hand, show nearly complete blocky spar cementation along with neomorphism of aragonitic skeletal grains (peloids were dissolved). Several marine hardgrounds with penecontemporaneous dolomite and/or phosphate and blackening also occur within the deeper-water facies. Each diagenetic zone in Clino and Unda is characterized by fabrics developed in one or more diagenetic environment. The larger-scale controls, therefore, are those that govern the diagenetic environment. For these young sediments, the most important are the depositional system (i.e., reef and platform versus deeper margin to slope) and the sea-level history. As a result, the diagenetic zones largely coincide with the depositional successions and seismic sequences identified in these cores. This study shows that the end product of marine to marine-burial diagenesis appears very similar to the end product of alteration in the phreatic meteoric environment i.e., a limestone composed of LMC and minor dolomite with micrite envelopes, moldic porosity, blocky spar cementation, and aragonite neornorphism. On this basis, a reevaluation is needed of the criteria for recognizing meteoric diagenesis in ancient carbonate sequences. Without clear physical evidence of subaerial exposure (e.g., caliche horizons or vadosecements) or chemical evidence of meteoric fluids (e.g., negative δ 18 O) great care is needed to identify meteoric diagenesis, In addition, the large-scale (> 100 m) sea-level lowstands of the latest Pleistocene did not result in significant phreatic meteoric diagenesis, perhaps because the fresh-water lens is too far below the recharge zone.