3. EFFECTS OF RELATIVE SEA-LEVEL CHANGES ON THE DIAGENESIS OF EOCENE SEDIMENT: NEW JERSEY SLOPE AND COASTAL PLAIN1Cecilia M.G. McHugh2,3 |
ABSTRACTEocene lithology recovered by the Ocean Drilling Program Legs 150 and 150X was studied to evaluate the effects of relative sea-level changes on the diagenesis of the sediment. Island Beach, Atlantic City, and Cape May boreholes were drilled on the coastal plain during Leg 150X, and Sites 902-904 and 906 were drilled on the continental slope (400–1200 m of water depth) during Leg 150. The shallow- to deep-water transect provides an opportunity to examine the diagenetic transformations of the sediment with increasing distance from the shoreline. Numerous episodes of diagenesis (with formation of micrite, sparry calcite, dolomite, and silica cements) typical of the near-shore, near-surface environment occurred in the coastal plain sediment. In contrast, less nucleation of crystallites, coarser crystal forms, and diagenesis below the bacterial sulfate reduction zone are characteristic of the upper slope Sites. Diagenesis is patchy at the middle slope, where some microfossils still maintain their primary cementation. Sediment adjacent to three main Eocene slope unconformities records physical and chemical changes that differ from those of the sediment within the sequences. The most extensive alterations occur in upper Eocene sediment beneath the contact with the upper Oligocene. This contact, marked by a seismic reflection that can be traced throughout the slope to the shelf and correlated to a sequence boundary, represents a major transition from an Eocene pelagic carbonate-dominated margin to an Oligo-cene hemipelagic siliciclastic-dominated margin. Lithologically, this transition is manifested in upper Eocene sediment by a decrease in the calcium carbonate and opal content and by an increase in detrital minerals. Diagenetic fluids transformed the unconformable surface of upper Eocene biosiliceous chalks into a mosaic of euhedral to subhedral dolomite rhombs that petrographic analyses suggest were deposited by highly saturated fluids at low temperatures. The upper Oligocene silty clays above contain abundant quartz, illite, kaolinite, and brown glauconites that indicate terrigenous sediment transport and reworking. More episodes of diagenesis, abundant former fluid migration routes, which suggest increased pore-fluid activity, and sediment reworking are associated with the two other Eocene unconformable surfaces. Energy dispersive X-ray analyses of former fluid pathways and the matrix of the sediment above and below these unconformable surfaces suggest that the sources for the fluids were local. These unconformities that separate the uppermost middle Eocene and the lower/middle Eocene have also been documented at other slope Sites and at the coastal plain, and appear to be widespread throughout the continental margin. |
|