Abstract:
ODP Leg 166 Sites 1003 and 1007 each recovered over 1200 m of slope facies along the western margin of Great Bahama Bank. The sediments alternate between dark, calcitic, pelagic-rich wackestones and lighter wackestones to grainstones with both aragonitic bank-derived bioclasts and pelagic components. Early marine-burial diagenesis is distinctly different in light layers. The light layers, with their initially greater aragonite content, were cemented early, while the dark layers remained unaltered to greater depths. Pressure solution in ODP Leg 166 is mainly seen as dissolution seams starting at depths of around 900 m but not becoming obvious until below 1000 m. Although the dark layers show minor seams at shallower depths (700-900), well developed seams appear in both layers at about the same depth. Fitted-fabric (also known as sutured contacts) appears in selected dark layers at significantly shallower depths (to 500 m) than the true seams. Despite an originally grain-supported fabric, the early cementation of the light layers protected them from compaction and distributed the pressure during burial. The uncemented dark layers compacted easily, sometimes changing a matrix-supported fabric to a grain-supported fabric where larger bioclasts (mainly foraminifers) concentrated pressure at grain-to-grain contacts. The effective pressure for the fitted-fabrics is approximately the same as that required for dissolution seams. The shallower depth is caused by the concentrated pressure of the grain-to-grain contacts which is allowed by the lack of early cementation. This distinction could help explain the widely varying depths suggested for onset of pressure solution in the literature.