PRESSURE SOLUTION IN OCEAN DRILLING PROGRAM LEG 166, SITES 1003 AND 1007: DISTRIBUTION AND CONTROLS

Determining the timing of pressure solution is usually complicated by having to extrapolate the depth at the time of alteration. Ocean Drilling Program Leg 166, on the western margin of Great Bahama Bank, recovered deep water carbonate sediments to a depth of 1300 mbsf where the current depth is also the maximum depth. We investigated sites 1003 (total depth=1300 m) and 1007 (total depth=1235.4 m) to determine the depth of pressure solution and the role of other factors in controlling its occurrence.

The sediment is an alternation of lowstand pelagic material, mostly low-Mg calcite, and highstand bank-derived material, with the typical mixed carbonate mineralogy. However, the current mineralogy of both lithologies is low-Mg calcite with trace amounts of quartz, dolomite, and aragonite. Lower carbonate values (=80-50%) correlate with dark pelagic-rich layers, indicating the presence of clays or other insoluble material. Higher percentages of carbonate (>80%) correlate with light layers of the core, rich in bank-derived components.

Recognizing the early stages of pressure solution is difficult. In thin section, thin dark wispy layers of concentrated organic or clay material appear at=300-500 meters depth in the pelagic-rich samples. With greater burial, thicker dark lines form around larger grains (usually pelagic foraminifera) and rare examples of grain-to-grain suturing occurs. By=900 meters, dissolution of grains is increasingly common shown by dark irregular lines truncating grains, again mainly in the dark layers. At=1000 meters, pressure solution seams first appear in both light and dark layers. These are 1-5 cm thick bands of dark material that have a higher concentration of large grains (usually pelagic foraminifera) than the surrounding sediment and show opaque lines truncating grains. These pressure solution seams extend across the entire width of the core and are easily identified in the hand sample.

Early stages of pressure solution appear first in the organic and/or clay-rich pelagic layers. However, unequivocal pressure solution seams first appear in both layers at=1000 m depth, suggesting that burial depth is the critical factor rather than insoluble content. Interestingly, these results are similar to those from chalks indicating original mineralogy is not important.