A generalized lithologic column for the base of the Pliocene (Unit I) and the uppermost Miocene (Unit II; Fig. 8) shows the major changes in sediment type during the transition from the Messinian evaporative environment, during which the gypsum-bearing sediments of Unit III dominated, to the pelagic, open-water environment of the Pliocene. Semi-quantitative XRD results (Table 2) document the significant mineralogical changes associated with these lithologic variations. In Table 2, sample types are keyed to the lithologies given in Figure 2 and Figure 8. Figure 9 shows depth profiles of the major phases present. Departures from the relatively uniform mineralogy characteristic of the lower Pliocene (see Tribble and Wilkens, Chap. 8, this volume) are apparent at a depth of 304.92 mbsf, above the Pliocene/Miocene boundary defined on the basis of planktonic foraminifers (305.22 mbsf; Iaccarino et al., Chap. 15, this volume). The abundance of clay minerals and, to a lesser extent, quartz, increase at the expense of calcite, and there is also an increase in the relative intensity of the dolomite peak. This excursion in bulk mineralogy corresponds to a sample from a parallel-laminated interval in Section 161-975B-33X-2 at about 102 cm (see Core 975B-33X photograph in Comas, Zahn, Klaus, et al., 1996, p. 525).
Below this depth, bulk mineralogy varies dramatically between compositions characteristic of the calcite-rich micritic intervals, and those of the clay- and quartz-rich silty clays and the sands. Intermediate lithologies, although quite different in terms of sediment components, have bulk mineralogies that are not too dissimilar to those of the overlying pelagic sediment. These variations in mineralogy are most distinct in plots of mineral ratios (Fig. 10). The interval surrounding the Pliocene/Miocene boundary is characterized by alternations of dark and light sediment (Fig. 8; also see Shipboard Scientific Party, 1996b, fig. 15, p. 126). Dark bands are marked by relatively high ratios of clay/calcite and quartz/calcite, low calcite abundance, and slightly elevated dolomite intensities (Fig. 9C-D and Fig. 10A-B). Light bands have clay/calcite ratios slightly above, and quartz/calcite ratios similar to, the background levels defined by the overlying pelagic sediments (Fig. 10A-B). Calcite concentrations are distinctly higher in the light bands than in the dark bands, but still somewhat lower than in the overlying pelagic sequence. Dolomite intensities are at background levels (Fig. 9C-D).
Below the banded interval, the core contains about 60 cm of finely interlaminated micrite and micritic silty clay (Fig. 8; see also Shipboard Scientific Party, 1996b, fig. 18, p. 127). Sample 161-975B-33X-3, 3 cm, in this interval has unusually high concentrations of quartz and dolomite, as well as a significant percentage of celestite (Table 2; Fig. 9B, D). The abundance of dolomite and celestite may indicate the presence of a diagenetic front. Brines diffusing upward from the evaporative facies would provide SO42- for reaction with Sr2+ possibly released by recrystallization of carbonates and result in precipitation of celestite. Dolomite is also a common diagenetic product of reaction of brines with carbonate minerals, although its coexistence with a high-quartz concentration indicates the possibility of a detrital source for the dolomite, as has been suggested for other Leg 161 sites (Tribble and Wilkens, Chap. 8, this volume) and for stoichiometric dolomite from Site 372 (Pierre and Fontes, 1978). The other two samples from the interval of interlaminated micrite and micritic silty clay have bulk mineralogies similar to the background Pliocene sediments (Fig. 9 and Fig. 10).
Below the interlaminated interval there are about 50 cm of interbedded micrite and micritic silty clay (Fig. 8; see also Shipboard Scientific Party, 1996b, fig. 17, p. 127). Samples from this interval represent the end-member compositions for the finely interlaminated sequences above and below. Two samples of micrite were analyzed (Samples 161-975B-33X-3, 63 cm and 68 cm; see Table 2). Both are nearly pure calcite, with only minor admixtures of dolomite, and in one case, quartz. No clay minerals were detected in the micrite (Fig. 9). The micritic silty clay end-member is represented by Samples 975B-33X-3, 86 and 92 cm (Table 2). These samples are enriched in total clay minerals, quartz, and dolomite and have low calcite abundance (Fig. 9). A third sample of micritic silty clay (Sample 975B-33X-3, 105 cm) has a somewhat intermediary composition (Table 2; Fig. 9). This interbedded interval is underlain by another finely interlaminated interval (Fig. 8).
The remaining section of Unit II consists primarily of sand-rich layers described in the petrologic description section above. The carbonate-volcaniclastic sand at a depth of 141-143 cm in Section 161-975B-33X-3 is noteworthy for its unusually high feldspar content (Table 2). Sample 975B-33X-CC, 7 cm, from the calcareous/terrigenous sand, contains abundant quartz, dolomite, and celestite (Table 2). The only X-ray detectable gypsum in Unit II sediments is from samples from Section 975B-33X-CC (Table 2).
The detrital component of Unit II sediments varies considerably in quantity, but a depth plot of the quartz/clay ratio (Fig. 10C) indicates a general constancy in the composition of the detrital material from the latest Miocene through the earliest Pliocene. The quartz-rich samples of the calcareous/terrigenous sand from Section 161-975B-33X-CC and the single sample from Section 975B-33X-3, 143 cm, are the only exceptions. In addition, although individual clay minerals were not quantified, illite and chlorite were consistently present in all clay-bearing samples.
A major difference between the sediments of Unit II and the overlying Pliocene Unit I is one not detectable via XRD: the source of the calcite. Figure 11 shows the cumulative percentages of all calcite components detected in smear slides for these sediments. At the same depth as the shallowest shift in mineralogy (304.92 mbsf), the percentage of foraminifers drops to near-zero values, the abundance of nannofossils drops markedly, and micrite becomes an important sedimentary constituent. Foraminifers (± bioclasts) are again abundant in the sand-rich lithologies near the base of Unit II.
Mineralogy for Unit III samples is reported in Table 3. For the gypsiferous sediments of Unit III, the quantification routine could not be used. For these samples, relative peak intensities are listed for the major phases. Weight percentages are reported only for two samples of clay and micritic silty clay that were interbedded with the gypsum. One sample of pure anhydrite was found (Sample 161-975B-33X-CC, 31 cm) corresponding to an interval of relatively soft, laminated sediment between two intervals of finely laminated gypsum. The remaining samples are all dominated by gypsum. Samples of gypsiferous chalk have the highest concentrations of nongypsum components, which include calcite, dolomite, and quartz. Pinch-and-swell and finely laminated varieties of gypsum generally have little or no detectable nongypsum components. The two samples of intraclastic microconglomerate (Samples 975B-34X-CC, 1 cm and 4 cm) also have relatively high concentrations of calcite, dolomite, and quartz.