Discrete downhole trends in color were documented at Sites 1213 and 1214, but a more mixed pattern was observed at Site 1207 (Fig. F2), which we questioned. Results of the reexamination and description of Site 1207 cores are given in Figure F5; these proved similar to the original shipboard descriptions with some minor variations in color (Fig. F2), verifying the mixed pattern. In general, Site 1207 (Figs. F2, F5) shows a general color trend of predominantly yellowish brown chert in Cores 198-1207B-1R through 10R, whereas Cores 198-1207B-10R through 32R display a wide range of chert colors, including grays. The gray cherts do not occur in Cores 33R through 37R, and then reappear in Core 38R.
Another question about the color patterns at Site 1207 concerned the wide variety of chert colors, from brown to black hues, observed in individual cores (Figs. F2, F5). Chert fragments were reexamined to determine if there is in fact a tendency for chert to be more variegated at Site 1207. Out of the 184 fragments described to create Figure F5, only 13% (24) were variegated; the remaining fragments exhibited only one color.
In general, the lithologies, mineralogies, structures, and textures of samples (Tables T2, T3) are similar to those described at Site 1207 (Bralower, Premoli Silva, Malone et al., 2002) and by previous workers for Cretaceous sedimentary rocks from Shatsky Rise (Keene, 1975) and other Pacific sites (Hein et al., 1981). As our focus is chert color, we looked for attributes associated with color and found grossly defined trends. In terms of biogenic components, radiolarian percentages were highest in the brown chert, intermediate in the gray chert, and lowest in the red chert. Foraminifer percentages decreased from reddish brown to brown to dark gray chert, whereas ostracodes were most common in the reddish brown cherts. Lamination and bioturbation also showed patterns, particularly in the gray lithologies. Distinct burrows were more common in the dark gray chert, porcellanite, and limestone; lamination was most common in the gray limestone. Diagenetic features were also analyzed; the only trend was that reddish brown chert showed the highest percentage of opal-CT lepispheres filling bioclasts. There was no trend in porosity type or abundance with color, which is more of a function of lithology, with the chalk/limestones having the highest percentage of matrix porosity and the cherts having the lowest.
Organic matter content was petrographically estimated (Table T3), and values ranged up to ~2%. Organic content in red chert samples was not distinguishable, owing to the presence of Fe oxides. Two brown cherts had moderate (~2%) organic matter content, whereas seven brown and three gray chert samples had low (~1%) organic matter content. Of the remaining chert samples, 11 brown, 15 gray, and 1 green had negligible (<1%) amounts of organic matter.
Geochemical data are presented in Table T4 and Figures F7, F8, F9, F10, F11, F12, F13, F14, F15, F16, and F17. In some samples elements generally considered as trace elements occur in sufficient quantities to be considered major elements (e.g., Ba and Sr) (Table T4) but are still discussed below with the trace components. A coefficient correlation analysis of XRF data was performed in order to determine significant positive (>0.80) or negative (less than –0.80) correlations among major elements and trace elements within the sample set (Table T5). Significant positive correlation exists among the following groups: TiO2, Al2O3, MgO, Na2O, K2O, Rb, Sr, Zr, and Zn; FeO, MgO and Cu; P2O5 and La; and V and Ba. A negative correlation exists between SiO2 and TiO2, Al2O3, MgO, CaO, Sr, Zr, and Zn.
As expected in a suite of chert, porcellanite, and chalk/limestone samples, the major elemental components are silica (SiO2), mainly representing opal (-A and -CT), chalcedony, and microquartz, and calcium (CaO), representing biogenic and authigenic carbonate components. Low aluminum content indicates only trace amounts of clay minerals (or feldspar) in these pelagic rocks. XRF data indicate some end-member lithologies, but many samples are mixtures of silica and carbonate phases. Sample mineralogy was verified using XRD techniques. The XRF and XRD data were used to reclassify the samples (Table T6). Three of the samples showed evidence of common to abundant cristobalite, confirming that the samples are porcellanite or porcellanitic chert. Four samples contain abundant calcite, confirming that they are limestones. Two are mixed limestone/chert. The remainder are chert.
To illustrate the above relationships, the weight percent SiO2 and CaO contents of the samples are plotted by lithology in Figure F7. All samples that significantly deviate from the 1:1 ratio line (e.g., Cores 198-1213B-25R, 10R, 198-1207B-47R, 198-1213B-21R, 19R, and 198-1207B-46R) contain high concentrations of barium (3,104–12,361 ppm) (Fig. F8). The samples with the greatest deviation (Cores 198-1213B-10R and 109-1207B-46R) tend to show higher concentrations of strontium (764–1026 ppm) (Fig. F9) and aluminum oxide (~3 wt%).
Major element distributions (Figs. F10, F11, F12, F13, F14, F15, F16, F17) show some trends among percent composition vs. lithology. Titanium percentages are generally low (0.18–0.006 wt%), aluminum content ranges up to 2.9 wt%, magnesium ranges up to 1.92 wt%, sodium ranges up to 0.69 wt%, and potassium ranges up to 0.80 wt%. These five components (Ti, Al, Mg, Na, and K) are most common in limestones and porcellanite and less abundant in chert. Limestones contain the most iron, but a few chert samples are also ferruginous. Iron percentages range up to 3.5 wt%. Manganese and phosphorous do not vary systematically with lithology. Manganese content ranges up to 1.6 wt%, whereas phosphorus ranges up to 0.49 wt%.
There are a few trends relating major element composition to chert color. With a few exceptions, gray cherts generally have lower concentrations of Fe and K than red and brown cherts. Brown cherts contain higher Mn or P and variable though moderate concentrations of trace elements. Gray cherts have lower Al and higher Ca. Chert color vs. aluminum content appears to be somewhat variable, but in general, gray cherts have the least amount of aluminum. Chert color shows no systematic relationship with magnesium, sodium, or titanium contents.
Several trace elements, such as Ni, Cr, La, Ce, and Ga, show no systematic relationship to lithology and major element percent composition (Table T4). Most trace elements, such as Sc, Rb, V, Zr, Nb, Zn, Y, Cu, Sr, and Ba, generally are most abundant in limestones and porcellanites, but show no apparent trend with chert color (Table T4; Figs. F8, F9).
XRF data (Table T4) were also analyzed to determine whether there were any correlations between major and trace element composition of chert and closely associated (same core) porcellanite or limestone. The samples selected for XRF analyses included six sets of chert and associated porcellanite or limestone from the same core segment or from the same core interval. These pairs include
With respect to major element compositions, several gross trends are apparent. The greatest difference in Fe content is seen between gray chert and its associated limestone, although both lithologies are generally enriched in Fe compared to other non-gray lithology counterparts. The gap between chert vs. limestone Fe content narrows in the green and brown chert-limestone pairs, and is even narrower between chert and porcellanite. Magnesium shows the same trend as iron. Other significant observations may have bearing on chert color. The limestone associated with the green chert has very high Mn, and the chert is also slightly enriched in Mn compared to other chert colors. The porcellanite (porcellanitic chert) associated with brown and red cherts has slightly less Mn than its chert counterpart. In addition to the red chert porcellanite pair in Core 198-1213B-7R, another red chert sample was analyzed. Notably, the brown chert has much higher P content.
Similarities in trace element composition were observed in three of the sample pairs. Limestone and green chert from Core 198-1213B-25R have similar contents of Cr (both 11 ppm), Sc (0 and 1 ppm, respectively), and Rb (9 and 8 ppm, respectively). The porcellanite (porcellanitic chert) and red chert from Core 198-1213B-7R have similar Ce contents of 13 and 10 ppm, respectively. The porcellanite and gray chert from Core 198-1207B-47R have similar Ni contents of 9 and 8 ppm, respectively. Aside from these trends, no other commonalities were observed.
Interpretations by shipboard scientists of downhole geophysical and geochemical logs at Sites 1207 and 1213 provide some additional information for understanding the stratigraphic distribution of chert in the section. This is especially important given the low recovery rates in the Cretaceous chert-bearing sections, so we attempt to correlate chert color distribution to these data.
The Formation MicroScanner (FMS) provides high-resolution electrical resistivity–based images of borehole walls, which allowed shipboard scientists to determine chert distribution, thickness, and percentage over the interval from 200 to 380 mbsf in Hole 1207B (Bralower, Premoli Silva, Malone et al., 2002). FMS data indicate that the chert is layered (continuous across borehole), not nodular (curved edge exposed in borehole), and that chert bed thickness ranges from 0.025 to 0.25 m (Fig. F18) (Bralower, Premoli Silva, Malone, et al., 2002). Note that the descriptive term layered is used rather than the term bedded, which has implications for the origin of the chert. Our addition of the chert color trends on Figure F18 shows that the thickest chert intervals occur within the uppermost yellow to yellow-brown chert sequence from 200 to 278 mbsf. The interval from 306 to 326 mbsf shows the highest percentage of chert, ranging up to 44%. Within this cherty interval, reddish hues dominate and gray chert is absent. Review of shipboard-prepared thin sections of chert within, above, and below this interval show no apparent textural or compositional differences across this zone. It is somewhat unfortunate that because of logging difficulties at other sites and time constraints (Bralower, Premoli Silva, Malone et al., 2002), the most detailed analysis of chert distribution could only be made for Site 1207, where the downhole color patterns are least distinct (Fig. F19).
The major natural gamma radiation peaks in the logged sections at Sites 1207 and 1213 (Fig. F19) coincide with the occurrence of an Oceanic Anoxic Event (OAE1a) section (see discussion in Marsaglia, this volume), where the gamma signal is associated with uranium-rich organic matter (Bralower, Premoli Silva, Malone, et al., 2002). Elsewhere, where the gamma signal is less dependent on uranium/organic content, relative peaks are more likely controlled by the distribution of potassium- and thorium-rich clay minerals. Locally, clay-rich tuffaceous intervals occur below and above OAE1a (Marsaglia, this volume), but elsewhere in the section the clay may be wind-blown nonvolcanic dust. Although not pictured, the patterns in porosity and density logging measurements mimic those of resistivity and gamma ray logs pictured in Figure F19. There is a shallower high-resistivity zone at both sites (gray shading in Fig. F19), which is marked by five significant peaks in resistivity, density, and porosity. These were interpreted by shipboard scientists (Bralower, Premoli Silva, Malone, et al., 2002) as intervals of well-lithified sediment, possibly densely spaced chert beds. These correspond to red-brown chert intervals, which, at least at Site 1213, accumulated more slowly than adjacent gray chert intervals. Thin section, XRF, and XRD data from samples from this interval corroborate their high silica content.
Discrete downhole trends in color were documented at Sites 1213 and 1214, but a more mixed pattern was observed at Site 1207 (Figs. F2, F3, F4). As discussed above, there are major similarities between the electrical resistivity log profiles for Sites 1207 and 1213, located >600 km to the south. However, a bed-by-bed correlation is not possible. The greatest differences are noted in the Aptian Selli OAE1a interval, where variable organic matter and volcanic input may have had an effect (see Marsaglia, this volume). The general similarity of logging patterns at Sites 1207 and 1213 suggests that the length of Shatsky Rise was affected by variations in sedimentation style, and biostratigraphic data presented in Bralower, Premoli Silva, Malone, et al. (2002) indicate that there was more pronounced variability in sedimentation rate over the same intervals at the southern site.