Few studies have examined the nature of chert color variation, particularly in deep-sea cores. Luedtke (1978), Fraser et al. (1972), and Hein et al. (1981) reported geochemical analyses of siliceous rocks (Table T1), but they did not focus specifically on the elemental differences among the different colors of chert. Hein et al.'s (1981) study is perhaps the most relevant to our discussion in that they analyzed some Cretaceous chert samples from Pacific sections (Table T1). They found that higher carbonate content in the chert samples correlated with high values of Mg, Ca, Mn, Sr, and Zr, whereas tuffaceous cherts were characterized by high values of K, Al, Zn, Mo, and Cr. Purer cherts had high SiO2 and boron values. No studies of Cretaceous chert-bearing sections have described alternating stratigraphic changes in color similar to those documented by Bralower, Premoli Silva, Malone, et al. (2002) on Shatsky Rise. However, Douzen and Ishiga (1995) reported stratigraphic changes in chert color from reddish to gray in Triassic and Jurassic outcrops of deep-marine sections near Japan and suggested that these represent changes in bottom conditions from oxic to anoxic.
Although trends in chert color with age and sedimentation rate have been documented at Sites 1207, 1213, and 1214 (Figs. F2, F3, F4) (Bralower, Premoli Silva, Malone et al., 2002), the combination of samples from Sites 1207 and 1213 provides the most complete temporal coverage of the Cretaceous chert-bearing intervals and represents the paleolatitudinal range from north (Site 1207) to south (Site 1213) across Shatsky Rise. We focus first on Site 1213, which has the most pronounced color variations over the longest stratigraphic record (Cenomanian to Berriasian). Shipboard analysis of color patterns within the Cretaceous chert-bearing intervals showed a distinct correlation between color and sedimentation rate throughout the section at Site 1213. Rough color zonations at Site 1213 (Fig. F3) are as follows:
56–85 meters below seafloor (mbsf) = red/brown,
85–189 mbsf = gray/black,
189–266 mbsf = red/brown,
266–400 mbsf = gray/black, and
400–447 mbsf = red/brown.
The sedimentation rates range from 12 to 21 m/m.y. in the gray/black zones and from 1 to 5 m/m.y. in the red/brown zones (Bralower, Premoli Silva, Malone, et al., 2002). Thus, it follows that the colors likely reflect sedimentation rates, with the reddish hues being associated with slower sedimentation and more oxidizing conditions.
In contrast, chert-bearing intervals at Site 1207 range from Campanian to Barremian in age, with only a minor gap at the Turonian/Coniacian boundary. There are less distinct color trends at Site 1207 and a tendency for the presence of both red/brown and gray/black chert in many cores. The indistinct color changes at Site 1207 may be a product of downhole contamination coupled with the overall lower sedimentation rates. Furthermore, as this was the first chert section described during Leg 198, some of the variation may be due to inconsistencies in description techniques.