Latest Cretaceous and Eocene Paleoenvironments in the Blake-Bahama Basin, Western North Atlantic

Abstract

At Site 534 in the Blake-Bahama Basin, western North Atlantic, an interval of 68 m of Maestrichtian (Upper Cretaceous) and upper middle to upper Eocene sediments consists of terrigenous siltstones, mudstones, and varicolored zeolitic claystones; minor recovery of micritic limestones, porcellanites, and quartzitic chert was made at this site as well. Comparisons with other Deep Sea Drilling Project (DSDP) sites in the western North Atlantic suggest that the following formations are present in this interval: Hatteras (Maestrichtian), Plantagenet (Maestrichtian and upper Eocene), Bermuda Rise (upper middle to upper Eocene), and the basal Blake Ridge Formation (upper middle to upper Eocene). Recognition of a Tertiary interval of the Plantagenet allows that formation to be divided into lower and upper informal units. Condensation makes this formal lithostratigraphic subdivision difficult.Together the formations record marked net condensed sedimentation (average rate ca. 2.5 m/m.y.) in strongly oxidizing bottom waters. From sedimentary structures and petrography, it is inferred that the terrigenous siltstones and micritic limestones were redeposited from the continental margin by turbidity currents. Chemical data plus petrography confirm relatively high plankton productivity during the upper Eocene. Much of the nonrecovered Eocene interval may represent chert and porcellanite. Fragments recovered were formed by replacement of relatively porous calciturbidites by opal-CT and quartz. Radiolarians in interbedded claystones rich in clinoptilolite show extensive dissolution. Relative to typical hemipelagic sediments, the claystones are enriched in many metals (Cu, Ni, Zn, Pb), particularly within manganese micronodules. The metal accumulation is related to a 30-m.y. period of slow net sediment accumulation, rather than to hydrothermal enrichment or to upward mobilization of metals from the underlying reduced Hatteras black shale facies.Elsewhere in the Blake-Bahama Basin, at Site 391, 22 km to the northwest, upper Eocene facies are missing, reportedly due to deep seafloor erosion of up to 800 m of the sedimentary succession. By contrast, the discovery that this interval is preserved at nearby Site 534 points to much less extensive seafloor erosion, possibly mostly in the Oligocene, which is missing at both DSDP Sites.

Mesozoic–Cenozoic Depositional Environments Revealed by Deep Sea Drilling Project Leg 93 Drilling on the Continental Rise off the Eastern United States: Cruise Summary

“…Our drilling indicates, therefore, that these lower beds of the Bermuda Rise Formation are more extensive than shown in Figure 13. Robertson (1983) reached the same conclusion for the Blake-Bahama Basin, where drilling at DSDP Site 534 also showed the Bermuda Rise Formation to be present.…”

“…The origin of these metals mentioned above in the Bermuda Rise Formation and particularly in the underlying Plantagenet Formation in the North American Basin has long been the subject of interest and debate (see summary by by Robertson, 1983). Lancelot et al (1972) suggested metal enrichment from volcanic exhalations.…”

Clay Sedimentology

“…The nature of iron oxides is partly similar in both cases, but the geochemistry considerably differs. Robertson (1983) studies comparable series in the same domain (Blake-Bahama basin), and attributes the relative increase ofmetals and the reddish color to a slow deposit in an oxidizing environment rather than to a hydrothermal influence. Lancelot et al (1972) stress the abundance of Al 2 0 3 and K 2 0 in Atlantic sediments, which they attribute to the abundance ofterrigenous minerals such as illite and kaolinite.…”

Late Aptian to Recent Sedimentological History of the Lower Continental Rise off New Jersey, Deep Sea Drilling Project Site 603

“…This formation was not recovered at Site 391 (Sheridan, Enos, et al, 1978), only 22 km northwest of Site 534 in the Blake-Bahama Basin. Robertson (1983) therefore concluded that the discovery of this interval of the Bermuda Rise Formation at Site 534 points to less extensive seafloor erosion than previously suggested by Jansa et al (1979). Robertson also noted that the distribution of the Bermuda Rise Formation is usually between the outer continental rise and the upper Eocene Mid-Atlantic Ridge.…”

“…Lancelot et al (1972) suggested that this metal enrichment was from volcanic exhalations; Murdmaa et al (1978) proposed that the metal enrichment was related to slow sediment accumulation; and Arthur (1979) suggested that the en- richment was the result of upward diffussion of the elements from the underlying black shales of the Hatteras Formation. Robertson (1983) analyzed multicolored claystones from the Plantagenet Formation recovered at Site 534 in the Blake-Bahama Basin. He concluded that slow accumulation of sediment in strongly oxidizing bottom waters may have been sufficient to allow metals to be scavenged from the seawater, and subsequent local diagenetic conditions within the sediment would alter the sediment chemistry, yielding color banding and mottling.…”