Sedimentology and Origin of Lower Cretaceous Pelagic Carbonates and Redeposited Clastics, Blake-Bahama Formation, Deep Sea Drilling Project Site 534, Western Equatorial Atlantic

Abstract

Drilling at Site 534 in the Blake-Bahama Basin recovered 268 m of Lower Cretaceous, Berriasian to Hauterivian, pelagic carbonates, together with volumetrically minor intercalations of claystone, black shales, and terrigenous and calcareous elastics. Radiolarian nannofossil pelagic carbonates accumulated in water depths of about 3300 to 3650 m, below the ACD (aragonite compensation depth) but close to the CCD (calcite compensation depth). Radiolarian abundance points to a relatively fertile ocean. In the Hauterivian and Barremian, during times of warm, humid climate and rising sea level, turbiditic influxes of both terrigenous and calcareous sediments, and minor debris flows were derived from the adjacent Blake Plateau. The claystones and black shales accumulated on the continental rise, then were redeposited onto the abyssal plain by turbidity currents. Dark organic-rich and pale organic-poor couplets are attributed to climatic variations on land, which controlled the input of terrigenous organic matter. Highly persistent, fine, parallel lamination in the pelagic chalks is explained by repeated algal "blooms." During early diagenesis, organic-poor carbonates remained oxygenated and were cemented early, whereas organic-rich intervals, devoid of burrowing organisms, continued to compact later in diagenesis. Interstitial dissolved-oxygen levels fluctuated repeatedly, but bottom waters were never static nor anoxic. The central western Atlantic in the Lower Cretaceous was thus a relatively fertile and wellmixed ocean basin.

Synthesis of Organic Geochemical Studies, Deep Sea Drilling Project Leg 93, North American Continental Margin

“…This scenario of downslope transport and redeposition is essentially the same as earlier proposed by Dean and Gardner (1982) for Site 367 in the Cape Verde Basin, by Dean et al (1984) for Site 530 in the Angola Basin, and by Robertson and Bliefnick (1983) for Site 534 in the Blake-Bahama Basin. Also, this scenario helps explain the variability in organic character present in the sediments of the western North Atlantic.…”

“…Quicker burial results in better preservation, even under oxygenated bottomwater conditions, through establishment of anoxic conditions in the sediments whenever organic matter supplies are sufficient to deplete pore-water oxygen levels. This phenomenon has led Habib (1983) and Robertson and Bliefnick (1983) to suggest that Mesozoic black shales at Site 534 result primarily from rapid sedimentation of terrigenous organic matter associated with turbidity currents and that the western North Atlantic need not have been anoxic during these episodes. In view of the abundance of turbidites at Site 603 and the generally terrigenous character of organic matter in sediments from both the Hatteras and Blake-Bahama Formulations, Rullkötter et al, Habib and Drugg, and Katz (all this volume) conclude that downslope displacement and rapid reburial by turbidity flows created the black shale deposits at this continental rise location.…”

Jurassic through Early Cretaceous Sedimentation History of the Central Equatorial Pacific and of Sites 800 and 801

“…In contrast to the ubiquitous oxygenation of the Pacific sediments, the Atlantic pelagic carbonates have preserved Late Jurassic through Early Cretaceous organic microfossils, enabling development of a regional high-resolution dinoflagellate stratigraphy Habib and Drugg, 1983), and display frequent intervals of laminated sedimentation with elevated organic-carbon content during the Valanginian through Aptian (e.g., Lancelot et al, 1972;Robertson and Bliefnick, 1983;Ogg et al, 1987;Huang, 1991).…”

Cretaceous ammonites from Fuerteventura, Canary Islands

“…In a larger context, these Lower Cretaceous turbidites reflect an interval of warm and humid climate, favouring the accumulation of siliciclastics during times of global sea-level highstands (Robertson & Bliefnick, 1983;Weissert, 1990), and are well documented along the western AtlanticTethyan seaway (von Rad & Sarti, 1986;Weissert, 1990). Interestingly Neocomites is also reported from comparable deposits of partly similar facies of the central Atlantic.…”