Cretaceous and Cenozoic Sediments from the Atlantic Ocean

Abstract

Leg 14 of the Deep Sea Drilling Project completed 9 holes; 7 were located off NW Africa, 1 in Ceara Abyssal Plain, and 1 on Demerara Rise. Quaternary and Pliocene sediments consist of nanno chalk oozes and brown clay. The facies boundary (CCD) is near 5000 meters. Sedimentation rates are between 10 and 30 meters/million years (m/My) for chalk ooze and 2 to 4 m/My for pelagic clay. Redeposition off the Amazon produces terrigenous and bioclastic turbidite sequences with rates increasing, on the average, from 50 to 150 m/My. During the Miocene the CCD is shallow, with restricted calcareous ooze and extended brown clay domain. Terrigenous and siliceous deposits are abundant off NW Africa. The Oligocene is characterized by a major hiatus; where present (Demerara Rise, foot of continental rise), it appears similar to the Miocene. Off NW Africa, Eocene sediments consist of brown clays, in part zeolitic, and greenish clays which, though rich in quartz and siliceous fossils, are less rich than Miocene sediments. Chert is rare. On Demerara Rise, the Eocene is represented by siliceous foram-nanno chalk. The Paleocene rests unconformably on Maestrichtian at Site 144, where the Cretaceous-Tertiary boundary was cored. Off NW Africa, brown and green pelagic clay and dolomite-clay (chert) cycles apparently belong to this period. The major sediment types of the Post-Cenomanian Cretaceous are zeolitic radiolarian, brown clay, black shale, limestone beds and chert layers. Redeposition is thought to play a major role in the origin of ümestones and cherts. Cenomanian carbonates consist mainly of nanno marl ooze, presumably deposited on an ancestral Mid-Atlantic Ridge. Calcareous carbonaceous sediments intercalated with limestone beds occur on Demerara Rise. Below the CCD, black shales, limestones and chert accumulated off NW Africa. Near the presumed boundary of ancestral ridge flank and continental rise, cyclic sequences of black shale and dolomite silts (in part ankerite) were deposited. Aptian and Albian sediments consist of black nanno marl off NW Africa and of shelly and quartzose calcarenite, and carbonaceous clay on Demerara Rise. The changing sediment patterns are related to changes in topography (continental drift, sea-floor spreading, sea-floor subsidence, orogenic activity including compressional buckling at the Gibraltar Fracture Zone) and changes in deep-sea circulation (carbonate under-saturation, oxygenation, nutrient concentration, bottom current strength), upper water circulation (from transAtlantic to intra-Atlantic currents, variations in upwelling, fertility), and redeposition processes.

Site 386: Fracture Valley Sedimentation on the Central Bermuda Rise

“…It seems probable that most of the "quartz" is chalcedony that has been diagenetically transformed from original biogenic opal via an intermediate stage of disordered cristobalite. This inference agrees with studies by Berger and von Rad (1972), who found that with increasing depth and age, opal and lussatite (disordered cristoba¬ lite) are finally transformed into quartzitic rock (see also Riech and von Rad, this volume).…”

Neogene Sand Layers off Northwest Africa: Composition and Source Environment

“…The seven sites cover a wide climatic range between the humid northern tropical belt in the south (Site 367: 12°30'N) and the humid Mediterranean belt in the north (Site 135: 35°1O'N, Site 370: 32°50'N). This distribution was shifted some 6° to 7° of latitude southward in the early Miocene, according to the reconstructions of plate motions (Francheteau, 1970;Berger and von Rad, 1972).…”

“…The DSDP Leg 14 sites are discussed by Berger and von Rad (1972). Neogene to Oligocene quartzose sand layers exist in Cores 135-SW1 and SW2, 138-1 and 2, 139-6 and 7, and 140A-1 and 2.…”

“…Physiographic setting of Sites 135,138 to 140 (Leg 14), 367,368,and 370 (Leg 41 Berger and von Rad (1972). generally suffer from a paucity of data.…”

“…Core recovery was much better at Site 368, but sandy layers rarely occur there. Table 1 summarizes the length of stratigraphic record represented by the cores (Berger and von Rad, 1972), and the distribution of core lengths at Sites 367, 368, and 370. In regard to this table, it must be stated that the stratigraphic and paleoenvironmental interpretations of this report…”