Western North Atlantic: Sedimentary Evolution and Aspects of Tectonic History

Abstract

Drilling results from DSDP Leg 43 and earlier legs in the western North Atlantic are synthesized to examine the sedimentary history and aspects of the tectonic evolution of the basin. In the Late Jurassic, reddish calcareous claystones and limestones were deposited in a pelagic environment near and above the CCD. Lime stone deposition persisted throughout the basin until the end of Neocomian time when a sharp rise of the CCD to less than 2800 meters occurred. This event was coeval with the beginning of "blackclay' ' deposition and formation of euxinic basin conditions at least below the CCD. The stagnant deep basin probably was created by the formation of circum-Atlantic deep-circulation barriers coinci dent in time with tectonic readjustments in the North Atlantic and the initial opening of the South Atlantic. At the same time, rudistcoral reefs flourished along the continental margin as far north as Nova Scotia. Although the CCD remained shallow until the Maestrichtian, the deep basin became oxygenated in the late Cenomanian/Turonian, probably because of the introduction of deep and bottom water from the South Atlantic, and multicolored clays were deposited at true pelagic rates of 1-3 m/m.y. Little terrigenous detritus entered the deep basin because of the Cretaceous trans gression and because of (now largely extinct) shelf-edge barrier reefs. Volcanic detritus from the then active New England Seamounts and from other background volcanism comprises a locally significant component of the multicolored clays. A sharp depression of the CCD to more than 5400 meters occurred in the late Maestrichtian to earliest Paleocene. This event resulted in basin-wide chalk deposition and may relate to the end-Cretaceous extinctions. Terrigenous detritus began to enter the basin in the early Paleocene, and locally deposited black clays indicate that deep-basin circulation was again sluggish. Highly siliceous sediments were deposited during the latest Paleocene to late Oligocene in response to developing abyssal circu lation, upwelling, and enrichment of nutrients in surface water by volcanism. The latter played an especially significant role in lateearly to early-middle Eocene rapid deposition of biogenic silica and development of "Horizon A" cherts. Siliceous turbidites flooded the basin during this interval, but the Bermuda Rise was isolated from the influx when it was uplifted during the middle to late Eocene. During the Oligocene to earliest Miocene, the abyssal circulation in tensified because of climatic cooling and tectonically controlled in troduction of bottom water from the Norwegian-Greenland seas; several hundred meters of sediment were eroded along the existing continental rise, forming a major unconformity. A sharp change to depositional conditions occurred in the early to middle Miocene, and rapid Neogene hemipelagic sedimentation controlled by moderately strong bottom currents is responsible for formation of the present continental rise and outer-ridge systems. The CCD has gradually deepened from about 4 km to more than 5 km during the Tertiary, but little carbonate is present in sediments other than on the Mid-Atlantic Ridge because of dilution by current-transported terrige nous debris. I " ' ' 'I DSDP BOREHOLES LEGS 1,2,4,11,43,44 40 e 1. Locations of Deep Sea Drilling Project sites in the western North Atlantic. Leg 43 sites are underlined.