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 k...

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

“…Paleodepths of the studied sites were estimated following the procedure of Tucholke and Vogt (1979). A uniform thermal subsidence of underlying oceanic crust, which was assumed to have initially formed at ridge depths of 2700 m, and of plateaus was computed by applying an empirical global curve (Parsons and Sclater, 1977, as modified by Marty and Cazenave, 1989-their "Case 2"), where Depth = 2700 m + 283 m Velapsed m.y.…”

“…Paleolatitude for sites distant to the Leg 129 region may have been affected by rotation of the Pacific Plate (discussed in text). Depths of seafloor (depositional surface) are computed using the procedure of Tucholke and Vogt (1979), which uses the empirical subsidence curve of oceanic crust (Parsons and Sclater, 1977, as modified by Marty and Cazenave, 1989, in their "Case 2") and assumes an initial ridge crest depth of 2700 m, hence: Depth of sediment surface = (2700 m + 283 m elapsed m.y.) [= crustal subsidence] -2/3 of accumulated thickness of compacted sediment.…”

Cretaceous Planktonic Foraminifers and Depositional Environments from the Ontong Java Plateau with Emphasis on Sites 803 and 807

“…The rapid deepening of the CCD through the middle Maastrichtian, noted in much of the world ocean (Thierstein, 1979;Tucholke and Vogt, 1979), is indicated by the upward increase in preservation, abundance, and diversity of planktonic foraminifers and the decrease in radiolarians. Deposition of the upper Campanian to lower Maastrichtian portion of the sequence took place at or near the CCD, with the occurrence of larger planktonic foraminifers dependent, in part, on the ameliorating effects of carbonate resedimentation (Figs.…”

Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid‐Atlantic Ridge

“…Approximate areas are based on these dimensions and reflect maximum area of mullion structure and maximum area which may have been affected by the fault, respectively. Maximum relief is measured from crest of megamullion to maximum depth in preceding depression or succeeding valley at fault termination; average relief is height above average seafloor depth for the associated crustal age, derived from a North Atlantic empirical crustal age/depth curve [Tucholke and Vogt, 1979]. zig is difference between maximum RMBA over megamullion and average RMBA of "normal" crust in the preceding or succeeding magmatic episode of spreading; NA, not available.…”