Mesozoic and Cenozoic Geology of the U.S. Atlantic Continental Slope and Rise

We have studied single·channel and \nmultichannel seismic-reflection profiles to identify and \ntrace key reflecting horizons and seismic sequences from . \nthe North American Basin into the U.S. Atlantic conti· \nnental rise and slope. Using seismic·facies analyses and \nisopach maps of seismic sequences together with availa· \nble borehole data, we interpret the geologic history of \nthe continental margin seaward of the shelf break. This \narea has a complex geologic record because deposition \nwas controlled by the interaction of a variety of sedimen· \ntary processes. These processes had their origins in both \nshallow-water (e.g., sea-level) and deep-ocean (e.g., abyssal· \ncurrent) impulses. Unfortunately, our interpretations are \nconstrained by borehole data only along the continental \nslope and the lowermost continental rise. Until the neces· \nsary samples are recovered in intervening areas, this \nreport serves as a preliminary analysis. \nfrom the time of initial sea·floor spreading in the \nMiddle Jurassic until the end of the Eocene, the sedimen· \ntary record was shaped principally by cross·slope processes \nsuch as turbidity currents and debris flows. The sedimen· \ntary prism that formed along the margin in the Middle \nJurassic probably was carbonate-rich, and it had a rela· \ntively uniform thickness with respect to the strike of the \nadjacent continental hinge zone. It is not clear whether \nthe margin was a carbonate ramp or had a fringing reef. \nIn contrast, by Late Jurassic time a shelf·edge reef was \npresent, and it controlled seaward dispersal of sediment. \nAbyssal fans formed at the foot of the continental slope \nbelow gaps in the reef trend!..notably seaward of the \nBaltimore Canyon trough and northeast of the Blake \nPlateau basin. Extensive construction of fans occurred \nthroughout the Early Cretaceous as segments of the reef \ngradually died or were overstepped by prograding shelf \nsediments. Seismic-reflection profiles show cut·and·fill \nstructures and presumably coarse-grained channel fill \nwithin the abyssal fans, but boreholes have sampled.only \nthe distal, fine-grained edges of these deposits. The bore· \nholes do show the dominant effect of deep-ocean processes \non the outermost margin, namely a sharp rise in the \ncalcite compensation depth (CCD) at the end of the \nNeocomian with subsequent deposition of carbon-rich \nblack sl!ales beneath intermittently anoxic bottom water. \nDuring the Late Cretaceous the basin again became \noxygenated, and high eustatic sea level caused flooding \nof the shelf. This restricted offshore transport of sediment, \nand a conformable drape facies was deposited across \nmost of the continental rise. In the Paleocene and espe· \ncially during the Eocene, seaward transport of sediment \nin turbidity currents reached its zenith. High productiv· \nity in surface waters, shelf erosion during sea-level low \nstands, and rapid sediment accumulation on the continental \nslope contributed to mass wasting that carried \nsediments up to 1,100 km across the broad continental \nrise and into the deep basin. \nThe dominance of cross-slope sedimentation processes \nwas dramatically curtailed at the beginning of the \nOligocene. Global cooling and the opening of the \nGreenland Sea to the Arctic Ocean allowed cool. strongly \ncirculating bottom water to enter the North Atlantic at \nthis time. The abyssal western boundary current that was \nformed along the U.S. margin deeply eroded the conti· \nnental rise and oversteepened the continental slope. \nCurrent speeds probably decreased during the Oligocene. \nThe flow, however, remained swift enough that the only \nsediment retained on the continental rise accumulated \nfrom large mass movements off the slope during a period \nof accelerated shelf erosion caused by a major sea-level \nlow stand in the mid·Oligocene (29 Mal. In the early \nto middle Miocene bottom-current speeds decreased \nfurther, the Gulf Stream swept sediment off the Blake \nPlateau. and a large clastic wedge prograded across the \nshelf of the Baltimore Canyon trough. These factors led \nto rapid deposition that formed the cores of the Blake \nOuter Ridge and another large sediment drift ( Chesapeake \nDrift) beneath the upper continental rise off New Jersey. \nThese drifts continued to develop under the influence of \ngradually decreasing current speeds during the late \nMiocene and Pliocene. At the same time, the growth of \ntwo secondary sediment drifts, the Hat teras and Bahama \nouter ridges, was accelerated, and very-well-developed \nsediment waves were formed there. The trend of gener· \nally decreasing bottom-current speeds was interrupted \nby short pulses of intense deep circulation in the late \nmiddle Miocene (12 Ma) and the late Pliocene (3 Ma). \nThese erosional pulses locally truncated beds on the \nflariks of sediment drifts, and they formed recognizable \nseismic boundaries (reflectors Merlin and Blue, \nrespectively) between seismic sequences. The pulses are \nprimarily responsible for modem sea-floor outcrops of \nMiocene and Pliocene sediments in ridge· flank locations. \nWhen Northern Hemisphere glaciation began in the late \nPliocene (3 Ma), downslope sedimentary processes again \nassumed an important role, particularly by incising sub· \nmarine canyons and channels into the continental slope \nand rise. The Quaternary sedimentary record has been \nshaped under the combined influence of these downslope \nprocesses and of contour-following bottom currents.