Edward P. Laine

Erosion of the Laurentide region of North America by glacial and glaciofluvial processes

Collection of seismic reflection data from continental margins and ocean basins surrounding North America makes it possible to estimate the amount of material eroded from the area formerly covered by Laurentide ice sheets since major glaciation began in North America. A minimum estimate is made of 1.62 × 106 km3, or an average 120 m of rock physically eroded from the Laurentide region. This figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift, Cenozoic marine sediments, and modern sediment loads of rivers. Most of the sediment produced during Laurentide glaciation has already been transported to the oceans. The importance of continental glaciation as a geomorphic agency in North America may have to be reevaluated. Evidence from sedimentation rates in ocean basins surrounding Greenland and Antarctica suggests that sediment production, sediment transport, and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps, such as the Laurentide. Low rates of sediment survival from the time of the Permo-Carboniferous and Precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental crust.

Geological effects of the Gulf Stream System on the northern Bermuda Rise

Abstract The Gulf Stream System contains a clockwise rotating set of bottom currents which influence the sea bed in the northern North American Basin. It is possible to interpret the present-day and historical record of current activity in this basin in terms of the deep flow of the Gulf Stream. This interpretation provides a much more satisfactory and consistent explanation of the geologic record than previous interpretations based strictly on the influence of the classic abyssal circulation. The present-day circulation of the Gulf Stream actively resuspends large quantities of sediment on and near the northern Bermuda Rise, and it is suggested that this resuspension may be due to the eddy field which is embedded in the westward flowing return flow. During the late Cenozoic the Gulf Stream System was responsible for depositing and shaping the major acoustic/sedimentological units on the relatively smooth surface of the Horizon A complex. Fine-grained, mont-morillionite-rich sediments derived from the chemically weathered saprolites of the Hudson and St. Lawrence drainage basins, were injected into the deep ocean basins by turbidity currents during the late Paleogene and the Neogene. Fine-grained turbidites from the St. Lawrence spread southward down the southeastern Sohm Abyssal Plain across the eastward-flowing Gulf Stream and its westward flowing return flows. Portions of these fine-grained sediments were entrained by the main and return flows and deposited downstream as acoustically non-laminated accumulations on the Corner and Bermuda Rises, respectively. The Hudson River input was injected into the deep basin through the Hudson Canyon System. The interaction of these fine-grained turbidites with the northward flowing deep flow of the Gulf Stream System led to the formation of the Gulf Stream Outer Ridge, here defined for the first time. During the late Neogene or early Pleistocene the Gulf Stream Outer Ridge was partially eroded by the Gulf Stream System and portions subsequently covered by coarse-grained Pleistocene turbidites. The coarse-grained illite-rich turbidites that flowed across the southeastern Sohm Abyssal Plain also crossed the main and return flows of the Gulf Stream System. The finer portions of these sediments were entrained and deposited as Pleistocene acoustically laminated sediments on the plateaus of the Bermuda and Corner Rises.

New evidence from beneath the western North Atlantic for the depth of glacial erosion in Greenland and North America

Abstract Interpretation of Deep Sea Drilling Project results and air-gun seismic profiles suggests that about 106 km3 of sediment have been eroded from eastern North America and southern Greenland and deposited in the adjacent North Atlantic since the beginning of continental glaciation. This volume is a minimum estimate which does not account for sediment beneath the continental shelf nor that portion carried south of the Blake-Bahama Outer Ridge by the Western Boundary Undercurrent. It represents erosion of about 100 m of solid rock and indicates that more than 90% of the sediment eroded from these areas was deposited as sands, silts, and clays in the adjacent western North Atlantic. Glaciation accounts for between 55 and 95 m of this average 100 m, and fluvial processes account for the remainder. The documented erosion in part substantiates W. A. Whites (1972, Geological Society of America Bulletin83, 1037–1056) hypothesis of deep erosion and exhumation of shield regions, but is not in agreement with the entire volume of erosion implied by his model.

Paleogene-Neogene depositional history of the middle U.S. Atlantic continental rise: mixed turbidite and contourite depositional systems

Abstract The construction of the middle U.S. Atlantic continental rise from late Paleogene to late Pliocene time included downslope sediment gravity flows and alongslope contourite deposition. Some previous studies have suggested that contour-following bottom currents were the dominant control on depositional patterns. Depositional relief from two large contourite drifts, largely built in late Miocene to late Pliocene time, continue to dominate present-day lower rise morphology. Detailed seismic sequence mapping presented here suggests that significant turbidite (fan) systems also were active and are preserved in the stratigraphic section. A single thick depositional sequence is mapped for Oligocene to middle Miocene sediments deposited when inferred fan systems off the Norfolk-Washington canyons were reworked by bottom currents along the lower rise to form the proto-Hatteras Outer Ridge. Detailed mapping of three late Miocene to late Pliocene depositional sequences indicates that fan-channel systems were active at the same time large contourite drifts were being constructed. Sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the Hatteras Outer Ridge and, through entrainment and resuspension, was an important source for regional contourite deposition controlled by the deep Western Boundary Undercurrent. We suggest that submarine fans and current-controlled sediment drifts may develop simultaneously as companion systems. Types of fan/drift interaction include current-only modification of fans, transitional fan-to-drift, and adjacent or overlapping submarine fan and sediment drift deposition.

High resolution isotope study of the latest deglaciation based on Bermuda Rise cores

Abstract A stable isotope and 14C investigation of carbonates from three late Quaternary cores with high rates of sedimentation from the northeastern Bermuda Rise has produced the highest resolution record of the glacial maximum to Holocene stable isotope change yet obtained from an open ocean location. The record includes a three-step “termination” and the first direct evidence of an early deglacial meltwater “spike” in the open sea.

A magnetic signature of bottom current erosion

Abstract To characterize the magnetic signal associated with bottom current erosion, we have conducted downcore measurements of magnetic texture and fabric in three cores containing known erosional hiatuses. Using biostratigraphic and magnetostratigraphic analysis, erosional hiatuses were identified for three cores from two current-dominated environments: the northern Bermuda Rise and the south Indian Basin. The cores were contiguously subsampled and the magnetic susceptibility ( K ), the anisotropy of magnetic susceptibility (AMS) and the anhysteretic susceptibility ( K ARM ) were measured. The AMS parameters h and q were used to describe the relative magnitude and the shape, respectively, of the representative susceptibility ellipsoid. The concentration-independent and dimensionless ratio K ARM / K was used to indicate relative changes in magnetic grain size. Hiatuses in the three cores were marked by unusually high values of h and q indicating a greater development of internal fabric with a more linear fabric representation; depressed values of K ARM / K indicated a coarser mean magnetic grain size. The magnitude of this signal, however, did not have a simple relationship with hiatus duration. t -test results statistically confirmed the existence of an erosional signature. To enhance the applicability of this technique, a quantitative discriminant model was constructed and calibrated using the magnetic characteristics of erosion. Since this model was constructed from samples of known ‘erosional’ and ‘non-erosional’ character, the discriminant functions could then be used as reference equations to identify erosion in other cores. Biostratigraphic and oxygen isotopic data from a 14 m core from the northern Bermuda Rise, extending back to isotopic stage 8 (∼ 250 ka BP), indicate two short intervals of low accumulation rate which, for this region, are most probably attributable to bottom current erosion. Magnetic data for this core were entered into the reference discriminant model and the two zones of suspected erosion were classified by the functions as being highly ‘crosional’ in character. Zones of probable erosion were identified at several other levels in the core. The timing of these short-term crosional events was found to coincide closely with glacial intervals, suggesting that bottom currents active on the northern Bermuda Rise were invigorated periodically during glacial maxima at c, 18, 58, 64, 85, 130–140, 165–180, 225, and 250 ka BP.

The genesis and character of benthic turbid events, Northern Hatteras Abyssal plain

Abstract Benthic turbid events, with particle concentrations of up to 1.5 mg l−1, occupy about 20% of a two-year record of near-bottom light attenuation made above the Northern Hatteras Abyssal Plain. Theyare most often associated with deep-sea mesocle eddies, although rare events occur during periods when southward contour flow predominates. In a temporal sense, the presence of deep-sea mesoscale eddies corresponds strongly to periods when cold-core Gulf Stream rings propagate through surface waters of the study area. Additionaly, over half of the benthic turbid events observed are initiated within a day of the passage of atmospheric storms with propagation speeds in excess of 800 km day−1.

Seismic stratigraphy of the northern Bermuda Rise

Abstract Analysis of low-frequency seismic profiles from the northern Bermuda Rise has led to the recognition of several seismic horizons and determination of a regional seismic stratigraphic framework. Horizon A∗ is the deepest regionally significant reflector identified, and Horizon A C , A T , and A V of the Horizon A complex are also mappable reflectors in the study area. Three additional regionally significant reflectors named Yellow, Purple and Silver have been identified in the area. These seven reflectors allow speculation on the sedimentary evolution of the area. Sedimentation below the Horizon A complex was primarily pelagic. The distribution of reflectors within the Horizon A complex indicates the area experienced turbidite deposition from both the U.S. east coast continental margin and nearby Bermuda, as well as pelagic deposition during the Eocene. Sedimentation subsequently has been dominated by at least two distinct periods of abyssal current flow. Initially bottom currents brought in sediments from the north or west, creating an anomalously thick accumulation of seismically poorly stratified sediments on the Rise. This period of accumulation was punctuated by an episode of erosion in latest Miocene or earliest Pliocene. The most recent sediments are seismically well-stratified sediments of primarily Quaternary age carried from the east by the Gulf Stream System.