Saeed Khodabakhsh

Depositional facies of late Pleistocene Heinrich events in the Labrador Sea

Late Pleistocene Heinrich ice-rafting events produced layers rich in ice-rafted debris in major parts of the North Atlantic north of 40°N. A high detrital carbonate content points to the Hudson Strait outlet of the Laurentide ice sheet as a dominant source of the icebergs. Heinrich events were coupled with short-term climate fluctuations during the last and penultimate glaciations and provide evidence for cryosphere-hydrosphere-atmosphere interaction in Pleistocene climate change. An unsolved problem with Heinrich layers has been their high concentration of fine-grained detrital carbonate (>80% of the total detrital carbonate), which cannot have been delivered by icebergs alone. We propose combinations of different processes that deposited four sedimentologically different types of Heinrich layers: ice rafting alone for the coarser, sand- to gravel-sized fractions and the fine fractions in distal regions (type IV Heinrich layers), whereas nepheloid flows deposited the bulk of the fine sediment in regions proximal to the Hudson Strait (type I Heinrich layers). On the Labrador slope, turbidity currents spilling over from canyons were also involved in transporting the fine-grained carbonate-rich material, causing an alternation of mud-turbidites and thin laminae of ice-rafted debris in type II Heinrich layers. On the levees of the Northwest Atlantic Mid-Ocean Channel, the thickness relationship is reversed: mud-turbidites deposited by occasional spillover of currents from the channel are thin and alternate with thicker laminae of ice-rafted debris (type III Heinrich layers).

Fine-grained sediment lofting from meltwater-generated turbidity currents during Heinrich events

Turbidity currents generated from sediment-carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater. From experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal. This ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows. Evidence for large-scale lofting of suspended sediment from the top of giant sand- and gravel-carrying turbidity currents in the Labrador Sea comes from two independent lines of observations: (1) The first is a distinct sedimentary facies consisting of stacked, centimeter-thick graded mud layers that contain grains of ice-rafted debris (IRD) supported by the mud. Deposition of these unusual layers requires a gradedlayer‐forming process that is slow enough to allow the incorporation of IRD; this is not possible with normal mud-carrying turbidity currents. (2) The second observation is the presence of a huge abyssal sand and gravel plain in the central Labrador Sea that received its sediment from bed-load‐rich meltwater discharges from the Hudson Strait outlet of the Pleistocene Laurentide Ice Sheet. These discharges turned into turbidity currents that released rising columns of freshwater that carried fine-grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater. Deposition from these slow turbid interflows would allow the incorporation of IRD in the accumulating graded mud deposits. The IRD-spiked graded mud facies is restricted to Heinrich layers within 300 km radius of the Hudson Strait ice-stream terminus, tying the sand-carrying turbidity currents via fine-grained sediment lofting to Heinrich events. Estimated total discharge volumes of individual currents are on the order of 10 3 km 3 , supporting the notion that Heinrich ice-rafting events were times of maximum meltwater generation.

CONTINENTAL SLOPE SEDIMENTATION ADJACENT TO AN ICE MARGIN. III. THE UPPER LABRADOR SLOPE

Abstract The upper Labrador Slope is the key area for sediment transfer from the northeastern margin of the Pleistocene Laurentide ice sheets (LIS) to the deep basin. It is subdivided from north to south into 8 sectors based on relief differences and echo-character on 750 line-km of continuous sleeve-gun and 3.5 kHz seismic profiles. High- to moderate-relief sectors 2 and 4 and, in part, 6 and 8 are seismically transparent and well stratified with continuous individual high-amplitude reflections and deep penetration; low-relief sectors 1, 3, 5 and 7 show strong bottom reflections and, with the exception of sector 1, low seismic penetration, poor stratification, and low-amplitude, if any, subbottom reflections. The lateral distribution of these alternating high- to moderate- and low-relief sectors reflects fundamental differences in the sediment transfer mechanisms through outlets from the LIS onto the slope. Low-relief slope sectors represent debrite and turbidite slopes and are located in front of ice-outlets on the slope and adjacent regions to the north. They originate from mass wasting on the upper slope of glacial detritus with a significant coarse component originally delivered as englacial or subglacial material and deposited in (end-) moraines or as bedload by subglacial or supraglacial rivers. High- to moderate-relief sectors originate from fall-out of suspended sediment from turbid surface-plumes (TSP) and preferentially occur off the southern half of outlets and south of major outlets, as exemplified by sector 2 south of the Hudson Strait outlet. This asymmetry is caused by the south-flowing Labrador Current, which entrains the buoyantly rising turbid meltwater-plumes at the glacier front. The present high-relief topography, which shows a dendritic pattern of upslope canyon branching, is the result of retrograde, headward canyon erosion by mass-wasting processes of an originally much smoother mud-blanket surface. The sediment is remobilized by slumping and entrained in debris flows and turbidity currents. The occurrence of TSP deposits on the high- to moderate-relief upper-slope sectors requires a summer sea surface not frozen over during major parts of the Pleistocene, including glacial maxima. TPS deposition on the upper slope, ice-rafting, and a plethora of ice-margin depositional phenomena, many of which give rise to mass-wasting and mass-flow phenomena, are the main features that make high-latitude continental slopes adjacent to continental ice-sheets different from their lower-latitude counterparts.

Sandy Submarine Braid Plains: Potential Deep-Water Reservoirs

Sandy submarine braid plains, like their fluvial counterparts on land, are sand-rich depositional environments that may display excellent reservoir characteristics in terms of sediment volume, porosity, and permeability. The submarine examples may be laterally associated with potential source rocks such as the fine-grained levee deposits of deep-sea channels. A side-scan sonar study of the central Labrador Sea revealed the existence of a more than 700 km long and up to 120 km wide submarine sand and gravel plain that has been supplied with sediment by high-density turbidity currents, possibly resulting from subglacial lake outburst flooding in the Hudson Strait. The side-scan imagery of parts of the plain displays a conspicuous streaky pattern of alternating high and low backscatter intensity. High-resolution 3.5 kHz seismic profiles and 12 kHz bathymetric profiles show that the pattern represents a furrow-and-ridge (erosional) or channel-and-bar (depositional) topography, similar to a braided alluvial plain. The furrows or channels have low acoustic backscatter, are less than 10 m deep, and are separated by ridges or bars having high backscatter. Some channels terminate in depositional lobes. Individual channels and bars (or furrows and ridges) are less than 100 m wide and can be followed up to 40 km downcurrent. On sleeve-gun seismic profiles, the total sand thickness appears to be between 200 m (proximal) and 100 m (distal). Piston cores from the plain recovered massive sand layers up to 4 m thick, buried under 1 m of Holocene hemipelagic ooze. Texturally, the sands and gravelly sands display a trend of improving sorting with increasing mean grain size. Some very coarse grained samples are moderately well sorted and almost matrix free. The flooding events that deposited the sands might be the submarine counterpart of Heinrich events but need not be restricted to such events. Radiocarbon ages of about 10 k.y. from the base of the ooze overlying the youngest sand gave a minimum age for the sand that is similar to the age of Heinrich event 0. Estimates for the discharge volume of individual events are poorly constrained and range from 103 to 105 km3. Braided channel patterns in deep-water (Begin page 1500) sandy depositional environments are not restricted to high latitudes but also have been identified in various submarine fan settings in the lower latitudes, for example, the Orinoco, Var, and Monterey deep-sea fans and in the Santa Monica Basin. The largest examples, however, are known from high latitudes, suggesting that melt-water discharge from continental ice sheets may favor the formation of this habitat of giant sands in the deep sea. The occurrence of sandy braided deep-water environments having favorable reservoir characteristics in a variety of tectonic settings makes this type of environment a potentially interesting deep-water target.

Relationship Between Petrographic Characteristics and the Engineering Properties of Jurassic Sandstones, Hamedan, Iran

To study the relationship between engineering properties and petrographic characteristics, 20 rock samples were collected from Jurassic sandstones in the Hamedan region, western Iran. The specimens were tested to determine uniaxial compressive strength, point load strength index, tangent modulus, porosity, and dry and saturated unit weights. Samples were also subjected to petrographic examination, which included the observation of 11 parameters and modal analysis. Based on the results of a statistical analysis, polynomial prediction equations were developed to estimate physical and mechanical properties from petrographic characteristics. The results show that textural characteristics are more important than mineral compositions for predicting engineering characteristics. The packing density, packing proximity and grain shape are the petrographic properties that significantly affect the engineering properties of samples. Multivariate linear regression analysis was performed, employing four steps comprising various combinations of petrographic characteristics for each engineering parameter. The optimal equation, along with the relevant combination of petrographic characteristics for estimating the engineering properties of the rock samples is proposed.

Glacimarine Drainage Systems in Deep-sea: The NAMOC System of the Labrador Sea and its Sibling.

The continental Pleistocene Laurentide Ice Sheet (LIS) had far-reaching marine influence in shaping the ocean-floor adjacent to ice margin. The basinwide submarine-canyon and deep-sea channel system of the Northwest Atlantic Mid-Ocean Channel (NAMOC) of the Labrador Sea is the submarine continuation of the drainge system of the LIS on land, forming an interconnected land/sea drainage system 6,000 km long, one of the word’s longest drainage systems of Pleistocene age. The submarine portion forms a dual system, consisting of the mud-dominated NAMOC with its tributaries and a submarine sandy braid-plain.