Sandy submarine braid plains: Potential deep-water reservoirs

Abstract

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.