Fluid Processes and Sea-Floor Response on a Modern Storm-Dominated Shelf: Middle Atlantic Shelf of North America. Part II: Response of the Shelf Floor.

Abstract Sediment flux on the Middle Atlantic Shelf can be described as an advective-diffusive process. Sediment grains are released to the dispersal system from erosion of the shoreface. These grains move about on the shelf surface in a “random walk” manner under the impetus of repeated storm events. However, statistics describing the hydraulic climate show that the grain trajectories are biased, random walk trajectories; the preferred direction of movement is along the shelf to the southwest. Grain size gradients reflect the southwestward transport, and grain size within each major shelf sector becomes finer to the southwest. Grain shape is a more durable grain characteristic than grain size, and tends to reflect early Holocene subaerial drainage patterns rather than modern dynamics. However, grain shape patterns are fading due to the diffusive process. Bed form patterns on the Middle Atlantic Shelf are similarly a response to intermittent, along-shelf (southwestward) sediment transport by storm-generated currents. Flow-transverse bed forms occur at several scales. Ripples (6–300 cm spacing) are ubiquitous. Evidence from time-lapse photography suggests that current ripples that were formed during storms were remade as oscillation ripples in the post-storm period. Megaripples (3–20 m spacing) occupy perhaps 10–15% of the shelf surface. In areas of fine to very fine sand, this class appears as hummocky megaripples, a bed form probably equivalent to the hummocky cross-strata sets seen in ancient sedimentary strata. Sand waves (20–200 m spacing) arc widespread on the shelf surface. In most areas, they are of less than 1 m amplitude, due to the relative infrequency of major flow events; however, in areas where the storm flow field is constricted and accelerated, they may be as high as 7 m. Sand ridges are flow-oblique bed forms with a spacing of 0.5–4 km, and a height of 5–10 m. They make northwestward-opening angles of 10–45° with the shoreline. Both the flow-transverse bed forms and the large flow-oblique sand ridges have steeper and finer-grained down-current (southeastward) flanks. The Holocene transgressive sand sheet, into which these bed forms have been impressed, ranges from 0 to 20 m thick. Its leading edge commonly lies at the base of the shoreface. It grows landward in response to storm currents, which sweep sand off the shoreface. The sand sheet lies disconformably on marine marginal, early Holocene and Pleistocene strata. Stratification in the transgressive sand sheet is complex. Vibracores and box cores reveal stacked sand strata, which commonly range from 10 to 50 cm thick. About 40% of the beds exhibit horizontal to low-angle inclined lamination. Inclined lamination may be equivalent to hummocky cross-stratification, although the relationship is difficult to establish in cores. Laminated beds tend to be graded (tend to fine upward). Graded beds at the sea floor have ripple cross-lamination or have burrows near the top, but this uppermost zone is missing, perhaps due to erosion, in deeper, graded beds beneath the sea floor. Other beds are massive to indistinctly mottled, and are assumed to be bioturbated. Massive and mottled beds become increasingly abundant at the expense of laminated beds in a seaward direction. About 10% of the beds reveal high-angle cross-stratification. The strata of the transgressive sand sheet can be divided into four classes: 1. graded strata are single-event storm beds deposited from suspension; they are most abundant on the down-current flanks of sand ridges; 2. hummocky strata are also single-event suspension deposits, their depositional environment was characterized by a high ratio of wave orbital current to mean current; 3. cross-strata sets are multi-event beds which develop on the up-current flanks of sand ridges; 4. lag strata, consisting of coarse sand, pebbly sand, or fine gravel, occur in the troughs between ridges. The sand strata of the Atlantic Continental Shelf are tempestites, deposited by geostrophic storm currents. Relatively dense near-bottom suspensions of sediment are generated by storms, but velocities measured within these suspensions arc directed primarily along the shore, rather than offshore. There is no evidence for high-velocity turbidity currents on the Atlantic Continental Shelf.