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Large-scale, single set cross-stratification, considered to have been produced by the upstream migration of antidunes, is present in the basal division of turbidite beds 2-3 m thick in the ß1 member of the Cloridorme Formation (Ordovician), Gaspe, Quebec. The local westward paleocurrent direction is approximately parallel with the east-west strike of the vertical beds, which crop out on the wave-cut platform.
The beds generally show 3 broad divisions. The basal division (2-15 cm) consists of very coarse-grained sand with a smooth or fluted base and sinusoid top (wave length from 45 to 100 cm and amplitude from 3 to 7 cm) and internal laminae inclined at a shallow angle (less than 15°) toward the east. Where flutes occur they give a paleocurrent direction toward the west. A second division consists of spindle or globular-shaped nodules ("pseudonodules") in an argillaceous matrix and commonly occurs above the basal division. In 1 bed, which occurs in a sequence with beds exhibiting ripple-drift cross-lamination, a ripple-drifted horizon is developed between the basal and pseudonodule divisions, the ripple-drift produced by a paleocurrent toward the west. The upper division consists of f ne-grained siltstone or shale.
Because the basal very coarse sandstone is a small percentage of the total thickness of any bed, it appears that the settling velocity of the coarser grains probably was reduced by a high concentration of fine suspended material which increased the viscosity and reduced the density difference between the coarse grains and the suspension; the result was a high transport rate and the formation and preservation of antidunes, because of a delicate balance between traction and suspension in the current. Calculations suggest that the current moved at velocities of approximately 1 m/sec.
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