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Temporal and Spatial Patterns of Rhythmite Deposition on Mud Flats in the Macrotidal Cobequid Bay-Salmon River Estuary, Bay of Fundy, Canada
In the inner part of the macrotidal, Cobequid Bay-Salmon River estuary, the fundamental sedimentation unit in the mixed flats and mud flats is a sand-mud couplet that is deposited during the flood or ebb half of a tidal cycle. The thickness of the mud layer is relatively constant between couplets, whereas the thickness of, and structure in, the sandy portion varies significantly because of differences in the current speed and tidal range between tides. Thus the deposits of successive tidal cycles are alternately thicker and thinner as a result of the diurnal inequality, and show cyclic variations in sand-layer thicknesses and structures because of the 14-day, neap-spring cycle. These structures are the mud flat equivalent of tidal bundles, and are a modern, process analogue for the tidal rhythmites described by various authors. In areas of slower sedimentation, the deposits of individual tides are not recognizable; instead, annual deposits are developed that consist of alternating, bioturbated (summer-fall) and laminated (springtime) sandy muds. Ice-produced deformation and ice-rafted pebbles mark the winter period.
Because the width of the muddy flats decreases into the estuary, the flats become progressively sandier headward, and short-term sedimentation rates increase from near zero at the seaward end, to 5 to 30 cm/year in the middle estuary, and to several metres per year in the inner, tidal-fluvial transition. Therefore, individual tidal couplets, the diurnal inequality, and neap-spring cycles are clearly visible only in the inner estuary where bioturbation is almost non-existent because of the low salinity and high sedimentation rates. The annual deposits are best developed in the middle estuary, whereas intensely bioturbated muds characterize the outer estuary. These longitudinal changes may provide a means of identifying ancient estuarine deposits, and of locating oneself within them.
The architecture of the fringing muddy sediments along the entire length of the tidal-fluvial transition consists of elongate, wedge-shaped bodies of essentially flat-lying tidal sediments separated from each other by steep erosional surfaces. Vertical aggradation predominates over lateral accretion, even on laterally migrating banks, and inclined heterolithic stratification (IHS) is rarely developed.
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