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Bedforms and Bars
Sedimentology of Some South Carolina Tidal-Creek Point Bars, and a Comparison With Their Fluvial Counterparts
The geometry of intertidal point bars in South Carolina’s predominantly mesotidal (tidal range 1.4 m to 2.6 m) marsh creek system is determined by tidal current ebb-dominance and meander geometry, primarily radius of curvature. The distribution of sedimentary facies is controlled by lateral segregation of flood and ebb currents set up by bar geometry and is further modified by tidal current time-velocity asymmetry. Ebb current dominance causes all point bars to be skewed toward, to trail away from the ebb side of a meander bend. This ebb dominance is purely a function of the entire inlet-marsh system and is present even in systems where freshwater influx is absent.
On tight meanders (r/w <2.5) point bars are relatively small with steep flanks and are fully welded to the bank. Chutes are absent. Semidiurnal current reversals generate no significant differences between flood and ebb current patterns, with the exception of turbulence associated with occasional ebb-oriented transverse bars. Bar surfaces are predominantly rippled fine sand with minor amounts of mud, and are bioturbated on their crests.
Intermediate meanders (2.3 < r/w <3) produce complex multilobed bars, with flood-dominant chutes and interlobe ramps. The chute and inner bar may be exposed to no ebb discharge and are composed of fine-grained sediments. The interlobe ramp displays a wide variety of structures and textures, ranging from flood-oriented sand waves (clean fine sand) to bidirectionally oriented ripples (muddy sand), depending on the bar orientation with respect to adjacent meanders. The outer bar comprises structures and textures similar to those on gentle meander point bars.
Gentle meanders (r/w >3) produce long narrow bars with relatively deep, flood-dominant chutes. Chute bars occur infrequently, but where present are ebb-oriented. Bar extremities are sand wave and megaripple fields. Their ebb- and flood-orientations (on the floodward and seaward ends, respectively) are preserved throughout the tidal cycle by shielding of the higher mid-bar area, where smaller bedforms and flat beds predominate. Mud occurs as drapes in larger bedform troughs and as thin beds on mid-bar crests. Chutes are silty muds, usually devoid of bedforms.
Point bars on very gentle meanders (r/w >>3) are fully welded to the bank, and resemble the “skew shoals” of fluvial systems. There is little or no current segregation on this type of bar. Gentle swales behind bar crests are filled with bioturbated muddy sand that is gradational into the adjacent marsh surface.
The fundamental contrast between fluvial and tidal channels is the difference in the temporal relationship between water surface elevation and the concommitant flow velocity. In fluvial systems highest velocities accompany the highest river stages. In most tidal channels, however, highest velocities occur at an intermediate stage. This contrast results in vastly different modes of floodplain development. Because tidal channels migrate more slowly than river channels, and because in tidal systems floods are far more frequent, a relatively larger proportion of tidal creek sediments occurs as overbank deposits.
Tidal creek and fluvial floodplain lithosomes are distinguishable not only on this basis of the relative importance of channel versus overbank deposits, but on the bases of fauna) diversity, bioturbate structures, orientations of cross-stratification, and characteristics of vertical point bar sequences that reflect tidal current segregation.
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