Sedimentary Structures

Chapter 7 Sedimentary Structures Abstract The exposed portions of the Thakuran River bed and the intertidal mid-channel bars reveal various bedforms of tidal origin. Sand flats with small and large-scale bedforms dominate throughout the downstream portion of the tidal shoals, whereas mudflats possess fewer physical structures. The presence of megaripple marks and ripple marks as bedforms indicates that the river extends to the upper part of the lower flow regime. Both the surface and internal structures are indicative of tidal influence. Surface structures like small ripple marks, linguoid ripples, megaripples, sandwaves, rill marks, rhomboid marks etc. are recognised. Internal structures like flaser and lenticular type are highly indicative of flood and ebb cycles. Zones with distinctive bedform characteristics can be delineated based on their morphological variations and

orientations. Moreover, 5–15 cm-wide intertidal belts are populated by hybrid bedforms. The structures indicating modification of bedforms due to the fluctuating velocity and depth during ebb and flood periods include: i) skewed spurs, ii) ripplefans, iii) flat-crested megaripples and sandwaves, iv) dissected channels and v) microdeltas. Key Words: Bedforms, Ripples, Megaripples, Sand waves, Ripplefans, Rill marks, Rhomboid marks, Skewed spurs, Flaser bedding, Swash marks, Backwash marks, Thakuran River, Sunderbans The Thakuran River channel in the Hugli-Matla estuarine complex of eastern India experiences a semi-diurnal, spring tidal range above 5 m near the mouth (Das 2015). The tidal prism covers the entire 80 km length of the river. Most of the sediments of the river are recycled by tidal currents due to the extreme paucity of freshwater discharge from the headland, except during the monsoonal months. The intertidal sand flats are ornamented by bedforms of various kinds and scales. Several scales of unsteadiness are marked in the tidal process. Among many such processes, the following are the most important: i) fluctuations in water depth and velocity associated with ebb and flood periods, ii) flow reversals associated with ebb and flood, iii) neapand spring-tidal flow variations, and iv) seasonal tidal variations with equinoctial cycles. A single bedform type and the assemblage of bedform populations display features due to one or more of these scales of unsteadiness at a particular point of time. Thus bedforms in a tidal situation often exhibit a quasi-equilibrium form under the effects of unsteadiness. With a purpose to examine the internal manifestations of bedforms generated of the mid-channel bars, several L-shaped trenches were dug during the time of emergence. Megaripples bedding, parallel stratification, horizontal stratification, reactivation surfaces in cross-bedded units and penecontemporaneous deformation structures have been encountered as internal structures.