Chapter: SEDIMENTARY STRUCTURES

Chapter: SEDIMENTARY STRUCTURES Gautam Kumar Das Synonyms Bedforms Bedding Definition Sedimentary structures. Primary or mechanical structures formed by physical processes in the sedimentary environment. Bedforms. Primary sedimentary structures formed by the interactions between turbulence of flow and sediment grains. Antidunes. Large-scale structures looking like plane beds formed after destruction of bedforms in the upper part of the higher flow regime. Megaripples. Large-scale sedimentary structures formed in the upper part of the lower flow regime. Ripples. Small-scale sedimentary structures formed in the lower part of the lower flow regime. 1

Bedding planes. Surface sedimentary structures. Bedding. Layering characteristics of the sequence. Sedimentary structures are surficial or internal, megascopic, three-dimensional features of sediments or sedimentary rocks (Pettijohn and Potter, 1964). These structures have been called mechanical or primary structures (Potter and Pettijohn, 1977) due to their formation by physical processes. In the modern environment, flow regimes at varying speeds and velocities produce different sedimentary structures, are called bedforms. Sands are deposited via a diverse suite of ripples, megaripples, sand waves, rill marks, rhomboid marks, backwash ripples, swash marks and current crescents in the central and marginal areas of estuary (Elliott, 1983). Ripple bedforms have a tendency to be occurred in the intertidal areas of mid channel bars or point bars experienced with the minimum tidal current velocities whereas, megaripples, sand waves are confined to the depressed zones of the tidal sandy bars. Small ripples are formed with the increase of flow velocity and these migrate in the direction of flow. With the continuous increase in flow velocity the small ripples may enlarge and change slope that give rise to the large scale bedforms megaripples. Gradually plane beds and eventually antidunes are formed as a result of destruction of megaripples at higher flow rates. Ripples and megaripples are the most commonly observed bedforms in the estuarine environments (Davis, 1983). The sand dominated middle to lower stretch of the estuary is significantly important where intertidal sand bodies with numerous bedforms of different scales get exposed after each tide. The exposed portion of the estuarine bed and the intertidal mid channel bars reveals various bedforms of tidal origin. Inherent unsteadiness in flow conditions, reversals of tidal currents and bedform-flow interactions act as the causal factors for the frequent changes in bedform architecture (Middleton, 1965; Conybeare and Crook, 1968). Thus bedforms, though not static, are permanent features which often exhibit a quasi-equilibrium form under effects of unsteadiness in a tidal situation of estuarine environment. Smaller bedforms quickly change their orientations, but large sandwaves do not, in response to flood-ebb change over (cf. Dalrymple et al., 1978; Collinson and Thomson, 1982). 2