ripples during rapid sediment accumulation. Bedding planes dip onshore 

 from wave-generated currents, while bedding associated with unidirec- 

 tional currents dips either onshore or offshore (Clifton 1976). 



Ripple symmetry. Inner shelf ripples can be symmetrical or asymmet- 

 rical. Symmetrical ripples have similar side slopes and are usually pro- 

 duced by waves and associated bidirectional currents of near similar 

 magnitudes (Reineck and Singh 1986). 



Asymmetrical ripples, or ripples with different side slopes, are formed 

 by bidirectional currents of different magnitudes (Reineck and Singh 

 1986). These bidirectional currents can be formed by both wave and tidal- 

 generated currents. Asymmetrical wave ripples occur especially in the 

 surf zone and shallow water under long period low waves, as the oscilla- 

 tory flow of water particles tends not to occur in a closed orbit. Net trans- 

 port of sediment occurs in the direction of wave propagation. Therefore, 

 there is significant unidirectional sediment movement associated with 

 asymmetrical wave ripples. Although both asymmetrical wave ripples and 

 current ripples have unequal side slopes, asymmetrical ripples bifurcate 

 while current ripples do not. Since the formation of bed forms on the 

 inner shelf environment is dominated by wave activity, the following dis- 

 cussion concerns wave ripples (ripples formed by wave-generated cur- 

 rents, also known as oscillation ripples) rather than current ripples 

 (ripples formed by tidal-generated currents). 



Sediment movement in symmetrical wave ripples is a function of wave 

 orbitals at the water surface, which flatten towards the bottom eventually 

 having only horizontal, and not vertical, movement. These ripples are es- 

 sentially straight-crested, have pointed crests, rounded troughs and fre- 

 quently show bifurcation. The occasional rounding of crests is a result of 

 the reworking of ripples as the current field changes characteristics. The 

 internal structure of wave symmetrical ripples is characterized by chev- 

 rons indicating two directions of transport (chevron bedding slopes away 

 from the crest and toward the trough of a ripple at equal angles). A more 

 detailed discussion of internal structure characteristics of wave-ripple bed- 

 ding can be found in Boersma (1970) and Reineck and Singh (1986). 



Clifton (1976), building on the work of Inman (1957) and Dingier 

 (1974), stated that the prediction of symmetrical ripple size, which is gra- 

 dational, is based on grain size, orbital velocity, and wave period. Three 

 types of symmetrical ripples include (Figure 14): 



a. Orbital ripples, which form under short-period waves and have ratios 

 between orbital diameter/grain diameter (d^/D) which are less than 

 2,000 (where ripple wavelength is dependent upon the length of 

 orbital diameter of the oscillatory current and is independent of 

 grain size). 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



51 



