50 



c. Small-scale ripple-foreset bedding. 



These three classes of bed forms can form from either wave- or tidal- 

 generated currents depending on the flow characteristics. 



Planar parallel laminae develop in shallow marine sands by: 



a. Sheet flow caused by the consistent flow of sand over a flat bed 

 during high-energy conditions (Davis 1983). 



b. Migration of long-crested ripple forms accompanied by a slow rate of 

 sediment accumulation. 



Deepwater sheet flow results from the high energy oscillatory flow of 

 large long-period waves (Clifton 1976), and the currents usually associ- 

 ated with geostrophic or downwelling currents. Shallow-water sheet flow 

 results from intense wave activity close to the shoreline and may show evi- 

 dence of shear sorting of particles of different size, density, or shape (less 

 velocity is needed to form sheet flow in fine sand than in coarse sand). 

 Other sedimentary structures associated with sheet flow include mica lami- 

 nae, convex-up shells, and little to no bioturbation due to wave reworking. 



The second cause of planar parallel laminae is the migration of ripples 

 accompanied with a slow rate of sediment accumulation known as slowly 

 climbing ripple stratification (or the internal structure formed in noncohe- 

 sive material from migration and simultaneous upward growth of long- 

 crested ripples). Climbing ripple stratification can be produced by either 

 currents or waves (Reineck and Singh 1986) of all periods, but only by 

 medium- to long- period waves (8 to 12 sec) in deeper water. The sedi- 

 mentary signature of the migration of ripple forms accompanied with a 

 slow rate of sediment accumulation includes poorly defined climbing rip- 

 ple foresets, shell lag deposits, concave up shells due to their tumbling 

 over ripple crests, and bioturbation. 



Medium-scale ripple foreset bedding is characterized by 6-cm-thick 

 foreset units in medium to coarse sand, which form due to the migration 

 of cuspate (three-dimensional) megaripples or the migration of long- 

 crested ripples if a rapid sedimentation rate is present. Lunate megaripple 

 migration produces cross-bedding, while long-crested ripple migration pro- 

 duces more tabular units (said of the shape of a sedimentary body whose 

 width/thickness ratio is greater than 50 to 1, but less than 1,000 to 1). The 

 foresets of medium-scale foreset bedding are oriented onshore in the direc- 

 tion of wave propagation suggesting the landward transport of sediment 

 associated with orbital asymmetry. 



Small-scale ripple foreset bedding is the most common structure near 

 the sediment water interface, but has a low preservation potential. This 

 type of bedding is characterized by foreset units less than 6 cm thick and 

 is produced by the migration of irregular asymmetrical wave ripples (to be 

 described in the following section) or by the migration of small-scale 



Chapter 4 Sedimentary Features/Stratigraphy ot the Inner Shelf 



