Swell-dominated conditions created the greatest hydraulic roughness of 

 all four cases. This was due to the existence of a thick wave boundary 

 layer with subtle ripples on a partially armored bed. 



In studies of the ancient geologic rock record, Brenchley (1989) and 

 Duke, Arnott, and Cheel (1991) state that hummocky cross-stratification is 

 part of a storm bed sequence characterized by an eroded base with a grada- 

 tional top, which includes the following activities (from bottom to top of 

 the sequence) (Figure 17): 



a. Waves interact with a relatively weak coast-oblique bottom current to 

 erode the muddy substrate. Simultaneously, shells and shell hash 

 carve tool marks in the mud and are deposited in swales). 



b. Coastal sand, moving as bed and suspended load under combined 

 wave and current bottom flow, is eventually transported offshore 

 resulting in the formation of horizontal lamination to low-angle 

 dipping sand (this also results in basal erosion). 



c. Formation of hummocky cross-stratification due to reworking of the 

 bed by storm processes. 



SUBSTRATE 



SHORE-NORMAL 

 FLOW SPEED 

 NEAR THE BED 



UJ 



a: 

 o 



X. 

 C/3 



Z 



CURRENT ° 



BED RESPONSE 

 BED STATE 



fNSTANTANEGUS 

 TMZ - AVERAGEO-" 



TIME 

 WANING OSCILLATORYI 



Firiw 



Figure 17. Probable sequence of events producing hummocky cross-stratification on ttie 

 inner shelf (after Duke, Arnott, and Cheel (1991)) 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



63 



