60 



Araott and Southard (1990) state that meter-scale, isotropic hummocky 

 cross-stratification is likely formed by large three-dimensional symmetri- 

 cal wave ripples produced by purely oscillatory flows and very strongly 

 oscillatory-dominant combined flows of storm waves. They documented 

 that the sedimentary response of the irmer shelf from pure oscillatory flow 

 at low speeds was small symmetrical vortex ripples. At higher current ve- 

 locities large, three-dimensional, round-crested bed forms with heights to 

 20 cm and spacings of decimeters to meters resulted. 



Hummocky cross-stratification varies with distance from shore and 

 water depth (Amott and Southard 1990). As energy decreases in an off- 

 shore direction, hummocky cross-stratification laminae tend to be less 

 deeply incised and dip at a lower angle. At nearshore locations, there is a 

 greater presence of wave ripples, and beds are lenticular (resulting from 

 high energy) and tend to erode at the top. At offshore locations where the 

 energy is less, the beds become tabular. In addition, wavelength and 

 height of hummocks are likely to decrease in an offshore direction. 



Amott and Southard (1990) found that superimposition of a steady cur- 

 rent with oscillatory motion produced significant changes in bed state. 

 Even a weak current caused bed forms to become asymmetric and mi- 

 grate; most of the combined-flow bed forms contained downstream- 

 dipping cross-stratification. Changes in the morphology of the ripples 

 were profound as currents increased. Currents of only 1-5 cm/sec, super- 

 imposed on oscillatory flows of 40-60 cm/sec, produced downstream- 

 dipping low-angle hummocky cross-stratification. For currents exceeding 

 13 cm/sec, hummocky cross-stratification occurred and dip angles were 

 formed near the angle of response (similar in morphology to high-angle 

 hummocky cross-stratification as described by Nottvedt and Kreisa 

 (1987). At higher oscillatory speeds (60-80 cm/sec), any non-negligible 

 current washed the ripples away, replacing them with a flat bed. How- 

 ever, Arnott and Southard (1990) state that a core current exceeding 

 95-110 cm/sec is needed to form large ripples exhibiting moderately steep 

 internal laminae in very fine sand. 



Examples. Greenwood and Hale (1980), in a study at New Brunswick, 

 Canada, using depth of disturbance rods, found that the depth of activity 

 at a bar is proportional to storm intensity. The seaward side of the bar 

 crest, which had maximum values of bed-level change due to large wave 

 heights, asymmetric oscillatory motion, and rip currents, eroded up to 

 35 cm. Meanwhile, the trough at the foot of the landward slope eroded up 

 to 37 cm due to scour by longshore currents. Accretion of up to 12 cm oc- 

 curred on the upper part of the landward slope in response to a decrease in 

 wave height due to breaking waves and increased water depth. In addi- 

 tion, accretion of up to 21 cni occurred on the upper seaward slope of the 

 bar, thus steepening both slopes and producing a seaward displacement of 

 the bar crest. Overall, the bar eroded during the storm, and sediment was 

 transported in multiple directions through megaripple migration. How- 

 ever, net transport of sediment was in an offshore direction. 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



