Storm-influenced bedding 



Types of storm -influenced bedding include the following: 



a. Hummocky cross-stratification - defined as laminae which are both 

 concave up (swales) and convex up (hummocks), possessing many 

 undulating erosion surfaces, and dip into the swales at angles of 

 approximately 15 deg (Brenchley 1985, 1989). The laminae are 

 oriented 360 deg, indicating that current orientation fluctuates over 

 an entire 360-deg circle. The beds, which thin over hummocks and 

 thicken over swales, appear similar when viewed from two faces 

 perpendicular to one another. Therefore, three-dimensional views 

 are required to correctly identify hummocky cross-stratification 

 (Brenchley 1985, 1989). 



b. Beds of laminated silt, usually only a few centimeters thick at most, 

 which fine upwards. 



c. Beds similar in nature to turbidites (where turbidites are defined as a 

 bedding sequence formed by a turbidity current or a bottom-flowing 

 current laden with suspended sediment and possessing a density 

 greater than that of the water which moves slowly down a 

 subaqueous slope (Bates and Jackson 1984)). These beds show 

 graded, parallel laminae or ripple drift lamination, commonly 

 formed below the wave base. 



Hummocky cross-stratification, also known as truncated wave ripple lami- 

 nae (Campbell 1966, 1971), is of utmost importance in the study of storm 

 deposits on inner shelf sedimentation/stratigraphy patterns. Studies con- 

 cerned with this subject include Campbell (1966, 1971), Harms (1975), 

 Hamblin and Walker (1979), Bourgeois (1980), Allen (1982), Dott and 

 Bourgeois (1982), Swift et al. (1983), Walker, Duke, and Leckie (1983), 

 Brenchley (1985, 1989), Duke (1985, 1987, 1990), Greenwood and Sher- 

 man (1984), Klein and Marsaglia (1987), Nottvedt and Kreisa (1987), 

 Swift and Nummedal (1987), Amott and Southard (1990), Higgs (1990), 

 Southard and Boguchwal (1990), and Duke, Amott, and Cheel (1991). 



Hummocky cross-stratification requires an increase in seaward sedi- 

 ment transport, and entrainment and deposition of sand on the continental 

 shelf above the wave base by storm-generated currents and waves 

 (Brenchley 1985). This bedding is usually formed by accretion as laminae 

 thicken over crests. However, some hummocky cross-stratification bed- 

 ding is produced by erosion when sediment is eroded from the hummocks 

 and is deposited and thickens in the swales. Brenchley (1985) questions 

 whether wave oscillatory currents or a combination of wave oscillatory 

 and unidirectional currents are needed to produce hummocky 

 cross-stratification. 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



59 



