G433 Review of sedimentary structures September 1 and 8, 2010 Fluid Parameters The three main parameters that determine the stable bedform in unidirectional flow conditions are: grain size flow velocity flow depth Several other parameters are equally important, though for most pure fluid flows on Earth, these parameters can be assumed to be constant. They include: m = fluid viscosity rf = fluid density rs = grain density g = gravitational constant Cohesive vs. non-cohesive sediments Hjulstrom Diagram Bedform phase diagram and hysteresis 2-D vs. 3-D structures Secondary flow created by bed roughness Aggradation vs. migration of bedforms Examples of climbing bedforms (unidirectional ripples) Dunes are similar to ripples, but dynamically distinct. Dunes Dune wavelengths commonly range from 0.6 m to hundreds of meters; heights range from 0.05 -10.0 m. Upper plane bed flow: intensive sediment transport over a flat bed Parting Lineation Antidunes occur in flows with sufficiently high Froude numbers. Antidunes Typically migrates upstream and shows little asymmetry. The water surface is strongly in phase with the bed. Commonly seen as train of symmetrical surface waves. flow migration Shoot and pool structures: Trains of cyclic steps occur in very steep flows with supercritical Froude numbers. The steps are delineated by hydraulic jumps (immediately downstream of which the flow is subcritical). hydraulic jump flow Bedforms in cohesive sediments Subaqueous bedforms in cohesive sediments: flutes and tool marks, including bounce, skip, groove, and chevron marks Gutter casts subaqueous, usually associated with storms Shrinkage cracks Incomplete, non-orthogonal, Ordovician Eureka Quartzite, W. Utah Shrinkage cracks subaerial desiccation Bedforms generated by surface waves • Surface waves transfer little mass but considerable energy • Surface waves define orbitals in fluid that have decreasing diameter with depth • Depth below which orbital diameters = 0 is termed wavebase • Deep water waves do not reach bottom • Shallow water waves do reach bottom; orbitals reaching bottom create a shear stress that oscillates back and forth as waves pass overhead • With sufficient shear stress, sediment grains will move, creating bedforms Wave orbitals deep water waves shallow water waves Movement of sediment by wave orbitals Unidirectional, combined flow, and oscillatory bedforms Wave ripples Hummocky cross-stratification (HCS) • Occurs in fine- to medium-grained sand • Produced by combined flow • Typically occurs below fair weather wavebase by larger waves produced during storms Physical features characteristic of HCS – hummocks (concave up features) and swales (concave down features) – psuedo-parallel laminations within hummocks and swales (although laminae may thicken into swales and thin over hummocks) – low angle (<15°) truncation surfaces HCS Eolian Dunes Sediment dynamics on dunes Grain flow deposits Grain fall deposits Wind ripple deposits Sediment gravity flows Turbidity currents: • particles are kept aloft in the body of the flow by turbulent suspension • density of flow greater than that of ambient fluid • both high density and low density turbidity currents exist Turbidite in flume Flume turbidite 2 Turbidites Liquified flows: • very concentrated dispersions of grains in fluid • usually result from shock of granular sediment (e.g. seismic shock) • grains kept in suspension by fluid pore pressure and from upward movement of expelled fluid Grain flows: •characterized by grain-grain collisions. •Little reduction of friction occurs in such flows, so they can only occur on steep slopes where the angle of initial yield has been exceeded. Debris flows: • slurry like flows in which large particles (up to boulders) are set in a fine- grained matrix • matrix has ‘yield strength’ which helps support grains during flow • matrix serves to lubricate grain irregularities so debris flows may occur on very gentle slopes Debris flow deposits.