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Recognizing Sedimentary Structures Flaser bedding Plane beds • • • • • • • • • Bi-directional pattern Intermittent flows cause alternating sand and mud layers Tidal environments, high energy Individual ripples and mud drapes Lenticular bedding • • • • Alternating sand and mud Tidal stands before flow reverses More mud than flaser beds Intertidal (foreshore) Laminar flows; parallel particle movement ‘Lamination’ in rocks Coarser grains, low velocities Finer grains, high velocities Current lineations Massive beds • No stuctures • Bioturbated zones Graded bedding • Normal; fining up • Settling out or decrease flow • Reverse; coarsening up Cross Stratification • • • • • Inclined layering that dips toward flow direction Cross laminations from current ripples Straight/sinuous or lingoid (unconnected) Planar-same direction; trough- sinuous to linguoid Dunes creates larger scale cross-beds Flow Regime Diagram • Can be used to estimate velocity, change in velocity of depositional flow. • A bedform stability diagram shows how the type of bedform that is stable varies with both grain size and flow velocity. • Boundaries change with water density and depth. Lower flow regime • Subcritical flow and change lies within antidune field. • Ripples dunes and lower plane beds. Upper flow regime • Plane beds and antidunes form. • Supercritical Flow • Bedforms in-phase with wave propagation. Vertical Sequences of Deltas Delta Plain Progradation • • • • • Sediment building out; progressively shallower • Shallowing up sequence; Coarsening up. • Sea level fall, change in channel path Avulsion. Transition between fluvial/alluvial environment. Branching and meandering of river as distributary channels. Channel sands and plain muds Brackish water Delta Front • Immediately forward of the channel mouth. • Coarsest drops first into progressively finer • Delta slope: 1-2 degrees (fine) Prodelta 30 degrees (coarse) • Suspended plume in front of the delta. • Lower density river waters • Fine grained sediment settles furthest from delta front • Gravity currents bring coarse down Retrogradation • Sediment erosion exceeds accumulation • Deepening up sequence; fining up • Sea level rise, constricted channel system River dominated River dominated Tide dominated Wave dominated Anthropogenic Factors Humans have been known to constrict the outflow of water and sediment. Preventing accumulation and channel migration that keeps the delta in equilibrium. As subsidence continues, and the channel is unable to avulse, the delta plain goes into retrogradation. Ice transport/deposition • • • • • High viscosity allows larger particles Produces laminar flow. Long period transport Significant around polar caps Greatest at times of extended glaciation. Air transport/deposition • • • • Low density limits load capacity. High viscosity limits grain size in load. Up to fine grained sand. Turbulent flow. Water transport/deposition • • • • The most significant medium. Viscosity allows up to cobble and boulder size Density allows for greater capacity of load. Turbulent flow to laminar flow Wave-Dominated Coastal Environment Continental shelf: • Offshore Zone: Below storm-wave base • Silt and mud, Extensive Bioturbation • Storm-silt layers; Tempesites • Weakly graded; suspension settling • Offshore Transition: • Sand, fine-grained, some silt, bioturbation, • sedimentary structures include hummocky cross stratificaton. • Transported shells; bivalves • Wave and current • Nearshore (shoreface): region between low-tide and fair weather base. • Mainly fine sand to medium • Fine laminations, crossbeds • Longshore bar/ridges oblique to coastline • Oscillatory wave • Foreshore: Surfzone, swash, between mean high and mean low • Sand, medium-fine grained • Land and seaward dipping laminae • Shoaling wave • Herringbone cross • Backshore: from berm to dunes • Fine grained sand with heavy minerals • Wind ripples up to dunes