Sedimentary structures Flashcards
Unidirectional flow causes what kind of bedforms?
Asymmetric ripples, dunes, antidunes
Bidirectional flow causes what kind of bedforms?
Straight crested symmetric ripples
Combined directional flow causes what kind of bedforms?
Hummocks and swales
What is the difference between a bedform and a sedimentary structure?
A bedform is the 3D thing (eg ripple) on the surface that moved, sedimentary structures is the internal thing (eg cross bedding) left behind.
How does cross bedding occur?
Migration of ripples and dune bedforms (often occur in sets)
What is tabular cross bedding?
Has a planar bounding surface, formed by migration of straight crested ripples and dunes.
What is trough cross bedding?
Has a curved bounding surface, formed by migration of curved ripples and dunes.
What is Flaser bedding and what does it indicate?
Thin mud streaks between sets. It indicates fluctuating floe - often tidal
What is convolute lamination/bedding? (deformation structure)
It’s complex folding of beds and laminations by plastic deformation of partially liquefied sediment. Common in find sand/silt.
What are flame structures? (deformation structure)
Tongues of mud projecting up into overlying layer. Caused by loading of water saturated mud so they get squeezed upwards into overlying (sand) layer.
What are groove casts?
Elongate ridges from infilling of erosional relief produced by things dragging/rolling along bed.
What are bounce marks?
Small indentations on the bed from intermittent contact (eg bouncing, skipping)
What are flute casts?
Elongate ridges with bulbous noses formed by infilling of current scours. Big nose points upstream. Often formed by turbitidy currents.
What are load casts?
Form by deformation of mud due to uneven loading.
What are stromatolites?
Organically formed laminated structures, have clay and silt particles trapped by cyanobacteria.
