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Structural Geology -Introduction Current ripple marks:(asymmetric) Current induced; show unimodal current direction. Usually have smooth crests and troughs Current ripple marks cannot be used as reliable younging indicator unless o heavy minerals preserved in troughs, or coarse particles preserved in troughs. Wind ripple posits in eolian systems can preserve coarse particles in the crest of the ripple Crossbedding: These features show internal layering inclined to the accumulation surface. There are several types of crossbeds in overall geometry. Symmetry of the crossbeds is the significant factor in the determination of younging direction o Symmetric ◇ Asymmetric graded bedding: A systematic variation in grain size in a layer from the bottom to the top of the layer. graded beds are generally formed when a swiftly fowing current carrying a range of particle sizes enters a calm depositional site. The largest particles settle faster(why?), forming a fining upward sequence Turbidite(Bouma sequence: Turbidity currents are density driven currents Most turbidity currents represent flows of sediment from the upper potion of a depositional basin into the lower parts of the basin. The currents flow as suspensions of particles more dense than the surrounding water. Turbidity currents have several distinct layers that demonstrate different flow rates(flow regimes) of the current. The most pronounced feature of a turbidity current is graded bedding in each layer as well as within the turbidite as a whole. The diagram below shows a classical Bouma sequence, in field settings the complete sequence is typically not Bed bottom markers Form on the top of beds. Molds mark the top of a layer, casts mark the bottom of a layer(sometimes called sole marks) Flute casts: Scour and fill features formed by flow over weak materials. Typically found in but not limited to, turbidites Groove casts: Trace of debris drug along bottom by current Last revised August 27, 1999 Fall 1999 4 ' " # 4 % 5 4 ' ! " # " 4 ' ( ' 6 ' ( ! 6 7 ! " 8# $ "9 $ #' . 7 7 : 7 "7 # 9 $ $ +# # ) ; . " # ; ' 7 6 '