Lecture 4 - Sedimentary Structures Flashcards
Define bedding:
Sedimentary layer thicker than 1cm (can be done in hours or 100s of years)
Define lamination:
Fine scale bedding, less that 1 cm thick
What effects the type of bedforms made?
Verlocity of the flow, grain size, depth of flow.
What can happen to these bedrocks to help us learn about the past
these structures can become fossilised
What does the absense of lamination show?
Bioturbidation
Can you draw a diagram about the formation of ripples?
Yes
What are the different bedform types?
Ripples
Sandwaves
Dunes
What gorms waves as aposses to ripples
Waves are formed by two currents meaning deposition and eroisin is equal on either side
Ripples. Slowest to fastest
straight crested
sinuous
catenary
Dunes slowest to fastst
lunate
lingouid
what are the differencens between two dimesnial and three dimesnal crests
straight crested ripples are the only 2D ones as they are just parrel lines
What is the Herring bone cross bedding associated with?
Tidal regieme, two undirectional currents at 180 degrees to each other. although it can be difficult to distingues between these waves and ripples.
In general what sets waves apart
Sharper crests, more symetrical cross section.
The above are straight unidrection currents what are the others more complex currents known as
Turbidity currents
What are turbidity currents
Sediment laden (turbid), fast flowing, common on seafloor. sediment often disturbed by storms or earth quakes.
Can you draw a flute mark? also explain a flute mark
Sand laded current erodes and marks cohesivce mud prior to deposition creating a half conical shape in the mud. These are then in filled with depoistion creating smooth conical flute marks.
What are groove marks?
Objects carried in flow groove into the mud, the space is filled in by the turbidite sediment
Wjat is graded bedding?
Graded bedding is where the larger rocks settle out first and then as it goes up the smaller sediments settle.
What is the sequence of these graded beds called
Bouma Sequence
What do mudcracks show?
Subarial exposure, in a dry climate causing the rocks to dry out.
What are trace fossils?
These are “traces” or marks in the sediment that are made by an activity, they are not classfied by organisms
What are the two mains groups of trace fossils?
1) formed on sediment surface such as tracks and trails EPIBENTHIC
2) formed within a sediment such as burrows ENDOBENTHIC
Sedimentation rates, what do borings or burrows show
There is zero sedimenation
Intenstly bioturbated sediment indicates
slow sedimentation
U shaped burrows inidcate…
rapid sedimenation
What can these be used to do (with regards to superposition)
They can be used to work out if there bed has been turned upside down (anticline)
