Sedimentary Structures Flashcards
(27 cards)
What are bedforms dependent on?
Velocity, depth, grain size
Generally, will you have mud with a high velocity environment?
No
What did Johannes Walther contribute to sedimentary structures? What is Walther’s law?
He discussed different facies. Walther’s Law states that a vertical sequence of sedimentary facies (groups of rocks with specific characteristics) reflects the lateral succession of depositional environments.
What is transgressive movement? Regressive?
Transgressive is when the shoreline moves land-ward. Regressive is when it moves towards the sea.
Generally in terms of grain size of a strat column, what do you associate with regressive and transgressive sea level changes?
For regressive, you have a coarsening up. For transgressive, it’s a fining up.
What kind of flow would allow a reverse graded bed (coarse on top)?
Debris flow.
What is planar bedding?
Flay, parallel to the surface. Sizes can be classified from very thickly bedded (1m) to thinly laminated (3mm).
A planar bed thinner than 1cm is ____________.
Lamination
What are the 6 types of ripples?
straight crested, sinuous, catenary, lunate, linguoid, wave-formed ripples (bifurcated)
How do you measure ripple index?
Take the length of the ripple (crest to crest) and then divide by the height.
What causes climbing ripples? What might cause it?
The sediment is moving so fast, with high sedimentation rate and flow rate, it causes the ripples to stack. Think storm events and floods.
What type of environment causes a hummocky cross stratification?
Storms deposit in shallow marine settings between normal wave base and storm wave base. 15 degree or lower angles
What are primary sedimentary structures?
Structures that occur in clastic sedimentary rocks and are produced by the same currents that deposit the sediment in which they are present. Their presence, scale and orientation reflect the conditions of transport and deposition.
What are secondary sedimentary structures?
They form after sediment is deposited. Produced by a wide variety of organic, physical, chemical processes.
What are bedforms?
Primary sedimentary structures like ripple marks, dunes, and plane beds that appear as 3-D features on the top surface of bedding planes.
What do flume studies show?
There is a predictable sequence of bedforms that depend on velocity, grain size, and depth of flow.
Where is the point of flow separation? Flow reattachment? Zone of reverse circulation?
Sep: the crest of a ripple or dune
Reatt: bottom
Rev: area of turbulance and backflow on lee side of ripple, below laminar flow of reattachment
study this flow strength diagram
Why are antidunes weird?
They are low undulating forms that reach 5m in spacing. Their crests are in phase with the surface waves so they migrate by accretion on the upstream side. Low dip angles, high flow velocity, they leave evidence of a flow direction contrary to flow direction of other indicators.
Tidal ripples are generally unidirectional, with weak backflow structures. Cross-beds are oriented in two directions, often with deactivation surfaces caused by the reversal of current direction during a tidal cycle. This is known as _______________.
Herringbone cross-bedding
What is a reactivation surface?
Often the migration of a ripple is interrupted, the ripple is eroded back and then buried by a new advancing bedform. Such interruption produces a tiny erosional surface between cross-strata which is a RS
The most distinctive features of tidal regions are caused by the mixing of sand and mud sized fractions from teh asymmetrical currents. Small lenses of sand in muddy beds, called ________ bedding occur when sand is trapped in troughs in the mud as sand waves migrate across a muddy substrate. If mixing produces minor mud layers in a sandy substrate, the pattern is called ______ bedding. An equal mixture of sand and mud characterizes ____ bedding.
lenticular, flaser, wavy
For wind transported sediment, deposition occurs on the ____ side, it does not scour because there is no zone of backflow. Water ripples erode on this side.
lee
____ marks, found on the bottom surfaces of beds, are usually casts or molds of depressions that were formed in the underlying beds by currents. The filling or ____ mark tends to have a high preservation potential
sole
