W8 L2 Sedimentary Structures

Question 1

Sedimentary structures useful as

Answer 1

indicators for eenvironment (ex. ripplee structures)

Question 2

Sedimentary Units: bed

Answer 2

commonly, but not always, horizontal
bounded by bedding planes
basic unit of sedimentary rocks
sediment source, climate and transport control type and amount of sediment deposited
changes in any of these causes a change in deposition
thickness: >mm to many metres (10-20 cm)

Question 3

strata

Answer 3

series of beds (bit less common term)

Question 4

a formation

Answer 4

sequence of consecutive beds sharing unique characteristics
minimum 20 m thick
the formation is the fundamental geological mapping unit

Question 5

Vertical Bedding

Answer 5

then tell younging direction

Question 6

Sediment transport, erosiomn, transportation, deposition GRAPH

Answer 6

plots grain diameter agains velocity

large part of diagramw here nothing happens → energy of current not high enough to move particles

Question 7

GRAPH range where transport happens

Answer 7

ange where transport happens
smaller particles the more range of transport
large particles only narrow range where they can be transported
can be adjusted through gradient change
this is where most sedimentary structures form
→ Sedimentary Structures are linked to their environment

Question 8

Erosion impact on geological record

Answer 8

no record, lose “time” in geological record because velocity leads to erosion

Question 9

Graph depends on/ tells us

Answer 9

sedimentary structures result from particular types of transport and flow regime
current velocity
grain size
minor velocity variations (in the centre of the diagram) create bedforms and structures
sedimentary structures strongly linked to their environment

Question 10

Flow Regime distinguish

Answer 10

distinguish between upper flow regime and lower flow regime
upper flow in contact to atmosphere
lower flow in contact with bedrock

Question 11

Ripples feature

Answer 11

Ripples as most prominent feature in current induced sedimentary structures

Question 12

Ripples are

Answer 12

Ripples are sediment modified by flowing water (or wind)

Question 13

Ripples indicate

Answer 13

Ripples indicate current direction, speed and depth

Question 14

Asymmetric ripples

Answer 14

asymmetric ripples – uni-directional flow (eg rivers, estuaries, sand dunes)

Question 15

Symmetrical Ripples

Answer 15

ymmetric ripples – bi-directional flow (eg shallow beaches)

1. unequal length stoss and lee sides; 2. equal length stoss and lee sides
2. smooth crests; 2. sharp crests
3. deep troughs; 2. shallow troughs

Question 16

straight crested ripples

Answer 16

straight crested ripples= uncomplicated flow, “perfect” ripples

Question 17

ladder ripples

Answer 17

adder ripples= last flow as water shallowed was perpendicular to earlier flow

Question 18

Cross Bedding definition

Answer 18

layering within a stratum and at an angle to the main bedding plane.

Question 19

foresets

Answer 19

foresets - sediment avalanches down the lee or slip face

Question 20

foreset boundaries

Answer 20

small-scale grain size or density differences = foreset boundaries

Question 21

main or master bed

Answer 21

main or master bed = major change in transporting current direction or sediment supply

Question 22

flow of foresets means

Answer 22

flow = dip direction of foresets

Question 23

Climbing Ripples

Answer 23

indicator of estuarine environment
constantly changing mixture of salt and fresh water, and by being dominated by fine sedimentary material carried into the estuary from the sea and from rivers which accumulates in the estuary to form mudflats.
when sediment supply is large, deposition predominates over erosion
ripple-crest migration is sideways and upwards

Question 24

Symmetrical Ripples

Answer 24

Symmetric ripples – wave oscillation
sharp ridges and concave-up troughs
either back-and-forth swash or ingoing/outgoing tides of equal strength

Question 25

Antidunes

Answer 25

higher velocity
Confluence of two fast flowing streams -
flow is left to right
antidunes are migrating right to left

Question 26

Bioturbation

Answer 26

structures in sedimentary rocks that have nothing to do with transport, these are controlled by lifee
sediment modified by biogenic activity
includes footprints and archaeological remains
trace fossils - not the organism itself, but marks it made during its lifetime
burrows
tracks
feeding traces
way up indicator
occurs after deposition
mark periods of slow deposition and other changes

Question 27

Desiccation Cracks

Answer 27

polygonal mudcracks – occur after deposition
curls up
desiccation
infilled = way up indicator
can also be seen as casts on the bottom of beds
find them where you have high evaporation, Intense evaporation produces very large-scale mudcracks

Question 28

Polar Desert and Periglacial Features

Answer 28

Deserts defined by amount of evaporation so yes polar deserts exist
permafrost and periglacial features indicate mean annual temperature -8oC
peirglacial means related to ice
ex. frost heaving, ice rings sorta thing

Question 29

Dessication cracks and proglacial deserts

Answer 29

intensely dry ground shrinks & forms polygonal cracks – a few cm to 10s of metres
can be enlarged by filling with water which expands on freezing
can infill with windblown sediment
former sites of glaciation revealed by crop marks following the old dessication cracks – patterned ground