Lesson 7: Conglomerates (and Rudites)

Question 1

describe and define rudites

Answer 1

definition: sedimentary rock composed of gravel sized sediments or 30% clast composition

description: greater than 2mm particle size

Question 2

2 classifications of rudites

Answer 2

sedimentary breccia
conglomerates

Question 3

two bases rudites are classified on

Answer 3

based on:
- percentage between matrix and clasts, and type of matrix
- homogeneity and clast composition

Question 4

discuss this definition of rudites:
large grain sediments cemented within fine grained matrix

Answer 4

“fine grained” can be removed from definition because it is really only infill between large clasts, it is rare for them to be completely absent (in that regard, it cannot be insignificant), but it eventually gets removed after diagenesis

tldr; the matrix is not important because they eventually get removed

Question 5

2 general subclassifications of rudites when based on the percentage between matrix and clasts as well as the type of matrix

Answer 5

1. grain/clast supported
2. matrix supported

Question 6

how many percent of clast composition must a rudite be to consider it grain/clast supported?

Answer 6

more than 50%

Question 7

how many percent of clast composition must a rudite be to consider it matrix supported?

Answer 7

less than 50%

Question 8

special classification of rudites that have more than 85% clast composition

Answer 8

orthoconglomerate

Question 9

what is the requirement for orthoconglomerates (aside from percentage of clasts), what is the implication?

Answer 9

the clasts must actually be touching each other, meaning they were supposed to be deposited at the same time

Question 10

special classification of rudites that have 30% (or 25%) to 5% clast composition

Answer 10

paraconglomerate/diamictite

Question 11

what is the issue with paraconglomerates/diamictites?

Answer 11

some authors don’t consider them to be under the rudite group (bc it has very little percentage of clasts)

thus, they can either just be termed:
- mudstone/siltstone/claystone with some gravel sized particles
- conglomeratic mudstone/siltstone/claystone

Question 12

2 general subclassifications of rudites when based on homogeneity and clast composition

Answer 12

1. polymictic conglomerate
2. oligomictic/monomictic conglomerate

Question 13

differentiate the subclassifications of rudites when based on homogeneity and clast composition

Answer 13

polymictic -composed of clasts from different sources/mineralogy/petrology

oligomictic/monomictic -composed of clasts from a single source/mineralogy/petrology

Question 14

expound on the definition of polymictic conglomerates

Answer 14

they don’t just mean different rock types, but it also means different composition of rocks.

in other words:
they do not have to be different as in igneous, metamorphic and sedimentary, they can be different as in from different igneous rocks: granite, basalt and diorite

Question 15

classification of rudites with clasts composed mainly of metastable or unstable minerals

Answer 15

petromict conglomerates

Question 16

why is it that petromict conglomerates are naturally under polymictic, but can also be classified as sometimes not under them?

Answer 16

this is because if the minerals/sources/petrology of the clasts are naturally different, that means that there is a big possibility that they are unstable with each other

NOT ALL PETROMICT ARE POLYMICTIC though because minerals from the same source can still be unstable with each other

Question 17

name the 5(+1) different depositional environments

Answer 17

1. deepwater (deep marine)
2. shallow marine and coastal
3. fluvial
4. alluvial
5. glacial

Question 18

examples of deepwater rudites

Answer 18

• basal portions of turbidites
• olisoliths

Question 19

what are olistoliths?

Answer 19

submarine landslide deposits

Question 20

another term for olistoliths

Answer 20

olistostrome

Question 21

why are olistoliths considered deep marine deposits when submarine landslides occur on the continental margins?

Answer 21

yes, it is true that submarine landslides do occur in shallow marine, but the sediments themselves get deposited eventually at the bottom of the ocean, thus deep marine

Question 22

3 subtypes of environments under shallow marine and coastal

Answer 22

1. wave-worked
2. wave-, storm-, and current-worked
3. tide-worked

Question 23

describe the setting of wave-worked environments (3)

Answer 23

• wave energy is sufficient for reworking and transporting sediments
• sediments tend to be well-sorted and well-rounded
• seaward dipping

Question 24

explain the reworking in wave-worked environments

Answer 24

abrasion, swash and backwash reworks the sediments and sorts them out

Question 25

example of wave-worked environments

Answer 25

at beaches, i.e. pebble beach, or that glass beach

Question 26

describe the setting of wave-, storm-, and current-worked environments (3)

Answer 26

• reworking is done by waves, longshore currents and rip currents (also storms)
• sediments tend to be poorly to moderately sorted (probably bc of the many sources of reworking)

Question 27

explain how tide-worked sediments are formed and how they are recorded

Answer 27

they are formed by marine transgression, thus happens over long periods of time (thousands of years). this is why it is often poorly documented (cannot be actually observed), and can only be seen through stratigraphic records

Question 28

tide-worked environments are somewhat related to what other environment?

Answer 28

wave-worked environment

Question 29

2 subtypes under fluvial environment

Answer 29

1. sheetflood (shallow braided stream)
2. stream flow

Question 30

describe sheetflood fluvial environment

Answer 30

• high and episodic energy (floodings)
• commonly clasts supported with silt or sand matrix

Question 31

describe stream flow fluvial environment

Answer 31

• channelized flow in deeper fluvial channels
• typically clast supported

Question 32

describe where deeper fluvial channels are exactly

Answer 32

downstream, from braided to just before muddy portions (floodplain) that are already purely silt and sand

Question 33

fluvial vs alluvial environments

Answer 33

fluvial: river related
alluvial: below the mountain, just out of the mountain

Question 34

describe alluvial sedimentary rocks

Answer 34

• high relief
• typically coarse grained
• clast-supported

Question 35

describe the energy change in alluvial environments

Answer 35

so basically from steep valleys, there is a sudden change of topography, meaning also sudden change in energy or current. thus the deposits are more sedimentary breccia

Question 36

explain fanglomerates

Answer 36

• debris flow type of deposition
• may have matrix supported with rapid erosion

Question 37

what are the rudites under glacial environments

Answer 37

meltout/lodgement conglomerate/tillite

Question 38

describe meltout/lodgement conglomerate/tillite

Answer 38

• little to no imbrications
• possibly faceted/striated (from the glacier sliding down the surface)

Question 39

metaconglomerates vs metamorphic conglomerates

Answer 39

metaconglomerates: any conglomerate undergoing metamorphism
metamorphic conglomerate: conglomerates with metamorphic origin

Question 40

something to note about when dating rudites

Answer 40

date the clasts, not the matrix because they can give different results. it would then be something like the time of formation vs time of burial (clast vs matrix)

Question 41

Aside from alluvial environments, what environment could fanglomerates form? And how?

Answer 41

In desert environments: larger particles get eroded immediately along with matrix

Question 42

Aka paraconglomerate

Answer 42

Diamictite

Question 43

Sheettflood is aka

Answer 43

Shallow braided stream