Lesson 7: Conglomerates (and Rudites) Flashcards
describe and define rudites
definition: sedimentary rock composed of gravel sized sediments or 30% clast composition
description: greater than 2mm particle size
2 classifications of rudites
sedimentary breccia
conglomerates
two bases rudites are classified on
based on:
- percentage between matrix and clasts, and type of matrix
- homogeneity and clast composition
discuss this definition of rudites:
large grain sediments cemented within fine grained matrix
“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
2 general subclassifications of rudites when based on the percentage between matrix and clasts as well as the type of matrix
- grain/clast supported
- matrix supported
how many percent of clast composition must a rudite be to consider it grain/clast supported?
more than 50%
how many percent of clast composition must a rudite be to consider it matrix supported?
less than 50%
special classification of rudites that have more than 85% clast composition
orthoconglomerate
what is the requirement for orthoconglomerates (aside from percentage of clasts), what is the implication?
the clasts must actually be touching each other, meaning they were supposed to be deposited at the same time
special classification of rudites that have 30% (or 25%) to 5% clast composition
paraconglomerate/diamictite
what is the issue with paraconglomerates/diamictites?
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
2 general subclassifications of rudites when based on homogeneity and clast composition
- polymictic conglomerate
- oligomictic/monomictic conglomerate
differentiate the subclassifications of rudites when based on homogeneity and clast composition
polymictic -composed of clasts from different sources/mineralogy/petrology
oligomictic/monomictic -composed of clasts from a single source/mineralogy/petrology
expound on the definition of polymictic conglomerates
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
classification of rudites with clasts composed mainly of metastable or unstable minerals
petromict conglomerates
why is it that petromict conglomerates are naturally under polymictic, but can also be classified as sometimes not under them?
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
name the 5(+1) different depositional environments
- deepwater (deep marine)
- shallow marine and coastal
- fluvial
- alluvial
- glacial
examples of deepwater rudites
- basal portions of turbidites
- olisoliths
what are olistoliths?
submarine landslide deposits
another term for olistoliths
olistostrome
why are olistoliths considered deep marine deposits when submarine landslides occur on the continental margins?
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
3 subtypes of environments under shallow marine and coastal
- wave-worked
- wave-, storm-, and current-worked
- tide-worked
describe the setting of wave-worked environments (3)
- wave energy is sufficient for reworking and transporting sediments
- sediments tend to be well-sorted and well-rounded
- seaward dipping
explain the reworking in wave-worked environments
abrasion, swash and backwash reworks the sediments and sorts them out
example of wave-worked environments
at beaches, i.e. pebble beach, or that glass beach
describe the setting of wave-, storm-, and current-worked environments (3)
- 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)
explain how tide-worked sediments are formed and how they are recorded
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
tide-worked environments are somewhat related to what other environment?
wave-worked environment
2 subtypes under fluvial environment
- sheetflood (shallow braided stream)
- stream flow
describe sheetflood fluvial environment
- high and episodic energy (floodings)
- commonly clasts supported with silt or sand matrix
describe stream flow fluvial environment
- channelized flow in deeper fluvial channels
- typically clast supported
describe where deeper fluvial channels are exactly
downstream, from braided to just before muddy portions (floodplain) that are already purely silt and sand
fluvial vs alluvial environments
fluvial: river related
alluvial: below the mountain, just out of the mountain
describe alluvial sedimentary rocks
- high relief
- typically coarse grained
- clast-supported
describe the energy change in alluvial environments
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
explain fanglomerates
- debris flow type of deposition
- may have matrix supported with rapid erosion
what are the rudites under glacial environments
meltout/lodgement conglomerate/tillite
describe meltout/lodgement conglomerate/tillite
- little to no imbrications
- possibly faceted/striated (from the glacier sliding down the surface)
metaconglomerates vs metamorphic conglomerates
metaconglomerates: any conglomerate undergoing metamorphism
metamorphic conglomerate: conglomerates with metamorphic origin
something to note about when dating rudites
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)
Aside from alluvial environments, what environment could fanglomerates form? And how?
In desert environments: larger particles get eroded immediately along with matrix
Aka paraconglomerate
Diamictite
Sheettflood is aka
Shallow braided stream
