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Sedimentary Petrology > Lesson 6: Introduction to Siliciclastic Sedimentary Rocks > Flashcards

Lesson 6: Introduction to Siliciclastic Sedimentary Rocks Flashcards

1
Q

explain the etymology of “siliciclastic” and connect to its definition

A

came from 2 words:
silicic -made of silicate minerals
clastic -made of fragments
= made up of silicate crystal/mineral fragments

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2
Q

explain the minerals considered as siliciclastic, what about silicified minerals?

A

whatever silicate minerals: feldspar, biotite mica, clay minerals
silicified minerals aren’t included in this because these are different. they form from alteration

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3
Q

how are siliciclastic sedimentary rocks classified?

A

mainly based on size, then secondarily the composition

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4
Q

illustrate the udden-wentworth scale

A

see slide 2

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5
Q

what are the 7 depositional environments of siliciclastic rocks?

A
  1. alluvial fan
  2. neritic
  3. marine bathyal and abyssal (turbidites)
  4. fluvial overbank
  5. eolian dune
  6. fluvial channel
  7. beach, offshore bar
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6
Q

describe alluvial fan environments for siliciclastic rocks

A
  • from immature to mid mature
  • characterized with sudden deposition
  • considered landslide deposits, most likely breccia, angular
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7
Q

describe neritic environments for siliciclastic rocks

A
  • immature to mid submature
  • shallow water deposits on continental shelf, pelagic
  • immature because it didn’t travel far, not rounded, not sorted
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8
Q

describe marine bathyal and abyssal environments for siliciclastic rocks

A
  • immature to mid mature
  • pelagic
  • sediments carried by turbidity currents
  • can still be immature even with bouma sequence because the sediments themselves aren’t mature in composition and shape
  • somewhat related to underwater landslides and surges, from high to low (which ofc isn’t typically mature)
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9
Q

describe fluvial overbank environments for siliciclastic rocks

A
  • immature to mid mature
  • river area
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10
Q

describe fluvial channel environments for siliciclastic rocks

A
  • end immature to start supermature
  • v shaped is more on the immature to submature, while the transport zone to deposition is more on mature
  • deltas are also included here
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11
Q

describe eolian dune environments for siliciclastic rocks

A
  • mid submature to supermature
  • because wind is an incompetent fluid, it can only carry the similarly small sized sediments, and thus deposit somewhat sorted rocks
  • doesn’t strictly mean on deserts, it just means windblown environments. it could also be high energy but just that the wind isn’t that strong to carry boulders yk?
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12
Q

describe beach, offshore bar environments for siliciclastic rocks

A

mid submature to supermature

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13
Q

describe the general mineralogy of siliciclastic sediments

A
  • silica-based
  • lacking carbon compounds (not completely absent but just very little)
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14
Q

how are siliciclastic sediments formed?

A

they are formed from pre-existing rocks, by breakage, transportation and redeposition to form siliciclastic rocks

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15
Q

what are the 4 indicators of immaturity based on mineralogy among sedimentary rocks?

A
  1. feldspars -arkosic/arkose
  2. lithic fragments -lithic
  3. clay minerals -muddy/argillaceous
  4. organic minerals
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16
Q

explain how lithic fragments indicate immaturity

A

ex. if quartz diorite fragment: immature because it didn’t go through a lot of weathering. it it had gone through good weathering, the plagioclase and feldspars (and other unstable minerals) in the diorite would have been weathered/dissolved away.

if quartz is the only one left. then that would mean it’s been thru a lot so it has good maturity

17
Q

when is a rock considered wacke?

A

only if clay/silt sized sediments. it have to have clay, feldspars and quartz as composition, it’s not mineral specific. it’s more size specific

18
Q

what does compositional maturity typically mean in siliciclastic sedimentary rocks?

A
  • it refers to the amount of stable minerals present
  • not exclusively, but typically, uses quartz in describing
19
Q

aside from quartz, what other minerals are used to describe compositional maturity?

A

garnet, tourmaline and others that are very hard, difficult to weather and not chemically reactive

20
Q

series that shows the predicted relative stability and weathering rates of rocks on the earth’s surface

A

Goldich Mineral Stability Series

21
Q

what is the most stable mineral based on the Goldich Mineral Stability Series

A

quartz.
usually the felsic minerals are the most stable (except for orthoclase feldspar)

22
Q

describe the stability of feldspars

A

as a whole, they’re not really stable at the surface because of they are very reactive to acid.

that is why typically we get clay from feldspars going through hydrolysis and hydration

23
Q

describe the mineral structure of clay minerals

A

mostly phyllosilicates, except for aluminum hydroxides like diaspores and bauxite minerals

24
Q

describe the composition and texture as well as give examples of IMMATURE sedimentary rocks

A
  • composition: feldspars, clay minerals, rock fragments
  • texture: poorly sorted; many sizes and/or angular fragments
  • examples: arkose/lithic breccia/conglomerate/arenite
25
Q

describe the composition and texture as well as give examples of SUBMATURE sedimentary rocks

A
  • composition: abundant quartz, immature minerals common
  • texture: sand is common but with silt, clay and larger sediments; grains begin to round
  • examples: subarkosic/sublithic arenite, quartz wacke, quartz conglomerate
26
Q

describe the composition and texture as well as give examples of MATURE sedimentary rocks

A
  • composition: purely quartz or stable minerals
  • texture: sand sized only with rounded sediments
  • example: quartz arenite
27
Q

what percentage of lithic fragments, clay minerals on sandstone would be considered immature or submature?

A

15% or more

28
Q

describe the composition and sorting+size of quartz arenite

A
  • purely quartz
  • uniform sand-sized particles
29
Q

why are breccias/conglomerates considered immature (i mean obviously duh but why?)

A
  • consider the transport duration and distance. breccias are usually just deposited near where they were eroded/broken off from.
  • also because it is a mix of large clasts and fine grained matrix, meaning poorly sorted.
30
Q

rocks under lutites/mudrocks

A

shale, siltstone, claystone

31
Q

rocks under arenites

A

sandstone

32
Q

rocks under rudites

A

conglomerates and breccias

33
Q

how can you determine maturity based on water properties?

A

acidic: feldspar won’t survive = mature
basic: feldspar will survive = immature, submature

Sedimentary Petrology (11 decks)

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