GEOL 340 Sedimentology and Stratigraphy
Lecture 10
1. Clastic Sandstones: Composition
2. Clastic Sandstone: Classification
3. Conglomerates and Shales
4. Provenance
Clastics: Composition (Tab. 5.1, p. 135)
I. Major Minerals
a. quartz (SiO2)
chert - micro- and polycrystalline quartz
b. feldspars
Alkali - potassium - K feldspars ( KAlSi3O8 solid soution [Kna]AlSi3O8)
more abundant in general
Plagioclase (NaAlSi3O8 albite solid solution CaAl2Si2O8 anorthite)
characteristic twinning
most abundant in seds derived from volcanic rocks
c. clay minerals
(Ka, Al, Si, Mg, Fe, OH, and H2O)
illite, smectite, kaolinite, chlorite, montmorillinite
primary and/or secondary
II. rock fragments
pieces of original source rock not yet completely disintegrated
III. accessory minerals - 1 to 2 % abundance
mica, biotite and heavy minerals
IV. Chemical cements (diagenesis)
Silicates
Clays
Carbonates
V. Matrix Minerals
Most common are the clay mineral groups (illite)
Shale Composition
rocks composed of mud-sized (< 60 mm)
chemistry reflects shale mineralogy
most are SiO2, Al2O3, and Fe2O3
Burgess Shale, Green River Shale
OAE's, Midcontinent cyclothem black shales
Clastics: Classification
Sandstone Classification (Williams et al., 1982)
walk through classification diagram in detail (Fig. 5.5, p. 144)
a. arenite = <5% matrix
b. wacke = argillaceous = >5% matrix
c. arkose = feldspar-rich
d. graywacke = matrix-rich, deep burial, chlorite, dark color
Impt! drop "gray" and use only "wacke"
Note: Cements and replacement crystals are products of Diagenesis!!!!!
So do not include these in the depositional classification.
Maturity: grain composition, size and sorting (Fig. 5.6, p.
145)
Conglomerates and Shales
Conglomerate Composition
same as sandstones but > 30% gravel sized particles
Conglomerate Classification (matrix-supported [diamictite]
versus grain-supported)
a. conglomerates and breccias
extraformational
intraformational
b. volcanic breccias
pyroclastic - explosive shattering
autobreccia - shattering during movement and colling
hyaloclastic - thermal shattering of glass in water
c. cataclastic breccia
landslides and slumps
tectonic breccia - gouge
collapse breccia
d. solution breccia
e. impact breccias
impact shattering
ejecta blankets
Classification of Shales = based on chemistry (Table 5.7, p. 157)
Provenance = deciphering the origin of sediments (Fig.
5.13, p. 159)
a. continental block
within continental masses
igneous, sedimentary and metamorphic rocks, few volcanics
b. magmatic arcs
zones of plate convergence with volcanic arcs
mainly volcanic sources
c. recycled orogens
zones of plate convergence with associated orogenic belts