1 EES 450: Sedimentary Geology ARENITES (SANDSTONES) AND THEIR CLASSIFICATION 20% - 25% of all sedimentary rocks Wacke – Sandstone with 10-15% matrix (silt-

1

EES 450: Sedimentary Geology

ARENITES (SANDSTONES) AND THEIR CLASSIFICATION20% - 25% of all sedimentary rocks

Wacke – Sandstone with 10-15% matrix (silt- and/or clay-size material between grains; e.g., with various clay minerals like chlorite, illite, etc.).

2



Quartz arenite Feldspathic arenite

Lithic arenite

3



Quartz arenite Feldspathic arenite

Lithic arenite

4



● Greywacke – Informal term for dark-grey, coarse-grained sandstones, consisting of poorly sorted, angular to sub-angular grains of quartz, feldspar, dark minerals and lithic fragments in a fine-grained, compact and clay-rich matrix. Often used to describe turbiditic sandstones. If possible, its use should be avoided.

5



● Turbidites – Result from turbidity current deposits, which are derived from underwater avalanches. Turbidite flows are responsible for distributing vastamounts of clastic sediment into the deep ocean.

6



● Greensands – Term for green sands or green sandstones applied to shallow marine deposits with large quantities of rounded, green grains (glauconite).

Roadcut – Lionel Mt. Sandstone(Cambrian), lower unit (TX).

Glauconitic Sandstone. Glauconite and quartz – plane light.

7



● Physical setting examples for different sandstone types.

Greywacke

Quartz arenite

Feldspathic arenite

Lithic arenite

8



● Common cements.

Calcite cementQuartz cement Hematitic cement

9



● Common cements.

Lower Silurian Red Mt. Fm. – quartz arenite

A – Sutured contact (red arrow) between quartz grains. Un-modified grain margins (black arrows) extend to overlap (red dashed lines) at the contact.

B – Same area, x-polarized light. Cements syntaxial on the two pressure dissolved grains meet with cement syntaxial to a third are a common point (black arrow). Suture – red arrow.

10



● Quartz.

- What do the terms “monocrystalline”, “polycrystalline” and “undulose extinction” mean? What, if anything, might the use of these terms tell us about the sediment or rock being examined?

Rounded monocrystalline quartz Polycrystalline quartz Polycrystalline quartz – undulose extinction

11



● Quartz.

- Monocrystalline quartz – Typically indicative of an igneous source rock – Often the major constituent (60% - 70%) of quartz arenites.

- Polycrystalline quartz – Often a relatively minor (< 40%) constituent of quartz arenites – Can indicate metamorphic source rock (quartzite) if polygonal crystal boundaries are evident and if a large number (3+) of quartz crystals form individual grains – Can also indicate a granitic source rock if fewer crystals form individual grains and no or very little intercrystal suturing is evident.

- Would you expect to see a difference in weathering stability between these quartz varieties? Why or why not?

12



● Feldspars.

Potassium feldspars - Orthoclase Microcline – x-hatched crystal twinning

13



● Feldspars.

Plagioclase feldsparsPlagioclase striations – polysynthetic twinning

14



● Accessory minerals. - Other common accessory minerals include pyroxene, corundum, zircon, tourmaline, garnet, magnetite and rutile.

- What can the dominance of particularaccessory minerals tell us, if anything?

Chlorite

Muscovite mica

Biotite Olivine

15



● Lithic arenites – Sandstones with > 5% lithic fragments, often having a clayey matrix.

Lithic arenite – Wolfville Fm. (Jurassic), at Red Head Nova Scotia. Top: plane light, bottom: x-polarized light.

16



● Common arenite matrices.

Clays Sericite – fine micas Micrite

Documents

1 EES 450: Sedimentary Geology ARENITES (SANDSTONES) AND THEIR CLASSIFICATION 20% - 25% of all sedimentary rocks Wacke – Sandstone with 10-15% matrix (silt-