Download Pet13Ss - West Virginia University

Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring
2007
Sandstones and Conglomerates – Clastic Sedimentary
Rocks
20-25% of stratigraphic record, but receive much more attention from
sedimentary petrologists than 25%
What geologists want to learn from Sandstones

Source area

rock type

direction

weathering environment

Transport

medium, energy

distance

Depositional environment

marine or non-marine

physical environment (beach, river, delta, etc.)
What clues are present in Sandstones?

Grain size

Grain shape

Grain sorting

Grain mineralogy

Sedimentary structures
Grain size

Detrital or clastic rocks have a huge range in grain size

We need a log scale to represent this wide size range

The Phi () Scale: = -log2 (mm)

mm = 2-

Each  step represents a doubling (smaller # or more neg.) or halving (larger #)
(memorize)
in size
For example

size in mm = 2- 

-6  26 mm = 64 mm

0  = 20 mm = 1 mm

2  = 2-2 mm = 1/4 mm = 0.25 mm

4  = 2-4 mm = 1/16 mm = 0.0625 mm
Size ranges are given names (See handout of Udden-Wentworth scale)
Gravel
larger than -1
> 2 mm
Sand
4 to -1
0.0625 mm to 2 mm
Mud
smaller than 4
mm
Clay
smaller than 8
mm m
Loose sediments can be separated by sieving

-2 (4mm)

-1 (2mm)

1 (0.5mm)

2 (0.25mm)

3 (0.125mm)

4 (0.062mm)

closed
Grain size comparator for lithified Sandstone
Sorting = range in grain size

Usually the size range (in ) that includes 2/3 of the grains
Sorting by comparison
Grain Shape

Sphericity - relative equidimensionality of three mutually perpendicular axes

Roundness - lack of sharp corners; larger grains round faster because of more
impacts
Mudrocks

Composed mostly of detrital material smaller than 4, i.e., smaller than 0.062 mm
or 62.5 m (mud=silt+clay)

Non-fissile (-stone) or fissile (-shale)

Named by proportion of Silt and Clay
> 2/3 silt
Siltstone
Silt-shale
1/3 to 2/3 silt
Mudstone
Mud-shale
>2/3 clay
Claystone
Clay-shale
Conglomerates

No agreement about the % of gravel sized material required to make a sediment a
GRAVEL or a sedimentary rock a CONGLOMERATE

Let’s say >30% gravel size material for a conglomerate

Small amount of pebbles or cobbles is very noticeable
Classification of Sandstones is based on Detrital Minerals

Quartz is most abundant sand-size grain

very stable in sedimentary environment

Feldspar may be abundant

may indicate rapid burial, dry climate, granite in source area

K-feldspar vs. Plagioclase?

Unstable Lithic Fragments are least abundant, but most informative

Chert, a stable lithic fragment, goes with Quartz
Accessory Minerals (other minerals)

Micas - may float in water; muscovite, especially is very stable

Heavy minerals - higher density than quartz and feldspar, some are very stable:
zircon, tourmaline, rutile, hornblende, garnet, ilmenite, magnetite, apatite,
pyroxene, etc. If loose sand or disaggregated sandstone is put in a heavy liquid
(s.g. 2.8-3.0) "heavies" sink to the bottom

Can be quite informative
Detrital Grain Types: Quartz
Chert is a stable lithic fragment, grouped with Quartz
Feldspars distinguished from quartz by alteration, twinning and perthite
What’s the large grain?
Lithic Fragments
Volcanic and Plutonic Lithic Fragments in a Sandstone