Download Pet13Ss - West Virginia University

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Igneous rock wikipedia, lookup

Sintering wikipedia, lookup

Sedimentary rock wikipedia, lookup

Mudrock wikipedia, lookup

Transcript
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