Sedimentology and Palaeontology Lecture 15: Siliclastics Flashcards
Define texture and the components of it in Sedimentology
Texture = grain characteristics
* Size
* Shape
* Sorting
Related to origins of the grains
and modification during transport
Define Fabric and the components of it in sedimentology
Fabric = arrangement of grains
* Alignment
* Bedding
* Lamination
* Grading
Related to depositional
processes
What does modal mean in grain sizes?
Modal class = most common grain size
- 1 mm = 1000 µm
- phi (ø) scale = -log2(grain size in mm)
Describe the normal grain classification system
Unlithified AGGREGATE Lithified ROCK
Granules-gravel-pebblesboulders Conglomerate
(> 2000 µm) /breccia
Rudite
Sand (62-2000 µm) Sandstone,
Arenite
Silt-mud-clay (< 62 µm) Siltstone-
mudstone-claystone
Argillite
What is the other grain classification system
Udden-Wentworth (grain size) classification
Define sorting in sedimentology
Sorting = standard deviation
of the grain size distribution
about the mean
If a sediment has a faster deposition then = more poorly sorted
Provenance = may already be well-sorted if it is from a Lsedimentary or equigranular crystalline parent rock
Describe sortings relationship with the distribution of grain sizes
Well sorted sediments have a clear modal grain size, and the majority of grains are of that size
A poorly sorted sediment have a less clear modal grain, and has various sizes
List the various grain shapes
Well rounded
rounded
sub rounded
sub angular
angular
very angular
Sphericity is a function of parent rock fabric
* Roundness is a function of abrasion during transport`
What is a matured sediment and a non mature sediment
Matured sediment has a matrix under 15%, good sorting, round grains and is Distal/far from parent rock.
Describe the compositional composition of Pettijohn
Relative % Q/F/L
* i.e. out of 100%
* Q50% (Quartz)
* F30% (Feldspar)
* L20% (Lithics)D
Define Rudites
Greater than 30%/over 30% of grains coarser than 2 mm (= clasts) DEand with finer-grained material (= matrix) in between
Describe rudite nomenclature/ rock types
- Conglomerate = rounded grains
- Breccia = angular grains
- Ortho- = clast supported
- Para- = matrix supported
- Monomict – clasts of same lithology
- Polymict = clasts of different lithologies
- Intraformational = clasts derived from within the
depositional system - Extraformational = clasts derived from different
lithostratigraphic units
Define
Siltstones
Mudstone
Claystone
Features
* Clay:silt
* Colour
* Smell
* Laminated/massive
* Parting/fissility
* Sedimentary structures
* Macrofauna
* Trace fossils
* Concretions
Define detrital clay
Detrital clays = derived from soils from weathering
* Mineralogy is a function of parent bedrock lithology and climate
Illite
Kaolinite
Chlorite
Smectite
