1. The Rock Cycle: Clastic Sedimentary Rocks
EARTH MATERIALS VI
The Rock Cycle: Clastic Sedimentary Rocks
Professor Peter Doyle

2. THE ROCK CYCLE

3. Relative proportions of rock groups
Sedimentary rocks: larger proportion of land surface
The greater proportion created by weathering, erosion & deposition
These are known as clastic sediments

4. SEDIMENTARY ROCKS
Clastic sediments from weathering & erosion of pre- existing rocks
Weathering can be physical (freeze-thaw) or chemical
Erosion & subsequent transportation by water, wind and ice before deposition
Biogenic and chemical sediments form as biochemical or inorganic chemical precipitates
E.g. shelly fragments in limestones; evaporites

5. Sediments to sedimentary rocks
Weathering structures seen in ancient terranes
Lithification due to:
Compaction (squeezing out water)
Diagenesis (precipitation of cements)
Transport shown by sedimentary structures

6. Mineralogy of sedimentary rocks
Clastic sediments
siliciclastics – quartz, feldspars, clay minerals, rock fragments
calciclastics – calcite, dolomite
Carbonate (biogenic) sediments
aragonite, calcite
chalcedony silica (flint) occurs as subsiduary
Evaporite (chemical) sediments
halite, gypsum and anhydrite
Dolomite, also as dolomitisation of calcite

7. Classification of clastic sediments: The Wentworth Scale
Grain Size Sediment Grade Rock
>256 mm Boulder  Conglomerate
256 – 64mm Cobble  (rounded clasts)
64 – 2mm Pebble  Breccia
  (angular clasts)
2 – 0.063mm Sand Sandstone
0.063 – 0.002mm Silt Siltstone
< mm Mud Mudstone / Shale
< 0.002mm Clay Claystone

8. Conglomerates & Breccias
Conglomerates - mainly rounded clasts
Ortho conglomerates - clast supported
Para conglomerates - matrix supported
Breccias - mainly angular clasts
Clast type defines rock type:
Monomict - clasts of mainly one composition
Polymict - clasts of various compositions
Diamict - clasts poorly sorted of various types
Matrix (sediment between major clasts)
sand and mud grade sediments
Cement binds clasts/matrix (Crystalline, diagenetic, typically calcite, silica

9. Finer grains between major clasts
Crystalline cement
Finer grains between major clasts
Grains floating in matrix
Grains touching

10. Calcite cement
Quartz clast

11. Diamict conglomerate
Orthoconglomerate
Monomict breccia

12. CLASTS have modal grain size in the sand grade ( 2mm – 0.063mm)
SANDSTONES
CLASTS have modal grain size in the sand grade ( 2mm – 0.063mm)
Coarse sand mm,
medium m
fine mm
Siliciclastic sands have clasts composed usually of quartz, feldspar, fine grained rock fragments or lithics
Mud grade matrix may be present
Crystalline diagenetic cements, e.g. quartz, calcite, clay minerals

13. Maturity Maturity is a function of sediment transport
Textural maturity refers to:
The degree of roundness of the grains
The amount of sorting of the grain sizes
Texturally mature sandstones have well-rounded and well-sorted grains, immature if not
Mineralogical maturity refers to the percentage of quartz grains
Feldspars break down with transport
Quartz grains more resistant
Mineralogically mature sandstones have mostly quartz grains
Arkose is mineralogically immature

14. Roundness Dry aeolian (airborne) grains Well-rounded & heavily abraded
Increasing Roundness=increasing maturity

15. Sorting Well sorted Poorly sorted
Increasing sorting=increasing maturity

16. Transport structures
Cross-bedding
Ripples

17. Mineralogy
Increasing quartz=increasing maturity

18. Classifying sandstones: mineralogy
Quartz
>95% Quartz =
Quartz arenite
<95% Quartz - divided on amount of feldspar or lithics
Rock fragments (lithics)
Feldspar

19. Arkose >25% Feldspar = arkosic composition
>25% Feldspar = mineralogically immature
Quartz
Feldspar
Lithics

20. PETTIJOHN CLASSIFICATION
Based on percentage of matrix
Matrix poor (<15%) - arenites
Matrix rich (>15%) – wackes
Wackes or (greywackes)
Tend to be dark in colour
Are poorly sorted

21. Examples in hand specimen
Light coloured
Light coloured
Light coloured
Dark coloured
Dark coloured

22. Red sandstone
Cross-bedded sandstone
Lithic sandstone
Polished sandstone
Quartz sandstone (arenite)

23. Fine grained siliciclastic rocks
Modal grain size <0.063mm
Grouped as Mudrocks (>50% siliclastic grains, <0.063mm)
Typical mineralogy
Clay minerals (sheet silicates),
quartz & feldspar
MUDROCKS may split (fissile) or not (massive)
Siltstones non-fissile, coarser grade, mica flakes common
Mudstones - massive, mud-grade
Shales - Fissile (splitting), mud-grade

24. Siliciclastic rocks
Increasing mud-grade grains

25. Shale