Shales sandstones and associated rocks Chapter 4
Pyroclastic versus Epiclastic Clast – a particle, or grain Epiclastic rocks are those composed of (nonvolcanic) particles of all sizes, clay to boulders Pyro – fire, volcanic Pyroclastic rocks are those composed of eruptive volcanic rock particles
Ch 4.1 mineral and rock grains textural and compositional terms Table 4.1
Friktion > 0,06 mm Cohesion < 0,06 mm Grade size in mm Boulder over 200 Cobbles 60-200 Gravel 2-60 pebbles 4-60 granules 2-4 Sand 0,06-2 Silt 0,002-0,06 Clay < 0,002 Grade or fraction
Surface charges and water that hold the sediment together clay and silt - charges are great compared to grain weight sand – charges weak compared to grain weight but capillary water important Cohesion
clay mineral grade size To avoid genetic inferences other terms are used: clay size - lutite, lutiteous, argillite, argillaceous sand size – arenite, arenaceous courser than sand size – rudite, rudaceous ? what two meanings has the word clay?
important but a lot of names based on composition – some are expansive others not Brucite - expansive Gibbsite Kaolinite – NOT expansive Montmorillonite (also called Smectite) expansive (deposition env. near shore) Illite – expansive (deposition env. deep sea) Bentonite – expansive (formed by weathering of volcanic ash) similar clay minerals chlorite halloysite vermiculite Clay minerals
Clay – fresh deposited – slides on slopes as little as 1 degree!!! Clay – fills in the bottoms of basins first Glacial and post glacial clay in Sweden Slide risk with clay
Glacial clay – rock flour produced by abrasion of rocks in the glacier Post glacial clay – “normal” clay – consists of clay minerals the product of weathering thus the clay rich rocks are not very effected by weathering Source of clay
Answer – p. 85 Table 4.2 the older they are the less expansive the clay is ?? How is the age of a clay rich rock related to the % or expected occurrence of swelling clays??
?? Post glacial clay?? ?? What could you say about the glacial clay deposits from the Weichselian Glaciation?
4.2 Lithification • To make into a rock • lithic, litho = rock
4.2 Lithification • consolidation – water squeezed out • compaction – air squeezed out • densification – both consolidation and compaction • diagenesis – both densification and cementation
Different strengths Weathering can “remove” cement quartz iron oxide calcite dolomite gypsum halite clay Types of cement – Fig 4.5,
Characteristics of cement • quartz – strongest • iron oxide - strong • calcite - soluble, crystalline intergrowths • dolomite – soluble but less than calcite • gypsum – extremely soluble • halite – extremely soluble • clay - (not true cement) can be leached by ground water
Classification Rock or Soil an engineer would classify a sediment that was either loose and unconsolidated or hard and “rocklike” as a soil if it lacked cement a geologist would classify it as a rock if it was pre-quaternary in age
Cementation of clay • movement of ground water is low • difficult to consolidate – squeeze out water • difficult for cement to migrate into voids
Cementation of clay How can thin layers of sand in clays and silts enhance lithification? Glacial clays are varved = winter layer and summer layer / some sand in summer layers
Consolidation of glacial clay If the water is caught in the basin – then it will support the clay If it is allowed to drain out – the clay will consolidate – resulting in subsidence of the ground surface
Consolidation of glacial clay Problems when a “basin of clay” is punctured and water is allowed to escape example: Stockholm area – Huddinge, slussen subway, and more!
Strength versus porosity • ?? How is rock strength and porosity related in clay rocks?? • Can you draw a simplified curve showing this relationship?? • p. 87, 88 Figure 4.6. The porosity decreases with time and depth of burial Thus the lower the porosity the stronger the rock
4.3 Description of some epiclatic rocks Rocks with grains coarser than 2mm • Conglomerate or Rudite • Breccia (fault breccia) • Tillite
Conglomerate or Rudite – Conglomerate or Rudite – more than 30% rounded particles larger than 2mm • deposition environments: rivers, mouth of streams, beaches, and colluvium
Conglomerate or Rudite – • physical character – bimodal, open work, imbricate structure Fig. 4.8, clast or matrix supported structure
Conglomerate or Rudite – • Imbricate structure
Conglomerate or Rudite – • not common but due to resistance to weathering they often stand out in the landscape as ridges
Breccia • more than 30% angular particles larger than 2mm • deposition environments - tallus and scree = rock fall, movement, glaciers, volcanic activity, landslides, meteorite impacts
Fault breccia • Fault breccia, fault gouge, mylonite • sheets of crushed material in a fault or fault zone • course angular rock fragments – breccia • fine clay, pulverized rock from intense grinding – mylonite • Very important with respect to permeability of “hard rocks”
Permeability • along grain boundaries • along faults, fractures and joints
Tillite • unstratified, unsorted soil deposited from glacial ice • depositional environment – glaciated areas • physical character – highly variable thickness both laterally and vertically and extremely variable grain-size distribution (boulder clay, gravel rich till)
Rocks with sand-size grains (0.06 to 2 mm) Sandstone and Arenaceous rocks sandstone often suggests that the grains are composed of quartz and feldspar arenites often are “sandstones” with grains other than quartz and feldspar (kalkarenite)
Texture • Texture refers to • Kornstorleksfördelning • Sortering • Kornform • Packning • Geometry of beds
Grain size Includes several qualities: • mean grain size • predominant grain size • range of grain size • Grain-size distribution • Sorting grain form plays a role!
range of paticles sorting % of different fractions grain size
Texture – packing of grains • grain – grain • matrix supported
grain form - maturity • the longer time a particle is transported • the better rounded it will become
question in notes on homepage Explain the concept of maturity or immaturity of sandstones. Give an example (name) of both a mature and immature sandstone. What is the main physical difference between these two.