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Controls Sandstone Composition (USA Significance (has sediments from great…
Controls Sandstone Composition
1. Climate
climate affects degree of weathering in provenance area
∴ eroded sediments from
same source rock
may exhibit
different composition
if climate differs
Differing compositional signals
may indicate
changing climate
i.e. changing palaeolatitude
Humid conditions
greater weathering than arid climate
∴ sands have
more quartz
lithics & feldspar dissolve / weather more readily
since more reactive than quartz
Arid vs Humid Sed Products
Arid
envmt
limited water ∴ minimal chemical weathering
physical erosion dominates
sand grains physically eroded from envmt
sand = relatively representative
of source rock composition
mix of granite components
i.e. individual fsp/qtz, granite lithics
undergone little chemical change
Humid
envmt
lotsa water ∴ greater chemical weathering
fsp/lithics = reactive ∴ dissolve
quartz rich rock results
sand = less true reflection of source material
Metamorphic Source Rock f(Climate)
Arid envmt i.e. Himalayas
i.e. Himalayas in Pakistan
Some quartz / plag / lithic fragments
Faithful record of source material
Humid envmt i.e. Dalradian metaseds, Scotland
Quartz-rich rock results
less representative
resistant grains preferentially preserved
more chem & phys weathering ∴ lithics & fsp removed pref
Provenance
= where rock components come from
~source/origin
2. Depositional environemnt
composition = f (depositional environment)
High energy envmt - reworks sediment
Abrades
soft grains (shale rip-up clasts)
Fractures
crystalline grains (fsp, calcite)
Increases quartz content
e.g. fluvial / beach envmts
Longer
transport
distance
greater
mineralogical maturity
greater
rounding
greater
sorting
USA Significance
has sediments from great variety of tectonic settings
California - Strike slip margin
Inner ranges - subduction
Utah - cratonic rocks
Rockies - orogenic belt