Physical Geology
Sedimentary Rocks
- sed rx are those which form by lithification of unconsolidated sediments.
- lithification is a process of compaction
- - weight of overlying seds causes material to reduced by 40%
- - increase press and temp
- - heat causes things to melt, like silica and calcite
- - these go into solution, fill spaces between remaining grains, then harden to form a glue: cementation
.
Silicate clastics:
- derived from the weathering of other material.
- or pieces of broken material glued together
- formed from deposition and lithification of these in rivers, lakes, streams, etc.
- grain size indicates energy level - how fast the water was moving.
- - small size indicates low energy: allows time for smaller and lighter particles to settle to bottom.
Shale: dark, quiet water, lagoon, lake, has layers (sheet structure)
Siltstone: also can't see grains, feels slightly gritty
Arkose: can see grains, near outcrop of grainite, dirty red, high feldspar content
Qtz sandstone: beach environment
Graywacke: dirty gray, a mix of stuff, formed in turbidity currents, can be fine on top and coarse on bottom.
Conglomerate: Mt. streams, rivers, round
Breccia: base of Mt. Slope, angular grains
Biogenic Sediments:
- formed by the accumulation of living critters
Limestones: CaCO3, fizzes when reacts with HCl
Chalk: warm, oceanic water, white, soft, made up of shells of microscopic organisms
Fossiliferrous Limestone: visible fossils, can be any color, also a warm shallow oceanic environment. Well consolidated frags well glued together.
Coquina: Formed in the surf zozne, poorly consolidated fragments loosely glued together.
Coal: black, shiny, light weight, forms in swamps, bogs.
Chemical Sedimentary Rx:
- formed by inorganic precipitation of dissolved material in water.
- tend to have a smooth (crystalline) texturem not clastic (things glued together).
- formed by changes in the chemistry of an environment (temp, press, etc.)
Limestone: No fossils, harder than chalk
Rock Gypsum: Evaporite, arid climate - basins, flats, out west, soft, light colored, not salty.
Rock Salt: Salty taste, evaporite
Chert: Jasper- red, flint- black, microcrystalline, deep ocean environment where CaCO3 from critters at shallow depths dissolve, but Silica precipitaties and settles to bottom of ocean.
-Sedimentary rx have several unique features:
- sorting:
- well sorted if the grains are all the same size
- poorly sorted if grains are different sizes
- strata or beds:
- layers of sed rx that record an environment of deposition
- layers can be of varying thickness
- bedding planes:
- separate the strata
- flat surfaces along which the rock will tend to break
- mud cracks, ripple marks, and cross-bedding
- all record the environment at time of deposition of sediments
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