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Understanding Earth Ch6 Notes
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Summary notes, colour coded, for the entirety of Understanding Earth Chapter 6 - Sedimentation - the relevant reading for Lecture 1
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Understanding Earth LEC 172 Lecture 1 ReadingCh 6; Sedimentation; Rocks Formed by Surface Processes
Surface Processes of the Rock Cycle:
Sediments/sedimentary rocks are produced by the rock cycles surface processes, after they’ve been moved
from the Earths interior via orogeny, and before they return to it via subduction.
Processes move materials from a source area (where sediments are created) to a sink area/sedimentary basin
(where they’re deposited in layers).
Surface Processes:
1) Weathering – processes which break down rocks on the surface to produce sediment particles.
a. Chemical = Minerals in rocks are chemically altered/dissolved.
b. Physical = Solid rock is fragmented via mechanical processes (freeze thaw/tree roots etc).
2) Erosion – when rock particles (formed via weathering) are dislodged & moved from the source area.
3) Transportation – processes which move sediment particles to the sink areas. (Water/Wind/Glaciers).
4) Deposition/Sedimentation – when sediment particles settle out as currents/winds slow, glacier edges
melt, or dissolved minerals precipitate out forming layers.
5) Burial – As additional layers build up, others are compacted and buried deeper with sedimentary
basins.
6) Diagenesis – physical and chemical changes induced by pressure, heat, and chemical reactions, that
cause sediments to lithify/convert into sedimentary rock.
Weathering & Erosion – Sources of Sediment:
Chemical weathering weakens rock and increases susceptibility to fragmentation. Physical weathering
produces smaller fragments, w/ larger surface areas exposed to chemical weathering. Together they create
solid particles and dissolved products which are carried via erosion.
End products are either Siliciclastic or Chemical/Biological Sediments:
Clastic Particles Particles, varying in shape & size, that are formed from the physical/chemical weathering of
pre-existing rocks and are transported and deposited as sediments.
* Siliciclastic Sediments = Produced via the weathering of rocks containing mainly silicate minerals.
* Chemical Sediment = A sediment formed at or near the site of deposition from dissolved materials that
precipitate from the water.
* Biological Sediments = A sediment formed at or near the site of deposition because of direct or indirect
mineral precipitation by organisms.
Bioclastic Sediments = A shallow-water sediment consisting of shell/skeleton fragments directly precipitated
by marine organisms, consisting of 2 calcium carbonate minerals – calcite & aragonite- in variable proportions.
Transportation & Deposition – The Downhill Journey:
After clastic particles & dissolved ions have been formed via weathering and dislodged via erosion, they are
transported to the sedimentary basin, over an exceptionally long time, downhill:
Rockfall from cliffs.
River transportation to oceans
Glacial Ice
Wind (which can blow to higher elevations, but overall gravity prevails)
, Currents as Transport Agents Air or Water Currents. 25b T of solid/dissolved sediments transported firstly
by rivers annually end up in oceans, whilst air currents transport much less. Currents carry them downstream
or downwind. The stronger the current/faster the flow, the larger particles it can transport.
Sorting = The tendency for variations in current velocity to segregate sediments according to size.
Deposition occurs when transportation stops and is driven by gravity, which works against the currents
ability to carry its load. Settling velocity is proportional to density (which is ~ same for clastic particles) and size
(larger settle faster in both water & air). As current strength/velocity determine the load size it can carry, as it
decreases larger particles are deposited 1st, with smaller ones later etc., …
Abrasion = the decreasing size and increasing roundness of larger particles during transportation as they
tumble/strike each other or underlying rock.
Oceans as Chemical Mixing Vats:
The driving force of chemical and biological sedimentation is precipitation rather than gravity, as the materials
are part of the aqueous solution.
The inflow (rivers/glacial melt) & outflow (evaporation at the surface) of water in the oceans is balanced over
shorter periods of time, as is the entry (rivers/rain/wind/glaciers/hydrothermal reactions with basalt at MOR’s)
and exit of dissolved minerals (chemical/biological reactions = precipitation to the sea floor).
Therefore, Ocean Salinity = total amount of dissolved materials per given volume of water – remains
constant.
e.g., Calcium dissolves from limestone/calcium-containing rocks when they’re weathered and transported as
dissolved calcium ions (Ca2+) to the oceans. Here they’re taken in by organisms and combined with carbonate
ions (CO32-) to form calcium carbonate shells etc. This original calcium leaves the oceans when the organisms
die, and their shells fall to the floor and accumulate as calcium carbonate sediments, which later turn into
limestone.
Sedimentary Basins; The Sinks for Sediments:
Sediments transported downhill, accumulate in depressions in the surface, which are formed via subsidence
= depression/sinking of a broad area of crust relative to the surrounding crust, induced partly by the weight of
sediments on the crust but mainly by tectonic processes.
Sedimentary Basins:
= A region where the combination of sedimentation and subsidence has formed thick accumulations of
sediment and sedimentary rock.
1) Rift Basins
= A sedimentary basin that develops at a divergent (Continental Rift Zone) plate boundary at an early stage of
plate separation as the stretching, thinning and heating of the continental crust results in subsidence.
A long, narrow rift, bounded by down dropped blocks of continental crust, filled with thick successions of
sedimentary rocks, and intrusive/extrusive igneous rocks (from basaltic lava eruptions).
E.g., Rio Grande Valley, East Africa or Jordan Valley, Middle East.
2) Thermal Subsidence Basins
= A sedimentary basin that develops in the later stages of plate separation as the lithosphere that was thinned
and heated during the earlier rifting stage cools, becoming denser and subsides below sea level.
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