4: Provenance of Siliciclastic Sediments

where do sediments come from, weathering/erosion

what does provenance mean? what else?

where do the sediments come from, reflected by weathering and erosion

what is sediment made of?

loose solids at the surface

what can loose solids at surface mean?

fragments of rocks, broken off minerals, shells, mineral crystals precipitated out of solution

how are fragments formed?

weathering from pre-existing sediments or rocks

what happens to fragments?

transported by air water or ice, deposited by settling, precipitated from solution, secreted by orgs

what do sediments form?

unconsolidated deposits on the earth’s surface

what sedimentary rock will gravel bar turn into?

conglomerate

what is bedrock?

solid rock underlying solids and surficial sediments at the surface

what is weathering?

at site breakdown of existing rocks or sediments by physical disintegration and/or chemical decomposition

what is an assumption of weathering?

exposure to atmosphere

what is erosion?

transport of weathering products away from their site of origin

what are the two modes of weathering?

physical and chemical

what is physical weathering?

disintegration of solid parent rock into smaller, loose pieces by various mechanical processes

what is chemical weathering?

decomposition of solid parent rock into dissolved or altered constituents or new compounds

what does chemical weathering occur most in?

aqueous solutions

where might chemical weathering be slow?

arid, dry environments, desert

what are examples of physical weathering?

freeze-thaw, exfoliation, roots, salt displacement

what is freeze-thaw?

frost shattering, water permeates into rocks through pore spaces or pre-existing cracks, then freezes and stresses the rock until it cracks

what is exfoliation?

rock breaks up into a thin surface, shedding

what is exfoliation most common in?

intrusive igneous rocks

what is a factor of root action as weathering?

root exchanged water with surroundings, affecting the rock and facilitating reactions on the surface and within the rock

what is salt displacement?

salt crystals expand as they grow, which will displace the material until they pop out of the ground

what are the types of chemical weathering?

simple solution, hydrolysis, oxidation and reduction, hydration or dehydration

what is simple solution?

minerals go into solution completely, without precipitation of other minerals

what is hydrolysis?

reaction between H+ and OH- ions of water and ions of silicate minerals, controlled by pH, minerals are partially dissolved, and mineral is produced

what is oxidation and reduction?

loss of an e- under oxygenated conds, gain of e- in water saturated/oxygen deficient conds below the water table

what is hydration and dehydration?

add or subtract water from existing minerals

what can microbes do in relation to rocks?

they can induce reactions that transform minerals/rocks or produce organic substances that react to decompose minerals/rocks

what is chelation?

removal of cations by forming org molecules with ring structures, these molecules keep ions in solution, the org molecules are soluble and wash away

what is karst topography? what is it the result of?

large mostly linear cracks from limestone dissolution, can be dolomite or gypsum, forms sinkholes, caves, and underground rivers

what is this? extra info?

conversion of feldspar to kaolinite, feathery plates are kaolinite, feldspar has the twinning and cleavage

what are the extremes of bowen’s reaction series? (sand/silt)

olivine is least stable, quartz is most stable

what are the extremes of bowen’s reaction series? (clay)

gypsum/halite is least stable, anatase/titanite/rutile/ilemenite/zircon is most stable

are clay minerals usually stable at the surface?

yes, if sediment has clay in it, it will stay, if it starts as other mineral, it will turn to clay

what does more surface area mean?

reactions can proceed more quickly

what are soils made from?

accumulated chemical and physical weathering of underlyng bedrock or sediment

what are peds?

the characteristics way that a soil crumbles, the pieces

what is the order of soil horizons?

O, A, E, B, C, R

what is the A horizon?

organic mixed with minerals

what is the E horizon?

bleached, organic matter and minerals are in solution, they flow right by

what is the B horizon?

enriched in organic matter, oxides, or clays

how is the B horizon enriched in organic matter?

the material is formed in upper layers turn into solution, then precipitate in B layer

what horizon is the weathering front in?

C

what do the soil horizons depend on?

climate, rainfall, type of substrate, slope

how does type of substrate affect soil horizons?

will affect downward movement of the weathering front

how does slope affect soil horizons?

steeper slope means soil growth progress will be washed away downhill

how does humidity affect soil horizons?

may cause intense leaching by organic acids resulting in silver and iron oxides precipitating, thick A and B horizons

what is the surface above the unconsolidated sediment? why?

weathering front, soil processes are weathering into the unconsolidated sediment

what is the surface below the unconsolidated sediment? why?

angular unconformity, bedrock is tilted, it was buried and tilted as it came back up, there is irregular erosion after this bedrock came back up

what is a possible story behind this?

the boulders in the sediment could have caused the irregular erosion in the bedrock, from a very fast flow, then the sediment got deposited and soil started forming on top

what are the controls on weathering?

chemical, physical, water chemistry, water availability, temp, climate, geomorphology

what are the controls on erosion?

wind, physiography, slope, flow velocity, vegetation, climate, fluid density

how is climate a control on erosion?

fast cold river in the north, transport by ice, frost wedging

what is a control by structure/texture of bedrock?

presence of fractures, rock cleavages, exposed surfaces

what is a control by drainage and topographic relief

the extent and direction of chemical weathering and erosion

what are the mechanisms of erosion that are gravity driven?

wind currents, rock fall, landslide, slump, debris flow, turbidity current, sheet wash, rills, gullies, streams, tidal currents, waves, oceanic currents

what can vegetation do as a control on erosion?

increase stability, decomposition of plants in soil, roots react with soil, induce chemical reactions

what are the 2 possibilities for uplift and erosion

once you erode a surface, physiography will result in uplift as compensation, or if there is uplift, then erosion will act on that area more than non-uplifted areas

what type of rocks are most likely to be sources for rock fragments in a sandstone?

finely crystalline volcanic rocks because they can yield sand-sized fragments

how will coarse crystalline structures with multiple minerals break apart?

will create smaller fragments containing only individual minerals

how will fine crystalline structures with multiple minerals break apart?

will create smaller fragments containing multiple minerals