Weathering and Sedimentation in the Rock Cycle Our geology so far has focused on internally-driven processes: plate tectonics, magmatism, metamorphism, orogeny. The rest of geology is driven by surface processes: the hydrologic cycle (rainfall, streams, ice), gravity, aqueous chemistry. Weathering and erosion are the processes that form and transport form sediment. Sedimentation, burial and lithification are the processes that transform weathering products into sedimentary rocks.1

Weathering and Sedimentation in the Rock Cycle A more detailed view of the surface-driven parts of the rock cycle shows the various steps between source rock and sedimentary product

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Weathering: decomposition of rocks There is a distinction between weathering and erosion: Weathering converts exposed rock to soil in place Erosion transports dissolved or fragmented material from the source area where weathering is occurring to a depositional environment . Most of the earths surface is covered by exposure of sediment or sedimentary rock, by area. But the sediment layer is thin in most places, with respect to overall crustal thickness, so sedimentary rock is a minor volume fraction of the crust (in part by definition: once buried to the mid-crust, sediments get cooked to metasediments).3

Weathering: chemical and physical The destruction of rocks at the Earths surface by weathering has two fundamental modes of operation: Chemical weathering is dissolution or alteration of the original minerals, usually by reactions with aqueous solutions Chemical weathering puts ions from the source minerals into solution for subsequent erosion by transport in flowing water as dissolved load.

Physical weathering is fragmentation into progressively smaller particles, from intact outcrop to boulders and on down to mineral fragments and sand grains. Physical weathering makes loose pieces of rock available for downslope movement by mass wasting or transport in flowing water as suspended or bed load.

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Chemical Weathering Chemical weathering is driven by thermodynamic energy minimization, just like chemical reactions at high temperature. The system seeks the most stable assemblage of phases. The differences are that (1) kinetics are slow and metastability is common; (2) the stable minerals under wet, ambient conditions are different from those at high T and P; (3) solubility in water and its dependence on water chemistry (notably pH) are major determinants in the stability of minerals in weathering.

A fresh rock made of olivine and pyroxenes will end up as clays and iron oxides, with other elements in solution A fresh rock made of feldspars and quartz will end up as clays, hydroxides, and quartz in most waters.5

Chemical Weathering

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Chemical Weathering The most common alteration product of feldspars is kaolinite, Al2Si2O5(OH)4, which serves as a model for the formation of clays by weathering generally. The reactions of feldspars to kaolinite illustrate some of the basic trends: K, Na, Ca are highly soluble and readily leached by chemical weathering. Excess Si can be removed as silicic acid although quartz is relatively insoluble. Al is extremely insoluble, and is essentially conserved as source rock is converted to clays. Weathering is a hydration process, leaving H2O bound in the altered minerals.

2 KAlSi3O8 + 9 H2O + 2 H+ -> Al2Si2O5(OH)4 + 2 K+ + 4 H4SiO4 Note the H+ on the left-hand sideonly acidic water can drive this reaction Natural waters are acidic due to equilibrium of carbonic acid with CO2 in the atmosphere CO2 (g) + H2O = H2CO3

2 KAlSi3O8 + 9 H2O + 2 H2CO3 -> Al2Si2O5(OH)4 + 2 K+ + 4 H4SiO4 + 2HCO3 Alteration of rock transforms acidic rainwater into neutral surface or ground water, with bicarbonate the dominant species (relative to CO2 and CO32). Mg and Fe2+ are also readily leached, but Fe3+ is very insolublethe ultimate residue of alteration of mafic rocks is hematite.7

Chemical Weathering Knowing the chemistry of reaction of minerals to kaolinite, it is possible to reconstruct from the dissolved ions in stream water the amount of each source mineral that reacted with the water.

Questions: How do you do the correction for atmospheric input? Do the source minerals in the Sierra Nevada all weather at equal rates?

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Chemical Weathering Some minerals are congruently soluble in acidic water, leaving no residue The most abundant is calcite: CaCO3 + H2CO3 = Ca2+ + 2HCO3 (the Tums reaction) Effects of dissolution (and precipitation) of calcite can be dramatic, to say the least.

Sinkhole

Speleothems

Karst terrain

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Rates of Chemical Weathering Many factors affect the rate at which a rock will weather, as summarized here. Some of these variables are local (e.g., source rock), some are global. These include temperature and pCO2, leading to the CO2weathering feedback cycle.

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Physical Weathering Anything that promotes disaggregration of a rock so that pieces can form soil or be eroded away by wind, water, or gravity transport is physical weathering. The distinction between physical weathering and erosion is subtle, but think of physical weathering as fragmenting the rock and erosion as carrying the fragments away; at times these may be the same event, of course.

Rocks that are jointed or faulted or have pre-existing weak zones are most easily weathered. Few of the stresses associated with physical weathering are significant compared to the tensile strength of intact rocks; something, has to start the process, either initial cracks and weaknesses or chemical attack on mineral cohesion.

Organisms, especially plants (think tree roots), are fond of breaking up rocks. Freeze-thaw, frost wedging, frost heavethe volume change between ice and water is effective in widening cracks in rock in suitable climates. Physical abrasion by flowing air or water, or more often by rock particles already mobilized by water or wind (think Fossil Falls). Tectonicsrocks caught in a fault zone are definitely undergoing physical weathering. Etc.11

Weathering feedbacks: chemical and physical Physical weathering and chemical weathering generally proceed in parallel in most environments. Physical and chemical weathering promote one another: Formation of cracks by physical weathering increases reactive surface area, promoting chemical weathering. Chemical weathering replaces intact interlocking minerals with weak clays or void space, making the rock easier to physically disaggregate, promoting physical weathering12

Weathering feedbacks: more generally Weathering of both kinds plays key roles in several feedbacks. Tectonics affects weathering through slopes and elevations, climate affects weathering through temperatures (via chemical kinetics and freezethaw), rainfall, pCO2, etc. Conversely, weathering and erosion affect tectonics and climate: Denudation by erosion must be isostatically compensated and so affect vertical motions of the crust Weathering controls water chemistry, courses of streams and groundwater, removes CO2 from the atmosphere, etc.13

Erosion and Transport Between weathering and sedimentation, matter must be transported from source to destination. This is erosion. We dealt with the landforms generated by erosion in the geomorphology lecture; here our concern is with the effects of transport on sedimentary rocks.

Modes of transport: Gravity (short distances and steep slopes) Wind (small particles only) Glaciers Water Surface runoff carries dissolved, suspended, and bed loads Groundwater flow only carries dissolved load

All these mechanisms carry products of physical weathering and insoluble residues of chemical weathering. Only water transport carries away leached soluble products of chemical weathering.14

Erosion and Transport Certain modes of transport physically modify and physically and chemically sort particles en route. Size sorting by surface water runoff flow:Current of a given velocity can generally carry all noncohesive particles smaller than a critical size; since current velocity drops with decreasing slopes from mountains to lowlands, it follows that sediments evolve from poorly sorted and coarse-grained near source to well-sorted and finer grained with increasing transport distance.15

Erosion and Transport Chemical sorting with increasing transport distance is like a continuation of chemical weathering during intermittent times when particles are temporarily deposited before further transport; most stable minerals are transported the furthest. Textures of particles are modified by abrasion during wind or water transport. Close to source particles are angular; far from source particles are rounded.

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Sedimentation Eventually transported particles and dissolved ions reach a place where they can be permanently deposited and accumulated. This is sedimentation. The sedimentary rocks that result from this accumulation are controlled by and record the sedimentary environment where they were deposited. We interpret ancient sedimentary rocks by comparison to modern environments where we can observe ongoing sedimentary processes and relate them to the composition, texture, and structure of the resulting rocks.

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Sedimentation Sediments and the environments in which they form are fundamentally divided into clastic and chemical: Clastic sediments are made of physically transported and deposited particles (they may later gain chemically grown cement during diagenesis) Chemical sediments are grown from solution, organically or inorganically; biochemical sediment more specifically refers to minerals grown from solution by organisms

In some cases the relationship between the environment and the character of the sediment is absolute and obvious (carbonate in reefs, boulder-strewn till in periglacial deposit, etc.); other cases are more subtle.18

Diagenesis The process of modification of newly deposited sediments into sedimentary rocks is diagenesis or lithification. Processes include: physical compaction by the pressure of overburden, accompanied by expulsion of pore waters Growth of new diagenetic minerals and continued growth of chemical sediments from pore waters. Dissolution of soluble elements of clastic rocks. Recrystallization and remineralization as water chemistry, pressure, and temperature evolve.

At the high-T and P end, diagenesis merges smoothly into the low-T and P end of metamorphism. The distinction is arbitrary.19

Sedimentary Rocks The preserved end-result of weathering, erosion, transport, sedimentation, and diagenesis is sedimentary rocks. Like sediments and sedimentary environments, the resulting rocks are divided into clastic (or siliciclastic or volcaniclastic, etc.) and chemical (or biochemical).

Clastic rocks are classified by particle size (and sorting) and composition.

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Sedimentary Rocks Chemical sediments are primarily classified, of course, by mineralogical composition.

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Sedimentary rocks and environmental information How do sedimentary rocks preserve information about their depositional environments? By composition, mineralogy and grain size, obviously, but also through sedimentary structure

Elements of sedimentary structure: Bedding Bed thickness, from finely laminated to massive

Vasquez formation: massive 30 m 30 cm

Burgess Shale: fine

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Sedimentary structure Character of bedding, from simple horizontal laminae to cross-bedding, ripples, soft-sediment deformation, or bioturbated. Cross-bedding indicates high and unidirectional current velocity, often winds in terrestrial settings, forming sand dune lee-slopes.

Ripple marks record back-and-forth action by waves in shallow water.

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Sedimentary Structure Mud cracks demonstrate drying-out of a thin layer of sediment fine enough to have significant cohesion. Definite proof of terrestrial setting or very shallow water marginal marine.

MODERN What about this structure? (Hint: it is not the surface of the Moon)

ANCIENT

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Sedimentary Structure Soft-sediment deformation indicates slumping or compression of layers before complete lithification.

Bioturbation is the vertical mixing of sedimentary layers by burrowing organisms. Evidence of such activity can be preserved on bedding surfaces as trace fossils. Indicative of water depth, availability of nutrients and oxygen, etc.

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Sedimentary Structure Graded Bedding: sorting of particle sizes within beds indicates time dependence and hence process of deposition An environment in which a episodes of high-energy transport give way to periods of low-energy transport gives normal graded beds: Alluvial settings, with wandering channels that fill up and become overbank deposits Continental slopes with turbidity currents

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Carbonate Rocks Most carbonate rocks are entirely biochemical sediment, made up of the body parts of calcite or aragonite-precipitating organisms Deep-sea carbonate ooze is made of foram shells Reef carbonates are made of coral reefs (usually) Stromatolites are formed by carbonate precipitation by microorganisms

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Tour of sedimentary environments

Let us go through each of the major categories of sedimentary environment, keeping in mind the relationship between observable processes in modern settings and the preserved features in ancient examples, and the ways in which observation of a sedimentary rock formation can be used to infer the type of setting and detailed information about it.

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Why study clastic sediment? Sedimentary rocks make up only 7.9% of the Earths crust.

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66% of the surface of the Earth is covered by sediment or sedimentary rocks. Humans interact with the Earth largely at or near its surface.

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Sedimentary rocks record the history of changing environments on Earth. Based on the recognition of the signature of changing environments over time, as preserved in the rock record. Environmental interpretation of rocks + Age of rocks = Earth History

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Environmental interpretation: The present is the key to the past. By examining the characteristics of various environments on Earth today we can interpret the environments in which ancient sediments were deposited.

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Age of rocks: Based on relative age (relative to associated rocks) or absolute age (radiometric dating).

Earth History: The history of changing environments on Earth.

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