Source of sediment: weathering and erosion
Sedimentary accumulations and recycling
The maximum known thickness of sediments deposited per unit time increases with decreasing age, and estimates of the volume of sedimentary rocks of different ages also show an exponential decrease with age. These trends are independent of the measure used: aerial extent, estimated volume, or maximum thickness.
This pattern was initially interpreted to mean that sedimentation rates have increased over geologic time due to:
• lower stands of sea level and consequent greater continental relief (Barrell, 1917)
• increased tectonic activity (Stille, 1936).
However, Gilluly and co-workers, in a series of papers from the late 40s onward, emphasised the effects of cannibalistic recycling and proposed that the observed thickness versus age trend reflects progressive erosion and recycling and decreasing preservation potential of older sedimentary rocks. Recycling accounts for a substantial proportion of sediment production. Estimates vary depending on the parameters used and the assumptions made, but they range from 65% to than 95% (reviewed in Cox, 1992). The "real" value will vary from age to age and place to place, but the average is probably somewhere between the two end-member extremes.
In addition, there is the "cover" issue: we are less likely to see preserved seds. that are older because they are buried. Also, older sediments are more likely to be deformed, metamorphosed, or melted.
The secular "megatrend" in the global sedimentary pile, in summary, seems to be a result of differential recycling within the sedimentary pile and exchange between the sediments and the crystalline or "primary" crustal rocks, that is, the plutonics, volcanics, and metamorphics that have insiumated or intermingled themselves with the sedimentary pile. Planetary evolution–progressive, irreversible changes in the earth’s atmosphere, hydrosphere, and biosphere, and presumably changes deep within the mantle and core as well–has modified and contributed to the secular megatrend. Superimposed on this, finally, are the local and shorter-term, though not necessarily less pronounced trends reflecting tectonic and geomorphic cycles, and other trends and cycles, long or short, that the earth experiences because it is part of the cosmos.
What are sedimentary rocks?
Sedimentary rocks form from material that is sedimented. Some small fraction of the sediment, such as coal, is wholly organic. Most of the rest is inorganic (although much of it is biochemical), and the constituents of sediment, whether mineral grains or biochemical mineral skeletal material, derives ultimately from the weathering of pre-existing rocks.
Sediment types
These are end-member classifications. Many rocks have elements of more than one sediment type.
Clastic: detrital material. Sand, mud, gravel etc. consisting of fragments of source rock and weathering products, mainly clay minerals. about 70-80% of all sediment.
Biogenic: shells and skeletons. Biologically precipitated minerals from chemicals in solution: phosphorites, carbonates, cherts
Chemical: direct chemical precipitates. Evaporites are the most important.
Volcaniclastic: or volcanogenic: fragmental volcanic material.
Where does sediment come from?
The ultimate source of all sedimentary material is crystalline rock of the crust. The continental crust dominates the sedimentary system although it represents only about a third of the area of the planet. Most of the material in circulation in sedimentary systems at the present time is derived by the erosion of pre-existing sedimentary rock, and much of the material in the rocks being eroded was in turn derived from older sedimentary rock, but eventually you get back to an ur-source of crystalline basement material.
Rocks are weathered and fragmented, and then they are eroded, or transported.
Products of weathering
Rocks at the surface of the earth, whether igneous, metamorphic or sedimentary, are broken down by a combination of physical and chemical weathering to produce:
Residues These are particles of the souce rock, and include rock fragments and mineral grains. These are the constituents of sandstones and conglomerates.
Secondary minerals Mainly clay minerals and oxides, produced by reactions between the primary minerals and water. These are the major constituents of mudrocks, although most muds also contain residual minerals grains (esp. quartz)
Solutes Soluble matierials released by weathering reactions. These are transported away from the source in solution, and are eventually deposited as evaporites or carbonates. They may (eg. Ca, Si) be extracted from the water by marine organisms and used to form skeletal material, in which case they may be deposited as biogenic cherts or limestones.
These materials have different physical properties, and so they tend to be transported and deposited by different mechanisms. This results in fractionation, so that the various sediment types tend to accumulate in different environments. Of course this is not an absolute fractionation: evaporites can form in arid inland basins, and coarse clastics can be deposited in the deep sea; but the majority of sediment is fractionated in this manner.
Mechanisms of Weathering
Weathering is the process by which rocks are broken down at or near the surface of the earth. There are two broad classes of weathering. They usually act together, but they are sufficiently different that they can be discussed and broken down separately.
Physical weathering
This is the process by which rocks are mechanically broken down into smaller fragments, without any significant change in chemical composition. In very cold or very arid climates, this is the dominant form of weathering.
Pummeling Entrained particles in flows (water, air or gravity) impact and bombard the substrate.
Air expansion: As waves advance against a cliff, they compress air that is trapped in rock cavities at the base. As the wave recedes, the air expands explosively, stressing the rocks, which ultimately may shatter.
Frost wedging When water freezes to ice, there is a volume change of about 9%. Water percolates into cracks and then freezes. When this happens repeatedly, the stress results in fracturing of the rock. This is the process that produces most scree on mountain sides: large angular blocks. In combination with chemical weathering, it can result in granularisation of granites: production of gruss.
Root wedging Roots of trees and shrubs growing into cracks and joints in rocks can exert sufficient pressure as they grow and thicken to crack the retaining rock.
Salt wedging In areas where salts are concentrated, esp. deserts and coastal areas, saline brines percolate into fractures in rock. Salts that crystallise out of these brines in confined spaces under pressure, exert a stress on the rock. Crystal growth stresses are dependant on rock porosity, and are especially effective in porous sedimentary hosts, where they can result in disaggregation of the rock along grain boundaries.
Diurnal T. changes It has been been postulated that daily temperature changes, causing differential expansion and contraction, could result in rocks fracture, especially in desert areas where the range in daily temperature can be very large. The basis for this is that rocks are poor conducters of heat. Therefore surfaces esposed to the sun's rays will expand more than the rock just a few cm. below the surface. This should lead to sheeting of the exposed surfaces, producing such phenomena as spheroidal weathering and exfoliation. Cited in support of this are the cracked and split rocks that are found following forest fires.
However, most studies indicate that if this process occurs, it is minor in its effects. Studies of antiquities in Egypt (where daily T. variations are very large) especially of toppled granite columns, indicate that those surfaces exposed to the sun are the least weathered of any of the surfaces (buried surfaces are most weathered). Also lab. studies (alternately heating and cooling rocks) produce very little change over hundreds of repetitions.
Pressure release As rocks are uplifted to the earth's surface, the overburden that lies above them is stripped away by weathering. The removal of the overburden weight causes the rock to expand, and a common result of this in isotropic rocks (notably granites) is the development of flat joints or fractures that are sub-parallel to the topographic surface. This is the process that (in association with chemical weathering) produces the massive granite domes of Yosemite, and Ayer's Rock in Australia.
Thermal contraction As rocks are brought from the warm depths to the cool surface they become cooler, and they contract. This is probably the mechanism by which joints form.
Chemical Weathering
Chemical weathering has been defined as the process by which a system involving rocks, water and air approaches equilibrium at or near the surface of the earth
Chemical weathering is the main mechanism by which sediment is produced. Rocks decompose at or near the surface of the earth largely by a series of chemical reactions, most of which involve water. Moisture speeds up the process of weathering enormously: compare the inscription on a 100-year old New England gravestone with the inscriptions on 3000-year old tombs in Egypt!
(1) It takes part directly in the chemical reactions
(2) It transports oxygen, carbon dioxide, organic acids and nitrogen acids to the surfaces of joints and fractures where the weathering reactions occur
(3) It removes the products of weathering to expose fresh rock surface so that weathering can proceed.
Mechanisms of chemical weathering
Dissolution this simple chemical process is the chief reaction that controls weathering of limestones, marbles and evaporites.
CaCO3 + H2CO3 = Ca2+ + 2HCO3-
Dissolved carbon dioxide in water forms a weak acid:
H2O + CO2 = H2CO3
This is why rainwater is such an efficient weathering agent.
Oxidation oxidation is the major weathering process for many non-silicate minerals, eg. pyrite:
2FeS2 + 7.5O2 + 4H2O = Fe2O3(s) + 4SO42- +8H+
The products are a new substance, iron oxide (haematite or limonite) and soluble sulphate.
NOTE: this reaction produces large amounts of acid: 8 moles of acidity for 2 moles of pyrite. This is the reaction behind acid mine drainage. The oxidation of the metals causes them to react with the water in a hydrolytic fashion, and this causes the acidity.
Oxidation of metals in crystal structures eg Fe2+ = Fe3+ disrupts the electrical charge balance in the structure, which promotes cation loss by other reactions to maintain neutrality, and this makes the mineral more susceptible to weathering by other mechanisms.
Hydrolysis The word means "water splitting", and this is the single most important reaction in chemical weathering. The dissolution and oxidation reactions described previously also involve hydrolytic steps. Hydrolytic reactions produce an excess of H+ or OH- ions in solution. We won't go into the chemistry of the reactions in detail, except to say that they involve reaction between water and mineral grains, the products of which are ionic complexes and new minerals (usually clay minerals and oxides).
Hydrolysis of non-silicate minerals is typified by the weathering of magnesite:
MgCO3 + H2O = Mg2+ + OH- + HCO3-
Simple silicate minerals also hydrolise to form simple ions. Forsterite (Mg-rich olivine) is a good example:
Mg2SiO4 + 4H2O = 2Mg2+ + 4OH- + H4SiO4
Silicate minerals that contain Al have more complex hydrolysis reactions that generally result in the formation of clay minerals as products. The simplest example is the hydrolysis of K-feldspar to produce kaolinite:
4KAlSi3O8 + 22H2O = 4K+ + 4OH- + 2 Al2Si2O5(OH)4 + 8H4SiO4
The effectiveness of silicate weathering reactions is increased by acid. Can write this reaction:
2KAlSi3O8 + 9H2O + 2H+ = 2K+ + Al2Si2O5(OH)4 + 4H4SiO4
This relates to Goldich’s comments on the effect of CO2 on feldspar hydrolysis.
Clay minerals (hydrated Al silicates) are the main secondary product of weathering of silicate rocks, and the major constituent of muds, and this kind of reaction is how they are produced.
Note that this reaction illustrates several features which are absolutely characteristic of weathering products:
(1) the solid residue is significantly more aluminous than the parental material
(2) lone ions are produced and generally removed in solution, to end up in evaporites or limestones, in bones or shells, or in diagenetic minerals elsewhere
(3) there is an excess of silica, which may be precipitated as collodal silica, participate in diagenetic reactions, or be incorporated into organisms
Biologic weathering
The development over geologic time of an extensive biological blanket has had a large effect on weathering of rocks, because biologic activity enhances greatly the inorganic weathering process.
Root wedging is estimated to be second only to frost wedging in importance as an agent of mechanical weathering. Not just tree roots: lichen hyphae are gelatinous, and expand and contract as they are wetted and dried, which has been shown to cause shale disintegration.
Most organic effects however are chemical. Worms burrowing in the soil ingest and process mineral grains, which are chemically attacked in thier guts. Also by loosening and overturning the soil, they promote the free passage of water, which enhances chemical weathering. Some land areas have as much as 107 worms/km2, so their effect is significant.
Experiments have shown that mineral decomposition (albite and muscovite) proceeds almost twice as fast in soil that has bacteria present (control: sterile soil).
Most of the earth's surface is within the biosphere, and therefore most soils contain abundant bacteria, root filaments etc. All aerobic organisms (including plants) produce CO2 during respiration, which acidises soil. Many plants excrete organic acids. Decomposition of organic material also produces organic acids.
Rates of weathering
The rate at which rock material weathers depends on:
Composition Different minerals have different chemical stabilities at the earth's surface. The general rule is, the higher the T and P at which the mineral crystallised, the more it is out of equilibrium with conditions at the earth's surface, and therefore the more susceptible it is to weathering. However, this is not an absolute rule: calcite forms quite readily at the earth’s surface, but also weathers rapidly; and diamond crystallises at mantle pressures and temperatures but is among the most durable of materials. The weathering sequence for common rock-forming minerals was determined by Goldich (1938). It is an empirical sequence, based on field observations of differential weathering, but it is the exact inverse of Bowen's reaction series for mineral crystallisation in magmas.
Grainsize Fine grained rocks provide more grain surface area for chemical reactions to proceed, and therefore weather faster than fine-grained rocks.
Climate Chemical weathering processes are more rapid and effective in warm, wet climates than in cold dry ones.
(1) Weathering reactions depend on abundant water and therefore proceed more rapidly when there is a bountiful supply.
(2) Weathering reactions are favoured by increased temperature: a general rule is that reaction rates double for a 10° C increase in temperature.
Chemical weathering is very retarded by cold, and polar weathering consists almost entirely of frost-heave-generated talus, with very little clay mineral or oxide production. In the humid tropics however, rocks are commonly deeply weathered and may be completely converted to clays and oxides down to depths of 10s of meters below the surface.
Estimates of the time required to weather 1 mm of fresh rock to a kaolinitic saprolite
Felsic Tropical semi-arid 65-200 yrs
Tropical humid 20-70
Temperate humid 41-250
Mafic Tropical humid 40 yrs
Temperate humid 70 yrs
Ultramafic Tropical humid 21-35 yrs
This table integrates across composition and climate.
Slope Several studies have shown that the steepness of terrane is a primary control on the extent of weathering. Sediment on steep slopes is quickly washed off into depocentres. Its residence time in the weathering system is short, and there tends to be little compositional alteration. Sediment on flat ground moves much more slowly from point of origin to depocentre, and is much more efficiently weathered. Johnsson et al. have shown that pure quartz arenites are produced in a single sedimentary cycle from basement source rocks where tropical weathering opperates on flat plains.
Biologic activity The amount of plant and bacterial activity plays a strong role in the rate of chemical weathering.
(1) There are more organisms per unit area in tropical environments than in temperate, and that plays at least as large a role as the increased temp. and humidity on driving the very efficient chemical weathering that characterises the tropics.
(2) The amount of biologic activity is also a function of time. Through much of the Phanerozoic, plants and animals have been widely distributed in terrestrial ecosystems, but prior to that they were not.
Changes in composition
Progressive weathering tends toward equilibrium. A completely weathered sediment will consist of quartz and clay minerals. The most intensely weathered material will have clays that are aluminous, and contain no other cations.
Changes can be monitored by comparing mineralogy
Also by comparing chemistry (clays usually)
Concept of immobile elements. Compare weathered and fresh, or assume Al. constant, and recreate original comp.
Products of weathering
Rocks at the surface of the earth are broken down by a combination of physical and chemical weathering to produce:
Residues These are particles of the souce rock, and include rock fragments and mineral grains. These are the constituents of sandstones and conglomerates.
Secondary minerals Mainly clay minerals and oxides, produced by reactions between the primary minerals and water. These are the major constituents of mudrocks, although most muds also contain residual minerals grains (esp. quartz)
Solutes Soluble matierials released by weathering reactions. These are transported away from the source in solution, and are eventually deposited as evaporites or carbonates. They may (eg. Ca, Si) be extracted from the water by marine organisms and used to form skeletal material, in which case they may be deposited as biogenic cherts or limestones.
Calculations show (Garrels and MacKenzie, 1971) that complete weathering of "average" continental crustal igneous rock (ie, granodiorite composition) produces these three components in the ratio
11 : 74 : 15
sandstone:mudstone:limestone
The actual makeup of the sediment mass on the globe (calculated by me)
20 : 58 : 22
sandstone:mudstone:limestone
The difference in the two sets of figures reflects a number of factors, the two most important of which are probably
(1) Weathering of rocks at the surface does not generally go to completion: rock fragments and non-quartz mineral residues are common in sedimentary rocks
(2) Most mud eventually gets deposited in the ocean basins. Of that, some significant but unknon fraction gets subducted, which will pull down the mudstone fraction.
Weathering in the early Paleozoic and Precambrian
The land surface before the Ordovician would have been a very unfamiliar-looking place.
"It's difficult to imagine land without life, because today even the harshest deserts have some of it. But for about 4 b.y., that's the way it was." (Van Andel, the First Green Spring).
From the time that the first continents were formed, probably about 4 by ago, until the early Ordovician, about 500 my ago (base of Ordovician is about 510 m.y.), life was confined to the oceans. The land surfaces were barren, bare rock. Probably no soil even, because the dirt that we are so used to see covering the lithosphere has a large proportion of organic material in it.
We believe that the first land plants evolved in the Ordovician, based on the some fossil spores, but the first actual land plant fossils are found in Silurian rocks. However, it wasn't until the Devonian that the terrestrial environment was well-colonised by plants.
Without plants there were probably no true soils, because most of what we consider soil has a very high organic component. Early Paleozoic and Precambrian regolith would have consisted solely of mineral matter: sand silt and mud. As a consequence, weathering was probably much less effective. Also, without roots to bind and hold soil in place, the transport of grains was probably more rapid. In modern environments, grains often get held up for long periods in alluvial plains or other environments. These long residence times in soils also give longer time for chemical weathering to occur.
So all in all, the average weathering grade of post-Silurian terrestrial sediment is probably significantly greater than that of pre-Silurian sediment.
The importance of water
Water is the main agent of weathering: freezing and thawing of water splits rocks apart; water as a liquid or a gas reacts with their constiuent minerals; and water nourishes the plants and other organisms that produce acids which promote chemical decomposition.
Submarine weathering
The weathering of submarine rocks occurs by the interaction between seawater and rocks of the ocean floor. It is most pronounced near the mid-ocean ridges, where agtive magma chambers close to the surface drive efficient hydrothermal circulation systems that pump water through the rocks of the ocean crust. The hot brines react with the mafic minerals and alter or weather them. This is a very important process, but is is not related to sediment production, so we will not consider it further.