Production of Mud and Silt High, wet mountains produce a lot of mud 2. 1 . Introduction Territories clay-sized material, less than 4 microns, comes mostly from the chemical weathering of rocks at the Earth’s surface, plus some contribution from volcanic ash and glacial rock flour. The origin of the silt component of mudstone is more controversial. Territories silt, 4 to 64 microns in size, has been thought by many to be largely the product of physical processes – fracture or chipping in transport, freezing and thawing, thermal expansion, exfoliation, release of confining pressure – all processes that favor size reduction.
Some territories silt may also be “born and not made”, when mudstone are deeply buried or become low-grade metamorphic rocks. Silt may also be formed biologically by the action of plants or animals to break up larger grains or to precipitate new silt-sized grains. Mudstone, especially from the Mesozoic and younger, contain clay- and silt-sized carbonate and fine siliceous debris of boogieing origin. These physical and biological processes are enhanced by the chemical transformation of parent materials, which releases both mineral particles and solutes from a rock. The flow diagram of Fig. 2. Incorporates many of Hess possibilities. The chemical processes that reduce mineral grains in size and transform primary minerals formed at depth into stable clay minerals are better understood both qualitatively and quantitatively than are the diverse physical and biological processes that produce silt-sized territories debris. This difference in understanding reflects the great fundamental difference in the two processes – clay minerals form from the slow but continuous transformations of the primary minerals to new, lower P-T phases at the Earth’s surface, whereas breakage and grinding, whatever their causes, understand.
Sources of territories mud and silt include soils; erosion of unconsolidated clays and silts by slope wash on interleaves; gully and stream bank collapse; volcanic ash, especially on convergent margins; the deflation of arid and semi-arid regions by wind, which deposits loess on land or silt and clay directly into a lake or ocean; and glaciations, which produces outcast and till on land and, at tidewater, marine fine-grained glacial deposits with dripstones. In addition, submarine weathering (Fee and MGM-rich sapient and celadon) and the formation of coagulation directly on the sea floor provide a small source.
S r—— Provenance Processes—-… Granites Volcanic BOOGIEING / CHEMICAL O) В«l r-“arcВ« … J Fragile : MIMICRIES’ . Row w I ‘:–4: SILT’ . ” SAND> ? Chemical Precipitates Robust Pellets Borings 2. 1 . Flow chart for the production of clay- and silt-sized territories and boogieing/ chemical debris 8 2. Production of Mud and Silt Of all of these, clay and silt-sized debris produced in soils, fines from volcanism, and glacial abrasion are the best understood and are the most important primary sources. To this add the weathering and erosion of ancient mudstone, which are abundant and disintegrate readily.
Above is a list of all the immediate sources. But where do these materials ultimately come from? Chiefly from the weathering of the two most common igneous rocks, granite and basalt, and from volcanic ejects. From these materials, feldspar, amphibole, pyroxenes and volcanic glass are the principal donors of clay minerals while the chief sources of silt-sized debris are quartz and feldspar, both ultimately derived from granites and gneiss. Thus silt is derived only from felsitic rocks, whereas clay is generated from both mafia and felsitic sources.
The rock cycle (Fig. 2. 2) is the ultimate control here – mud is reinstated to a basin, becomes indurate into mudstone with deep burial, and with advanced metamorphism, is transformed into a gneiss or granite. When uplifted and weathered once again, these crystalline rocks yield mud, silt, sand, and gravel and a new cycle starts. A smaller cycle exists at sea – the formation of sapient and celadon from basalt on newly formed mid ocean ridges and their later emplacement into accretion wedges and incorporation into magma along active margins. 2. 2.
Sedimentary Differentiation The conversion of granite, gneiss, basalt, or an arose into mud, silt, sand and gravel f variable composition is the result of sedimentary differentiation at the Earth’s surface. Sedimentary differentiation is broadly defined as all those changes during weathering and transport that segregate detrimental minerals by size and sorting – and thus by chemical composition. The concept of sedimentary differentiation is an old one, but in recent years we have come to understand much better what controls both its efficiency and the great contrasts in surgical mineralogy that occur at the Earth’s surface.
Sedimentary differentiation starts in the profile of weathering in humid limited (more than 50 CM of rainfall per year) and follows the sequence below (Charley 1989, p 22) Parent rock + Action deficient rainwater -+ Secondary clay minerals plus quartz, Fee and Mn oxides + Export of solutions rich in actions and dissolved silica The processes represented by this equation transform large primary minerals termed at high P-T conditions into tine-grained secondary clay minerals stable at low P-T conditions (see Box 2. 1 and Appendix A. ) so that finally insoluble minerals such as quartz, kaolin, and aluminum and iron oxides accumulate at the Earth’s surface. Virtually all these transformations occur in soils Appendix A. 3) or in alluvium as it sits in flood plains in transit to the sea. Although not explicit in the above equation, the residence time that the primary mineral spends in the zone of weathering is all-important – the longer this time, the greater the likelihood of mineralogical transformation between a mountain range and a distant basin.
There are also two other outcomes – weathering in semi-arid (10 to 50 CM rainfall) and arid (less than 10 CM) climates, where the above pattern does not apply. In semi-arid regions An+ and K+ are both mainly in solution, but Ca++, MGM*+, and Hashish only partially. Consequently, mineralogical transformation is less complete. In arid regions or regions with excess irrigation, many soils do not follow the above depletion equation because evaporation exceeds precipitation and calcite and evaporation minerals are precipitated to form crusts such as calculate and gypsters.
Sometimes and mixed layer clay minerals are also formed. This reverse process also occurs in low lying, poorly drained areas where actions and sill- Sources Aeolian dust – Lake or ? Sea – Soil an?glacial interleaves Chaw?Nell / / ash debris Minor submarine tightness and weathering Rock Cycle so . “”enter Differ,. ,. Sedimentary basin Segregation by size and mineral stability tab’s.. Uplift and erosion q. SSI Burial and Metamorphism ‘17,9 Crystalline Co mud and silt Fig. 2. 2. Simplified sources of territories (above) and rock cycle (below) 2. Production of Mud and Silt 9 Box 2. . Clay Minerals and Weathering Clay minerals (Appendix A. 3) chiefly form via the weathering of primary minerals in soils in the following way H+ + primary mineral intermediate clay mineral + solutions gibbets + solutions. Three examples of the above general equation are H+ + K-feldspar H+ + muscovite H+ + glass -7-7-7 elite elite smitten smitten kaolin kaolin Bessie, gibbets, gels (allophones) smitten Hollister kaolin gibbets. The needed H+ comes via release from water, which is facilitated by excess CA H2O O H+ +OH-, CA *H2O 0 WHICH 0 carbonic acid H+ + Hoc; . Carbonate ton Thus the more CA dissolved in the water (supplied by bacterial respiration), the faster the weathering process. Another factor, and probably more important, is the total flux of water through the soil system; the larger the flux, the greater the tendency for these reactions to move to the right, a process that always converts complex crystal structures into simpler ones. Conversely, with minimal H+ all these sections are sluggish or stall. This H+ or its proxy, rainfall, is a key underlying driving force in weathering and the production of clay minerals.
Two other factors are time – with enough time even slow reactions go to completion – and, of course, starting materials in the source rocks. Hence, clay mineral compositions depend on residence time, rainfall, and source rocks. The above reactions tend to be reversed during burial, for example elite and quartz form at the expense of smitten (Chap. 6). IAC leached from uplands are added back to the clay mineral lattices. 2. 2. 1 . Residence Time, Relief, and Rainfall The interrelationships of residence time, relief and rainfall are well displayed in a simple matrix (Fig. . 3). The tectonic stability of the site of weathering is the key factor that controls residence time. There are two end members – sat blew cartons and passive continental margins tort one pole, while convergent margins form the other. Cartons and passive margins typically have low relief and relatively gentle slopes – except for some residual plateau-type mountains and some marginal, rift-related, escarpments along coasts like Brazil. Convergent margins, by contrast, have high relief and high summits plus steep instable slopes and deep valleys.
Compound- Wet highlands Major donor -large volumes of silt and mud with expandable and layered clays in suspension plus large volume of diverse chemical species in solution Minor to moderate donor – small to moderate volumes of mud and silt plus suspended and chemical loads of diverse composition. Minor export of Aeolian clay and silt Wet lowlands Minor donor – small volumes of mud with gibbets and kaolin in suspension plus restricted chemical species in solution Dry highlands Dry lowlands Negligible donor except for Aeolian export of clay and silt of diverse compositions.
Appreciable export of Aeolian clay and silt Fig. 2. 3. Simplified 2 x 2 matrix of role of highlands and lowlands (relief) versus rainfall (wet/dry) on sediment production. Although other factors exist, these are the most important 10 2. Production of Mud and Silt Fig. 2. 4. Schematic of revering silt and mud (coarse stipple) and loess derived from High Asia (After Salary et al. 1998, Fig. 5). Published by permission of the authors and Elsevier Science inning this effect is a high proportion of mechanically unstable volcanic ash and hydrothermal altered volcanic rock.
Continent-continent collisions produce the iratest volume of mud. High Asia, the product of the India-Asia collision, is a superb example of a great source of detritus, especially fines (Fig. 2. 4). Convergent margins are of three types: ocean-ocean (island arcs), continent-to-ocean (Andean) and continent-to-continent (Himalayan) margins. In all three, uplift (sustained by statistic refinancing) is rapid and occurs chiefly by faulting. Consequently, erosion is rapid and large volumes of detritus are generated over long time spans, where climates are wet.
Under these conditions, bedrock materials in the zone of weathering have short accidence times and detritus is composed mostly of mineralogical immature debris. The great contribution, about 70%, of southeast Asia to the world’s total erosion output (Mailman and Dade 1983′ Fig, 4) is explained by its high relief and steep slopes combined with high monsoonal rainfall. Here large rivers such as the Ganges, Apparatus, Mekong, and Roadway play important roles, but it is also easy to visualize a simple Andean margin with high monsoonal rainfall and many short rivers contributing large volumes of mud directly to a sea or ocean.
An example would be the Cascade and Olympic Mountains of the Upset Sound area. Met. Rainier is a prodigious producer of fingered detritus. Not only do its eruptions generate large volumes of ash, but hydrothermal activity on the flanks of the mountain produces a severe alteration of the original coarse-grained volcanic rocks into unstable, clay-rich deposits that are a major contributor to the giant mudflows the area has experienced, some of which have reached the sea Cohn et al. 003). On the other hand, on stable cartons and passive margins, uplift is mostly either proceeding or sea level controlled. Except for a few areas of active rifting in narrow belts along continental margins or within the carton, erosion rates re slow, residence times are long, and non-glacial soils are well developed and millions of years old. Here sediment output is low and, in tropical climates, such landscapes will yield mostly quartz, kaolin, gibbets and Fee and Mn oxides (Fig. . 5) – the most stable end products of weathering (Edmond et al. 1996). Thus in the absence of glaciations on cartons and passive margins, a chemical landscape will prevail as long as rainfall exceeds evaporation and there is ample water to flush through the soil system. Consequently, such penalized, non-glaciated, low-relief rattans and passive margins are insignificant sediment donors, except where they are crossed by a large river sourced in far distant, bordering mountains.
This is well illustrated in Brazil at the Junction of the Colonies and ROI Negro Rivers at Unmans, Amazons, where they Join to form the Amazon River (Fig. 2. 6). The Colonies, sourced on the steep, high-rainfall, eastern slopes of the Andes, is always brown and turbid because of its large suspended load whereas the ROI 11 Fig. 2. 5. Thin literate developed over leached quartz sand in Bah, Brazil Q) Quill . N. Banded iron formation 2300 OZ O o. Q) OZ Cell 2500 Cell
