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Chapter 7 - Formation of Sedimentary Deposits

Introduction

Many gems and precious stones are extracted from sediments. Most gems originally formed in other geologic settings (igneous and metamorphic) and are transported by erosion to locations where they accumulate along with other materials. In some cases, the original source (host rock) of the gems may no longer exist or may not be exposed at the surface. The processes of weathering and erosion are important components to making allow gem-bearing sedimentary deposits to form. Sediments are solid fragments of inorganic or organic material that come from the weathering of rock and soil erosion, and are carried and deposited by wind, water, or ice. A few semiprecious materials (minerals, rock, and fossils) do form and occur in sedimentary rocks.

Weathering produces sediments, erosion moves sediments.
Erosion is the mechanical processes of wearing or grinding away materials on a landscape by the action of wind, flowing water, or glacial ice. Deposition is the process of sediments settling and accumulating from a moving fluid (wind, water, or ice). Once sediments have accumulated in a stable setting they can gradually undergo compaction and cementation to form sedimentary rocks Sedimentary processes and the sediments and rocks they produce are part of the Rock Cycle (Figure 7-1).
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The Rock Cycle
Fig. 7-1. Sediments, sedimentary rocks and sedimentary processes are part of the Rock Cycle.

Weathering

Weathering is the gradual destruction of rock under surface conditions. Weathering may involve physical processes (mechanical weathering) or chemical activity (chemical weathering). Biological activity can also result in weathering that can be construed as mechanical, biological, or both.

Weathering processes can begin long before rocks are exposed at the surface. This is true in most places on the earth surface where rocky outcrops (bedrock) is not exposed. In addition, weathering and erosion can take place simultaneously, perhaps most obviously in settings like rivers in flood, or waves crashing on a beach.

Mechanical weathering involves all processes that collectively break rocks into smaller pieces (see examples in Figures 7-2 to 7-6). Mechanical weathering includes all forms of mass wasting—a general name for processes by which soil and rock move downslope under the force of gravity. Mass wasting includes sudden events such as rock falls, landslides and avalanches—to long-lasting processes including slow movements of massive slumps or the slow creep of material down hillsides. Mechanical weathering takes erosional grinding as fast-moving flood waters moves boulders and sediments down stream valleys and where wave action batters rocks and sand along a shoreline. Rocks are shattered by earthquakes and volcanic explosions, the expand and split when erosion unloads overburden on compressed rocks that were previously deeply buried. Rocks will split when water freezes and expands in cracks. Rocks exposed on the surface are subject to expansion and contraction caused by daily heating and cooling (particularly effective in arid environments). Mechanical weathering is also caused by organic activity—the breakdown and movement of rock and soil caused by expanding tree roots, burrowing, feeding activity, etc.

The mechanical breakdown of rocks increases the surface area (per unit area) increasing the available surface area where chemical weathering can take place (see Figures 7-7 and 7-8.).

Chemical weathering involves the breakdown (decomposition, decay, and dissolution) of rock by chemical means. Dissolution is the action or process of dissolving or being dissolved, moving soluble components of materials into solution. Leaching is the process of dissolving and removing the soluble constituents of soil or rock near the land's surface. Water flowing under the influence of gravity caries dissolved materials away, ultimately adding to the saltiness of the oceans or deposited as salts, such a such as an inland desert basin.

Chemical weathering also involves other chemical reactions including hydrolysis, hydration, oxidation, and carbonation. Hydrolysis is the chemical breakdown of a compound due to reaction with water. Hydration is the process of combining with water to a molecule. Oxidation is the process of combining elements with oxygen ions. A mineral that is exposed to air may undergo oxidation. Carbonation is saturation with carbon dioxide (as soda water).

Decaying organic matter releases carbonation and organic acids that enhance the chemical reactivity between rocks and groundwater. All these processes are happening around us (Figure 7-8).

In most surface and near surface settings, mechanical and chemical weathering is taking place simultaneously. Much of it takes place in environments where wetting and drying periods take place. The chemical breakdown of rocks is most rapid in warm and humid climatic conditions persist. Mechanical weathering processes dominate in cold setting where daily heating and cooling, and freezing and thawing cycles occur frequently in winter months.
rockfall Gulkana Glacier, Alaska
Fig. 7-2. Gravity drives mass wasting. In this case, a rock fall, breaks big pieces into fragments. Fig. 7-3. Glaciers (moving ice) scours bedrock and carry away large quantities of sediment.
Waterfall Ano Nuevo beach
Fig. 7-4. Flowing water transports, grinds fragments, and erodes landscapes. Fig. 7-5. Wave action along shorelines grinds rocks into fragments.
fallen tree with soil and rocks in roots surface area
Fig. 7-6. Tree roots grow into cracks and expand; when a tree falls material is ripped up and made accessible to surface erosion. Organic acids released by decaying matter assist chemical weathering. Fig. 7-7. The mechanical breakdown of rocks increases surface area (per unit volume). Increased surface area increases the space for chemical weathering processes to take place.
Rusty old cars in Panamint Valley, California Chemical weathering is enhanced along fractures in the bedrock
Fig. 7-8. Rusting old cars illustrate the same chemical weathering process that break down rocks. Fig. 7-9. Chemical weathering is enhanced along fractures in the bedrock where water seeps through.
Different minerals weather in different ways. Minerals that form under high temperatures or high pressures may not be stable in the surface environment. Of the common rock-forming minerals, quartz is perhaps the most stable in the surface environment because is both hard and relatively insoluble in surface waters. In contrast, mafic minerals and feldspars have metallic elemental components (including Na, Ca, K, Fe) that can easily dissolve in water or react with oxygen or water to form minerals that are more stable in the surface environment. The feldspars, micas, and mafic silicate minerals ultimately break down to form clay minerals. Iron that does not dissolve will hydrate or oxidize, essentially become brown-colored minerals in soil (limonite and hematite). Rust that forms on an old car is mostly the mineral limonite (Figure 7-9).

Weathering Products of Common Minerals

Common Minerals Common mineral insoluble sediment soluble content
quartz (SiO2) quartz sand and silt silica (increases in hot water)
mafic minerals (olivine, pyroxene, amphibole, biotite) clays, hematite and limonite (rust) salts of Mg++, Na+, K+, Ca++; soluble iron- oxides Fe++, Fe+++
feldspars clays salts (Ca, Na, K)
gypsum (CaSO4) none Ca++. SO7--
Fig. 7-10. Common Minerals calcite (CaCO3) none Ca++, HCO3- (increases in cold water)
Weathering and erosion are continuous processes in the surface environment, enhanced by the presence of water (the "universal solvent"). Surficial deposits composed of weathered materials takes on a variety of forms (see Figure 7-11).

Regolith
is the name for a layer of loose rock debris resting on bedrock, constituting the surface of most land. Regolith can become soil with the introduction of organic residues and ongoing weathering. Once rock weather into sediments they can begin to move. Colluvium is a general term applied to loose and incoherent surficial deposits, usually at the base of a slope and brought their chiefly by gravity. Eluvium or eluvial deposits are accumulations of weathered rock fragments and soils that are derived by in-situ weathering (particularly leaching of soluble materials) or weathering plus gravitational movement and accumulation. The process of removal of materials from geological or soil horizons is called eluviation or leaching.

Once sediments are moved by water to a new location it is called alluvium—alluvium is general term for unconsolidated sediments deposited by flowing water such as on stream channel beds, flood plains, and alluvial fans. The term applies to stream deposits of "recent times." The migration of sediments from upland regions to the ocean basins can take a very long time. Sediments can eroded and re deposited many times along the journey. Deposition is the process of sediments settling and accumulating from a moving fluid (wind, water, or ice).

Sediments are classified into two groups: Clastic and Non-clastic

Clastic sediments are composed of fragments (detritus) derived from older rocks. Clastic sediments include gravel, sand, silt and clay (combinations of sand silt and clay are "mud"). Clastic sediments are classified by the dominant "clast" size in a deposit (a clast is a rock fragment); see Figure 7-12. Clastic sediments can become clastic sedimentary rocks (or often called "detrital sedimentary rocks").

Non-clastic sediments
are composed of materials precipitated from water or produced by biological activity (skeletal material, respiration, and excretion). Non-clastic sediments and sedimentary rocks include carbonates, evaporites (salts), coal, bone beds, and other organic and inorganic chemical deposits. (See below: Gems and Semi-precious Stones From Sedimentary Rocks of Non-Clastic Origin.)

How do sediments become sedimentary rocks?

Sediments can become "lithified" into sedimentary rocks once they've been deposited in a stable setting where burial, compaction, and cementation can take place. The processes, collectively called diagenesis, typically takes place slowly over time but rates depend on many factors including the chemistry of the sediments and groundwater passing through the sediment, and how quickly or deeply burial takes place. Deposits of unconsolidated sediments typically have high "porosity"—pores are open spaces between grains filled with gas or fluids (water or in some cases, petroleum). "Compaction is the process of gravitation consolidation of sediments, decreasing the volume of pore space between particles of sediment and increasing hardness. Cementation involves processes that harden sediments through the precipitation of minerals in pore spaces between grains of rock and mineral fragments, binding them together (Figure 7-13). Common minerals that form cement include quartz, calcite, hematite, magnetite, and clays. The cementing minerals are slowly deposited between grains by groundwater.

Sedimentary rocks
are exposed throughout the world, covering about half of the exposed land on the earth surface. This "sedimentary cover" was deposited mostly in coastal environments, in shallow seas flooding continental basins, on continental shelves and in ocean basins along the margins of continents; they are also exposed in mountainous regions (Figure 7-14). Most of these sedimentary rocks formed in the last several hundred million years. They contain all the oil, natural gas, coal, and many mineral resources essential to our modern world. They also preserve the "fossil record"—fossils, preserved in sediments deposited at the times when ancient life forms lived and died. Sedimentary rocks preserve stories of ancient environments and stories of changing landscapes through time.

The mile-thick sequence of sedimentary rock formations exposed by erosion in the Grand Canyon is an exceptional example of the sedimentary cover preserved on the North American continent (Figure 7-15). The sedimentary rocks exposed in the Grand Canyon represent sediments deposited in environmental setting ranging from shallow marine seaways to coastal sand dunes that accumulated long before the uplift of the Colorado Plateau region.
Weathering processes
Fig. 7-11. Weathering involves many processes occurring at or near the surface environment.
Classification of sedimentary rocks
Fig. 7-12. Clastic grain sizes
Cement fills in pore space between mineral grains
Fig. 7-13. Cement fills in spaces between mineral grains.
Map of the world showing the location of volcanoes
Fig. 7-14. Map of geologic provinces of the world.
Sedimentary rock formations exposed in the Grand Canyon
Fig. 7-15. Sedimentary rock formations exposed in the Grand Canyon.

Clastic Sediments and Clastic Sedimentary Rocks

Gravel is rock particles that have been moved by moving water. Gravel usually consists of a mix of the more durable and most abundant rock types in the sediment source areas (Figure 7-16). Gravel deposits typically occur along stream valleys close to mountainous source areas and along rocky coastlines with high wave action.

Conglomerate is a sedimentary rock composed of cemented gravel. It consists of rounded to subangular fragments (larger than 2 mm in diameter) set in a fine-grained matrix of sand or silt, and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay; the consolidated equivalent to gravel (Figure 7-17).
Gravel bar along Coyote Creek in Morgan Hill, California conglomerate
Fig. 7-16. A gravel deposit exposed in a dry stream bed during a dry season. Fig. 7-17. Conglomerate formed from an ancient stream gravel deposit.
Sand goes through degrees of refinement at it moves away from source areas. Sand deposits near mountain ranges may be enriched in feldspars. Volcanic regions may produce sand enriched in dark minerals. "Mature" sand that has traveled long distances in streams, blown by wind, or worked by waves will be enriched in quartz and individual grains will be very well rounded and well sorted (see below). Large sand deposit accumulate along stream valleys, on beaches, barrier islands, and offshore bars, and in dune fields in coastal areas and in desert environments (Figure 7-18).

Sandstone
is a sedimentary rock formed by the consolidation and compaction of sand and held together by a natural cement, such as silica, calcite, and iron-oxide minerals (Figure 7-19).
Quartz sand beach Outcrop of Navajo Sandstone near Tuba City, Arizona
Fig. 7-18. Sand is winnowed (sorted) and accumulates on a beach by wave action Fig. 7-19. Sandstone outcrops exposed in Utah's Canyonlands National Park
Mud is a general term lumping together sediments consisting of a mix of clay, silt, and may contain sand. Mud may be an unsorted mix of fine grain materials. Mud accumulates in quiet water setting where coarser materials have settled out elsewhere (Figure 7-20). Mud-rich accumulations are common in river delta regions, swampy coastal regions, tidal flats, and in lake and deep water settings.

Mudstone
is a fine-grained sedimentary rock formed from the compaction and cementation (litification) of muddy sediments rich in silt.

Shale is a soft, finely stratified sedimentary rock that formed from consolidated mud rich in clay minerals and can be split easily into fragile plates, such as along bedding plains (Figure 7-21).
Austrailian mudflats Shale exposed in Capitol Reef National Park
Fig. 7-20. Mud accumulates in quiet-water environments such as these tidal flats Fig. 7-21. Shale and mudstone outcrops in Utah's Capitol Reef National Park

Deposition and Depositional Environments

Deposition is the process of sediments settling and accumulating from a moving fluid (wind, water, or ice). Sediments will erode anywhere where forces associated with currents are strong enough to dislodge and move sediment particles. The "heavier" the particle, the more energy it take to move. Water is perhaps the primary mover of materials on earth. Wind typical can only move small particles of dust-to-sand-sized particles. Water can move particles of any practically any size if the current is strong enough. Moving glacier ice (though limited to mountainous regions and high latitude regions during Ice Ages) can move materials including house-sized blocks of rock. In general, as a fluid speeds up materials will erode. As the fluid slows down materials will settle out and be deposited. Over time, sediments will accumulate and possibly become buried, becoming permanently deposited until possibly exposed to surface erosion at some later period in geologic time. Places where sediments accumulated are called depositional environments. Depositional environments occur in many different geologic settings on land and under water (examples are illustrated in Figure 7-22).
Sedimentary environments
Fig. 7-22. Sedimentary depositional environments occur on land and under water.
 
"High-Energy" and "Low Energy" Depositional Environments: Flowing water is the dominant force causing erosion and deposition on Earth (with human mining and construction activity rapidly closing on that claim!). The faster the water moves, the "higher the energy" in a physical setting. As flowing water increases in speed, the more it may become turbulent, increasing its ability to lift and move particles. Fast moving water can carry materials of different particles ranging from boulders and gravel to finer materials (sand, silt, and clays). Flowing water also sorts sediments by size and density. "High-energy environments include river channels, beach and shallow offshore environments (Figures 7-23 to 7-25). Flowing water may let larger materials settle and be deposited while finer materials are carried away and deposited in "quieter" (low energy environments) (Figures 7-26 to 7-28).

Different sedimentary environments have different "energy" characteristics that may change from time to time. The forces of energy in a stream will increase as the volume of water increases, such as during flood. The same is true of beach and offshore bar environments. As wave energy increases, the greater the amount of energy translates into shoreline erosion and the moving of sediments to quieter and deeper offshore settings. Wave action separates sand from courser and finer fractions, building up or eroding beaches with changing conditions.

High Energy Depositional Environments

river beach environment reef
Fig. 7-23. River channels Fig. 7-24. Beaches Fig. 7-25. Coral reefs
Weather (climate) and wave energy are variable factors in "high-energy" environmental settings. Sediments are constantly being deposited or eroded in these settings.

Low Energy Depositional Environments

lake environment swamp Elkhorn Slough estuary
Fig. 7-26. Lake (lacustrine) Fig. 7-27. Swamp Fig. 7-28. Tidewater marsh
Slow-moving currents prevent coarse-grained sediment from migrating into in low-energy depositional environments. Fine materials can be carried long distances before they can settle out in the absence of waves and currents.

Unique characteristics of sedimentary deposits

Sediments preserve other characteristics that may tell information about the environment where they occur. Sediment particle shapes, degree of sorting, bedding characteristics are typically unique to different geologic settings.

Rounding of sediment grains

When particles are moved by running water they become rounded (Figure 7-29). The corners hit first and are worn down. The sharp edges are also pounded. The particles may become round boulders or pebbles. Bits of sand move with them. As the water slows the largest particles drop out first, making deposits of round boulders and pebbles called conglomerate. The smaller particles are swept away downstream (unless they are trapped between or beneath the large particles).
Roundness of grains
Fig. 7-29. Rounding of sediment grains: The farther a particle is moved, the more rounded and spherical it should become. Angular particles tend to be deposited close to their source (Image from Powers, 1959).
 

Sorting

This sediment moving process of running water sorts particles by size and to a lesser degree by shape. This is called sorting (illustrated in Figure 7-30). Sediments exposed to longer transport or exposure to currents and waves tend to be more sorted by shape and size.
Sorting
Fig. 7-30. Sorting of sedimentary particles.
The amount of sorting depending on the conditions and amount of time at which the stream works on the particles. Obviously particles of the same mineral that are more rounded and sorted have traveled further. The sediments sorting, roundness, and sphericity could act as a clue to following either modern or ancient alluvial rocks to their ultimate source. For example, very well sorted and rounded materials may suggest a source from an older sedimentary rock rather than from igneous rock. Sand from rivers and stream are very different from sands associated with beach and sand-dune deposits (see Figures 7-31 to 7-35).
sand from an upland stream is rich in feldspars River sand rich in quartz fragments Beach sand rich in quartz grains Beach sand rich in microfossils Dune sand
Fig. 7-31. Sand from a mountain stream may be rich in poorly sorted and angular grains of feldspars, quartz, and other minerals. Fig. 7-32. With erosional transport over long distances, river sand will become enriched in quartz as feldspars decay. Fig. 7-33. Beach sand is enriched in well rounded and well sorted quartz grains. Fine materials are winnowed out. Fig. 7-34. Beach sand in many tropical settings may be enriched in shell material, including microfossils. Fig. 7-35. Wind-blown dune sand is typically very well sorted and very well rounded, polished to frosted grains of mostly quartz.

Lamination and bedding

Sediments are deposited in layers ranging from paper-thin sheets to massive beds tens to hundreds of feet thick! A laminae (or lamination) is a layer of sediment or sedimentary rock layer only a small fraction of an inch (less than a centimeter) in thickness (see Figure 7-36). Thin lamination is typically associated with fine-grained sediments deposited in quiet or slack-water environments, such as in a lake basin or offshore below the influence of waves and strong currents. Bedding is the smallest division of a sedimentary rock formation or stratigraphic rock series marked by well-defined divisional planes (bedding planes) separating it from layers above and below (see Figure 7-37).
lamination bedding, Bisti National Monument, NM
Fig. 7-36. Lamination in shale. Each laminae my be an annual cycle of deposition. Fig. 7-37. Bedding exposed in a canyon carved in flat-lying sedimentary rocks.
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9/17/2014