Chapter 6 (Chapter 7 – current textbook) Sedimentary RocksSediments + Transport = Ocean Deposition

Weathering & Soil ReviewRocks + Weathering + Erosion = Sediments

The history of Earth’s Internal Processes is recorded in the Igneous and Metamorphic rocks.

The history of Earth’s External Processes is recorded in the Sedimentary rocks.

After rocks are degraded by weathering, the weathering products (sediments) are transported by river systems and then deposited in ocean basins. Longshore currents distribute sediments along the coast forming the continental shelves. Wave action separates the sediments by grain size. As more sediments accumulate, compaction + cementation = sedimentary rock.

A few sedimentary rocks are preserved from lakes and other continental environments, but most were deposited on continental shelf environments.

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Processes that produce sedimentary rocks:Diagenesis – Changes that occur after deposition, includes Recrystallization, Lithification, Compaction, & Cementation.

Recrystallization – Less stable minerals change to more stable, e.g., Aragonite becomes Calcite. Mild heat and pressure may also cause recrystallization of existing minerals grains.Lithification – sediments become sedimentary rock through compaction & cementation

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Compaction – Overlying weight causes downward pressure, closing pore spaces between particles, decrease in volume. Clays may lose 40% of pore space with compaction. This parallel packing of clay “plates” decrease porosity & permeability (the ability to pass liquids).

4Clay “plates”

Cementation – Ground water + dissolved minerals percolate through compacting sediments. Minerals are deposited along grain margins until grains become cemented. Fe oxide, silica, and Calcite are most common cements. Cement

5Sand grains

Most common cements for sandstones are silica, iron oxide, calcite.

Diagnostics –

Fe oxide – reddish brown color,

Silica - hardest cement, will not “fizz”

Calcite - effervesces on exposure to HCl.

The texture of a sedimentary rock, its grain size, sorting, and rounding is a function of its origins.

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Major types of Sedimentary Rocks – Clastic and Chemical

Clastic (detrital) sedimentary rocks are composed of mineral and rock fragments and clays derived from pre-existing rocks. Physical weathering yields the fragments, while chemical weathering of silicates yields the clays.

Major types of clastics – Shale, Siltstone, Sandstone, Conglomerate or Breccia – based on Particle Size (Table 7.1, pg. 214)

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Major constituents – Clay and Quartz are most common.

Less common – Feldspar and Mica, & accessory minerals.

Grain size is related to energy conditions during transport. Larger size = Higher energy.

Sorting – similarities of grain size, related to length of transport.

Roundness – degree of angularity of grains, relates to type and length of transport.

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Shale – deposited in quiet waters, composed of minute particles, indistinguishable w/o High Magnification. Clay refers to size (<1/256 mm) and also to a class of minerals. Shale has the characteristic of being “fissile”, i.e., splitting into thin, tabular layers. Claystone or Mudstone usually breaks with a concoidinal (curving) fracture.

Shales & Clays are the most common sediments/sedimentary rocks.

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When clay particles “drop out” of suspen-sion in quiet water, initially the grains may be randomly oriented.

As compaction increases, the flat clay particles assume a nearly parallel

alignment, producing fissility.

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Common quiet environments for clay deposition include lakes, river floodplains, lagoons, outer continental shelf.

Shale color - related to depositional environment.

Light colored – oxidizing environment, i.e., well-oxygenated with good circulation.

Dark colored – reducing environment, contains organics. High organic content may serve as source bed for petroleum.

Red colored – likely deposited in a continental environment, river delta, tidal flat.

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Shale commonly splits easily and

forms talus slopes.

Shales and clays are mined to

produce pottery, brick, tile,

ceramics, and absorbents.

In some cases, shale layers may serve as glide plains for horizontal thrust faulting in areas of lateral compression.

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Siltstones are composed of particles between 1/256 mm & 1/16 mm. Clays feel smooth when rubbed between fingers, silts feel slightly gritty to the touch. Individual grains are difficult to identify.

Siltstones are deposited in slightly higher energy environments than clays, usually in tidal flats and continental shelf environments (shoreward of clays). Siltstones composed of eroded & re-deposited ash may also be deposited in calderas (remember sample from the Eagle Mts., west Texas).

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Sands and sandstones are composed of particles 1/16 mm to 2 mm in diameter, i.e., “sand” refers to size w/o regard to mineralogy.

The larger size (compared to clay) allows observation of individual grain characteristics, e.g., sorting and roundness.

Sorting, the degree of size similarity, is related to mode of transport (wind, water, ice), energy level, and length of time in transport.

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Beach sands are usually well-sorted

because of a long transport and

“washing” by wave action.

Fluvial/alluvial sands are less well-

sorted, with occasional gravel

beds (the result of occasional floods).

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Quartz is the dominant mineral in most sandstones, these are called quartz sandstones. Source area – mature, well weathered continent. Other varieties of sandstone include:

“Arkose” – sandstone with >25% feldspar. Angular, poorly-sorted grains suggests nearby granitic source rocks in a relatively dry environment.

“Graywacke” a dark sandstone with >15% silt/clay matrix and rock fragments. Very poor sorting and angular rock fragments suggests rapid erosion, short transport, rapid burial.

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“Conglomerate” – consists of rounded pebbles (“river rock”), suggesting a long transport time, deposited in a high energy environment (large river, mountain stream, accumulation of gravel along a rapidly eroding seashore.

“Breccia” – consists of angular pebbles, suggesting a nearby source and rapid erosion & deposition.

Most conglomerates & breccias are deposited in continental settings.

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“Chemical” Sedimentary Rocks – derived from the precipitation of dissolved minerals in water.

Inorganic – minerals precipitate because of evaporation and/or chemical activity. Examples: Evaporites (salt, gypsum), chert, travertine or cave onyx (stalactites, etc.), oolitic limestones

Organic – minerals precipitate because of organic activity. Examples: Limestone, Dolostone, Diatomites, bedded cherts. Coal is included in this category.

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Evaporites – minerals that precipitate from “super-saturated” (mineralized) water in enclosed basins.

Salt – NaCl

Gypsum – CaSO4

Inflow of water

No external drainage, only evaporation

Shallow sea inflow during high tide, storms

Evaporation from coastal basin

Playa lake basins be-tween mountain ranges, especially in Basin and Range Province.

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Coal – composed of compressed, altered, but not decayed plant remains.

Deposited in a “reducing” (oxygen-poor) environment, often in coastal delta settings. Sulfurous, acidic setting is inhospitable to most bacteria. Sulfur is usually deposited as pyrite or marcasite. This oxygen-poor environment may also be conducive to deposition of some uranium compounds.

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Progression of organic maturation (see p. 226)• Peat – poor energy source, better for mulch• Lignite – use only when necessary• Bituminous – most common• Anthracite – best, but least common

Because anthracite has undergone some metamorphism, the rocks containing the layer may be deformed along with surrounding rocks, more difficult to mine.

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Limestone – primarily formed by the organic activity of certain algae and other organisms that remove calcite (CaCO3) from water for their skeletons or exo-skeletons.

Limestones are deposited under the following conditions: Warm, clean, shallow water, within the photic zone. There are a few freshwater limestones.

Usual sites of deposition are: Outer margins of continental shelves; Shallow carbonate platforms. At the edge of continental shelf or carbonate platform, biologically “built” structures (bioherms, reefs), composed of skeletal debris and living corals, may be present.

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Modern reefs are dominated by Scleractinid Corals. Other organisms, e.g., Rugose and Tabulate Corals, archaeocyathids, bryozoans, calcareous algae, tubular molluscs (rudistids), have been responsible for past reefs. Reefs attract other living organisms, yielding greater species diversity (biodiversity).

A good example of an ancient reef is the Permian-aged Capitan Reef, Guadalupe Mts., Texas & New Mexico (west of Carlsbad Caverns).

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This reef was built by algae, sponges, and bryozoa. Skeletons help trap sediments, aid in build-up. http://www.nps.gov/gumo/gumo/geology.html

Back reef Reef Fore reef

Basin facies

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The Permian Reef encircled the Delaware

Basin (left), a portion of the shallow sea covering parts of

western N.A. during…cont.

…the Permian Period. The reef facies in the Guadalupe Mts. are exposed because of faulting.

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Coquina texture limestone with sand matrix, probably deposited along a shoreline.

Dolostone – composed of the mineral dolomite, originally deposited as a lime-stone, groundwater conditions cause the partial removal of Ca++ and its replacement by Mg++ to form CaMg(CO3)2

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Chert – microcrystalline silica (chert, flint), precipitated from seawater (minor) or biochemically by siliceous, microscopic diatoms (below) and radiolaria.

Volcanic ash may also serve as a source of silica (for chert).

Sometimes silica will replace the calcite in limestone (including the fossils).

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Decreasing Energy

Environments of Deposition – geographic setting for sediment accumulation. Facies (pg. 234) – characteristics relate to Depositional Environment

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Other examples of sedimentary environments are shown on Fig. 7.19, pp. 230 - 231.

Box 7.2 article – discussion of recovery of continental shelf cores for paleoclimatic study. The thickness of layers, the nature of the sediments, the organics (including fossils and microfossils), and isotopic study of certain minerals = proxy data.

The sedimentary characteristics + fossils offer evidence of environmental conditions.

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Examples of Sedimentary Environments

Red – continental environments

Blue – transitional

environments

Black – marine environments

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Continental environments – Eolian (windblown), fluvial, alluvial, alluvial fans, playa lakes, paludal (cave), glacial settings, fresh water lakes, early rift deposits.

Marine environments – continental shelf, continental slope and rise, abyssal plain, deep sea trench (not often preserved).

Transitional environments – beaches, tidal flats, deltas, lagoons.

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Sedimentary structures – Internal and External. External – those structures seen on the surface.

Mudcracks Ripple marks

See also Figures 7.25 and 7.26 pg. 237

In sedimentary rocks, raindrop impressions, salt casts, and trace fossils (footprints, feeding trails, etc.) can be preserved on bedding plane.

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Internal sedimentary structures – those that are seen in cross-section, i.e., from the side. Examples are cross-bedding (below) and Figure 7.22, the horizontal layering w/in the Grand Canyon (Chapt. 1 and Fig. 7.3, pg. 216), and graded bedding.

The cross-beds at left were

deposited in sand dune deposits.

Fossils in sedimentary

rocks – evidence of past life.

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Major types of fossils –

Body fossils – remnants of organisms preserved, e.g., shells, bones, etc..

Trace fossils – evidence of life activities preserved, e.g., footprints, burrows, fecal pellets.

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