#15 Sedimentary Rock

Sedimentary rock forms by lithification of sediment layers (pieces of pre-existing rock or minerals chemically precipitated from water) deposited at Earth's surface under water, air, or ice. Sediment = clastic (detrital), chemical inorganic, and chemical organic.

Thickness of sedimentary rocks averages ~2 km (11% of crust and 0.1% of Earth). Radiometric age of oldest (meta)sedimentary rock = ~3.8 billion years, sedimentary rocks formed as early as 4 b.y.a. Why not 4.6 b.y.a.?

 

I) Importance (Benefits and hazards to society? Geology?)

 

 

 

 

 

 

 

 

 

 

 

Sedimentary Rock - Formation Processes

Step 1 = Weathering, produces clastic (detrital) sediment and dissolved ions (for chemical sediment). Steps 2 and 3 = Erosion and Transportation, removal and carrying away of detrital grains and dissolved ions from weathering sites by rivers, wind, and glacial ice. Step 4 = Deposition or Chemical Precipitation, depending on sediment type. Step 5 = Lithification (Diagenesis), conversion of sediment to sedimentary rock.

Transportation (Influence on detrital grain shape and size) - Initial detrital sediment is commonly angular. Why?

includes resistant and non-resistant mineral grains. Examples of each?

 

During transportation, rocks and mineral grains are broken and rounded by turbulent action of rivers or wind; non-resistant minerals break and dissolve. Grain size and sorting (variability of grain sizes) of sediment depends on nature and energy level of transportation medium. Wind transports and deposits very well sorted sediment (silt only) because finest grains are carried away and medium to coarse grains are not easily transported by wind. Glacial ice, landslides, and flooding rivers transport and deposit poorly sorted sediment (diamicton) because wide range of grain sizes can be moved due to high energy (or strength) of transporting medium. Geologists examine sorting in sedimentary rocks to determine transportation medium.

Precipitation/Deposition - Dissolved ions are carried by liquid water until change in temperature, pressure or chemical composition causes mineral precipitation or if living organism extracts ions to form CaCO3 shell or SiO2 skeleton. Usually chemical inorganic sediment is deposited where it settles, chemical organic sediment usually deposited only after organism dies or discards shell. Most sediment is detrital, carried by rivers (~10 billion tons of detrital sediment is carried to oceans every year), wind and glacial ice also carry and deposit detrital sediment. Deposition of detrital sediment occurs when moving fluid (water or wind) slows down; larger, heavier grains settle first and smaller, lighter grains are carried farther and deposited last. Examples = mountain river flows into plains and produces wedge shaped deposit (alluvial fan, photo #1, #2), river enters lake or ocean (no current) produces delta (photo-Nile).

Origins of Sedimentary Minerals

(1) Inheritance (detrital) - mineral formed in another area in previous stage of rock cycle; information on sediment source area (provenance)

(2) Neoformation (authigenesis) - mineral formed in place (precipitation from solution or reaction of pre-existing mineral); information on in-situ conditions (deposition or diagenesis)

(3) Transformation - mineral retains some of structure while undergoing chemical reaction; information on provenance plus in-situ conditions

Sedimentary Structures (loads of excellent photos) - features in sedimentary rocks that reveal conditions under which sediment was deposited. Examples = bedding (stratification or layering, photo #1, #2-inclined), ~horizontal banding due to change in grain size, mineral content or color. Clear break, bedding plane, marks end of one depositional event and beginning of another.

Graded bedding (photo #1, #2)= single sediment layer showing variation in grain size from coarse at bottom to fine at top, due to rapid deposition of sediment with variety of sizes in ~still water - densest (largest) grains settle first, smaller grains settle later. Graded beds commonly occur during deposition of turbidity current, rapidly moving dense mixture of sediment and seawater. Turbidity currents produced at edge of continental shelf, sediment carried by rivers becomes unstable and dislodged by earthquakes. Turbidity currents can move very fast (up to 60 km/hr), when slows down, deposits graded bed.

Cross-beds (photo, photo set-scroll to #3, 6-8, 10) = sediment deposited at non-horizontal angle, form when sediment drops from moving fluid (wind, river, or beach). Wind produces dunes, rivers and beaches produce ripples (< dunes). Ripple marks (photo, photo set-scroll to #11-16, 18) = wavy linear features on top of sediment produced by wind or moving water, can be symmetrical or asymmetrical.

Mudcracks (photo - modern, photo set-scroll to #19-22; photo - ancient, photo-scroll to #23) - fractured muddy sediment that forms when it dries due to shrinkage.

Lithification (Diagenesis) involves physical changes (compaction - pressure of burial < sediment pore space) and chemical changes (cementation - precipitate mineral cements, usually quartz, calcite, or Fe-bearing minerals, in pore spaces) that harden sediment; also get recrystallization (convert unstable minerals to more stable ones, e.g., aragonite to calcite).

Sedimentary Rocks - Nature and Origin - subdivided by those involving deposition of clastic sediment (clastic sed. rocks) and precipitation of chemical sediment (chemical sed. rocks); also organic sed. rocks (involving plant or animal activity). We will use clastic, inorganic chemical and organic chemical sedimentary rocks, not just organic and chemical.

A) Clastic Sedimentary Rocks (link #2) - classified based on grain size. Detrital mineral composition gives information on sediment source area or provenance (area undergoing weathering and erosion to provide sediment).

Sediment grain sizes include coarse-grained gravel (>2 mm) (granule = ~pea, pebble = ~walnut, cobble = ~softball, boulder > ~bowling ball), medium-grained sand (0.064 - 2 mm), fine-grained (microscopic) silt (0.004 - 0.063 mm), and very fine-grained (microscopic) clay (<0.004 mm or <4 m). Clay sediment = very fine grains, usually includes clay minerals, quartz and others, clay mineral = fine-grained sheet silicate such as kaolinite, smectite, and illite. Rock names = conglomerate, breccia, sandstone, siltstone, and shale/mudstone.

Mudstone (photo) = most common sedimentary rock, > 50% of all sedimentary rock; consists mostly of clay sediment (rich in clay minerals and quartz) with massive bedding. Shale (photo) = fissile mudstone (splits in thin sheets). In shale, clay minerals are aligned ~parallel to bedding; in mudstone, ~random orientation due to action of burrowing organisms. Mudstone and shale in Pennsylvanian cyclothems (sequence of sedimentary rocks that includes sandstone, mudstone, coal, shale, and limestone) of this area. Mudstone/shale form in quiet waters, e.g., deep lakes, lagoon (swamp), deep ocean basin, and floodplains after floodwaters recede. Commonly, mudstone is black from ~high organic carbon content. Mudstone used to make brick, ceramics, and cement. Oil shale contains abundant waxy organic matter (kerogen = precursor to oil). Kerogen in oil shale could provide USA with oil for ~500 years (USA has 2/3 of world's oil shale). Not economically feasible to use this energy (big environmental impact, large volume of waste rock, much water needed to process). Most abundant clay minerals in mudstone = illite, smectite, and kaolinite. During diagenesis (deep burial for long time), both smectite and kaolinite react to illite.

Siltstone (photo) slightly larger grains (gritty to teeth), much more quartz and less clay minerals.

Sandstone, ~25% of all sedimentary rock, subclassified on presence/absence of matrix (detrital, fine-grained minerals between sand grains). Sandstones with >5% matrix = wackes and sandstones with <5% matrix = arenites. Arenites can have cement (mineral precipitated in pore space, usually quartz or calcite). Sandstone classification involves composition of framework (sand) grains, quartz vs. feldspar vs. lithic (rock) grains.

Sandstone names have two modifiers, e.g., quartz arenite, feldspathic arenite, and lithic wacke. Quartz arenite (photo) = >90% quartz (and resistant heavy minerals such as zircon, rutile, and tourmaline), usually well rounded; High or low maturity?

Forms in tectonically stable environment and far from tectonic plate boundary, e.g., shallow epicontinental sea, beach, or dunes. Example = St. Peter sandstone.

Feldspathic arenite (arkose) (photo) >10% poorly sorted and angular feldspar (mostly K-spar weathered to kaolinite); Mature or immature sediment?
Forms near feldspar-rich source rock (e.g., granite) in rivers.

Lithic wackes (graywacke, photo) rich in rock fragments (volcanic, low-grade metamorphic, and fine-grained sedimentary rock grains), "salt and pepper" look; found at convergent plate boundary, near orogenic belts (many source rocks) and turbidites in accretionary wedge. Mature or immature sediment?

Conglomerate and breccia have largest grains; conglomerate (photo)  - rounded grains and breccia (photo) - angular grains. Both usually have matrix and/or cement. Conglomerate = mountain rivers (fast, nearby sediment source); breccia = high-energy environment with little transport, e.g., landslide, meteorite impact site, or sinkhole. Diamicton (rock = diamictite, photo) "matrix-supported" unsorted sediment; forms as landslide or glacial deposit.

B) Chemical Sedimentary Rocks - classified by mineral composition (also grain size or grain type): inorganic chemical (mineral precipitation in water) and biogenic/organic (from activity of plants and animals). Limestone (inorganic and organic), dolostone (inorganic only), evaporites (inorganic only), chert (inorganic and organic), and coal (organic only). To get inorganic mineral precipitation, water must be supersaturated with respect to mineral (concentration of dissolved ions > saturation or equilibrium state, mineral precipitation removes excess dissolved ions).

Limestone ~10 - 15% of sedimentary rock, forms by biologic and inorganic processes. Modern environments = shallow, warm, marine water of tropical shelf (inorganic) and reef (organic).

In tropical shelf (e.g., Bahama Banks, photo), seawater is ~saturated with respect to calcite. Origin of oolitic limestone (photo) = warm, upwelling, turbulent seawater that loses CO2 gas, --> supersaturation with respect to CaCO3 and calcite or aragonite precipitation as spheres (ooliths).

Ca2+ + 2HCO3- --> CaCO3 + H2O + CO2 (i.e., H2CO3)

Reef environment (photo) involves organisms (e.g., coral and non-reef building organisms like brachiopods, crinoids, gastropods, and bivalves) that extract Ca2+ and CO32- from seawater to form calcite or aragonite shell (fossiliferous limestone, photo).

Limestone (many classification schemes) can contain large grains (shells, ooliths, dried-out mud clasts, or peloids) with calcite or aragonite matrix (micrite = carbonate mud, photo) or calcite or aragonite cement (spar, photo). Other limestone names = chalk (photo), crystalline limestone, and coquina (photo).

Dolostone (photo) contains dolomite (CaMg(CO3)2); controversial origin. Most dolostone forms during diagenesis (after deposition) by limestone alteration with Mg-rich water:

Mg2+ + 2CaCO3 --> Ca2+ + CaMg(CO3)2

Limestone precursor evidence = relict textures (preserved/altered fossils, photo). One environment = evaporated (very saline and dense) seawater, which sinks into porous limestone, converts to dolostone. Seawater has abundant Mg and good Mg source, need circulation through limestone.

Evaporites evaporation of (usually) seawater (contains ~3.5% dissolved salt, mostly Na+ and Cl-); gypsum rock (CaSO4 .2H2O, photo) and rock salt (NaCl, photo). During seawater evaporation, get precipitation of gypsum first, gypsum is less soluble than halite. If 1 km of seawater evaporates, get ~14 m of halite and ~0.75 m of gypsum. Paleozoic evaporites in North America contain up to 1 km of gypsum and 350 m of halite! Get up to 750 m of halite in Silurian rocks in Michigan basin and in Late Cenozoic Mediterranean basin get >2 km of halite. Need many volumes of seawater.

Chert = fine-grained quartz of biogenic (organic) and inorganic origin; bedded chert (contains layering, photo) and nodular chert (photo). Origin of bedded chert = accumulation of fine-grained silica sediment (siliceous ooze) in deep ocean environment. Siliceous sediment from tiny organisms (radiolaria, photo  = animal; diatom, photo = plant) that extract silica from seawater and secrete silica skeleton. Nature and distribution of other marine sediments:

pelagic clay ("clay ooze") = detrital muds from

carbonate ooze = tiny CaCO3-secreting organisms (foraminifera, photo) that live mainly in

glacial sediments from

Siliceous ooze converts to chert (by recrystallization) during burial. Nodular chert occurs in limestone or dolostone and forms during burial due to flowing, acidic groundwaters that dissolve carbonate and precipitate quartz.

Coal = organic C-based, derived from plant debris; fresh-water swamp (stagnant, O2-depleted waters prevent oxidation of Corg), e.g., Everglades in FL or Okefenoke in GA; need deep burial to convert plant debris to > grades (ranks) of coal ranging from peat (compacted plant debris, photo, ~50% C), lignite (brown coal, photo, ~70% C), bituminous (dull black, photo, ~80% C), and anthracite (shiny black, photo, ~90% C); Sulfur (pollutant) enters coal if sea level >, floods plant debris with seawater, rich in sulfate, which is reduced to sulfide.

"Reading" Sedimentary Rock - use information about rock (grain shape, size, sorting and composition), fossils, and sedimentary structures to determine depositional environment (conditions in which sediment is deposited at Earth's surface) and geologic history. Many kinds of depositional environments, divided into continental, marine, and transitional:

1) Continental environments = mostly detrital origin, include rivers, lakes, deserts, glaciers, and caves;

Consider rivers -

floodplains -

glacial -

2) Transitional Environments = characterized by action of waves, tides, and ocean currents, which break soft grains and shells and carry fines offshore;

Consider beaches -

Also get deltas, lagoons (low energy, shallow water, protected from waves by barrier island), estuaries, subduction zones

3) Marine Environments = can be shallow (<200 m = shelf), or deep (>200 m, slope and ocean bottom), sunlight penetrates only ~50 m so shallow ocean has abundant life,

Consider shallow and warm marine -

deep marine -

In different locations, can get variety of sediment deposited at same time due to different environmental conditions (e.g., river channel, floodplain, oxbow lake); Sedimentary facies = set of characteristics (grain shape, size, sorting and composition) that distinguish one sediment (sedimentary rock) from nearby units deposited at same time (e.g., Fig. 6-28). Therefore, you can look at horizontal variations in sediment (sedimentary rock) to see how environmental conditions changed laterally but at same time. You can look at vertical variations in sediment (sedimentary rock) to see how environmental conditions changed over time but at same location.

Environmental Application - Porosity and permeability of sediment and rocks - Porosity = volume of pore space compared to total volume of rock or sediment. Permeability = ability of earth material to transmit fluid (groundwater, oil), which is related to degree of connection of pore spaces. Earth material with high permeability = aquifer (desirable for groundwater and oil), earth material with low permeability = aquitard/aquiclude (desirable for siting waste disposal site).

Rank in terms of porosity (phi or n) and permeability (k):

1) Gravel vs. sand vs. mud (well-sorted sediment)

 

 

Relationship between grain size and porosity?

Relationship between grain size and permeability?

2) Glacial till (unsorted sediment, gravel mud) vs. sand

 

 

3) Sand vs. sandstone

 

4) Uncemented sandstone vs. unfractured granite

 

 

5) Uncemented sandstone vs. fractured granite

 

 

Effect of nature of fracture on permeability (size and connection).

 

Examples of different kinds of fractures in earth materials?