"sedimentary rocks" - come from weathered pre-existing rocks &/or organic materials

• the weathered material has been transported, deposited, and lithified

• sedimentary rock accounts for 5% (by volume) of the Earth's outer 16 km

• sedimentary rock comprises a relatively thin, discontinuous layer in the uppermost portion of the crust

• 75% of all rock outcrops on the continents are comprised of sedimentary rock

• sedimentary rock sometimes contains evidence of past environments and of the mechanisms involved in their formation (e.g., the how the sediments were transported)

• many sedimentary rocks have important economic value (e.g., coal, oil & gas, fertilizers, construction material)

• There are two types of sedimentary rocks:

• Detrital Sedimentary Rocks - are formed from solid particles - derrived from both physical and chemical weathering
• (weathering: the disintegration and decomposition of rock at or near the surface of earth; erosion: the incorporation and transportation of material by a mobil agent, such as water, wind, or ice)
• Chemical Sedimentary Rocks - are formed from soluble material, produced largely from chemical weathering
• these dissolved materials are precipitated by either inorganic or organic processes

• weathering & erosion of preexisting rock, generates fine-grained bits and pieces of (solid) material - "sediment" - (more specifically, "clastic sediments")

• this sediment is comprised mainly of quartz, feldspar, & clay minerals, & some micas

• these materials are then transported (by wind, water, gravity, & ice) elsewhere & deposited

• the sediments are laid down in horizontally layered beds > which form strata

• ("stratigraphy" - the study of the sequences, & spacing of these strata in order to determine the age and origin of the rocks)

• the sediments are then compacted, cemented, or hardened ("lithification") to form sedimentary rock

• detrital rocks are classified on the basis of particle size

• particle size provides useful information about the deposition environment

• currents of air or water sort the particles by size - the stronger the current the larger the particle size carried

• In order of increasing particle size: shale, sandstone, conglomerate or breccia

shale - comprised of silt- and clay-sized sediments

• very fine-grained; individual particles can't be identified without magnificaton
• account for over 1/2 of all sedimentary rocks
• deposition occurs in a low-velocity environment (lakes, river floodplains, lagoons, the deep-ocean basins)
• fossils are relatively common
• clay packs tightly in layers (laminae), which allows for poor cementation and a "weak" rock
• because shale packs tightly, it often forms a barrier to water/oil and gas
• shale does not produce prominent outcrops
• mudstone and siltstone

Shale

Sandstone

• sedimentary rocks in which sand-sized grains predominate
• accounts for 20% of sedimentary rocks
• quartz ("quartz" sandstone) (because of its durability) is the predominant mineral in most sandstones

• the degree of similarity in grain size - "sorting" - tells us about the depositing environment; well sorted vs. poorly sorted
• windblown deposits are typically better sorted than are deposits resulting form wave action; and wave action sorts better than stream action
• streams, waves, and wind tend to round the particles; and very rounded particles imply that long-distance transport took place
• conversely, angular grains indicate that the material was transported a short distance, and by some other medium (e.g., ice)

Sandstone

• conglomerates consist largely of gravels
• particles range in size from large boulders to pea-size
• conglomerates are poorly sorted (because they often contain sand and mud sized particles as well)
• are deposited in turbulent (mountain) streams and areas of strong wave action; are found in association with landslides and glacial activity as well
• if the large pieces in the rock are angular, the rock is called "breccia" - indicates that little transport of the sediment took place

Conglomerate

Breccia

• are formed from material that had been dissolved into solution - important in oceanic environments

• this material is transported in solution & then is precipitated out of the solution - forms "chemical sedimentary rock"

• e.g., limestone (CaCO₃); and dolomite (CaMg(CO₃)₂

• these sedimentary rocks can later be dissolved in solution (weathered) & assume unique structures - "Karst Landscapes"

......Inorganic vs. Organic (biochemical)

Chemical Sedimentary Rock:

• precipitation occurs in two ways: (1) via "inorganic" processes
• (e.g., evaporation; chemical reactions);
• and (2) via the organic (life) processes of marine organisms - "biochemical" processes"
• (e.g., when sea dwelling organisms extract calcite (CaCO3) and silica (SiO2) from seawater to build their shells; when the organisms die, their silica/calcite shell collect on the ocean floor and form biochemical sediment

Rocks:

(1) Limestone

• accounts for ~10% of the total volume of sedimentary rocks & is the most common chemical sedimentary rock
• is comprised mainly of calcite (CaCO3)
• is formed by either inorganic or organic processes
• marine biochemical forms (90%)are the most common forms of limestone:

• "coquina" - (a limestone) comprised of poorly cemented calcite-rich shells

• sedimentary rock - e.g., "chalk" - a soft porous rock composed of the shells of microscopic sea organisms

• Inorganic Limestones:

• form when the concentration of CaCO3 increases to the point where it precipitates
• examples: "travertine" (seen in caves);

• is a type of chemical sedimentary rock comprised of microcrystalline silica (or quartz)
• some chert is precipitated from seawater to form nodules of chert - thus it's inorganic
• most chert is believed however to be organic in nature - derrived from the silica-rich skeleton's of marine organisms (diatoms and radiolarians)

• is different from other chemical sedimentary rocks
• it is comprised of large amounts of plant material that has burried - initially in a swamp - an oxygen poor environment, which limited decomposition
• high temperatures and pressures drove-off volatiles and changed the plant material to > peat > lignite > bituminous > anthracite (metamorphic rock)

• form when evaporation is the mechanism triggering precipitation
• e.g.'s,: halite (NaCl), gypsum (plaster) (CaSO4), the mineral sylvite (potash) (used as a fertilizer)

• compaction & cementation work hand-in-hand
• cementing materials: silica, calcite, iron oxide

• strata (beds) - sedimentary rocks are laid down in layers
• bedding planes - a nearly flat surface separating two beds of sedementary rock; each plane marks the end of one deposit and the beginning of another having different characteristics
• cross bedding - when sediments do not accumulate in a horizontal fashion
• fossils, ripple marks, mud cracks - clues to past environments