SEDIMENT AND SEDIMENTARY ROCK
Weathering yields particles and dissolved substances which form raw materials for sedimentary rocks.
Sedimentary Rocks composed of sediment -- loose (unconsolidated), solid particles produced by chemical or mechanical weathering, chemical processes, or organisms (fig. 6.3).
weathering --> erosion --> transportation --> deposition --> lithification = sedimentary rock
2 types of sediments:
-detrital- rock particles and mineral grains weathered and eroded from preexisting rock
-chemical- chemical precipitation of mineral from solution
-Sedimentary particles classified on basis of size (clay, silt, sand, gravel) (Table 6.1).
II- Transportation and Deposition
Once sediment formed (by weathering/erosion), it is transported (by water, wind, etc.).
During transport, sedimentary particles modified by:
-rounding- angular particles made round by abrasion (fig. 6.4a).
-sorting- sedimentary grains selected and sorted according to size by transporting agent (fig. 6.4b).
-well sorted: all particles are about the same size
-poorly sorted: wide range of particle sizes
Deposition- occurs when transporting agent loses the energy to transport particles.
Depositional Environment- a geographic area where sediment is deposited (fig. 6.5).
-continental- stream, lake, desert, glaciated areas...
-transitional- delta, lagoon, beach...
-marine- continental shelf, deep ocean basin...
Lithification- process that turns sediment into sedimentary rock (fig. 6.6); involves:
-compaction- weight of overlying sediments reduces amount of pore space.
-cementation- precipitation of cement (calcite, quartz, hematite, limonite) between grains of sediment.
IV. Types of Sedimentary Rock
5% of earth's crust, cover most of the seafloor and about 75% of the continents.
Like sedimentary particles, classified as detrital and chemical.
-detrital classified mainly on basis of particle size:
-chemical classified on basis of composition and texture (clastic or crystalline): Table 6-2.
Made up of detritus- gravel & sand
-clastic texture- composed of fragments / clasts; all detrital and some chemical sedimentary rocks (some limestones) have this texture (fig. 6.8a).
Composed of gravel-sized particles
Conglomerate- rounded gravels
Sedimentrary Breccia- angular gravels
Sandstone (1/2 – 2 mm)
Mudrocks- silt and clay
Chemical and Biochemical Sedimentary Rocks
Derived from solutions. They may have: crystalline- interlocking mineral crystals; most chemical sedimentary rocks (and no detrital) show this texture (fig. 6.8b), or clastic texture.
Limestone & Dolostone (carbonate rocks)
Evaporites (rock salt & rock gypsum)
Chert (microscopic crystal of quartz)
V. Sedimentary Facies
Body of sediment deposited in a specific environment (Fig. 6.16)
Analysis- determine how or in what depositional environment sedimentary rocks deposited in. Clues:
-3-D shape (geometry) of rock body- "blanket", "shoestring", etc.
-textural characteristics- grain size, rounding, sorting, etc.
Sedimentary Structures- form from physical and biological processes operating in environment of deposition; can use to help determine ancient environment of deposition.
-strata or beds- parallel layers in rock separated by bedding planes; usually a result of a pause in deposition or change in grain size (figs. 6.19).
-graded bedding- grain size within a bed decreases upward; result of rapid deposition by a turbidity current (fig. 6.20).
-cross-bedding- layers inclined to main beds; indicate deposition in a current (figs. 6.21).
-ripple marks- ridges on surface of sediment (bedding plane); deposition in a current (fig. 6.22).
-mud cracks- polygonal cracks formed in clay-rich sediment when it dries; indicate alternating wet and dry conditions (fig. 6.22).
Fossils- remains or traces of ancient organisms; helps us determine environment of deposition (certain organisms lived in only certain environments [shallow marine, deep ocean, etc.]) (figs. 6.24, 6.25).
-body fossils- remains of organisms, usually hard parts (shells, bones, teeth); soft parts (tissue) rarely preserved.
-trace fossils- traces of organisms, tracks, trails, and burrows (fig. 6.24b).
VII. Natural Resources Associated with Sedimentary Rocks
-sand & gravel, clay, limestone, evaporites (salt, gypsum), quartz (silica) sand, phosphate, diatomite, coal, uranium, hydrocarbons (petroleum, natural gas), etc.
Hydrocarbons- need a source, reservoir, and a trap to accumulate in economic quantities (fig. 6.29).
-source- remains of undecomposed microscopic organisms deposited in sediment (usually black shale).
-reservoir- a permeable rock where hydrocarbons accumulate (if a cap rock is present).
-trap- area where hydrocarbons accumulate; reservoir overlain by a cap rock required; traps may be structural or stratigraphic (Fig. 6.29).
Oil Shale- small particle rock and kerogen
Tar Sands- sandstone with asphalt like hydrocarbons fill pore spaces.
Coal (fig. 6.30)