Chapter 6

 

SEDIMENT AND SEDIMENTARY ROCK

 

 

 

I.                                           What is Sediment?

 

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

 

Sediment transport- the movement of detrital and chemical sediment by natural processes.

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...

 

 

III- Transform of Sediment to Sedimentary Rock

 

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:

Table 6-2.

-chemical classified on basis of composition and texture (clastic or crystalline): Table 6-2.

 

Detrital Sedimentary Rocks

 

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).

Table 6.2
Conglomerate and Sedimentary Breccia

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)

 

Coal

 

V. Sedimentary Facies

 

Body of sediment deposited in a specific environment (Fig. 6.16)

VI. Environmental

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)

 

Uranium