Sedimentary Rocks Flashcards
What are sediments and sedimentary rocks?
Sediments
Loose grains and chemical residues of Earth materials including rock fragments, mineral grains, parts of plants or animals, and rust. All sediments have a source where they were produced by the biochemical processes of plants or animals, or by chemical and physical weathering of rocks.
Chemical weathering
decomposition or dissolution of Earth materials. Ex: halite (sodium chloride) dissolves to form salt water.
Physical (aka mechanical) weathering
Cracking, scratching, crushing, abrasion or other physical disintegration of Earth materials. This process causes big rocks to be disintegrated into clasts, including rock fragments and mineral grains. It causes the breakdown of animal shells
Clasts
Broken pieces - including rock fragments and mineral grains.
Lithification
The hardening of sediment/clasts to produce rocks. This process occurs as layers of sediment are compacted (pressure hardened), or cemented (glued together by tiny crystals or chemical residues precipitated from fluids in the pores of sediment)
Precipitation of minerals
It’s possible to form a dense hard mass of inter-grown crystals directly, as they precipitate from an aqueous solution.
Ocean water
The most common aqueous solution and variety of salt water on Earth. As it evaporates, a variety of minerals precipitate in a sequence. First to form is aragonite (calcium carbonate. Gypsum forms when 50-75% of ocean water has evaporated. Halite (table salt) forms when 90% has evaporated.
Sedimentary Rocks
Broken into different classifications based on what type of material they are made of (composition). The composition of a sediment or sedimentary rock is a description of the types and abundance of the grains it’s made of. There are 3 main types: detrital, biochemical and chemical.
Detrital Rocks
Detrital sedimentary rocks contain clasts. These are fragments or pieces of rock or minerals. The composition of clastic sedimentary rocks is divided into three types - clay/silt, sand and gravel.
Biochemical rocks
Biochemical sediments and rocks are composed mostly of the remains of organisms, such as shells, plant fragments and carbon.
Chemical Rocks
Chemical sediments and rocks are composed mostly of intergrown mineral crystals precipitated from aqueous solutions and chemical residues.
Textures of sediments and sedimentary rocks
The texture of a sediment or sedimentary rock is a description of it’s constituent parts and their sizes, shapes and arrangement. The texture is a small-scale property of a rock, but determined many of it large-scale properties, such as the density, porosity or permeability.
Clastic texture
Clastic texture means a rock consists of clasts. In detrital rocks, it is called siliciclastic and the clasts are rock fragments. In biochemical rocks it is called bioclastic and the clasts are fragments/shells of organisms. The size and form of the clasts can be used to determine the velocity and direction of current in the sedimentary environment the rock was formed. Fine, calcareous mud settles in quiet water. Gravel and larger clasts are only deposited by fast-moving water.
Matrix or cement
Between the clasts of the rock can be composed of a matrix or cement (cement can be crystals or precipitated minerals).
calcareous
from old English cealc “calk, soft white limestone, lime, plaster, pebble. Possibly borrowed. from Latin: calcarius - of or pertaining to lime
Clay and silt
Less than 1/16 mm. Clasts are not visible to the unaided eye.
Sand
clasts between 1/16 and 2 mm in size
Sediment size
Gravel is greater than 2 mm. For context - 1 mm is the size of a pencil lead. 2 mm is the coloring surface of a crayon. 5 mm is a pencil eraser. The 1/16 mm is the boundary between silt and sand, and divides particles visible to the human eye (sand) and particles that are too small to be recognized in the field (silt and clay).
Sorting (well or poorly)
distribution of grain sizes for different rock types. When all clasts are generally the same size the rock is ‘well-sorted’, when there is a large difference between grain size the rock is ‘poorly sorted.’
Chemical texture
Crystalline and microcrystalline textures. Sedimentary rocks that form when crystals precipitate from aqueous solutions have a crystalline texture, either clearly visible or microcrystalline (too small to identify). As crystals grow, they interfere with each other and form an intergrown and inter-locking texture that holds the rock together
HCl
Hydrochloric acid is used to test whether a rock is calcareous or not. Calcareous rocks effervesce in HCl
Sedimentary Structures
a variety of structures formed at the time of deposition or by physical processes, or by activities of plants and animals. Sedimentary structures can be used as indicators of the environments where they normally form today. (Think Rift Valley and how we can extrapolate the environment Australopithecus lived in)
Layering of sediments
Sediments and sedimentary detrital rocks are characterized by bedding or stratification, which occurs when layers of sediment, or beds, with different particle sizes or compositions, are deposited on top of one another. These beds range from millimeters or centimeters thick to meters or many meters. most bedding is relatively horizontal, though some form at high angle relative to horizontal.
Cross bedding
consists of beds deposited by wind or water and inclined at ranges of as much as 35 degrees from horizontal. Cross-beds form when sediment particles are deposited on the steeper, downcurrent (leeward) slopes of sand dunes on land or sandbars in rivers and on the seafloor. Cross-betting patterns in wind-deposited
