Geological Definitions

Anaerobic
containing very little oxygen.

Anoxia
the condition of containing very little oxygen.

Aragonite
a mineral consisting of CaCO3. Aragonite is less stable than calcite under pressures at the surface of the Earth. Calcite precipitation is often inhibited, however, and both modern and ancient seawater are supersaturated with respect to calcite. With high enough supersaturations, aragonite can also precipitate. Aragonite is common in many modern carbonate sediments as shells and as crystals that precipitated from sea water.

Archean
adjective implying conditions between 2.5 billion years ago and 4.0 billion years ago. The oldest rocks are about 4.0 billion years old defining the older boundary. The younger boundary is defined by an apparent difference in geological attributes of rocks older than 2.5 billion years and younger than 2.5 billion years. These differences may be due to our incomplete knowledge of the rock record rather than to a dramatic change in the Earth.

Banded Iron-formation
a type of rock that is very rich in iron. Typically, it contains iron oxides, iron silicates, and/or iron carbonates as well as the mineral chert.

Calcite
a mineral consisting of CaCO3. Sometimes Mg2+ substitutes for Ca2+ forming Mg-calcite. Sea water is supersaturated with respect to calcite, and calcite is common in many modern and ancient carbonate sediments as shells and as crystals that precipitated from sea water.

Carbonate
a group of minerals usually consisting of a divalent cation and CO3. Shells and corals are made of carbonate. The two most abundant carbonate minerals in today's oceans are calcite and aragonite, two different arrangements of CaCO3. Dolomite is another common carbonate mineral in ancient sediments.

Cement
referred originally to crystals that grew between sand grains and "cemented" them together. Now, the term "cement" more generally refers to crystals that grew in voids.

Chert
a form of the mineral quartz that has very small crystal sizes. The chemical composition of both chert and quartz is SiO2.

Conformable Contact
a boundary between two layers of rock that does not mark a major change in sedimentary conditions. In other words, the rocks below and above the contact were deposited in similar enivornments.

Craton
an old, very stable block of continental crust.

Dolomite
carbonate mineral consisting of MgCa(CO3)2. It is the most stable carbonate mineral in the oceans, but it precipitates very slowly and is almost never present. Instead, it is found predominantly in ancient sediments where it forms when the sediments are buried. "Dolomite" also refers to rock consisting of mostly of the mineral dolomite.

Facies
rocks with a group of characteristics that imply a certain origin or history.

Formation
a group of rocks with similar characteristics that can be recognized in the field and whose distribution can be plotted on a map.

Ga
billion years old

Grainstone
carbonate rock consisting of grains of carbonate sand without finer-grained carbonate (micrite) between them.

Limestone
rock consisting of mostly the minerals calcite and aragonite.

Lithofacies
rocks with a group of characteristics that imply a certain origin or history.

Ma
million years old

Mesoproterozoic
adjective implying conditions between 1 billon years ago and 1.6 billion years ago.

Micrite
very fine (1-5 microns) carbonate crystals (either calcite or aragonite). It composes carbonate mud. Micrite can precipitate from seawater or form from the breakdown of larger carbonate grains. Most modern micrite is the result of calcifying algae.

Microbialite
a sediment that is demonstrably influenced by micro-organisms. It is similar to the term "stromatolite", but explicitly implies a biological origin or influence.

Microspar
crystals that are 4 to 10, sometimes 50 microns in diameter. This term is usually, but not exclusively, used for carbonate minerals.

Neoproterozoic
adjective implying conditions between 540 million years ago and 1 billion years ago.

Ooid
a grain consisting of multiple coatings of carbonate (usually calcite and/or aragonite) that precipitated on a nucleus. They are usually round and smooth. They form in shallow water depositional environments.

Oolite
a rock made up of ooids.

Paleoproterozoic
adjective implying conditions between 1.6 billon years ago and 2.5 billion years ago.

Peritidal
adjective implying that the rocks were deposited in an area associated with a tidal flat.

Phanerozoic
adjective implying conditions between 540 million years ago and the present. (see discussion of "Precambrian" for information on the 540 million year old date)

Precambrian
adjective describing time before the Cambrian Era which was the time in Earth history when the first macroscopic life was found in the fossil record (in England when historical geology was first developing). The Precambrian-Cambrian boundary has recently been shifted in absolute time by Sam Bowring, John Grotzinger, and Beverly Saylor at MIT. It is now placed at about 540 million years ago. Thus, Precambrian time covers all rocks older than 540 million years old.

Proterozoic
adjective implying conditions between 540 million years ago and 2.5 billion years ago.

Siderite
a mineral consisting of iron and carbonate: FeCO3.

Shale
fine-grained, sedimentary rock composed of particles smaller than about 60 microns. Particles consist predominantly of minerals containing silica such as quartz and clays.

Stromatolite
The most common usage is: a laminated, relief-forming structure of a biogenic, specifically microbial, origin. Usually they are found in carbonate sediments, particularly Precambrian sediments. It is usually very difficult to demonstrate a biological origin, and care should be taken not to assume a biological origin for all "laminated, relief-forming structures" in carbonate rocks.

Sulfide
a mineral consisting of a metal atom bonded to a sulfur atom.

Supratidal
adjective implying that the rocks were deposited on a shoreline above the normal high-tide level.

Subtidal
adjective implying that the rocks were deposited in water deeper than the normal low-tide level.

Taxis
a motilitity response to a stimulus. For example, many cyanobacteria can sense and move towards light. Such behavior would be a "taxis" as in "phototaxis".

Transgression
an encroachment of seawater onto what was previously above sealevel, i.e. a sealevel rise.

Wave Base
the depth at which wave activity influences sediment deposition on the sea floor.



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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616
sumner@geology.ucdavis.edu