Bear Gulch Limestone
Encyclopedia
The Bear Gulch Limestone in Montana
is a fossiliferous lagerstätte
, a limestone layer laid down in the Mississippian epoch of the Carboniferous
period, about 318 mya. This lens
of limestone was laid down in a surrounding matrix that indicates a landscape of mudflats and braided channels (linear sandstone
seams) in fresh and brackish water, in an arid climate (gypsum
formation). Magnetic orientation of particles
relative to the bedding planes of the strata suggest a latitude of about 12°N, a tropical
siting.
In the Bear Gulch Limestone, many genera of fossil fishes and some invertebrates were occasionally preserved in detail when they died and sank to the bottom of a shallow marine or estuarine
lagoon. In such cases, conditions on the silty bottom preventing the usual bacterial decomposition are usually invoked, characterized as close to anoxic
, thus suppressing ordinary bacterial decomposition; however, fossils of fishes of distinctly bottom-dwelling type are present, and traces of worm burrows or similar bottom disturbance show that sufficient oxygen was available in the upper levels of bottom silt. Pulses of hypoxic or anoxic conditions and perhaps periodic episodes of heavy siltation—the lagoon laid down up to 90 ft/30 m of limestone during its existence— may have been involved.
The numerous genera and many body types represented by fossils are witnesses to a highly diversified ecology
with many available niches to be filled by specialists.
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
is a fossiliferous lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
, a limestone layer laid down in the Mississippian epoch of the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period, about 318 mya. This lens
Lens (geology)
In geology a lens is a body of ore or rock or a deposit that is thick in the middle and thin at the edges, resembling a convex lens in cross-section. Adjective: "lenticular"....
of limestone was laid down in a surrounding matrix that indicates a landscape of mudflats and braided channels (linear sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
seams) in fresh and brackish water, in an arid climate (gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
formation). Magnetic orientation of particles
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
relative to the bedding planes of the strata suggest a latitude of about 12°N, a tropical
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
siting.
In the Bear Gulch Limestone, many genera of fossil fishes and some invertebrates were occasionally preserved in detail when they died and sank to the bottom of a shallow marine or estuarine
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
lagoon. In such cases, conditions on the silty bottom preventing the usual bacterial decomposition are usually invoked, characterized as close to anoxic
Anoxic sea water
Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. This condition is generally found in areas that have restricted water exchange....
, thus suppressing ordinary bacterial decomposition; however, fossils of fishes of distinctly bottom-dwelling type are present, and traces of worm burrows or similar bottom disturbance show that sufficient oxygen was available in the upper levels of bottom silt. Pulses of hypoxic or anoxic conditions and perhaps periodic episodes of heavy siltation—the lagoon laid down up to 90 ft/30 m of limestone during its existence— may have been involved.
The numerous genera and many body types represented by fossils are witnesses to a highly diversified ecology
Paleoecology
Paleoecology uses data from fossils and subfossils to reconstruct the ecosystems of the past. It involves the study of fossil organisms and their associated remains, including their life cycle, living interactions, natural environment, and manner of death and burial to reconstruct the...
with many available niches to be filled by specialists.