Tar pits, sometimes referred to as asphalt pits or tar seeps, are large asphalt deposits. They form in the presence of petroleum, which is created when decayed organic matter is subjected to pressure underground. If this crude oil seeps upward via fractures, conduits, or porous sedimentary rock layers, it may pool up at the surface. The lighter components of the crude oil evaporate into the atmosphere, leaving behind a black, sticky asphalt. Tar pits are often excavated because they contain large fossil collections. Tar pits form above oil reserves, and these deposits are often found in anticlinal traps. In fact, about 80 percent of petroleum found on Earth has been found in anticlinal traps. Anticlines are folds in stratigraphic layers in which each half of the fold dips away from the crest. Such structures are usually developed above thrust faults or in tectonic regions where the land is bending and folding. If the structure above the concave-down fold (arch) is a non-porous rock or aquitard, such as shale, it is considered an anticlinal trap. If there is a fault or fracture in the overlying strata above the oil reserve, the oil may migrate to the surface. This is possible by capillary fringe and because oil is less dense than water.
Chemistry Tar pits are pools of asphalt. However, at the beginning of their formation, they are not necessarily sticky and dense. The pools were composed of crude oil that originated below Earth's surface. Crude oil is a mixture of heteroatom compounds, hydrocarbons, metals, and inorganic compounds. Heteroatom compounds are organic molecules that contain elements that are not carbon or hydrogen, while hydrocarbons contain only carbon and hydrogen. Crude oil is less viscous than asphalt because it contains a higher percentage of light hydrocarbons. Light hydrocarbons include the following alkanes: methane, ethane, propane, and butane. These molecules have very low molecular weights. Crude oils may also contain some inorganic impurities, such as CO2, H2S, N2, and O2. At the surface, these light molecules may evaporate out of the crude oil, leaving behind the heavier, stickier molecules. Asphalt, or bitumen, usually contains hydrocarbon molecule chains with 50+ carbon atoms. The longer the hydrocarbon chain, the more viscous it becomes, and the boiling point increases. Evaporation is an important process in the formation of tar pits. A reservoir of light crude oil on Earth's surface can be reduced by up to 75% of the initial volume just after a few days, forming asphalt as the resulting product. For medium crude oils, the volume may be reduced by 40%. Crude oils will evaporate differently depending on their chemical composition. The average composition of a bitumen sample by weight is 80.2% carbon, 7.5% hydrogen, 7.6% oxygen, 1.7% nitrogen, and 3.0% sulfur.
Notable tar pits
La Brea
The La Brea Tar Pits is located in Los Angeles. The petroleum that is seen on the surface is sourced from the Salt Lake Oil Field reservoir and the oil sands in the Repetto and Pico formations. These oil deposits were formed during the Miocene Epoch when marine plankton organisms accumulated in an ocean basin. Over time, sediments buried the organisms 300 to 1000 meters below Earth's surface, subjecting them to high pressures. This process turned the organic matter into oil. The 6th Street Fault that cuts through the Salt Lake Oil Field is the conduit that feeds the La Brea Tar Pits. Petroleum migrated to the surface over time, trapping and preserving animals and plants for the past 50,000 years.
Carpinteria
The Carpinteria Tar Pits are located in Tar Pits Park in Carpinteria, California. These tar pits were predicted to have formed during the Pleistocene. During an excavation project, 25 plant species were recovered along with 55 species of birds and 26 species of mammals. Springs of tar still ooze to the surface through fractures in the underlying stratigraphic layers of marine shale.
Binagadi asphalt lake
The Binagadi Asphalt Lake is located in Azerbaijan. This tar pit is known for preserving the heads and bodies of multiple cave lions, a mammal that flourished in the Pleistocene. A well-preserved horse skull was also found in the Binagadi asphalt lake. It is estimated to be 96–120 thousand years old. It is on display at the Natural-Historical Museum of Azerbaijan in Baku, Azerbaijan.
Pitch lake
The large, bowl-like depression filled with bitumen in Trinidad and Tobago. The lake has a maximum depth of 250 feet with an area of 100 acres, making it the largest deposit of solid bitumen on Earth. The lake is cold and dense near the shores, and it has a top layer that can be walked on. Underneath this skin, the asphalt is continuously churning. The lake gets gradually softer and hotter near the center where the bitumen begins to bubble. The gas being released in the middle of the lake is largely methane and an ample amount of carbon dioxide. Pitch Lake formed thousands of years ago in the presence of tectonic activity. Movement along a fault created a fracture that tapped into an oil and gas reservoir deep within the crust. The oil and gas seeped upward to Earth's surface through the fracture over time, creating Pitch Lake. Due to this constant replenishment of the oil and gas, the lake has a slight current. The current is largely unnoticed because the top layer of Pitch Lake is mostly solid.
Fossils
… excerpt ends here. Continue reading the full article.
