Illinois - Galena
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2022-09-19 15:57:16
Illinois - Galena
Illinois - Galena
Galina is one of the most charming towns in the Midwest, with many historic buildings dating back to the pre-Civil War era. It's also reportedly famous for some ghosts whose tours will take you to the most haunted places, perfect for Halloween. But the highlight of fall is the Galena Oktoberfest in early October, with plenty of beer, sausages, live polkas and beer mug enduros， sports fitness .
Mazon Creek Siderite Mine, Illinois, USA - 17th of a series of exceptionally buried fossils
The Mazon Creek specific burial fossil site is located in northeastern Illinois, USA, and was formed in the estuarine delta environment of the Carboniferous Pennsylvania Period (300 million years ago). A large number of exquisite animal and plant fossils are produced here, mainly preserved in siderite (FeCO3) nodules near Carboniferous coal seams. These siderite nodules coat the entire body of the organism, including various biological hard and soft parts. The preserved organisms include both terrestrial flora and fauna (known as the Braidwood biota) and marine life (known as the Essex fauna).
Paleogeographic studies have found that during the Carboniferous Pennsylvania period, most of North America was in the tropics, Illinois was near the equator at that time, and most of the state was covered by an ancient ocean deep into the land. The Francis Creek Shale overlying Mazon Creek is the widely distributed Colchester coal seam, where siderite nodules contained in the shale were discovered during mining. The Francis Creek Shale contains a series of discontinuous detrital mudstone, siltstone and sandstone sequences with large variation in thickness, up to 25-30 meters at the thickest point. The presence of fossil siderite nodules is the hallmark of the Francis Creek Shale. The sedimentary cycles of marine and non-marine facies indicate that this place used to be an estuarine delta environment that continued to grow into the sea, and biological tissues were deposited there when transgression or flooding occurred. The study found that after a transgression event, a larger flood event occurred here and resulted in the deposition of the Francis Creek Shale. Then some small rivers carried some organic-rich muddy sediments, which broke through and eventually flowed into seawater.
Penn Paleogeography
Mazon Creek preserves a large number of animal and plant fossils, which better reflect the terrestrial and coastal ecosystems at that time. The Braidwood biota includes a series of complex terrestrial ecosystems and delta freshwater ecosystems, while the Essex fauna is mainly a marine ecosystem near the estuary, which also shows that the delta was an estuary delta deep into the ocean at that time (as it is now estuary of the Yellow River).
Yellow River estuary
The Braidwood fauna produces more than 140 species of insects (Heterologus, Thesoneura), Euphoberia, centipedes, carapaces, spiders (Orthotarbus, Pleophrynus) and more. Organisms in freshwater include fish, amphibians, bivalves (such as mussels and clams), shrimp-like crustaceans (Cyzicus), swordtails (Euproops horseshoe crabs, also known as horseshoe crabs), arthropods (Acanthotelson, Palaeocaris, Anthracaris), etc. The discovery of ostracods, lungfish, sharks and other fossils also indicates that the offshore rivers may also have
The Essex fauna includes various polychaetes as well as crustaceans, cephalopods with mollusks, jellyfish (Essexella), hydras, tube jellyfish, multiplates, swordtail (Limulus), limulus (Lepidoderma sea scorpion) , echinoderms, many species of fish including sharks, and a strange looking animal, the Tullimonstrum gregarium.
Mazon Creek flora is very rich in species, about 400 species of plants in 30 genera have been found, and they are very well preserved! Most fossils are well-preserved biological structures, such as bark, leaves, stems, seeds, etc. They belong to some extinct or still existing plant groups, such as ferns (Pteridophytes), cuneiform plants ( Sphenophytes), ferns, seed ferns, and primitive conifers and ginkgoes are preserved.
Palaeoenvironmental studies have found this estuary delta to be a swamp-like environment in the tropics, and the mudstone-to-sandstone cycle at Mazon Creek suggests that there were periodic floods and tides. There is a clear demarcation between the Braidwood biota and the burial of the Essex fauna, indicating that the environment here has undergone a sudden change (hypoingression).
A unique feature of the Mazon Creek burial is that the fossils here are preserved in siderite nodules, and the soft bodies are particularly well preserved. The vertical tree trunk fossils preserved in situ indicate that these creatures may have been buried suddenly, and the aquatic creatures in the estuary may have been buried and killed by a large amount of sediment brought by the flood, especially the existence of burrows such as bivalves. The sudden burial of a large amount of sediment has occurred here. Most of the fossils are well preserved and show no signs of destruction, suggesting that these organisms were likely buried rapidly and without later bioturbation. The mollusks may have been buried while they were alive. Some people think that the anaerobic environment here inhibits the spoilage of microorganisms and leads to the preservation of mollusk fossils (this explanation may be problematic, the author prefers to think that mollusk preservation is a rapid mineralization. effect).
Mazon Creek fossils are primarily preserved as solids or imprints in siderite nodules. Plant fossils are mainly preserved as entities, and their interiors are filled with calcite, kaolinite, sphalerite and biological residual carbon. Shelled mollusks, cephalopods, sea cucumbers and other soft bodies are sometimes preserved as solid bodies, and jellyfish are preserved as overlapping compressed bodies, reflecting that biological soft bodies have been compressed by the surrounding soil. Arthropods and worms are usually preserved as imprints, sometimes degraded epidermis or chitin shells, and sometimes their internal organs.
80% of the composition of siderite nodules is carbonate, which indicates that siderite mineralization may have occurred in the early stage of diagenesis. The core of mineralization around the organism may be formed in the process of biological spoilage. The formation of siderite depends on three variables: the presence of iron ions; the rapid burial of organic matter; and, third and most important, microbial activity in the nodule centres without a seawater sulfate supply. It has thus been explained that the sulfur-reducing bacteria depleted the initial sulfates in the inter-sedimentary water, and the activity of the methanobacteria led to the onset of siderite mineralization.
In summary, the formation of the Mazon Creek idiosyncratic burial may be due to the following: rapid burial of large amounts of sediment, insufficient supply of sulfate by the estuary, supply of iron ions, and the presence of large amounts of organic matter (most of these explanations ignore the early effects of mineralization)
Siderite is not difficult to guess from the word "rhombus" in the name. It is a carbonate mineral, and the word "siderite" comes from the Greek word "sideros", which means "iron" - siderite is subcarbonate. Iron, component FeCO3: It will chemically react with concentrated hydrochloric acid or hot hydrochloric acid to generate bubbles, and it is easily oxidized and transformed into limonite and goethite in the natural environment.
The siderite mineral belongs to the hexagonal crystal system, the mineral density is 3.6 ~ 3.9, the Mohs hardness is 3.5 ~ 4.5, it has a shell-shaped or staggered fracture, and three groups intersect to form an excellent cleavage of rhombohedron. Most of the mineral crystals are translucent to opaque. It has glass to silk luster, colors are gray, yellow, yellow-brown, light brown, reddish-brown, etc., streaks are off-white, as for the crystal appearance, it is mainly a rhombohedron with an arc-shaped surface - the crystals are mostly flat and slightly Leaf-like, "scalenohedron" shaped siderites are relatively rare, while mineral aggregates appear in grape-like, spherical, associative, stalactite-like, earthy, or vein-filled forms. In addition, siderite will have strong magnetic properties when heated, and will release CO2 and convert it into limonite.
There are two main reasons for the formation of siderite. First, exogenous origin, produced in sedimentary rocks: most of these layered clastic sedimentary rocks contain organic components from organisms - such as (black) shale, coal seams, etc., in other words, siderite is in low oxygen. Second, it is formed in the hydrothermal veins of medium temperature to low temperature: siderite is commonly found in metamorphic sedimentary rocks and is a gangue mineral formed after hydrothermal accumulation; in addition, it may also appear in pegmatites siderite. Common symbiotic minerals are: quartz, pyrite, limonite, goethite, chalcopyrite, sphalerite, cryolite, Galena, barite, calcite, dolomite, fluorite, etc.
The world famous siderite deposits are: Poland, Czech Bohemia (Bohemia), Harz Mountains and Freiberg in Germany, Lorraine in France, Cornwall in England, Biera Baixa in Portugal, Pennsylvania, Michigan, Utah, Ohio Eastern State, Colorado, Roxbury, CT, Franklin, NJ, San Bernardino County, CA, Ladysmith, Wisconsin, Antler Mine, Arizona, New York, Yukon Rapid Creek, Francon Quarry, Montreal, Canada, Mont Saint-Québec Hilaire, Minas Gerais, Brazil, Huancavelica, Peru, Tatasi, Bolivia, Broken Hill, Prospect Hill, New South Wales, Australia, Tsumeb, Otavi, Namibia, Ivigtut, Greenland. Among them, the siderites produced in Britain, France and Germany have the best appearance.
Siderite is a relatively minor iron ore, most of which have low iron content, and are only valuable for industrial mining in some areas. In addition, siderite can also be used as a mineral collection - some siderites are iridescent due to the overlying goethite, while some siderites may have been completely replaced by limonite, forming siderite "skin" (appearance)", a pseudomorph of limonite "bone (component)".
Siderite belongs to the calcite group of minerals, and the minerals in the group are isomorphous to each other: due to the similar crystal structure of each mineral, they have many similar physical properties, including: belonging to the hexagonal system, trigonal (sub)crystalline System (trigonal) - the crystal form is mostly rhombohedron or scalenohedron, there are three groups of well-developed rhombohedral cleavage, and the transparent rhombohedral crystal has double refraction phenomenon. In fact, the cations in the mineral composition can completely replace each other to form a series of solid solutions, so the distinction between minerals may become more difficult.
As mentioned above, siderite will be produced in sedimentary rocks with organic components, such as black shale and coal seams. Let us imagine the formation environment of siderite: an ancient swamp area, many plant fragments, etc. It is a hotbed for coal mines and coal formation in the future. Because there is water and dissolved iron in this environment, it is an oxygen-deficient environment, so it is also suitable for the formation of siderite, which is coal-bearing. The reason why siderite is common in sedimentary rocks!
The siderite in these sedimentary rocks is mostly produced in layers or concretions (nodule, concretion). The so-called concretion is formed by the accumulation of siderite crystals, covering a core, and then covering and growing to the outer layers. , this core is mostly other minerals, such as: pyrite, sphalerite, chert, etc., but in the shale in the Mazon Creek area of Illinois, USA, the siderite nodules are not coated with minerals, but Plants and animals living in the ancient swamp area with the source of coal minerals! - This kind of siderite nodules covering animal or plant fossils is most famous in Illinois, but in addition, other regions such as western India are also produced, and it is not only seen in one place.
