| Knox Supergroup | |
|---|---|
| Stratigraphic range: Late Cambrian–Dapingian ~497–470.0 Ma | |
_1_(44862884385).jpg) Sphalerite from the Knox Superroup, Tennessee | |
| Type | Supergroup |
| Unit of | Sauk |
| Sub-units |
|
| Underlies | Ancel Group,Wells Creek Dolomite |
| Overlies | |
| Lithology | |
| Primary | Dolomite |
| Other | Limestone andsandstone |
| Location | |
| Region | Indiana, Illinois, Kentucky, New York, Ohio, Pennsylvania, West Virginia |
| Country | United States |
| Type section | |
| Named for | Knox County, Tennessee |
| Named by | Safford (1869, p. 151) |
TheKnox Supergroup, also known as theKnox Group and theKnox Formation, is a widespreadgeologic group in the Southeastern United States. The age is from theLate Cambrian to theEarly Ordovician. Predominantly, it is composed ofcarbonates, chieflydolomite, with somelimestone. There are alsocherty inclusions as well as thin beds ofsandstone.[1][2]
TheKnox lies beneath aunconformity known as the Knox Unconformity. There is well-developedkarst topography at the top of the Knox Supergroup. This demonstrates a prolonged period of erosion. This has modern economic impact because of forming reservoirs for hydrocarbons as well as some mineral deposits.[2]
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TheEverton Formation is ageologic formation in northernArkansas throughMissouri,Illinois andIndiana that dates to the middleOrdovicianPeriod.[3] It was named by Ulrich (1907) for exposures found aroundEverton, Arkansas. Fossils ofConodontsParaprioniodus costatus andLeptochirognathus quadratus indicate Whiterockian age.[4]Unconformities separate this formation from the underlyingPowell Formation and the overlyingSt. Peter Sandstone Formation. The Everton Formation is composed primarily ofdolomite,limestone, and somesandstone as well as a zone ofbreccia. There are local thick sections of a friable sandstone. The grains are nearly identical to the overlyingSt. Peter Sandstone, and may cause some confusion in identifying the different formations. The sand grains are medium grained, well rounded and are frosted. Fossils ofostracods,cephalopods,gastropods,bivalves,trilobites, andbryozoans are preserved within the formation.[5]
Five namedmembers of the Everton Formation are recognized (in stratigraphic order):
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TheBeekmantown Group is the upper unit of theKnox Supergroup.[6] It is a late Cambrian to lower–middle Ordoviciangeologic group that occurs in the eastern Canada (Québec) and northeastern United States,[7] datable from itsconodont fauna. It containsdolomiticsandstones andcarbonates from just off land from the palaeocoastline. Most likely the ArbuckleArbuckle Group of Oklahoma, andEllenburger Group of Texas are equlivant to the Beekmantown.[8]
The usage of the term is diverse and depends on the state or region in question, and the group encompasses differentformations in different regions. In some states it is considered a formation rather than a group, but the lithology varies by region.[9]
InPennsylvania the Beekmantown is broken down into theBellefonte Formation at the Knox Unconformity, followed by the either the Axemann Formation or theNittany Dolomite, with the Stonehenge Dolomite at its base. The Stonehenge is stratigraphically equitant to the Chepultepec Formation.[6]
Zinc is mined in outcrops found inVirginia. Mostly in Augusta, Rockingham, and Shenandoah counties. The largest is the Bowers-Campbell Mine.[10]
Within the breccia zonegalena is extracted along with small amounts ofsphalerite andpyrite.[8]
The Beekmantown is quarried in many locations for aggregate and rock.[11]
TheChepultepec Formation is anEarly Ordovician-agedgeological formation in theAppalachian regions of Eastern North America. Also known as theChepultepec Dolomite, it is a unit of the UpperKnox Group, overlying theCopper Ridge Dolomite and underlying the Longview-Kingsport-Mascot sequence. The formation was named by E. O. Ulrich in 1911 for the town ofChepultepec (nowAllgood).[6]
The Chepultepec Formation is a primarilylimestone anddolomite formation, the earliest formation of theOrdovicianperiod in its area. Further north, it is equivalent to theStonehenge Formation of theBeekmantown Group.[12] The formation was first described from Allgood, and has also been found inTennessee andVirginia.[13] In Virginia, the Chepultepec Formation has a habit of forming large naturalarches, includingNatural Tunnel inScott County andNatural Bridge inRockbridge County.[14][15]
This group includes theOneota Formation and theShakopee Formation.
TheOneota Formation is ageologic formation in the upperMidwest (United States) includingMinnesota,Illinois,Indiana andWisconsin. It preservesmarinefossils dating back to theOrdovicianperiod.[16]
TheSugar Loaf landmark located inWinona, Minnesota is made of Oneota Dolomite.
TheShakopee Formation is ageologic formation inIllinois,Indiana,Wisconsin, andMinnesota. It preservesfossils dating back to theOrdovicianperiod. It is named after the town ofShakopee, Minnesota, where the formation can be seen in bluffs along theMinnesota River.
The formation is broken up into two members, the New Richmond and Willow River Members. The Shakopee was deposited in a peritidal setting and is divided by an unconformity. The unconformity left river valleys that were filled withsiliciclastic material that was covered by marine deposits. Facies-defining cycles are divided into grainy carbonate, muddy carbonate, sandstone, shale, andStromatolitic–Algal matBoundstones.[17]
Stromatolite reefs are a common occurrence in the Shakopee and are well documented throughout. Some of the recognized species of stromatolites arecryptozoon minnesotense, cryptozoon liberta and cryptozoon rosemontensis.[18]