Geological group in Wisconsin
The Sinnipee Group is a geological group in Wisconsin. It consists primarily of sedimentary carbonate rocks. Primarily made of
dolomite, it also has
limestone as a secondary component and can even have shale imbedded with it. It was formed in the
Ordovician period and has three rock members:
Galena, Decorah, and
Platteville formations.
[1]
Formations
The
Platteville Limestone is one of the formations of the Sinnipee Group. It is primarily made of
limestone. It lies over the
Glenwood Shale. In many places, the Platteville Limestone has
dolomitic
mottles. This member is heavily jointed and the
mottles happened before the jointing. This particular member is being quarried heavily. While not as full as the
Decorah Shale, the Platteville Limestone is a large layer that has
bryozoans,
brachiopods, clams, snails,
cephalopods, and
trilobites that can be found in the limestone sediments. The Platteville Limestone is easily seen along the roads following the
Mississippi River due to
erosion.
The
Decorah Shale is another formation of the Sinnipee Group. It is primarily made of
fossiliferous
shale. It lies on top of the
platteville limestone formed by the shallow sea that covered central North America. Because of its chemistry it tends to erode rapidly. Since the
Decorah Shale is
fossiliferous the
Decorah Shale layer is often used as a place for amateur fossil hunters to begin their collections. It also serves as release of phosphate ions into the ground water system if
acid rain is introduced to it. This allows, in some cases, for new
minerals. The
Decorah Shale formation is made of three members. These members are the Spects Ferry, an argillaceous limestone,
[2] argillaceous dolomite,
[3] Calcareous shale and argillaceous limestones.
[4]
The
Galena Group is also a formation of the Sinnipee Group. It is primarily made of
Limestone and is deposited on the
Decorah Shale. The
Galena Group is considered fossiliferous. It is believed that the
Galena Group was deposited in a calm marine environment.
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The
strophomenid brachiopod
Strophomena from the Sinnipee Group in Dane County
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The imprint of the
nautiloid
Trocholites from the Sinnipee Group in Dane County
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An unidentified
orthid brachiopod from the Sinnipee Group in Dane County
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A bivalve mollusk called
Saffordia from the Sinnipee Group in Dane County
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A large nautiloid cephalopod called
Beloitoceras belonging to the group
Oncocerida from the Sinnipee Group in Dane County
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A mid sized bivalve mollusk called
Similodonta from Dane County
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A mass death plate of
rhynchonellid and
strophomenid brachiopods as well as
modiomorphid bivalves from Dane County
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The imprint of a
bryozoan, a colonial invertebrate that is still alive today from the Sinnipee Group
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A partial stem belonging to the crinoid
Pycnocrinus from a riverbed belonging to the Sinnipee Group
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The imprints of the
pygidium and lower
thorax of
Isotelus, the
cephalon of
Thaleops, a brachiopod and a bryozoan from the Sinnipee Group.
Counties
The Sinnipee Group covers around 25% of
Wisconsin,
[5] including part or all of the following counties:
Brown,
Calumet,
Dane,
Dodge,
Fond du Lac,
Grant,
Green,
Green Lake,
Iowa,
Jefferson,
Lafayette,
Marinette,
Oconto,
Outagamie,
Rock,
Walworth,
Waukesha,
Winnebago.
Formation
Formation of the Sinnipee Group took place in the
Ordovician. It is believed that this group formed from a shallow sea that once covered much of
North America. Each of its formations have their own pattern to their deposition and each is slightly different however they are unified by a shallow sea.
Landscape
Because of being mostly made of dolomite and limestone the landscape has become a
Karst environment. This is important because the
karst environment influences the Water movement in the rock layers. The Sinnipee Group has many Springs and several cave systems including
Cave of the Mounds. This environment even allows for
disappearing streams. Another reason this environment is important is because it has so many places to store water and that water is almost a limitless resource. The water leads to better agricultural yields as well as affecting our land such as watered lawns, and some protection from droughts or floods.
Overburden
The Sinnipee Group over burden consists of
glacial till from the
Laurentide Ice Sheet that is around 11,000 years ago. However, this till only covers to the
driftless zone and does not extend where the glaciers did not.
See also
References
External links