Iron ooids in southeastern Minnesota are found scattered in thin beds as unconsolidated grains and in calcareous rocks in the uppermost beds of the Decorah Formation (Upper Ordovician, Sandbian). In this thesis, I am studying the genesis of iron ooids of the Decorah formation and looking into the origin of the iron used to from the ooids. Iron ooids are small, commonly well sorted, medium sand sized, less than 1mm in diameter, oblate spheroidal sedimentary grains. Iron ooids are found mostly unbroken with concentric layers of iron oxides, hydroxides or oxyhydroxides around a nucleus of a bioclast, lithoclast or peloids. They were made from volcanic ash that enriched the sea water in iron minerals and then precipitated around grains as they rolled around on the ocean floor. During the Ordovician, parts of North America, then called Laurentia, were covered in an epeiric sea in which the ooids were formed. The Decorah iron ooids were probably created above storm wave base and below normal wave base in occasionally agitated shallow marine environments.

The accumulation of iron available for the creation of Decorah iron ooid lamellae probably originated from the weathering of volcanic debris and ash from the Taconic orogeny. The evidence for this is the presence of bentonite beds throughout the Decorah. The bentonite ash beds found throughout the Decorah Formation are a result of the Taconic orogeny to the southeast. During the Taconic orogeny, Laurentia, Baltica, and a volcanic arc collided to the southeast forming the Taconic Mountains, which have since eroded.

Modern iron ooids, found at the foot of a volcano in Indonesia, have been discovered to have similarities to Ordovician iron ooids. The similarities further indicates that iron comes from the ash from volcanoes. The Ordovician iron ooids of the Decorah formation are not heavily researched or described; therefore, they are a primary focus of this study.


Wilson, Mark




Geology | Paleontology | Sedimentology | Stratigraphy | Tectonics and Structure | Volcanology


Iron ooids, Iron, ooids, Decorah Foramtion, Decorah, Ordovician, Minnesota, Volcanic

Publication Date


Degree Granted

Bachelor of Arts

Document Type

Senior Independent Study Thesis



© Copyright 2017 Etienne V. Fang