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IGS - Winneshiek Lagerstätte Field Excavation

Winneshiek Lagerstätte Field Excavation

August 2010

The Middle Ordovician Winneshiek Lagerstätte near Decorah is one of Iowa’s most interesting paleontological and geological sites because of its unusual fossil components and extraordinary fossil preservation (igsb.uiowa.edu/Browse/Lagerstatte/Lagerstatte.htm). The fossil site was first reported in 2006, and has been considered as a “discovery of a decade in Ordovician paleontology” by many paleontologists. In addition to fossils, preliminary research has implied that the Lagerstätte is located in a hypothesized impact structure which was formed by a meteorite in the Middle Ordovician. Although research on the fossil fauna and deposit has been progressing, the Winneshiek Lagerstätte was not excavated and systematically investigated until the research project was awarded funding by the National Science Foundation (NSF) in 2010.

The investigation is a cooperative research project led by the Iowa Geological and Water Survey (IGWS). Three IGWS staff, Dr. H. Paul Liu, Robert McKay, and Dr. Brian Witzke, and Dr. Derek Briggs from Yale University comprise the research team. Following the proposed schedule, systematic excavation of the fossil site was the major goal of the team in 2010. Before the excavation, Dr. Briggs came to IGWS and checked previously collected fossil specimens and visited the fossil site at Decorah. During his stay, the research team had two group meetings to discuss the excavation, future research, possible achievements, and the research schedule.

With the help of a local construction company, excavation of the fossil site was conducted during the week of August 23, 2010. During the excavation, a temporary dike was built to divert water flow away from the excavation area, and water was continuously pumped out to make a 10 m x 25 m area for stratigraphic investigation and sample collection. A backhoe excavator carefully dug into the riverbed and exposed a section of shale with a total thickness about of 4 m below the contact of the Winneshiek Shale and the overlying St. Peter Sandstone (see stratigraphic chart). In addition to the research team members, Dr. Thomas Marshall from IGWS and Charles Monson assisted in the fieldwork, and Dr. Caroline Davis (IGWS) collected geophysical measurements at the excavation site. More than 2 tons of shale sample, as well as some good fossil specimens, were collected from the excavated section. All samples were transported to Iowa City and are stored in the IGWS warehouse for preparation and research. According to the landowner’s request, the excavated trench was backfilled after the collection and investigation was completed.

The Winneshiek Shale samples are now being prepared and checked for fossils. Lab analyses of both fossils and shale lithology are being done, and collaboration with other scientists is proceeding. It is expected that more well-preserved invertebrate and vertebrate fossils will be discovered from the collection, and several important aspects of Ordovician faunal evolution, paleoenvironment, and local geological history will be resolved by this research project.

This material is based upon work supported by the National Science Foundation under Grant No. 0921245.

The stratigraphic setting of the Winneshiek Shale in the study area

Site investigation with Dr. Briggs before the excavation

Construction of the temporary dike

Pumping and bailing of water

Site measuring

Trenching with the backhoe

Field discussion

In situ investigation

Stratigraphic measuring

Sample collecting

Geophysics study

Site refilling

Water recovering

Happy team

Samples moved to IGWS warehouse