| Winneshiek Shale | |
|---|---|
| Stratigraphic range:Darriwilian | |
 Winneshiek Shale strata exposed via a temporary dam (a-d) | |
| Type | Formation |
| Underlies | St. Peter Sandstone |
| Overlies | unnamedbreccia unit |
| Thickness | 38 m[1] |
| Lithology | |
| Primary | shale |
| Location | |
| Region | Upper Midwest |
| Country | United States |
| Extent | Iowa |
| Type section | |
| Named for | Winneshiek County, Iowa |
| Named by | Liuet al., 2006 (as "Winneshiek Lagerstätte")[1] |
 Location of the Decorah crater in Iowa, the only area where the Winneshiek Shale is found | |
TheWinneshiek Shale (originally theWinneshiek Lagerstätte) is aMiddle Ordovician (Darriwilian-age)geological formation inIowa. The formation is restricted to theDecorah crater, animpact crater nearDecorah, Iowa. Despite only being discovered in 2005, the Winneshiek Shale is already renowned for the exceptional preservation of its fossils. The shale preserves a unique ecosystem, theWinneshiek biota, which is among the most remarkable Ordovicianlagerstätten in theUnited States.[2] Fossils include the oldest knowneurypterid,Pentecopterus,[3] as well as giantconodonts such asIowagnathus andArcheognathus.[4]
The Winneshiek Shale is a thin and geologically homogeneous package of dark grey to greenish-brown sandyshale. Drill core data has estimated a maximum thickness of 38 meters,[1] though in most areas its thickness is only about 18–27 meters.[3] The shale is replete withpyrite and organiccarbon.[2] It lies solely within the Decorah Structure, a 5.6 km (3.5 mile)-wide probable impact crater near Decorah, Iowa. Within the crater, the shale overlies a much thicker unnamed unit mostly composed of impactbreccia. The impact which formed the crater occurred after the deposition of theShakopee Formation and before the deposition of theSt. Peter Sandstone, bracketing the Winneshiek Shale between those two formations. The St. Peter Sandstone is separated from the Winneshiek Shale by anunconformity, indicating that most of the crater fill had been eroded away by the time of the sandstone's deposition.[5] This characteristic was not initially recognized, and the Winneshiek Shale was first believed to be a subunit of the St. Peter Sandstone.[1]
Apart from boreholes, the shale is only accessible at a few thin outcrops along theUpper Iowa River. A temporary dam constructed in 2010 allowed for the collection of numerous fossils from a 4-meter interval of the shale.[2] Fossils are primarily preserved asbiomineralized shells or carbonaceous films, typically representative of hard parts with little soft-tissue preservation. Nevertheless, someCeratiocaris specimens havephosphatized gut contents, and softbromalites are preserved asapatite structures. The shale'sdepositional environment is reconstructed as a calm marine basin orestuary with ananoxic, low-pH seabed.[2][1]
The Winneshiek Shale has noradiometric dating and little overlapping fossil content with nearby formations, making precise age estimates difficult. The overlying St. Peter Sandstone is firmly lateDarriwilian in age based on itsconodont fauna. The Winneshiek Shale shares only a few taxa with other formations, namelyMultioistodus subdentatus andArcheognathus primus. Both of these conodonts were originally known from the mid- to upper-DarriwilianDutchtown Formation ofMissouri. The mid-Darriwilian global stage corresponds to the lateWhiterockian regional stage in North Americanbiostratigraphy. Conodont material similar toIowagnathus grandis is also known fromSiberia.[4]
The Darriwilian age of the Winneshiek Shale is supported bychemostratigraphy trends tabulated from drill cores. There is a gradual positive trend in organicδ13C values going up the shale. This may be correlated with the lower half of the mid-Darriwilian isotope carbon excursion (MDICE), a chemostratigraphic event observed worldwide. The MDICE is preceded by the lower-Darriwilian negative isotope carbon excursion (LDNICE), which likely occurred at the same time as the Decorah impact. Other fossil locales showing a similar negative excursion have an approximate age of around 465-467 Ma. Moreover, the Decorah impact may have occurred at the same time as numerous other mid-Ordovician impact craters in North America and around theBaltic Sea. This brief spike inasteroid impacts and meteorite abundance, the “Ordovician Meteor Event”, likely occurred due to the break up of theL-chondrite parent asteroid. It may be connected to theGreat Ordovician Biodiversification Event (GOBE), a major increase in animal diversity during the mid-Ordovician.[6]
The correlation between the MDICE and the positive excursion found in the Winneshiek Shale has been questioned. Small organic carbon isotope excursions may be influenced by local environmental or ecological conditions, rather than worldwide events. Moreover, numerous excursions occur in every time period, so there is no reason to assume that the Winneshiek record is specifically correlated with the MDICE and LDNICE. The connections between the Decorah impact, other impacts worldwide, and the GOBE have also been criticized for their reliance on imprecise age estimates.[7] Criticisms of the correlation between the MDICE and the Winneshiek excursion have been countered with the argument that alternative explanations have no direct evidence within the strata.Multioistodus subdentatus was presented as a mid-Darriwilian fossil supporting the Winneshiek Shale's biostratigraphic connection to areas with similar chemostratigraphy, despite the lack of direct biostratigraphic overlap.[8]
The fossil content of the Winneshiek Shale, known as the Winneshiek biota, is distinctive when compared to that of most Ordovician fossil sites. There are very few fossils ofbenthic animals (those which live their lives on the seabed) such astrilobites,echinoderms,brachiopods,bryozoans, orcorals. Instead, the Winneshiek biota consists ofnektonic animals (which swim in open waters) or nektobenthic animals (which swim close to the seabed). The most common animal remains areconodont elements andbromalites, which make up >50% and >25% of recovered fossils, respectively.[1][2] Some of the bromalites contained conodont elements, and the bromalites themselves were likely made by eurypterids or larger conodonts.[9] Other fossils includechelicerates,[3][10] bivalvedcrustaceans,[11][12]algae,[13]linguloid brachiopods,[1] a singlegastropod specimen,[11] and head shields from armoredagnathans (jawless fish).[1][2]
Color key
| Notes Uncertain or tentative taxa are insmall text; |
| Arthropods of the Winneshiek Shale | ||
|---|---|---|
| Taxon | Notes | Images |
| Ceratiocaris winneshiekensis[11] | A commonphyllocaridancrustacean, one of the oldestceratiocaridids |  |
| Crustacea indet.[11] | An indeterminate bivalved crustacean, possibly anotostracan orDouglasocaris-like crustacean | |
| Decoracaris hildebrandi[11] | A large bivalved crustacean, possibly the oldest knownthylacocephalan |  |
| Iosuperstes collisionis[11] | An indeterminate bivalved crustacean |  |
| Leperditiidae? indet.[11] | An indeterminate bivalved crustacean, possibly a leperditiidleperditicopidan |  |
| cf.Lomatopisthia[11] | Apalaeocopidanostracod | |
| Palaeocopida indet.[11] | An indeterminate palaeocopidan ostracod | |
| Pentecopterus decorahensis[3] | The oldest knownmegalograptideurypterid |  |
| Podocopida indet.[11] | An indeterminate podocopidan ostracod | |
| Winneshiekia youngae[10] | Adekatriataneuchelicerate similar to eurypterids andchasmataspidids | |
| Chordates of the Winneshiek Shale | ||
|---|---|---|
| Taxon | Notes | Images |
| Archeognathus primus[4] | A common giantarcheognathidconodont |  |
| Astraspis sp.[1] | An armoredagnathan (jawless fish) |  |
| Conodonta indet.[4] | Multiple unnamed or undescribed conodont taxa | |
| Iowagnathus grandis[4] | A common giantiowagnathid conodont |  |
| Multioistodus subdentatus[4] | A conodont | |
| Otherorganisms of the Winneshiek Shale | ||
|---|---|---|
| Taxon | Notes | Images |
| Cladophorales indet.[13] | An unnamed large-celled filamentousalgae, similar toCladophora | |
| Gastropoda indet.[11] | An indeterminate gastropod | |
| Linguloidea indet.[1] | An indeterminate linguloidbrachiopod | |