Where: Allamakee County County, Iowa (43.3° N, 91.4° W: paleocoordinates 19.7° S, 61.3° W)
• coordinate based on political unit
• outcrop-level geographic resolution
When: Elgin Member (Maquoketa Formation), Maysvillian (452.0 - 449.5 Ma)
• Basal 15 cm phosphorite in Elgin Member, Maquoketa Formation, at Dubuque Formation contact.
• group of beds-level stratigraphic resolution
Environment/lithology: deep subtidal ramp; lithified, dolomitic, pyritic phosphorite
• The strong tendency for some groups to be preserved articulated while echinoderms are not suggests a rather low sedimentation accumulation rate, but this is not a true starved basin situation. The pitted and mineralized Maquoketa-Dubuque contact resembles many other corrosion surfaces in the Upper Mississippi Valley Galena Group (Witzke, 1985b), and likely formed in a submarine, not subaerial environment. If the suggestion of Brown (1974) that upwelling of oxygen-poor, phosphate-rich waters into a typical carbonate shelf edge produced the diminutive Maquoketa faunas is correct, then this community, with a significant but small echinoderm component, marks the transition from the highly affected Palaeoneilo fecunda-Nuculites neglectus Community (with almost no echinoderms) to the more normal marine Thaerodonta recedens-Iowacystis sagittaria Community. Benthic assemblages have not been defined for many Ordovician communities as yet, but a position analogous to BA 3-4 is probable (see also Witzke & Glenister, 1987, for further discussion).
• The lithology is a poorly consolidated to pyrite-cemented dolomitic phosphorite ranging in thickness from 0.1 to 0.3 m. Most common are small (<2 mm) subsphaeroidal concentrically laminated hydroxylapatite clasts, but less well rounded phosphate clasts up to 15 cm in length and phosphatized fossils are also abundant. Locally quartz geodes ranging in size from 1 cm to more than 25 cm in length are common, e.g., at the type locality. The geodes contain minor sulfide mineralization (wire marcasite) and common calcite, ferroan dolomite, fluorite, and selenite crystals in the often largely hollow interiors... This community appears to have been subject to some current activity, but not frequently enough to remove selectively particular size clasts. The deposit is very poorly sorted, at least basally, with clay to gravel-sized clasts intermixed, some or all of which may be phosphatized.
Preservation: replaced with phosphate
Primary reference: T. J. Frest, C. E. Brett, and B. J. Witzke. 1999. Caradocian-Gedinnian echinoderm associations of Central and Eastern North America. Paleocommunities--a case study from the Silurian and Lower Devonian 638-783 [M. Foote/K. Koverman/P. Wagner]more details
Purpose of describing collection: paleoecologic analysis
PaleoDB collection 13261: authorized by Michael Foote, entered by Kimberly Koverman on 04.02.2002
Creative Commons license: CC BY (attribution)
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