Platteville and Decorah Formations at the McGregor Quarry by Brian Witzke and Greg Ludvigson, Senior Research Geologists Geological Survey Bureau, Iowa,  Department of Natural Resources and Norlene R. Emerson University of Wisconsin - Madison, Wisconsin

Note:  We appreciate the cooperation of the City of McGregor in providing access to this now abandoned quarry.  McGregor utilizes this quarry area for certain types of fill and disposal.  The quarry lies just outside of the park boundary, but the strata exposed here are generally better displayed than the same units within the park (as at Bridal Veil Falls).  Quarries are potentially dangerous places, and the trip leaders will advise a safe approach to the spectacularly complete succession of Platteville and Decorah formations (see Figure).  Fossil and rock collecting by trip participants is possible at this quarry site (collecting of any kind is not permitted in the park).

Introduction to the Geology of the McGregor Quarry

  

St. Peter Sandstone and Glenwood Shale

The short road leading from the highway into the quarry and the adjacent slopes and drainages expose strata of the upper St. Peter Sandstone.  These sandstone strata are overwhelmingly composed of quartz sand, although some clay material is present in the uppermost sandstones.  Low-angle crossbeds are faintly visible in some of the homogeneous fine-grained sandstone beds.  The thin Glenwood Shale interval occurs above the St. Peter Sandstone, although it is rarely exposed due to its soft and easily weathered character.  A portion of the Glenwood Shale, a green-gray noncalcareous shale, is visible in a small exposure on the north side of the entrance road. 

 

Platteville Formation

The prominent quarry walls at this locality belong to the Platteville Formation, and the floor of the quarry is developed on the basal unit of the Platteville Formation.  The lower portion of the quarry walls exposes dolomite strata of the Pecatonica Member.  Although it may not be immediately apparent, most of these dolomite beds contain small molds of fossil material, now left as hollow spaces in the rock.  The basal unit contains some scattered quartz sand, and the overlying beds contain some argillaceous (clay) and shaley material.  The Illinois Geological Survey has subdivided the Pecatonica Member (which they elevate to a formation in Illinois) into several smaller-scale stratigraphic units, and some of these units are recognized at the McGregor Quarry (Chana, Dane, New Glarus members; see Figure and unit descriptions).  A widespread hardground surface (a submarine disconformity surface) is seen at the top of the Pecatonica Member.

The upper portion of the Platteville Formation differs significantly from the underlying Pecatonica dolomites, and is primarily composed of fossiliferous wavy-bedded limestone beds capped by an interval of dolomitic limestone.  This interval has been named the McGregor Member, whose name derives from the City of McGregor; this quarry locality is herein designated the type locality of the member.  These strata are spectacularly fossiliferous, with exceptional preservation of a diverse assemblage of bottom-dwelling invertebrate animals represented.  The wavy-bedded interval is commonly referred to as the Mifflin Member (or formation) in Minnesota and Illinois, but these beds are generally included as part of the McGregor Member in Iowa.  Trip participants are encouraged to examine these limestones and their contained fossils.  Brachiopod shells, trilobite fragments, snails, crinoid debris, bryozoans, and large ostracodes are especially abundant.  The upper dolomitic limestone ledges (units 13-16) have been termed the “Grand Detour” Member (or formation) in the Illinois classification, but the use of this stratigraphic term may not be appropriate in northeastern Iowa.  The upper part of Unit 16 includes dense extremely fine-grained limestone that breaks with a glassy fracture; this lithology is often associated with the Quimbys Mill Member of the upper Platteville Formation at localities in Wisconsin and Illinois, a unit also termed the “glass rock” by the historic lead miners in the Lead-Zinc district.  It is possible that these upper strata at the McGregor Quarry may correlate with a much thicker upper Platteville interval in Illinois which includes the Grand Detour, Nachusa, and Quimbys Mill formations.

The top of the Platteville is marked at the quarry bench above the main quarry walls.  The contact with the overlying Decorah Formation is a sharp planar surface capped by a pale colored (whitish to yellow-orange) bentonite about 1 to 2 inches thick.  This is the Deicke K-bentonite, and widespread volcanic ash layer found throughout the eastern and central United States and northern Europe.  At this quarry, the Deicke bentonite marks the base of the Decorah Formation.

 

Decorah Formation

Strata of the Decorah Formation (lower Galena Group) are nicely displayed in the upper part of the quarry above the upper bench.  These strata are further discussed in the contribution by Ludvigson et al. in this guidebook.  We will access this section by walking up the wooded ridge, and carefully descend a slope into the upper quarry.  Immediately above the Deicke bentonite is a thin limestone bed with a truncated hardground surface at its top; this hardground includes small vertical borings made by an unknown animal, and a truncated shell of a large nautiloid cephalopod can be seen on this surface.  This thin interval represents one of the southernmost known exposures of the Carimona Member, a limestone unit that reaches thicknesses to about 6 feet in Minnesota.  Southward from Pikes Peak, the Carimona limestones are generally absent in the Iowa outcrop.

The overlying interval of green-gray calcareous shale, which is partially covered in places, includes scattered thin beds and lenses of brachiopod-rich limestone (some containing other fossils, especially bryozoans).  Layers of individual shells are also seen in places within the shales.  This shale interval is known as the Spechts Ferry Shale, the lower member of the Decorah Formation, and it is the most widespread shale unit in the formation.  A second bentonite, the Millbrig K-bentonite, occurs a short distance above the base of this shale, but it may require some digging to expose it.

Wavy- to nodular-bedded limestone strata occur above the Spechts Ferry Shale, and these comprise the Guttenberg Member of the Decorah Formation.  These beds resemble in many respects the limestones of the underlying McGregor Member, and their contained fossils are equally spectacular.  Large blocks of Guttenberg limestone are evident in places in the quarry talus, and beautifully preserved brachiopods and other fossils can be seen.  The wavy beds of the Guttenberg are separated by thin shaly layers and partings.  Although not evident at this site, probably due to oxidation and weathering, these thin shaley layers are composed of organic-rich brown shale.  Skeletal grains (especially brachiopods) generally become more abundant but more broken and abraded upward through the Guttenberg, and upper Guttenberg strata lose the wavy-bedded aspect and are seen in more horizontal beds.  Two thin bentonites (altered volcanic ashes) may be present in the Guttenberg succession at this quarry.  The Elkport K-bentonite is seen immediately below the wavy bedded interval, in the upper part of a thin phosphatic limestone and shale unit (termed the Garnavillo Member in the Illinois classification).  A thin light-gray shale in the upper Guttenberg may represent the Dickeyville K-bentonite.

The upper part of the Decorah succession at the quarry contains notably more shale than underlying Guttenberg strata, and this interval is known as the Ion Member in Iowa.  Unlike the Guttenberg, the Ion includes significant amounts of green-gray calcareous and fossiliferous shale.  These shales interbed with argillaceous to shaley limestone beds, many containing abundant brachiopods and other fossils.  A shaley zone near the top includes scattered large bryozoans, including the hemispherical form known as Prasopora.  Prasopora occurs at this stratigraphic position across a broad area of the Upper Mississippi Valley.

Near the base of the Decorah Formation, in the Guttenberg and Spects Ferry members at the McGregor Quarry, a group of geoscientists has investigated a  geologically brief episode of changes in the 13C/12C ratios of inorganic and organic carbon in stratigraphic succession.   Also identified in other portions of the geologid record, these carbon isotope excursions record global perturbations in the earth's carbon cycle.  A positive carbon isotope excursion, first recognized in the Middle Ordovician Decorah Formation of eastern Iowa, has since been recognized in correlative carbonate strata in other regions.  The widespread appearance of this chemostratigraphic event, along with a coincident decrease in d13Ccarbonate - d13Corganic matter values (D13C) during the excursion suggest that the event was global in extent, recording a brief drawdown of atmospheric pCO2 resulting from increased marine organic carbon burial.  Recent analysis of the carbon isotopic chemistry of chlorophyll-based photosynthetic biomarkers in the discovery site in eastern Iowa further supports the idea that the event recorded a global drawdown in atmopheric pCO2.  To view a report on the investigation of this carbon isotope excursion at the McGregor Quarry and a comparison of results to isotope values observed in correlative strata at other sites in the region, click here.

Dunleith Formation

The highest beds in the quarry differ significantly from underlying Decorah strata and are included within the Dunleith Formation of the Galena Group.  These beds are notably less argillaceous and more dolomitic.  Strata of the Dunleith Formation are best seen in the nearby Pikes Peak Quarry (see Stop description).

The stratigraphic succession of the Platteville and Decorah formations seen at the McGregor Quarry can be closely compared with other sections in the region.  A graphic section of the McGregor Quarry and X56 roadcut section north of Guttenberg is illustrated in the Figure to provide a basis for such stratigraphic comparisons.  Many individual beds and units can be correlated across large areas (like those shown on the figure), indicating that similar processes effected deposition across vast regions of the seafloor.  Even so, a general northwestward increase in shale content in Iowa and Minnesota within the Decorah Formation indicates that shale source areas and shorelines once lay in that direction.  Clays derived from those areas did not reach into the far offshore areas, where limestone deposition prevailed.  As those land areas were inundated by the expanding seaway, shale influx ceased altogether later during Dunleith deposition.

 

  GRAPHIC SECTION OF THE STRATA EXPOSED AT THE
PIKES PEAK QUARRY AREA
(SEE WRITTEN DESCRIPTION BELOW)

click on section to view higher resolution image for printing click on section to view higher resolution image for printing Figure 2.  A Graphic illustration of stratigraphic section exposed at the McGregor Quarry. See legend for explanation of symbols used. The X56 roadcut section north of Guttenberg is included for comparison.

Click here for written description of the rock section
at the McGregor Quarry

 

 

 

Stop 10 is in Downtown McGregor

         

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