Galena Formation



The Galena Formation or group in the UMV is an interesting unit. In Illinois and Iowa the Galena Group consists of the Dunleith and Wise Lake Formations while in Minnesota the Galena Formation consists of the Cummingville, Prosser and Stewartville Members (in ascending order).

The Galena Formation (speaking from a Minnesota perspective) records a transgressive – regressive sequence commencing with the underlying Decorah Formation, reaching maximum water depth (highstand) in the Prosser Member and shallowing again through the Stewartville Member and lowstand in the overlying Dubuque Formation before commencing the next transgressive cycle.

As mentioned in the description of the Decorah: “As the Galena transgression continued the shale facies, represented by the Decorah stepped further northwestward replaced by carbonates in the form of the Cummingsville member (Dunleith) of the Galena”. The Cummingsville Member, in proximity to the type area (southern Olmsted County), exhibits a sawtooth pattern of slightly to moderately dolomitic limestone-shale sequences. Upon cursory examination this unit appears to be void or nearly void of fossils but under careful examination beds within this unit can be very productive, yielding a wide variety of both trilobites and echinoderms. Trilobites found in the Cummingsville include: Illaenus americanus (Thaleops laurentiana), Calytaulax callicephalus, Ceraurus pleurexanthemus, Failleana sp., Sphaerocorphe sp, etc. Hardgrounds (intraformational non-depositional surfaces) do occur within this unit but not as commonly as the overlying Prosser but the hardgrounds encountered within the Cummingsville are extremely productive surfaces to explore for echinoderms.

The Prosser Member represents the deepest water deposited unit within the Galena. The unit is the purist lime unit and probably represents lower shelf deposition. As noted above hardground surfaces are abundant within this unit, generally appear burrowed and many exhibit a thin pyrite coating on the surface. Unlike those in the Cummingsville the hardgrounds in the Prosser do not produce echinoderms for the most part. There are several bentonite layers within the Prosser and much work is being done to assign correlations to other Shermanian age rock units around the country. The most common trilobite found in the Prosser is Anataphrus borreaus (Isotelids are thought to be deeper water inhabits). Other trilobites include Ceraurus pleurexanthemus, Flexicalymene senaria, Dolichoharpes dentoni, Eobrontus lunatus, Encrinuroides, Ceraurinus marginatus, Amphilichas sp. and rare Sphaerocorphe and Holia (see the faunal list for a full accounting).

The Stewartville Member is the uppermost member of the Galena Formation and is a dolomite rock unit representing a shallowing of the sea level. The unit is best known for the abundant large gastropods Maclurites and Hormotoma as well as many Receptaculites (Fisherites). Because of the dolomitization that this unit underwent trilobites are uncommon though molds and cast of larger specimens can be found in the unit.