EFFECTS OF FOSSIL LAG DEPOSITS ON CONODONT DISTRIBUTION IN THE MIDDLE DEVONIAN (GIVETIAN) MILWAUKEE FORMATION

The Milwaukee Formation is a Middle Devonian (Givetian) bioturbated, shaly dolostone restricted in geographic extent to the northern part of Milwaukee County. Conodonts recovered from the Milwaukee Formation correlate with the hermanni and disparilis zones, and the subterminus Fauna. The Milwaukee Formation has abundant brachiopods, bryozoans, crinoids, tentaculids and other fossils, which occur primarily in fossil lag deposits, thin packstone layers varying in thickness from 1 to 50 cm. Lag deposits with fining upwards sequences and laminated mudstones directly above them are interpreted as storm deposits. Others may be the result of storms, but lack these features or were concentrated by currents.

The purpose of this study is to determine if these fossil lag deposits affected the distribution of conodont elements or faunas. A nearly complete section of the Milwaukee Formation was analyzed using core from MMSD borehole I30-NS-DS-7. The core was divided into 54 one-foot sections. From each one-foot section the fossil lag deposits were separated from the non-lag or “normal” deposits using a rock saw. The 108 resulting samples were weighed and dissolved in dilute formic acid and more than 1500 conodonts were recovered.

A parasequence of increasing lag deposits-interpreted as a shallowing upwards trend-was identified by comparing the percentage, by weight, of lag deposits in each one-foot section of core. This shallowing upwards trend continues from the base of the Milwaukee Formation to the unconformable contact of the Berthelet and Lindwurm members. Above the unconformity is a deepening upwards/shallowing upwards cycle of decreasing lag deposits, which reach a minimum (the maximum flooding surface) and then begin increasing again.

Conodont yields from the base of the formation to the Berthelet―Lindwurm unconformity average more than 50 conodonts per kg, but above this section the Milwaukee Formation becomes more argillaceous and conodont yields drop to less than 4 conodonts per kg. Below this unconformity, icriodid elements are inversely correlated with lag deposits while polygnathid conodont abundances remain relatively constant indicating icriodid conodonts had a preference for low-energy, carbonate environments in the Milwaukee Formation.