PENTAMERID BRACHIOPOD INTERVALS AND THEIR RELATIONSHIP TO DEPOSITIONAL SEQUENCES IN THE SILURIAN (LLANDOVERY) OF EASTERN WISCONSIN AND NORTHEASTERN ILLINOIS

Three prominent intervals of pentamerid brachiopods occur as coquinas in Llandovery strata in eastern Wisconsin and northeastern Illinois. Two of these intervals mark specific positions within depositional sequences. The lowest interval, characterized by Virgiana, occurs as a biostromal coquina and in a zone around large algal mounds in the lower Mayville Dolomite (Rhuddanian). The two other Llandovery pentamerid intervals occur as bank-like coquinas in level-bottom environments. Abundant Virgiana characterize the middle interval at the top of the Mayville Dolomite (Rhuddanian) on the north. To the south, this may coincide with a Platymerella interval in the Elwood Dolomite (Rhuddanian). Pentamerus characterizes the upper pentamerid interval in the Manistique Dolomite (Aeronian) on the north and at the top of the Kankakee Dolomite (Aeronian) on the south. Stricklandiid brachiopods co-occur at this horizon throughout the study area. The middle and upper pentamerid intervals occur within transgressive sequences preceding deposition of pure carbonate highstand sediments.

In the northern part of the study area, the pentamerid intervals are separated predominantly by intertidal-supratidal laminites and shallow subtidal deposits. This pentamerid-laminite cyclicity has caused stratigraphic confusion and regional miscorrelation in the past. In northeastern Illinois, the pentamerid intervals are separated by restricted subtidal and peritidal deposits. The two upper pentamerid intervals extend throughout the study area despite these generally deeper water conditions on the south. Sequence boundaries are more difficult to recognize on the north because subsidence maintained accommodation space. Moreover, features characteristic of such shallow depositional environments on the north may be mistaken for more significant exposure surfaces related to sequence boundaries.

During the Wenlock, pentamerid intervals once again become prominent in carbonate mound environments. A noted exception is the “Pentamerus” interval in the upper Laurel Limestone and its equivalents in southwestern Ohio, southern Indiana, and north-central Kentucky. In that region, pentamerid coquinas occur in a bank-like level-bottom setting similar to those in the middle and upper Llandovery occurrences in the study area.