Maquoketa Formation



The Maquoketa formation of Richmondian age, exposed in the UMV, has caused quite a few conundrums concerning its depositional history. My stab at trying to succinctly describe it is as follows: following deposition of the Dubuque formation sea levels increased significantly causing the carbonate shelf to shift toward the north and northwest most likely inundating the Transcontinental Arch and forming a carbonate shelf extending from the present southeast Minnesota all the way to northern Canada. Unfortunately, the following late Ordovician (Hirnatian) ice age caused sea levels to drop significantly allowing erosion of much of this shallow water carbonate over higher elevations.

What remains, in the UMV outcrop belt, preserves the transition zone from carbonate shelf through the slope facies to deep water deposition in the Illinois basin.

The Elgin member of the Maquoketa formation is the lowest member and the only unit of the formation preserved in southeast Minnesota and the border area with northeast Iowa. Here the carbonate deposition exhibits a shallowing upwards sequence moving from a low diversity fauna (primarily graptolites and asphid trilobites) to a more shallow water benthic community with a diverse fauna of brachiopods, pelecypods, nautiloids, trilobites, crinoids, sponges and solitary corals. The rock is primarily micrite (carbonate mud) and for trilobite collectors the primary collecting technique is breaking rock. Large specimens of Isotelus (6 – 10 inch) can be found with persistence but skelites (individual thoracic segments, pygidia, librigem, hypostomes, etc.) of individuals that would rival the world’s biggest trilobite (Isotelus rex) are not uncommon. Also among the dominantly Isotelids can be found rarer forms including Flexicalymene, Gravicalymene, Ceraurus, Ectenaspis and Brachiyaspis.

Shale increases down-slope and up-section as one moves south into Iowa. The Elgin member in Fayette County, Iowa is described as consisting of 66 to 79 feet of micritic limestone with shale interbeds. Shale increases upward through the section. Many, if not all, of those thin (2 – 5 inch) carbonate beds, including the classic Anataphrus vigilans zone represent distal tempestites or turbidites. This is supported by the taphonomy of Anataphrus trilobites that can be found in the carbonate beds. Most are found at right angles or odd angles to the bedding plane and are easiest to find by breaking the carbonate beds and looking on the edges.

The upper Elgin and overlying Clermont shale yield along with Anataphrus, Flexicalymene, Calyptaulax, Ceraurus, Bumastoides, Nahannia, Ceraurinus, Sphaerocorphe, a couple of species of Amphilichas and a rare Scutelid.

Unfortunately, the increased shale in the formation means that outcrops are usually the result of construction or quarrying activity and do not persist, usually becoming grassed over in a few years.