Geodiversity Atlas—Great Lakes I&M Network Index

The pre-glacial Cenozoic is likewise undocumented in GLKN parks and the GLKN as a whole. The geologic record does not resume until the glaciations of the Pleistocene, and succeeding glaciations usually obliterated the deposits of the previous episode, so most of what we know for a given location pertains to the most recent glaciation and following events. Quaternary deposits are not handled consistently in the literature: in some cases they are put into formations, but in others they are described only in terms of their depositional process (“alluvium”, “lacustrine”, etc.). Formations reported from GLKN units include the latest Pleistocene Copper Falls Formation and primarily early Holocene Miller Creek Formation at APIS (Thornberry-Ehrlich 2015); the West Campus Formation at MISS (Stone 1966); and the Copper Falls, Cromwell, Miller Creek, New Ulm, Pierce, River Falls, Trade River, and West Campus Formations at SACN (National Park Service 2018).

There is practically no record of the Mesozoic in GLKN parks, or elsewhere in the GLKN for that matter, except for Western Interior Seaway rocks in northern and western Minnesota. A reworked Late Cretaceous ammonite has been found at a locality in MISS (Cobban and Merewether 1983); the most likely origin is from Western Interior Seaway rocks to the northwest, from which it was either directly transported by glaciers or by fluvial action after some glacial transport. At the Devil’s Lake State Park unit of ICAG there are local gravel deposits on unglaciated East Bluff (some with Ordovician, Silurian, or Devonian fossils) thought to have been deposited by Cretaceous or Cenozoic fluvial activity when the surrounding land surface was much higher (Clayton and Attig 1990; “East Bluff Member of the Windrow Formation” of Andrews 1958).

By the beginning of the Paleozoic, approximately 540 million years ago, the rift basins had long been filled with sedimentary rocks and significant erosion had occurred in some areas, wearing down ancient Precambrian bedrock in places such as near Taylors Falls in SACN and the Devil’s Lake State Park unit of ICAG; this Cambrian exhumation can be identified due to the presence of conglomeratic facies between the ancient bedrock and typical Cambrian rocks (Clayton and Attig 1990; Tweet and Santucci 2018). Shallow continental seas began to encroach during the Cambrian. Most of the Cambrian deposition in GLKN parks occurred during the late Cambrian. The Jordan Sandstone is exposed in MISS (Tweet 2014). PIRO has outcrops of the late Cambrian–Early Ordovician Munising Formation (Blewett 2012). SACN is located in a classic study area for the late middle–late Cambrian of North America, and includes outcrops of the Mount Simon Sandstone, Eau Claire Formation, Wonewoc Sandstone, Tunnel City Group (Franconia Formation of older references), St. Lawrence Formation, and Jordan Sandstone (Tweet and Santucci 2018). Nonnetwork IATR and ICAG have some areas of Cambrian bedrock outcrops, primarily at the Devil’s Lake, Interstate, and Mill Bluff State Park units of ICAG and associated segments of IATR (Black 1974). These have, respectively, the Parfreys Glen Formation (Clayton and Attig 1990); the Tunnel City Group and conglomeratic facies (Wirth et al. 1998); and the Mount Simon, Eau Claire, and Wonewoc Formations and Tunnel City Group (Clayton 1989).

After a geologically brief marine retreat at the close of the Cambrian, the seas returned in the Early Ordovician, leading to the widespread deposition of carbonate rocks in the GLKN. Further sea level changes were followed by an even more extensive advance in the Late Ordovician, producing heavily fossiliferous carbonates and shales punctuated by bentonite beds representing massive volcanic eruptions. The same three GLKN parks with Cambrian rocks (MISS, PIRO, and SACN) are also represented in the Ordovician. MISS has outcrops of the Early Ordovician Prairie du Chien Group, and of the St. Peter Sandstone, Glenwood Formation, Platteville Formation, Decorah Shale, and Cummingsville Formation from the Middle–Late Ordovician marine advance (Tweet 2014). PIRO has the period-crossing Munising Formation mentioned above and the Au Train Formation, generally dated to the Early Ordovician (Miller et al. 2006) but reportedly including Late Ordovician fossils at a locality in the park (Oetking 1952). SACN has widely distributed outcrops of the Early Ordovician Prairie du Chien Group and a small area with the St. Peter Sandstone, Glenwood Formation, Platteville Formation, and Decorah Shale preserved by faulting (Tweet and Santucci 2018). Non-network IATR and ICAG have Ordovician rocks at the Cross Plains State Park unit, including the Prairie du Chien Group, St. Peter Sandstone, and Platteville Formation (National Park Service 2012). Events after the Ordovician are poorly known in the GLKN parks, none of which have any exposed bedrock younger than the early Late Ordovician Cummingsville Limestone found in MISS. Silurian and Devonian bedrock is present at INDU and SLBE but is buried by Quaternary surficial deposits (see discussion in Hunt et al. 2008). Younger Paleozoic rocks are widespread in some areas outside of the parks within the GLKN, such as the well-known Michigan Basin, but these and other postOrdovician formations are represented in GLKN parks only by eroded material, often transported by glaciers and/or washed up from offshore. Examples of such detrital material include stones with Silurian and Devonian fossils found at INDU and SLBE, and fossiliferous Paleozoic stones found at APIS and ISRO (Hunt et al. 2008).

The GLKN is within the craton, or stable ancient core, of North America, which coalesced from smaller crustal blocks between about 2.5 and 1.0 billion years ago. Approximately 1.1 billion years ago, the North American craton began to rift but stopped, leaving faults and thick intervals of igneous rock in an “n”-shaped scar perhaps from as far as Texas to Alabama, passing through the GLKN from southwestern Minnesota to the present location of Lake Superior and then south through Michigan. The presence of the rift zone is still felt in the GLKN; Lake Superior is within a structural basin formed by the rift, and the rift’s ancient faults have helped define the St. Croix Valley.

By far the oldest rocks in a GLKN park are found in VOYA, which has outcrops of igneous and metamorphic rocks of late Archean age, between 3.0 and 2.5 billion years old. Paleoproterozoic rocks are exposed at GRPO as the Rove Formation (Thornberry-Ehrlich 2019), and at the non-etwork NPS-affiliated ICAG, where the Baraboo Quartzite of late Paleoproterozoic age is exposed at the Devil’s Lake State Park unit as part of the exhumed Baraboo Range (Clayton and Attig 1990). Outcrops of Mesoproterozoic rocks can be found at five of the GLKN network parks, as well as at IATR, ICAG, and KEWE. These rocks are either igneous in origin, representing the Midcontinent Rift, or sedimentary, filling the structural basin left by the rift. APIS has in ascending order the basinfilling Orienta Sandstone, Devils Island Sandstone, and Chequamegon Sandstone (Thornberry-Ehrlich 2015). At GRPO, the Pigeon River Diabase intruded the Rove Formation during rifting (Thornberry-Ehrlich 2019). ISRO has outcrops of the Portage Lake Volcanics and the younger basinfilling Copper Harbor Conglomerate (Thornberry-Ehrlich 2008). PIRO has outcrops of the Jacobsville Sandstone (Blewett 2012), a sedimentary unit of uncertain late Mesoproterozoic– Neoproterozoic age. SACN crosses a variety of Precambrian bedrock units, but glacial and other surficial deposits conceal most of them. There are small outcrops of the Clam Falls Volcanics (sometimes lumped in or identified as the Chengwatana Volcanics) and overlying Copper Harbor Conglomerate (Tweet and Santucci 2018). Among the non-network units, KEWE has a similar complement of Mesoproterozoic basalts followed by basin fill (Copper Harbor Conglomerate, Nonesuch Formation, Freda Sandstone, and Jacobsville Sandstone). Most bedrock exposures in IATR and ICAG are buried by glacial and other surficial deposits (Black 1974), which reflects these units’ establishment for Ice Age features. However, rift basalts are exposed in the Interstate State Park unit of ICAG (Black 1974), which is also one of IATR’s trailheads.