ARCHAEOCYATHS FROM THE NORTHERN MARGIN OF THE ILLINOIS BASIN, AND THEIR IMPLICATIONS FOR THE AGE AND DEPOSITIONAL SETTING OF THE LOWER MT. SIMON SANDSTONE AND “ARGENTA FORMATION”

Recently discovered fossils from northeastern Illinois provide new age constraints on the basal sedimentary units of the Illinois Basin (ILB), including the lower Mt. Simon Sandstone (LMS) and the underlying “pre-Mt. Simon” or “Argenta Formation.” Fragmentary archaeocyaths were identified in cuttings from the McCoy #1 well in Will County, Illinois, along the northern margin of the ILB. Specific identification is not possible due to the condition of the material, but morphological elements such as an intervallar syrinx and intercommunicating inner-wall canals indicate an age range within Cambrian Stages 3–4, specifically Botoman–Toyonian (~510–516 Ma). These specimens are the oldest fossils ever found in Illinois, and constitute the first unambiguous evidence that rocks of the Sauk I supersequence are preserved within the ILB. Fossil-bearing horizons occur in both the LMS and Argenta, implying that the top of the LMS corresponds to the Sauk I–II unconformity. This indicates a significant hiatus between deposition of these units and deposition of the upper and middle Mt. Simon, which are thought to be Marjuman (497–504.5 Ma). The LMS and Argenta at McCoy #1 were deposited at least partially in shallow shelf environments off the southern shoreline of Laurentia. Correlative strata are apparently confined to a “proto-Illinois Basin” in eastern Illinois and western Indiana, thought to be related to post-Rodinia rifting. Core from Argenta and LMS intervals at the Illinois Basin – Decatur Project (IBDP) site, in central Illinois, shows some marine influence but with a stronger terrestrial signature than the McCoy succession. Depositional, mineralogical, and diagenetic differences between the Argenta and LMS at IBDP have previously been interpreted as evidence of a hiatus between the LMS and Argenta. However, similarities in the LMS and Argenta fossil assemblages at McCoy #1 suggest that any such hiatus may have been brief.