Identification of the late Homerian Mulde Excursion at the base of the Salina Group (Michigan Basin, USA)

Abstract

Constraining the age of strata is a fundamental source of uncertainty in the study of sedimentary rocks, particularly in restricted basins that generally lack index fossils. An illustrative example of this is the evaporite‐bearing Salina Group in the Michigan Basin. Our integrated study of facies, paleontology, and stable isotope geochemistry from the base of the Salina Group in Wisconsin addresses long‐standing chronostratigraphic uncertainty surrounding these units. Conodont samples from the basal boundary interval (Racine‐Waubakee formation contact) produced non‐diagnostic ‘disaster’ and ‘recovery’ faunas typical of both the Mulde (Homerian) and Lau (Ludfordian) events. Strontium isotope analysis (87Sr/86Sr) of these conodonts from five horizons just below the boundary yield values between 0.70844 and 0.70850 confirming a Homerian age. Multiple carbon isotope profiles through this interval confirm the presence of a 2.5–3‰ positive excursion. Cumulatively these data constrain the base of the Salina Group in Wisconsin to the Mulde Excursion interval (late Homerian). This integrated study provides a sound initial step towards a deeper understanding of the processes of Silurian evaporite formation in the Michigan Basin.

Utility of organic carbon isotope data from the Salina Group halite (Michigan Basin): A new tool for stratigraphic correlation and paleoclimate proxy resource

“…This is observed in the new halite-derived carbon isotope data set from the Michigan Basin. The composite curve in Figure 6 is in general agreement with the global curves of Cramer et al (2011Cramer et al ( , 2015 and Sullivan et al (2016). As mentioned, Kuglitsch (2013) identified Kockelella walliseri slightly above co-occurring elements of Kockelella ranuliformis and Ozarkodina sagitta rhenana from the lower Lockport Formation, at 4540 ft-4580 ft in the Weinert #2-6 core, correlative with the lowest positive CIE (orange and green bars in Figs.…”

“…However, as mentioned earlier, studies on the southern and eastern basin margin show that the Niagara-Salina transition occurred during upper Wenlock (Homerian) time (Cramer, 2009;Swift, 2011;Kleffner et al, 2012;McLaughlin et al, 2013;Sullivan et al, 2016). In a study from the western margin (modern-day Wisconsin), Sullivan et al (2016;their fig. 5) successfully constrained the Mulde event CIE from the correlative Niagara-Salina Group boundary by demonstrating Homerian 87 Sr/ 86 Sr ratios in conodont elements recovered from the acme of the positive CIE, thus constraining this event.…”

“…The Niagara barrier and pinnacle reef system is capped by up to 762 m (2500 ft) of interbedded anhydrite, halite (Fig. 1D), and organic-rich carbonate mudstone of the Salina Group, representing a major change in paleoenvironment and local sea level (Mesolella et al, 1974;Sullivan et al, 2016). The Salina Group is divided into repeating sequences of evaporites, carbonates, and shales identified as Salina units A-0 to G (Mesolella et al, 1974).…”

“…4 in Brett et al, 1995). On the southern and western margins of the Michigan Basin, this lithological transition appears older, constrained to the upper Homerian (Wenlock) by the conodonts Ozarkokina bohemica longa and Pseudooneotodus linguicornis (NE Indiana and Ohio; Kleffner and Rexroad, 1999;Cramer, 2009;Swift, 2011;Kleffner et al, 2012) and by the onset of the Mulde event CIE (NE Indiana, Ohio, and Wisconsin; Cramer, 2009;Swift, 2011;Kleffner et al, 2012;Sullivan et al, 2016). However, this apparent difference in stratigraphic position for the base of the Salina Group between the eastern United States and the Michigan Basin area is not an implied diachroneity, rather, it is an indication that this lithological transition is complex and is less precisely known in the New York area.…”

“…6), temporal constraint is hampered by a lack of viable conodont data from the Niagara-Salina Group boundary in the Michigan Basin center (Klug, 1977;Kuglitsch, 2000;Sullivan et al, 2016), and a lack of age data from the Salina Group. However, as mentioned earlier, studies on the southern and eastern basin margin show that the Niagara-Salina transition occurred during upper Wenlock (Homerian) time (Cramer, 2009;Swift, 2011;Kleffner et al, 2012;McLaughlin et al, 2013;Sullivan et al, 2016). In a study from the western margin (modern-day Wisconsin), Sullivan et al (2016;their fig.…”