Distribution of exceptional biotas in large-scale depositional sequences in Silurian rocks of the Great Lakes region

Although rare, exceptional biotas occur at several horizons in Silurian (Llandovery-Ludlow) rocks of the Great Lakes region. They are characterized by similar taxa, including algae, "worms", graptolites, conularids, and a variety of non-calcified arthropods, which differ sharply from the usual shelly benthos in these rocks. Generally, each horizon occurs in an atypical depositional environment distinct from dominant normal marine settings in these rocks. The exceptional biotas are most commonly associated with one of two lithologies: organic-rich argillaceous carbonates or fine-grained laminated carbonates. Both of these lithologies are poorly fossiliferous, contain few shelly benthos, and occur in very specific positions within a series of large-scale and long-term depositional sequences that characterize Silurian carbonate platform rocks of this region. Each sequence spans tens of thousands of years or more, beginning with an initial widespread flooding event and ending with a significant sea level drop and local emergence. Initial stages of each sequence, including exceptional biota horizons, commonly are associated with a pronounced positive delta (super 13) C isotope excursion, which coincides with other known Silurian excursions. Each sequence appears to represent a global sea level change that is glacial-eustatic in nature. The particular depositional environments associated with exceptional biotas appear to have developed at specific times during each of these sequences. Those found most commonly within the fine-grained laminated lithology were deposited under local hypersaline conditions that followed initial sea level fall at the end of each sequence. The exceptional biotas found in the organic-rich argillaceous lithology were deposited during initial transgression of a sequence under anoxic conditions, which accompanied the restricted circulation associated with flooding of an irregular surface. The compositional similarity of these biotas through time suggests that they comprise long-ranging, but seldom fossilized, taxa that typically are preserved only during episodes of exceptional preservation. These episodes correlate with unique conditions that existed at specific times and positions within depositional sequences related to glacial-eustatic sea level fluctuations.