Fossils and Stratigraphy of the Upper Ordovician Standard in South Eastern Indiana

摘要

Given the lack of economic deposits, the Upper Ordovician rocks in and around the Cincinnati region, including southeastern Indiana, have received remarkably consistent attention from geologists since the mid to late 1800s. This is, largely, because they are among the most richly fossiliferous deposits in the world. Fossils are intrinsically interesting if for nothing more than their beauty. The following plates include some of the most common fossils and some of the most sought-after fossils that might be encountered on the fieldtrip. With the exception of two photos, the fossil figures were taken from Cummings (1907). The abundance of fossils makes the deposits a convenient natural laboratory, and recent studies include the ecological dynamics of species migration (the Richmondian invasion; e.g. Stigall, 2010), the exploration of continent-scale evolutionary relationships (e.g. Jin 2001; 2012), and the day-to-day interactions of extinct forms (Dattilo et al. 2010; Freeman et al. 2013).In this guidebook you will see hints of a complex history of stratigraphic nomenclature. Early stratigraphic work by Cummings (1907) in Indiana and others in the immediate area of Cincinnati (summarized by Caster et al., 1955) relied heavily on fossil content to correlate relatively thin units over large areas. In the 1960s (e.g. Peck, 1966; Brown u26 Lineback, 1966), an emphasis on the facies concept and the strict separation of lithostratigraphy and biostratigraphy inspired a proliferation of new named units that tend to follow political boundaries like state lines. The resulting correlation chart (Cuffey, 1998: copied herein) is a bit confusing, in part because it reflects the concept that lithologic units are facies mosaics and that tracing thin units for long distances is impossible. With the advent of event stratigraphy and sequence stratigraphy, the concept of “stratigraphic surfaces” was added to the geologist’s lexicon. Older stratigraphic approaches were revived and revised in a new sequence stratigraphic system (e.g. Holland and Patzkowski, 1996). Ongoing work is sequence stratigraphic in basis and has resulted in the extension and refinement of the earlier stratigraphic system, as well as the elimination of “state line stratigraphy” (e.g. Brett u26 Algeo. 2001).Underlying stratigraphy is sedimentology, and the key sedimentological question in the Cincinnatian is the origin of shelly limestone beds intercalated with mudstone beds, as well as small scale cycles that consist of alternating limestone and mudstone rich phases. If these meter-scale cycles are so extensive that they can be traced individually across the Ohio, Kentucky and Indiana outcrop area, how are they generated and how is it that they don’t disappear into a mosaic of facies. Since most shell beds contain abundant evidence of reworking, and since the area was in the tropical storm belt during the Ordovician, these beds and cycles have long been interpreted as storm beds or “tempestites” that formed from storm winnowing (Kreisa, 1981). More recently arguments have been made in support of basin-scale fluctuations in the supply of mud from the Taconian Orogen (Brett et al., 2008; Dattilo et al., 2008, 2012) as the principle cause of bedding, with ubiquitous storm (or tsunami?) reworking playing only a minor role.