New Evidence for Reconstructing the Marine Faunal Assemblage of The

New evidence for reconstructing the marine faunal assemblage of the Decorah Formation (southeastern Minnesota and northeastern Iowa, USA): A qualitative survey of microfossils Phillip J. Varela Senior Integrative Exercise March 11, 2009 Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, Minnesota Table of Contents Abstract Introduction............................................................................................................1 Background............................................................................................................1 Ordovician Laurentia...................................................................................1 The Great Ordovician Biodiversification Event..........................................4 Geologic Setting......................................................................................................5 Stratigraphy of the Decorah Formation......................................................5 Paleoenvironment and Depositional Setting...............................................8 Field and Laboratory Methods...........................................................................13 Microfossil Survey Results..................................................................................14 Conulariids..................................................................................................17 Chitinozoans................................................................................................20 Conodonts....................................................................................................20 Scoleconodonts............................................................................................25 Reevaluation of Faunal Assemblage...................................................................27 Acknowledgements..............................................................................................29 References Cited...................................................................................................29 New evidence for reconstructing the marine faunal assemblage of the Decorah Formation (southeastern Minnesota and northeastern Iowa, USA): A qualitative survey of microfossils Phillip J. Varela Carleton College Senior Integrative Exercise March 11, 2009 Advisor: Clinton A. Cowan, Carleton College Department of Geology ABSTRACT Three Late Ordovician (Mohawkian) outcrops of the Decorah Formation in southeastern Minnesota and northeastern Iowa contain abundant fossils representing a diverse shallow-water marine biotic community. Although some macrofossil communities have been documented and analyzed at great depth, many microfossil communities remain virtually unknown. In order to establish a complete understanding of the paleoenvironmental conditions and dominant biologic communities, it is necessary to establish a better understanding of microfossils. A total of nine shale samples from Kenyon, MN, Rochester, MN, and Decorah, IA were analyzed for microfossil content. From this survey, I identify five groups of poorly documented or poorly understood macro- and microfossils, all of which are not easily found in hand sample and are only recoverable after processing for microfossil collection. Fossil groups include chitinozoans, conodonts, conluariids, Linguliformea (phosphatic inarticulate brachiopods) and scolecodonts. The goal of the survey is to add to the growing knowledge of the biotic community that defines the Decorah Formation and to integrate these poorly understood fossil groups with the previously established faunal assemblage. Based on the inferred lifestyles of microfossils found within the Decorah Formation, this study maintains the paleoenvironmental parameters of previously established reconstructions and adds insight into the representative biotic community. Keywords: Decorah Formation, microfossils, chitinozoan, conodont, conulariid, Linguliformea, scolecodont, paleoenvironment 1 INTRODUCTION Examination of the distribution of microfossils in three outcrops of the Late Ordovician (460.9 - 443.7 Ma) Decorah Formation, including an outcrop not previously investigated for microfossils, helps constrain the paleoenvironment and depositional setting in southeastern Minnesota and northeastern Iowa. The Decorah Formation contains an abundance of well-preserved macrofossils and trace fossils, and the biostratigraphic distribution of these fossils has been examined in great detail (see summary in Sloan, 1987). This report does not focus on macrofossils; rather, it surveys the microfossil content within the lower members (Spechts Ferry, Guttenberg, Ion) of the Decorah Formation to further constrain the known faunal assemblage and assess the paleoenvironment based on parameters for organism life styles. Through this qualitative microfossil survey, I attempt to provide insight for further understanding the general environments and biological communities represented within the Decorah Formation. BACKGROUND Ordovician Laurentia The Ordovician Period (488.3±1.7 to 443.7±1.5 Ma; Gradstein et al., 2004) is characterized by generally high sea levels, with global transgressions leading to the generation of shallow epeiric seas (Barnes, 2004). Most of the large landmasses were situated in the southern hemisphere (Achab and Paris, 2006), with the exception of Laurentia (largely modern North America), which straddled the equator. Paleographic reconstructions (Fig. 1; Witzke, 1980; Young et al., 2005) place what are now most of eastern and central North America anywhere 2 EQUATOR N 30° S Fig. 1. Late Ordovician paleogeography of Laurentia (Modified from Witzke 1980). Study area is outlined. 3 between 10° and 30° S latitudes during this time. Consequently, Laurentia is considered to have been tropical during most of the Ordovician, with very little continental migration or rotation (Cocks et al., 2004). The eastern margin of Laurentia is marked by the development of a convergent tectonic setting, resulting in subduction, microterrane collision, and volcanism during Late Ordovician time (Mac Niocaill, 1997). This tectonic activity was responsible for the Taconic Orogeny, a mountain building event attributed to the genesis of the Appalachian mountain belt (Achab, 2006). K- bentonites, widespread ash beds altered to potassium-rich clays characteristic of volcanism along an active tectonic margin (Huff et al., 1992), are well known in Mohawkian sequences, and Emerson et al., (2004) emphasize their stratigraphic, paleontological, and sedimentological implications in the Decorah Formation, as K-bentonite correlation has provided a framework for biostratigraphy. The global climate of the Ordovician is generally considered a transition from an early greenhouse state to a later icehouse state, terminating with extensive glaciations and a mass extinction event (Barnes, 2004). Because shallow epeiric seas covered much of the Ordovician world continental areas, sea surface temperatures are important for understanding the nature of oceanographic and climatic systems. Recent studies of the marine 18O record from conodont apatite (Trotter et al., 2008) and calcite (Tobin et al., 2002) assume sea surface temperatures between ~42°C and ~22°C. The lower end of these temperatures falls within the range of present day sea surface temperatures (Trotter et al., 2008). 4 The Great Ordovician Biodiversification Event The Paleozoic Era experienced two major radiations of global biodiversification. The first is known as the Cambrian Explosion, which saw a rapid appearance of complex organisms and diversification at the phylum and class levels (Droser and Finnegan, 2003). The second is the Great Ordovician Biodiversification Event (or the Ordovician Radiation), which led to a significant increase in organism biodiversity at lower taxonomic levels (Miller, 2001). During the Ordovician Radiation, organism family and genus diversities increased three- to fourfold relative to Cambrian communities (Trotter et al, 2008). Most significant to the character of the seafloor was the diversification of skeletal organisms such as brachiopods, bryozoans, cephalopods, conodonts, corals, crinoids, graptolites, ostracodes, stromatoporids, and trilobites. The dominance of these organisms greatly changed the marine environment from trilobite-dominated to brachiopod-dominated soft substrates, and echinoderm- and bryozoan-dominated hard substrates developed (Droser et al, 1997; Harper, 2005). In reef ecosystems, tabulate corals and stromatoporids began to dominate over sponges in the Ordovician and continued throughout the Paleozoic Era. Diversification of marine organisms induced important paleoecologic changes, such as the development of new modes of life (Harper, 2005). Droser et al. (1997) propose the term “Bambachian megaguilds” for the adaptive strategies of organisms originally described by Bambach (1983). During the radiation, new megaguilds (organism life styles) developed and there was an increase in sessile epifaunal (e.g. suspension feeders) and mobile epifaunal (e.g. detritivores, 5 herbivores, carnivores) organisms (Harper, 2005). There is a clear transition from a primarily infaunal community in the Cambrian to a mixed infaunal and epifaunal community in the Ordovician. The timeframe of this biodiversification event is estimated to have lasted 25 million years (Harper, 2005), and although the chronostratigraphy for the Ordovician is well established, there remains difficulty in

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