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Microendolithic Structures from the Fort Payne Formation (Lower

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Microendolithic Structures from the Fort Payne Formation (Lower Microendolithic structures from the Fort Payne Formation (Lower Mississippian), Kentucky and Tennessee: Implications for the paleoenvironment of carbonate mud- mounds A thesis submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Master of Science in the Department of Geology of the College of Arts and Sciences by Jeffrey Hannon B.A. University of Cincinnati June 2010 Committee Chair: D.L. Meyer, Ph.D. i ABSTRACT: Resin cast embedment and naturally occurring internal molds have revealed for the first time an assemblage of microendoliths from the Early Mississippian Fort Payne Formation. Heterotrophic microendoliths Orthogonum lineare, O. giganteum, and Saccomorpha clava dominate this unit, as well as the phototrophic traces Ichnoreticulina elegans and Fascichnus dactylus, a chlorophyte (Ostreobium quekettii) and cyanobacteria (Hyella caespitosa), respectively, that were found within a variety of fossil substrates, including solitary corals, bryozoans, brachiopods, and gastropods, with a notable absence in all echinoderms. The Fort Payne from the Lake Cumberland/Dale Hollow region of southern Kentucky-northern Tennessee contains a suite of fossiliferous green shale deposits with isolated locations of enigmatic carbonate mud-mounds. The latter, commonly referred to as ‘Waulsortian’ mounds, have been the center of much speculation in the past few decades. Paleoenvironmental parameters in which formation occurred have only been indirectly inferred. Using the aforementioned microendoliths, a sense of the light availability within the Illinois Basin can be determined. The ichnofaunal assemblage uncovered is representative of light conditions spanning from dysphotic to aphotic. Using previously constrained depth parameters that indicate a depth of no more than 100m, a dysphotic environment suggests that the water column was eutrophic, presumably due to high productivity within the Illinois Basin. This would seem to corroborate several other inferences about the Early Mississippian, including the growing number of suspension feeding echinoderms, in addition to the restructuring of phytoplanktonic communities and increased nutrient flux from a developing terrigenous floral weathering regime in the Early Mississippian. ii iii TABLE OF CONTENTS SECTION PAGE TITLE…………………………………………….. .............................................................................................................. i ABSTRACT ...................................................................................................................................................... ii TABLE OF CONTENTS....................................................................................................................................iv LIST OF TABLES/FIGURES ............................................................................................................................. iii CHAPTERS CHAPTER 1 – Introduction ............................................................................................................................ 1 CHAPTER 2 – Method .................................................................................................................................... 6 CHAPTER 3 – Results ................................................................................................................................... 12 CHAPTER 4 – Discussion .............................................................................................................................. 17 CHAPTER 5 – Conclusion ............................................................................................................................. 21 REFERENCES ................................................................................................................................................ 22 82 iv List of Tables and Figures FIGURE 1- Locality map of field area…………….………………..…………………………….2 FIGURE 2- Block diagram of reconstructed geology…………………………….………………3 FIGURE 3- Cross section of local geology……………………………………..………………...4 FIGURE 4- SEM photographs of natural microendolith structures……………………………,.10 FIGURE 5- SEM photographs of resin embedment microendolith structures…………….…….11 FIGURE 6- SEM photograph of Fascichnus dactylus…………………………………………..13 FIGURE 7- Infestation frequency based on substrate within natural mold..……………………15 FIGURE 8- Infestation frequency on natural bryozoans molds……………………..…………..16 TABLE 1- Total microendolithic inventory……………………………………………………..12 v CHAPTER 1- INTRODUCTION The Lower Carboniferous (Mississippian) is known worldwide for an abundance of shallow- to deep-water limestone mounds, referred to as bioherms, carbonate buildups, mud-mounds, or even reefs (Bosence and Bridges, 1995). They are unlike typical reefs formed by biogenic skeletal intergrowth or framework, and instead contain carbonate mud with varying amounts of interstitial bioclasts, large skeletal material, or peloids (Meyer et al., 1995; Monty, 1995). Similar carbonate mounds occur in the Lower Carboniferous globally, and are typically referred to as Waulsortian mounds (Lees and Miller, 1995), after a locality in Belgium. In south-central Kentucky and north-central Tennessee (Fig. 1), the Lower Mississippian Fort Payne Formation is represented in a suite of naturally exposed outcrops and highway road cuts. Carbonate buildups are commonly present in the area and are characterized by the presumed growth of autochthonous carbonate mud in the form of large mounds, subsequently colonized by vast communities of benthic organisms such as filter feeding crinoids and fenestrate bryozoans, brachiopods, corals, mollusks, and other, less common taxa (Meyer et al., 1995; Krause, 2004). An important and unresolved aspect of the carbonate buildups is the environment of formation. The water depths of the mounds within the Illinois Basin have been constrained based on rare occurrences of photoautotrophs and the lack of storm influenced sedimentary features. Dasycladacean algae have been detected, but are rare (less than 1% of constituents) (Ausich and Meyer, 1990) and only indicate a depth somewhere in the photic zone, up to ~60 m (maximum depth range of dascylads), assuming the algae were autochthonous to the mound. To provide further, perhaps more detailed evidence to constrain water depth, microendolithic structures, intricate networks of borings produced by microscopic organisms of various trophic lifestyles that colonized stable carbonate structures, can be used as depth indicators for ancient basin analyses (Gektidis, 1999). The Fort Payne Formation provides an opportunity to use microendoliths to analyze water depth. In particular, a detailed analysis of Fort Payne microendoliths in the light of information available on a published microendolith photic zonation chart (Glaub, 1994) can clarify the photic levels within the Ft. Payne and, in conjunction with independently derived estimates, can help demonstrate the eutrophy of the ancient water column. An analysis of the microendolithic communities throughout the green shale facies immediately 1 surrounding carbonate mud-mounds will help clarify the environmental conditions of carbonate mud deposition. Geologic setting In the study area, the Fort Payne Formation is a suite of dolosiltstones, shales and carbonates deposited during the Early Osagean Stage of the Mississippian (Lewis and Potter, 1978). It represents the siliciclastic progradational infilling of the Illinois Basin comprising the Borden Delta within the Mississippian epeiric sea (Krause, 2004). The Borden Delta is known Fig. 1. Locality map with approximate geographical location of for its extensive deposits Mississippian Borden Delta Front. Proximity of each sampling locality of siliciclastics (siltstones to the delta front is shown. 61N=Renox Creek; BF9=Blacks Ferry; DH=Dale Hollow; OB=Owens Branch. and shales) and its location shifted during the Mississippian away from the study area (Ausich and Meyer, 1990; Krause, 2004). Underlying and surrounding the carbonate mounds is a succession of fossiliferous green shales with interbedded tabular packstones, the result of storm influenced, periodic reworking of the Borden Delta siliciclastics (Fig. 2). During the early Osagean (Fig. 3) local portions of the Borden Delta were abandoned as it moved northwards, resulting in episodic deposition from storm-influenced reworking of the Borden siliciclastics. Areas of exceptional fossil preservation within the shale, due to rapid burial events further support this theory (Krause, 2004). Lowstand 2 Fig. 2. Block diagram of the Fort Payne Formation facies from the Illinois Basin slope. DHW/E=Dale Hollow West/East; LC=Lake Cumberland. Modified from Terry 1990. effects on pre-existing highstand deposits included channeling and deformation from slope reorganization, resulting in isolated areas of mounding within the green shale. The carbonate buildups were initiated and built up during a marine transgression, nucleating on the topographic highs of green shale mounds, and terminating at the maximum flooding surface during the early highstand. Green shale deposition resumed during the ensuing highstand, blanketing the mound and repeating the process during another sea level oscillation cycle (Fig. 3; Krause, 2004) . The composition of the atmosphere was changing dramatically during this time. Levels of atmospheric O2 and CO2 were changing drastically, with the largest and most abrupt decline in CO2 and rise in O2 seen
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