University of Florida Thesis Or Dissertation

PALEOENVIRONMENTAL ANALYSIS OF MONTBROOK: AN UNUSUAL FOSSIL LOCALITY FROM THE LATE MIOCENE IN NORTHERN FLORIDA

MICHAEL JOSEPH ZIEGLER

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2019

To Dorothy Helvak

ACKNOWLEDGMENTS

I would like to formally thank all the enthusiastic volunteers of the Montbrook Fossil Site during the 2017-18 season for their interest and support during fieldwork. I thank Florida

Geologic Survey staff for selflessly sharing resources and expertise. I thank my Thompson Earth

System Institute: Moonshot Project 2019 Cohort lab group for assisting in the collection of research materials. Similarly, I thank my undergraduate researchers for preparing microfossils used in this study. I thank my graduate advisor and committee for pushing me to be a stronger researcher and communicator of science. With love, I thank my family for endless encouragement and laughter.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...... 4

LIST OF TABLES ...... 7

LIST OF FIGURES ...... 8

LIST OF ABBREVIATIONS ...... 10

ABSTRACT ...... 11

CHAPTER

1 MONTBROOK SITE INTRODUCTION ...... 12

Research Inquiry ...... 12 An Exceptional Miocene Locality ...... 13 Florida Platform During the Miocene ...... 16

2 LITHOSTRATIGRAPHY AND FACIES MODELING ...... 24

Regional Stratigraphic History ...... 24 Methods ...... 27 Microfacies Analysis ...... 27 Smear Slide Analysis ...... 28 Sediment Core Analysis ...... 30 Results ...... 31 Microfacies Analysis ...... 31 Rippled sand unit ...... 32 Massive fossiliferous gravel unit ...... 33 Wavy cross laminated unit ...... 35 Heterolithic flaser unit ...... 37 Smear Slide Analysis ...... 39 Sediment Core Analysis ...... 43 Discussion ...... 46 Smear Slide Analysis ...... 46 Micofacies Analysis ...... 51 Sediment Core Analysis ...... 55 Conclusions ...... 58

3 ICHNOFOSSILS AND BIOTURBATION INDEX ...... 83

Methods ...... 84 Results ...... 86 Discussion ...... 87

Conclusions ...... 90

APPENDIX

A PALEOMAGNETISM ...... 96

B EDUCATION, OUTREACH AND COLLABORATION ...... 100

LIST OF REFERENCES ...... 103

BIOGRAPHICAL SKETCH ...... 114

LIST OF TABLES

Table page

2-1 Summary of facies assignments in Montbrook outcrop...... 80

2-2 Summary of smear slide results with patterns of presence and absence of major sediment constituents...... 81

3-1 Summary of BI analysis including percentage area of primary sediment structures (P.S.) and secondary sediment structures (S.S) with associated depth...... 95

LIST OF FIGURES

Figure page

1-1 Map of the Montbrook Fossil Site in Florida. Levy County is subset to illustrate the location of Montbrook as an area of interest in north-central Florida ...... 20

1-2 Faunal reconstruction of Montbrook, October 28, 2019. Courtesy of Rachel Keeffe ...... 21

1-3 Age distribution of Miocene and Pliocene fossil localities in Florida ...... 22

1-4 Geologic map of Florida modified from Scott et al. (2001) with Neogene fossil localities. Subset illustrates prominent fossil localities in north-central Florida and their proximity to Montbrook...... 23

2-1 Geologic cross-section of Florida from Scott et al. (2001) featuring the north to south transect ...... 61

2-2 Montbrook fossil site overview highlighting the various areas of interest within and around the locality...... 62

2-3 Representative outcrop and key geologic features observed from the lower step ...... 63

2-4 Representative outcrop and key geologic features observed from the upper step ...... 64

2-5 Correlated lithostratigraphic columns of upper and lower steps based on site description and microfacies analysis where depth is measured in centimeters from the surface ...... 65

2-6 Representative section of rippled sand unit (facies A) ...... 66

2-7 Representative section of massive gravel unit (facies B) ...... 67

2-8 Representative section of wavy cross laminate sand unit (facies C) ...... 68

2-9 Representative section of heterolithic flaser unit (facies D) ...... 69

2-10 Feldspar grains observed from smear slide samples in XPL ...... 70

2-11 Heavy mineral grains observed from smear slide samples in PPL ...... 70

2-12 Quartz grains with pyrite framboids observed from smear slide samples in PPL ...... 71

2-13 Palygorskite observed from smear slide sample in XPL ...... 71

2-14 SEM images of artifacts that contain needle-like minerals inside their hollowed structures ...... 72

2-15 SEM images of microfossils found in situ ...... 72

2-16 SEM images of preferentially sorted phosphatic-rich grains ...... 73

2-17 FGS sediment core 19731 drilled north of Montbrook ...... 74

2-18 FGS sediment core 19732 drilled south of Montbrook ...... 75

2-19 FGS sediment core 19733 drilled west of Montbrook ...... 76

2-20 Fence diagram of FGS sediment cores correlated by lithology and plotted against elevation ...... 77

2-21 SEM images and EDS data collected on representative phosphatic materials ...... 78

2-22 Schematic cross-section of a tide-dominated estuary and distribution of lithofacies resulting from transgression of the estuary, followed by estuary filling and progradation of sand bars or tidal flats from Dalrymple et al. (1992) ...... 79

3-1 Methodology of calculating Bioturbation Index ...... 92

3-2 Ichnofossil morphologies preserved in heterolithic deposits ...... 93

3-3 Appendage fragments of ghost shrimp from Montbrook specimens ...... 93

3-4 3D schematic conceptualizing the strata, facies, architectural bedding features and bioturbation in the Montbrook outcrop ...... 94

A-1 Characteristic Remanent Magnetism (ChRM) inclination and declination data where filled circles denote normal polarity and open circles represent reverse polarity...... 97

A-2 Representative examples of Zijderveld vector plots illustrating the unstable nature of Montbrook demagnetization data ...... 98

A-3 Paleomagnetism results of susceptibility-temperature analysis ...... 99

B-1 SEFS participants picking Montbrook sediment for microfossils, June 10, 2019. Courtesy of Dr. Bruce MacFadden...... 101

LIST OF ABBREVIATIONS

ASL Above Sea Level cmAd Centimeters Above Datum cmbd Centimeters Below Datum

EDS Energy Dispersive Spectroscopy

GFS The Gray Fossil Site

GYFC Gainesville Youth Fossil Club

FGS Florida Geologic Survey

FLMNH Florida Museum of Natural History

IHS Inclined Heterolithic Strata

Ma Million Years Ago

MBS Montbrook Fossil Site

NALMA North America Land Mammal Age

PPL Plane-Polarized Light

SEFS Scientist in Every Florida School

SEM Scanning Electron Microscope

SSD Soft Sediment Deformation

SLF Sea Level Fall

SLR Sea Level Rise

TESI Thompson Earth Systems Institute

XPL Cross-Polarized Light

Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science

PALEOENVIRONMENTAL ANALYSIS OF MONTBROOK: AN UNUSUAL FOSSIL LOCALITY FROM THE LATE MIOCENE IN NORTHERN FLORIDA

Michael Joseph Ziegler

December 2019

Chair: Bruce MacFadden Major: Geology

Montbrook is an extremely fossiliferous late Miocene to early Pliocene deposit located on the Ocala Platform in Levy County, Florida. The site has been excavated by the Florida Museum of Natural History since its discovery in 2015, producing over 50,000 catalogued fossils. Taxa such as Teleoceros sp., Hexameryx simpsoni, Rhynchotherium sp., and Borophagus hilli constrain the age, making it the only late Hemphillian locality in Florida with a predominantly terrestrial fauna. However, the fossil assemblage is mixed, spanning from terrestrial vertebrates to freshwater and marine fossils. This initial site formation analysis was conducted to reconstruct the paleoenvironmental narrative with an exploratory multiple method approach. Smear slides of the cores place Montbrook within the Hawthorn Group, although they constitute a unique environmental facies not yet described. In order to place the site in a regional geologic context, three deep sediments cores were drilled and correlated with known local deposits. Sediment cores revealed the vertical boundary of underlying bedrock is Eocene age Avon Park and Ocala

Limestone formations, as well as associated karstic features. Montbrook is a complex marginal environment, representing fluvial to estuarine deposition, with intermittent marine tidal influence, offering some of the first direct insights into transitional terrestrial ecosystems from

5.5–5.0 Ma in the southeastern USA.

CHAPTER 1 MONTBROOK SITE INTRODUCTION

The Montbrook fossil locality is a late Miocene site in eastern Levy County, Florida (Fig.

1-1), approximately 40 km south of the Florida Museum of Natural History (FLMNH) in

Gainesville. Discovered in 2015, Montbrook (FLMNH site LV070) is considered one of the most prolific paleontological sites in the southeast United States in regard to the quality and sheer abundance of vertebrate fossils. Excavation during concentrated field seasons have produced over 50,000 catalogued specimens, to date. University of Florida faculty, staff and students manage onsite logistics, whereas the vast majority of fossil collection efforts are sustained primarily by public volunteers totaling over 11,000 person-hours. Because the site lies on private land, initial efforts were focused primarily on recovering as many fossil specimens as possible.

As a product of these tremendous recovery efforts, Montbrook possesses a sizable dataset of physical vertebrate specimens available for research. Despite the considerable recovery efforts of fossils over the last four field seasons, remarkably, much of the site remains to be excavated because the lateral and vertical boundaries of this fossiliferous deposit have yet to be defined.

Research Inquiry

The goal of this study is to investigate the paleoenvironmental conditions and geological processes associated with the formation of the Montbrook fossil site. Due to its relatively new status as a fossil locality, effectively nothing has been published on regarding the geologic setting and sedimentological processes recorded at Montbrook. Here, this study employs multiple methodologies to reconstruct and interpret the paleoenvironmental narrative of

Montbrook via facies, sediment core association and microfossil assemblage analysis of the sediments. Though the main approach focuses on sedimentological analyses, this study helps to further specify the environmental niches occupied by decapod bioturbators which could lead to a

more specific paleoecological reconstruction than has been previously possible. Using a more holistic approach to draw inferences about past ecological conditions, this study addresses the following research questions: 1) what are the paleoecological implications associated with the ancient depositional environment of Montbrook based upon facies analysis, 2) does the ichnofossil assemblage and bioturbation index further support the environmental interpretation at

Montbrook, 3) how does the localized litho-stratigraphy of Montbrook fit into the regional

Miocene-Pliocene geology of Florida and 4) will sediment core descriptions provide insight to the lateral/vertical continuity of fossil-bearing strata of Montbrook and refine the spatial boundaries of the site?

An Exceptional Miocene Locality

The Montbrook locality supports a mixed fossil assemblage, spanning from terrestrial vertebrates to freshwater and marine fossils. Fossil remains at Montbrook consist chiefly of vertebrate specimens and fragmentary elements of invertebrates. There are measurable concentrations of trace fossils present in Montbrook deposits and will be discussed thoroughly in subsequent chapters. Published on the official FLMNH website, Montbrook has yielded a diverse array of identifiable vertebrate fossils that are represented by approximately 122 species (“Faunal

List,” 2019). A faunal reconstruction of Montbrook features a portion of the diverse fossil taxa found at the site with an emphasis on terrestrial specimens (Fig. 1-2). Previous research of

Montbrook specimens described a new genus and species of heron, aptly named Taphophoyx hodgei (Steadman and Takano, 2019) as an homage to landowner, Eddie Hodge. Other charismatic fauna includes the smilodontin Rhizosmilodon fiteae, alligator gar Atactostues spatula, peccary Protherohyus brachydontus and giant otter Enhydritherium terraenovae.

Additionally, other than freshwater fish, the most abundant fossil specimens found belong to the slider turtle Trachemys inflata and proboscidean Rhynchotherium sp. Vertebrate biochronology,

specifically the co-occurrence of taxa such as Teleoceros sp., Hexameryx simpsoni,

Rhynchotherium sp., and Borophagus hilli suggests a late Hemphillian (Hh4) North America

Land Mammal Age (NALMA) for the site and an estimated geochronological age of 5.5 to 5.0 million years ago (Tedford et al., 2004; Hulbert, 2019; Steadman and Takano, 2019). Although, it is important to note that some Montbrook fossils represent older and younger ages, so the current estimate of 5.5 to 5.0 million years ago (Ma) should be interpreted carefully and likely represents a larger span of time.

In an attempt to further refine the temporal boundaries of Montbrook deposits, this study used paleomagnetic analysis as a proxy for dating. Results of these analyses were equivocal and unable to contribute to the temporal constrain of Montbrook deposits. Although, paleomagnetic data revealed the presence of magnetic minerals preserved in the sediment samples which have paleoenvironmental implications (Appendix A). Determining the age range of Montbrook sediments merits further inquiry but is confidently defined as a Mio-Pliocene locality. Overall,

Montbrook is temporally situated around the transitional period between the latest of the

Miocene and earliest of the Pliocene epochs offering a unique opportunity to investigate what fauna inhabited the Florida landscape around 5.5 to 5.0 Ma. Most notably, the fossils of

Montbrook make it the only late Hemphillian locality in Florida with a predominantly freshwater/terrestrial fauna (Fig. 1-3).

Florida has abundant Neogene fossil localities, although this study specifically acknowledges sites comparable to Montbrook in regard to temporal and geographical setting.

Regionally, five fossiliferous localities in north-central Florida have Hemphillian NALMA vertebrate assemblages (Fig. 1-4). Palmetto Microsite and Whidden Creek localities are included within the composited Palmetto Fauna (FLMNH site PO005) and represent the only other source

of late Hemphillian age fossils recorded in Florida (MacFadden, 1986; Morgan, 1994; and Webb et al., 2008). The Palmetto Fauna largely comprises marine vertebrate cartilaginous fish.

Moreover, the terrestrial mammals collected from this region are mostly represented by isolated teeth and skeletal elements (MacFadden, 1986). Conversely, Montbrook is predominantly producing semi-articulated and partially associated terrestrial and aquatic specimens from freshwater and brackish-water habitats. The Love Bone Bed (FLMNH site AL001) represents a deposit consisting of fluvial sediments and diverse fauna representing estuarine, freshwater and terrestrial habitats (Webb et al., 1981). Although the environmental setting of Love Bone Bed parallels the habitats of Montbrook fauna, taxa at the site indicate the former is latest

Clarendonian NALMA (Woodburne, 2004). Mixon’s Bone Bed (FLMNH site LV009) and

McGehee Farm (FLMNH site AL027) represent deposits from the early Hemphillian NALMA.

While Mixon’s Bone Bed represents a low energy closed aquatic environment with no marine vertebrates, McGehee Farm represents a fluvio-estuarine environment (Rojas, 2012; Hulbert,

2013). Temporally, Moss Acres Racetrack (FLMNH site MR012) is closer to Montbrook in age but considered to be a sinkhole deposit (Lambert, 1997).

On a broader scale, Montbrook compares favorably with another notable locality outside of Florida that also straddles the transitional boundary between Miocene and Pliocene epochs.

The Gray Fossil Site (GFS) in Tennessee, USA is a unique late Neogene locality that has slight faunal assemblage overlap with Montbrook. In contrast, however, the GFS depositional environment is widely regarded to represent an extensive paleosinkhole system preserving multiple cycles of sediment filling (Worobiec et al., 2013). Florida is notorious for sinkhole fossil deposits due to plentiful limestone outcrops, dissolution and the associated formation of extensive karstic features. Remarkably, Montbrook is unique in that it does not reflect another

sinkhole deposit, but rather, it provides some of the first direct paleoenvironmental evidence of near-shore terrestrial ecosystems from the subtropical late Miocene of the southeastern USA.

Florida Platform During the Miocene

The Florida Platform is considered to be a tectonically quiescent and relatively stable environment located on the passive margin of the North American plate (verbatim Randazzo and

Jones, 1997), although it has sustained some structural and lithological change during the

Neogene Period. Carbonate deposition was extremely prevalent on the Florida Platform prior to

Miocene time when the region reflected a shallow marine setting. This depositional environment is preserved predominantly via the carbonate dominated terrain of modern-day Florida and composed of thick beds of limestone/dolostone. In north-central Florida, remnants of Eocene to

Miocene carbonate deposits are influential in shaping the modern landscape as a result of karstic processes. In the geologic record, Neogene to Holocene strata document a prominent transition from carbonate to siliciclastic dominated deposition on the Florida Platform (Evans and Hine,

1991), where an influx of siliciclastic sediments deposits intermixed with and spilled over carbonate environments. Montbrook is composed of Neogene siliciclastic sediments representing one of these pervasive deposits, the Hawthorn Group. Montbrook sediments have produced a diverse assemblage of fossilized material and provide a rare opportunity for understanding the paleoenvironmental conditions during the late Miocene and early Pliocene, a significant transition period in Florida that experienced changes in landscape, sea level, and climate (Haq et al., 1987; Scott, 1992, 1997; Zachos et al., 2008; Hine, 2009; Hine et al., 2017; Adams, 2018).

During the early Miocene, the broad Florida Platform was characterized by a considerable influx of siliciclastic sediments as a result of a few factors, namely sediment supply and accommodation space. These influential factors were the result of a trio of environmental changes working in concert with one another: 1) rejuvenated uplift in the southern Appalachian

Mountains, 2) sustained warmer/wetter climate, and 3) lowered sea level during Messinian Event

(Missimer and Maliva, 2017). Tectonic induced uplift of the southern Appalachians Mountains during the late Miocene would provide an increase in topographic relief of the surrounding landscape (Gallen et al., 2013), prompting periods of sustained erosion and a major sediment source. Moreover, the Messinian Event partially overlaps with the suggested late Miocene age of

Montbrook. The Messinian Event was an episode of oceanic change where the Mediterranean

Sea was isolated from the Atlantic Ocean. Although drivers causing the desiccation of the

Mediterranean Sea are debated, it is widely thought that the closure of the Gibraltar Strait and a severe drop in eustatic sea level (~60 meters) around 6.0 to 5.4 Ma (Cita, 1982; Duggen et al.,

2003; Miller, 2009) contributed. Together, inland mountain building and sea level fall could provide a hydraulic gradient subject to phenomenal transport of detrital sediment from the southern Appalachian Mountains to the low-lying ancient Florida Platform. Thus, the provenance of many Miocene siliciclastic deposits in Florida are sourced from the Appalachian

Mountains. With this in mind, the residence time of sediment deposits in central Florida are significant and grains were likely subject to sustained mobilization under subaqueous conditions.

High fidelity models of global seal level demonstrate periods of rise and fall during the

Miocene (Haq, 1987; Miller et al., 2005). These fluctuations in sea level often deposit characteristic sequences of rock in the geologic record where sea level fall (SLF) is commonly associated with regressive settings and sea level rise (SLR) with transgressive settings

(Catuneanu, 2002). The Messinian Event records a dramatic lowering of eustatic sea level and is recognized as a high-magnitude marine regression (Loutit and Kennett, 1979) that partially overlaps with the current age estimate of Montbrook. Due to lowland topography and proximity to the sea, the Florida Platform has been subjected to frequent sea level fluctuations and shifts

between periods of transgression and regression during the Miocene in-part recorded by cyclic deposits of Hawthorn Group sediments (Missimer and Banks, 1982). Scott (1988) reports that changes in Miocene sea level was the primary factor controlling the extent of Hawthorn Group sediment deposition. Moreover, vast deposits of Hawthorn Group sediments covered the Florida

Platform during a major transgression in the Middle Miocene and were subsequently eroded and redeposited (Huddlestun, 1988; Scott, 1988). Due to the unconformable nature of Hawthorn

Group deposits, interpreting where on published sea level models Montbrook sediments belong is impracticable without a more refined age of the site. Rather, this study focuses on analyzing

Montbrook sediments to interpret depositional environments and any spatial variation.

Even though the Florida Platform is generally considered to be tectonically quiescent, previous research has recognized geologic features associated with uplift and subsidence

(Opdyke et al., 1984; Adams et al., 2010) as a result of karst unloading induced isostatic uplift.

Due to the ubiquitous nature of limestone deposits, carbonate rock dissolution may have influenced the deposition of siliciclastic sediments on the Florida Platform and uplift of Plio-

Pleistocene ridges in northern Florida (Smith and Lord, 1997). Although, large-scale uplift in northern Florida is likely due to the influence of many factors including karstification and mantle dynamics (Woo et al., 2017; Adams, 2018), which may account for the modern elevation of

Montbrook. Compared to the relative low topography of Florida, Montbrook is situated high at

~70 feet above sea level (ASL). These features would be considered minor in comparison to those found in the dynamic Appalachian region, but indicate that the Florida Platform has sustained and reacted to tectonic influences since the Neogene (Randazzo and Jones, 1997).

Other than displaying geomorphological evidence, Montbrook sediment also records unusual architectural structures like folding and soft sediment deformation. Previous research conducted

on Hawthorn Group sediments have documented similar structural irregularities within this dynamic deposit (Scott, 1988). In subsequent chapters, Hawthorn Group sediments and structural irregularities at Montbrook are analyzed at the micro-, meso- and macroscopic scale in order to interpret the paleoenvironment represented by this unique fossil deposit.

Figure 1-1. Map of the Montbrook Fossil Site in Florida. Levy County is subset to illustrate the location of Montbrook as an area of interest in north-central Florida.

Figure 1-2. Faunal reconstruction of Montbrook, October 28, 2019. Courtesy of Rachel Keeffe. Taxa include A) Taphophoyx hodgei (heron), B) Teleoceros sp. (short-legged rhino), C) Hexameryx simpsoni (six-horned pronghorn), D) Pseudhipparion simpsoni (three- toed horse), E) Rhynchotherium sp. (gomphothere), F) Hemiauchenia edensis (dwarf llama), G) Enhydritherium terraenovae (giant otter), H) Trachemys inflata (slider turtle), I) Atractosteus spatula (alligator gar), J) Alligator mississippiensis (American alligator), K) Apalone sp.(softshell turtle), L) Borophagus hilli (hyena dog) and M) Rhizosmilodon fiteae (saber-toothed cat).

Figure 1-3. Age distribution of Miocene and Pliocene fossil localities in Florida. These prominent fossil localities on the Florida Platform with associated NALMA have been modified from Hulbert (2001) and Woodburne (2004). A black box around the site name signifies the current existing temporal-boundaries of the fossil localities and associated symbols correspond to Figure 1-4.

Figure 1-4. Geologic map of Florida modified from Scott et al. (2001) with Neogene fossil localities. Subset illustrates prominent fossil localities in north-central Florida and their proximity to Montbrook.

CHAPTER 2 LITHOSTRATIGRAPHY AND FACIES MODELING

Montbrook sediments are described at the micro-, meso-, and macroscopic scale in order to emphasize the spatial interrelationship of excavated fossiliferous deposits and the unexplored deposits surrounding the site. At the microscopic level, smear slide analysis revealed patterns in sediment grain composition via petrographic and SEM techniques. Scaled up to the mesoscopic level, facies analysis detailed the lithology and accompanying structural features of Montbrook outcrops in the active excavation area. The classification of sediments into facies and stratigraphic packages were defined to interpret the depositional history of Montbrook and its fossiliferous deposits. Moreover, macroscopic level analysis utilized visual core descriptions to correlate peripheral sediment columns and define the vertical boundaries of subterranean bedrock. This multiple method approach allowed for a more holistic paleoenvironmental exploration of sediment deposits in order to determine the depositional settings of the deposits at

Montbrook and how they fit into the larger framework of Florida geology.

Regional Stratigraphic History

The broad stratigraphic framework of the Florida Platform has been well established through extensive large-scale mapping projects (e.g. Scott et al., 2001). Due to topographical low relief and limited exposures of successive strata, the majority of geologic formations have been defined by surficial sedimentary deposits and subsurface evaluations through sediment core drilling, phosphate mining and the excavation of fossil localities. Across the Florida Platform,

Paleogene carbonate deposits are generally overlain by Neogene sedimentary deposits. Strata of

Miocene and younger age deposits in north-central Florida are characterized by large amounts of clastic sediments, clay, and sand, in contrast to the dominant carbonate lithology of the older

Cenozoic rocks (Espenshade and Spencer, 1963). Specifically, in Levy County, the terrane is

predominantly composed of Eocene Ocala limestone outcrops, Miocene-Pliocene Hawthorn

Group sediments and undifferentiated Pleistocene/Holocene deposits (Fig. 2-1).

This study focuses on Hawthorn Group sediments because there are documented occurrences of these deposits in Levy and surrounding counties and the sediments align temporally with current age estimates of Montbrook (Vernon, 1951; Scott, 1982, 2001). It should also be noted that the deposits at Montbrook merit further inquiry because, historically, what constitutes Hawthorn Group sediments has been ill-defined. The change in nomenclature from the original classification of Hawthorn Formation to Hawthorn Group sediments is well documented in literature and highlights some of the problems associated with these deposits

(Huddlestun, 1988). The Hawthorn Group is considered one of the most misunderstood units in the southeastern United States because it encompasses a high variability of sediments including alluvial, marine, terrestrial and deltaic beds in southern Georgia and northern Florida (Puri and

Vernon, 1964; Scott, 1982). While the purpose of this study is not to disentangle this longstanding legacy issue in Florida geology, it recognizes the geological significance and historical connection. In conjunction with the sedimentological analyses preformed in this study, literature on local deposits like Hawthorn Group and Alachua Formation sediments helps provide a more detailed report on the lithology, stratigraphy and depositional history of Montbrook sediments (Vernon, 1951; Pirkle, 1956; Puri and Vernon, 1964; Scott, 1988; Huddlestun, 1988;

Lazareva, 2004).

Prior to being subsumed into Hawthorn Group sediments, the Alachua Formation was a prominent Miocene geologic unit in north-central Florida with a type locality located approximately 8km northeast of Montbrook at the Mixon’s Bone Bed (Webb, 1964). Alachua

Formation sediments were largely described as a combination of sands and clays with occasional

silicified limestone boulders and varied sizes of phosphatic grains (Pirkle, 1956). One of the characteristics used to distinguish between the Alachua and Hawthorn formations is a more terrestrial fossil assemblage from the Alachua Formation versus a predominantly marine fossil assemblage from the Hawthorn Formation (Vernon, 1951). Some Alachua Formation deposits were interpreted as residuum from the erosion of Hawthorn Group sediments which were redeposited into sinkholes and other areas of increased accommodation space (Pirkle, 1956). As will be developed here, Montbrook is not considered to be a sinkhole site and supports a mixed faunal assemblage predominantly composed of terrestrial taxa. It is certainly possible that

Montbrook deposits are residuum of Hawthorn Group sediments that constitute unique environmental facies not yet described or recognized in the literature.

In clastic sedimentology, facies is an essential term that can seem ambiguous, but here it is generally defined as distinct sedimentary deposits discernable via physical, chemical or biological attributes. Simplified, facies are deposits of sediment with specified characteristics

(Reading, 1986). In this study, facies analysis focused on lithology, bed/lamina thickness, bedding architecture, nature of overlying and underlying contacts, body and ichnofossils as well as any recognizable post-depositional features. Montbrook sediments are classified into facies using multiple methods in order to best interpret the depositional history of the site at the micro-, meso- and macroscopic scale. Smear slide, facies and sediment core analyses were conducted to interpret processes, depositional environments, and spatio-lateral continuity of Montbrook deposits. This study presents a high-resolution site formation analysis and facies interpretation supported by the assessment of representative strata, architectural elements, smear slide granulometry and a subsurface exploration of transecting sediment cores.

Methods

Microfacies Analysis

During the initial field season at Montbrook in 2015, 1.0 by 1.0meter squares were established to form a gridded plot system anchored to an onsite datum. This coordinate system enables reliable location referencing of excavated fossils and exposed stratigraphy within the site. During the 2018 field season, a semi-contiguous vertical section of Montbrook sediments was identified in the northwest corner of the site and selected as the outcrop featured in this study (Fig. 2-2). To expose the sedimentological features of the outcrop, trowels were used to clean off the outermost layer. This excess sediment, overburden and float material was removed from the study area and deposited in the spoil piles outside of the pit. Fossils found in situ were recorded before removal. Due to onsite accessibility, this representative outcrop was described as two correlated exposures and referred to as step one (lower) and step two (upper). In total, the outcrop measured 2.74 meters vertically and 4.0 meters horizontally (Fig. 2-3 and 2-4). Utilizing the previously founded datum, stratigraphic sections were measured, described and photographed. Originally measured in respect to the site datum, samples were labeled as either centimeters above datum (cmAd) or centimeters below datum (cmbd). In addition, bounding surfaces, depositional geometries and lithologies were recorded on-site and analyzed in the sedimentology lab at University of Florida. Facies were grouped within stratigraphic section partially based on association, interpretations and symbology in Miall (1985). Starting from the basal most section of the outcrop, sediment samples were collected in sequential 5cm increments. Once secured in Whirl-Pak® bags, samples were labeled with unique sample IDs and associated stratigraphic positions. A total of 50 samples were obtained for sediment composition and microfacies analyses. Although these samples constitute the area of interest in this study, this

sampling process was repeated four more times at adjacent sediment columns for sedimentological comparison.

Smear Slide Analysis

Analysis of unlithified sediments was conducted via smear slide and petrographic microscopy. Smear slides were prepared based on modified methods outlined sensu Rothwell

(1989). Standard microscope slides were cleaned with Kimtech Science™ wipes and alcohol to remove any artifacts that could be confused with anomalous sediments. A drop of deionized water was placed on the slide and ~.05 to 1.0 mm3 of sediment removed from the sample bags with a wooden toothpick. This small quantity of sediment was distributed onto the slide and dispersed in the water producing a translucent slurry. When there were no visible sediment clumps, the slide was placed on a hotplate until all the water evaporated. Norland Optical

Adhesive 61 was applied as a cement because, once set, it has an almost identical refractive index as quartz which makes it easier to isolate uncommon constituents. Microscope coverslips were secured, and the prepared slide was exposed to a black light to set the UV curing cement.

Smear slides offer a proficient analytical method to identify sediment composition and offer a semi-quantitative approach to measure relative lithological changes in a measured section (Kelts,

2003). Even so, there are few consolidated image-based references specifically for smear slide analysis. Integrated Ocean Drilling Program (IODP) manuals (Marsaglia et al., 2012, 2015) were utilized as visual atlases during the identification process. With a Nikon Eclipse E600 POL petrographic microscope, smear slide components were examined in plane-polarized light (PPL) and cross-polarized light (XPL), which helped emphasize sediment grain features. Initially, slides were viewed at 100x to isolate a representative area and then focused in at 400x when recording common sediment attributes used in presence-absence data. Accompanying software,

Nikon ACT, enabled image capturing. Selected grains described from smear slide analysis

warranted further analysis. In order to determine composition and small-scale structure of these common grains, high fidelity images of representative sediments were captured under the Zeiss

EVO MA10 scanning electron microscope (SEM). As part of the same material analysis SEM system, preliminary elemental data were collected using the EDAX Apollo silicon drift Energy

Dispersive Spectroscopy (EDS) detector and accompanying software.

Typical petrographic slides have a uniform thickness in which sediment grains share the same viewing plane, but smear slides do not have a standard thickness. Due to this variability, this study limits the use of common descriptive petrographic measurements like pleochroism and birefringence as supportive evidence in grain descriptions. Rather, slides were investigated to determine any recognizable grain feature patterns in micromorphology, relief, composition, or alteration. Microscopic analysis of sediment smear slides revealed a few patterns and consistencies of constituent clastic and diagenetic minerals throughout the stratigraphic succession. Because most smear slides had substantially less than 100% sediment cover, analysis relied strictly on presence or absence, rather than measures of proportionality as a semi- quantitative approach to sediment composition. Slides were coded as 0, 0.5, or 1 based on representation of predetermined grain characteristics, where 0 denotes complete absence, 0.5 denotes a minor presence, and 1 denotes a strong presence. Smear slides are linked to stratigraphic positioning. Overall, the samples span 0–270 cm below the surface where the upper step represents 0–150 cm and the lower step represents 170–270 cm. The ~20 cm hiatus in data represents the distance between upper and lower step of the measured outcrop during sample collection. In order to clearly illustrate any trends based on described features, all coded measurements are displayed graphically with supportive images that were captured while under the microscope.

Sediment Core Analysis

The Florida Geologic Survey (FGS) drilled three sediment cores into unexplored substrate surrounding exposed Montbrook deposits represented by core numbers: 19731, 19732 and 19733. Locations of cores are documented in latitude and longitude: north core 19731

(latitude: 29° 18’ 29.2”; longitude: 82° 25’ 51.9”), south core 19732 (latitude: 29° 18' 24.4"; longitude: 82° 25' 53.7") and west core 19733 (latitude: 29° 18' 26.7"; longitude: 82° 25' 54.4").

A total of 201.5 feet of core was removed, logged and stored at the FGS central repository in

Tallahassee, Florida. Due to FGS protocol, all sediment cores are measured in feet in order to better correlate with elevation data. Thus, there is a unit discrepancy between the sediment section described within the Montbrook fossil site and the FGS cores. Records of the core summaries can be accessed at the FGS well and exploratory borehole query website and searched via core number. When exposed to subaerial conditions core samples had the opportunity to dry, oxidize and form an outer layer. Before a descriptive core analysis could be conducted, the outer layer of the cores needed to be carefully removed in order to expose contiguous sections.

Working up the core, altered surface sediments were removed laterally in an attempt to eliminate vertical mixing of sediments and best preserve any original structure. Unconsolidated material removed was directed towards either side of the core to maximize the exposed surface. High fidelity photographs were taken indoor and outdoor in order to highlight any variability as well as secure the best digital representation of the core samples for reference. A visual description analysis was conducted on the cores based on physical and chemical characteristics of sediment grains. Core sediments were carefully hydrated with deionized water in order to increase moisture content on surface and sharpen coloration. Although color is often used as an indicator of sedimentological change throughout a core, lithology and granulometry served as the primary focus in this description. Underlying limestone formations were recorded in all sediment cores

and Alizarin Red (1%) was utilized in order to distinguish between calcium carbonate limestone

(CaCO3) and calcium magnesium carbonate dolomite CaMg(CO3)2 deposits. Once described, minimally destructive sediment samples were removed from prominent sediment beds for smear slide analysis and comparison of Montbrook material.

Results

Microfacies Analysis

Strata described in this study are predominantly composed of siliciclastic sediments, specifically, dominated by sand and clay with a minor carbonate influence. Representative stratigraphic columns (Fig. 2-5) of the upper and lower outcrops are correlated and illustrate the association of coded Facies, characteristic sediment architecture and location of fossils within the measured section. All vertebrate and invertebrate body fossils as well as trace fossils were recorded for stratigraphic position, excavated and identified to the highest hierarchical classification. Although all fossil taxa are utilized as supportive data in facies descriptions, chapter three elaborates on the ichnofossil assemblage to provide further paleoenvironmental evidence. For reference, all assigned facies codes are outlined on the stratigraphic column alongside their associated environmental flow regime.

Two depositional environments, constituting different regimes, have been interpreted from the 2.74-meter outcrop of sediments. The different regimes are proposed with regard to the environmental processes inferred from and associated with the facies: 1) the fluvially-dominated regime (minor facies) and 2) the tidally-dominated regime (major and most prevalent facies).

The fluvially-dominated regime comprises two facies: a rippled sand unit (facies A) and a predominantly massive gravel unit (facies B). While, the tidally-dominated regime is composed of two facies: a wavy cross-laminated unit (facies C) and, most notably, a heterolithic flaser unit

(facies D). Table 2-1 summarizes the different facies and associated features. Each facies has

been assigned a facies code based on Miall (1985) and are represented by; Fine Laminated Sands

(Fl), Crudely Bedded Gravel (Gm), Massive Gravel (Gms), Fine Crossbedded Sands (Sl) and

Rippled Sand (Sr). Davis (1983) defines a depositional system as an assemblage of process- related sedimentary facies. Collectively, the depositional environments defined in this study assign Montbrook to a fluvial-estuarine depositional system and the four associated facies are described herein.

Rippled sand unit

Facies A (Fig. 2-6) is the basal most deposit and consists predominantly of fine-grained rippled sands. In section, the deposit is approximately 47 cm thick with an overall southward dip.

Moderately sorted angular to subrounded sand grains range in size from 1.0–3.0 Ø and average around 2.0–2.5 Ø. Sedimentary structures vary from structureless homogeneity to thin (1–5 cm) undulating beds with discontinuous clay laminae (1 cm). The concentration of isolated clay lenses (1–3 cm) are ubiquitous but increase up section. Interbedded sand and clay units are often truncated and form irregular minor trough-crossbedding and classified as flaser bedding.

Sediments are inversely graded and coarsen upward into a gradational contact with the overlying gravel beds of facies B. Furthermore, an erosional and irregular contact is present between facies

A and an adjacent sand filled cavity. The extent and nature of the lower contacts are unknown because the boundary of facies A was not exposed during fieldwork. Even so, a partial rib of the gomphothere Rhynchotherium sp. was recovered from within this measured deposit. Moreover, further down section (~335 cm below the surface) three other vertebrate fossils were unearthed in subsequent excavations. These included carapace elements from the turtle Trachemys inflata as well as a femur and partial pelvic element of Rhynchotherium sp. Field analysis of strata and in situ femur fossil revealed a dip of 23S and 21S, respectively. Correlation between dip of

strata and positioning of the fossils could reflect a moderate southward paleoflow of this unit under subaqueous conditions during genesis, although a single observation is hardly conclusive.

Trace fossil burrows are found throughout this unit.

Facies code Sr is assigned to this unit and described herein. Sigmoidal boundaries of facies A and shallow trough-crossbedding are usually linked to the migration of small to large- scale ripples which suggest sustained energy flow and corresponding channel movement (Miall,

1985, 1996). Bedform migration recorded via ripple deposits are common microform features in fluvial settings. In this setting, sustained flows are prone to mobilize bedload and suspended sediments via traction currents (Simons et al., 1965). Observed flaser bedding is characteristically defined by ripple cross lamination and thin clay lamina (Bhattacharya, 2008) that imply periods of sustained current activity and calm subaqueous conditions. Although primarily associated with tidal environments, flaser bedding occurs in fluvial environments and any depositional setting where ripples may form (Davis, 1983).

Massive fossiliferous gravel unit

Facies B (Fig. 2-7) is matrix supported and consists of very coarse sand, minor deposits of carbonate fragments and preferentially sorted gravel sediments. In section, the unit is approximately 28 cm thick and has a low-degree southward dip comparable with the basal most bed. Poorly sorted subangular to rounded grains range in size from -1.0–3.0 Ø and average around 0.5–1.0 Ø. Sedimentary structures are generally lacking to crudely bedded with isolated clay lenses (1-3 cm). Although, there is a visual distinction between the lower unconsolidated gravel layer and the capping semi-indurated gravel layer is heavily oxidized. A gradational contact with a moderate elipson-shaped surface is present with both the underlying facies A and overlying facies C. Moreover, this highly fossiliferous unit supports a diverse assemblage of microfossils including ichthyoliths (fish teeth and denticles).

Facies codes Gm and Gms are assigned to this unit and described herein. Thick lag deposit packages of poorly sorted sands and gravels from facies B were laid down during periods of increased energy in a depositional setting not conducive to the uniform accumulation of clay laminae. Gm is characteristically massive to crudely bedded sands and gravels. A crudely bedded to structureless deposit implies that deposition was rapid and relatively short-lived (Boggs,

2006). Semi-ambiguous gradational contacts between facies A, B, and C make vertical boundaries difficult to identify. The ill-defined margins observed from one facies to another could indicate a genetic relationship during deposition and potential mixing zones. Transitioning from facies A to B, measured sediments generally increase in grain size up-section. Graded bedding is also associated with Gms and likely the result of sufficient sediment supply during storms or episodic events (Rahman and Tahir, 2019). Specifically, inverse graded sediments could indicate a change in in-channel deposition from traction current to sediment gravity flow dominated deposition. In a fluvial setting, sediment gravity flows occur when large quantities of loose sediments are mobilized via failure and liquefaction on a sloping surface (Miall, 2010).

There is a sediment feature cutting through facies A and B. This feature consists almost exclusively of fine-grained massive to poorly bedded sands. In section, the deposit is approximately 21 cm thick. Well sorted subangular to subrounded sand grains range in size from

1.5 – 3.0 Ø and average around 2.0 – 2.5 Ø. Sediment structures are generally lacking but there are a few faint planar beds in this deposit. Contacts with adjacent facies A and overlying facies B are both sharp and erosional with an undulating boundary.

No facies code is assigned to this deposit because it is interpreted as an isolated sedimentary feature but is described herein: This deposit of semi-structureless sand moderately fines upwards and is likely a product of bioturbation. The feature cut through facies A and B

which created well-defined erosional contacts and provides no real evidence of a contemporaneous relationship with the surrounding sediments during deposition (Colby, 1964).

Quite possibly, a void was created after the deposition of facies A and B and subsequent sediment accumulation filled in this space of accommodation.

Wavy cross laminated unit

Facies C (Fig. 2-8) consists prominently of interbedding fine-grained sands and clays and are classified as heterolithic strata. In section, the deposit ranges in thickness from approximately

42 cm in the lower step and approximately 21 cm in the upper step. Moderately to poorly sorted sand grains are subangular to subrounded. Ranging in size from 1.0 – 3.5 Ø, the sands average around 2.0 – 2.5 Ø. There is a noticeable gradational contact with overlying deposits of facies D distinguishable, in part, by an oxidized layer of semi-consolidated sediments. Medium to coarse pebble-sized (-3 – -5Ø) clasts composed of indurated sands occur within and just below the oxidized boundary. Overall, sedimentary structures are dominated by thin (<1 cm) clay laminae and thicker (1-5 cm) undulating sand beds that accrete vertically as wavy bedded deposits. These interbedded sand and clay strata are often truncated and form irregular trough crossbedding with semi-continuous drapes of clay. Many of the laminae and beds are traceable, whereas others terminate into a section of mottled material at the confluence of a prominent onlap feature.

Although, this discontinuity of layers elucidates a minor fault within the bedding sequence based on vertical displacement. The convoluted nature of these strata is associated with the numerous architectural elements observed in this unit, especially the soft sediment deformation (SSD).

Facies codes Sl and Sr are assigned to this unit and will be described herein. This deposit is composed of heterolithic sediments characterized by wavy bedding that originate during alternating intervals of steady fluid flow and slack water (Martin, 2000). Alternating deposits of fine rippled sands and clays produce interbedding and associated low angle (<10) crossbedding.

Wavy bedding is ideally composed of equal sand and clay components that model horizontally continuous strata (Reineck and Wunderlich, 1968). Undulating beds and laminations are often disturbed by bioturbation. Moreover, syn- and post-depositional processes like SSD, onlap and mottling obscure lateral continuity in this deposit and succeeding strata.

Under subaqueous conditions, sediments are saturated with water which can alter the integrity of deposits. During this state of liquefaction or fluidization, loosely consolidated sediments are prone to displacement prior to consolidation. This alteration of sediments may produce an array of SSD structures including the convolute bedding and slump folds observed in this section. Slumping structures occur worldwide and are common in various depositional settings including continental, coastal and marine environments (Shanmugam, 2017). The slumped structures at Montbrook laterally displaced sediments from their original bedding and formed folds. Literature favors earthquakes as being the prevailing trigger for slumping, but it can form as a result of a number of mechanisms including when sufficiently saturated sediments are deposited in areas with sustained sedimentation rates and steep slopes (Moretti and Sabato,

2007; Byun et al., 2019). Regarding this deposit, the underlying gravel unit is banked and could have provided the inclined surface necessary for slumping. Conversely, if the laminated sand and clay deposits had already been deposited, the gravel unit could have been induced into the system as a loading feature. In this scenario, the gravel unit would produce folded fine-grained material as it settled into the environment and consequently partially enveloped (Byun et al.,

2019). There is little evidence supporting this because there is a lack of compressed laminations underlying the gravel unit or erosional contacts with overlying strata. It is important to note that these slump folds could also be interpreted as another deformation structure known as convolute bedding. Similar to slumping features, convolute bedding occurs in a wide range of

environmental settings. Convolute structures, like folds, are widely regarded as are penecontemporaneous features that form within recently deposited beds/laminae that are still under the influence of fluid currents (Dzulynski and Smith, 1963; Ghosh et al., 2012). Overall, the SSD structures observed within this deposit were influenced heavily by fluid flow from either or, more likely, both fluvial and tidal influenced regimes.

Deposits influenced by SSD at Montbrook are most noticeable when viewed from the southward facing exposure, whereas the westward facing exposure emphasizes another prominent architectural feature known as onlap. Here, packages of heterolithic sediments appear to ramp up the sigmoidal shaped boundary, accumulate in the lower lying area, and truncate into the higher sloped strata. Onlapping is a common feature within transgressive systems and has been recognized in sequences of stacked fluvial and tide-dominated deposits (Plink-BjöRklund,

2005). Transgressive deposits amass in periods of relative sea level rise where inland environments can experience periods of inundation. Heterolithic deposits of this and succeeding facies are characteristic of tidal and fluvially-dominated environments where both would be directly influenced by periods of transgression. Specifically, the transgressive surface of an estuary can record a change in facies from fluvial to tidal-estuarine deposits with onlap terminations (Allen and Posamentier, 1993).

Heterolithic flaser unit

Facies D (Fig. 2-9) consists predominantly of interbedding medium and fine-grained sands and clays. Sediments in this unit are similar to those in facies C since they constitute another heterolithic deposit, but these strata are distinctly classified with flaser bedding.

Heterolithic facies can be divided into three sub-facies, flaser, wavy and lenticular bedding based on percentage of sand and mud (Rahman and Tahir, 2019). Based on the predominance of sand through this sequence, the heterolithic deposits of facies D is dominated by flaser bedding

structures. In section, the deposit is approximately 135 cm thick and represents the largest sections of conformable strata in this study. Moderate to well sorted sand grains are angular to subrounded. Ranging in size from 1.0 – 3.0 Ø, the sands average around 1.5 – 2.0 Ø.

Sedimentary structures are dominated by vertically accreting flaser bedded sediment packages composed of thin (<1 cm) clay laminae and thicker (1-11 cm) sand beds. In general, strata have undulating semi-continuous surfaces, but often lateral continuity is obliterated by bioturbation, water escape structures, and Liesengang banding.

Facies codes Fl and Sr are assigned to this unit and described herein. These heterolithic deposits experience fine lamination and minor rippling and likely represents an environment that promotes both the deposition of sand-dominated beds during periods of persistent flow as well as settling of overbank or waning flood deposits. Overall, this facies owes its deposition to a low energy environment that would selectively deposit by suspension settling (Boggs, 2006). This claim is further supported by the internal organization of vertically accreting sediments which are represented by alternating couplets of sand and clay that appear to have an inherent patterning. Alternation of thin-clay layers indicate a cyclic depositional process which is common in tidal influenced inner estuarine sediments (Kuecher et al., 1990). In subsequent field seasons, deposits of sediments above this measured section have been correlated to these heterolithic structures. From cross-section, these deposits revealed low degree (~17–28º) bedding which is interpreted as inclined heterolithic strata (IHS), a common sedimentological term that denotes heterolithic deposits generated on a sloped surface (Thomas et al., 1987).

Areas that lack sedimentary structure present but not common in this deposit. Miall

(1996) suggests that the lack of sedimentary structures can be attributed to rapid sedimentation, dewatering after deposition or even biological activity such as syn- or near syndepositional

bioturbation. The unstructured and mottled material in facies D are heavily altered by burrow features and Liesegang rings. Tidally-dominated environments often experience periods of flooding which can introduce brackish waters. During this intermittent inundation, deposited sediments provide fresh surfaces that are prone to bioturbation (Dalrymple and Choi, 2007).

Moreover, the units that encompassing the upper outcrop step, as a whole, are more oxidized than underlying units and experience Liesegang rings. Deposits that develop color banding associated with Liesegang rings are not a result of primary sedimentary stratification but rather artifacts of sediment saturation (Marko et al., 2003). As a pervasive diagenetic feature observed in this deposit, Liesegang rings at Montbrook are likely products of groundwater movement through the semi-indurated sediments.

Smear Slide Analysis

Petrographic analysis of sediment smear slides under plane-polarized (PPL) and cross- polarized light (XPL) revealed multiple trends in constituent clastic and diagenetic mineral composition based on stratigraphic position within the measured Montbrook outcrop. Due to the ubiquitous presence of quartz within these deposits, only feldspar, the detrital heavy mineral zircon, siliceous debris, and clay fractions were recorded as clastic components. Within the diagenetic component, surface structures identified as pyrite framboids and altered clay minerals were determined, while the biogenic component included phosphatic material, fossil fragments and moderately worn ichthyoliths. Data of clastic, diagenetic and biogenic sediments are presented herein with an emphasis on recognizable patterns throughout stratigraphic succession.

In order to illustrate results of this study, the presence-absence schematic (Table 2-2) is complemented by images captured underneath the microscope (Fig. 2-10 to 2-16).

Quartz grains dominate the composition of all sediment samples analyzed and are best recognized due to low relief and conchoidal fractures, whereas the feldspars have strong

cleavage and twinning. When viewed in XPL, polysynthetic twinning was the most recognizable feature identified from feldspar grains. For examples, the cross-hatched appearance of twinning in two directions is a diagnostic feature likely representing microcline or anorthoclase (Fig. 2-

10). When identifying feldspars, any untwined feldspars present could have led to misidentification, but this specific error is likely insignificant and does not merit remediation.

Both quartz and feldspars recorded consistent occurrences of mineral inclusions and vacuoles.

Although not as prevalent as quartz, feldspathic grains were found throughout the majority of the samples with a marginally higher occurrence moving up section.

Heavy minerals are generally not considered clastic material, but these grains are assumed to be originally sourced from outside of the Montbrook depositional system and are classified as detrital sediment. These heavy minerals are partitioned into two groups based on common micromorphologies where one group represents tabular grains and the other spherical grains (Fig. 2-11). Many of the non-opaque tabular grains are classified as apatite, a very common phosphatic heavy mineral that lacks cleavage. When viewed in PPL, both groups of heavy minerals had high relief and translucent to opaque appearance. The presence of tabular heavy minerals is moderately consistent, but there is a slightly stronger concentration in the sediment samples from stratigraphically lower sections. Tabular grains from 0–140 cm are found intermittently, generally less altered, and range in size from 50–110 um. Whereas, tabular grains from the lower sections, specifically 200–230 cm, generally exhibit signs of more wear and are fractured. Similarly, spherical heavy minerals are found recurrently in sediment samples from higher up in in the section (25–130 cm) and found in higher concentration in the lower stratigraphic sections (135–250 cm). Detrital zircon are recognized as durable heavy minerals

found within the sediment samples and are worn. Found infrequently above 105 cm, zircons are moderately uncommon, yet relatively higher in abundance below 105 cm.

The final presence-absence focused on as clastic components were sediment grains coated in clay on slides that had observable amounts of siliceous debris. Clay coating and siliceous debris were recognized by their oxidized appearance, amorphous morphology, and extreme fine-fraction sediment size. Although siliceous debris can generally be found on most all smear slides, there is a stronger concentration of clay coated grains in the upper section of the outcrop. Specifically, from 0–100 cm there is an observed increase in clay content and siliceous debris with the greatest concentration of clay coated grains measured from 0–55 cm.

Overall, the features making up the diagenetic component of smear slides offered a stronger contrast in presence-absence. Diagenetic components of the sediment samples were represented by two main recognizable features, pyrite framboids and palygorskite. Pyrite framboids were identified as bulbous and opaque structures located on the surface of select sediment grains (Fig. 2-12). There was a higher concentration of pyrite framboids from the slides representing stratigraphically lower samples around 175–265 cm. Whereas, samples from above this vertical boundary experienced intermittent presence and absence of pyrite framboids.

The altered clay mineral, palygorskite, was interpreted based on their diagnostic needle- like structures and was observed coating sediment grains as well as in the void spaces between grains (Fig. 2-13). There was a strong dichotomy in samples that contained palygorskite and those absent of the mineral. Samples from the lower section contained palygorskite, with a specifically high concentration of around 170–220 cm. Conversely, the mineral was found in much lower concentrations in samples overlying 170 cm and relatively absent above 110 cm.

The morphology of this material support the interpretation of palygorskite, but it is important to note that the aluminum-rich crystals are exceptionally long for authigenic clay material.

Similar in structure to palygorskite, other needle-like minerals are recorded from deposits around ~195 cm and below. SEM imagery documents these minerals are often found coating phosphatic grains, isolated as sediment debris, or as internal structures (Fig. 2-14). Material analysis of the minerals found within weathered artifacts that have high concentrations of phosphorous and elaborated upon in the discussion section. These phosphorus-bearing minerals are interpreted as crandallite and/or wavellite. It is likely that these minerals formed as products of weathering.

Mineralized during diagenesis, fossilized bone fragments and fish teeth are recognized as phosphatic organic debris and represent the biogenic component of this analysis. There was a distinct dichotomy in presence-absence within the biogenic component of these sediments.

Phosphatic fragments had high relief and generally had a deep orange hue. One prominent ichthyolith was recognized in the 230–235 cm smear slide based on the curved conical shape commonly represented by fish teeth. A few representative examples of biogenic material commonly found in the sediment column were imaged on the SEM (Fig. 2-15). All major concentrations of phosphatic fragments or ichthyoliths were documented at and below 195 cm.

Based on these smear slides, sediments from overlying strata essentially lacked any observable biogenic material. It is important to note that apparent lack of fossil material in strata overlying the arbitrary 205 cm below the surface does not mean that these sediments are not fossiliferous.

On the contrary, fossils have been found from these deposits but, similarly, are quite sparse compared to underlying more productive deposits.

Admittedly, classifying all of the phosphatic material as biogenic is a bit misleading because there are observable deposits of phosphatic sands that seem to be confined to the sediments below 195 cm. These phosphatic sediments have extreme preferential sorting and are composed chiefly of coarse sand to gravel granule sized semi-spherical grains (Fig. 2-16). SEM imagery allows comparison of a representative grain to one that is fractured. Externally, there is little difference between the two grains. Contrast between the two specimens is most recognizable internally, where the lack of internal structures is revealed. Interpreted as francolite, this cryptocrystalline material is largely composed of calcium, oxygen and phosphorus. This interpretation is followed up with more supportive data in the discussion section.

Due to their larger size, ranging from 1.0Ø to -1.5Ø, smear slides were not the best method to collect accurate presence-absence measurements of the preferentially sorted phosphatic-rich grains. Instead, sediment samples from each 5cm interval were analyzed via microscopy. Data produced from these observations reveal that the rounded francolite grains generally trend with the presence-absence of phosphatic fossil fragments throughout the sediment column. Overall, the boundary of rounded francolite grains only extends 10 cm past the defined boundary of fossil fragments. Due to the stratigraphic overlap, co-occurring diagenetic and biogenic phosphatic material are lumped together.

Sediment Core Analysis

Over 200 feet of core was drilled from three peripheral locations around the central

Montbrook deposit and described at the FGS based on visual core descriptions. Cores were strategically placed just north (19731), south (19732) and west (19733) of Montbrook proper and provided insight towards defining the possible extent of the fossil-bearing deposits outside of the current excavation site. Analyzed core samples provided higher resolution of subsurface geology through correlation of carbonate bedrock and overlying clastic sediments. Similar to the facies

and smear slide analyses, variations in lithology, bed/lamina thickness, color, bedding architecture, nature of overlying and underlying contacts and other pertinent sediment characteristics were recorded to refine stratigraphic context of these deposits (Fig. 2-17 to 2-19).

Lithological data collected from the sediment cores were compiled into correlated transects that inferred vertical and lateral boundaries of core deposits (Fig. 2-20). In general, the cores had a thin covering of organic rich overburden and composed predominantly of semi-indurated siliciclastic sediments underlain by thick deposits of carbonate rock. Described deposits were recognized across sediment cores with exceptionally distinct transitions between the siliciclastic and carbonate sources. In total, these beds are lumped into four major deposits categorized as clayey sands, marl, biogenic limestone and dolomitic limestone.

The sediment column constructed from core 19731 had a total depth of 56 feet and was almost divided equally between clayey sand and biogenic limestone deposits (Fig. 2-17).

Siliciclastic beds generally lack structure and are described as massive with extremely gradational contacts. The lack of internal structure is likely due to the semi-unconsolidated nature of these sediments and, in part, an artifact of drilling disturbance. Patterning and the appearance of mottled sediments is attributed to oxidation. Poorly to moderately well sorted sands predominantly consist of fine to medium sized grains ranging in size from 0.0–3.5 Ø and average around 1.5–2.0 Ø. There is slight variability in the concentration of clays throughout the column but are commonly present where there are calcareous sands observed. Fluctuating clay content could be attributed to the presence of interbedded clays and sands, although the siliciclastic sediments are generally well mixed and defined as clayey sands. There is a sharp boundary between these clayey sands and the underlying biogenic limestone deposit at approximately 28 feet below the surface.

Constituting the longest sediment column, core 19732 had a total depth of 103 feet and was dominated by biogenic and dolomitic limestone deposits with minor deposits of clayey sands and marl (Fig. 2-18). The thin deposits of clayey sands were often calcareous and semi- mottled. Ranging from poorly to well sorted, these sands predominantly consist of fine to medium sized grains ranging in size from 0.0–3.5 Ø and average around 1.5–2.0 Ø. Underlying calcium carbonate rich marl deposits share a gradational contact with overlying sands and a convolute contact with the underlying limestone deposit. Moreover, the limestone deposit contact is approached carefully because there was evidence of alteration in the core potentially as a result of extremely unconsolidated sediments or hitting a cavity during drilling. The basal most deposits of core 19732 are split into biogenic and dolomitic limestones based on drastically different compositions and are distinguished from one another at ~81 feet below the surface.

These two carbonate deposits are described in more detail in the discussion section.

Core 19733 is very similar to 19731 because it splits its composition almost equally between clayey sand and biogenic limestone deposits over the 42 feet of sediment (Fig. 2-19).

Moderately well sorted sands predominantly consist of fine to medium sized grains ranging in size from 1.0–3.5 Ø and average around 2.0–2.5 Ø. Under the microscope both 19733 and 19731 had observable amounts of heavy mineral grains and phosphatic material. There is a slight deviation in average sediment size from 19731, but the overall composition is almost identical.

Calcareous sands are prevalent in most measured sand deposits with variable amount of clays.

The contacts between siliciclastic beds that constitute the clayey sand deposits are extremely gradational and overlay a cavity where data was not collected due to prominent void space within the core from 21.5 feet to 25.5 feet below the surface. Underlying the cavity, biogenic limestone represents the remainder of sediment core 19733.

Discussion

Montbrook sediments were described at the micro-, meso-, and macroscopic scale in order to emphasize the spatial continuity of onsite deposits and untouched material surrounding the site. At the microscopic level, smear slide analysis via petrographic microscope and SEM techniques revealed trends in constituent material. In general, diagenetic material tended to concentrate in the lower facies (A, B and partially C), whereas overlying facies (C and D) experienced intermittent amounts of diagenetic and heavy mineral material but had higher concentrations of oxidized clay material. Scaled up to the mesoscopic level, analysis of

Montbrook outcrops detailed lithology and accompanying structural features and categorized five distinct facies that, collectively, define a fluvial-estuarine depositional environmental.

Moreover, macroscopic level analysis utilized visual core descriptions to correlate peripheral sediment columns and define the vertical boundaries of underlying limestone deposits that are interpreted as Ocala and Avon Park Formations. This multiple method approach allowed for a more holistic paleoenvironmental exploration of sediment deposits across analyses. Patterns of consistency and variability are further discussed herein.

Smear Slide Analysis

Smear slide analysis offered a relatively quick semi-quantitative tool to recognize constituent clastic and diagenetic mineral compositions and any associated patterns in sediment distribution throughout a stratigraphic succession. Based on clastic, diagenetic and biogenic components, smear slides from Montbrook revealed a few trends in data based on mineral presence-absence. Some of these interpreted trends served as indicators of environmental setting and were linked to depositional processes.

Siliciclastic sediments composed predominantly of quartz and detrital feldspar are found in a large variety of depositional environments due to their ubiquitous presence and worldwide

distribution. Being some of the most common mineral components in sandy environments, the presence of quartz and minor influence of detrital feldspar in Montbrook sediment is expected.

Feldspar is generally less durable than quartz and less resistant to chemical weathering.

Susceptible to this chemical alteration during weathering, feldspathic grains tend to be abraded and fragmented during transport (Nichols, 2009). Due to the relatively low occurrence of feldspar grains in the samples, Montbrook sediments could have been subjected to chemical weathering and/or moderately long residence times prior to deposition. The increased concentration of feldspars higher up section could represent a lower energy environment more conducive to the preservation of feldspathic grains. Moreover, the tendency of clay covered grains and siliceous debris to concentrate in the upper stratigraphic section provide further evidence for steady to intermittent periods of quiescent environmental conditions.

Heavy minerals are detrital components of siliciclastic sediments characterized as high- density grains that comprise a minor fraction of sands. Unlike feldspathic grains, heavy minerals are generally more durable and resistant to chemical weathering. In the microscope, translucent to opaque heavy minerals generally have high relief and tend to be fragmented with preferential morphologies (tabular and spherical). Higher concentrations of heavy minerals in the more basal sediment samples could be linked to fluid flow and tractive movement during deposition. In their geologic survey report, Brady and Jobson (1973) detail that the even the transport of smaller high-density grains are more resistant to movement than larger low-density grains during sediment transport.

Diagenetic alteration of sediments is common and likely a major proponent of Montbrook deposits due to water level fluctuation as well as syn- and post-depositional processes. This alteration is recognized and documented by previous paleomagnetism analyses conducted as part

of this study (Appendix A). Further evidence of diagenetic alteration is found in the form of pyrite framboids. Pyrite framboids usually adhered to larger sediment grains and are viewed as opaque semi-spherical aggregates underneath the microscope. In sedimentary deposits, pyrite framboids are widely considered to have formed under subaqueous conditions during deposition, or during diagenesis (Sawlowicz, 1993). These structures have been recognized as a dominant form of syngenetic and diagenetic pyrite in oxic, hypoxic and anoxic environments meaning that the occurrence of pyrite framboids is likely the result of many processes rather than a narrow set of physiochemical conditions (Wilkin and Barnes, 1997; Suits and Wilkin, 1998). The large concentration of pyrite framboids observed in samples from lower in the stratigraphic section and the intermittent presence in the upper sections trend with heavy minerals. However, caution should be applied when using presence of pyrite framboids as an indicator of depositional environment, especially in systems where sediments are exposed to fluctuating environmental conditions (Roychoudhury et al., 2003). Previous paleomagnetism analysis on magnetic susceptibility were conducted as part of this study. Results of pilot samples revealed that

Montbrook sediments contain traces of magnetite which, in a reducing environment, often reform as pyrite (Appendix A).

One of the most observable patterns in the smear slides is the strong dichotomy between the presence and absence of palygorskite in different samples. These mineral deposits of palygorskite have a diagnostic needle-like fibrous appearance and tend to concentrate in smear slide samples lower in the stratigraphic sequence. Palygorskite is a rare magnesium-bearing clay mineral reported from a spectrum of sedimentary environments, including those under freshwater, brackish water, saline and hypersaline conditions (Singer, 1979). Although widely regarded as a rare mineral, vast deposits of palygorskite occur across Florida in association with

Hawthorn Group sediments (Lane, 1987; Scott, 1988; Lazareva, 2004). It is likely that the palygorskite documented from these sediment samples is derived from weathering and transportation of detrital Hawthorn Group clays. Palygorskite has been interpreted to form diagenetically in peritidal environments via replacement and dissolution or precipitation (Ryan et al., 2018). Moreover, palygorskite is common in arid to semi-arid climatic conditions with alkaline fluids rich in Si, Al, and Mg that could be sourced from a concentration of groundwater or cyclical flooding and subsequent evaporation (Galán and Pozo, 2011). Florida clays with associated deposits of palygorskite are often interbedded with phosphatic sands and attributed to diagenetically altered phyllosilicates or through direct crystallization (Isphording, 1973). Smear slides with recorded palygorskite concentrations have a strong stratigraphic overlap with slides that produced phosphatic material.

Phosphatic materials are common constituents in many sediments across the Florida

Platform and formed as a product of change in a dynamic environment. Specifically, changes in eustatic sea level are typically associated with the formation of extensive phosphatic deposits in

Hawthorn Group sediments during the Miocene (Espenshade and Spencer, 1963; Riggs, 1979;

Fountain et al., 1993). Sediments containing phosphatic materials are commonly linked to terrestrial pebble deposits in the Central Florida Phosphate District (CFPD). The CFPD is situated in south-central Florida and contains sediments classified as the Bone Valley Member deposits of the Hawthorn Formation. Although Montbrook likely does not contain Bone Valley

Member sediments, phosphatic grains and minerals have been recognized as essentially ubiquitous features in Hawthorn Group sediments in northern Florida (Scott, 1988). The phosphatic materials at Montbrook were represented by preferentially rounded grains, fossil

fragments, and needle-like minerals that formed inside of hollowed structures. Elemental data collected on these aided in their identification (Fig. 2-21).

The three components in Figure 2-21 highlight the diversity and prevalence of phosphatic material from the lower sections measured at Montbrook. Preferentially sorted grains are generally semi-spherical with slight deviations in shape that ranged from compressed oviform to nodular. In cross-section, the grains have no internal structure and are described as cryptocrystalline. Calcium and phosphorus rich compositions support the classification of the carbonate-fluorapatite mineral francolite. Carbonate-fluorapatite is one of the most common phosphate minerals in Florida deposits and has been found in Alachua and surrounding counties

(Sever et al., 1967; van Kauwenberg et al., 1990). The preservation of rounded phosphatic grains at Montbrook ranged from white to brown and were generally polished to frosted. Overall, the appearance of these phosphatic grains suggests long residence times in a kinetic environment or heavy weathering.

Rounded phosphatic grains were often coated with needle-like minerals. Similar in structure to palygorskite, these minerals were not only observed as matrix and coating francolite grains but also as internal structures. Conceivably, these structures reformed as products of diagenesis and are interpreted as wavellite. Both wavellite and crandallite are phosphorous- bearing minerals that are reported to form in association with weathering francolite (Lazareva,

2004). Similar to the other phosphatic components at Montbrook, wavellite is found in heaviest concentrations at and around strata defined as the poorly-sorted gravel material of facies B. The preferentially sorted phosphatic grains could indicate remobilization or erosion through source material prior to deposition at Montbrook. Subsequent weathering of francolite material could have supported the reformation of delicate needle-like wavellite structures as internal

components. Wavelite and crandallite deposits are well documented constituents of weathered phosphatic material from Hawthorn Group sediments in Alachua and surrounding northern

Florida counties (Espenshade and Spencer, 1963; Blanchard and Denahan, 1966; Blanchard,

1972; Williams et al., 1977).

Micofacies Analysis

Restricting Montbrook to either an estuarine or fluvial system would be made simple if diagnostic characteristics in the described facies were representative of one particular environment. However, analyzed facies revealed features associated with both tidal and fluvial environments. Each stratum has the potential to reveal unique attributes that vary from a normalized facies model, but that does not mean assigning an environment is impossible. In order to construct a useful facies model, it is necessary to distill away local variability and retain only the common features (Walker, 1984). Collectively, Montbrook facies are interpreted as marginal fluvial-estuarine deposits associated with a tidally influenced environment.

In the broadest sense, an estuary was defined by Pritchard 1967 as a “semi-enclosed coastal body of water which has free connection with the open sea and within which sea water is measurably diluted with freshwater from land drainage.” This definition of an estuary mainly focuses on the brackish water component, which aligns with the mixed faunal assemblage found at Montbrook as it is interpreted to support an array of taxa from strictly fresh or saline to brackish water environments. However, the goal of this chapter was not to parse out ecological attributes of Montbrook fossils, but rather, to infer depositional setting of its outcrops. This being said, a more modern geology-centric definition of an estuary provided by Dalrymple et al. (1992) state that “an estuary is a transgressive coastal environment at the mouth of a river, that receives sediment from both fluvial and marine sourced, and that contains facies influenced by tide, wave and fluvial processes” and will be utilized herein.

Under this definition, the origin of estuarine deposits can result from transgression and relative sea level rise as well as from fluvial deposits influenced by tidal conditions. Dalrymple et al. (1992) elaborates that the intensity of these influences varies based on positioning within an estuarine system where tidal processes decrease up-estuary and fluvial processes decrease down- estuary. Conceptual models of idealized wave and tide dominated estuary facies are well documented and illustrate the typical tripartite structure of both estuary types (e.g., Boyd et al.,

1992; Dalrymple et al., 1992; Daidu, 2013). In a tripartite structure, an estuary will generally have a lower boundary that is controlled primarily by marine processes, a central mixing zone with fluvial and marine influence and an upper boundary that is dominated by fluvial processes with tidal or wave influence. Comparison between Montbrook fluvial-estuarine facies and conceptual facies model counterparts (e.g., Dalrymple et al., 1992; van den Berg et al., 2007), support the classification of tidally influenced deposits from an upper estuary (Fig. 2-22). The caveat being that the section described in this study is order(s) of magnitude smaller than the broad-scale models used for comparison. Although facies classifications and associations are independent of scale, it is important to note the limited exposure of the Montbrook outcrop. The succession of sedimentary deposits from Montbrook largely correspond to the conceptual tide- dominated estuary model published by Dalrymple et al. (1992) wherein the basal-most unit is composed of alluvial channel deposits and is overlain by tidal-fluvial deposits that generally produce a coarsening upwards trend. Overlying tidally bedded sands and mud can often occur as flat-lying strata or as IHS (Dalrymple et al., 1990, 1991). Moreover, Montbrook sediments from facies C and D are composed of heterolithic deposits at an incline with observable planar to flaser bedding.

On the other hand, the described section at Montbrook has sedimentological features that do not necessarily align with the idealized tidally-dominated facies model. For example, prominent SSD and associated slumping and microfaulting are clear deviations from the idealized estuary facies model. Even so, localized processes and regional influences often create exceptions. For example, transgressions and oscillating sea levels that occurred during the

Miocene caused localized reworking of clastic sediments and slumping in northern Florida as a product of weathering and erosion (Espenshade and Spencer, 1963). It is possible that these discrepancies are a result of secondary processes and are influenced by the morphology of limestone deposits underlying the site. This will be discussed further in the sediment core discussion section. Even though the tidally influenced sediments from Montbrook vary from the idealized schematic, zonation of sedimentary facies in an estuary environment is broadly applicable and typically independent of tidal range (Flemming, 2011). The fluvial-estuarine classification of Montbrook deposits and associated facies is further supported by their physical structures including heterolithic deposits, flaser bedding, minor slumping and onlap features.

Deposition can be influenced by changes in water level, energy, and fluid flow in a sedimentary system. Under subaqueous conditions, Montbrook experienced fluctuations in deposition as a result of periods of sustained flow where mobilized sediments were preferentially sorted and periods of quiescence where clays would settle. As these processes repeated, couplets of sand beds and clay laminae were recorded as heterolithic deposits. Specifically, the low degree bedding angle of the heterolithic deposits at Montbrook are interpreted as IHS.

Environmentally, IHS features in sedimentary deposit are widely associated with tidally influenced river systems and estuary filling (Thomas et al., 1987; Nouidar and Chellaï, 2001;

Longhitano et al., 2012). Although, similar strata have also been reported to occur in a variety of

fluvial environments where episodic flooding of overbank areas produces heterolithic deposits

(Dalrymple, 2010; Gugliotta et al., 2016). As a marginal environment, Montbrook sediments share features, like IHS, that are found in both fluvial and estuarine settings, but are further refined based on other diagnostic structural features.

Prominent flaser bedding structures are abundant in the tidally-dominated facies of

Montbrook and superimposed on the IHS deposits. Flaser, wavy, and lenticular bedding structures are sediment packages on a gradational spectrum based, in part, on the ratio of beds/laminae of sand and mud/clay. Under this scheme, flaser bedding structures are composed predominantly of sand, whereas wavy bedding is approximately equal parts sand and mud/clay, and lenticular bedding contains less sand than mud/clay. Montbrook fluvial-estuarine sediments demonstrate both isolated semi-planar and amalgamating flaser and wavy bedding with little cross stratification. Flaser and wavy bedding are most closely associated with estuarine and tidally influenced depositional environments (Reineck and Wunderlich, 1968; Thomas et al.,

1987; van den Berg et al., 2007; Diadu, 2013). Other influential works recognize that these heterolithic features form in fluvial environments or in similar depositional setting where ripple bedding occurs (Davis, 1983; Martin, 2000; Bhattacharya, 2008).

Estuaries predominantly form during transgression where rising sea level conditions with rising relative sea level (Cattaneo and Steel, 2003; Dalrymple et al., 2011). Moreover, Miocene sea level fluctuations were the primary factor influencing the extent of Hawthorn deposition in

Florida where the state experiences a few major periods of transgression (Scott, 1988).

Diagnostic in a transgressive environment, onlapping sediments often accumulation during sea level rise as sediments are being transported further inland (Catuneanu, 2002). The wavy sediments of facies C have a prominent onlapping sediment feature where conformable strata are

truncated by adjacent inclined strata. Overlying the uppermost fluvial unit, the onlapping feature is part of the transitional boundary between fluvially-dominated and tidally-dominated facies.

Allen and Posamentier (1993) report that, in an estuary, a transgressive surface may record an abrupt change in facies from fluvial deposits to finer tidal-estuarine muddy sands with onlap terminations.

Ichnofossils also offer evidence supportive of the fluvial-estuarine interpretation and will be discussed in chapter three more thoroughly. Preserved burrows and fossil fragments suggest that the mud burrowing shrimp (Family Callianassidae) were present within the Montbrook sediments. Bioturbation generally increased moving up section but is mostly composed of a uniform burrow morphology. In the upper section of a fluvial-estuarine marginal environment, the ichnofossil assemblage tends to be increasingly monotypic (Greb and Martino, 2005).

Overall, Montbrook facies resemble a transitional environment from an upper estuary where fluvial processes are dominant and there is intermittent tidal influence.

Sediment Core Analysis

FGS Sediment cores drilled around Montbrook allowed the broadest spatial examination of deposits with over 200 feet of core was drilled north, south and west of the fossil site.

Examination of these three cores therefore provided a broader understanding of Montbrook subsurface geology and the associated sediment deposits. Deposits were lumped into four major deposits categorized as clayey sands, marl, biogenic limestone, and dolomitic limestone based on lithology and evaluation of constituent material. Descriptions of core samples allowed correlation of across sediment cores and emphasized transitions between the siliciclastic and carbonate sources. In doing so, Eocene age limestone units were identified as Ocala Limestone and Avon Park Formation.

The distinction between the two carbonate deposits, biogenic and dolomitic limestone, is based on a few diagnostic features associated with well-known formational units in Florida. The biogenic limestone is an underlying deposit in each of the core samples starting between 40 and

50 feet ASL. Classified as a bio- or pelsparite wackestone to packstone, these limestone deposits are generally composed of more than 10% grains to grain supported (Folk, 1959). A muddy matrix is often supported by grains composed of preferentially sorted miliolid foraminifera (0.0–

1.0 Ø). Other biogenic material observed in these deposits are minor amounts of coral and echinoderms interpreted as Fibularia vaughani. Moreover, trace fossils of bivalves, echinoderms and gastropods create sections of limestone that are moldic to vuggy with occasional presence of druzy crystals. Although largely well indurated, areas of relatively little cementation, high porosity or bioturbation are described as friable. Specifically, these biogenic limestone deposits are interpreted as Ocala Limestone.

The dolomitic limestone deposit occurs only in core 19732 and are positioned 8 feet below sea level. Classified as a dolomite wackestone, this carbonate deposit has a crystalline structure that can be described as sucrosic with intercrystalline pores. There is little to no presence of fossils, but evidence of bioturbation preserved environmental influences occurred prior to the induration of sediments. Unlike the Ocala Limestone, these dolomitic deposits have observable parallel and minor crossbedding. Parallel bedding is best emphasized by repeated lamina of carbonized organic material. Fossil sea grass communities are rare in the geologic record, but commonly preserve as thin deposits in the Avon Park Formation and are represented by genera Thalassodendron and Cymodecea (Ivany et al., 1990). These diagnostic features help classify dolomitic limestone deposits from core 19732 as part of the Avon Park Formation.

In facies and smear slide analyses, the measurement and collection of sediment samples were limited to the 2.74-meter outcrop. These deposits serve as a general analogue for

Montbrook sediments where spatial continuity is inferred from this section based on the application of Walther’s law. Under this assumption, facies recognized vertically throughout the section should reflect the same succession of facies laterally (Middleton, 1973). FGS cores determined variation in lateral/vertical continuity between siliciclastic sediments sampled in the area of active excavation and peripheral locations.

Overlying siliciclastic deposits from 19733 and 19731 are composed of clayey sands and similar in lithology to the argillarous sand deposits at Montbrook. Although phosphatic debris was observed within core 19731, representative phosphatic francolite grains and wavellite minerals associated with the fossiliferous facies A and B were not recognized. The lack of representative Montbrook features in the core section could be due to channelized environment that reflect a localized deposit. It is possible that the disconnect between sediments analyzed from within the site and sediment cores from around the site reflects a channelized environment that and localized deposition. Preferential deposition of fossil-bearing Montbrook sediments could have been influenced by the subsurface geology of limestone deposits and any associated karstic features. A prominent cavity in the Ocala Limestone is documented from the deposits of core 19733. The Ocala Limestone is one of the most permeable units of rock in Florida and exhibits extensive karstification that influences the topography of overlying deposits (Miller,

1986; Scott, 2011).

According to onsite GPS data, core 19731 sits 72.27 feet above sea level (ABS), whereas core 19732 is at 63.58 feet and 19733 at 70.91 feet. Research by Vernon (1951) states that the crest of the Ocala uplift runs in the northwest-southwest direction and eastern Citrus and Levy

Counties are on the margin of the major uplift. Elevation ABS is not consistent amongst the cores with the northern score higher and southern lower which creates a low-degree dip southward. This southward trend is best recognized in the thickness of siliciclastic sediments where cores 19731 (north) and 19733 (west) are comprised of thick deposits (~25 feet) of clayey sands. In comparison, core 19732 (south) mainly is composed of marl with a thin (~5 feet) deposit of clayey sand. These siliciclastic sediments dramatically taper southward from the north and west directions.

Conclusions

This sedimentological study offers the first paleoenvironmental analysis of Montbrook deposits. As a transitional terrestrial ecosystem, Montbrook records changes in environmental setting and reflects a fluvial-estuarine system produced in conjunction with sea level fluctuation.

Florida has experienced periods of submersion and emersion during the geologic past due to changes in global climate and eustatic sea level that heavily influenced the geology of the state.

Periods of transgression and regression often record as changes in depositional environment at the micro, meso and macroscopic scale. Through analysis of grain composition, bedding architecture, and facies this study reports on these changes and provides a more comprehensive paleoenvironmental interpretation of Montbrook.

Patterns of clastic, diagenetic and biogenic constituents recognized in compositional smear slide studies complement this facies analysis and provide further support. Preferentially sorted francolite grains and associated weathered phosphatic minerals are typically restricted to the more basal gravel bed facies. It is likely that francolite grains were sourced from outside the

Montbrook system and products of remobilization in a high-energy fluvial environment. Once settled, diagenetic processes ensued and allowed for the reformation of wavellite minerals as internal structures and products of weathering.

Prominent syn- and post-depositional features in the Montbrook outcrop help piece together the paleoenvironment. Conformable fluvial deposits transition from a wavy bedded sand to massive gravel unit and highlight the active ancient landscape. Fossiliferous gravel beds were deposited under high-energy subaqueous conditions as an episodic event. The site records a transgressive environment where fluctuating sea levels deposit tidally-dominated sediments on the Montbrook landscape. As the extent of tidal influences rise, onlapping sediments are deposited at Montbrook and record intermittent deposition of clays. Furthermore, tidal conditions deposited sediment packages composed of sand-clay couplets. Heterolithic deposits record periods of sustained fluid flow followed by calm conditions conducive to the settling of clay in an upper estuary setting .

Sediment cores drilled at Montbrook revealed that underlying limestone deposits are composed of Eocene age Avon Park and Ocala Limestone formations. Moreover, associated karstic features found in the Ocala Limestone included a cavity. Undulating surfaces and accommodation space prior to uplift created a depositional setting that promoted deposition.

Montbrook deposits represent residuum of Hawthorn Group sediments that formed in a fluvial- estuarine setting. Sediment cores revealed similar clayey sand deposits but were not able to definitively correlate them to fossil-bearing units at Montbrook. Therefore, the distribution of fossiliferous deposits at Montbrook is unknown. Deposits north and west of the site are more likely to contain fossiliferous sediments than deposits in the south. Further understanding of the potential to find fossils at Montbrook would be enhanced by drilling a core within the current site of excavations and directly to the east. This possibility is unlikely at present due to the current fossil excavations.

The Montbrook sediments represent a productive and dynamic environmental system that has experienced changes in depositional settings throughout its history. Facies analysis determine that these sediments constitute two major facies, a fluvially-dominated and a tidally-dominated environment. Collectively, the association of facies at Montbrook suggest the site formed when the Florida Platform experienced fluctuating sea levels. Specifically, this transitional system likely formed as a result of substantial fluvial processes with intermittent tidal influence in a transgressive environment during the Miocene of Florida.

Figure 2-1. Geologic cross-section of Florida from Scott et al. (2001) featuring the north to south transect. Dashed lines outline the Alachua and Marion County section and the depositional units associated with their subsurface geology include Hawthorn Group sediments, Ocala Limestone and Avon Park Limestone.

Figure 2-2. Montbrook fossil site overview highlighting the various areas of interest within and around the locality. Specifically, A) is an area overview illustrating the position of FGS sediment core drill sites and corresponds to Figures 2-12 to 2-20. The subset B) is a site overview with 1m by 1m gridded plots where the representative stratigraphic section of Montbrook was sourced from and corresponds to Figures 2-3 to 2-9. Photo courtesy of author.

Figure 2-3. Representative outcrop and key geologic features observed from the lower step. Staff is 1.0 meter in height with each white and balck segment representing 10 cm increments. The west-facing lower step measures ~100cm. The stratigraphic sequence represented here can be observed in other sections of site, but not all architectural features are ubiquitous. The observed features include: L-1) ripple marks, sigmoidal bedding, and minor crossbedding, L-2) large burrow or void infilled with semi- structurless sand, L-3) fossiliferous gravel, L-4) soft sediment deformation (SSD) and convolute bedding, L-5) onlap wedge and heterolithic bedding and L-6) ichnofossil burrows and interbedded deposits of sand/mud. Photos courtesy of author.

Figure 2-4. Representative outcrop and key geologic features observed from the upper step. Staff is 1.0 meter in height with each white and balck segment representing 10 cm increments. The west-facing upper step measures 155cm vertically from contact with the lower step to the modern surface. The deposit is composed of heterolithic sediments of alternating sand and clay layers. U-1) wavy bedding and semi-parallel laminae have undulating surfaces and often truncate into one another. Liesegang rings cut through bedded strata and likely record water movement through sediment. U-2) flaser bedding and ichnofossils are found in increased concentrations in the upper step. Photos courtesy of author.

Figure 2-5. Correlated lithostratigraphic columns of upper and lower steps based on site description and microfacies analysis where depth is measured in centimeters from the surface. Vertebrate and trace fossils found in situ are recorded. Facies codes are based on Miall (1985).

Figure 2-6. Representative section of rippled sand unit (facies A). Both the back bars denote 10cm scale. Sedimentary features are examined in situ and reported from A) the west facing section where the circled object is a partial rib of Rhynchotherium sp. and B) the south facing section where the dashed line represents characteristic ripple marks and the solid line represents minor crossbedding. Overall, the heterolithic deposits of facies A have observable flaser bedding with clay laminae/lenses. Photos courtesy of author.

Figure 2-7. Representative section of massive gravel unit (facies B). Both the back bars denote 10cm scale. Sedimentary features are examined in situ and reported from A) the west facing section where the dashed line represents the undulating boundary of the oxidized bed of semi-indurated gravel capping the unit and B) the south facing section where the solid line represents the gradational contact with the underlying rippled sand unit (facies A). Gravel deposits of facies B are fossiliferous and contain observable lenses of clay. Photos courtesy of author.

Figure 2-8. Representative section of wavy cross laminate sand unit (facies C). Both the back bars denote 10cm scale and the circled features represent preserved ichnofossil burrows. Sedimentary features are examined in situ and reported from A) the west facing section where the solid line represents crossbedding and B) the south facing section where the dashed line represents SSD and associated folds. Overall, the heterolithic deposits of facies C are characterized by truncating sand and clay strata and prominent architectural bedding features like SSD, folding and onlap. Photos courtesy of author.

Figure 2-9. Representative section of heterolithic flaser unit (facies D). Both the back bars denote 10cm scale and the circled features represent preserved ichnofossil burrows. Sedimentary features are examined in situ and reported from A) the west facing section where the flaser bedding is observable as couplets of larger beds of sand with clay laminae and B) the south facing section where the dashed line highlights the low degree (~17–28º) inclined nature of the heterolithic strata. Photos courtesy of author.

Figure 2-10. Feldspar grains observed from smear slide samples in XPL. White arrows point to feldspar grains where A) demonstrations unidirectional polysynthetic twinning observed at a depth of 115 cm and B) demonstrations bi-directional polysynthetic twinning observed at a depth of 215 cm. Photos courtesy of author.

Figure 2-11. Heavy mineral grains observed from smear slide samples in PPL. White arrows point to heavy mineral grains where A) features a typical tabular heavy mineral surrounded by oxidized clay coated grains observed at a depth of 50 cm and B) features two more spherical heavy minerals observed at a depth of 225 cm. Photos courtesy of author.

Figure 2-12. Quartz grains with pyrite framboids observed from smear slide samples in PPL. White arrows point to pyrite framboids where A) features a few framboids surrounded by grains moderately coated in oxidized clay recorded at a depth of 95 cm and B) features an amalgamation of pyrite framboids recorded at a depth of 215 cm. Photos courtesy of author.

Figure 2-13. Palygorskite observed from smear slide sample in XPL. White arrows point out a quartz grain covered in the palygorskite material as well as it within the matrix at a depth of 200 cm. Photo courtesy of author.

Figure 2-14. SEM images of artifacts that contain needle-like minerals inside their hollowed structures. Both specimens were recovered from a depth of 195 cm where the white scalebar denotes 100 um. A) illustrates the typical preservation state of these hollowed specimens and B) is a specimen fractured just prior to imaging and surrounded by needle-like mineral debris. Images courtesy of author.

Figure 2-15. SEM images of microfossils found in situ. All featured microfossils were collected from 205 cm or below in the measured section: A) is the fragmental appendage of an invertebrate from a depth of 215 cm where the scalebar denotes 200 um, B) is a fish tooth from a depth of 205 cm where the scalebar denotes 50 um and C) is a denticle from a depth of 210 cm where the scalebar denotes 50 um. Images courtesy of author.

Figure 2-16. SEM images of preferentially sorted phosphatic-rich grains. Both specimens were collected from a depth of 230 cm and the white scalebar denotes 100 um. A) is a typical rounded grain where the white arrow points out some of the needle-like structures covering the structure and B) is a fractured grain where the white arrow illustrates a lack of internal structure. Images courtesy of author.

Figure 2-17. FGS sediment core 19731 drilled north of Montbrook. The black bar denotes one foot and segments are labeled sequentially where A) is the sediment closest to the surface from 0.00–9.25 feet below the surface, B) 9.25–19.25 feet, C) 19.25–34.90 feet, D) 34.90–46.00 feet and E) 46.00–56.00 feet at which the core terminates. Photos courtesy of author.

Figure 2-18. FGS sediment core 19732 drilled south of Montbrook. The black bar denotes one foot and segments are labeled sequentially where A) is the sediment closest to the surface from 0.00–13.95 feet below the surface, B) 13.95–26.33 feet, C) 26.33–36.08 feet, D) 36.08–45.33 feet and E) 45.33–56.33 feet, F) 56.3365.33 feet, G) 65.33–74.50 feet, H) 74.50– 93.33 feet and I) 92.33–103.00 feet at which the core terminates. Photos courtesy of author.

Figure 2-19. FGS sediment core 19733 drilled west of Montbrook. The black bar denotes one foot and segments are labeled sequentially where A) is the sediment closest to the surface from 0.00–8.00 feet below the surface, B) 8.00–17.00 feet, C) 17.00–32.50 feet with a prominent cavity around 21.50–25.50 feet and D) 32.50–46.00 feet at which the core terminates. Photos courtesy of author.

Figure 2-20. Fence diagram of FGS sediment cores correlated by lithology and plotted against elevation. Here the northern core (19731) has an elevation of 72.27 ft ASL, southern core (19732) has an elevation of 63.58 ft ASL and western core (19733) has an elevation of 70.91 ft ASL.

Figure 2-21. SEM images and EDS data collected on representative phosphatic materials. Phosphatic specimens are represented by A) a preferentially rounded grain, B) needle-like minerals and C) a fossil fragment. Wt% shows relative concentration of elements from the sample and At% shows normalized concentrations of atomic data. ZAF corrects raw peak intensity data and converts to quantitative concentrations. Images courtesy of author.

Figure 2-22. Schematic cross-section of a tide-dominated estuary and distribution of lithofacies resulting from transgression of the estuary, followed by estuary filling and progradation of sand bars or tidal flats from Dalrymple et al. (1992).

Table 2-1. Summary of facies assignments in Montbrook outcrop. Interpretation is based on lithology, granulometry, bed/lamina thickness, color, bedding architecture, nature of overlying and underlying contacts, body and ichnofossils, and any recognizable post- depositional features. Here laminae are defined as 1 cm following the precedent set by McKee and Weir (1953). Facies Grain Size (Ø) Attributes Interpretation A) Sr 2.0 – 3.0 Minor cross-bedding with low- Sandy bedform with minor degree southward dip. Rippled presence of overbank fine Rippled Sand Clayey Sand sand bed with discontinuous clay sediments. Deposited during a Unit laminae. relatively calm environment with altering water levels/directions. B) Gm & Gms -1.0 – 3.0 Berm feature with massive to Gravel berm feature formed from marginally bedded gravels. a high energy environment. Massive Gravel Sandy Gravel Embedded fossils present are semi- Likely episodic due to the Unit worn. Clay lenses (~1-3 cm). general lack of structure and many isolated fish fossils.

C) Sr & Sl 2.0-3.5 Onlapping sediments and SSD with Lateral accretion deposits associated folding/micro-faulting. thrusted over underlying gravel Wavy Cross Clayey Sand Semi-mottled via bioturbation. beds. Clastic wedge and onlap Laminated Unit linked with transgression during SLR (Allen and Posamentier, 1993). D) Fl & Sr 2.0-3.5? Vertically accreting heterolithic Tidal influence with intermittent sediment packages. Interbedded periods of shallow water low- Heterolithic Clayey Sand sand/clay couplets with sharp energy deposition where fine Flaser Unit contacts. Liesengang ring banding grained clastic sediments can present. Plane and ripple bedding. settle producing flaser bedding.

Table 2-2. Summary of smear slide results with patterns of presence and absence of major sediment constituents. Coded presence-absence of smear slide sediments based on relative concentration where tabular heavy minerals are represented by a (T) and rounded an (R). Phosphatic material represents both fossil fragments and rounded phosphatic-rich grains. White boxes represent complete absences (-), light grey boxes represent moderate presence (/), and dark grey represents strong presence (+).

Depth Ox. Clay Heavy Heavy Pyrite Phosphatic (cm) Feldspar Coated Mineral (T) Mineral (S) Zircon Framboid Palygorskite Material (F) 0 / + - / - - - - 5 - + / - - - - - 10 / + / - - - - - 15 + + - / - - - - 20 / + - - / / - - 25 / + + + - - / - 30 / + + + - + - - 35 + + / - - - - - 40 + + / / - - - - 45 + + + / - / - - 50 + + + / - + - - 55 / / + + + - - - 60 / / / + - / - - 65 / / + - / / - - 70 / / / + / + - - 75 / / / + - / - - 80 / / / - - - - - 85 / / / / - - - - 90 / / / / - / - - 95 / + + / - + - - 100 / + + + - / - - 105 - / + + / / - - 110 + / / / - + / - 115 + / + + + / / - 120 / / / + / / / - 125 + / + + + - - - 130 + / / / - / - - 135 + / - / - + / - 140 / / + / / - - - 145 / / - + - / / - 170 / / + + - / + - 175 / - + + - + + -

Table 2-2. Continued. Depth Ox. Clay Heavy Heavy Pyrite Phosphatic (cm) Feldspar Coated Mineral (T) Mineral (S) Zircon Framboid Palygorskite Material (F) 180 - / + - - + + - 185 + / + + / + + - 190 + / + / / + + - 195 + / + + / / + + 200 / / + + / / + + 205 / + + + - + + + 210 / / + / / + + + 215 + - + + / + + + 220 + - + / - / / + 225 / - + + / + - + 230 / - + / / + / + 235 - / + + / + - + 240 / / + + - + / + 245 / / + + / + + / 250 + / + / - / / / 255 + / + / - + - + 260 / / + / + + - + 265 / / + + / / + /

CHAPTER 3 ICHNOFOSSILS AND BIOTURBATION INDEX

Ichnofossils are preserved traces of ancient life that often serve as records of how flora and fauna interacted with their environments (McIlroy, 2004). As residual clues of environmental interaction, ichnofossils are found at a variety scales and concentrations that create unique structures (“icnhofabrics”) within sedimentary systems. In an active environment influenced by bioturbation, the primary sediment fabric can get replaced by the ichnofabric to varying degrees. Bioturbation reworks sedimentary deposits via organism interactions that occur as a combination of syn- and post-depositional processes and is recognized as a major component in facies analysis interpretation (Taylor et al., 2003). As a compliment to the facies analysis conducted in the previous chapter, this section aims to provide further paleoenvironmental evidence by conducting a bioturbation index (BI) study on an assemblage of ichnofossils found at the Montbrook fossil site.

The majority of trace fossils found from within the measured sedimentary section at

Montbrook are interpreted as burrows created by decapod crustaceans. Specifically, it is likely that the burrows were created by ghost shrimp belonging to the Family Callianassidae. This interpretation is based on the very common occurrence of Callianassidae fossil appendage fragments in Montbrook deposits. Furthermore, ghost shrimp are recognized as infaunal bioturbators and environmental engineers that can considerably influence the substrate of soft sediments settings (Berkenbusch and Rowden, 2006). Ghost shrimp can disturb conformable strata through the process of creating burrowing structures. As physical structures, burrows are formed through the removal of sediment from its original position and subsequent infill displaces deposits further (Frey and Pryor, 1978).

Environmental engineers, like ghost shrimp, can have a dramatic influence on the preservation quality of bedding features in an outcrop. Depending on the degree of disturbance, the inherent organization of sediments can range from undisturbed to completely homogenized by bioturbation. As a way to formally measure the affect organism activity has on substrate preservation, indices of bioturbation have been standardized (e.g. Droser and Bottjer, 1986;

Taylor and Goldring, 1993). In this chapter, a BI analysis was conducted to determine if ichnofossils support the fluvial-estuarine interpretation of the Montbrook outcrops and to examine how the succession of bioturbated sediments compare to the previously described facies assignments.

Methods

During initial fieldwork associated with the sedimentological analyses conducted in chapter two, high fidelity photographs were taken of the measured outcrops to document the sedimentary deposits. In this study, those images were utilized in a quantitative BI analysis to determine the degree of alteration of Montbrook sediments. Similar to the facies analysis in chapter two, both the upper and lower portions of the measured sections were included in this study to gain a more comprehensive idea of any change over successive stratigraphic deposits. In order to compare the disruption represented in these deposits, 50cm by 25cm vertical cross- section plots were determined sensu Droser and Bottjer (1986) with some variation with regard to updated post-processing methods that are discussed herein.

In total, nine sediment plots were created and analyzed to determine the percentage area of primary and secondary sedimentary structures. Five plots were analyzed from the upper step spanning from 15 to 140cm below the surface and four plots were from the lower steps spanning from 165 to 265cm below the surface. Plots were chosen from a contiguous vertical section within the measured outcrop from chapter two. The initial plot starts at 15cm below the surface

because the undulating surface created void spaces where sediments would not be included in the entire plot and not conducive to a percentage area calculation. Primary fabric constitutes physical sedimentary structures like observable bedding and are generally considered to not be bioturbated. Whereas, secondary fabric consists of bioturbation structures classified as burrows or mottled material. In this study, mottling is partially interpreted as a product of prominent slump and soft-sediment deformation features.

In order to ensure consistency, a uniform scale was determined from onsite measurements and translated to the sediment plot images. Utilizing image editing software FIJI and Adobe Illustrator the boundaries of these sediment disturbances were traced to isolate as categorized features and emphasized further by increasing the contrast of images. The area of bioturbation and mottled features were measured using the scientific image processing software

FIJI. Because the sediment plots were uniform at 50cm by 25cm, their area was a consistent

1250cm2. From this, the portions attributed to bioturbation or mottled material could be calculated. Remaining percentages area were attributed to primary structures and strata that was seemingly unaltered by bioturbation or mottled material. It is important to note that the strata component included massive deposits even though they are generally structureless. With percentage area of bioturbation calculated, indices of bioturbation were applied based on the classification scheme from Taylor and Goldring (1993). In this scheme, BI grading ranges from 1 to 6 and is assigned based on percentage of sediments bioturbated where grade: 0 = (0%), 1 = (1-

4%), 2 = (3-30%), 3 = (31-60%), 4 = (61-90%), 5 = (91-99%) and 6 = (100%). Figure 3-1 provides an example of the described methodological processes based on a section of sediment included in the BI analysis.

Results

Two different ichnofossil morphotypes are interpreted from the sedimentary section at

Montbrook (Fig. 3-2). The less common morphotype preserved is an elongated tube that branches vertically and ranges in size from 2 to 9cm long. Branched components deviate in size from the, otherwise uniform, tube diameter which averages around 1 to 3cm. Moreover, the rounded branched components of the tube tend to be shallow and terminate into adjacent sediment deposits. The other morphotype is represented by a rounded tube feature that ranges in diameter from <1 to 2cm. Together, these ichnofossil features altered Montbrook sediments as syn- and post-depositional processes and make up the bioturbation component of this study.

The degree of bioturbation Montbrook sediments experienced are not constant throughout the sediment section where bioturbation features tend to increase located up section. Reported BI values assigned to plots (1.1–1.9) are displayed in Table 3-1 and are correlated with depth.

Specifically, BI has a narrow range of 2–3 when incorporating both percentage area of burrow structures (%Burrows) and mottled material (%Mottled) in the analysis. A closer look at these two components, reveal that %Burrows and %Mottled are inversely related where %Burrows increases up section and the %Mottled largely increases down section. Plots 1.6–1.9 are lower in the section and have large concentrations of mottled material that are recognized as the dominant factor making up the percentage area that was bioturbated. Exclusively focusing on distinguishable burrow features as secondary processes, %Burrows results potentially reflect a more accurate representation of bioturbation in Montbrook sediments. Even so, BI values and

%Burrows are generally lower down section (plots 1.7 and 1.8) when compared to plots of successive strata higher in the sedimentary section.

In order to analyze the influence, if any, depositional setting has on BI, sediments plots were correlated with the facies described in this study and modeled (Fig 3-4). Data collected in

chapter two provided facies descriptions with associated stratigraphic levels. When BI plots are correlated with the facies interpretations, plots 1.1–1.6 are comprised of sediments described as deposited in a tidally-dominated regime and plots 1.7–1.9 are comprised of sediments from the fluvially-dominated regime. The fluvially-dominated components tended to have very few distinguishable burrow features and strata that appeared heavily mottled. In contrast, the tidally- dominated components tended to have the highest percentage area of primary structures.

Moreover, these plots documented the highest percentages of burrow features with the apex being at plot 1.3.

Decapod fossils found in situ at Montbrook are common occurrences and mainly represented by fragmental appendages found through the picking of sediment matrix (Fig. 3-3).

Only in situ fossil appendage elements have been found from Montbrook and no body fossils.

These appendages represent the claw of ghost shrimp and range in size from range in size from

2mm to 5mm. Fossil specimens interpreted as Callianassidae from the Montbrook section and

FLMNH database are laterally compressed and taper distally. Appendage elements are further identified as either the upper (dactylus) or the lower (propodus or fixed finger) component of the claw of a ghost shrimp (Hyžný and Klompmaker, 2015). Determining between minor or major chela is difficult because fossils are fragmentary and not commonly found in association with other appendage or body components. (Hyžný and Klompmaker, 2015). Although identification past family level is not discernable at this time, there is a strong taxonomic affinity towards

Callianassidae at Montbrook.

Discussion

Ichnofossils observed in the Montbrook outcrop were categorized based on two major morphologies, i.e., a rounded tube and elongated branched tube features. The orientation of preserved burrows is largely dependent on the bedding plane, and Montbrook burrowing tubes

extend vertically through strata with little variation horizontally. Therefore, specimens are not considered to be positioned in bedding plane view because continuous vertical sections of branched tubes can be viewed in cross-section. It is possible that the rounded tube structures represent a cross-sectional view of an elongate branching tube or a burrow branched horizontally. Hyžný et al. (2015) reports that horizontal bending of burrows is common feature at bifurcation points in the tunnel system. The branched component of the elongate ichnofossil features are interpreted as bifurcation points in a shallow burrow system. Modern

Thalassinideans use these enlarged areas as turn-around points within the burrow and can be represented by blindly ending branches (Frey et al., 1978). Under this interpretation, these two burrow morphologies likely constitute a monotypic burrow feature.

Both Thalassinoides and Ophiomorpha are ichnogenera that have similar branching tube morphologies and are interpreted as dwelling burrow features created by crustaceans (Martin,

2013). Ophiomorpha is an ichnofossil that has been interpreted as densely branching structures that create extensive dendric tube systems (Löwemark et al., 2016). Similarly, Thalassinoides has a branching tube morphology. Historically, the distinction between Thalassinoides and

Ophiomorpha tends to be a bit ambiguous because the major distinction is based on characteristics of pellet wall construction (Frey et al., 1978). Inherently, this generalized distinction is subjected to biases depending on the quality of ichnofossil preservation. Monaco et al. (2007) report that some horizontal segments of Ophiomorpha that lack this bumpy pellet façade resemble Thalassinoides, but the vertical portions are typically lined. Even so, due to poor preservation quality and unconsolidated nature of the sediments, excavating or creating epoxy resin cast of ichnofossil structures to distinguish characteristic features of ichnogenera

Thalassinoides and Ophiomorpha was not feasible. The ichnofabric preserved at Montbrook

reflects an extensive system of burrowing features that reasonably formed in an environment inhabited by decapod bioturbators.

Although Montbrook sediments likely represent an extensive system of burrowing produced by multiple bioturbators, Hyžný, et al. (2015) state that Callianassidae form the most intricate and widespread burrow systems of all known fossorial shrimps. Currently, fossil decapods at Montbrook are only represented by in situ Callianassidae elements and were presumably the dominant bioturbating organisms. Ghost shrimps are the most likely suspect, although identifying the producers of these burrows without body fossil preservation within the structure should be approached cautiously (Hyžný, 2011). In modern times, burrowing features created by decapods are common in near coastal settings. Morphologically, the closest extant analogue to fossil Ophiomorpha are the burrowing structures made by the mud shrimp,

Callianassa major (Family Callianassidae), which predominantly lives in coastal settings in southeastern USA (Nagy et al., 2016). Moreover, Callianassidae are some of the most successful invaders of brackish-water environments and are able to tolerate environments with fluctuating salinity conditions (Hyžný et al., 2015). Facies analysis of Montbrook sediments interpret the paleoenvironment to be a fluvial-estuarine that would be intermittently prone to brackish-water conditions.

The depositional setting appears to have an influence on BI sediments plots 1.1–1.9 when correlated with the Montbrook fluvial-estuarine facies model (Fig. 3-4). Secondary fabric was composed of percentage area of identifiable burrows and mottled material. Much of the lamination-destroying mottling could be attributed to bioturbation, however, the mottled material for lower lying strata is likely associated with prominent deformation features. Specifically, fluvial-dominated strata had low %Burrows due to high-energy gravel deposits of facies B.

There is a general increase in %Mottled down section. Moreover, extensive secondary features associated with the fluvial deposits (e.g. soft-sediment deformation and slumping) were recorded from plots 1.7-1.8 and are interpreted as artifacts of bedding and not attributed to bioturbation.

Because the standard BI values incorporate %Mottled, plots 1.5 to 1.7 should be interpreted carefully. Comparably, there is strong increase in percentage area of identifiable burrow features moving up section (Table 3-1) from fluvial- to tidally-dominated facies and are best recognized cutting through successive laminae in the heterolithic deposits of facies C and D.

Conclusions

The excavations at Montbrook have produced a mixed faunal assemblage, spanning from terrestrial to freshwater and marine fossils. Previous chapters outline some of the common vertebrate taxa that are found from these environmental settings (see chapter one), and this chapter focuses on the ichnofossils as supportive evidence. In this chapter, BI analysis determined alteration of sediments from burrowing structures generally increase up section and are most prevalent in the tidally-dominated facies C and D. Specifically, the heterolithic deposits from plots 1.1–1.3 are the most concentrated. Moreover, BI of fluvially-dominated facies B

(plots 1.7-1.8) were not as conducive to the habitation and/or the preservation of fossil burrows.

Sediments record moderate bioturbation and are likely a product of extensive burrowing from decapod crustaceans, Callianassidae. Ghost shrimp are regarded as the primary environmental engineers at Montbrook and often are found in situ or in picked sediment deposits. Fossorial shrimp are effective at altering soft-sediment deposits. Burrow features increase in concentration over the transition from fluvial to estuarine sediments. Intermittent deposition of heterolithic sand and clay packages could provide fresh surfaces to burrow into and ample material to fill in relic tunnels. Moreover, ghost shrimp are tolerant to and often inhabit

brackish-water conditions which further supports the classification of Thalassinoides and/or

Ophiomorpha as possible ichnogenera.

As common constituents of deposits in brackish-water environments, Thalassinoides and

Ophiomorpha occur in fluvial-tidal settings, often associated with Skolithos ichnofacies that form during periods of higher sea level (Martin, 2013; Díez-Canseco et al., 2015).

Sedimentological features such as onlapping and heterolithic strata at Montbrook reflect a transgressive environment favorable for sustaining an estuary. Low variation among Montbrook ichnofossils suggest an assemblage skewed more toward being monotypic. Greb and Martino

(2005) reports that low diversity of an ichnofossil assemblage reflects an upper estuary environment in a fluvial-tidal depositional setting.

Figure 3-1. Methodology of calculating Bioturbation Index. Semi-quantitative process of calculating primary and secondary fabric percentage areas using a section of sediment from the plot 1.3 (65–90cm below the surface) in the upper step where: A) represents the 50cm by 25cm plot where the black scale bar denotes 5cm, B) the burrow features and mottled material emphasized by tracing over field photographs in Adobe Illustrator and C) calibrated percentage area measurements taken in FIJI. Photos courtesy of author.

Figure 3-2. Ichnofossil morphologies preserved in heterolithic deposits. This section represents 65–90 cm below the surface where the solid line arrow shows the rounded tube feature and the dashed line shows the elongate branched tube feature. Black scale bar denotes 5cm. Photo courtesy of author.

Figure 3-3. Appendage fragments of ghost shrimp from Montbrook specimens. A) FLMNH specimen 285846 is identified as a propodus and B) FLMNH specimen 285835 is identified as a dactylus where the scale bar represents 5mm. Photos courtesy of FLMNH Invertebrate Paleontology Collection. C) Represents SEM imagery of a microfossil appendage fragment picked from around 215cm below the surface where the scale bar denotes 1mm. Image courtesy of author.

Figure 3-4. 3D schematic conceptualizing the strata, facies, architectural bedding features and bioturbation in the Montbrook outcrop. BI is correlated with facies and associated sedimentological features: A) inclined heterolithic bedding, B) burrow features, C) soft-sediment deformation folding, D) onlap, E) wavy bedding and F) massive sand filled feature cutting through the wavy bedded sand and fossiliferous gravel units. Photos courtesy of author.

Table 3-1. Summary of BI analysis including percentage area of primary sediment structures (P.S.) and secondary sediment structures (S.S) with associated depth. Values are based on Taylor and Goldring (1993) where BI is determined based on the cumulative percentage of secondary structures (mottled material and burrow structures). Plot ID Depth (cm) % Burrows % Mottled % P.S. % S.S BI 1.1 15-40 3.985 28.977 67.038 32.962 3

1.2 40-65 4.26 19.919 75.821 24.179 2 1.3 65-90 6.631 5.751 87.618 12.382 2 1.4 90-115 1.071 29.767 69.162 30.838 3 1.5 115-140 0.772 15.304 83.924 16.076 2 1.6 165-190 3.049 29.958 66.993 33.007 3

1.7 190-215 0.929 29.395 69.676 30.324 2 1.8 215-240 0.083 25.771 74.146 25.854 2 1.9 240-265 0.65 30.631 68.719 31.281 3

APPENDIX A PALEOMAGNETISM

Continuous records present in sedimentary outcrops potentially makes them an important focus in paleomagnetism (Opdyke and Channell, 1996). Montbrook sediments were collected in situ and tested for paleomagnetic analysis in an attempt to refine the temporal position of the fossil bearing deposits. Changes in Earth’s magnetic field are recorded in sedimentary rocks via depositional or chemical remanent magnetization and are most recognizable as the ‘flip’ between normal and reverse polarities (Merrill et al., 1996). According to the geomagnetic polarity timescale of Cande and Kent (1995), there are periods of reversals during the polarity chrons

(C3n.4n and C3An.1n) which overlap the age range assigned to Montbrook based on biostratigraphic expectations. Paleomagnetism analysis does not produce an absolute age, but by comparing the pattern of polarity(ies) present at Montbrook to an established timescale, it could bridge the gap between absolute dating and biochronology. Thus, in an attempt to further refine the chronology at Montbrook, I attempted to do paleomagnetic analyses of the sediments from this locality. Although the results were inconclusive, they are documented here.

In total, 52 block samples were collected from four sites within the measured stratigraphic section described in this study. Prior to collection, large samples on in situ sediment were stabilized with an epoxy resin and orientations were recorded. In the University of Florida sedimentology lab, large samples were cut down to the requisite 1-inch3 cubes, wrapped in aluminum foil and placed in a shielded room at the paleomagnetic laboratory to prevent the acquisition of and dispel present viscous remanent magnetizations. Samples were measured for natural remanent magnetization (NRM), and the variation of magnetic susceptibility (X) with temperature (k-T curves). NRMs were measured with a 2-G discrete cryogenic magnetometer and accompanying CryoPC3 software. Susceptibility variation with temperature was measured

via AGICO Kappabridge KLY-3 magnetic susceptibility meter with a furnace attachment in an argon atmosphere in order to potentially prevent alteration during the experiment. A subset of pilot samples were subject to demagnetization analysis using alternating field (AF) and thermal demagnetization (TD) techniques.

Analysis of NRM yielded scattered results where the majority of data points had north- directed declinations and moderately steep, moderate down-inclinations (Fig. A-1). Applying a

45° cutoff to the data to eliminate outliers results in a mean direction of D=7.4°, I=+41.4°

(k=8.3, a95=11.5°; n=22). Because secular variation is likely average by the several meters of section sampled, we can calculate a paleolatitude of ~24° N, which agrees within error with the recent latitude of Montbrook (present day 29.4° N). This lower paleolatitude may be a result of secondary magnetic components affecting the NRM or inclination shallowing of the sediments.

Figure A-1. Characteristic Remanent Magnetism (ChRM) inclination and declination data where filled circles denote normal polarity and open circles represent reverse polarity. Rejected samples shown in read, mean and 95% confidence interval on mean direction shown (Koymans et al. 2016).

Demagnetization analysis produced ambiguous results (Fig. A-2) that ended up quickly reaching the noise levels of the magnetometer. Montbrook sediments contain weakly magnetized constituent materials with low magnetic susceptibility. The magnetic instability of Montbrook samples is likely attributable to sediments enduring heavy diagenetic alteration or lack of sufficient quantities of stable magnetic minerals, such as magnetite or hematite. Susceptibility- temperature analysis demonstrates that the magnetic material carrying the NRM alters during demagnetization (Fig. A-3). Specifically, the material reaches zero magnetic susceptibility around 400°C, and then on cooling the sharp rise at 580C shows the presence of magnetite

(Dunlop and Ozdemir, 1997). Thus, even in an argon-atmosphere experiment, the alteration of the samples produces magnetite. In a reducing sedimentary environment, magnetite is prone to dissolve and reform as sulfides which, as a result, can essentially yield anoxic sediments that are nonmagnetic (McElhinny and McFadden, 2000). Pyrite (FeS2) is an extremely common sulfide.

In the smear slide analysis section of this study, pyrite framboids structures were recognized as a major diagenetic component in Montbrook sediments.

Figure A-2. Representative examples of Zijderveld vector plots illustrating the unstable nature of Montbrook demagnetization data. Here, A) is an AF demagnetization plot where mT is milliteslas and B) is a thermal demagnetization plot. In both plots, tick marks are measured in increments of 0.1 milliamps per meter (mA/m).

Figure A-3. Paleomagnetism results of susceptibility-temperature analysis. Magnetic mineral determination with alteration on heating producing magnetite (Curie point around 580C) where temperature is plotted against magnetic susceptibility (Kt) (Tauxe et al., 2018). Red and blue lines denote stages of heating and cooling, respectively.

Overall, Montbrook sediments appear to be diagenetically altered and not conducive to utilizing paleomagnetism analysis as a proxy for dating. Therefore, conducting further paleomagnetic studies at this locality likely will not yield interpretable results. However, paleomagnetic analysis produced some useful information on the state of magnetic minerals preserved in sampled sediments and paleoenvironmental conditions.

APPENDIX B EDUCATION, OUTREACH AND COLLABORATION

Fossils and the research practices associated with paleontology have served as an appealing gateway to integrate scientific research into both formal and informal educational programs (Grant et al. 2016). Fossils at Montbrook serve multiple purposes because they are used for institutional research at the FLMNH as well as in numerous outreach activities serving the public, educators and K–12 students. Montbrook inherently lends itself to place-based educational opportunities and K–12 development since it is largely excavated by volunteers from the public, including Florida educators and students. For example, after attending the summative workshop of an NSF-funded research experience for teachers at the FLMNH, a local teacher created the Gainesville Youth Fossil Club (GYFC). Since its inception in 2017, more than two dozen K–12 students have participated in GYFC activities, interacted with faculty, staff and students from the FLMNH, and been involved in group excavations at Montbrook.

Because there are many underserved and underrepresented communities surrounding

FLMNH that might not have the ability to attend an onsite excavation, screening sediment for microfossils is an activity used to promote off-site interaction with the Montbrook site.

Fossiliferous sediment from Montbrook is abundant and utilized as an anchoring event in an inquiry-based activity that can be used at every K–12 level. This screening activity has been particularly successfully when implemented at a pretrial detention center in the southeast United

States because it is a very tactile experience promoting hands-on-learning in what is normally an environment with minimal stimulation. For those who are limited by geographical boundaries, the FLMNH is adopting innovative practices and technologies to help mitigate the lack of accessibility to Montbrook. Utilizing photogrammetry as a proponent of outreach for my research project, 3D models of the Montbrook site were reconstructed and uploaded to the digital

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3D repository, www.sketchfab.com. As an online hub, this educational model is open-source and accessible to anyone with internet connection allowing for a virtual tour of the excavation site.

Through the Thompson Earth Systems Institute’s (TESI) Scientist in Every Florida

School (SEFs) Project, four Florida K-12 educators aided in the data collection of my master’s research thesis during a week-long professional development in the summer of 2019. As part of the collaboration, SEFS participants searched through Montbrook sediment to isolate and identify microfossils (Fig. B-1). Specifically, SEFS participants were attempting to find any foraminifera within the Montbrook sediments for use as paleoenvironmental indicators. It is likely that paleoenvironmental conditions or diagenetic alteration of Montbrook sediments were not conducive to the preservation of foraminifera. Even so, other microfossils and phosphorous- rich material were discovered through this process. Specimens found during the SEFS professional development were applied to the facies analysis in chapter two.

Figure B-1. SEFS participants picking Montbrook sediment for microfossils, June 10, 2019. Courtesy of Dr. Bruce MacFadden.

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Contributing to the data in this study, SEFs participants gained authentic research skills that prompted the development of K–12 resources influenced by their personal experiences. All too often, academic research reaches a terminus once published upon which is a huge disservice to the K–12 educational community. Thus, forging academic partnerships with the SEFS participants is mutualistic opportunity to incorporate novel research within the K–12 system while promoting scientific literacy and state-wide interest in Montbrook.

Overall, the two analytical techniques from Appendix A and B did not yield the desired results. Nevertheless, these studies and their data are reported here in order to document that they did not work as hoped. Even so, unintended discoveries associated with these analyses were in part fruitful to my research and a result of educational outreach and collaborative opportunities.

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BIOGRAPHICAL SKETCH

For Michael Ziegler, an interest in the natural science began during adolescence while growing up in Georgia and carried through his tenure as an Eagle Scout while in the Boy Scouts of America. Graduating from Grayson High School in 2012, he was accepted to Georgia College and State University where he pursued a bachelor’s degree in environmental science and geology. During his undergraduate career, Ziegler studied taphonomic alteration of Pleistocene megafauna under paleontologist Dr. Alfred Mead. While enrolled at Georgia College, he interned at the Smithsonian Tropical Research Institute as part the University of Florida’s 2015

Panama Canal Project. Following undergraduate, Ziegler earned a fellowship with George

Washington University’s Koobi Fora Field School and conducted paleoanthropology research in

Kenya. Afterwards, he returned to the United States and taught K–12 science in public schools around Atlanta, Georgia. Pursuing his passions for science research and education, he studied at the University of Florida as a graduate research assistant in the Dr. Bruce MacFadden vertebrate paleontology lab. His research focused on paleoecology and sedimentology. Through which, he described the previously unknown ancient environment of a Florida fossil locality, Montbrook.

While enrolled, Michael Ziegler led educational workshops at the Florida Museum as part of the idigfossils, myFossil and Thompson Earth Systems Institute team. Furthermore, these outreach opportunities were implemented globally. Most notably he collaborated with the Institute for the

Study of Mongolian Dinosaurs to co-lead a paleontological professional development in

Mongolia and an inclusivity workshop in Australia at the 2019 Society of Vertebrate

Paleontology conference. Michael Ziegler earned his master’s degree through the Department of

Geology and Florida Museum of Natural History in December 2019 and plans to continue his education.

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