High-Resolution Facies Analysis of Deltaic Subenvironments of the Pennsylvanian Atoka Formation, Arkansas

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1997 High-Resolution Facies Analysis of Deltaic Subenvironments of the Pennsylvanian Atoka Formation, Arkansas. Tanwi Gangopadhyay Louisiana State University and Agricultural & Mechanical College

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Recommended Citation Gangopadhyay, Tanwi, "High-Resolution Facies Analysis of Deltaic Subenvironments of the Pennsylvanian Atoka Formation, Arkansas." (1997). LSU Historical Dissertations and Theses. 6462. https://digitalcommons.lsu.edu/gradschool_disstheses/6462

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. HIGH RESOLUTION FACIES ANALYSIS OF DELTAIC SUBENVIRONMENTS OF THE PENNSYLVANIAN ATOKA FORMATION, ARKANSAS

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

The Department of Geology and Geophysics

by Tanwi Gangopadhyay B.Sc., University of Calcutta, 1988 M.Sc., University of Calcutta, 1991 August, 1997

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UMI 300 North Zeeb Road Ann Arbor, MI 48103

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ACKNOWLEDGMENTS

Sincere thanks goes to Dr. Arnold Bouma for the support and guidance he

provided as the author’s major advisor. The author appreciates his input during field

seasons and suggestions in organizing and documenting the present work. Deep

appreciation also goes to Dr. Barun K. Sen Gupta for his guidance on analysis of

microfossils, for suggestions in preparing this document, and for serving as the minor

advisor to the committee. Dr. James Coleman is especially thanked for his time in

accompanying the author to the field and introducing the basics of delta processes.

Dr. Harry Roberts deserves special praise for providing information on deltaic geology

through his course and on occasions when the author sought specific help. The author

owes much thanks to Dr. John Wrenn for serving in the committee and providing

laboratory facilities and suggestions for palynological analysis. Dr. Ray Ferrell was

very cooperative in providing help and information regarding X-ray and smear slide

analysis of clays. Dr. Randall Hall has provided valuable time to serve as the graduate

school representative in the committee. Heartfelt gratitude goes to Charlie Stone, from

the Arkansas Geological Commission, for introducing the author to the geology of the

Arkoma Basin and for his immense help during field seasons. Rick Young, Wanda

LeBlanc, and post doctoral fellows Gang Lu and Xiaogang Xie deserve special thanks

for their help in the rock preparation, clay analysis, paleomagnetic, and microprobe

laboratories, respectively. Martine Hardy, fellow graduate student and friend, is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thanked for her sincere help in palynological analysis. Mary Lee Eggart and Clifford.

P. Duplechin were extremely helpful in preparing some figures for this document.

The author thanks the Department of Geology and Geophysics, Louisiana State

University for supporting her as a teaching assistant for five years. The Arkansas

Geological Commission and the Gulf Coast Association of Geological Societies

provided funds that helped defray field expenses in 1993 and 1995, respectively.

Pamela Bome, Richard Loftin, Thomas Buerkert, Ricky Boehme, Kush

Tandon, and David Hinds, her fellow graduate students deserve special thanks for their

support and help during trying times. Most importantly, much thanks goes to the

author’s husband Debnath, for providing devoted support and motivation.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ACKNOWLEDGMENTS...... ii

LIST OF TABLES...... vii

LIST OF FIGURES______via

ABSTRACT...... xiii

CHAPTERS 1 GENERAL GEOLOGY OF THE ARKOMA BASIN AND INTRODUCTION TO THIS DISSERTATION...... 1 1.1 Introduction ...... 1 1.2 Objectives and Approaches ...... 2 1.3 Previous Work on Deltas ...... 5 1.4 General Geology and Stratigraphy of the Arkoma Basin ...... 7

2 DEPOSITIONAL FRAMEWORK OF THE PENNSYLVANIAN DELTAIC DEPOSITS IN ARKANSAS...... 16 2.1 Introduction ...... 16 2.2 Deltaic Model of the Atoka Formation versus Alternative Models ...... 16 2.3 Fluvially-Dominated Deltaic Model for the Atoka Formation ...... 20 2.3.1 Basic morphologic and stratigraphic framework of various types of deltas from modem and ancient examples ...... 22 2.3.2 Establishing a Fluvial-Dominated Origin for the Atoka Formation Deltas ...... 31

3 DESCRIPTION AND INTERPRETATION OF DELTAIC SUBENVIRONMENTS...... 35 3.1 Introduction ...... 35 3.2 Methods ...... 35 3.3 Description of Outcrops and Characterization of Deltaic Deposits from Various Subenvironments ...... 36 3.3.1 Freshour Quarry ...... 36 3.3.2 Conway Roadcut ...... 47 3.3.3 Second Morrilton Roadcut (2MOR) ...... 54 3.3.4 Third Morrilton Roadcut (3MOR)...... 59 3.3.5 Fourth Morrilton Roadcut ...... 65 3.3.6 Boston Mountain Roadcut ...... 68 3.3.7 Searcy Quarry...... 74

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 ADDITIONAL APPROACHES IN THE CHARACTERIZATION OF DELTAIC DEPOSITS FROM VARIOUS SUBENVIRONMENTS...... 79 4.1 Trace Fossils ...... 79 4.1.1 Introduction ...... 79 4.1.2 Identification ...... 80 4.1.3 Interpretation ...... 88 4.2 Palynology ...... 91 4.2.1 Introduction ...... 91 4.2.2 Methods ...... 93 4.2.3 Results...... 96 4.2.4 Discussion ...... 98 4.2.5 Conclusions ...... 104 4.3 Clay Diagenesis ...... 105 4.3.1 Introduction ...... 105 4.3.2 Methods ...... 106 4.3.3 Results and Identification ...... 109 4.3.4 Interpretation ...... 114 4.3.5 Diagenetic Implications ...... 118 4.4 Provenance and Tectono-Sedimentary Interpretations Based on Petrographic Analyses ...... 120 4.4.1 Inroduction ...... 120 4.4.2 Methods ...... 120 4.4.3 Results...... 121 4.4.4 Interpretation ...... 124 4.4.5 Late Paleozoic Tectonic Evolution of the Basin ...... 127 4.5 Systematic Variations in Grain-Orientation Related to Depositional Processes in Deltaic Subenvironments, Using Anisotropy of Isothermal Remanent Magnetization ...... 130 4.5.1 Introduction ...... 130 4.5.2 Methods ...... 132 4.5.3 Results...... 134 4.5.4 Discussion ...... 136 4.5.5 Conclusions ...... 141

5 STACKING PATTERNS IN THE ATOKA FORMATION WITHIN A SEQUENCE STRATIGRAPHIC FRAMEWORK...... 143 5.1 Introduction ...... 143 5.2 Dataset...... 143 5.3 Regional Stratigraphy in a Global Cyclostratigraphic Perspective ...... 144 5.4 Discussion ...... 148

6 CONCLUSIONS...... 161

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES...... 173

APPENDIX A PRESENTATION OF MEASURED PROFILES...... 190

B X-RAY DIFFRACTION DATA...... 199

C PRESENTATION OF ANISOTROPY OF ISOTHERMAL REMANENT MAGNETIZATION DATA...... 208

D PLATE SHOWING PHOTOGRAPHS OF PHYTOCLASTS...... 210

VITA ...... 212

vi i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

Table 2.1 Variability in sedimentary characteristics from river-dominated, tide- dominated, and wave-dominated deltas based on information from recent deltas and deltas from the rock record ...... 34

Table 4.1 Identified trace fossils, their probable trace makers with behavioral traits, and association with specific environments as documented in the present study...... 87

Table 4.2 Key criteria of phytoclast degradation stage determination ...... 95

Table 4.3 Categories of kerogen particles identified ...... 96

Table 4.4 Result of the point counting shown in percentages. Am. Str. and Am.-Nstr represent Amorphous Structured and Amorphous Non-structured Phytoclasts, respectively ...... 97

Table 4.6.Composition of clay parameters used for NewMod simulation ...... 107

Table 4.7. Calculated percentages of various mixed layer clay assemblages from subenvironments as determined by the NewMod simulation study ...... 115

Table 4.8. Results of traditional point counting. Abbreviations Ch, PQ, Cm, Mat, M, and Chi. represent Chert, Poly quartz, Cement, Matrix, Mica, and Chlorite, respectively ...... 122

Table 4.9. Results of the Gazzi-Dickinson point counting method in percentages.... 122

Table 6.1 Sedimentary characteristics from various deltaic subenvironments observed during the present study. The outcrop names and their depositional affinities are tabulated versus different sedimentary characters ...... 171

vii f

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

Figure 1.1 Map of Arkansas showing the position of the Arkoma Basin. Inset map shows the study area and the locations of the different outcrops ...... 3

Figure 1.2 Hypothetical cross-sections depicting the tectonic evolution of the southern margin of North America ...... 8

Figure 1.3 Synthesized paleogeographic maps depicting the evolution of the southern margin of North America ...... 10

Figue 1.4 Correlation chart showing the stratigraphy of the southwestern Ozark Region, Arkoma Basin, and Frontal and Central Ouachitas during the late Paleozoic ...... 12

Figure 1.5 Schematic representation of depositional environments of Atoka Formation in the Arkoma Basin ...... 13

Figure 1.6 Generalized stratigraphic column of the area depicting the sedimentation rates corresponding to the Stanley, Jackfork, and Atoka formations ...... 15

Figure 2.1 Characteristic geologic features of modem transgressive and regressive barrier island shorelines ...... 18

Figure 2.2 Schematic diagram illustrating the threefold division of deltas into fluvial-dominated, wave-dominated and tide-dominated types ...... 21

Figure 3.1 Outcrop photograph of the east wall of Freshour Quarry illustrating the thickening upward nature of beds...... 37

Figure 3.2 The outlay of Freshour Quarry illustrating the east, west, north, and south faces ...... 37

Figure 3.3 Schematic diagram representing the salient features from measured sections from the west and east faces...... 38

Figure 3.4 A mud-lump, visible as disharmonically folded shales disturbing the stratification of the overlying delta front deposits on the north-face ...... 41

Figure 3.5 Gamma-ray profile from the west face of the quarry along measured section WFH demonstrating an ideal coarsening-upward package ...... 42

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.6 Depositional model of the deltaic facies at Freshour Quarry ...... 43

Figure 3.7 The gradual transition between delta front deposits (DF1) and distributary mouth bar deposits (DMB1) showing a thickening-upward trend ...... 45

Figure 3.8 The sharp boundary separating distributary mouth bar deposits of the first cycle (DMB1) from delta front deposits of the second cycle (DF2) ...... 46

Figure 3.9 Distributary mouth bar sandstones of the second cycle (DMB2) overlain by black shales of interdistributary bay fill origin (IDBF) ...... 48

Figure 3.10 Hummocky cross stratified sandstone from the Conway Road cut section ...... 49

Figure 3.11 Gamma-ray pattern from the Conway Road cut section representing a coarsening-upward trend ...... 52

Figure 3.12 A model demonstrating the depositional setting of the Conway Road cut...... 54

Figure 3.13 An outcrop photograph showing the sharp based sandstone resting on the thinly bedded black shales ...... 55

Figure 3.14 A representative photograph of the lower interbedded shale-siltstone interval with persistent sand laminae and lenticular bedding...... 56

Figure 3.15 The gamma-ray pattern measured from the second Morrilton Roadcut ...... 58

Figure 3.16 The third Morrilton Road cut (3MOR) represented by shales below (far right) and massive sandstones above ...... 59

Figure 3.17 Schematic representation of the intervals in profile 3MOR ...... 60

Figure 3.18 The measured gamma-ray profile from Road cut 3MOR, representing an overall coarsening-upward package ...... 63

Figure 3.19 Photograph showing Unit 1 sandstone overlying the lower shale interval with a sharp erosive base ...... 66

Figure 3.20 The gamma-ray pattern from the fourth Morrilton Road cut ...... 69

Figure 3.21 Characteristic heterolithic bedding from Unit 1 represented by interbeds of shale-siltstone and sandstones ...... 70

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Figure 3.22 The characteristic marker bed represented by wavy based sandstone that is heavily bored ...... 71

Figure 3.23 Large-scale cross-bedding and bedform geometry from Unit 4 at 32m in the section ...... 72

Figure 3.24 The gamma-ray profile for the measured section at Boston Mountain represents a coarsening-upward trend ...... 73

Figure 3.25 The conspicuous channel cut high up on the south wall of the quarry 77

Figure 4.1 Circular, branching and almond shaped epichnial trace fossils (marked by arrow heads) ...... 81

Figure 4.2 Sketches showing the most common horizontal traces on the delta front bedding surface namely B l, B2, B3, B4, B5, B7, and B 8 ...... 82

Figure 4.3 Gyrochorte as seen on delta front bedding plane from the Freshour Quarry ...... 83

Figure 4.4 Conostichus as seen from the 4MOR section at Morrilton, marked by arrows...... 86

Figure 4.5 Scalarituba from the upper bedding surface of a sandstone block (marked by arrows) at Searcy Quarry ...... 87

Figure 4.6 Distribution of trace fossils in Fentress-Rockcastle sequence and in rocks of deltaic origin described in the literature ...... 89

Figure 4.7 Distribution of different phytoclasts and spores from the present study...... 98

Figure 4.8 X-ray powder diffraction pattern of sample TG3MOR smear preparations ...... 110

Figure 4.9 Backscattered electron micrograph of sample 4MOR5 and X-ray spectra of selected area ...... 112

Figure 4.10 Comparison of XRD pattern produced by ethylene glycol saturated sample TG4MR3AE ...... 116

Figure 4.11 Proposed model for diagenesis used in interpretations in the present study. Original model derived from Wilcox sandstone cements ...... 119

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.12 A) QFL, B) QmFLt, Q QpLvLs, D) QmPK diagrams showing compositional variations in Atokan sandstones ...... 123

Figure 4.13 QFR diagram showing compositional variations of Atokan sandstones ...... 124

Fgure 4.14 Detrital modes (QFL) of the Stanley, the Jackfork, and the Atoka formations (present study), Q=Qm+Qp; F=P+K; L=Lv+Ls+Lm ...... 127

Figure 4.15 Paleogeographic map of the Ouachita Trough imposed on the present day state boundaries ...... 129

Figure 4.16 Magnetic anisotropy data represented in the form of anisotropy ellipsoid...... 133

Figure 4.17 Equal area stereonet plots of sample maximum (Kmax. on primitive circle) . and sample minimum (K^m cluster near center) axes of anisotropy ellipsoids ...... 135

Figure 4.18 Percent anisotropies from the three depositional settings: (a) distributary mouth bar, (b) delta front, and (c) crevasse splay ...... 137

Figure 4.19 Representative isothermal remanent magnetization acquisition curve. The pattern reflects a mixture of hematite and magnetite as the magnetic carrier phases ...... 137

Figure 5.1 Subsidence and sedimentation rate curves for the Paleozoic rocks of Arkansas ...... 146

Figure 5.2 The Paleozoic part of the Exxon Sea-level Curve ...... 147

Figure 5.3 “Slug” diagram depicting the geometries of various depositional systems organized into sytems tracts related to a complete sea-level cycle .. 147

Figure 5.4 Coastal onlap curve for the Pennsylvanian Morrowan, Atokan, and Desmoinesian epochs ...... 149

Figure 5.5 Cross sectional representation of depositional units from the Morrowan and Atokan time ...... 151

Figure 5.6 Morrowan paleogeograpnic map of the Ouachita Basin ...... 152

Figure 5.7 Well-log data from the subsurface of the Atoka Formation. Stacking patterns are inferred on the basis of the established correlations ...... 154

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.8 Generalized stratigraphic column for the Ouachita Basin, with relative sea- level, precipitation, and tectonic curves ...... 155

Figure 5.9 Stratigraphic column showing the coal-bearing formations (Upper Atoka, McAlester and Savanna formations) ...... 159

Figure 5.10 Summary diagram based on the results from the present study- depicting the depositional stacking pattern of Pennsylvanian deposits ...... 160

xii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

The present research was aimed at comparison and distinction between various

deltaic subenvironments from Pennsylvanian deposits of the Arkoma Basin, Arkansas.

Detailed facies analysis of deltaic deposits reveal significant variation in sedimentary

characteristics between different subenvironments. A multi-pronged approach,

including documentation of measured sections and gamma ray profiles in the field and

subsequent laboratory analysis of samples, was undertaken.

The Atoka Formation deltaic deposits have been established primarily as

representing fluvially-dominated deltas. Interdistributary bay fill deposits are splintery

shales with abundant siderite nodules. Crevasse splay sandstone units are ripple cross

laminated and coarsen and thicken upward to a maximum of 60cm thick beds.

Distributary channel deposits have sharp erosive bases and are composed of either an

active or a passive fill. Active fill is characterized by large scale cross-bedding while

passive fill is predominantly parallely laminated. Distributary mouth bar deposits show

coarsening and thickening-upward sequences with tabular beds grading upward into

more lenticular beds with occasional well developed cross-bedding. Delta front

deposits are heterolithic with ripple cross-laminations and intense bioturbation.

Prodelta deposits are predominantly shales with laterally persistent silty laminations

that occasionally form starved ripples.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Zoophycos ichnofacies are well represented in distributary mouth bar and delta

front deposits. Skolithos-Cruziana ichnofacies are represented in delta front and

interdistributary bayfill deposits, the latter with the unique presence of Conostichus.

Amorphous non-structured dominated kerogen assemblages are present in all

the subenvironments suggesting long transport possibly by fluvial effluence.

Lycospora spp. dominated palynomorphs suggest coal-forming, swampy vegetation in

the lower deltaic plain.

Anisotropy of isothermal remanent magnetization (IRMA) analysis was

conducted which revealed systematic variations in magnetic grain long axis (K^ax)

alignment Distributary mouth bar deposits show flow normal, while delta front and

crevasse splay deposits show flow parallel Kn«r orientations. Distributary mouth bar

deposits have 19% and delta front and crevasse splay deposits have 8% and 13%

anisotropies, respectively, associated with sedimentary fabrics.

The Atoka Formation deltaic deposits have been placed within a sequence

stratigraphic framework. The composite section of the Atoka Formation reveals

retrogradational, progradational, and aggradational parasequence stacking patterns

deposited in a transgressive to highstand systems tract

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1

GENERAL GEOLOGY OF THE ARKOMA BASIN AND INTRODUCTION TO THIS DISSERTATION

1.1 Introduction

Fluvially-dominated, mud-rich deltas are characterized by a diverse set of

depositional sunenvironments. Determination of a full suite of criteria that

characterizes each of them is the key to the understanding of the spatial distribution

and temporal stacking of the subenvironments. Research along these lines, based on

observations in the rock-record, has been limited and the number of approaches that

have been taken to determine subenvironmental recognition criteria is unsatisfactory.

Regarding ancient deltas, the available publications (e.g., LeBlanc, 1975) are normally

limited to the local geology and stratigraphy of the delta concerned. The published

literature on the ancient deltas deals with a few subenvironments, which covers only a

small subset of the total spectrum of subenvironments seen in fluvially-dominated,

mud-rich deltas. Although lithologic descriptions are common, detailed integrated

analyses of sedimentary structures, grain-size variations, grain-fabric, bioturbation, etc.

for different subenvironments are rare to non-existent (Woodrow and Sevon, 1985;

Pulham, 1989; Harris, 1989). On the other hand, detailed facies analyses are available

for most of the recent deltas from all over the world. For example, an enormous

amount of literature is present on the modem Mississippi Delta in this context (Fisk,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2

1955; Coleman and Gagliano, 1964; Coleman and Prior, 1981; Coleman and Roberts,

1991).

The present study is aimed at high resolution facies analysis of the various

subenvironments from the paralic lower deltaic plain to subaqueous distal parts of a

late Paleozoic delta complex. The Pennsylvanian deltaic deposits of the Morrowan and

the Atokan epochs in Arkansas provide an ideal setting to attempt such a study.

Mississippian and Pennsylvanian strata form a nearly continuous belt of rocks

deposited in a late Paleozoic foreland basin, 1980km long and are exposed along the

Appalachian Valley and Ridge Province, the Ouachita Mountains, the Arkoma Basin,

the Ozarks region, and the Marathon Uplift (Link and Roberts, 1986). This study was

conducted on part of Morrowan and lower, middle, and upper Atokan deltaic deposits.

The Freshour Quarry north of Jacksonville, three roadcuts south of Morrilton, roadcuts

at Boston Mountain and Conway, and the Searcy Quarry show excellent outcrops that

enable detailed deltaic facies analysis (Fig. 1.1).

1.2 Objectives and Approaches

The objectives and approaches of the study are outlined in this section.

1. The primary objective was to generate criteria for distinguishing between

deposits formed in various deltaic subenvironments from the rock-record. A multi

pronged approach was used to attain this goal. The Pennsylvanian deltaic deposits

from the Morrowan and the Atokan epochs in Arkansas provide an ideal setting to

attempt such an objective. Similar studies have been done from the recent deltas but

detailed analysis of deltaic subenvironments from the rock-record has not been done in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3

STUDY

M^rTfcl

Figure 1.1 Map of Arkansas showing the position of the Arkoma Basin. Inset map shows the study area and the locations of the different outcrops (modified after Gangopadhyay and Heydari, 1995).

sufficient detail. The multi-pronged approach used in this study includes a variety of

field and laboratory techniques. Specific sedimentary features such as, grain size,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4

sedimentary structures, bedding geometries, gamma-ray profiles, vertical trends, and

trace fossils were recorded in the Held. These were integrated to develop criteria for

recognition of various deltaic subenvironments. On the basis of the primary

observations, standard names were assigned to the deposits, such as distributary

channel, distributary mouth bar, delta front/distal bar, prodelta, bay fill etc., in

accordance with observations published from Holocene deltas. Subsequently,

laboratory techniques were applied, on samples collected from the field, involving

palynology, clay mineralogy, petrography, and rock-magnetic grain-fabric

determination to substantiate the Held-based inferences. Some of these laboratory

techniques were also applied to develop an understanding of the roles of physical,

chemical, and biological processes on sediment deposition in various deltaic

subenvironments. The integration of results from such a variety of approaches has not

been done before in the facies analysis of deltaic deposits from the rock-record.

2. The Pennsylvanian middle and upper Atoka Formation deposits in the

Arkoma Basin have long been documented as deltaic deposits (Zachry, 1983;

O’Donnell, 1983). But a detailed analysis of their sedimentary characteristics from the

point of view of establishing whether they represent deltas with fluvial, tidal, or wave

dominance, has not been done. The present study evaluates the importance of fluvial-,

wave- and tidal-dominance or influence in the Atokan deltas, based on comparing the

present field-observations and conclusions with other deltaic deposits from the recent

and ancient rock-record.

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3. The Atoka Formation deltaic deposits have been previously studied

primarily from a lithostratigraphic perspective. The present study places the Atoka

Formation in an allostratigraphic framework under the regional effects of relative sea-

level oscillations, tectonics and sediment supply. Well-logs from the Arkoma Basin

have been used to attain this goal. The study of the Atoka Formation sheds light on the

preservation of deltaic facies and their temporal and spatial stacking pattern. The

deltas represented in the Atoka Formation are very similar in scale and setting to the

modem day Mississippi Delta. The observations and conclusions from the present

study are hence applicable to the facies analysis of late Cenozoic deltaic deposits,

especially from the Gulf of Mexico province.

13 Previous Work on Deltas

A large volume of work is available on deltas from the literature. Examples can

be found in case studies of modem and ancient deltas that have been published in

SEPM Special Publication 15 (1970) as well as three compilations of deltaic papers by

Whateley and Pickering (1989), Shirley and Ragsdale (1966) and by Broussard (1975).

The last two publications mainly concentrate on modem deltas around the world.

Limited information is available concerning ancient deltas, most of which elaborate on

the the local geology and stratigraphy involving the delta concerned. Although

lithologic descriptions are common, detailed integrated analyses of sedimentary

structures, grain-size variations, vertical trends, bioturbation, etc. focusing on different

subenvironments are rare. Few well documented facies analysis studies of ancient

deltas, such as those of Ferm (1970), Balsley (1980), Wright (1986), and Pulham

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6

(1989), are available. On the other hand, detailed facies analyses of recent deltas

around the world have been done. An enormous amount of literature is present on the

modem Mississippi Delta in this context (Fisk, 1955; Coleman and Gagliano, 1964;

Coleman and Prior, 1981; Coleman and Roberts, 1991). Pioneering works were carried

out by Fisk (1955), working with the sedimentary framework and the factors

controlling the growth of the modem birdfoot delta and late Quaternary deposition; by

Scruton (1960) on the order and pattern of delta building and characteristic

stratigraphic sequences, formed as a delta cycle; by Coleman and Gagliano (1964)

emphasizing cyclic sedimentation in the Mississippi River deltaic plain evident from

vertical and lateral distribution of environmentally controlled facies and delta growth;

by Kolb and Lopik (1966) on depositional environments of the Mississippi River

deltaic plain; by Coleman and Wright (1975) interpreting depositional facies in deltaic

sediments resulting from interaction of the dynamic processes that modify and disperse

transported riverine sediments; by Coleman and Prior (1981) also on the deltaic

environments of deposition, various factors and resulting sedimentary patterns; by Fisk

and McFarlan (1955) on Late Quaternary deltaic deposits of the Mississippi River, by

Welder (1959) on processes of deltaic sedimentation in lower Mississippi River; by

Coleman (1988) on dynamic changes and processes in the Mississippi RiverDelta;

and by Coleman and Roberts (1991) mainly on Quaternary sedimentation and extreme

complexities in deltaic deposition associated with the late Wisconsinan sea-level

changes and several others.

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1.4 General Geology and Stratigraphy of the Arkoma Basin

The evolution of Paleozoic stratigraphy in the central part of Arkansas is a

product of opening and closing of a Paleozoic ocean basin. The Ozarks, Arkoma

Basin, and Ouachita Mountains in Arkansas are genetically related late Paleozoic

cratonic dome (Ozarks), a foreland basin (Arkoma Basin), and a thrust-belt system

(Ouachita Mountains), respectively (Fig. 1.1). A model of the evolution of the

Arkoma-Ouachita system is presented below. For convenience of presentation of this

tectonic model, Houseknecht and Kacena (1983) divided the evolutionary history of

the region into five “time-slice” segments, each of which is represented by a schematic

cross section in Figure 1.2. Accumulation of Paleozoic sediments in the Arkoma-

Ouachita system originated during latest Precambrian and earliest Paleozoic when

continental rifting (Fig. 1.2A) resulted in the opening of a proto-Atlantic ocean basin

(Fig. 1.2B). For most of the Paleozoic, carbonates and siliciclastics were deposited

within and along the margins of this ‘Ouachita Ocean, Trough or Basin’. The dome

and the foreland basin areas were broad continental shelves separated by a narrow

slope from this deep basin to the south. The ocean basin began to close forming a

southward subducting complex during early Mississippian to early Atokan time (Fig.

1.2C). Subduction of this newly formed oceanic crust beneath the southern continent

Llanoria resulted in the formation of an accretionary wedge to the south accompanied

by sediment deposition in a foreland basin over a downwarped Cambrian to

Mississippian carbonate shelf to the north. The deep marine Ouachita basin was

closing tectonically by northward advancing thrust sheets of the accretionary wedge

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South North

B ^ - l m m

A: Late Pracambrian • Early Palaozoic B: Late Cambrian - Early Mississippian C: Earty Missisaippian • Earty Atokan 0 : Earty Atokan - Middle Atokan E: Late Atokan

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Exotic TnnaHionil d u al

J OuacNtcAccratfonaryWMgo

Figure 1.2 Hypothetical cross-sections depicting the tectonic evolution of the southern margin of North America (modified after Houseknecht, 1986).

during early to middle Atokan time (Fig. 1.2D). By late Atokan time the Ouachita fold

belt and the Arkoma foreland basin were established (Fig. 1.2E).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9

In east central Arkansas 300 to 10,000m of Pennsylvanian strata were

accumulated in parts of the Ozarks, Arkoma and Ouachita Basin (Link and Roberts,

1986). In the Ozarks and the Arkoma Basin, these rocks overlie aboutl800m of shelfal

lower Cambrian to Mississippian rocks that in turn rest on Precambrian granitic

basement The Pennsylvanian section is 300m thick in the northern Arkoma Basin and

ranges from Morrowan to Desmoinesian (lower to middle Pennsylvanian) in age. The

equivalent section in the Ouachita Basin to the south (Ouachita Facies) is over

10,000m thick and overlies about 6500m of Ordovician to Mississippian basinal strata.

There are marked differences in the lithologic character, thickness, and depositional

pattern of age equivalent units between these two areas.

An appreciation of the influence of tectonics on sedimentation during the late

Paleozoic is the key to the understanding of the regional and local facies distribution.

After synthesizing the work of many researchers Houseknecht and Kacena (1983)

created four paleogeographic maps that depict the evolution of southern margin of

North America during the Chesterian, Morrowan, Atokan and Desmoinesian. These

maps are presented in Figure 1.3. By Chesterian time (late Mississippian), most of the

structural elements within and associated with the Arkoma-Ouachita system were

formed. Collision of the two continental masses caused uplift of the Ouachita belt

along its juncture with the Appalachians. Alternating carbonate and siliciclastic

deposition continued to characterize deposition throughout the northern part of the

remnant ocean basin, including the Illinois Basin and the northern shelf area, or

Arkoma Basin (Fig. 1.3 A). Vast quantities of clastic sediment were shed through

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. !

alluvial systems into the Black Warrior basin, a developing foreland basin. The

Ouachita trough was a major sediment sink, where the thick pile of Stanley Formation

turbidites accumulated. Emergent structures include the Ozark Dome, Nashville

Dome, and the Appalachians to the east The Wichita Uplift and the Pascola arch did

Figure 1.3 Synthesized paleogeographic maps depicting the evolution of the southern margin of North America during the Chesterian, Morrowan, Atokan, and Desmoinesian epochs (modified after, Houseknecht and Kacena, 1983).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. not become active until Morrowan and Desmoinesian times, respectively. As a result

of continued convergence between North America and ‘Llanoria’ during the

Morrowan, the remnant ocean basin became more constricted and is depicted as a

predominantly shallow water linear seaway (Fig. 1.3B). Orogenic uplifts in the south

(Ouachita belt) and east (Appalachians) resulted in increased fluvial-deltaic

sedimentation in the Black Warrior and Illinois Basins. Large volumes of terrigenous

sand and shale were supplied to the northern part of the basin from the Ozark Dome.

The early-middle Atoka period is the time of imminent collision between North

America and the northward advancing subduction complex (Fig. 1.3Q. Curvilinear

flexures and associated normal (growth) faults in the south and west regions of the

Arkoma Basin developed in response to both subduction and loading. Fluvial-deltaic

and shallow-marine deposition continued around the basin. The growth faulting in the

Arkoma Basin resulted in a change from a more classic, Atlantic type, passive margin

to an unstable, Gulf Coast-type margin during this time of very rapid sediment

accumulation along the southern edge of the North American continent. By late

Atokan time, collision was complete and the most severe structural deformation had

ended (Fig. 1.3D). The overall structural configuration is about similar as it is today.

Orogenic highlands (Ouachitas) extended from east Texas through Oklahoma-

Arkansas-Mississippi and northeastward to the Appalachians.

The correlation chart in Figure 1.4 summarizes stratigraphic nomenclature for

the Carboniferous strata of the region. The youngest Paleozoic strata that crop out in

Arkansas are early to middle Pennsylvanian (Morrowan, Atokan, and Desmoinesian)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12

in age. The present study examines middle to upper Atokan and upper Morrowan

rocks. On the Ozark shelf of northeast Oklahoma, the Morrowan units are thin,

dominantly carbonate or mixed carbonate/clastic sequences of the Sausbee and

CENTRAL FRONTAL SOUTHWESTERN OUACHITAS OUACHITAS OZARK REGION co OKLAHOMA OKLAHOMA OKLAHOMA ------» I I . -M co hm njb n omm* n n o m m omm ARKANSAS - 1947______1947 SuBwrtindlM«gy.1979

ATOKA FORMATION FORMATION nmr Member rAPANUCKA JOHNS Sfc^FM. VALLEY WAPANUCKA SHALE FORMATION ■art lAt UNION VALLEY JACKFORK •SPRINGER’ - CROMWELL 1 1 GROUP FORMATION PENN.CANEY V.

mt.

W0VACUL1TE BOONE GROUP

Figue 1.4 Correlation chart showing the stratigraphy of the southwestern Ozark Region, Arkoma Basin, and Frontal and Central Ouachitas during the late Paleozoic (modified after Sutherland and Manger, 1979 and Sutherland, 1988).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. McCulIy formations (Fig. 1.4). They are unconformably overlain by fluvio-deltaic

elastics of the Atoka Formation. The Oklahoma Morrowan units grade laterally

eastward into the Hale and Bloyd formations in northwest Arkansas. The Morrowan

strata on the northern shelf range in thickness from less than 66m in northeast

Oklahoma to over 650m in east central Arkansas. Basinward to the south, these units

are known as the Jackfork Formation and Johns Valley Shale. The lower Atoka

Formation overlies Morrowan units in Oklahoma and northwest Arkansas.

Within the Arkoma Basin, the Atokan strata recorded several different depositional

environments ranging from a deep water submarine fan facies Gower Atokas) in the

south to a shallow water deltaic facies (middle and upper Atokas) to the north (Fig.

1.5). The lower Atoka consists of thin fluvio-deltaic facies in the north and thick

turbidite facies in the south. By upper Atoka time, fluvial-deltaic deposition dominated

Figure 1.5 Schematic representation of depositional environments of Atoka Formation in the Arkoma Basin (modified after Houseknecht, 1986).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14

all areas of sediment accumulation. This study is concentrated only on the

deltaic facies of upper Morrowan and middle and upper Atokan Formation in the

Arkoma Basin.

The term Arkoma Basin was introduced in the late fifties for the asymmetrical

structural depression between the southern flank of the Ozark uplift (the Boston

Mountains) and the thrust front of the Ouachita Mountains (Cline, 1960). The Arkoma

Basin by virtue of being a foreland basin, is a hybrid feature in sedimentological and

structural characteristics. It displays a continental basement overlain by an incomplete

Cambrian to early Mississippian platformal sequence without any major thickening

being evident on seismic sections southwards toward the Ouachita thrust belt (Arbenz,

1989). However, major post-basal-Atokan thickening of the Atoka southwards across

the entire Arkoma Basin is well documented (Stone et al., 1981). This thickening is

accomplished by south tilting, and growth faults of lower and middle Atokan age,

dominantly down to the south. The position of maximum depocenter of the sediments

can not be ascertained, but that the subsidence exceeded sediment supply in the deeper

parts of the Arkoma Basin is witnessed by the deep water shale and turbidite flysch

facies of the massive lower to middle part of the formation. The rate of deposition was

extremely high. Evidence clearly show a northward shift of the depositional axis

during the middle Pennsylvanian when the major infilling of the basin took place from

east and north by deltaic progradation (Morris, 1974 a and b).

The general stratigraphy of the study area is shown in Figure 1.6. The

Mississippian Stanley Shale is overlain by Pennsylvanian Jackfork Formation and

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OZARKS. T im e Ouachita Mts. o— ARK. VALLEY — o

— 3,000 3,000'

6.000-j — 6,000 Meters

Joftn»V»««v —9,000 Jackfortc

Sediment Stanley —12,000 Accumulation Rate

—15,000 Meters

Figure 1.6 Generalized stratigraphic column of the area depicting the sedimentation rates corresponding to the Stanley, Jackfork, and Atoka formations.

Johns Valley Shale. These are unconformably overlain by fluvio-deltaic elastics of the

Atoka Formation. Relative thickness and very low sediment accumulation rate suggest

that a period of starved basin sedimentation existed from the late Ordovician to early

Mississippian followed by a period of rapid sedimentation during the Pennsylvanian

(Fig. 1.6). Houseknecht (1986) used thicknesses, after correction for compaction, and

estimated sediment accumulation rate at I06m/my for the Stanley Formation,

420m/my for the Jackfork Formation, and lOOOm/my for the Atoka Formation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2

DEPOSITIONAL FRAMEWORK OF THE PENNSYLVANIAN DELTAIC DEPOSITS IN ARKANSAS

2.1 Introduction

This chapter will focus on establishing the Atoka Formation deposits primarily

as deltaic in origin. A deltaic versus barrier island origin of the sand packages is going

to be discussed in particular. A discussion will then be presented on establishing the

Atoka Formation deltas as primarily dominated by fluvial processes rather than wave

and tidal dominance based on the present and previous works using both modem and

ancient examples. The Atoka Formation deltaic deposits will then be placed

qualitatively within the existing tripartite delta classification scheme of Galloway

(1975). This chapter provides a general framework within which the specific details of

the subenvironments recognized during this study (discussed in Chapter 3) in the

fluvially-dominated Atoka Formation deltas can be placed.

2.2 Deltaic Model of the Atoka Formation versus Alternative Models

The outcrops studied from Arkansas cover units corresponding to the

Morrowan (Searcy Quarry), lower Atoka (Boston Mountain), middle Atoka (Freshour

Quarry and Conway road cut section), and upper Atoka (three roadcuts south of

Morrilton, named as 2MOR, 3MOR, and 4MOR in southward direction) formations.

Most of these rock sequences show a gradual thickening-upward trend. They comprise

progradational patterns with a relative increase of coarse elastics higher up in the

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section. This also is demonstrated by funnel-shaped gamma-ray patterns measured

from outcrops, using a hand-held scintillometer. Fore-stepping parasequence sets are

observed from subsurface well-logs (Haley, 1982). Plant debris and organics are

commonly present which indicate derivation of sediments from a terrestrial source

possibly transported to the ancient shoreline by fluvial effluence. Petrographic studies

support a northern cratonic provenance for the sediments. Paleocurrent directions

measured from primary sedimentary structures support a general southward-directed

paleoflow associated with the sourcing channels. Zachry (1983) showed coarsening-

upward log motifs from gamma ray and spontaneous potential logs from various

successions in the middle and upper Atoka Formation. The oil and gas charts by Haley

(1982) also show small, but distinct coarsening-upward packages present in the upper

part of the lower, middle, and upper Atoka. Lenticular sand body geometry observed

from the oil and gas charts reflects a point sourced depositional style with the sand

package thickening away from the shoreline.

Two kinds of barrier islands, prevalent in the stratigraphic record, are

transgressive and regressive types. Most modem barrier islands are the transgressive

type (best demonstrated by barrier islands around the modem Mississippi Delta)

associated with the Holocene sea-level rise. They have a sheet-like geometry, low

preservation potential and are rare in the ancient rock record (Moslow, 1984).

Transgressive barrier islands are characterized by thin sequences of bioturbated

lagoonal muds overlain by horizontally-bedded washover-foreshore sands of storm

origin (Fig. 2.1a). The common occurrence of barrier overwash sands overlying

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backbarrier silty-sands and salt marsh peats (e.g., Cape Lookout, North Carolina)

indicate landward migration during evolution of the islands. Clean, well-sorted,

parallel-laminated sand, overlying organic-rich, fine-grained lagoonal sediments,

makes them unique (Moslow, 1984). Seaward prograding regressive barrier island

complexes, on the other hand, are less common in a modem setting. They are well

documented from the rock record, highly depositional with lenticular geometry, and

have a high preservation potential (Moslow, 1984). These are characterized by thick

coarsening-upward sequences of burrowed to cross-bedded fine sand (Fig. 2.1b).

Representative depositional environments include beach ridge, backshore, foreshore

TRANSCRESSIVEl REGRESSIVE

Rs>Rd Rsl Rs

Figure 2.1 Characteristic geologic features of modem transgressive and regressive barrier island shorelines (after Moslow, 1984).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19

and transition zone. The majority of barrier island complexes (both types) are

composed of relatively thick (8-10m) prograding shoreface sands which interfinger

with offshore sediments (Moslow, 1984). The migration of tidal inlets along the

shoreline results in significant reworking of both types of barrier islands (transgressive

and regressive), depositing thick sequences of coarse cross-bedded sand. Based on the

preservation potential of modern barrier islands, it can be extrapolated that the

deposits studied herein would have been preserved as a shoal which is the ultimate fate

of the barrier island series for the modem day Mississippi River Delta (Penland et al.,

1988). If the Atoka sediments were of barrier origin, one would expect evidence of

prolific biogenic activity, marine fauna, and only a meager amount of organics in the

sediments, all of which were rarely recorded during the present study. If the overlying

sandstone represents a barrier island complex, the shale underlying the major sand

packages would be a marine shale which is not supported by the fauna, flora, and

sedimentologic criteria. Barrier island deposits from both the present and rock-record

are restricted to 8-12m thickness (Moslow, 1984). However, the depositional cycles

observed during the present study commonly exceed 20m in thickness which is more

suggestive of progradational deltaic deposits. The base of barrier islands (both

transgressive and regressive types) does not show any consistent pattern (sharp or

gradational), whereas the bases of the sand bodies from the present study are

consistently gradational. Sedimentologic criteria, used to preclude the barrier island

origin of the Atokan Formation deposits are 1) absence of lagoonal and marshy rooted

deposits , 2) presence of coal in associated deposits of the middle and upper Atoka,

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3) absence of shells, 4) lack of wave ripples and hummocky cross stratification (HCS),

and 5) absence of glauconite and calcitic cement in the sandstones.

2 3 Fluvially-Dominated Deltaic Model for the Atoka Formation

A deltaic system is a three-dimensional, rock-stratigraphic unit composed of

many delta lobes, deposited as a part of a major cycle of terrigenous sediment influx.

Delta morphology and stratigraphy are primarily the product of an interplay between

fluvial sediment input and reworking of these sediments by marine processes. The

sources of marine energy are manifold, including ocean and wind generated currents,

density currents, tidal currents, storm and wave surges. Deltaic deposits however, are

primarily modified by tidal currents and wave energy (Galloway, 1975). Marine deltas

can thus be characterized in terms of three end member types - 1) flu vial-dominated,

2) wave-dominated, and 3) tide-dominated deltas. Figure 2.2 illustrates this tripartite

subdivision of deltas into fluvial-, wave-, and tide-dominated end members. Modem

fluvial-dominated deltas include the bird-foot lobe of the Holocene Mississippi Delta.

The Sao Francisco and Ganges-Brahmaputra deltas exemplify typical wave- and tide-

dominated deltas, respectively. Holocene marine deltas exhibit a continuous spectrum

of morphologic types. In an early stage, a delta may consist of a single depositionally

active lobe; however, as it progrades, the site of the maximum deposition shifts,

producing a more complex three-dimensional stratigraphic entity (Fisher et al., 1969).

Moreover, deltaic systems commonly show long-term evolutionary trends as

progradation extends into increasing water depth or as conditions change from net

regressive to net transgressive (Galloway, 1975). Pennsylvanian deltas of north-central

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rwfvr

FUMAL ,aoumed

T yam

/ M n — JOB \ / KMC i i A / ammo

8 y a W«ME O ER G Y FLUX TOE ENERGY FLUX /A?

OtHmcma Figure 2.2 Schematic diagram illustrating the threefold division of deltas into fluvial- dominated, wave-dominated and tide-dominated types. The relative importance of sediment input, wave energy flux, and tidal energy determine the morphology and internal stratigraphy of the delta (after Galloway, 1975).

Texas evolved from fluvial-dominated elongate to wave-dominated lobate to cuspate

as progradation continued across a shallow platform to deeper marine water

(Galloway, 1975). Analyses of modem deltas show that the interplay of the fluvial and

marine processes produces a great variety of delta types. Fisher et al. (1969)

categorized these deltas into two broad groups, high-constructive and high-destructive

deltas. High-constructive deltas are the typical fluvially-dominated deltas that result in

progradational packages. Morphologically, high-destructive/marine-dominated deltas

consist of two distinctive end members depending on whether wave or tidal energy

predominates the reworking processes.

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2.3.1 Basic morphologic and stratigraphic framework of various types of deltas from modern and ancient examples

Subenvironments and facies of deltaic deposits are controlled by physical

processes operative on riverbome sediments brought to the shoreline. ‘River-

dominated’ deltas experience high volumes of sediment and water discharge, high

sedimentation rates and develop unstable depositional slopes. These deltas tend to

prograde relatively rapidly to form a broad deltaic plain. Rates of sedimentation may

exceed rates of subsidence or sea-level rise for extended periods of geologic time,

resulting a laterally extensive progradational wedge. ‘Wave-dominated’ deltas undergo

rapid reworking and dispersal of sediments in the shoreline zone by marine processes.

They prograde more slowly, reflecting a dynamic balance between sediment supply

and sediment redistribution. Therefore, a more delicate balance exists between the rate

of sedimentation and the rate of subsidence or sea-level rise to determine the fate of

wave-dominated deltas.

Fluvial-Dominated Delta: Lateral migration of distributary channels and

channel abandonment in fluvial-dominated delta plains result fining-upward

sequences. Plant debris are abundant throughout, with peats forming in the upper part

of the sequence. Flood-generated processes are principal means of sediment supply

which produce features such as natural levees, crevasse channels, and crevasse splays.

Interdistributary shallow waters are influenced by locally generated wind waves that

produce isolated ripples or lenticular lamina (Coleman and Gagliano, 1965). Fluvial-

dominated delta fronts are characterized by two kinds of sediment-laden flows

(hyperpycnal and hypopycnal) that play an important role after they escape the river

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confinement at the mouth of the distributary channels into the receiving basin (Bates,

1953). Hyperpycnal flows are denser than the ambient basin water and tend to flow

beneath the basin water as density currents, causing sediments to bypass the shoreline,

thus restricting the development of a delta. Hypopycnal flow is less dense than the

basin water and is the most common in modem marine deltas. It has been documented

from the Mississippi and Po deltas by Scruton (1956) and Nelson (1970). Three

processes related to hypopycnal flows, namely buoyancy of outflow, inertial processes

related to outflow velocity, and frictional processes resulting from outflow interact

with the sediment-water interface at the distributary mouth (Wright, 1977). Combined

in different proportions, they produce a series of outflow dispersion models for

hypopycnal flows in areas where rivers enter basins with limited wave or tidal energy

field as discussed below:

1. Inertia-dominated river mouths form where high-velocity bed-load rivers

enter a fresh water basin. Sediment dispersion produces an elongate steep-fronted

Gilbert type delta mouth bar. This process is of limited significance for river deltas in

marine basins but it can be of importance during brief high-discharge periods and

rivers flowing into lacustrine basins.

2. Friction-dominated river mouths form where rivers enter a basin with

shallow inshore waters. Frictional interference between the flow and the sediment

surface increases the spreading and deceleration of the outflow jet, producing a

triangular middle ground bar at the mouth of the river, which causes the channel to

bifurcate.

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3. Buoyancy-dominated river mouths form where river water extends into the

basin as a buoyancy-supported surface plume; they are restricted to the marine basins.

This is favored by the presence of relatively deep-water channels and moderately deep-

water fronting the river mouth, thus reducing the role of frictional mixing of water

masses and producing an elongate mouth bar which projects a considerable distance

into the basin with a gently dipping slope (Elliott, 1991). It is common for river

mouths to be dominated by buoyancy processes when discharge is low and more

influenced by frictional and inertial processes during high discharge periods (e.g.

South pass, Mississippi Delta, Wright and Coleman, 1974).

Tidal-Dominated Delta: Tidally influenced distributary channels have a low-

sinuosity, flared river mouth form with a high width-to-depth ratio which contrasts

with the almost parallel sided fluvial distributary channels in areas of low tidal range

(Elliott, 1991). Tidally dominated deltaic plains are characterized by dune bedforms

within the distributary channels and a maze of sand bars. In the modem Mahakam

Delta side-attached alternate bars are characteristic of low-sinuosity fluvial and tidally

influenced channels (Allen et al., 1979). In the lower central part of these channels

flow-aligned bars are common. These features resemble the linear tidal current ridges

of other tidal-dominated deltas such as the Ganges-Brahmaputra and Mekong

(Coleman, 1969; Coleman and Wright, 1975). Upper delta plain distributary channels

pass upward into organic-rich clays of the mangrove swamps disturbed by rootlets,

whereas near-shoreline tidal channel sequences terminate in flat laminated coastal

barrier sands (Weber, 1971; Oomkens, 1974). In the Rhine Delta the trough cross-beds

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with reverse paleoflows pass upwards into heterolithic facies comprising flaser

bedding. Characteristic features include bimodal flow direction and frequent small-

scale vertical facies variations reflecting short-term fluctuation in intensity and

direction of tidal currents (Oomkens and Terwindt, 1960; Terwindt, 1971b).

Interdistributary areas include lagoons, minor tidal creeks, and intertidal-supratidal

flats which are sensitive to climate. In the Niger Delta, mangrove swamps (vegetated

intertidal flats) are dissected by tidally influenced distributary channels and a complex

pattern of sinuous tidal creeks (Allen, 1965d). hi the arid climate of the Colorado

River Delta, Gulf of California, interdistributary areas of desiccated mud and sand

flats, with localized salt pans, are common (Meckel, 1975). Tidally-influenced delta

fronts show that the shoreline and the distributary mouth areas are often an ill-defined

maze of tidal current ridges, channels and islands, which may extend for a

considerable distance offshore before giving way to the delta front slope (e.g., Ganges-

Brahmaputra; Coleman, 1969). Tidal-current ridge sands at the top of the delta front,

coarsening upward sequence are composed of bi-directional trough cross-beds, with

occasional clay drapes and numerous minor channels.

Both the Klang River Delta, Malaysia (4.3m mean spring tide), and Ord River

Delta, western Australia (4.75 - 6.0m mean spring tide), exhibit funnel-shaped mouths

in the lower course of the distributary channel (Coleman and Wright, 1975). The

distributary mouth bar is extremely widespread, merging from all river mouths due to

a very high littoral drift. Large linear tidal ridges are oriented normal to the shoreline.

The channel mouths and immediate offshore areas are choked with well-sorted fine to

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medium sands forming sandy shoals. These ridges form due to an interaction between

flood- and ebb- dominated bedload transport (Coleman and Wright, 1975). The high

tides result in diurnal inundation of large parts of these deltas. Broad saline mangrove

and nipa palm swamps are well developed adjacent to the channels (Coleman et al.,

1970). Interdistributary areas are characterized by in-situ peats in the Klang Delta and

barren mud-cracked flats in the Ord Delta. The high evaporation rate in the latter

results in intercalation of thin stringers of evaporites within the silty clay sediment.

Wave-Dominated Delta: The delta fronts are characterized by high wave

energy impinging upon the fluvial discharge resulting in concentration of sand at the

shoreline. Localized river mouth bars, ornamented by landward oriented swash bars,

may form but commonly most of the sand is transported by longshore drift to produce

a nearly continuous fringe of beach sand. Deltas in most cases are characterized by a

regular beach shoreline with only a slight deflection at the distributary mouth, and with

a relatively steep delta front slope. River mouth bars do not develop and bathymetric

contours parallel the shoreline (Elliott, 1991). Progradation involves the entire delta

front rather than particular point sources on the shoreline and, therefore, is slower. The

Upper Cretaceous Blackhawk Formation in the Book Cliffs, Utah, provides evidence

of an ancient wave-dominated deltaic shoreline (Balsley, 1980). Sediments brought to

the river mouth were extensively reworked by onshore and longshore processes. Each

of the major cycles is characterized by a sequence starting with prodeltaic shale,

overlain by delta front sandstones that increase in grain size, bed thickness and

bedform energy upward. The coastal plain sequence of coal, mudstones with oyster

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shells, and thin fine-grained sandstones, records deposition in swamps and bays.

Interchannel areas show sheet-like spits formed by high wave energy. Coeval

distributary channels tend to be straight or of low sinuosity filled by fine mudstones to

coarse sandstones (Levey et al., 1980). The Castlegate Sandstone unit, which overlies

the Blackhawk Formation, shows similar characters supporting wave-dominated origin

of deltaic deposits (Van de Graff, 1972). The Upper Cretaceous Point Lookout

Sandstone in Utah, provide another example (Wright, 1986) with, slightly different

succession of facies. The lower shoreface is characterized by hummocky cross

stratified (HCS) sandstones, wave ripples, abundant macro fauna (including

ammonites, shark teeth, pelecypods). The upper shoreface shows coarsening-upward

textural trend and broad gende trough (swaly) cross-stratification or parallel

lamination. This change from HCS to trough cross stratification reflects the upward

change under the influence of marine processes from oscillatory wave action to

unidirectional flow (Harms et al., 1982). The recognition of laterally equivalent delta-

front sandstones is supported by an increase in the amount of organic debris relative to

the adjacent shoreface sandstone, decrease in sorting values and faunal activity,

increase in occurrence of lag material and loading structures in the delta front units,

and observed changes in sedimentary structures (presence of parallel and small scale

ripple lamination as opposed to HCS in the shoreface). The foreshore zone is

characterized by low angle, wedge to parallel seaward-inclined lamina sets. The next

higher unit forms continuous sheets composed of amalgamated channel deposits. The

upper Cretaceous San Miguel Formation, Maverick Basin, South Texas, exhibits net-

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sandstone patterns showing development of strike-oriented sandstone bodies (Weise,

1979). Sand-feeder (fluvial) systems are occasionally preserved as dip-aligned net

sandstone trends in the updip sides of the strike-oriented sandstone bodies. The

preserved shapes of the sandstone bodies show a wide spectrum including wave-

influenced lobate, cuspate, and strike elongated deltas in order of an increasing

importance of wave processes (Weise, 1979). The changing balance between the rates

of sediment input, wave energy, and sea-level rise produce cuspate wave-dominated

deltas to slightly lobate wave-influenced deltas.

The Sao Francisco Delta in South America and Senegal Delta in Africa are

examples of modem wave-dominated deltas, characterized by a steep offshore slope.

This high wave energy produces a smooth delta shoreline with only minor protrusions

at the river mouth (Coleman and Wright, 1975). The Sao Francisco coastal plain is

comprised of large, broad, sandy beaches characterized by clean, well sorted,

quartzose sand and aeolian sand dunes. The composite stratigraphic section consists

essentially of sand (-50%) with the total exclusion of river-produced sand bodies. The

delta plain in the Senegal Delta exhibits barrier-beach-dune ridges, oriented parallel to

the coast and separated by broad organic-filled swales with local salt pan

accumulations (Coleman and Wright, 1975). In an overall sequence, bioturbated

fossiliferous mud passes upward into an alternating mud, silt and sand package which

Anally passes into well sorted, parallel to low angle laminated sands representing a

high energy beachface (Coleman and Wright, 1975).

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Fluvial and wave-dominated delta fronts are characterized by a smooth cuspate

or arcuate beach shoreline. Localized protruberances in the vicinity of the distributary

mouth are composed of subdued mouth bars flanked by beach ridge complexes.

Present day examples occur in the Danube, Ebro, Nile, and Rhone Deltas (Elliott,

1991). The overall coarsening-upward succession of facies shows bioturbated offshore

clays passing gradually upward into fine laminated silt, and discrete beds of sand with

ripple lamination (Oomkens, 1967,1970; Maldonado, 1975). The uppermost, well

sorted, horizontally bedded sand unit is deposited by near-shore processes.

Fluvial and tide-dominated delta: The Holocene depocenter of Yangtze Delta,

eastern China, can be classified as a tide and fluvial-dominated delta. Stanley and

Chen (1993) identified three main facies, namely the lower delta plain (estuary, tidal

flat, marsh, fluvio-marine channel and chenier deposits), delta front, and prodelta. The

isopach maps from the Holocene show distinctly linear, northwest-southeast trending

(closely parallel to the lower coarse of the Yangtze) sand bodies. The zone of

maximum thickness extends in lobe-like fashion seaward of the estuary.

Wave and tide-dominated deltas are exemplified by the Rhine and Niger deltas.

Distributary channels show a general decrease in width-to-depth from the coast

inwards. The width-to-depth ratios vary from 55-180 for the Rhine Delta, while the

same for Niger varies from 38-200 (Oomkens, 1974). The sediments deposited in the

Nigerian tidal channel is supplied by longshore currents and by rivers. The tidal origin

is shown by the occurrence of a few marine organisms in the basal part of the channel

fill and by persistence of thin clay beds (mud drapes) throughout the cross-bedded

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sands. In contrast, Rhine tidal channel fills are composed of a rich, mixed, displaced

fauna that is supplied only by longshore currents (Noorthoom van der Kruijff and

Lagaaij, 1960). Coreholes in the Rhine Delta indicate that the thickest sand units were

deposited in coast-normal channel complexes. Niger Delta sands show similar trends.

But in these two deltas the coast-normal sand patterns are not due to a predominance

of fluvial supply but rather due to the presence of a strong coast-normal tidal current

system (Oomkens, 1974).

The Burdekin Delta of Queensland, Australia, is characterized by an extremely

erratic discharge related to the arid climate and precipitation. Wave power and tidal

range are considerably higher resulting in broad, sandy tidal flat deposits (Coleman

and Wright, 1975). The active channel comprises numerous bifurcating distributaries,

bell shaped river mouths, and sandy shoals and large bedforms within the channel.

Within the zone of tidal inundation, currents show bimodality. Beach ridges alternate

with mangrove-infested swales. In contrast to the channel sand, which are oriented

normal to the shoreline, the beach ridges are oriented parallel to the general shoreline

trend.

Fluvial-wave-tide influenced delta fronts are characterized by tidal currents

frequently operating in conjunction with wave processes. The shoreline is composed of

wave-produced beaches or cheniers separated by tide-dominated distributary channels

and mouth areas. Offshore bathymetric contours and facies belts parallel the shoreline

although slight protrusions at the vicinity of the distributary mouths are evident

(exemplified by the Irrawaddy and Orinoco Deltas). The Eocene Queen City

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Formation in the east Texas embayment represents a depositional record of riverine,

tidal and wave interaction (Hobday et al., 1979). Small, high-constructive shoal water

deltas and crevasse splay subdeltas developed. Barriers originated either as destructive

components of delta abandonment or as contemporaneous strike-fed features marginal

to the main delta complex. Extensive backbarrier or bay-margin intertidal and subtidal

flats, and shoals, reflect the interplay of tidal and wave generated processes.

2.3.2 Establishing a FIuvial-Dominated Origin for the Atoka Formation Deltas

The outcrops in Arkansas represent very well-developed coarsening- and

thickening-upward sequences. Cycles with bioturbated silt laminated shale at the base

passing upward into bioturbated siltstone/fine-grained sandstone and medium bedded

fine-grained sandstones above, representing prodelta, delta front, and distributary

mouth bars respectively, comprises ideal fluvial-dominated deltaic sequences.

The relative abundances of fine grain size, organics and bioturbation are

indicative of fluvial dominance during deposition. Fossil mudlump (mud diapers) are

preserved as mound-like features in delta front and distributary mouth bar sediments.

This feature, an analog to observations from the Recent, further establish these

deposits as fluvial-dominated deltaic bodies. The preservation of mud diapers also

indicates a lack of significant wave energy. The presence of small-scale cross

lamination in delta front deposits, wispy lamination (i.e., discontinuous wavy

lamination) and large-scale cross bedding in the distributary mouth bars, all indicate

the predominance of unidirectional current activity, and support the fluvial dominance

model. The lack of mud drapes, bi-directional current markers, and wave ripples,

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suggest insignificant tidal and wave influence, respectively. Hummocky cross

stratification and lam-scram stratification commonly preserved in wave/storm

dominated shorelines are rare to absent in the outcrops of the Atoka Formation,

suggesting a dominance of fluvial processes.

The absence of marine fauna in the deposits indicates a terrestrial origin of

sediments, hi addition, the presence of characteristic shallow/brackish water trace

fossils ( Conostichus, Bergaueria) in some of the outcrops imply the influence of more

brackish water over pure saline waters on the deposits. Predominance of amorphous

non-structured kerogen (> 80%) in almost all depositional subenvironments indicate a

mature stage of degradation reached by prolonged transport. In addition, a small

percentage of spores (< 10%) in all the lithofacies may also support long distance

sediment transport, possibly under fluvial dominance. Table 2.1 summarizes the

distinguishing characters that were used to establish a fluvial-dominated origin of the

Atokan deltaic deposits.

Facies belts in fluvially-dominated deltas form lobate geometries in response to

sediment influx associated with point sources. Wave-dominated deltas, on the other

hand, develop shore-parallel facies belts. Sandstone isolith and porosity maps from the

study area, based on subsurface correlation, reveal such lobate geometry (Zachry,

1983). According to him, the concentration of sandstone maxima in lobate and

elongate belts, and the development of shaly successions away from those belts, is

suggestive of sandstone accumulation in highly constructive delta systems that

prograded southward across the northern and central part of the Arkoma Basin. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. axis of these lobate deltaic sandstones trend S-SW which is approximately normal

the paleoshoreline.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. length > width Distributary mouth bar, Delta front, Distributarychannel Interdistributary bay andthickening- upward Mudlump present, and fill High Lobate/dip elongated (Zachry, 1983) Atoka (Present (Present Atoka Study) fill,Crevasse Splay 50-65m coal in higherunits Moderate to well Moderate Abundant rare Gradual coarsening Cross bedding, ripple cross lamination, cut Beachridge, Shoreface thickening- lamination, wave Very high Low Wave-dominated Length= width 40-130m upward Aolian dune, Swales, Foreshore Parallel ripples, tabular Very well sorted Poor Nomudlump, coal rare shellscommon (shallow marine) elongated Coarsening and cross beds, HCS Cuspate, strike flaserbedding Variablebed, mud drapes, Steeply SlopingModerate Not determined High (climate based) No mudlump, Coal (climate based) (estuarine) Tide-dominated Long linearbars oriented normal to shoreline Tidal channel, Tidal bars Length > width 30-100mthick Fining upward (associated with Herringbone bidirectional cross Moderately sorted Abundant, rootlets channels) Tidal flat, Mangrove swamp 180mthick oriented normal to shoreline Distributary channel, Distributary mouth bar, Delta front thickening-upward Interdistributary bay, Moderate, rootlets High Elongate to lobate Length> width, 50- Well Sorted Moderate shells rare shells abundant Crevasse Splay, Marsh Cut and fill, trough cross beds, ripple cross-lamination Coarsening and 0.4°) Bioturbation Organic content Specialcharacter Mudlump,coal Characters River-dominated Dominant Scale/extent (plan view) Subaqueous Profile Gently sloping (0.2- Sedimentary Sedimentary structure Sorting Sand to shale ratio BiogenicCharacters Macrofauna deposiliona! element Vertical Trend Sand body geometry Associated Facies characters Table 2.1 Table 2.1 river-dominated, andtide-dominated, Variability from characteristics wave-dominatedsedimentary in deltas based sidecomparison. for on information from et recent Coleman deltas al., deltas (Fisher from the rock record and Gagliano,and 1969; 1964; Galloway, Observationsdeltaicstudiesthe and deposits from previous the from Atokan present tabulated are 1975). along

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DESCRIPTION AND INTERPRETATION OF DELTAIC SUBENVIRONMENTS

3.1 Introduction

Various deltaic facies exposed in different outcrops in the Arkoma Basin were

studied as part of the present research. The outcrops are Searcy Quarry (Monowan),

Boston Mountain road cut (Lower Atoka), Freshour Quarry and Conway road cuts

(Middle Atoka), and the three Morrilton road cuts - 2MOR, 3MOR, and 4MOR

(Upper Atoka). The location of the outcrops is given in Figure 1.1. The entire

spectrum of depositional subenvironments, spanning the lower delta plain to prodelta,

are exposed in this composite set of outcrops. The observations and results presented

in this chapter are primarily field-based, the next chapter presents the laboratory

results. The interpretations regarding the depositional subenvironments are essentially

based on field observations and literature reviews, and have been corroborated by

laboratory analyses.

3.2 Methods

Detailed observations were made on the lithology of each facies with special

emphasis on grain size, sand/shale ratio, sorting, sedimentary structures, bioturbation

(intensity and types) in order to generate a working set of criteria. At each locality,

vertical profiles were measured documenting variations in lithology, sedimentary

structures, bioturbation patterns, bedding geometry, and nature of contacts.

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Observations on vertical and lateral trends depicting coarsening/fining,

thickening/thinning, were also recorded on the profiles. Gamma-ray measurements

were performed with a hand-held scintillometer to record relative vertical changes in

sand/shale ratio. Grain size was determined in the laboratory using a Macintosh based

image analysis program called NIH Image .

3.3 Description of Outcrops and Characterization of Deltaic Deposits from Various Subenvironments

3.3.1 Freshour Quarry

Description: This quarry shows an excellent set of outcrops of various

subenvironments of a fluvially-dominated delta, comprising of two delta cycles

overlying each other. This relationship is visible in the three-dimensional exposure

made possible by the differently oriented quarry walls. Stratigraphically, this quarry

represents the middle part of the Atoka Formation. A field photograph of the outcrop

is shown in Figure 3.1. The ouday of the quarry is presented in Figure 3.2.

Measured section from the west face of the quarry (WFH, Figs. 3.2 and 3.3;

Appendix A) starts with 9m of interbedded heterolithic siltstone and shale. The

amount of shale decreases upward up to 13.5m, with abundance of silty/sandy

interbeds increasing at the expense of the interbedded shales. Lower beds are 5-10cm

thick which gradually thicken upward to 15-25cm thickness at about 21.7m of the

section. Above this level 25-30cm thick beds are common. Organics are commonly

dispersed throughout the massive sandstone beds. The upper surface of the beds are

very commonly wavy to ripple-laminated and mica-rich. Mica-rich, silty

shales are common as partings (0.5-2cm thick) in between beds. Up to 38m of the

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Figure 3.1 Outcrop photograph of the east wall of Freshour Quarry illustrating the thickening upward nature of beds. The section exposed is 16m thick (as of 1996).

ocmftr ROADCUT

rm m tK S t

;o.M8ij*a=]

auvmr

Figure 3.2 The outlay of Freshour Quarry illustrating the east, west, north, and south faces. The abandoned quarry across Highway 5 is also shown.

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78m Organic-rich, splintery shale, siderite, pyrite 63.8i

25-30cm, flat-bedded, 27m fU sandstones, 50 cm at top 53.4m Thin-bedded, bioturbated7 24.5m alternating siltstone-sandstone 46m 18.5m 25-30cm, lenticular beds

25-30cm sandstones, flat -bedded, massive with partings

EFH 21.7m 15-25m sandstones

5-10cm sandstones

O m H H H i Heterolithic composition

WFH

Figure 3.3 Schematic diagram representing the salient features from measured sections from the west and east faces (WFH and EFH). For details see text and Appendix A.

section the fine sandstones (fL) have the same characters with distinct wavy to almost

flat contacts and occasional partings. Above 38m slight wedging of beds can be

observed with occasional burrows, identified as Paleophycus, parallel to the bedding.

Wedging increases upward with common bidirectional lateral pinch-out of beds. These

lenticular, concave-upward, sandbodies show abundant current ripples associated with

the upper surfaces of the beds. Micas and organics accentuate the wavy-parallel to

ripple lamination. At around 46m a sudden change in color, bedding thickness and

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abundance of burrowing is observed. A package of thin-bedded alternations of silty

shales and very fine sandstones are present with a very sharp boundary at 46m.

Bioturbation is abundant, representing both crawling and burrowing activity, identified

as Planolites. Rip-up clasts are occasionally present Upward, the layers commonly

pinch and swell with ripple/wavy lamination disturbed by heavy burrowing. The beds

gradually coarsens and thickens upward to 25-30cm thick even beds of fine (fU)

sandstone, which thicken further to 50cm massive sandstone units towards the very top

of this interval (63.8m). These units are very abruptly overlain by very organic-rich

splintery black shales with abundant sideritic nodules. The overlying shale section is

14m thick with coaly luster that soils the hand. Shiny lamination surfaces are

commonly marked by mica and pyrite.

Profile EFH from the east face of the quarry (Fig. 3.3; Appendix A) shows a

similar distribution of incompletely exposed facies. The basal units are medium to

thickly-bedded (10-50cm), tabular, massive fine sandstone units with some wavy

laminations. Contacts between the beds are slightly erosional or contain thin clay

partings. In general, burrows are absent but some of the upper contacts show distinct

evidence of burrowing. Overall, the package shows coarsening and thickening upward

trend up to 18.5m. The next interval up to 24.5m has a sharp base and is lithologically

characterized by alternation of thinly-bedded and cross-laminated (5-8cm) sandstones

and laminated siltstones or shales. These deposits are heavily bioturbated, often

resulting in partial destruction of the primary sedimentary structures. At some places

remnants of primary stratification in the form of ripple laminations are visible.

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Organics are abundant in the shales. The upper surface of the bedding planes show

numerous varieties of trace fossils as well as evidence of burrowing (both vertical -

2cm scale, and inclined, as discussed in Chapter 4). A finely laminated shaly deposit

occurs as a mounded feature on the north face of the quarry and shows disharmonic

folding that has disturbed the original layering (Fig. 3.4).

The gradual increase in grain size (i.e. sand-shale ratio) from the lower to the

upper part is evident in the gamma-ray profile shown in Figure 3.5. The first transition

is gradual, straddling 21.7m in the section and the next break is sharp at 46m. These

changes are marked by arrows in the profile in Figure 3.5.

Interpretation: Subenvironments of a fluvially-dominated delta, exhibiting two

complete delta cycles nested together on top of each other with a lateral offset are

exposed in the Freshour Quarry (Fig. 3.6). The deltaic facies include distributary

mouth bar, delta front/distal bar, prodelta and interdistributary bay fill deposits. The

outcrop exposes deposits that form in the subaqueous part of a modem river-

dominated delta which is characterized by basinward fining of sediments. Sands and

coarser elastics are deposited at or near the distributary channel mouths while finer

grained sediments settle farther offshore primarily out of suspension from the water

column.

From the west wall of the quarry the lower 21.7m of the section shows a

gradual upward increase in grain size and bed thickness (from 5-10 to 15-25cm),

comprising delta front deposits of the first cycle. The delta front/distal bar deposits

form the seaward sloping margin of the advancing deltaic sequence and it underlies the

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Figure 3.4 A mud-lump, visible as disharmonically folded shales disturbing the stratification of the overlying delta front deposits on the north-face of the quarry.

distributary mouth bar deposits of the same cycle with a gradational contact in

between. The progressive coarsening and thickening upward implies a gradual

approach of the primary sediment source (e.g., distributary channel) near this site. The

corresponding vertical increase in sand/shale ratio is shown in the gamma-ray profile

in Figure 3.5. At the mouth of a distributary channel in modem deltas, basinward

outflow of low-density turbid water leaves the confines of the channel and flows out

over the denser saline water. It expands by virtue of being less dense (buoyancy

difference) and loses its velocity. Coarser sediments settle rapidly out of bedload and

from suspension, depositing the bulk of the coarser detritus in the vicinity of the

distributary channel mouth, forming a major sand body - a distributary mouth bar

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WTWIIWH 70 ~ i ■ i

Bedded sandstone GO

Sendstone “ggg with =scce lenticular geometry o 40 X

Sandstone with 30 tabular geometry

Siltstone- 15 sandstone interbeds

3§ il 10

100 200 300 400 Gamma Count (cps)

Figure 3.5 Gamma-ray profile from the west face of the quarry along measured section WFH demonstrating an ideal coarsening-upward package with a break at ~ 46m.

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Figure 3.6 Depositional model of the deltaic facies at Freshour Quarry. PD, DF, DMB and, IDBF represents prodelta, delta front, distributary mouth bar and interdistributary bayfill deposits, respectively. The window of view at Freshour Quarry is depicted

(Fisk, 1955; Coleman and Gagliano, 1964, 1965; Coleman and Prior, 1981).

Accumulation rates are extremely high, probably higher than any other

subenvironments associated with a delta. From 21.7 to 38m, the bed-thickness

increases to about 30cm with distinct tabular bedding. The overlying interval between

38 to 46m is composed of lenticular-bedded sandstone with common cut and fill

structure. This implies increased dynamics of sedimentation related to active

deposition from a nearby distributary channel mouth. The 21.7-46m interval represents

distributary mouth bar deposits of the first cycle (DMB1). The 46 to 53.4m interval

represents delta front deposits of a second cycle (DF2) characterized by thin-bedded

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bioturbated interbeds of siltstone and sandstone similar to beds in the 0 to 21.7m

interval of this section. The sharp base of the 46-53.4m interval implies that this

represents the onset of a new cycle. From 53.4 to 63.8m the section is characterized by

25-3 Ocm thick flat bedded fine (fU) sandstones that reach a maximum thickness of

50cm at the top of this interval. The base of this interval around 53.4m has a

gradational character. This (53.4-63.8m) interval represents the distributary mouth bar

of the second cycle (DMB2). From 63.8 to 78m, the section is comprised of black,

splintery, organic-rich (coaly) shale with abundant siderite nodules and pyrite. This

interval is interpreted as interdistributary bay fill marshy deposits which implies

abandonment of the second delta cycle and colonization by marsh/bay fill

subenvironment

The western face inside the quarry, hence represents a distributary mouth bar of

the first cycle (DMB1), followed successively upward with a sharp contact by delta

front deposits of the second cycle (DF2). The DF2 grades upward to the distributary

mouth bar of the same cycle (DMB2) capped by interdistributary bay fill to marsh

deposits. The DMB1 is exposed in both the eastern and western faces and is underlain

by delta front/distal bar deposits of the same cycle (DF1) with a gradational contact in

between (Fig. 3.7). The DF1 is exposed on the northern face of the quarry and at the

roadcut section outside the quarry. The northern face shows an extensive bedding

surface of DF1 with profusely developed bioturbations.

The eastern face inside the quarry also represents distributary mouth bar

(DMB 1,0 - 18.5m) facies overlain by the delta front (DF2, 18.5-24.5m) facies with a

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Figure 3.7 The gradual transition between delta front deposits (DF1) and distributary mouth bar deposits (DMB1) showing a thickening-upward trend. The section seen is approximately 6.5m thick.

sharp contact in between. DF2 grades upward to the distributary mouth bar (DMB2,

24.5-27m) of the same (second) cycle as seen in the west face.

The ideal vertical sequence of a fluvially-dominated delta should be prodelta

overlain by delta front/distal bar which in turn is overlain by a distributary mouth bar.

This type of vertical succession reflects progradation resulting in a coarsening- and

thickening-upward cycle. As discussed earlier, the roadcut outside the quarry shows

DF1 overlain gradually by DMB 1 which is overlain very sharply by DF2 (Fig. 3.8).

Delta front deposits (DF2) are seen to overlie the distributary mouth bar deposits

(DMB 1), though the expected sequence would be the opposite if they belonged to the

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Figure 3.8 The sharp boundary separating distributary mouth bar deposits of the first cycle (DMB 1) from delta front deposits of the second cycle (DF2).

same delta cycle. Using the modem analog, one delta lobe is active at the distributary

mouth at a given time (Coleman and Prior, 1981), and deposits from younger,

adjacendy developing delta lobes may juxtapose with each other in time (applying

Walther’s Law). This scenario is exposed at Freshour Quarry and is depicted in the

depositional model shown in Figure 3.6 showing abandonment of one delta cycle and

occupation of the site by the fringes of another delta lobe which is active at a later

time. The distributary mouth bar visible in the eastern and western faces,

corresponding to the first cycle (DMB 1), is overlain by delta front and distributary

mouth bar sediments of the adjacent active delta lobe corresponding to the second

cycle (DF2 and DMB2). The DMB1 is thickest in the west face (23m) which thins to

about 18m in the east face of the quarry within a lateral distance of 820m. The

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distributary mouth bar of the second cycle (DMB2) is thinner than the stratigraphically

lower cycle (DMB 1) as shown in Figure 3.6. In the Freshour Quarry the thickness of

DMB2 varies from 2.5m in the east face to about 10m in the west face which is a

reflection of lateral delta lobe switching from the previous site, slightly west Further

evidence supporting this conclusion is available from the abandoned quarry on the

other side of Highway 5 (towards the west) where the same DMB2 seems to thicken

even more (see Fig. 3.2). The precise thickness from the abandoned quarry was not

measured because of poor accessibility to the section, being mostly under water. For

the upper (second) cycle therefore, the fringe of the second delta lobe is encountered at

Freshour Quarry. This second cycle of deltaic deposition is overlain by a 14m thick

section of interdistributary bay fill/marshy deposits of the same (second) cycle, in the

west face of the quarry (Fig. 3.9). This implies total abandonment of the second delta

cycle or farther progradation of the second delta lobe basinward.

In the north face of the quarry, the DF1 deposit is discordandy intruded by of

underlying prodelta sediments of the same cycle (PD1) with disharmonic

folding (see Fig. 3.4). It exemplifies a fossilized mud diaper exhibiting similar features

akin to mudlumps forming at the mouth of the modem Mississippi channel mouth bar

(Morgan, 1961; Morgan etal., 1968).

3.3.2 Conway Roadcut

Description: This section is 38m thick and is presented in Appendix A. An

overgrown interval below the section of 5m was not recorded in the profile. The

lowermost 2m of the measured section is characterized by well-laminated splintery

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Figure 3.9 Distributary mouth bar sandstones of the second cycle (DMB2) overlain by black shales of interdistributary bay fill origin (IDBF). Author on ramp for scale.

shale with occasional siltstone beds (0.5cm thick). An upward increase in the number

of siltstone beds and thickness from 0.5 to 2cm is noted within this interval. Between 2

to 4m the section is characterized by small coarsening and thickening-upward

packages with a progressive increase in the percentage of siltstone. Siltstone beds are

undulatory with thickness of the individual beds varying between 2 to 4cm. Occasional

lenticular and ripple laminations are common. From 4-6m siltstone beds become

thicker (6-7cm) upward with silty shale present only as partings. At the very top (at ~

6m) of this interval a 25cm thick bed with swaly and wavy bedforms are present (see

Conway section in Appendix A). This bedform is identified as hummocky cross

stratification (HCS, Fig. 3.10) and was noticed for the first time in the section. From 6-

10m the section is characterized by the appearance of very fine sandstones. As a

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whole, this interval shows very fine sandstone/siltstone with occasional silty shale

partings. The percentage of fine sandstone increases upward in number and in

thickness of the beds. Internally, the beds are wavy and ripple laminated and bedding

contacts are foliaceous with an amalgamated nature. Amalgamated, variably thick

units increase and continue up to 14m of the section with wavy contacts between beds.

Around 14m, several subunits (average thickness 7-8cm) of very fine (vfU) to fine (fL)

amalgamated, wavy-laminated sandstone, with squashed out remnants of micaceous

and organic rich partings, are observed. At 15m, hummocky cross stratification (HCS)

reappears in fine (fL-fU) sandstone. Individual units are amalgamated with abundant

wavy laminations. From 16-20m, a heavily wavy-laminated unit, has amalgamated

Figure 3.10 Hummocky cross stratified sandstone from the Conway Road cut section.

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wavy discontinuous beds, that are characterized by hummocky cross stratification. The

upper bounding surface at 20m is a major wavy surface marking the boundary between

cross bedded units above and wavy laminated to hummocky cross stratified units

below. From 20m onward, profuse development of cross bedding is noted. It starts

with fine (fU) sandstones and is gradually overlain by medium (mL) sandstone. Beds,

10-12cm thick, wedge out with wavy upper and lower surfaces mimicking a dune

field. Amalgamated units prevail with occasional partings. Shale clasts are common

from 22-24m and are mostly concentrated in discontinuous crumbly lenticular zones

with poorly developed wavy laminations. Internally, the beds are ripple-laminated with

occasional bioturbation common at the very top of the interval. The section continues

upward as well-bedded fine sandstone (5-13cm thick), with occasional partings, that

are internally cross-laminated and cross-bedded. Beds are wedge shaped with

occasional rip-up clasts. Up to 33m, beds nonuniformly thicken upward from ~10cm at

the base to 30cm at the top. Lensing and wedging out of individual beds are common

and the upper surface of the beds contain abundant mica and organic matter. Internally,

the beds are cross-bedded. Occasional l-2cm thick partings are common. Flattened

rip-up clasts often are common. At 35m a massive bed with a distinct amalgamation

contact is present. At 37m a 5-15cm thick silty shale unit is present and it is overlain

by an internally massive to faintly cross-bedded unit continuing to the very top of the

section at 38m.

<1 _

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The measured gamma-ray pattern from the outcrop (Figure 3.11) demonstrates

a gradual coarsening-upward package with a subtle break at 20m indicated by the

arrow in Figure 3.11.

Interpretation: The section starts with a 5-6m overgrown unmeasured shale

with abundant bioturbation implying diverse fauna! activity. It is overlain by a few

meters of section characterized by alternating silty shale and siltstone with the number

and thickness of siltstones increasing up section. It suggests an increasing amount of

sediment influx to the site with a resultant increase in coarser elastics towards the top.

The presence of abundant ripple and wavy laminations in the very fine sandstone units,

slightly higher in the section, implies increasing current activity on the sediments. The

very fine sandstone is gradually overlain by fine (fL) sandstone with hummocky cross

stratification (HCS) and occasional ripple lamination. The presence of hummocky

cross stratification (HCS) in this unit implies deposition between fair weather and

storm weather wave base by frequent suspension and redeposition of sediments which

were possibly affected by storm processes (Wright, 1986; Dott and Bourgeois, 1982).

The lowermost shaly unit (not measured) below the base of the section probably

indicates offshore or offshore-transition facies while the hummocky cross stratified

unit represents lower shoreface facies. An abundance of amalgamated contacts in a

thickening-upward sequence between 10-20m of the section implies frequent influx of

coarser sediments to the site and strong currents eroding any finer grained sediments

during the slack period intervening higher energy episodes.

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illlUl

Cross* ■ »-» -* o c q q c q sandstone

: 20

Hummocky cross stratified sandstone

0 100 200 300 400

Gamma Count (cps)

Figure 3.11 Gamma-ray pattern from the Conway Road cut section representing a coarsening-upward trend with a lithological break around 20m.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53

At 20m, the conspicuous surface marks the boundary between the cross-bedded

units above from hummocky cross stratified and wavy laminated units below. This

change from HCS to trough cross stratification reflects an upward change from marine

processes with oscillatory wave action to unidirectional flow, respectively. In the

lower units above the major surface rip-up clasts are common with abundant large

scale cross bedding. It implies high energy associated with unidirectional currents.

Presence of occasional bioturbation around 24-26m for the first time in the section

indicates occasional breaks in sedimentation. The abundance of cross-bedding and

ripple cross lamination increasing higher up in the section suggests that this entire unit

possibly represents deposition in the upper shoreface.

The Freshour Quarry and the Conway roadcut sections both represent the

Pennsylvanian middle Atoka Formation and likely represents time-equivalents of each

other. As already discussed, the Freshour Quarry shows deposition of two cycles of

fluvial-dominated deltaic bodies. The Conway section represent deposition in

interdeltaic wave-dominated coastal areas without a major influence from a fluvial

point sources. This scenario is depicted in Figure 3.12, where the location of the

Conway outcrop is inferred to be on the shoreface laterally outward along the shoreline

from a delta, represented by the Freshour Quarry. The relative high thickness of

shoreface sandstone compared to Gulf Coast barrier island successions, is most likely

due to pronounced subsidence rates during the middle Atokan time that generated the

greater accomodation space. The activity of numerous growth faulting at that time can

also account for such abnormal thickening of the shoreface sandstone body.

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Lower • %

v;.:.%;; Oelio Ploin £,'£*>’> v »

• Lower -y ^ Coostol Ploin / • • • / >-SF . y f CONWAY

Figure 3.12 A model demonstrating the depositional setting of the Conway Road cut. Laterally developing shoreface (Conway) and delta lobes (Freshour Quarry) are commonly present along shorelines (after Wright, 1986). The possible locations of the two sections are depicted in the model. DF, USF, LSF, and OT represent delta front, upper shoreface, lower shoreface and offshore transition, respectively.

3.3.3 Second Morrilton Roadcut (2MOR)

There are four roadcuts at Morrilton, three of which were investigated in the present

study. They are named as 2MOR, 3MOR and 4MOR from the north to the south.

Outcrop 1MOR represents the same sequence as 2MOR, being exposed on two limbs

of the same fold. The respective road cuts 2MOR and 3MOR are separated by 608m

and 3MOR and 4MOR by 144m, measured along Highway 9. All of these outcrops

correspond to the Upper Atoka Formation.

The 2MOR outcrop shows 42m of section starting with a 33m section of shale

overlain by 9m of sandstones with a very sharp boundary in between (Figure 3.13; for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. measured section 2MOR see Appendix A). The lower part of the section (0-33m) is

characterized by monotonous shale, alternating with occasional silty shale and fine

siltstone. Millimeter-scale laminations in the shale section are very common with

rare bioturbation ( Planolites) often obscuring the primary stratification. Parallel to

Figure 3.13 An outcrop photograph showing the sharp based sandstone resting on the thinly bedded black shales.

cross-laminated siltstone and very fine sandstone in the shale is shown in Figure 3.14.

Occasional l-1.5cm thick siltstone to very fine sand lenses with discontinuous ripple

cross laminations are abundant. Bioturbation from the shale interval is very prolific in

the lowermost part of the section. The 33m of shale in the section is comprised of 0.5

to 1.5m thick coarsening-upward packages.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This monotonous shale section is overlain by 9m thick sandstone with a sharp

flat base without any evidence of erosional scouring into the underlying unit. Within

the total 9m the lower 2m is comprised of medium bedded tabular sandstones with

well developed parallel laminations. Overlying the tabular bedded interval, the rest of

the sandstone body (upper 7m) is characterized by a common presence of bidirectional

Figure 3.14 A representative photograph of the lower interbedded shale-siltstone interval with persistent sand laminae and lenticular bedding.

lateral wedging out of beds and lenticular bedding geometry. Flat bedding planes are

almost lacking. The beds are thinner in the uppermost part of the section. The gamma-

ray pattern from the outcrop is presented in Figure 3.15 which represents high counts

for the lower interbedded shale-siltstone package and lower counts for the sandstones

across the very sharp boundary at 33m.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57

Interpretation: The section from 0 to 33m of monotonous shale with occasional

millimeter scale silty laminations represents normal depositional characteristic for an

area immediately offshore of a sediment point source. The persistence of these silty

laminations over considerable lateral extent is a distinctive character observed from

prodeltaic deposits in modem deltas (Coleman and Gagliano, 1964; Coleman and

Prior, 1981). Deposition in a prodelta setting is invoiced for the 0 to 33m section at

2MOR. Very fine ripple cross-laminated sandstones in the form of lenticular bedding,

and bioturbation ( Planolites) support the prodelta interpretation. The presence of 0.5

to 1.5m thick coarsening-upward packages in this shaly section possibly represents

increasing flow within each package related to seasonality in discharge. Very fine

sandstones near the top of these 0.5 to 1.5m thick packages represent an increase in

sediment influx associated with river floods.

At 33m in the section, the sharp based very fine (vfU) sandbody represents

deposition in a nearshore setting possibly a beach or spit. The sharp base possibly

indicates a hiatal surface with or without erosional removal of the upper part of the

prodelta sequence. This sharp contact is a likely a candidate for a higher order

sequence boundary since proximal deposits (beach/spit) overlie distal deposits

(prodelta). These higher order sequences are present in the Upper Atoka Formation

as discussed in Chapter 5. The beach interpretation for the 2m interval (33-35m) is

supported by the tabular bed geometry, very well developed parallel laminations, and

good sorting. The uppermost part of this section from 35-42m characterized by very

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58

Top Sandstone,

«*«*«?

’tnceibedded

siltstone

100 200 300 400 Gamma Count (cps)

Figure 3.15 The gamma-ray pattern measured from the second Morrilton Road cut. The sharp lithological break is clearly represented at ~ 33m.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fine (vfU) sandstones with lenticular geometry and cross-lamination has depositional

signatures that are indicative of unidirectional flow related to some channel activity.

3.3.4 Third Morrilton Roadcut (3MOR)

Description: This roadcut comprises a 38m thick section. Figure 3.16 shows an

outcrop photograph of the third Morrilton roadcut The measured section is presented

in Appendix A and Figure 3.17 represents a schematic summary. The lower 32.5m of

Figure 3.16 The third Morrilton Road cut (3MOR) represented by shales below (far right) and massive sandstones above, comprising a coarsening and thickening upward package. The overall section is 38.5m thick.

the section shows a coarsening-upward package starting with a 19.5m thick interval of

splintery shale, overlain by a 13m thick sandstone (Fig. 3.17). The uppermost 6m of

the section is composed of dark gray shales. The lower 19.5m is characterized by

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60

grayish black to dark gray splintery shale with an abundanca of organics. Silty

laminations (millimeter scale) are common with rare lenticular silty laminations. These

laminations are occasionally disturbed by tube-shaped burrows ( Planolites) which are

parallel to the bedding. Sideritic nodules are abundant throughout this lower shale (0-

19.5m thick); they are brownish gray in color. Within this shaly section at 8m and 19m

from the base there are two 10-15cm thick reddish brown carbonate shell-hash layers

(Fig. 3.17). These beds are extremely rich in marine fauna, namely crinoids,

bryozoans, brachiopods, and possibly some forams. These two carbonate layers are

also associated with cylindrical, vertical burrows ( Conostichus), 3-6cm long and

perpendicular to the general bedding orientation. Towards the upper part of the

3MOR

38.5m Dark gray splintery shale bioturbated, concretions

Ripple laminated fine sandstone, commonly amalgamated. 19.5m

Carbonate hash

Dark gray splintery shale bioturbated, concretions

Carbonate hash

Figure 3.17 Schematic representation of the intervals in profile 3MOR.

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shaly interval (0-19.5m), a thickening and coarsening upward trend is observed with

occasional 2cm thick fine sandstone beds that show pinch and swell structure with

very small scale cross lamination. The bedding thickness gradually increases upward.

The shaly interval is overlain by a 13m thick (19.5-32.5m in the section)

sandstone unit characterized by a gradual increase of bed thickness and grain size

(from £L to fU sandstones). The entire unit (19.5-32.5m) is tabular bedded, with bed

thickness increasing from S-lScm at the base (~ 19.5m) to about 50cm at the top

(~32.5m; average 40cm with the maximum thickness being 80 cm). The base of the

sandy section is highly bioturbated (with perpendicular burrows, mainly associated

with partings in between thicker beds). Burrow activity diminishes upward. The

sandstone body shows evidence of water escape structures (at 27.5m from the bottom),

occasional wavy laminations and abundant ripple cross lamination which increases in

dimension upsection. Occasional mud clast/chip impressions, representing rip-up

clasts are observed. The upper surface of the bedding planes, where visible, are

micaceous. Around the mid section (~26m of the overall section) of the sandstone

body, the beds are almost amalgamated with total exclusion of finer grained partings.

The sand to shale ratio is very high. Overall the sandstone unit shows a thickening and

coarsening-upward trend. Rootlet structures are very common in the uppermost quarter

meter of the sandstone body.

The sandstone terminates at 32.5m and is overlain by a 6m thick section of

splintery shale with the same characteristics as the one down section (the lower most

package) and with a very low sand to shale ratio.

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The measured gamma-ray profile from the third Morrilton roadcut is presented

in Figure 3.18. It shows an coarsening-upward package overall within the section and

specifically within the upper sandstone interval.

Interpretation: The lower deltaic plain in modem fluvial deltas crevasse splays form as

clastic wedges that are stacked on top of each other, sandwiched between

interdistributary marsh and bay fill shale deposits (Welder, 195S; Coleman and

Gagliano, 1964). Each crevasse splay forms initially as a result of breaching a levee of

a distributary channel during flood. Lateral outflow through the crevasse and into the

bay gradually increases through successive floods and finally reaches a peak of

maximum deposition. Subsequently the flow wanes and the depositional site is

abandoned. Subsidence (compaction) takes over and the newly deposited sand body

becomes inundated, thereby reverting back to a bay environment thus completing the

cycle. Continued subsidence and repetition of the same physical processes result in the

stacking of several cycles on top of each other, eventually building a thick sequence of

bay fill deposits (Welder, 1955; Coleman and Prior, 1981).

The measured section (3MOR) starts with a thick section of splintery shale

which is organic-rich, occasionally bioturbated and shows abundant siderite nodules. It

gradually coarsens upward into a thick sandstone unit. The lower shale may be

representing an interdistributary bayfill while the upper sandstone package represents a

crevasse splay deposit. Interdistributary bayfills form in a brackish water setting in the

present day. Within the interdistributary bayfill shale there are two 10-15cm thick

fossiliferous silty/very fine sand beds. The fossils are mostly broken, indicating

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3M 0R SECTION

Gamma Count (cps)

Figure 3.18 The measured gamma-ray profile from Road cut 3MOR, representing a overall coarsening-upward package with conspicuous coarsening within the upper sandstone (I9-32.5m).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64

significant physical reworking in a high energy environment. Possible predatory

activity could also have caused fragmentation of the shells. But the pervasive nature of

shell fragmentation indicates the storm related hypothesis to be more likely. The

carbonate hash beds have obvious coarse-tail grading, the matrix is very fine sand to

silt. The fossil components are restricted to the two distinct carbonate beds only and

not found in the shales. The carbonate beds are comprised of phylloid algae, crinoid

columnals and other echinoid fragments, brachiopods, bryozoans, molluscans and

forams (identified as a questionable Endothyrid; B.K. Sen Gupta, personal

communication, 1997) all of which are marine. These carbonate hash beds are

interpreted to represent marine detritus transported locally into the bay possibly by

overwash during storms. These episodic events break the general quiescence in the

interdistributary bay. The carbonate hash beds have abundant burrowing ( [Conostichus)

specifically in the lower part of the beds. The interdistributary bay organisms possibly

fed on the nutrients supplied by these quickly deposited allochthonous storm beds of

marine origin. Abundant marine organisms are found with washover deposits on the

bay side of barrier islands from present day storm dominated coastlines e.g., Santa

Rosa Island, Pensacola during Hurricane Opal in 1995 (Stone et al., 1996).

The shaly section is gradually overlain by very fine sands (vfU) with the lower

parts of the sand body being highly bioturbated implying animal activity as a result of

abundant nutrients supplied by flood-related sediment influx in the bay. Due to

weathering the beds exhibit a apparently massive outlook but at closer inspection they

show ripple lamination. Beds are commonly tabular in geometry and thicknesses

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increase upsection associated with increased amalgamation, indicating a progressive

increase in energy associated with rapid deposition. Presence of water-escape

structures supports this interpretation and indicates a high rate of deposition in a water

saturated system. Rootlet structures are common at the very top of the section

signifying colonizing by plants after the sand dominated system was abandoned. The

crevasse splay unit is overlain by dark gray splintery shales which indicate that the

normal sedimentation processes in the interdistributary bay is reestablished after the

abandonment and consequent submergence of the crevasse splay sand body.

3.3.5 Fourth Morrilton Roadcut (4MOR)

This roadcut represents a 45m thick section, characterized by a lower 22.4m

dark splintery shale, overlain by a 22.6m thick sandstone package. This section

corresponds to the Upper Atoka Formation. The lower shaly section has similar

characteristics as the third Morrrilton roadcut (3MOR). It is composed of grayish black

to dark gray splintery shales with millimeter-scale siltstone laminations, occasional

burrows, abundant organics, and siderite nodules. The sand to shale ratio is very low.

Wavy lamination is occasionally present with fine-scale burrows. Presence of pyrite,

coaly luster, and leaf impressions are found midway through this shaly section. This

lower shaly section is abruptly overlain at 22.4m by a 22.6m thick sandstone with a

sharp erosive basal contact (Fig. 3.19). The 22.6m thick sandstone body is divided into

five units based on unique characteristics.

Unit 1 starts at 22.4m and continues up to 25m. It has a very sharp base.

Distinct evidence of basal erosion is present. This unit is characterized by lenticular

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bed geometry. Rip-up clasts are associated with the bases of most of these beds. Fewer

rip-up clasts are present towards the top of these beds. The unit is predominantly

massive, amalgamated, with very small burrows. The percentage of burrows increases

in the upper part of this unit

Figure 3.19 Photograph showing Unit 1 sandstone overlying the lower shale interval with a sharp erosive base. The sandstone is 2.5m thick.

Unit 2 begins at 25m and is 5m thick (up to 30m). The lower one meter is very

thinly laminated. Abundant ripple lamination grades toward climbing ripples.

The upper part is thick bedded with abundant ripple lamination. Each bed is an

aggregate of two or more subunits. Rip-up clasts are occasionally present. An overall

thickening-upv/ard trend of beds from a few centimeters to 20-30cm is evident. Mostly

horizontal and some inclined burrows are associated with bedding surfaces.

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Unit 3 begins at 30m and is 4.5m thick. This is a tabular bedded unit with less

frequent ripple cross laminations. Occasional burrows are still present in partings

associated with the upper parts of thicker beds.

Unit 4 at 34.5m is 6.5m thick. This unit is comprised of fine sandstones (fL)

with lenticular beds and large scale cross-bedding. Accretion-style bedding characters

are commonly observed. Upwards within this interval, bed-thickness and wedging out

increase. Burrows are common near the base of these beds. A water escape structure

was observed at the very top of this unit.

Unit 5 starts at 41m and is 4m thick. It is characterized by a thickening-upward

package from a few cm to a maximum of 15cms. Burrows are abundant and are

associated with wavy upper contacts. This unit is internally ripple cross-laminated.

The lower part of each bed represents an aggregate of thin layers associated with

occasional shale clasts. Wedging out of beds is rare. Midway through this section a

water escape structure is present

Grain-size analysis of the sandstone section (22.4 to 45m) shows that a subtle

fining-upward trend is present from fine upper (fU) sandstone at the base to fine lower

(fL) sandstone at the top.

The gamma-ray profile measured from this road cut is presented in Figure 3.20.

This profile shows an abrupt coarsening corresponding to the base of the upper

sandstone body, with otherwise high gamma counts from the lower shaly interval and

low counts from the sandstones.

! ___

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Interpretation: The lower part of the section (0-22.4m) characterized by silt to

very fine sandstones with splintery shale, likely represents a bayfill setting. The

presence of frequent siderite nodules, leaf impressions, and occasional pyrite streaks

and lack of marine fauna support the bay fill origin.

The upper part of the section from 22.4 to 45m is inferred to represent a

channel fill that records various stages of activity. The base of this interval at 22.4m is

erosional, marked by a distinct contact which sequesters the coarsest sediments found

in the Morrilton outcrops. Progressively higher up in the section numerous flow

regimes are represented. For example, high suspended load to bed load deposition is

characterized by climbing ripple lamination in Unit 2 (25-30m); bar formation, lateral

switching, and flow partitioning represented by lateral wedge-outs and accretions are

observed in Unit 4 (34.5-41.2m). An overall fining-upward (fU to fL sand) trend in

grain size is observed from the base (~22.4m) to the top (-45m) which supports the

channel fill origin for this interval.

3.3.6 Boston Mountain Roadcut

Description: This roadcut shows 63m of section presented in Appendix A. The

entire section is divided into six units of variable thickness, based on observed

depositional characters. They are numbered Unit 1 to Unit 6 from base to top.

Unit 1 (0-17m) is characterized at the base by an alternation of dark gray shale and thin

(2-3cm) siltstones to fine (fL) sandstones (Fig. 3.21). The siltstones and sandstones are

characterized by fine-scale ripple cross-laminations. The grain size coarsens upward

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4M0R SECTION

Upper sandstone

Lower shale

100 200 300 400 Gamma Count (cps)

Figure 3.20 The gamma-ray pattern from the fourth Morrilton Road cut. The sharp base of the upper sandstone interval is represented by distinct break in the profile marked by the arrow at 22.4m.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70

from 4m onward up to 17m with simultaneous increase in thickness of the sandstone

beds. This unit is heavily burrowed.

Unit 2 (17-19.2m) is characterized by tabular sandstone. Very thin partings are

present in between thicker beds giving them an apparent amalgamated appearance.

Figure 3.21 Characteristic heterolithic bedding from Unit 1 represented by interbeds of shale-siltstone and sandstones.

Internally, the sandstones are ripple cross-laminated. This unit is also heavily

burrowed.

Unit 3 (19.2-30m) is characterized by lenticular sandstone bodies as opposed to

tabular beds which were abundant in the unit below. Fine-scale ripple-lamination,

which was the dominant sedimentary structure in the lower part of the section, is rare

in this interval. At 24m, a very distinctive composite bed, act as a marker bed (Fig.

3.22). The lower half of this bed is heavily ripple-laminated with poorly developed

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Figure 3.22 The characteristic marker bed represented by wavy based sandstone that is heavily bored. The scale sits on the marker bed.

mud drapes. The upper half of this bed is densely bored. Boring is very characteristic

in the section at Boston Mountain in this particular interval upward.

Unit 4 (30-45m) is characterized by abundantly bored sandstones with large-

scale cross-bedding (Fig. 3.23). Molds of borings (preserved as holes) which line the

foreset beds of large-scale cross bedding are common. This unit is characterized by

relatively thick beds (averaging 25-50cm).

Unit 5 (45-5 lm) is characterized by massive, amalgamated sandstone units

with evidence of large scale bedforms. At 45m, beds are distinctly massive with

amalgamation contacts occasionally showing wedging and large-scale bedforms (at

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Figure 3.23 Large-scale cross-bedding and bedform geometry from Unit 4 at 32m in the section.

51m a large-scale bedform is evident, dimensions 2.5m wavelength by Im wave

height). Occasional borings are present.

Unit 6 (54-63m) is characterized by a non-distinctive massive bed. The very

top of the section at 63m comprises a 3m thick massive medium-grained (mL)

sandstone and is the coarsest grain size observed in the present study.

The measured gamma-ray profile from the Boston Mountain road cut is

presented in Figure 3.24. It represents a gradual coarsening-upward profile.

Interpretation: The lowermost part (0-4m) of the section, composed of

interbedded shale and siltstone, is interpreted to represent a prodelta deposit. The bulk

of the prodelta deposit is not exposed, being present below the base of the exposed

section. The gradual coarsening and thickening observed within the interval 4 to 17m

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45 Cross- bedded sandstone 40

SAS53 25

S iS S S

Interbedded siltstone- sandstone

Interbedded , shale- siltstone

- I - 1 i I 0 100 200 300 400 Gamma Count (cps)

Figure 3.24 The gamma-ray profile for the measured section at Boston Mountain represents a coarsening-upward trend.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 74

(part of Unit 1) most likely represent deposition in a delta front setting. The delta front

deposits continue into Unit 2 (17-I9.2m) and exhibit evidence of active progradation.

The presence of well preserved cross-laminations and amalgamated contacts

represents enhanced depositional activity in the upper reaches of a delta front deposit

Unit 3 (19.2-30m), composed of sandstones with lensoid bedding geometry and

greater bed-thickness, most likely represents transition into a distributary mouth bar

deposit The presence of abundant borings and occasional mud laminae are suggestive

of tidal influence. The distinctive bed at 24m, referred to as a marker horizon (see

description and Appendix A), is heavily bored and was very likely associated with

deposition during reworking of a flooding surface. Units 4 and S (30-5 Im) are

characterized by medium-grained (mL) sandstones with large scale cross-bedding and

are interpreted to be distributary channel margin deposits with some tidal influence.

Tidal influence is evident from the lining of reactivation surfaces of large-scale cross

beds related to slack periods between ebb and flood stages. Large-scale bedforms

observed in Unit 5 are also suggestive of a tidal origin for this interval. The uppermost

unit (Unit 6), characterized by massive, medium grained (mL) sandstones with

amalgamation surfaces, represents a distributary channel fill near the axial part.

3.3.7 Searcy Quarry

Description: The rocks of this quarry corresponds to the Morrowan Bloyd

Formation. It is 3 lm thick, the measured section is presented in Appendix A. This

quarry, with well exposed western and southern faces, offers an opportunity to study

the three dimensional variability of a prograding deltaic sequence. The situation is

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similar to the Freshour Quarry (see Section 3.3.1). The north and south wall represent

the strike section of the delta while the east and west walls represent dip sections.

A shale interval is present below the base of the section where the profile

measurement starts. The entire section has been divided into four parts from a

descriptive point of view.

Unit 1 (base to 6m) is composed of fine (fL) sandstone with wavy bedding.

They are approximately 10-15cms thick (maximum 40cm). The beds are characterized

by wavy, discontinuous partings with or without thin veneers of organics. Sedimentary

structures present are wavy to ripple cross-laminations. Occasional interbeds, 5- 10cm

thick (to as low as l- 2cm), are characterized by dark silty shale with very fine sand,

occurring as discontinuous pinch and swell bedding. Burrowing (identified as

Scalarituba, a locomotive burrow) is present as a fine-scale biogenic structure. Besides

Scalarituba other finer scale burrows are also present.

Unit 2 ( 6- 15m) is composed of fine (fU) sandstone beds with an average

thickness of 15-20cm. Overall thickness increases upward up to a maximum of 0.6m

with tabular bedded geometry. Thicker beds have 0.5-2cm thick siltstone partings rich

in micas and organics. Scalarituba is present and is not restricted only to bedding

planes. Beds in this interval are current ripple laminated. The upper bounding surface

is a major contact onto which lower most beds of the next upper unit (Unit 3) are seen

to downlap.

Unit 3 (15-25m) is characterized by a thickening- and coarsening-upward (fL-

fXJ) interval. The lower beds are 5-7cm thick and progressively thicken upward to 30-

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45cm at around 20-2 lm. Near 25m in the section the beds further thicken to 60-

100cm. Internally, the beds are less bioturbated and nonuniformly ripple laminated.

Organic-rich silty/shaly partings are commonly present which are less than 0.5-2cm in

thickness. From-18m to the very top of the unit beds are massive in appearance but

internally ripple cross-laminated. The scale of cross-lamination is larger than below

but lower in abundance. The top of the bedding surfaces, as observed from loose

blocks, are linguoid ripple laminated. At 18m and above burrows are practically

absent.

Unit 4 (25-3 lm) is very similar to the lower interval. Profuse wedging of beds

is present Sedimentary structures in the form of larger scale cross bedding are present.

Bed thickness is roughly 25cm or higher (maximum up to 50cm), but lateral variability

is high due to the wedging and lenticularity in beds. The channel fill above the

erosional cut in the south wall (Fig. 3.25) is 4m thick. The lower most (-1.5m) fill

is characterized by fissile, parallel laminated, shaly siltstone with intermittent thin

laminations of very fine sand. From 1.5m upward to 4m in the channel fill, 10- 12cm

thick fine-grained sandstones appear interbedded with previously described shaly

siltstone facies. Thickness of sandstone beds progressively increases toward the top of

the unit to -30cm with shaly/siltstone interbeds progressively decreasing upward. The

channel base is characterized by a major erosional scour with ~4m erosional relief

within a lateral span of 60m, as observed from the southern face. The bed below the

sedimentary fill has an abundance of rip-up clasts.

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Figure 3.25 The conspicuous channel cut high up on the south wall of the quarry. The channel fill is 4m thick.

Interpretation: The overall section at Searcy Quarry represents a progradational

deltaic cycle with a discontinuity at the top of Unit 2. Units 1 and 2 represent delta

front deposits characterized by wavy, discontinuous interbeds of siltstone and very tine

sandstone. Formation Micro Scanner (FMS) logging by Schlumberger (C. Stelting,

personal communication, 1997) at this location revealed very thin-bedded interbeds of

siltstone and shale below the quarry floor. This subsurface package represents prodelta

deposits corresponding to the exposed delta front (Unit 1 and 2). Units 3 and 4 overlie

a discontinuity surface, observable in outcrop, which very likely developed in response

to an updip channel avulsion. These two units (3 and 4), are composed of tabular and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lenticular bedded fine sandstone, respectively, and are distributary mouth bar deposits

related to the inferred updip avulsion episode. The implied nearshore high-energy

conditions of deposition is supported by the presence of large-scale cross bedding and

Ophiomorpha burrows in the Unit 4 sandstones. The genetically related distributary

channel is exposed high up on the southern face of the quarry, resting unconformably

over its own distributary mouth bar deposits. This represents the extreme

progradational nature of this delta cycle. The channel base has an erosional relief of

4m over a lateral distance of 60m. The channel, comprised of a passive fill of

predominantly fissile shale, marks the abandonment of this delta cycle.

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ADDITIONAL APPROACHES IN THE CHARACTERIZATION OF DELTAIC DEPOSITS FROM VARIOUS SUBENVIRONMENTS

In addition to the field-based observations and inferences, numerous laboratory

techniques were undertaken to demonstrate the variations in physical, chemical, and

biological processes from various deltaic subenvironments. The multi-pronged

approach includes ichnology, palynology, clay mineralogy, petrology, and sedimentary

fabric determination using rock magnetics. The primary observations and inferences

that led to the determination of systematic variations between deposits from numerous

deltaic subenvironments are presented in this chapter.

4.1 Trace Fossils

4.1.1 Introduction

Trace fossils (or ichnofossils) are biogenic sedimentary structures and include

tracks, trails, burrows, borings, fecal pellets and other traces made by organisms. They

provide valuable informations regarding paleoenvironment and ecology. Three

fundamental sedimentologic processes interact to produce sedimentary deposits which

are sedimentation, bioturbation (sediment mixing), and erosion (Ekdale et al., 1984a).

The physical character of any deposit depends on the balance among the rates of these

three processes. When the sedimentation rate exceeds that of bioturbation, primary

stratification dominate. When the reverse is true, primary stratification is obscured (or

obliterated) and biogenic sedimentary structures predominate.

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Pennsylvanian trace fossils in the Ouachita Mountains are precious because

many of these are the only representations of soft bodied animals otherwise virtually

unknown in the fossil record (Chamberlain, 1978). Conostichus, the burrow of a

Pennsylvanian sea anemone, is an example. Moreover, trace fossils provide ethologic

and ecologic information about benthic organisms that could not be derived from

either the soft or the hard parts of the animals. In an otherwise unfossiliferous rocks as

the ones investigated, importance of trace fossils proved to be particularly useful.

4.1.2 Identification

All the outcrops of the study area were inspected for trace fossils which were

identified at the generic level (Hantzschel, 1975; Chamberlain, 1971a; Bromley, 1996)

and placed in their respective ethologic groups (described by Seilacher, 1953; Ekdale

et al., 1984a; Frey and Pemberton, 1985; Bromley, 1996). Identities of corresponding

trace making organisms were inferred (Table 4.1).

Trace fossils are most abundant and diverse in the Freshour Quarry, especially

on a bedding plane exposed in the southward dipping north wall of the quarry (see Fig.

3.2). Traces from other outcrops, although not as common, were also studied.

Two broad groups of traces are present in the Freshour Quarry: vertical to

nearly vertical burrows and horizontal traces. The ichnogenera are described below.

A. Vertical Burrows: 1) Bergaueria (Prantl, 1946): These are vertical to nearly vertical

burrows, circular to elliptical in plan, cylindrical to hemispherical in cross section with

round bases (Fig. 4.1). They have smooth unomamented walls with ‘U-shaped’ cones

with subequal length and diameter. Most of them are tilled up with a sandy plug (in

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some cases iron rich, as evident from the brown coloration or rim around the plug).

These are classified into four size groups based on their maximum diameter in plan

views as Al, A2, A3 and A4 with 2.5-4,1.5-2.5,1-13, and < 1cm in diameter,

respectively.

Figure 4.1 Circular, branching and almond shaped epichnial trace fossils (marked by arrow heads) identified as Bergaueria, Thallassinoides, and Lockeia, respectively.

2) Conostichus (Lesquereux, 1876): Morphologically similar to Bergaueria

burrows, are common in the Morrilton roadcut sections. The difference between

Bergaueria and Conostichus is the ornamented wall of Conostichus in the form of

transverse constrictions and longitudinal ridges and furrows. Otherwise these have

similar conical to subconical sandstone structures with a more ‘V-shaped’ cones for

Conostichus.

Bergaueria and Conostichus are formed by activities of actinian anemones

(Chamberlain, 1978). Ethologically they represent resting traces or cubichnia

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(Seilacher, 1953). It is easy to confuse other types of burrows with Conostichus such

as worm burrows. Branson (1960) pointed out that life habits of Conostichus are much

like those of marine worms. However, no living or fossil worm is known to have the

duodecimal symmetry of Conostichus (Chamberlain, 1971a). Worm burrows can be

layer parallel to near parallel. Common vertical burrows of worms are generally U-

shaped tubes (Bromley, 1996).

B. Horizontal traces: The horizontal traces of the Freshour Quarry are

classified into the following types depending on their shapes and sizes. Each one is

assigned a letter for easier description purposes. The most common ones are shown in

Figure 4.2.

B2B1 B3

B4 B5 B7

B8 Scale 1 cm

Figure 4.2 Sketches showing the most common horizontal traces on the delta front bedding surface namely Bl, B2, B3, B4, B5, B7, and B 8 .

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1) Gyrochorte (Heer, 1965): These are designated as B1 (Fig. 4.3). Typical

morphology of an epichnial (traces of the upper surface of the casting medium)

specimen consists of twin, parallel, straight to tortuous, hemi-cylindrical lobes,

tangentially adjoined creating a narrow median furrow. Better preserved specimens

reveal the characteristic oblique biserial transverse lobe segmentation (Fig. 4.3). They

are commonly 2-10cm long and 3-6mm wide. These were probably made by

gastropods and identified as “snail trails” (Howard, 1972). These represent locomotion

or crawling traces or repichniaof Seilacher (1953).

Figure 4.3 Gyrochorte as seen on delta front bedding plane from the Freshour Quarry.

2) Lockeia (James, 1879): These are designated as B2 (Fig. 4.2). Typically

these are 2-3 mm wide, short (1-1.5cm), straight to curved (not winding), stout, and

elevated traces tapering to sharp points at both ends. Seilacher (1953b) recognized

small almond shaped hyporeliefs (reliefs at the base of the bed) as a result of a small

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deeply burrowing bivalve which commonly presses sand into the underlying mud with

its foot or shell. Similar abundances of Lockeia have been reported from distributary

mouth bar deposits of Pennsylvanian age from Indiana (Archer and Maples, 1984), and

Carboniferous of England (Collinson and Banks, 1975). These represent resting traces

or cubichnia of Seilacher (1953).

3) Curvolithus (Fritsch, 1908): These are designated as B3 and are long (7-

8 cm, up to 20cm), very narrow (width l- 2mm), smoothly curved to wavy, elevated

(epichnia) or depressed (hypichnia) forms. They are interpreted as the mold of the base

of an overlying trilobes burrow of Curvolithus type (Heinberg, 1970; Chamberlain,

1971a); these seem to be crawling trace or repichnia of shallow water organism.

4) Thallassinoides (Ehrenberg, 1944): These are designated as B4 (Fig. 4.1)

and are long (7-20cm), moderately narrow (1-1.5cm), irregularly cylindrical,

occasionally winding, traces preserved both as depressions (hyporelief) and elevations

(epirelief). These are smooth-walled burrow systems with common Y- to T-shaped

branching (see Figs. 4.1 and 4.2). These very thinly lined to essentially unlined

horizontal burrow systems are characteristic of fine grained coherent substrates, in

which wall reinforcement is unnecessary. They generally are regarded as dwelling or

domichnia and/or feeding or fodinichnia burrows of decapod crustaceans.

5) Planolites (Nicholson, 1873): These are designated as B5 (Fig. 4.2) and are

short (2-4cm, occasionally up to 8 cm), elliptical in cross sections, and are also present

in the Mornlton road cut sections. These are characteristically tubular, unlined, rarely

branched, straight to tortuous, smooth walled burrows. Fillings are essentially

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structureless, differing in lithology from the host rock. Ethologicaily these are regarded

as feeding structures or fodinichnia (Frey and Pemberton, 1985) and as grazing traces

or pascichnia (Ekdale, 1985; Bromley, 1996) formed by mobile endobionts.

6) Paleophycus (Hall, 1847): These are designated as B 6 (Fig. 4.2) and are

bifurcating tuning forks with the fork length being 1.5cm (total length > 2cm). They

characteristically are distinctly lined, essentially cylindrical, predominantly horizontal

burrows with the sediment fill typically of same lithology and texture as the host

stratum. Wall linings are smooth. These are interpreted as dwelling structures or

domichnia. This ichnogenus is also found in distributary mouth bar deposits of the

Freshour and Searcy Quarry.

7) Rusophycus (Hall, 1852): These are designated as B7 (Fig. 4.2) and are short

(lcm long), narrow (0.25cm wide), stout, and dented that have bigger wheat-grain like

appearance. These are possible Rusophycus representing resting traces or cubichnia of

arthropods. Linck (1942) restricted its occurrence in brackish water. Similar traces in

marine Paleozoic beds are probably made by trilobites.

8 ) Gordia (Emmons, 1844): These are designated as B 8 (Fig. 4.2) and are

smooth trails of uniform thickness or mud lined grooves which has a tendency for

looping. Possibly these are made by some vagile deposit feeders such as worms and

represent crawling traces or repichnia.

Besides the Freshour Quarry, other outcrops were also inspected for trace

fossils. Abundant Conostichus were documented associated with alternating

lithologies in the third and the fourth Morrilton (3MOR and 4MOR) roadcut sections

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(Fig. 4.4). Planolites are also common mainly associated with shales. In the Searcy

Quarry a horizontal chevron trail, was observed associated with the delta front and the

distributary mouth bar deposits. These are identified as Scalarituba (Weller, 1899, Fig.

4.5). They represent crawling (repichnia) to grazing (pascichnia) traces of some kind

of worm. Planolites are common in finer-grained parts of the section. Near the top of

the distributary mouth bar deposits Ophiomorpha (Lundgren, 1891) burrows were

observed. These are distinctly lined with agglutinated pelletoidal sediment. Infill is

generally structureless and of similar composition as the host rock. It represents

dwelling (domichnia) burrows of decapod crustaceans, especially some species of

shrimps. The most common burrows in the Boston Mountain section are questionable

Paleophycus (?). Table 4.1 lists all the traces found in different outcrops.

Figure 4.4 Conostichus as seen from the 4MOR section at Morrilton, marked by arrows. i

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% 7 - v. i '• "v £-~ \

Figure 4.5 Scalarituba from the upper bedding surface of a sandstone block (marked by arrows) at Searcy Quarry.

Table 4.1 Identified trace fossils, their probable trace makers with behavioral traits, and association with specific environments as documented in the present study. Locations Name Genera Trace maker Ethology Environment Freshour A1-A4 Bergaueria Actinian Domichnia Delta front Quarry anemones Cubichnia Freshour B1 Gyrochorte Gastropods Repichnia Delta front Quarry Freshour B2 Lockeia Bivalve Cubichnia Delta front Freshour B3 Curvolithus (?) Repichnia Delta front Freshour, B4 Thallassinoides Decapod Domichnia Delta front Morrilton crustacean Fodinichnia Freshour, BS Planolites Mobile Fodinichnia/ Delta front Morrilton endobiont Pascichnia BayfUl Freshour, B6 Paleophycus Mobile Domichnia Delta front Boston endobiont Distributary Mountain mouth bar Freshour B7 Rusophycus (?) Arthropod Cubichnia Delta front Freshour B8 Gordia Worm Repichnia Delta front Morrilton Conostichus Anemone Cubichnia Bay fill Searcy Scalarituba Worm Repichnia Delta front Quarry Pascichnia Searcy Ophiomorpha Decapod Domichnia Distributary Quarry crustacean mouth bar

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4.1.3 Interpretation

Individual trace making organisms have evolved through time while their basic

behavior remained essentially unchanged. For example, deposit feeders are preadapted

to quiescent environments with abundant nutrients and do not like turbid-water

settings. The opposite is true for suspension feeders. Similarly locomotion traces

(repichnia) are preserved only under a strict set of environmental conditions. This

ability to discern behavioral trends of benthic organisms greatly facilitates

environmental interpretations from the rock record. Significant information regarding

depositional environments, that were extracted from ichnofauna, include

sedimentation history in terms of rates of deposition, identification of ichnofacies and

their corresponding depth of occurrence.

Based on several studies Miller and Knox (1980) proposed a model for the

distribution of trace fossils in Pennsylvanian deltaic deposits from Fentress-Rockcastle

sequence in Tennessee. The model is shown in Figure 4.6. Results from the present

study shows a remarkable match in terms of identification of different ichnogenera as

well as their occurrences in particular subenvironments. Some facies-restricted

ichnofauna (e.g., Conostichus), which are only present in the interdistributary bays, has

also been documented in thin-bedded sandstones interbedded with shales from other

Pennsylvanian deposits (Chamberlain, 1978). Proximity to the shoreline with implied

agitation, oxygenetion, bathymetry, and substrate were probably important factors of

distribution. The trace making organisms (anemones) must have preferred a consistent

muddy substrate but also desired the sand texture.

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DELTA PLAIN DISTRI BUTARY DELTA MOUTH FRONT BAR PRO- I DELTA BASIN

Asteriacites ...... Bargaueria ~ Conostichus ■■ ■ Helminthopsis ...... Planolites — — ...... -- ...... : ...... Rhizocorallium...... Scalarituba...... Zoophycos ...... 1------1------1------1------y

Figure 4.6 Distribution of trace fossils in Fentress-Rockcastle sequence and in rocks of deltaic origin described in the literature (dotted pattern; modified from Miller and Knox, 1980). Ranges of occurrences of ichnogenera from the present study (solid line) are also depicted.

According to Rhoads (1975), one of the most important variables controlling

the distribution of bottom-dwelling organisms is the nature of substrate, in terms of

grain size, organic content, and sedimentation rate. Slow continuous deposition results

in complete bioturbation of the sediment with no evidence of primary lamination. In

restricted basins the anaerobic condition restricts bioturbation with common

preservation of original stratification. Rapid continuous deposition (in prograding

beaches, deltas) prohibits biogenic activity. Presence of varieties of traces associated

with the delta front deposits from the present study, hence implies a relatively lower

rate of sedimentation of these deposits than the overlying distributary mouth bars.

Upper bedding surfaces in distributary mouth bar deposits in comparison to that of the

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delta fronts show minimal biogenic activity suggesting longer hiatal periods

represented by delta front bedding planes. Traces tend to be rare in sequences

associated with high sedimentation rates showing mainly fiigichnia (escape burrows),

cubichnia (resting traces), and domichnia (dwelling burrows) with sturdy agglutinated

walls (Ekdale et al, 1984a). Lined burrows, namely Paleophycus, and strong walled

Ophiomorpha present in the distributary mouth bar deposits of the Freshour Quarry

and Searcy Quarry, respectively, indicate high rate of sedimentation associated with

these deposits.

The Cruziana and Skolithos ichnofacies of Seilacher (1953) are very intimately

associated in the Atoka Formation in the Arkoma Basin indicating shallow marine to

intertidal, including interdistributary flats (Chamberlain, 1978b). The trace fossils

show a broad range of form and behavioral habit, including crawling trails of

gastropods ( Gyrochorte), resting traces of bivalves ( Lockeia), and resting traces of sea

anemones ( Conostichus). The predominance of Conostichus, Lockeia, Gyrochorte,

Thallassinoides, occasional Ophiomorpha and other gastropod-like trails provide

strong evidence of an estuarine to nearshore molluscan fauna. In other areas it ranges

from nearshore through offshore bar.

The Zoophycos ichnofacies of Seilacher (1953) in the frontal Ouachitas occurs

in thickly bedded massive sandstones displaying high-angle cross bedding. The

assemblage has a low diversity consisting mainly of Scalarituba, Planolites and

Paleophycus. The Zoophycos assemblage generally indicates an intermediate depth at

the transition from shallow shelf to deeper basin. The presence of Zoophycos in the

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Bloyd and Atoka Formation gives evidence that Zoophycos producers were capable of

inhabiting very shallow water environments during the Paleozoic (Osgood and Szmuc,

1972; Miller, 1979; Miller and Johnson, 1981). At this point a more precise

bathymetry, other than below wave base and above turbidite sedimentation, seems to

be difficult to determine.

Dam (1990b) from his study in shallow marine deposits of the lower Jurassic

Neill Klinter Formation grouped 34 ichnotaxa in a series of 11 ichnocoenoses. He

interpreted them in the light of their trophic and ethological properties and found a

strong correlation with sedimentary environments. The distribution reflects changes in

factors controlled by water depth, bottom water oxygenetion and environmental

stability. His Curvolithus and Planolites association fairly matches with the association

of ichnogenera from the present study characterized by Gyrochorte, Thallassinoides,

Planolites, Paleophycus, Curvolithus, Ophiomorpha, and Gordia. This assemblage

indicates aerated, low to medium energy distal delta setting between fair weather and

storm weather wave bases (Bromley, 1996).

4.2 Palynology

4.2.1 Introduction

Palynology is the study of organic walled microfossils which includes kerogen

(i.e., acid-resistant organic matter) and palynomorphs (e.g., spores, pollen, acritarchs,

etc.). Paleopalynology is important for sedimentological interpretations and

palynofloras have been a part of the geologic record for more than 1 billion years.

During the last twenty years or so, there has been an upsurge in the use of kerogen

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(sedimentary organic matter) for the interpretation of depositional environments.

Palynomorphs can be sensitive indicators of sedimentary processes and sediment

provenance. Linked with this is the value of kerogen analysis for identifying

hydrocarbon source rocks (Batten, 1996). Characteristics of the vegetation from which

sedimentary organic matter is derived can be indicated by the spores and pollen

occurring in the deposits.

‘Facies’ is a body of sediments of a specified character that represents a

particular environment of deposition. Prefixes are commonly added to limit or clarify

the sense in which it is used. ‘Palynofacies’ are facies based on the distribution of

organic matter (e.g., woody particles and palynomorphs) occurring in a sedimentary

deposit The word palynofacies encompass the total complement of kerogen recovered

from sedimentary rocks by palynological processing techniques (Batten, 1996).

The present study concentrates on phytoclasts (fragmented plant material),

spores, and acritarchs. The occurrence and composition of organic matter in

sedimentary rocks reflect an enormous number of variables that influence the

terrestrial or aquatic environments in which the organics are generated, their transport

to the depositional site, and alteration before and/or after deposition. Sedimentary

environments differ in terms of their physical, chemical, and biological attributes and

the way they affect kerogen. Each organic particle is affected (similar to the other

sedimentary particles) by the dynamic forces within the depositional environments

(transportation and deposition of both autochthonous and allochthonous components,

reworking and recycling). In addition, each clast is affected by chemical changes

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during burial. These chemical changes may be caused by physical (especially thermal),

biological (biodegradational) or chemical (pore water interaction) activity (Hart et al.,

1994). The end result is that a specific environment may contain a kerogen spectrum

that reflects the origin and diagenetic history (in terms of thermal maturation and

burial diagenesis) of the organic matter in the sediment (Hart et al., 1994).

The purpose of the present study is to test whether there is a systematic pattern

of kerogen distribution in the subenvironments of a Pennsylvanian delta. Palynofacies

study is aimed at an independent evaluation of different subenvironments with which

to test the results from sedimentologic studies of different depositional

subenvironments of the Morrowan and Atokan delta. Moreover, the existing literature

on some of the shaly sections (e.g., in 3MOR and 4MOR) studied, suggests a marine

origin without any evidence. This study was aimed at testing the validity of the

previous interpretations of the shaly sections.

4.2.2 Methods

1) Kerogen Processing technique: Hand specimens collected during the field

work in Arkansas were used. The Prolabo Microwave Digestion Unit of the Center for

Excellence in Palynology (CENEX), Louisiana State University, was used to process

the samples. The computer controlled digestion unit can be programmed to add

reagents (e.g., HF, HCL, HN03) as needed and to heat samples by means of

microwave radiation to provide complete digestion of the crushed sedimentary rock

samples. Noncalcareous samples, siltstones and shales, were digested in concentrated

HF. Resistant samples, showing incomplete digestion after the first step, were then

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. treated with hot HNO 3. The resulting digested residue was rinsed and centrifuged

several times until complete neutrality was attained and sieved through a 10 pm sieve

with an ultrasonic probe. Calcareous samples were treated with hot HCL followed by

HF for complete digestion. Some samples required a heavy liquid separation to

remove undigested minerals after sieving. Two double mounted slides of the > 10 pm

residue were made per sample. To make the double mounted slides a few drops of

dispersant, poly-vinyl alcohol (PVA) were added to a coverslip. A drop of sieved

residue was added, mixed with the PVA using a toothpick, spread well and dried. A

drop of mounting adhesive (“Cover Bond”) was placed on a slide and the coverslip

was turned over with the sample side down and attached to the slide.

2) Kerogen classification/identification: The degree of degradation

(preservational stage) of organic particles is an important parameter for characterizing

depositional environment. For the present study, classification scheme by Hart (1986)

for preservational stages of phytoclasts was used. Hart and Hardy (in preparation)

redefined the key criteria for each of the degradation stages in order to facilitate the

systematic identification of kerogen particles.

Four phytoclast degradation stages were defined: well preserved, infested,

amorphous structured, and amorphous non-structured. The key criteria of each

degradation stage are shown in Table 4.2. This classification system has already been

applied in various depositional environments of the modem Mahakam Delta with

successful results (Hardy and Wrenn, 1996).

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Table 4.2 Key criteria of phytoclast degradation stage determination.

Preservation stage of Criterion 1 Criterion 2 phytoclasts - Well preserved Transverse wall - No gelification - Infested preserved - Gelification - Amorphous structured Transverse wall not - Fibrous cellular structure - Amorphous non preserved - No structures structured

The different stages of phytoclast degradation are briefly defined below.

1. Well preserved phytoclasts are angular in outline with partial to complete

preservation of transverse walls. Gelification of cellular structures is rare. Fungal

attack and bacterial pitting on cell walls are not common.

2. Infested phytoclasts are angular in outline with transverse walls partially preserved

(Plate 1, Appendix D). Cellular gelification is abundant but mostly incomplete.

Occasional bacterial pitting and fungal piercing is indicated by cellular wall disruption.

3. Amorphous structured phytoclasts have a partially preserved structural framework

with remnants of the original fibrous cell structure and they lack transverse walls

(Plate 1, Appendix D). Cellular gelification ranges from moderate (ghost structures

present) to complete. Evidence of fungal and bacterial attack are usually destroyed

during prolonged degradation.

4. Amorphous non-structured phytoclasts lack a structural framework and fibrous

character (Plate 1, Appendix D). Cellular gelification is complete.

Categories of organic remains that were identified and counted during the

palynofacies analysis are listed in Table 4.3.

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Table 4.3 Categories of kerogen particles identified.

Name Preservation stage Description Phytoclast Infested See above Amorphous structured Amorphous non-structured Opaques Black to the edges, glowing surface Spores Palynotnorph (Organic walled microfossils) Acritarch Palynotnorph Amorphous Undifferentiated Structureless, heterogeneous, fluffy Organic material with diffused edges. Matter

3) Counting Technique: At least 300 kerogen particles were counted on two

double mounted slides for each sample. The abundance of the distinctive organic

particles is expressed as percentage within a sample (Table 4.4).

4.2.3 Results

Palynologic samples were studied from interdistributary bay fill, passive

channel fill, distributary mouth bar, delta front, and prodeltaic deposits. Kerogen

assemblages from all subenvironments of the Atokan delta contain significant amounts

of amorphous non-structured phytoclasts Figure 4.7 and Table 4.4.

Most spore assemblages from various subenvironments were dominated by

Lycospora spp., but occasional specimens of Densosporites sp., Florinites sp., and

Laevigatosporites spp. also were observed (Plate 1, Appendix D).

The kerogen assemblages can be divided into three palynofacies PI, P2 and P3

(Table 4.5). PI and P2 are further subdivided into two subpalynofacies namely Pla,

Plb, P2a, and P2b.

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Palynofacies PI: It is characterized by 80% or more amorphous non-structured

and opaque kerogen. Subpalynofacies P la is amorphous non-structured dominated

Table 4.4 Result of the point counting shown in percentages. Am. Str. and Am.-Nstr. represent Amorphous Structured and Amorphous Non-structured Phytoclasts respectively. Sample # Lithology Infested A m . S tr. Am . O paque Spores •Fluffy’ N -Str. P articles M aterial 3M O R95a Shale 0.7 13 30 5 3 3 2.7 0 3M OR95b Shale 2.3 17.3 7 9 3 3 1 3 1.7 4M O R 95a Shale 1 3 16.8 76.6 0.98 3.6 0.65 4M O R95b Shale 0.65 20 615 16.4 1 3 0 4M O R 96a Shale 0 11.9 413 45.8 0 3 2 0 SFH95d Shale 30 33.2 28 3 3 5 5 0 SFH 95b SUty 4 9 13.9 4 1 3 35.7 4 5 0 Shale SQ8 Shale 0.33 14.4 72.9 12 0 0 3 2 SQ9 Siltstone 1.6 14.7 7 6 5 6.8 0 0 3 2 EFH3 Siltstone 9 273 32 25 4.6 1.6 EFHSa S iltstone 8.4 32.6 465 11.6 0.9 0 OFH96a Shale 2.3 28.9 40.4 14.8 1 3 5 0 BM a.96 S iltstone 0 9 5 56 34 0 0 NFH95c Shale 1.9 17.4 44.6 29 6.8 0 2MOR95a Shale 033 113 79 3 3 6 0 3M O R f C arbonate 7 5 205 60.7 6.7 0.3 4 3 hash

Table 4.5 Palynofacies identified from the present study and list of representative samples covering different subenvironments. ‘IDBF’ and ‘DMB’ indicate interdistributary bay fill and distributary mouth bar deposits, respectively Palyno Sub Environment Sam ples C haracter -facies palyno facies PI P la IDBF 3MOR95b, 4MOR95a, Amorphous non- Passive Channel fill 4MOR95b, SQ8, SQ9, structured dominated P rodelta 2M O R95a (> 6 0 % ) P lb ID B F 3MOR95a, 4MOR96a, Subequal amorphous D elta front BMa96, NFH95c, non-structured and P rodelta SFH 95b opaques P2 P 2a IDBF SFH95d Infested dominated (> 30% ) P2b DMB EFH3, EFHSa, OFH96a Amorphous structured Delta front and spore dominated (> 30% ) P3 ID BF 3MORf “Fluffy” material present

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(>60%) while subpalynofacies Plb shows subequal proportion of amorphous non-

structured and opaques (- 30% and more).

Palynofacies P2: It contains - 35% or more infested and amorphous structured

kerogen taken together. Subpalynofacies P2a shows dominance of infested kerogen

while P2b is amorphous structured and spore dominated.

Palynofacies P3: It is dominated by “fluffy” material (Amorphous Organic

Matter, AOM) and remains of unknown organisms.

4.2.4 Discussion

Almost all the samples processed show a significant amount of amorphous

non-structured kerogen (30-80%). Thermal alteration (catagenesis and metamorphism)

of organic matter due to the increasing geothermal gradient associated with increasing

depth of burial, causes organic matter to change color in a predictable sequence.

100%

70% - B "fluffy" material B spore 60% ■■ ■ opaque BAm. non-structured 50% - BAm. structured Ano, □ Infested

Figure 4.7 Distribution of different phytoclasts and spores from the present study.

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Gelification refers in part to the formation of amorphous humic gels (Batten, 1996).

According to him, these substances may be produced during the complex process of

decay and degradation of plant tissues that can result in either the preservation of

original features or homogenization of structure and thus produce amorphous non-

structured kerogen.

Three major palynofacies were recognized from the present study, PI, P2, and

P3.

Palynofacies PI is characterized by ‘degraded’ kerogen and contains more than

80% amorphous non-structured and opaque particles taken together. This degraded

assemblage reflects long distance of transport and deposition of allochthonous material

far from the source area. In addition, spores were common (< 5% in most samples)

irrespective of lithology e.g., shale, silty shale, or siltstone and this too may reflect

long distance of transport prior to deposition. The presence of only the most resistant

kerogen types (e.g., lignin, woody tissue and opaque particles) and the presence of

large, heavy spores indicates deposition nearshore or in the fluvial system. Two

subpalynofacies of PI are distinguished - amorphous non-structured dominated (Pla)

versus subequal proportions of amorphous non-structured and opaques (> 30% - Plb).

Most of the samples representing this subpalynofacies (Pla) come from

interdistributary bay fill clay sediments, and passive channel fill shale and siltstone.

Similar kerogen was found by Beckman (1985) who showed that fresh and brackish

marshes of the Atchafalaya Delta are characterized by a high abundance of amorphous

non-structured materials.

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The dominance of opaques in P lb and relatively low abundance of amorphous

non-structured is intriguing. Opaque particles, particularly charcoal, are typically

porous, buoyant and are generally capable of prolonged flotation, especially in sea

water because of its higher density (Harris, 1958). They are progressively more

physically comminuted with transportation and are resistant to further degradation.

The association of a high relative abundance of ‘black wood’ or ‘opaque phytoclasts’

with high energy ancient environments has been documented by many studies (e.g.,

Batten, 1973a, 1982b; Barnard and Cooper, 1981; Nichols and Jones, 1992; Williams,

1992). High energy environments, such as channels, levees, proximal delta fronts,

estuarine, and near shore marine situations, contain abundant opaque particles. Hart et

al. (1994) found abundant opaque particles in channel, levee, and distributary mouth

bar deposits of the modem Mississippi Delta. The opaque dominated assemblages in

some of the samples from the present study must indicate similar association with

relatively coarse-grained, high energy facies, such as been found by Fisher (1980),

Nagy et al. (1984), and Parry et al. (1981). This supports the interpretation of a delta

front origin for the opaque dominated samples. Some of the opaque dominated

samples from the present study come from upper reaches of interdistributary bays.

Habib and Miller (1989) found a similar occurrence of an opaque dominated kerogen

assemblage characterizing subaerial delta plain and interdistributary coastal facies. The

predominance of opaque particles from distal sediments, such as prodelta from the

present study, may be negatively correlated with sediment accumulation rate

presumably due to reduced supply (decreased runoff and redeposition) of fresh non-

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opaque phytoclasts and the selective preservation of the refractory opaque material

(Hellem et al., 1986). The absence of opaque dominated assemblage in the distributary

mouth bars from this study can be explained by the fact that in humid climates,

nearshore settings close to active fluvio-deltaic sources result dilution of opaque

content due to an overall higher supply of larger and fresh phytoclasts (Hellem et al.,

1986). At the same time, dilution of overall volume of kerogen in association with

abundant elastics due to a very high rate of sediment influx, might also have played a

significant role.

Palynofacies. P2. consists of ‘less degraded’ kerogen, more than 40% of which

is infested and amorphous-structured phytoclasts combined. This may reflect fresh

input of organic matter flushed into the system. Two subpalynofacies were

distinguished - subpalynofacies P2a is dominated by infested (> 30%) phytoclasts with

abundant pyrite, while the other (P2b) is dominated amorphous-structured phytoclasts

and spores. Subpalynofacies P2a is characterized by sample SFH 95d which represents

an isolated interdistributary bay. Pyrite formation requires local reducing conditions

with occasional marine influence bringing in sulphur from nearby open ocean. In

conditions of lowered oxygen levels, consumption of organic matter by metazoans is

reduced or eliminated. Denitrification by anaerobic bacteria takes place causing

degradation (Batten, 1996; Tyson, 1995). One of the products of sulphate reduction is

hydrogen sulphide (H 2S) which may react with iron to form pyrite especially during

slow deposition in anoxic conditions (Berner, 1978,1981, and 1984). Pyrite is

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typically most abundant in marine and brackish peats (Cohen et al., 1984; Batten,

1985a; Lin and Morse, 1991; Brown and Cohen, 1994).

Subpalynofacies P2b shows a dominance of amorphous-structured phytoclasts

and spores. Input of organic matter and spores which probably underwent less

transportation and quick deposition (suggested by their good preservational stage), and

the absence of pyrite in this palynofacies supports the interpretation of these rocks

forming in distributary mouth bar and delta front settings. Most of the spores present

are Lycospora spp., with a few specimens of Laevigatosporites spp., Densosporites

spp. and Florinites. Lycospora spp. and Laevigatosporites spp. are miospores

associated with Carboniferous peat-forming plant communities (coal swamps).

Lycospora were produced by Lycophyte trees (e.g., Lepidostrobus growing in swamps)

comprising the swamp forest in the delta plain and the flood plains. Laevigatosporites

were produced by Calamitales, Sphenopsides and ferns which were pioneering plants

in the swamps. Densosporites sp. are spores of herbaceous Lycopodes, which grew in

brackish water marshes. Florinites sp. were produced by Cordaites and Conifers. The

dominance of Lycospora spp. implies derivation from a swamp forest growing on a

delta plain composed of Lycophyte and ferns. Similar development of lycopod,

cordaitean, and sphenopsid floras in the Illinois basin during the Atokan time was

explained as proximity to marine influences (Phillips et al., 1984). The presence of

pyrite and the spores discussed above indicate a quiet bay environment, isolated from

significant siliciclastic input but with brackish/saline water and surrounded by delta

plain swamp. The selective presence of spores in the delta front (OFHa) and

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distributary mouth bar deposits (EFH3 and EFHSa) indicates sorting and concentration

of the spores in shaly samples. Similar spore sorting has been reported by Tschudy

(1969) from Lake Maracaibo sediments where larger trilete fem spores are mainly

restricted to the vicinities of river mouths. Such trends are partly a reflection of the

preferred humid habitat of most pteridophyte plants, but seems due largely to more

limited spore transport. The commonest spores found from this palynofacies are the

Lycospora with thin equatorial structures (lighter). This spore composes the main

miospore assemblage. In ancient sediments, Hughes and Moody-Stuart (1967) have

noted that proximity to deltaic source areas is indicated by the presence of and relative

abundance of large spores, spore tetrads, spore masses and megaspores. This also was

observed by Davies et al.(1991) in Early Carboniferous sediments. Subpalynofacies

(P2b) is not as well preserved as P2a and does not show evidence of pyrite, which may

indicate more transportation but not for long distance.

Palynofacies P3 most probably exhibits a marine influence. The fluffy material

may be remains of marine organic matter (AOM). Remains of some organisms are also

present in the sample. The absence of acritarchs may be indicative of non-open marine

situation. However, they were not expected in profusion because they were very low in

abundance during middle Pennsylvanian and preferred quiet water settings where

shales accumulated (D. Atta-Peters, personal communication, 1997). In their study

from Carboniferous marine transgressions, Van de Laar and Fermont (1990) did not

find any acritarchs or other marine indicators in the palynological samples. However,

marine macrofauna like crinoids and Goniatites were found. The sample representing

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the marine influenced palynofacies from the present study also contains a prolific

marine macrofauna, including crinoid columnals, bryozoa, phylloid algae, and

articulate brachiopods. The presence of amorphous algal material can be indicative of

marine deposits.

4.2.5 Conclusions

Results from the palynofacies analysis reinforces some of the interpretations of

subenvironments in the Pennsylvanian deltas. The dominance of amorphous non-

structured kerogen in almost all the lithofacies indicates long transport and deposition

far from the source area. This supports the interpretation of the delta being fluvially-

dominated (see Chapter 2). Most of the previous literature regarding the study area

suggests a marine origin for all the shaly sections around Morrilton. Sedimentological

data indicates a brackish water origin for all of the shaly sections. Palynological study

further corroborates that interpretation. The carbonate hash deposit (3MORf) was

formed as a ‘overwash ’ bed from a nearby shallow marine environment by storm

surges. The presence of AOM material, which probably indicates a marine algal origin,

also suggests a allochthonous origin for this bed. The dominance of Lycospora spp. in

the palynomorph assemblage supports the interpretation that the lower deltaic plain

was swamp dominated. Though coal occurs in none of the outcrops from the present

study, abundant Carboniferous coal-forming swampy vegetation surrounding the delta

plain supports the interpretation of these deposits being formed during a transgression

or as part of a transgressive system tract (see Chapter 5).

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4.3 Clay Diagenesis

4.3.1 Introduction

The interpretation of the geologic history of sedimentary rocks relies on

techniques developed primarily for coarse-grained materials (mainly sandstones), the

essential aspects being mineralogy, grain size, and spatial distribution of minerals.

Finer grained rocks (e.g. shales) have generally been neglected because of lower

resolution and depth of Held of optical microscopes to identify clay minerals and to

reveal the details of clay particle associations. This shortcoming has been overcome

with the introduction of electron microbeam devices such as the scanning electron

microscope (SEM), and the electron microprobe (Ferrell and Carpenter, 1990).

The depositional environment has a strong control on the early diagenesis of

sedimentary rocks and varies with the intrinsic physical parameters of the rocks, such

as grainsize and sorting (Stonecipher and May, 1990). DiMarco et al. (1986), while

working on the Cubit’s Gap crevasse splay subdelta in the Mississippi Delta, tried to

determine the control of depositional mechanics on clay mineral composition. They

showed that clays in closely associated sandy, silty and muddy lithofacies in crevasse

splays have the same original composition and therefore concluded that any

differences in the < 2 |im clay mineral fraction in similar ancient sequences may be

wholly due to diagenesis and not to the physical segregation of clays in the

depositional environment (Gibbs, 1977). The present study was conducted on

siliciclasdc rock samples collected from late Paleozoic deltas encompassing subaerial

lower delta plain (e.g. distributary channel, interdistributary bay fill, and crevasse

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splay) and subaqueous lower delta plain (e.g., distributary mouth bar, delta front/distal

bar, and prodelta). An attempt was made to test whether there is any environmental

control on the clay mineral assemblage. The purpose of this part of this study was to

characterize different deltaic subenvironments in terms of their clay mineral content, to

explain their origin and differentiate between the detrital and diagenetic component of

the clay mineral (< 2 pm) fraction in terms of composition and occurrence within the

rock.

4.3.2 Methods

X-ray powder diffraction (XRD) analyses were performed on the clay fraction

(< 2 pm) of shales and siltstones to test for variation in the composition of clay

minerals among subenvironments. Scanning electron microscope (SEM) and

microprobe spot analysis were performed on siltstones and sandstones from each of

the subenvironments to supplement the X-ray results. Microprobe studies were also

employed to compare and distinguish between the detrital and the authigenic

component in the clay fraction.

For XRD analysis, hand-specimen sized shale/siltstone samples were collected

in the Held from various subenvironments. They were crushed into small pieces, and

gently ground using a mortar and pestle. Two splits were made at this stage, one went

through the micronizer for bulk sample analysis while the other half was suspended in

a 0.1 w t % sodium phosphate solution for peptizing and extraction of the < 2 pm clay

fraction. The clay thus extracted by settling was centrifuged and concentrated for

smear slide preparation and XRD analyses. The slides were analyzed using a Siemens

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D5000 X-ray diffractometer. Four separate treatments were carried out; air-dried (AD),

ethylene glycolated (EG), and heated to 300°C (H) and 550°C (T).

The XRD diffractograms were then matched against NewMod mineral

reference fries (Moore and Reynolds, 1997) generated with a computer program called

Mulcalc 1.0 for Windows. With Mulcalc a wide spectrum of XRD patterns were

generated by varying selected physical and chemical parameters of the mixed-layer

clays such as layer type, proportion of the layers, ordering, Fe and K content, etc. to

get the best visual fit with the observed XRD patterns. The factors used in creating the

mineral reference fries with resulting composition of the clay mixes are shown in

Table 4.6. hi the Table ‘Sil Fe’ and ‘hydroxide Fe’ stands for the distribution of Fe

between the octahedral sites of the silicate and hydroxyl layers of chlorite like

minerals. Ordering in mixed layer clays is represented by Reichweite (R) which

expresses the probability of finding identical layers next to each other (Moore and

Reynolds, 1997). Change of R from 0 to 1 indicates increased ordering of the mixed-

layer clays. The Fe content of a mixed clay defines the number of Fe atoms per two or

Table 4.6. Composition of clay parameters used for NewMod simulation.

C lay T ype I T ype 2 % % Sil H ydro H ydro O rdering Fe K m ix Clay C lay T ype Type Fe xide xide (R) t 2 Fe lay er

CH LFe T ri-tri T ri-tri 50 50 2 25 1 0 chlorite chlorite DM Dimica Dimica 50 50 1 0 0 Kaol Kaol 50 50 • 0 CHL T ri-tri T ri-tri 50 50 1 1 1 0 chlorite chlorite DMICA Dimica Dimica 90 10 0 0. 0. 2 6

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three octahedral sites (Fe = 0.2 for DMICA in the following table indicates that there

are o.2 Fe atoms per two octahedral sites). The K content similarly defines the number

of K atoms per half-unit cell.

The spatial distribution, chemical composition, and identification of clay

minerals in sedimentary rocks from various environments were determined with digital

images produced in an electron microprobe. For this study the microprobe technique

was especially useful for illustrating the various composition of clay minerals and their

occurrence within the rocks and whether they were detrital or diagenetic. Comparison

with X-ray powder diffraction results (first part of the study) clearly shows that these

two methods are complementary. The XRD provides reliable identification, but its

utility at the specific particle level is restricted.

For microprobe analysis thin sections were made from rock samples, polished

and coated with a thin, conductive layer of carbon to reduce problems associated with

charging. The first step required acquisition of a backscattered electron image (BSEI)

with a distinct gray-level contrast between grains of varying composition and epoxy

filled pore spaces. The brightness of the digital signal produced by backscattered

electrons is directly proportional to the mean atomic number of the elements within the

area bombarded by the electron beam. Direct correlation with atomic composition is

not always possible because of the loss of signal at the edges of the grains and

variations due to surface roughness. In a BSEI image, minerals with high atomic

number elements are readily identified because of their white appearance against a

gray background. The brightest areas in the images are heavy minerals. Clay minerals,

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quartz framework grains, and quartz overgrowths are medium-gray features in the

micrographs while the pores are represented by dark black areas. Differences in

composition of clay mineral particles were revealed by varying shades of gray (darker

shades generally corresponded to illite and lighter shades to chlorites). These images

were used to obtain a preliminary identification o f the minerals present

4.3.3 Results and Identification

Interdistributarv bav fill: The samples analyzed were 3MOR1,3MOR6 (both

shales) and 3MOR4 (crevasse splay sandstone). Sample 3MOR1 is composed mainly

of illite, some chlorite, and kaolinite or a second chlorite of a different composition.

Diffractograms (Fig. 4.8) for different treatment show presence of illite (001 peak at

9.97A, 002 peak at 4.96A. The graphs show presence of chlorite (001 peak at 14A,

002 peak at 7.06 A , and 003 peak at 4.7A d-spacing). The peak at 7.06A can be a

combination of chlorite 002 and kaolinite 001 or a compositionally different chlorite

002. The peak at 3.5A suggests contribution from chlorite 004 and kaolinite 002 or a

chemically different chlorite. Sample 3MOR4 shows predominance of chlorite, with

some illite (Appendix B). Presence of a superstructure at 28A d-spacing can be

attributed to smectite and some illite. Presence of smectite in the clay assemblage is

also supported by the fact that there is a tiny peak at approximately 12A in airdried

(AD) which shifts to a higher d-spacing values in ethylene glycolated (EG) treatment

indicating expandable character of the clay (smectite). Sample 3MOR6 shows

dominance of illite (001 at 10A, 002 at 4.9A), followed by chlorite (001 at 14A, 002 at

7A, 003 at 4.7A , and 004 at 3.5A) and some kaolinite (001 a 7A, 002 at 3.5A) or a

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o tfo 4s r i i i i i i r ‘i > "1ST * 8 IU " 1 ^ ' ' ’"^T ' ' * ' ’ r allD^P" v » i 9 CHL K ‘

wWi wWi D CHL ; CHL D • • I i i • r ■ Figure 4.8 X-ray powder diffraction pattern of sample TG3MOR smear preparations. 4.8TG3MORethylene A) preparations. smear Figure glycol powder sample X-ray of diffraction B)pattern saturated, air- CH-chlorite, hour. (ILL-illite, one K-kaolinite). for oneC)dried, for hour, D) heated 500°C to heated to 300°C

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chemically different chlorite (Appendix B). The bulk sample for all of these support

these interpretations (Appendix B). The microprobe spot analysis of crevasse splay

sandstone 3MOR4 and 3MOR2.96 (at the top of the interdistributary bayfill clays)

shows illite (mainly detrital with some authigenic) and two compositionally different

chlorites (corresponding to the detrital and authigenic component; Appendix B).

Authigenic clays mainly occur as pore filling clays.

Distributary Channel: The three shale samples from the bayfill deposits

underlying the channel sandstones analyzed are 4MOR2,4MOR3A, and 4MOR3B.

Sample 4MOR2 shows a dominance of illite, some chlorite, kaolinite or chemically

different chlorite. The break in the peak at 3.53A clearly reflects the contribution from

two clays minerals, either a combination of chlorite and kaolinite or two

compositionally different chlorites (Appendix B). The chlorite 001, and 003 peaks are

low intensity but distinct Sample 4MOR3 A shows mainly illite. The breaks in peaks

at 7 A and at 3.5A indicate the presence of two clay minerals. Sample 4MOR3B shows

the same result except that it may have a trace of smectite (peak at 17A in EG which

shifted from - 15A in AD treatment; Appendix B)). The bulk data for the samples

show an abundance of quartz with illite, chlorite, and kaolinite (Appendix B). The

initial results were reinspected through SEM and microprobe spot analysis in

sandstones 4MOR5 and 4MOR6 which show the presence of illite (detrital), two types

of chlorites, and the lack of kaolinite. Chlorites occur as detrital as well as authigenic

components (Fig. 4.9) in the clays but with two distinct compositions. Thus breaks in

the peaks at 7A and 3.5A in the diffractograms must correspond to two chlorite

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4MOR5

Figure 4.9 Backscattered electron micrograph of sample 4MOR5 and X-ray spectra of selected area. A) Backscattered image; B) X-ray spectrum of area I; Q X-ray spectrum of area 2 . Chemical element symbols identify the peaks in the spectra.

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varieties as opposed to a combination of chlorite and kaolinite. Only in a few cases

illite has also been found as authigenic clay. All authigenic clays occur as porefilling

clays.

Distributary mouth ban Samples analyzed are EFH9, EFH91, WFH9 (all

siltstones). All samples have same clay assemblage as shown in Appendix B. All have

abundant illite (with approximately same amount) and some chlorite. Microprobe spot

analysis of sandstone EFH5a shows that almost all clay minerals (detrital and

authigenic) are illite (Appendix B). Chlorite is present in the rock but in negligible

quantity. Sample WFH3 shows that illite is the most common detrital component

along with some chlorite while the latter is comprised almost exclusively of the

authigenic variety.

Delta front/distal ban Results of shale samples NFH2, SFH 8 , SFH18 are

shown in Appendix B. They show monotonous occurrence of illite with some chlorite

and slightly varying proportions between samples. Microprobe analysis (of sandstones

SFH3.95 and NFH3) shows dominance of illite with some chlorites squeezed between

other ffameworking grains due to compaction (Appendix B). Authigenic clays

(chlorites in most of the cases) generally fill the pore spaces in the sandstones

analyzed.

Prodelta: One shale (2MOR2) and two siltstones 2MOR1 and 2MOR3A were

analyzed from the prodeltaic deposits. The first two samples have mainly illite with

some chlorite (Appendix B). Sample 2MOR3A shows mainly chlorite with some illite

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(Appendix B). The bulk data for all of the samples of the prodelta show dominance of

quartz, with illite and some chlorite (Appendix B).

Microprobe analysis of samples 2MOR95.5 and 2MOR95.8 from the sandstone

body overlying the prodeltaic shale and siltstone show presence of only illite as the

detrital component (Appendix B). No authigenic clays are developed in the pore

spaces. Rocks are well compacted with well developed silica overgrowth.

Compactions must have played a significant role in not producing any

diagenetic/authigenic clays in the rocks.

Computer calculated NewMod reference parameters are shown in Table 4.6.

Simulated diffractograms were generated combining different proportions of these

hypothetical mixed layer clays (from Table 4.6) to find comparable matches for

observed diffractograms from the present study. The results were successful (Figure

4.10 and Appendix B). This approach resulted in estimation of percentages of different

mixed layer clay types in creating the assemblages of clays found in various

subenvironments (Table 4.7). The results from the NewMod study matched fairly with

the XRD and microprobe analysis except for the absence of kaolinites from the coarser

grain size fraction.

4.3.4 Interpretation

Results from the XRD together with microprobe spot analysis from the rocks

from different subenvironments of the study area show uniform composition of the

clay mineral fraction. The detrital component of the clay fraction is dominated by illite

with occasional chlorites, while the latter constitutes the diagenetic/authigenic

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Table 4.7. Calculated percentages of various mixed layer clay assemblages from subenvironments as determined by the NewMod simulation study. Sam ple # Environment C H L F e DM Kaol 3MOR1 Interdistributary Bay 8 3 9 0 5 1.2 3M O R 4 Interdistributary Bay 74.4 25.6 3M O R 6 Interdistributary Bay 7.6 90 23 4MOR2 Distributary Channel 8.4 86.8 4.8 4M O R 3a Distributary Channel 6.8 88.7 45 4M O R 3b Distributary Channel 8.8 88 3.2 2M OR1 Prodelta 14.6 85.4 2M O R 2 Prodelta 18.6 81.4 2MOR3a Prodelta 0.006 60 39 CHLDMICADM EFH 9 Distributary Mouth Bar 2 9 5 70 EFH91 Distributary Mouth Bar 28.6 71.4 WFH9 Distributary Mouth Bar 26 74 N FH 2 Delta Front 9.8 90 .2 SFH 8 Delta Front 33 66.7 SFH 18 D elta F ront 27.9 72.1

component of the clay mineral fraction. Spotl et al.(1993), in their study of clay

mineralogy and illite crystallinity of the Atoka Formation from the Arkoma Basin, had

similar results where the mineralogy of the < 2 pm fraction of the mudrocks was fairly

monotonous and composed of illite, Fe-chlorite, kaolinite, quartz, and traces of

feldspars. Illite crystallinity values were fairly high and show little variation

throughout the entire maturity range. According to the authors illite was exclusively of

the 2M1 polytype, suggesting a predominantly detrital origin. The sandstone samples

especially those from the distributary channel and the crevasse splay from the present

study, have two different compositions of detrital chlorite and authigenic chlorite. This

difference in chemical composition is depicted as higher aluminum to silicon and

higher magnesium to iron ratios in the detrital variety of chlorite in the sandstones

(Fig. 4.9). This is attributed to the variation in aluminum content in the octahedral sites

of the chlorite. This interpretation is supported by several studies from the

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700 TG4MOR3A: KaorUM +Nn.ltoCHLFtrOOM O 600 newmodmix2 500 TG4MR3AE 400 300

200

100

0 10 20 3040 50 60 ZTHETA

500

TWFH9E:CtirQ.07+DllllGa*020tC0 newmodmix2 400 TGWFH9E

300

100

0 0 10 20 30 40 50 60 ZTHETA

Figure 4.10 Comparison of XRD pattern produced by ethylene glycol saturated sample TG4MR3AE (a) and TGWFH9E (b) with patterns computed for NEWMOD mix2. Mix compositions are shown above the respective figures.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 117

literature on chemical analysis of authigenic/diagenetic chlorites. Hillier and Velde

(1991) have shown that diagenetic chlorites are more siliceous, have lower total

Fe+Mg, and lower octahedral occupancy. The low total of octahedral cation is due to

vacant octahedral sites indicating that low temperature diagenetic chlorites have a

dioctahedral component and result from the necessity to assign more Al3+ to the

octahedral sheet than to the tetrahedral sheet. Boles and Franks (1979) found

diagenetic chlorites having compositions between di-octahedral and trioctahedral

varieties. In all of the diagenetic series, octahedral A1 is, almost without exception,

greater than tetrahedral A1 (Hillier and Velde, 1991). This excess of A1 in the

octahedral sites is due to the relatively high Si/Al ratio of most diagenetic chlorites.

During the passage of diagenesis to metamorphism, however, octahedral occupancy

appears to increase until eventually chlorites become fully trioctahedral. As they

become less siliceous with the onset of metamorphism, tetrahedral A1 increases at the

expense of octahedral A1 so that total A1 is more or less conserved. Jahren and

Aagaard (1989) showed that diagenetic chlorites exhibit definite compositional trends

with increasing temperatures. Tetrahedral A1 increases substantially from 100-160°,

and the octahedral vacancy decreases correspondingly. These observations indicate

continuous recrystallization of authigenic clays during increasing diagenetic grade.

According to Hillier and Velde (1991) most diagenetic chlorites reported from the

literature are Fe-rich, with an almost complete range between Fe and Mg-rich

varieties. There is a correlation between Fe/(Fe+Mg) and Al. The most Fe-rich

chlorites are the most aluminum rich and exclusively Si-poor, while the most Mg-rich

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chlorites are least aluminous. The diagenetic chlorites from the present study have a

similar compositional trend.

Some distributary mouth bar sandstones show illite flakes with different

shades of gray (Appendix B). The darker parts are low in potassium while the brighter

parts are higher in potassium content which can be an artifact of smaller versus larger

grain size of the illite respectively (R.E. Ferrell, personal communication, 1997).

4.3.5 Diagenetic Implications

From the study, it is clear that the < 2 pm fraction of all samples (irrespective

of subenvironments in the deltaic setting) are dominated by quartz and illite with some

Fe-rich chlorite. Percentages of illite and chlorite vary in the different

subenvironments. The high percentage of illite (> 80%) for most of the samples (Table

4.7) may indicate contribution from two sources. They are primarily detrital and are

derived from the Appalachian source (Spotl et al., 1993) which acted as a secondary

contributor to these deltaic sediments apart from the northern craton. Petrographic

studies (Section 4.4) on sandstones from different environments show that there is

abundant quartz cementation along with presence of significant amounts of micas and

chlorites and an almost total lack of feldspars. Authigenic illites were probably

generated by the smectite to illite transformation reaction (Boles and Franks, 1979)

which provided silica to the pore water system (Fig. 4.11). Silica may also be provided

by intergranular pressure solution due to a high degree of compaction. This silica thus

released, was precipitated as quartz overgrowth/cement in the sandstones. The iron

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DETRITAL----- K-FELDSPAR AND MICA, BREAKDOWN^—

0 50 100 150 200 TEMPERATURE *C

Figure 4.11 Proposed model for diagenesis used in interpretations in the present study. Original model derived from Wilcox sandstone cements by Boles and Franks (1979).

that was liberated by the same reaction probably transformed any preexisting

kaolinites to Fe-rich chlorites which is very abundant in the samples (Table 4.7).

This study produced promising results which can be attributed to facies control

on the diagenetic clay mineral assemblages in different subenvironments, though it is

difficult to comment on the extent of such facies control. The detrital clay fractions

show monotonous compositions for all the subenvironments, however, the varying

percentages of the detrital and authigenic clays, imply that facies had significant

control on the resultant clay fractions. For instance, the distributary channel and the

interdistributary bayfill/crevasse splay deposits, that were mainly affected by

fresh/brackish water, have two compositionally different chlorites as the detrital and

authigenic components and some kaolinites in the bay-fill shales together with detrital

illites. But the facies that were influenced by marine pore waters (e.g., distributary

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 120

mouth bars, delta front/distal bars, and prodelta) have chlorite (mostly authigenic in

origin) as well as detrital illite and no kaolinite. The illite composition, however, can

not be related to any such facies control and must have been deposited in the

sedimentary basin with other detrital elastics.

4.4 Provenance and Tectono-Sedimentary Interpretations Based on Petrographic Analyses

4.4.1 Introduction

Petrographic studies can be used 1) to determine probable source of the rocks,

2) to assist in lithofacies analyses, 3) to provide insights into the nature of the rock

fabric, 4) to shed light on diagenetic history, including grain alteration and

cementation, 5) to assign sedimentary basin to its proper plate tectonic framework, and

6) to delineate paleoclimatic conditions. Four outcrops were sampled from excellent

roadcuts south of Morrilton, and from a quarry north of Jacksonville to pursue the

petrography of the sandstone. The objectives were 1) to delineate the compositional

variations of the deltaic lithofacies, 2) to interpret the tectono-sedimentary conditions,

3) to determine the provenance of the different lithofacies, and 4) to shed light on the

late Paleozoic tectonic evolution of the basin.

4.4.2 Methods

Twenty samples from interdistributary bay fill, crevasse splay, distributary

channel fill, distributary mouth bar, delta front /distal bar, and prodelta

subenvironments were analyzed. The Gazzi-Dickinson point counting method

(Dickinson and Suczek, 1979) was applied to interpret the tectonic settings during the

sedimentation of the deltaic deposits. The results are plotted in QFL, QmFLt, QpLvLs,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 121

and QmPK triangular diagrams of Dickinson and Suczek (1979). Here Q is the total

quartz and it includes monocrystalline quartz (Qm), polycrystalline quartzose lithic

fragments (Qp), and cherts; F is monocrystalline feldspar grains and includes

plagioclase (P) and K-feldspar (K); L is unstable lithic fragments which includes

volcanic/metavolcanic types (Lv) and sedimentary types (Ls); Lt is the total lithic

fragments and is equal to L+Qp. Compositional analyses were conducted on 400

grains per thin section. For each triangular representation, the percentages of the

frameworking grains were recalculated to 100%. Traditional point counting technique

was employed for classifying the rocks using Folk’s (1974) scheme and were plotted

on QFR diagram. In this diagram Q is the sum of Qm and Qp, F is the sum of P and K,

and R includes chert and all other types of rock fragments namely volcanic (Lv),

sedimentary (Ls), and metamorphic (Lra).

4.4.3 Results

The results from the study show that all the lithofacies are dominated by quartz

(Gangopadhyay and Heydari, 1995). Monocrystalline quartz is the most abundant (71-

82%; Table 4.8). Polycrystalline quartz comprises a minor fraction of frameworking

grains (3-7%; Table 4.8). The low feldspar content is conspicuous (K-feldspar 2-5%;

plagioclase feldspar 0-4%). Rock fragments (5-12%) are comprised of almost equal

proportions of sedimentary and metamorphic varieties. Among the accessory minerals

micas are more abundant than others. Zircon is present in rock types of all the

subenvironments. The results of the Dickinsonian point counting are shown in Table

4.9. The composition of all sandstones from the study fall within the recycled orogen

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4.8. Results of traditional point counting. Abbreviations Ch, PQ, Cm, Mat, M, and Chi represent Chert, Poly quartz, Cement, Matrix, Mica, and Chlorite, respectively.

Sam ple# Qm Ch PQ KP Lv Ls Lm Cm Mai M Op Z r Chi EFH2 49.5 15 5.0 0.0 0.0 0.0 6 2 1.4 1.4 12 75 3.6 25 0.0 EFH3 63.7 1.7 13.2 25 0.0 0.0 7.1 5.7 15 2.7 1.7 0.0 0.0 0.0 EFH 5 695 12 9.4 0.0 0.0 0.0 3.0 4.0 0.0 12 55 1.7 2 2 0.0

EFH6 66.8 19 6.1 0.0 0.0 0.0 4.6 25 2.7 0.0 5.4 4.4 29 5 4 EFH 7 65.4 12 13.4 32 0.0 0.0 5 5 35 3.0 4 5 12 0.0 0.0 0.0 WFH3 67 5 0.0 7 2 0.0 15 0.0 5 5 25 1.7 1.2 6.0 0.0 25 1.4 WFH7 60.8 3.1 62 0.0 0.0 0.0 3.0 2.8 4.7 2.6 7.1 2.4 15 15 3MOR3 61.8 0.0 3.0 15 0.0 0.0 5 5 5.4 95 45 42 3.7 15 0.0 3MOR4 65.1 0.0 4.0 2.9 0.0 0.0 43 52 5.0 8.1 25 0.0 0.0 0.0 NFH1 622 1.0 3.0 1.8 1.0 0.0 3 3 13 3.8 95 7.0 3.8 0.0 15 NFH3 55.1 0.0 4.7 12 2 5 0.0 4 5 0.0 7.4 8.6 3.7 75 4.7 0.0 WFH11 43.6 0.0 3.0 0.0 0.0 0.0 5 5 2.0 0.0 15.4 16.4 53 45 25 4MOR5 59.1 2.0 8.0 35 1.2 0.0 2.7 32 5.4 3.7 45 27 15 0.0 4MOR6 66.1 0.0 8.4 15 2 5 0.0 4 2 0.0 25 15 32 25 5.0 0.0 4MOR7 532 1.6 4.1 3.6 4.1 15 3 5 4.1 7.4 52 5 2 0.0 0.0 1.4 4MOR10 615 0.0 7.7 1.7 1.4 2.8 5.0 2.8 6.1 28 55 0.0 0.0 1.1 4MOR12 72.0 12 5.7 0.0 0.0 0.0 3 2 42 22 1.7 42 3.0 1.7 0.0 SFH1I 655 1.0 2.4 0.0 0.0 0.0 3 5 55 1.7 5 5 6.9 52 2.4 0.0 SFH16 555 0.0 4.7 0.0 1.0 0.0 3.7 4.7 0.0 10.0 10.0 6.7 35 0.0

Table 4.9. Results of the Gazzi-Dickinson point counting method in percentages.

Sample Q FL Qm F L t Qp Lv Ls Qm P K # EFH 2 91.9 05 7.8 845 05 15.4 555 0.0 44.7 99.7 05 0.0 EFH 3 855 57 151 69.0 5 7 285 57.1 0.0 459 96.2 0.0 3.8 EFH 5 905 1.1 8.4 78.4 l.l 205 75.4 0.0 24.6 98.6 1.1 0.4

EFH 6 956 0.6 6.8 81.6 0.6 17.9 77.1 0.0 259 995 0.4 0.4 EFH 7 875 35 9.2 71.7 35 24.7 64.8 0.0 35 2 955 0.0 4.7 WFH3 89.2 56 8.2 805 56 175 655 0.0 34.7 96.9 5 7 05 WFH 7 87.1 51 10.9 75.6 51 22.4 60.9 0.0 39.1 97.4 l.l 15 3MOR3 865 5 6 115 853 2.6 15.1 26.1 0.0 735 96.9 0.0 25 3MOR4 855 4.1 10.6 80.4 4.1 155 351 0.0 67.9 94.1 0.0 4 5 NFH1 88.9 3.7 7.4 835 3.7 158 42.1 0.0 57.9 95.8 15 5 7 NFH3 90.0 5.6 4.4 856 5.6 11.9 625 0.0 375 93.7 4 5 2.1 WFH11 83.7 1.7 145 775 1.7 20.9 36.7 0.0 635 97.8 0.7 15 4MOR5 84.0 6.2 9.8 71.9 6.2 21.9 67.2 1.6 315 951 1.9 6.1 4MOR6 87.8 4.6 7 5 77.4 4.6 18.0 62.1 0.0 37 9 94.4 35 2.1 4MOR7 73.9 9.6 165 66.7 9.6 23.7 30.4 10.1 59.4 73.9 6.8 5.9 4MORIO 83.1 3.7 13.3 73.8 3.7 256 84.7 3.8 115 955 2.2 2.6 4MOR12 90.6 0.0 8.6 856 0.0 165 685 0.0 31.7 99.0 1.0 0.0 SFH11 87.7 0.0 11.8 835 0.0 16.2 50.0 0.0 50.0 99.5 0.0 0.0 SFH16 86.6 1.4 12.0 79.4 1.4 19.1 57.7 0.0 425 98.2 1.8 0.0

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Qm

60 Qm

tncraaming Ratio of Maturity of Stability from Continanu l

Figure 4.12 A) QFL, B) QmFLt, Q QpLvLs, D) QmPK diagrams showing compositional variations in Atokan sandstones. Q=Qm+Qp; Q=total quartz; Qm=monocrystalline quartz; Qp=polycrystalline quartz+chert; F=P+K; F=monocrystaIIine feldspar; P=plagioclase feldspar; K=K feldspar. L=Lv+Ls; L=lithic fragments; Lv=volcanic rock fragments; Ls=sedimentary rock fragments; Lm=metamorphic rock fragments; Lt=Lv+Ls+Lm; Lt=all lithic fragments.

provenance (Fig. 4.12). The results of traditional point counting shows that the

sandstones have a general composition of Q 82 .90 F3.to R5-19 and are classified as

sublitharenites according to Folk’s (1974) scheme (Fig. 4.13).

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.Quaraarenite Subfeldsareaite -Sublitharcnlte

Figure 4.13 QFR diagram showing compositional variations of Atokan sandstones. Q=Qm+polycrystalline quartz; F=P+K; R=chert+Lv+Ls+Lm (from Folk, 1974). Q=total quartz; Qm= monocrystalline quartz, F=monocrystalline feldspar grain, P=plagioclase feldspar; K=K-feldspar; R=lithic fragments; Lv=volcanic rock fragments; Ls=sedimentary rock fragments; Lm=metamorphic rock fragments.

4.4.4 Interpretation

Source rocks, climate, relief, transport, and diagenesis have long been regarded

as principal controls on sandstone composition. These factors will be discussed while

addressing the specific source area (s) pertaining to the middle and upper Atokan

deltaic deposits.

Middle and upper Atokan sandstones consist mainly of abundant quartz,

suggesting a cratonic derivation of the deltaic sediments. Contrary to the accepted

model of deposition of elastics in a foreland basin where sediments are derived from

the foreland thrust belts, the sediment source in this case was located toward the

northern cratonic side. Paleocurrent and petrographic data show that, during the

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Pennsylvanian, sediments were derived from the north, northeast, and east across the

ancestral Illinois Basin, the Ozark Uplift and the Black Warrior Basin (Link and

Roberts, 1986). Pulses of uplift associated with the Nashville and the Ozark Domes

were probably significant sediment contributors. Low abundance but persistence of

metamorphic rock fragments imply that the drainage network feeding the deltas also

tapped the Appalachian orogenic belt as a secondary but significant donor. An

Appalachian provenance for rocks in the Ouachita Fold Belt after middle Ordovician

time also was stressed by Gleason et al. (1994) based on Nd-isotopic data from the

lower Atokan turbidites. Morris (1974a) suggested that the lower Atoka (deeper water

turbidites) represents a thorough mixing of sediments derived from the North

American craton and the Appalachians (through the Black Warrior Basin) by sediment

gravity flows. The present study in the Arkoma Basin, however, is on the deltaics

(middle and upper Atoka), at the head of the alluvial system flushing sediments

through the ancestral Illinois Basin. Hence, a limited mixing of the sediments from the

Appalachian orogen with the northern cratonic input is suggested (Gangopadhyay and

Heydari, 1995).

The lack of feldspars may suggest removal of this mineral in the source area by

the climatic factors. Climate has a major effect on composition of sandstones in low

relief areas where tectonism is low, as in the case of many cratons and passive margins

(Potter, 1978; Franzinelli and Potter, 1983). Low latitudinal disposition of the

sedimentary basin in the study area, along with the presence of coal beds in associated

Atokan deposits indicate a warm and humid paleoclimatic conditions which may be

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responsible for removal of chemically unstable feldspar grains by weathering in the

source area.

Relief generally determines the length of time the source materials can remain

in the zone of weathering. According to Hayes (1979), low relief in the provenance

results in lowered erosional rates, thereby increasing the relative efficiency of chemical

weathering to remove the chemically unstable components at or near the source. Also,

the relative tectonic stability of cratons permit chemical and mechanical weathering to

reduce feldspars and mafic minerals thus enriching the sand fraction in quartz.

Long distance of transport can be a controlling factor in accomplishing the

dearth of feldspars in the sandstones. It has been presumed that feldspars are more

subjected to abrasion and destruction during transport than other minerals, (Pettijohn

et al., 1987). Short distance between the source area and the site of deposition implies

that mechanically unstable grains will not be greatly reduced during transportation into

the basin. However, larger low gradient streams seem to be capable of transporting

feldspars long distances without any significant loss. Russell (1937) showed that the

Mississippi River sediments contain 25% feldspars near Cairo, Illinois. After

transportation of about 1800 km to the delta, the sediments still contain 20% feldspars.

Similarly, feldspars do not generally disappear as a result of wave reworking and

longshore transport, although some reduction in volume has been reported by Mack

(1978).

Diagenesis can result in removal and/or alteration of some of the important

components beyond recognition, including feldspars. In this study, the dearth of

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feldspars is not attributed to the diagenetic removal because of the lack of supporting

evidence from petrographic observations.

4.4.5 Late Paleozoic Tectonic Evolution of the Basin

Compositional variations between the Stanley, the Jackfork, and the Atoka

sandstones provide an interesting synthesis of the late Paleozoic tectonic evolution of

the Arkoma Basin. The Stanley sandstones are fine-grained and consist mainly of

quartz with significant amounts of metamorphic rock fragments, some feldspars, and

abundant matrix (Morris, 1989; Fig. 4.14). The Jackfork sandstones are typically

quartzose, cleaner, well-sorted, and slightly coarser grained than the Stanley

/ ' / / \ Recycled Orogen Continental Block /V dp \ Provenance Provenance \ 7 A V '

s o / / V \ s o

/ I MagmaticAicN. \ 7 X^Pnvenance \ \ > r V Stanley Gr. (Late Mississippian)

Jackfork Fm. (Early Pennsylvanian)

|||||f|j|j|| Atoka Fm. (Middle Pennsylvanian)

Figure 4.14 Detrital modes (QFL) of the Stanley, the Jackfork, and the Atoka formations (present study), Q=Qm+Qp; F=P+K; L=Lv+Ls+Lm (data for the Stanley and the Jackfork formations from Lowe, 1989).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 128

sandstones (Morris, 1989; Fig. 4.14). The Atoka sandstones show abundant quartz

with significant amounts of metamorphic/sedimentary rock fragments and are typically

micaceous. In spite of deposition in a foreland basin adjacent to a thrust belt, the

Stanley sandstone compositions plot on the continental block provenance. The

Jackfork sandstones cluster around apex Q indicating a cratonic block provenance.

The Atoka sandstones with known cratonic derivation plot on the recycled orogen field

(Gangopadhyay and Heydari, 1995). These compositional patterns of sandstones with

known cratonic regimes suggest that the Dickinsonian diagrams are in this case

insensitive to the true regional tectono-sedimentary scenario. Similar results are found V.

in the literature where data from provinces with known tectonic affinity do not fit the

Dickinsonian tectono-sedimentary interpretations. Potter (1986), in his study of South

American beach sands, showed that in the Argentine association, active margin

components masked the overwhelming passive margin components.

Lithologic, petrographic, and paleocurrent data suggest that the sediment

transportation was mostly from the south and southeast during the Stanley

sedimentation but dominantly from the north during the Jackfork (Morris, 1989) and

the Atoka deposition. The composition of the southerly derived Stanley sandstones

suggests that these sediments were derived from an uplifted craton/passive margin

which lay south of the Ouachita Basin (Lowe, 1989). The accretionary prism and

volcanic arc did not provide a major source for the Stanley sediments. Once in the

basin, sediments were carried by turbidity currents in a southwesterly direction parallel

to the trough axis to form submarine fans. The Stanley sandstones were followed by

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the clean, well-sorted sandstones of the Jacicfork and maintained the same mode of

deposition by sediment gravity flows (Fig. 4.15). However, abundance of quartz in the

Jackfork indicates a shift in direction of sediment supply, from the south during the

Stanley time to the northern cratonic province during the Jackfork time. Rapid

subsidence, accompanied by down-to-the-south normal faulting along the northern

margin of the basin, occurred during the deposition of the lower Atoka. By middle

Atoka time the trough with its maximum subsidence and sediment accumulation

shifted northward into the area of the Arkoma Basin. The supply of the elastics kept

pace with basin subsidence and produced a very thick clastic wedge in the form of

deltaic deposits.

MO MISS

ARK KA /

Figure 4.15 Paleogeographic map of the Ouachita Trough imposed on the present day state boundaries, showing the derivation of the Stanley sediments from the south and the Jackfork sediments from the north and northeast (modified after Chamberlain, 1978).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. L30

The distribution of sandstone compositional types has been interpreted to

indicate a supply from two major sources. More lithic and feldspathic sands (Stanley)

were derived from orogenic provenance on the south and southeast, whereas more

quartzose sands (Jackfork and Atoka) were derived from the northern craton (Morris,

1974b). The contribution from the Appalachian orogenic belt, however, was much

more important during the Atokan time than in the older Jackfork Formation time due

to the activation of the Alleghanian front (Graham et al., 1976).

4.5 Systematic Variations in Grain-Orientation Related to Depositional Processes In Deltaic Subenvironments, Using Anisotropy of Isothermal Remanent Magnetization (IRMA)

4.5.1 Introduction

Rock magnetic techniques have been used to study sedimentary grain fabrics

for the last few decades (Hamilton and Rees, 1970a; Jackson and Tauxe, 1991). Rock

magnetic grain-fabric studies have focused on both Recent and ancient sedimentary

rocks (Tarling and Hrouda, 1993; Lu and McCabe, 1993). The grain anisotropy results

obtained from these studies have also been used as proxies to infer paleocurrent

directions from sedimentary rocks which lack traditional current markers (Ellwood and

Ledbetter, 1979; Schieber and Ellwood, 1988). Results of grain lineations determined

by more conventional optical methods like photometry and petrography when

compared with magnetically determined fabrics have yielded concordant results (Taira

and Lienert, 1979).

Magnetic data for any sample may be expressed as a triaxial ellipsoid whose

major, intermediate and minor axes represent the relative susceptibility magnitudes

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131

and azimuths (Wolff, 1989). The total ellipsoid shape and orientation reflects the

overall directional fabric of the sample, as expressed by magnetic mineral grains

(Khan, 1962). Rock-fabric determination by anisotropy of isothermal remanent

magnetization (IRMA) has this underlying assumption that is substantiated by repeated

studies (McCabe et al., 1985). The IRMA technique uses a strong field to magnetize a

sample along different sample axes. The field used is strong enough to saturate

magnetic particles. Any preexisting magnetization residing in the magnetic grains is

completely destroyed by this laboratory treatment. The magnetization that is induced

will be strongest parallel to the predominant long-axis of the magnetic grains in the

sample (C. McCabe, personal communication, 1997). The presence of anisotropic

grains is a basic requirement for this method to work but the directional coherence of

long-axes in the samples is also important for a sample to have good bulk anisotropy.

A review of various techniques for measuring the orientation of the long-axis of non

magnetic and magnetic grains and their mutual relationship is presented by Basu

(1997).

Deposition on a horizontal surface is primarily controlled by gravitational

settling of grains in the absence of currents. Platy (oblate) grains lie on the

depositional surface or the bedding plane. The more rod-like (prolate) grains will also

lie on the bedding plane producing a foliation but with their long axes randomly

dispersed. The resultant fabric will be strongly oblate and is confined to the bedding

plane with the minimum axes of the grains being perpendicular to the bedding. In the

presence of slope and lack of currents the more rod-like grains tend to roll down

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 132

gradient. The grains that actually roll down are dependent on their prolateness and the

dip and roughness of the depositional surface. As a result a lineation perpendicular to

the direction of maximum slope is superimposed on a strongly oblate fabric. With

increase in slope the superimposed lineation increases until the angle of repose for the

grains is reached and slumping sets in. In such a case the fabric mimics one affected by

currents.

The oblate and prolate grains respond in two different fashions in the presence

of currents (Tarling and Hrouda, 1993). Firstly, the oblate grains are lifted at their

leading edges, resulting in imbrication. The minimum axes of the grains are tilted by

increasing amounts (up to 20°) from the vertical, as velocity increases. Secondly, a

lineation is imposed by the more prolate grains on the primarily oblate fabric, the

lineation direction being dependent on the current velocity and the slope of the

depositional surface. The long axes of the anisotropy ellipsoid will lie on the bedding

plane and the short axes will be perpendicular to it (Fig. 4.16). The grain long-axis

orientation is either parallel or perpendicular to paleoflow directions. The present

study discusses the relationship between grain long-axis orientation and flow

characteristics from the various deltaic subenvironments. This is the first study that has

attempted to infer depositional signatures from magnetic grain long-axis orientation at

the subenvironment scale in deltaic deposits.

4.5.2 Methods

Oriented samples were collected from different locations in the Atoka

Formation. The samples were selected from three subenvironments namely,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 133

Minimum Axis

Bedding Plane Maximum Axis

Figure 4.16 Magnetic anisotropy data represented in the form of anisotropy ellipsoid. A depositional fabric has the maximum axis parallel to the bedding plane and minimum axis perpendicular to it

distributary mouth bar, delta front/distal bar, and crevasse splay deposits. Multiple 2.2

x 2.5 cm cores were cut from the oriented samples for the rock magnetic study.

Isothermal remanent magnetization (IRM) was imparted along nine specified sample

axes using a pulse magnetizer set at a saturation value o f450 milli Teslas (mT).

McCabe et al.’s (1985) method was used to calculate the anisotropy ellipsoid. The

remanent magnetization was measured with a cryogenic magnetometer and the

component due to IRM was found out by vector subtraction of the base level natural

magnetization. The procedure is so set up that the anisotropy is measured with

sufficient redundancy and the best fit anisotropy tensor was determined using a least

squares method. The tensor obtained was used to find calculated susceptibilities along

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 134

nine axes and residual (observed minus calculated) susceptibilities along the axes were

computed. Small residuals indicate that the data is of high quality and the assumption

that the anisotropy may be depicted by a triaxial ellipsoid is valid. A computer

program was used to determine the best fit anisotropy tensor. Percent anisotropy was

measured for each of the samples and is represented by 100(Kn.!.»-Km.n)/K...t. where

Kmax, Kuu, and Kmm are maximum, intermediate and minimum anisotropy axes

(McCabe et al., 1985). The determination of the carrier phase of magnetization in the

sandstones was done by stepwise IRM acquisition, starting at 50mT and going up to

llOOmT.

4.5.3 Results

Subenvironments from which samples were collected to determine the

depositional fabric are distributary mouth bar and delta fronts comprising major delta

lobes from Freshour Quarry and crevasse splay deposits from a road cut (3MOR) at

Morrilton. Table B.l (in Appendix B) presents the Kmax, Kint, Kmm, lineation (L),

foliation (F), and percent anisotropy for each specimen representing the various

subenvironments. Kmax. Kjnt, and Kmm are the maximum, intermediate and minimum

axes of anisotropy ellipsoids. Figure 4.17 represents the equal area stereonet plots for

Kmax and Kmin for each subenvironment. In all of these plots the Kmax is nearly

horizontal or have very shallow plunges and the Kmm axes are vertical or have steep

plunges, together representing very well developed depositional fabrics. The

distributary mouth bar samples have a mean Kmax lineation with an azimuth of NE-

SW. The mean azimuth of the Kmax lineations from the delta front samples is directed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.17 Equal area stereonet plots of sample maximum (1 L. t on primitive circle) and sample minimum (Kmm cluster near center) axes of anisotropy ellipsoids. Open and solid symbols represent upper and lower hemisphere projections, respectively. (a) distributary mouth bar samples, (b) delta front samples, and (c) crevasse splay samples. Paleocurrent directions measured from the Held are shown with arrows in the figures.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 136

WNW-ESE. The crevasse splay samples have a mean azimuth of NW-SE.

The percent anisotropies from the distributary mouth bar, delta front and crevasse

splays form distinct populations. The distributary mouth bar samples have the highest

mean anisotropy of 19.16 %. The crevasse splay and delta front siltstones and

sandstones have mean anisotropies of 12.84 % and 8.03 %, respectively. The percent

anisotropy data is represented in Figure 4.18. The carrier phase of magnetism was

determined to be a mixture of magnetite and hematite. The determination is based on

IRM acquisition curves that represent gradual increase in intensity with applied field

and reaches near saturation intensities only at high field strengths. A representative

IRM acquisition curve is shown in Figure 4.19. Observations using scanning electron

microscopy reveal that the magnetic particles are 5-12 pm long. They are dispersed in

the matrix indicating their detrital origin. An intrinsic anisotropy with aspect ratios of

4:1, is associated with the magnetic particles

4.5.4 Discussion

The relationship between the directions and paleocurrent flow from the

distributary mouth bar, delta front and crevasse splay deposits are discussed on the

basis of relative importance of fractional effects and varying flow velocities.

Kmar vector and paleocurrent directions: The mean Kmax azimuth from distributary

mouth bar deposits is NE-SW. The paleocurrent measurements based on foreset

bedding from the distributary mouth bar is directed SE. A mutually perpendicular

relationship is hence observed between the Km„ lineation and paleocurrent direction.

Delta front deposits exhibit a mean WNW-ESE Kmax azimuth. The paleocurrent

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137

Distributary Mouthbar 5 - a Data ■ n = 1 7 iJn

3- DeltafrontData Crevasse Splay Data 2- n=19 6 -

% Anisotropy

2 -

% Anisoropy

% Anisotropy

Figure 4.18 Percent anisotropies from the three depositional settings: (a) distributary mouth bar, (b) delta front, and (c) crevasse splay. The number of samples, mean anisotropies and standard deviations are represented.

IRM Acquisition

120

100

80 -

£ 40

0 200 400 800 800 1000 1200 FMd(mT)

Figure 4.19 Representative isothermal remanent magnetization acquisition curve. The pattern reflects a mixture of hematite and magnetite as the magnetic carrier phases.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. direction in the delta front deposits is directed toward SE. A low angle oblique to near

parallel relationship is hence observed between the Km« lineation and observed

paleocurrent direction.

The roughly perpendicular relationship between the distributary mouth bar and

paleocurrent direction suggests deposition from traction dominated fast flowing

currents. These fast flowing traction-dominated currents reworked the long axis of the

ferromagnetic grains into a flow normal alignment Bottom friction between the

riverine effluence and the depositional surface as observed from the Mississippi River

delta is very characteristic at distributary mouth bars especially during periods of high

discharge (Wright and Coleman, 1974). A similar depositional process is envisaged for

the distributary mouth bar deposits at the Freshour Quarry leading to flow normal Km*

orientations. Similar flow-perpendicular grain lineations have been observed in fast

flowing deep sea channels by Ellwood and Ledbetter (1979) and Rees (1965). The

amalgamated nature of sandstones and overall lack of fine-grained sediments (see Fig.

3.2) suggest traction dominated flows; this and the possibility of the modifying effects

by wave reworking at the river mouth would also tend to promote development of the

mutually perpendicular relationship in the distributary mouth bars. The delta fronts are

characterized by suspension settling from buoyant plumes that expand due to flow

unconfinement immediately offshore from distributary channel mouths. As a result,

the delta front deposits undergo limited fractional reworking. The low angle oblique to

near parallel relationship between K n», azimuths of delta front deposits and

paleocurrent flow is suggestive of such low velocity currents with limited traction. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 139

lack of traction to low traction flow velocities are unable to rework the long axis of

ferromagnetic grains sufficiently to develop a mutually perpendicular relationship with

flow direction. The limited traction effect is very likely a signature of marine waves

since the fluvial effluent fails to reach the sediment water interface at the depositional

site of delta fronts. The resultant effect is near parallelism of lineation and

paleocurrent direction. The delta front samples have a larger directional spread in the

Kmax orientation which is attributed to effluent flow divergence in an unconfined

system.

The crevasse splay deposits at the third Morrilton road cut (3MOR) exhibit a

mutually parallel relationship between the lineation and paleocurrent direction,

the latter being directed WNW as observed from ripple cross laminations. The mutual

parallelism between Kmr Iineations and paleocurrent flow is supposedly related to

traction influenced currents of moderate velocity. There is lesser dispersion in Km-*

orientations with respect to paleocurrent flow, suggesting uniformly directed currents

during deposition.

Percent anisotropy and depositional flow velocity: Percent anisotropy has been

used as a proxy for relative variations in paleoflow velocities between associated

facies (Ellwood and Ledbetter, 1979). The delta front deposits have a mean anisotropy

of 8.03 % (o= 1.94) as opposed to 19.16 % (a = 13.5) for the distributary mouth bar

sandstones. The lower value is suggestive of lower flow velocities during deposition of

delta fronts than in distributary mouth bars. The crevasse splay deposits at 3MOR have

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a mean anisotropy of 12.84 % {a- 4.28) which is suggestive of flow velocities

intermediate between the distributary mouth bars and delta front samples.

The higher percent anisotropies in the distributary mouth bar samples is

possibly a combination of high velocity fractional flows with active reworking,

possibly with some contribution from incoming waves. The combined effect enhances

the percent anisotropy. The bulk of the distributary mouth bar samples have high

anisotropies (> 18%) but a few are very low (< 10%; see Appendix B), which leads to

a relatively high scatter (

magnitude of anisotropy possibly reflects the active nature of the depositional site with

intervening reduced activity periods. The active nature of deposition is indicated by the

presence of foreset bedding and cut and fill structures, implying significant effects of

traction. The higher percent anisotropy of -19 % in the distributary mouth bar samples

is a result of deposition in such a dynamic environment. Similar high ranges in percent

anisotropies have been observed from active depositional settings, like channel fills,

by Ledbetter and Ellwood (1980).

The percent anisotropies in the crevasse splay and delta front deposits are

considerably lower (p. = 12.84 and 8.03, respectively) than the distributary mouth bar

samples, with well clustered values (<7 = 4.28 and 1.94, respectively). The lower values

are due to limited traction from lower velocity flows and possible lack of significant

wave reworking. The delta front deposits are characterized by small scale cross

lamination, lenticular lamination and wavy laminations which represent limited effect

of traction which may be due to marine waves. The mean anisotropy of 8.03 % in the

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delta front deposits is a result of deposition under these conditions. The crevasse splay

deposits have cross laminations in the thinly bedded lower part which increase in scale

in the thickly bedded upper part. The lower part represents deposition under limited

traction and upper reaches indicate deposition under comparatively increased flow

velocities. The intermediate anisotropy value of 13 % in these sandstones has

developed under these settings.

4.5.5 Conclusions

The orientation of magnetic particles are difficult to evaluate in the light of

depositional processes because of a variety of operative factors. The difficulty in

evaluating the data also originates from the fine grain-size of the magnetic particles,

which are 8-10 pm in size. This study incorporates the integrated results of both Kmax

directions and percent anisotropy to interpret the relationship between grain-

orientation and depositional processes tied to field based depositional interpretations.

In spite of the difficulty in evaluating the effects of certain factors there is a consistent

pattern that is observed when the IRMA results are discussed within the framework of

sedimentary characteristics observed in the field e.g., sedimentary structures and

current directions. The IRMA results independently corroborates the field-based

observations. The following conclusions have been drawn from this study.

1. A combination of sedimentologic and rock-magnedcally determined

sedimentary fabric analyses was used to demonstrate the similarity in the depositional

processes between crevasse splay subdeltas and major delta lobes. Variations in grain-

size, bed-thickness, and sedimentary structures indicate increased flow velocities

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upsection in both subdeltas and delta lobes. Results from rock-fabric studies, using

anisotropy of isothermal remanent magnetization, are consistent with this inference:

percent anisotropy increases upsection in both subdeltas and major delta lobes,

coherent with increasing paleoflow intensities.

2. Well developed sedimentary fabrics are represented in the samples with the

long axes (Kmax) of anisotropy ellipsoids lying on the bedding plane and short axes

(Kmin) of anisotropy ellipsoids oriented normal to bedding.

3. The Km», lineation systematically varies with flow direction. Mean Kmax

vector directions from distributary mouth bar deposits are approximately flow normal.

Mean Km,, direction for delta front and crevasse splay deposits are roughly flow

parallel. The mean anisotropies for delta front and its overlying distributary mouth bars

are - 8 % and - 19 %, respectively, while that for upper reaches of crevasse splay

subdeltas is ~ 13 %. Integrating the Kmax direction with respect to observed paleoflow

directions and percent anisotropies, inferences based on depositional style can be

made.

4. The flow normal Kmax direction and high percent anisotropies (- 19 %) from

distributary mouth bars indicate deposition dominantly from traction flows with

associated high flow velocities. The flow parallel K^* orientation and lower percent

anisotropies (~ 8 %) from the delta front deposits indicate lesser dominance of traction

and lower flow velocities. The flow parallel Kmax orientation from the crevasse splay

deposits and intermediate percent anisotropies (- 13 %) indicate a mixed traction and

suspension mode of deposition from flows with moderate velocity.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTERS

STACKING PATTERNS IN THE ATOKA FORMATION WITHIN A SEQUENCE STRATTGRAPHIC FRAMEWORK

5.1 Introduction

This chapter deals with the sequence stratigraphic framework of the

Pennsylvanian deposits of the Ouachita Trough and Arkoma Basin. The present study

has placed the Atoka Formation in an allostradgraphic framework within the

background of global eustasy, associated tectonics and local sediment supply. This is

an important contribution by the author since no work regarding the sequence

stratigraphy of the Atoka Formation exists. Sequence stratigraphic interpretations from

the late Paleozoic in Arkansas were restricted to the well-studied Jackfork Formation

only (Coleman et al., 1994). The shallow-water and fluvial deposits of the Atoka and

Hartshome/McAlester Formations have not been previously studied in this regard.

This chapter specifically discusses the Atoka Formation, however, the underlying

Jackfork, Johns Valley, Bloyd and the overlying Hartshome and McAlester

Formations are also discussed to make the study complete.

5.2 Dataset

Discussions in this chapter are based primarily on subsurface data of the Atoka

Formation from well-logs. Well-log data from the west-central part of Arkansas were

collected from various oil companies and compiled in a regional scheme by Haley

(1982) published by the United States Geological Survey. The interpretations are

based on a total of seventy six well-logs from one east-west line and four north-south

143

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 144

oriented lines revealing regional scale cross sections. Observations made in the Held

during the present study, from the Lower Atoka (Boston Mountain road cut), Middle

Atoka (Freshour Quarry, Conway road cut) and Upper Atoka (Morrilton road cuts)

formations have been used to substantiate the interpretations that are primarily based

on subsurface correlation. USGS coal-bed data from Arkansas has been used in this

study (Haley, 1982) since the occurrence of coal-beds are predictably tied to base-level

fluctuations (Flint et al., 1995).

5.3 Regional Stratigraphy in a Global Cyclostratigraphic Perspective

The Ouachita trough is composed of a thick basin-fill succession that was

initiated during the Cambrian and terminated at the end of the Pennsylvanian. There

was a dearth of clastic sediment input during the Lower Paleozoic. Deposition took

place in an open sea where carbonate and other non-clastic (e.g., novaculite)

accumulation was prevalent. Tectonic subsidence was negligible during the early

Paleozoic time period. The rate of sedimentation increased in response to the onset of

the Ouachita Orogeny at the beginning of the Mississippian period (Chamberlain,

1978). This was associated with increased subsidence rates. During the Pennsylvanian,

generation of accomodation space was extremely high along with very high clastic

input when the Ouachita Orogeny was at its peak.

Deep water elastics and their shallow water feeder systems of Missisippian to

Pennsylvanian age are preserved in outcrop in the Arkoma Basin, Arkansas. The

Mississippian strata are exclusively characterized by sediment gravity flow deposits

and associated basinal shales, while the Pennsylvanian is composed of sediment

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gravity flow, deltaic/shallow water, and fluvial deposits. This style of basin-fill

succession especially from the Pennsylvanian, represented by the stratigraphically

lower deep-water deposits (Jackfork Group and Johns Valley Shale), successively

overlain by shallow water/fluvial deposits (Atoka, followed by McAlester and

Savannah Formations; see Fig. 1.4), reflect two long-term (third-order; Vail et al.,

1977) sequences. These third-order sequences are in turn composed of short-term

oscillations evident from well developed cyclothems from the Pennsylvanian (fourth-

order; Heckel, 1986). The fourth-order fluctuations are evident from stacked

parasequences comprising two third-order sequences in the Pennsylvanian. The third-

order sequences are bound by unconformities of regional to subregional extent, from

the base of the Jackfork, Atoka and Hartshorne formations (Sutherland, 1988).

Sediment supply, subsidence and eustasy, all contributed with varying importance to

generate the observed successions of the Mississippian and Pennsylvanian. The high

sediment supply coupled with enhanced subsidence (Fig. 5.1 a and b), both as a result

of the Ouachita Orogeny, led to phenomenal sedimentation rates (e.g., lOOOm/m.y

during Atoka deposition; Houseknecht, 1986). The Pennsylvanian is also known to be

an Ice-House period characterized by high frequency, high-amplitude, glacio-eustatic

cycles much like the late Cenozoic (Fischer, 1984).

The Paleozoic part of the Exxon sea-level curve shows the Pennsylvanian to

lower Permian forming a super sequence (Fig. 5.2). A super sequence is a major

unconformity-bound unit composed of several sequences. An ideal sequence is shown

in Figure 5.3. The Atokan epoch in Arkansas is characterized by 15-18 sandstone units

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M.Y. -250 -300 MISSISSIPPI** -350 DEVONIAN SILURIAN -400 -450 ORDOVICIAN ® -500 ARBUCXLE MARATHON CAMBRIAN -550 TBINESSEE APPALACHIANS UTH0GENET1C SUBSIDENCE O f OCEANIC CRUST CumubttwlNcImm

Figure 5.1 Subsidence and sedimentation rate curves for the Paleozoic rocks of Arkansas, (a) Graphs showing cumulative sediment thicknesses of assorted Paleozoic sections versus time and (b) Graphs showing rates of sedimentation after compaction, versus time. Curve 1 shows lithogenetic subsidence of oceanic crust after Sclater et al., 1971. Curves 2 ,3 ,4 ,5 and 6 represent subsidence and sedimentation rates in parts a and b, respectively. Data from 2 to 6 are after Elias, 1956; Wilmarth, 1957; King, 1937; Morris, 1974; Swingle and Luther, 1964; and Neuman, 1960, respectively.

separated by shale (Zachry, 1983). These shale-bound sandstone units can be referred

to as parasequences (Van Wagoner et al., 1988), developed in a shelfal setting in

response to sea-level fluctuations, within a general rising trend. Thirty-two equivalent

units called ‘mesothems’ have been recognized from northwest Europe by

Ramsbottom (1977) that are thought to relate to sea-level perturbations also within an

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RELATIVE CHANGES OF SEA LEVEL PERIODS MSMG FALLING1 PRESENT SEA LEVEL PERMIAN PENNSYLVANIAN MISSISSIPPIAN DEVONIAN SILURIAN

ORDOVICIAN

CAMBRIAN

PRECAMBRIAN Figure 5.2 The Paleozoic part of the Exxon Sea-level Curve (after Vail et al., 1977).

zu oM l SHALLOW DEEP

LEGEND

SURFACES SYSTEMS TRACTS (SSI SEQUENCE KXMOAMES HST HK3HSTAND SYSTEMS TRACT S B 1) - TYPE 1 TST TRANSGRESSJVE-SYSTEMS TRACT SB 2 )-TYPE 2 M * indMd^nllair (IN tOLSI DOWNLAP SURFACES 1ST » LOWSTANO SYSTEMS TRACT M » indMd-nOtY HR (unl/ co| • lop mass now/cfiamwl pgc m pregndYig compMx fTSI TRANSGRESSRIE SURFACE m l I c o - n u n How/channel avmfcank deposits IN m lOMNmdlM le • I a ■ c SMVV « SMELF-MARGM WEDGE SYSTEMS TRACT

Figure 5.3 “Slug” diagram depicting the geometries of various depositional systems organized into sytems tracts related to a complete sea-level cycle (after Vail, 1987).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148

overall eustatic sea-level rise. Ross and Ross (1988; Fig. 5.4) recognized more than

seventy third-order cycles in the Carboniferous and Permian, which they group into six

second-order sequences, and speculate that the amount of sea-level change for the

third order cycles was between 100-200m. These cycles are often bound by widely

traceable unconformities signified by incised valleys, karst surfaces, and regoliths

(Hallam, 1989).

5.4 Discussion

Overall during the Morrowan and Atokan times the shelfal sequence is

characterized by transgressive-regressive cycles (Stone, 1968) which resulted in

stacked parasequences. The stacking of back-stepping, fore-stepping and aggradational

parasequences was in response to high frequency fluctuations of relative sea-level tied

primarily to active tectonics (including development of intrabasinal structures, e.g.,

formation of syndepositional growth faults) and glacially controlled eustasy. During

the lower most part of lower Atoka tectonic subsidence was moderate and sediment

input comparatively low which resulted in predominantly retrogradational (back-

stepping) parasequence sets. During middle and later parts of early Atoka tectonic

subsidence and sedimentation were both extremely high resulting in aggradational to

progradational parasequence sets. During middle and especially the upper Atokan

times there was a retardation in tectonic subsidence with reduced sediment supply

which formed aggradational parasequence sets. The following discussion relates to the

different formations and their response to changing relative sea-level.

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COASTAL ONLAP CURVE AS AM INDICATION OF EUSTATIC SEA LEVEL CHANGES

SCALE

Figure 5.4 Coastal onlap curve for the Pennsylvanian Morrowan, Atokan, and Desmoinesian epochs, within the overall pattern for the Carboniferous and Permian Periods (after Ross and Ross, 1988).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Jackfork. Johns Valiev and Blovd Formations: The Mississippian to

Pennsylvanian boundary is marked by a major sea-level fall characterized in the

Ouachita Trough-Arkoma Basin by lowstand submarine fan deposits on the basin floor

(Jackfork Group) and an unconformity on the equivalent shelf (Sutherland, 1988; see

Fig. 1.4). This unconformity represents a major Type 1 sequence boundary that is

globally correlatable, e.g., in England, North Africa, China, and Australia (Hallam,

1992). In Illinois and Kentucky a relief of as much as 134m is developed on the pre-

Pennsylvanian surface (Hallam, 1992). This conspicuous sea-level fall is correlated to

the initiation of a major phase of Gondwanaland glaciation. The duration of this

distinct hiatus was approximately 4.5 m.y. (Veevers and Powell, 1987). The Jackfork

Formation represents a lowstand systems tract and is characterized by basin-floor fans

of a submarine fan depositional system in the deeper parts of the Ouachita trough

(central and west-central Arkansas e.g., Big Rock Quarry and DeGray Spillway,

respectively). A relative sea-level rise at the end of the Jackfork Formation is recorded

by the Johns Valley Formation which exhibits slope-like depositional affinities (slump

blocks, deformational features etc.) as shown in Figure 5.5. The Johns Valley

Formation most likely represents the slope-fan component of the lowstand systems

tract of which the Jackfork Formation represents the basin-floor fan. The lowstand

wedge or lowstand prograding complex of the same sequence is represented by shelf-

edge deltaic facies of the Bloyd Formation to the north (north/northeastern Arkansas,

e.g., Searcy Quarry) when sea-level started to rise during the late lowstand.

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OUACHITA 6E0SYNCLME ARKOMA BASIN e a m u L ouachitas fhomtal ouacmitas

£100 Marik

aaAIW— allalaa

Vaflay S a h ,

OKLAHOMA

Sllilif 6r««f

Figure 5.5 Cross sectional representation of depositional units from the Morrowan and Atokan time. Location of the cross-section is shown in the inset map to the right. Jackfork Group and Johns Valley Shale represent basinal deposits and the Atoka Formation represents deltaic and shelfal deposits (after Sutherland and Manger, 1979).

The Atoka Formation: The Atoka Formation has been divided into sandstone

units numerically numbered from Division 1 through 109 (Haley, 1982; Haley and

Stone, 1996). The following discussions on the Atoka Formation are based on

identification of parasequence stacking patterns by the author that are referred to by

the numbered units.

Division 109-97: This lowermost part of the Atoka is marked by a relative sea-

level fall which is recorded in the stratigraphic column by an unconformity in the

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Jackfork Isopach Cl *1000 ft.

Figure 5.6 Morrowan paleogeographic map of the Ouachita Basin. Modified from Coleman, 1990,1994; Johnson et al., 1988; Houseknecht and Kacena, 1983; Morris et at., 1979, Roberts 1985.

adjoining Ozark shelf (Sutherland, 1988; see Fig. 1.4). This unconformity is not as

pronounced and extensive as the one below the Jackfork Group, and possibly

represents a Type 2 sequence boundary. Hence the lower Atoka marks the initiation of

another sequence (i.e. unconformity bound depositional unit; Van Wagoner et al.,

1988). During the time through which the unconformity developed on the shelf, the

deeper parts of the basin accumulated deep water elastics in the form of turbidites and

debris flow deposits that constitute basin-floor fans and slope fans (C. Stone, personal

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communication, 1997). Subsequent to the deposition of deep-water elastics in the

basin (lower part of lower Atoka) the outer shelf experienced the accumulation of

heterolithic deposits in the form o f shelf-edge deltas (upper part of lower Atoka)

constituting a lowstand wedge or prograding complex, using the terminology of Vail

et. al. (1991). This initial turnaround of sea-level at the late lowstand and active

subsidence generated accomodation space on the shelf translating the depocenter

northwards from the basin floor to the shelf (lower Atoka Formation). The thick

(-65m) progradational package (delta front to distributary mouth bar) at Boston

Mountains is overlain by a distributary channel deposit and constitutes a depositional

unit during this time. This outcrop is perched high up on the footwall block of a large

growth-fault (Mulberry Fault) in north-central Arkansas (see Section 3.3.6 for details).

The later part of deposition of the lowstand wedge is marked by progressively

thinning-upward sand-body thicknesses (parasequence set marked by progressive

thinning upward) constituting a retrogradational or back-stepping parasequence

stacking pattern terminating at the top lowstand surface or the transgressive surface

(Vail et al., 1991). The back-stepping parasequence set is represented by the interval

between Division 109 (Spiro sandstone unit) at the base to Division 97. This interval is

represented in Figure 5.7 (Haley, 1982; Zachry, 1983) and is characterized by

progressive upsection decrease of parasequence thicknesses, from approximately 36 to

21m, going upward from Division 109 to 97, respectively. This is a consistent pattern

that can be observed from Arkansas Oil and Gas Charts 1 to 5 (Haley, 1982). Figure

5.7 is a representative section from Chart 3 demonstrating the essence of the

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Arkansas Oil ind Gas Chaft 3

C5 Basin ward C6 C7

C 4

t o—

SO— l to 0 2 to

' Progradation

T . 05 . J* S f t .

T.0> SjKtt ft» Figure 5.7 Well-log data from the subsurface of the Atoka Formation. Stacking patterns are inferred on the basis of the established correlations. Sandstone unit numbers are represented along tie-lines. A retrogradational pattern is observed from sandstone units 109 to 97, progradational pattern from 95 to 62, and aggradational pattern from 62 to 28. Data used from Arkansas Oil and Gas Chart 3, showing cross- section profiles C4 to C7 (Haley, 1982).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 155

depositional stacking patterns discussed in this chapter. The back-stepping

parasequence pattern is probably related to the initiation of a relative rise in sea-level,

that was more pronounced during the deposition of the middle and upper parts of the

Atoka Formation. This regional representation of the pattern is shown in Figure 5.5

(Sutherland and Manger, 1979). The relative rise in sea-level is demonstrated in Figure

5.8, modified after Coleman et al. (1994) which is based on compilation of results

from Ross and Ross (1987), Cecil and Eble (1989), Arbenz (1989), Sutherland

Figure 5.8 Generalized stratigraphic column for the Ouachita Basin, with relative sea- level, precipitation, and tectonic curves (modified after Coleman et al., 1994).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 156

and Manger (1979). This marks the lowest part of lower Atoka deposits as part of a

lowstand to late lowstand systems tract

Division 95 to 62: This division conforms to the middle and upper parts of the

lower Atoka Formation and encompasses Division 95 (Vernon and Casey sandstone

units) at the base to Division 62 (Morris and Tackett sandstone units; Haley, 1982;

Haley and Stone, 1996) at the top. Active tectonics associated with the collision

between paleo North America and the northward advancing southern continent of

‘Llanoria’ (Fig. 1.3.) during this time resulted in a period of extremely high subsidence

rates (see Fig. 5.1). The rate of relative sea-level rise increased at approximately the

same rate during the middle Atoka as in the later part of lower Atoka (Ross and Ross,

1988). Abundant sediments were shed from the northern hinterland (Ozark and

Nashville domes), which assumed greater relief in response to flexural bulging

associated with pronounced subduction to the south. Normal/growth faults developed

along the southern part of the Arkoma Basin in response to subduction and loading by

northward advancing thrust-sheets. Fore-stepping parasequence sets resulted from

active progradation which is demonstrated by a vertical increase of parasequence

thicknesses, from around 27m for Division 95 at the base of the Middle Atoka

Formation to 106m for Division 62 at the top (Fig. 5.7; Haley, 1982). The middle and

upper parts of the lower Atoka Formation represents deposition in the middle part of a

transgressive systems tract, when relative sea-level was gradually rising (Coleman et

al., 1994; see Fig. 5.8). Steady but slow rise of sea-level during this time concurrent

with pronounced tectonic subsidence/growth faulting (creating immense accomodation

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space) and paroxysmal uplift to the north (providing enormous amount of sediments)

resulted in active progradation of deltaic parasequences across the shelf.

Division 62 to 4: It is composed of the interval from Division 62 (Carpenter B

sandstone unit) to Division 4 corresponding to the middle and upper Atoka Formation

(see Fig. 5.7; First Atoka Sand Unit; Haley, 1982). This cycle is characterized by more

aggradational parasequences primarily in response to very high tectonic subsidence

rates creating enormous accomodation space), concurrent with relative slowing down

of the sediment influx from the north. The decrease in sediment input could possibly

be related to reduced tectonic emergence of the northern cratonic area, relative rise in

sea-level, changing climate resulting in reduced precipitation, or most likely a

combination of the three (Coleman et al., 1994). These units are predominantly

characterized by aggradational parasequence set as demonstrated by the uniformity in

parasequence thicknesses (~ 75-9 lm; Haley, 1982) from the middle and upper Atoka

Formation (Fig. 5.7). Sediments were still being derived from the north and northeast.

The uniform thickness of parasequences reflect a stable equilibrium between sediment

supply and accomodation space generated by the interaction between subsidence and

sea-level fluctuation. The Freshour Quarry outcrop represents two such stacked

parasequences from the middle Atoka Formation (see Section 3.3.1). Thin

discontinuous yet conspicuous coal bands appear for the first time during the middle

Atoka Formation. Coal beds are more numerous and extensive during the upper Atoka

Formation (Haley, 1959,1978). Development of coal-beds are primarily tied to base-

level elevation (Flint et al., 1995). An episode of abrupt relative rise in sea-level

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during the later Atokan time resulted in a sudden elevation of base-level which was

conducive to the development of numerous coal beds (Fig. S.9). The constancy of

accomodation space, resulting in aggradational parasequence sets and the presence of

coal beds, indicate deposition of the middle and upper Atoka Formation under the late

transgressive systems tract (TST) to early highstand systems tract (HST).

During the Desmoinesian, Hartshome and McAlester formations conditions

were even more conducive for the development of economic coal deposits promoted

by rise in sea-level (Fig. 5.9). The rise stabilized in a relative highstand during the

Desmoinesian epoch (see Fig. 5.4).

The stacking patterns of depositional units within the Jackfork Group, Johns

Valley Shale, and Atoka Formation is summarized in Figure 5.10. The subaerial hiatus

at the base of the Jackfork Group represents a Type 1 sequence boundary, while that

below the Atoka Formation possibly represents a Type 2 sequence boundary. The

back-stepping (retrogradational), fore-stepping (progradational), and aggradational

stacking pattern in the lowermost part of lower Atoka, middle and upper parts of lower

Atoka, and middle and upper Atoka Formation is demonstrated in Figure 5.10.

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Cava rial coal

1610 Coal

Coal 500- 1620 N- -£2SL_3S Coal

Coal Coal

1500- 9000

Figure 5.9 Stratigraphic column showing the coal-bearing formations (Upper Atoka, McAlester and Savanna formations) and coal-beds from west-central Arkansas (after Haley, 1959).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160

Middle and Upper Atoka Formation Aggradation

= £ >

Middle and Uppermost Early Atoka

Progradation

Retrogradauon

Lowermost Early Atoka

Correlative Conformi

Johns Valley Formation

Jackfork Formation

Sequence Boundary Correlative Conformity

Distance

Deep-water elastics and slope deposits IWIH Subaerial Hiatus

Deltaic parasequences Stacking Pattern

Figure 5-10 Summary diagram based on the results from the present study depicting the depositional stacking pattern of Pennsylvanian deposits in Arkansas, with special emphasis on the Atoka Formation. Sequence boundaries at the base of the Jackfork and Atoka formations are shown.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6

CONCLUSIONS

The present study on Pennsylvanian Morrowan and Atokan deltaic deposits

was aimed at characterization of different subenvironments. The first conclusion

reached confirms the fluvial dominated origin of the observed sedimentary sequences.

Other conclusions deal with detailed sedimentary characterization of various

subenvironments from these fluvially-dominated deltas. A summarized extract of the

results from the study is shown in Table 6.1.

1. The deltaic deposits from the Morrowan and Atokan epoch document

fluvially dominated deltas over wave- and tide- dominated types. Ideal progradational

sequences are preserved, consisting of prodelta deposits successively overlain by delta

front/distal bar and distributary mouth bar deposits. The sequences show well

developed coarsening- and thickening-upward trends as observed in other ancient and

recent fluvial dominated deltas. The presence of small-scale cross laminations in delta

front, and large scale cross bedding in distributary mouth bars and distributary channel

fill, indicate predominant unidirectional currents supporting fluvial dominance.

Sandstone isolith and porosity maps, based on subsurface correlation, reveal lobate

shore-normal sand bodies, flanked laterally by shaly successions that further support

fluvial dominance. Evidence for the influence of wave and tidal activity are, however,

seen in some outcrops.

161

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2. Interdistributary bayfill deposits are characterized by dark gray splintery

shales with occasional millimeter-scale laminations and rare lenticular laminations.

The total thickness of the deposits varies from 18 to 20m. These shales contain

abundant siderite nodules and organics. The sand to shale ratio is low depicted by very

high gamma-counts. Bioturbation is abundant; traces especially the horizontal types

(Plcmolites) are common in the lower bayfill sections comprised of silty laminations.

Another trace fossil identified as Conostichus is documented in the upper bayfill

successions associated with a gradual increase in coarser sediment influx. The lower

contacts of the interdistributary bayfill deposits are sharp whereas the upper

boundaries are commonly transitional with overlying crevasse splay deposits, where

present. Bayfill shales are commonly associated with carbonate hash beds composed

of fragmented marine fossils representing overwashed storm deposits. The

interdistributary bayfill shales gradually coarsen upward and are overlain by crevasse

splay deposits (documented in the 3MOR section).

3. Crevasse splay sandstones are comprised of fine grained (fL to fU), tabular

bedded sandstones with overall coarsening and thickening-upward trends. Bed

thicknesses vary from 10 to 30cm on an average, with a maximum thickness as high as

80cm. Thickness of the crevasse splay deposits are - 1 lm with a high sand to shale

ratio. Crevasse splay sandstones are amalgamated and show ripple cross laminations

which increase in scale upsection. Amalgamation contacts are associated with rip-up

clasts. Bioturbation is restricted to the lower part. Rootlet structures on top of the

crevasse splay sandstones are a common diagnostic character, indicating colonization

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by plants after abandonment of the sand body. The basal contact of crevasse splays is

gradational while the upper contact is sharp. The crevasse splay sandstones are

overlain and underlain by bayfill shales.

4. Distributary channel fill deposits show variability in their sedimentary

character and are divided into two groups, active and passive channel-fills. Active

channel fills are comprised of fine (fL to fU) and medium (mL) grained sandstones.

Bedding thickness varies from 10 to 30cm and may be as thick as 300cm. The total

thickness of active distributary channel fills varies from 20 to 30m. Individual

sandstone beds are characterized by a lensoid geometry associated with profusion of

lateral wedging (cut and fill structure). Large-scale cross bedding is the predominant

sedimentary structure. Basal surfaces are characterized by erosional scour with

abundant rip-up clasts. The channel fills demonstrate an overall fining-upward trend.

Passive distributary channel fills are characterized by finer elastics, e.g., shale to

siltstone with rare occurrence of fine-grained sandstones. Beds are usually less than

Scm thick with parallel laminations. Sand to shale ratio of these units are very low.

5. Distributary mouth bar deposits are documented from several outcrops.

These deposits are characterized by fine-grained sandstones. Average bed thickness

ranges from 10 to 30 cm reaching a maximum of 150cm in the upper parts of the

deposits. Total thickness of these deposits varies from 18 to 28m. Sand to shale ratio is

very high. Beds are commonly tabular in geometry and exhibit wavy to wispy

laminations. However, bed lenticularity, cross-bedding and occasional cut and fill

structures are also present Organic matter occurs dispersed in the beds. Silty to shaly

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partings, associated with thicker beds, are common with occasional rip-up clasts and

rare bioturbation. Occasional Paleophycus and Opbiomorpha burrows are documented

in these distributary mouth bar deposits. The basal contact of these deposits with the

underlying delta-front sandstones are gradational while the upper boundary is sharp.

Distributary mouth bar deposits characteristically exhibit thickening and coarsening-

upward trends.

6. Delta front/distal bar bed thicknesses vary between 5 to 8 cm. Total thickness

of these deposits ranges from 10 to 20m. Delta fronts are characterized by alternation

of thinly bedded very fine grained sandstone to siltstone and shale. Delta front beds

have a wide range of bed geometries, both tabular and pinching/swelling are observed.

Ripple cross laminations and wavy laminations are present in the very fine-grained

sandstones. Upper bedding surfaces are often intensely burrowed. Both horizontal and

inclined burrows and crawling traces are abundant Vertical to inclined burrows in the

delta front deposits are identified as Bergaueria. Commonly occurring traces are

identified as Thallassinoides, Planolites, Gyrochorte and Lockeia. Intense bioturbation

obscures most of the primary sedimentary structures.

7. Prodelta bed thicknesses vary from 1cm to a maximum of 2cm. Total

thickness of these deposits may be as high as 33m. These deposits show persistent

parallel lamination of siltstones within the shale. It occasionally shows lenses of very

fine-grained sandstone with discontinuous ripple cross-laminations or starved ripples.

Prodelta deposits are composed of several small scale (0.5 to 1.5m thick) coarsening-

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upward packages. Bioturbation, is abundant especially in the lower part of the

deposits, the most common type being PUmolites.

8 . Trace fossil analysis from the deltaic deposits reveal systematic variation

and distribution that are facies dependent Trace fossils are most abundant in the delta

front sediments, including Bergaueria, Thallassinoides, PUmolites and Gyrochorte.

The presence of the clay-lined Paleophycus and strong walled burrows Ophiomorpha

in the distributary mouth bar deposits indicate a high energy environment associated

with very high rates of sedimentation. Conostichus is specifically restricted to

interdistributary bayfill deposits. Trace fossil assemblage matches Curvolithus

association which is intermediate between Skolithos-Cruziana and Zoophycos

ichnofacies and indicates deposition between fair and storm weather wave base. The

Zoophycos ichnofacies represented by Scalarituba, PUmolites, and Paleophycus,

indicate deposition in shallow to intermediate, shelfal water condition represented by

distributary mouth bar and delta front deposits. The Skolithos-Cruziana ichnofacies,

represented by Conostichus, Lockeia, and Thallassinoides, indicate deposition in a

nearshore condition including delta front and interdistributary bay fill deposits.

9. Distribution and type of kerogen in various subenvironments corroborate

interpretations independently based on sedimentary characters. Long distance fluvial

transport, is indicated by the dominance of amorphous non-structured assemblages in

all subenvironments. This supports the fluvial-dominated origin of the deltaic deposits.

Carbonate hash beds in bay fill deposits contain fluffy material which suggests a

marine origin. This supports the storm related allochthonous overwash origin of these

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beds in an interdistributary bay. Lycospora spp. dominated assemblages suggest

deposition of sediments in a lower delta plain covered by a typical Carboniferous coal-

forming swampy vegetation.

10. XRD studies, together with microprobe spot analysis, from rocks of

different subenvironments show monotonous composition of the clay mineral fraction.

The detrital component is dominated by illite and rare chlorite in the bayfill and delta

front deposits. The clay mineral fractions in the distributary mouth bar deposits are

exclusively composed of illite. The authigenic component is characterized by the

exclusive presence of chlorite in all subenvironments. The fresh water dominated

depositional environments, e.g., crevasse splay and distributary channel deposits,

however, have two chemically different chlorites corresponding to the detrital and

authigenic varieties. Lower aluminum to silicon and magnesium to iron ratios in

chlorites of authigenic origin have been documented from the distributary channel and

crevasse splay deposits. Smectite to illite transformation reaction may have acted as

the source for the silicon ions that give rise to quartz cementaion in the sandstones.

11. Detailed petrographic studies were classify the sandstones and

quantitatively determine their compositional variations to evaluate tectonic influence

on the deposition of the Pennsylvanian Atoka Formation. The rocks have a

composition of Qg 2-90 F3.10R5.19 and are classified as sublitharenites. Quartz is

dominated by monocrystalline types (82-71%) with minor amounts of polycrystalline

variety (3-7%). Feldspars constitute a minor proportion (K-feldspars 2-5%; plagioclase

feldspar 0-4%); and lithics (5-12%) are composed of metamorphic and sedimentary

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fragments. Paleocurrent data and a high percentage of quartz suggest that the

sediments are primarily derived from the northern craton. However, metamorphic and

sedimentary rock fragments imply the Appalachians to have acted as a secondary

source area. The high quartz content, and a dearth of feldspars indicate that a warm

and humid climate with associated low relief extensively altered the primary

mineralogic signatures from the source area..

12. A combination of sedimentologic and rock-fabric analyses was used to

demonstrate the similarity in the depositional processes between crevasse splay

subdeltas and major delta lobes. Variations in grain-size, bed-thickness, and

sedimentary structures indicate increased flow velocities upsection in both the

subdeltas and delta lobes. Results from rock-fabric studies, using anisotropy of

isothermal remanent magnetization, are consistent with this inference. Well developed

sedimentary fabrics are represented in the samples with the long axes (K^m) of

anisotropy ellipsoids lying on the bedding plane. The short axes of anisotropy

ellipsoids are perpendicular to bedding. The Kmax lineation systematically varies with

flow direction. Percent anisotropy increases upsection in both the subdeltas and major

delta lobes, coherent with inferred increase in paleoflow intensities based on

sedimentary structures. The mean Kmax vector direction from distributary mouth bar

deposits are approximately flow normal. Mean Kmax direction for delta front and

crevasse splay deposits are roughly flow parallel. The mean anisotropies for delta front

and its overlying distributary mouth bars are - 8 % and ~ 19 %, respectively while that

for the upper reaches of crevasse splay subdeltas is ~ 13 %. Integrating the Kmax

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direction data with respect to observed paleoflow directions and percent anisotropies,

inferences based on depositional styles have been made. The flow normal Knuu

direction and high % anisotropies (- 19 %) from distributary mouth bars indicate

deposition dominantly from traction flows with associated high flow velocities. The

flow parallel Kmax orientation and lower % anisotropies (~ 9 %) from the delta front

deposits indicate lesser dominance of traction and lower flow velocities. Flow parallel

Kmax orientation from the crevasse splay deposits and intermediate percent anisotropies

(~ 13 %) indicate a mixed traction and suspension mode of deposition from flows with

moderate velocity.

13. The Pennsylvanian deposits of the Arkoma Basin have been placed in a

sequence stratigraphic framework based on interpretations from subsurface well logs

and outcrop observations. The Jackfork and Atoka Formations represent two

unconformity-bound depositional sequences. The Jackfork, along with the Johns

Valley Formation represent a lowstand systems tract (LST) comprised of submarine

fan and slope, elements respectively. The lower part of lower Atoka, middle and upper

part of lower Atoka, and middle and upper Atoka Formation, comprised of deltaic and

shallow marine parasequences, reveal retrogradational, progradational, and

aggradational stacking pattern, respectively. The lower part of lower Atoka represents

a low stand to late lowstand systems tract (LST). Middle and upper part of lower

Atoka Formation represent deposition during an early to middle transgressive systems

tract (TST), respectively. The middle and upper Atoka Formation represent deposition

during the late transgressive to early highstand systems tract (HST).

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14. The comparison between ancient and modem deltas is often difficult.

Extraction of the relative importance of tidal, wave or fluvial influence from the rock

record is difficult when any one of these processes is not dominating. Preservation

factor plays a crucial role in studies dealing with the rock record in this regard. Coastal

and nearshore sectors are especially complicated because of the numerous interactions

between several factors. The lack of continuous outcrops, and post depositional

erosion that eliminates key surfaces and units, make facies analysis difficult from the

rock record. Minor changes in relative sea-level cause transgression and regression

which are often difficult to distinguish from autocyclic changes. Intense bioturbation

by a diverse fauna in nearshore conditions obscures primary sedimentary structures

and makes facies determination difficult. Post-depositional changes, such as

compaction, burial diagenesis, and weathering, further add to the difficulty in

interpretation. Weathering obscures sedimentary structures with consequent increase

in the degree of difficulty in determining depositional facies. Working with ancient

deposits such as the Pennsylvanian also restricts application of microfossils in

distinguishing different sedimentary environments. All that is generally possible is a

detailed examination of outcrops and a comparison with modem environments by

means of literature survey to reach the best possible conclusion regarding syn- and

post- depositional processes. Outcrop study provided better visual aspects than core

analysis. Total absence of body fossils from deposits in the present study with only a

sparse microfauna, made the task even more difficult The multi-pronged laboratory

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. analyses conducted in the present study provided supplemental data that helped

constrain the major conclusions.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1cm, 1.5cm Boston Prodelta us clayey lamination Freshour, 2MOR Mt. Monotono shale and silly shale maximum 30*33m lamination a. parallel b. lenticular lamination occasional ripple ousripple lamination /starved c. discontinu cross table con’d. 12m Distal bar Searcy, Boston Delta front/ sandstone alternation Freshour, Very fine and silt/shale 5-8cm a. pinch and swell to tabular bedding lamination b. lenticular lamination c. wavy burrowing e. Organics d. intense l0-30cm !8-28m 1.3m Mouthbar Searcy Quarry, Boston Mt. Distributary Finesand Freshour, average, maximum lamination a. wavy/wispy b. silty/shaly partings c. occasional cutand fill tabularbedded in dip section, large scale in strike d. Massive cross bedded section very very fine Shaleto siltstone to Passive Searcy Quarry sandstone 5-6cm max. Channel fill 4m lamination a. parallel 10-30cm Ml. Active Fill Fine tomedium sand 20-30m upclasts average Channel 4MOR, Boston b. ripplecross lamination a. abundantrip structure c. Cut and fill cross bedding ripples f. water escape d. large scale structure e. climbing IO-30cm average,IO-30cm 3MOR maximum0.8m sandstones lamination Crevasse Splay b. wavy bedding f. water escape a. amalgamated c. ripple d. Rootlet top structure structureat the clasts e. abundant rip up l8-20m 10-llm Silty shale Finesand Freshour 3MOR,4MOR, with silty lamination mm scale Interdistributary bay fill b. abundant siderite nodules a. splinteryclay c. organics and rock type Location Grainsize Bed Total thickness Sedimentary structures thickness Character riarnes andtheir riarnes depositional tabulated are affinities characters. Significant variationssedimentary versus different are presentbetween depositsfrom subenvironments.different and Chapterdetails. forconclusions 3 See Table 6Sedimentary .1 characteristicsvarious from observed present The during the study. deltaic outcrop subenvironments

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Is) in lower up part coarsening packages (.5-1,5m, throughout abundant sharp small for long silty lamination continuous lateral distances thickening moderateintense very low gradational and intense coarsening alternation bioturbation oflithology, occasional gradational with same thickening laminated massive thickly bedded, wavy cycledeposit coarsening and low very high thickening filled with finer and sediment sharp, erosive coarsening erosivecut fining and thinning bedding occasional not visible fining up, scour and fill, cross very few at the high high bottom thickening gradational sharp, erosive coarsening and rootlet structures at the top very low sharp towards thetop nodules slightly coarsening Splintery habits of shales with sideritc Sand/shale Bioturbation abundant ratio Lower contact Trend up Special character

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Zachry, D.L., 1983, Sedimentologic framework of the Atoka formation, Arkoma Basin, Arkansas, in: D.W. Houseknecht, ed., Tectonic-Sedimentary Evolution of the Arkoma Basin: SEPM Midcontinent Section, v. 1, pp. 34-52.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A

The measured sections are presented in this Appendix. The sections are presented in sequence from the West Face of Freshour Quarry (WFH), East Face of Freshour Quarry (EFH), Conway Roadcut, second Morrilton Road Cut (2MOR), third Morrilton Road Cut (3MOR), fourth Morrilton Road Cut (4MOR), Boston Mountain Road Cut, and Searcy Quarry The legend that goes with the measured sections is also provided. For details refer to Chapter 3. For the matched gamma-ray profiles also refer to Chapter 3.

LEGEND

UTHOLOGY

l&WSij medium sandstone sandy sit

tine sandstone (b lg g j] sWsfltstone [pv-j-lj shale/mudstone

very fine/silty sandstone eisyey silt carbonate hash

PHYSICAL STRUCTURES

current ripples '■O bough cross^traL scour ^ dimbing ripples wavy parallel lams. .V rip up clasts jv. water escape str. ^ erosional cut arid fffl contact —n> amalgamation contact = parallel laminations o lenticular bedding ^ massive X otganics TS load casts = 5^ hummocky cross-stat. low angle tabular bedding /w i convolute bedding accretion bedding

UTHOLOGIC ACCESSORIES Sid Siderite Odd shed fragments Py Pyrite

ICHNOFOSSILS a planolites § bored hardgraund J:* rootlets c s palaeophycus 0 Ophiomorpha

CONTACTS > ■ sharp undulating w w w scoured

190

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> C * I JflfS f! O in CC o i IflM ]1 i

i < v ^ i Ik

XX X X t xxxxxxxx: m x : xx xx xxxx: xxxx: xxxxxxxx: m x x x x : xxxxxxxx: xxxx: X3C30CX3C36X) 9PCXXX3xxxx: xxxxxxxx:K X X X X X X X 3 ‘MOMxxxx: xxxxxxxx:

12 i 1 ■ 1 I i f s |! llfP ^ !t! I i

li'ffdt'ftl >fl u /ss i i s. t 11/11/ i s / 1 i / I

xxxx; xxxx xxxx xxxx: xxxx: xxxx: xxxx xxxx xxxxxxxx: xxxx; vdHf

fU to mL sst, av. 10-12cm beds wedging with upper and lower Shaletiasis commonthroughout (mostly concerAiaiedYri 4 bed s with partings. Internally m assive to faintly cross bedded. One massive bed with distinct amalgamation contacts. 5-15cm thick shsHty ale break. Ripple crest 210/65. amalgamated.Toward the base of this unit major cross bedding. surfaces wavy mimicing ripple field. Partings common but mostly laminations present. discontinuous crumbly lenticular zones). Poorly developed wavy developed cross bedding. bloturbation at the very top for the first time in the section.Well section.Beds are wedge shaped. Rip up clasts occasional. laminated and cross bedded. Bloturbation in the middle of the rich and micaceous. Occasionalto cross partingsbedded. 1-2cm Occasional thick. Internally ripple flattened rip up clasts common. top. Lensing and wedging out common. Top of every bed organic Towarco the top relatively thinner beds-10-12cm with parting. ■ ■ Well bedded (5-.13cm) with occasional partings, Internally cross • Nonuniformly thickening up from -10cm at the base to 30cm at the \ Internally ripple laminated, shaleclasts common mid way, CONWAY ROADCUT CONWAY ESN 20 22 26 2 4 30 28 . x: -: •< -: tv granule UWiWlW BIZK ^VVVVVVVVV : i l l l i 8xs8r a n ORAZH ORAZH

ti I st, av. 10-12cm beds wedging with upper and lower 3 HCS common throughout the unit. Wavy laminated unit with amalgamated contact within b eds. Very fine sandstonefeiltstone with occasional skty shale partings, 3hate ctasls common throughout (mostly concerrtrateO SmaD coarsening and thickening uppackages of skty shale and fU to mL (organic and mecaceous).Wavy laminattton common throughout. units below. Major wavy bounding surface (w ave length 1.5-2m beds, internally HCS. Top surface is a major one marking boundary alleast) thickness of beds. Internally ripple and wavy laminated. Bedding between cross bedded units above and wavy laminated to HCS contacts still present with wavy lamination. amalgamated contacts with squashedpu: .emnants ol parting % of line sandstone increases upward with increasing # and stratification, HCS) siltstone with percentageupward. Siltstone of siltstone beds increasing are wavy with fromthickness 50 to 80 at individual% beds Rare siltstone towards the bottom. (large scale bedform, long amplitude identified as hummocy cross lamination are common. a s parting. At the very top 0.25m thick bed with sw alyand wavy bed contacts wavy. contacts foliaceous with amalgamated nature. thickness from 0.5-2cm. Otherwise splintery shale well laminated. amalgamated.Toward Ripplethe base crest ol this210/65. unit majorcross bedding. laminations present. varyingbetween 2-4cm, Occasional lenticularlamination and tipple bloturbation at the very topdeveloped for the first time in crossthe section.Well bedding. surfaces wavy mimicing ripple field. Partings common but mostly discontinuous crumbly lenticular zones). Poorly developed wavy Several subunits (av. Th. 7-flcm) vfU to r ssLAbundant Siltstone beds getting thicker (6-7cm) upward with sHtyshale only Internallyconstituted by amalgamated units of variable thickness, # of fine silt b eds increases towards the top, with increasing Heavily wavy laminated unit. Amalgamated wavy discontinuous ft-sst.toward the base goes uptofU at the top.Amalgamation \ internally nppie laminatea, snale ciasts common mia way, 10 16 14 18 22

wy xxxxxxxxxx: : xxx xxx.xxxx;xxx : ixxxxxxxxx; ::;xxxxxxxxx: x-:xxxxxxxvx.: •' •' :xxxxxx:rMxx '. ••• -:;^ xx x x x x : : : iKKXxxxxxaa &XXXXXXXW& ; xx x x x.• :S!>n% xxxxxxxxxx:

At 37.6mAt well developed x-bedding present. Surface resembles Contacts sharp andflat, very slightlywavy.Bipple-x stratification thickness of sand 4cms siltstonecontains wed developed toll-lamination lamination.Belowwavy 26 m shale/siltstone overwhelms sandstone. starved ripples lamination),(lenticular bottom contactflatter and wavier uppercontact of sandstone layerscommon,Shale and Above 26m sand increases but subordinate than shale/sDt.MaxImum sandstone.Sandstone layers pinch and swell and stringersform a s in accretion bedding largerand better developed towards the top of each bed. resulting in wavy lowercontact on subsequently overtying the sand unit. sandstone. Scaleof x-bedding Is much higher. Lateral wedging out of individual beds unlike flat andcontinuous'beds atthe very base of the beds foliated due to x-lamlnation? distinctcurved lowercontacts resemble smileyfaces. Upperparts of - Composite of beds thicknessranging In 10-30cm, Moderately Monotonous alternation ofshale, sUtstonerfine to very fine 39m At waterescape structure. Uppercontacts around38m start getting wavier dueto tippling 2MOR SECTION 2MOR ESN ?" B t V ’—O m t V ^ e» V” *• V” ^ e» t W VV V ** ** «!_« t •- granule «p ' ^^ ^TTTTT T T T T T7TTT T i T TTTTTTTT t t T t 5555SSS »r n h h l uvi-ivvv" r cUv •—** •—** rrrrr rr-. ; ; I I sand U mill mill vcmfvl vcmfvl B QKAIN SXU QKAIN

Micaceous uppercontact redappearance.with theWithin bed wavycontacts suggestssurfaces. ripple P.C, Contacts gradational, medium dark gray, micro scale micro gray,Contacts dark gradational, (mm) medium Laminations micro scale, Laminations micro occassionallaminations. lanlicular developed.SMtstones, microcross waS iMaminations & Effect increases, of rippling starved ripples. ParaMwell lamination Smalcoarseninguppackage,from0.5to 1,3m.,burrow%overall Pinchand sa lsand (5-€cms) shale, in dueIMams, to rippling. with x-tams. ripple with uppercontact and flat bottom contact. laminationswell developed. Ripple crossmicrointervals laminations resemble discontinuous, Ripplestaminaltone. lenticular we* Indicating erosion. N11SWpredominant, but bimodaldirections present? Base of thin N105W. developedshalyin layers and and common sandyin sSty layers. P.C. developed layers In M ,5cm sbutabsent thick in layers <1cm thick. sand swaley suggesting at 1,6m) has loading?loading unM( Underlying (?) occursas swales (tOcmw.f) same. Aggregate ot sandstones, with amalgamated uppercontact, ^ Finesand, thttena up. P.C.N110W. y - Monotonous shale and siltstone alternation, sed.str. same. t Interspersedthrough the wholecm -1,5cm horizon 1 very line sand shale, splintery laminated dark wavy layers sidy weH within Very \ — ■ Severalcontacts the within bed, depicting uppercontact ripply.

Carbonate hash layer marks the beginning of sandstones above Splinteryshale. laminations. to high rates of sedimentation. amalgamated sandstones more common. Mudclast/chip impressions. Top of beds micaceous. Above this level have ripple and wavy laminations.rare. Bed thicknessAbove this traction Increases structure withare total exclusion of liner materials and wavy laminations. ■ ■ Overall sand higher,% - sediments churned, Veryfine sands with burrows (cylindrical), layerswarp around it. - Occasional fine sand of 2cm. pinches and swells with cross - Micaceous top ot bedding planes. Less than lOcms thick beds - Contacts distinct, somewhat wavy. % of burrowing decreases due - Sandstone beds getting thicker on an average 5-25 cms. Upperpart characterized by root zones. 3MOR SECTION 3MOR Y T 20 2 6 22 r o t 2 4 3 0 . 2 8 3 2 34 36 3 8

B BMPS t -sand -sand -ai.lt U E S N R S I -cobble -pebble -granule o T A R —clay *«• t t *«• « Hill vcm fv GRAIN GRAIN 8IZR

Overall sand % higher, sediments churned. Eight layers from 4.7S to 7.4m , at regular Intervals of 30-75cm s. Veryfine sands with burrows (cylindrical), layers warp around it. Occasional streaky patches of siderite nodules Carbonate hash layer marks theOccasional beginningof sandstonesfine sand of 2cm. pinches aboveand swells with cross Splintery shale Greyish black to dark gray splintery shale Carbonate hash layer This is common through out. laminations. - le- 1 fi 1 14- 20 12- 22- ■10- O s S E s ■IKKKKHrX p i

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The parting at 41m equivalent tocondensed section??equivalent at 41m Upper The parting top. 8the very continues tyupto which S abate Baaeat Link clasts, developedx-lamination. poorly very present. Lower parts aggregateol parts severalshale present.beds.small Lower Very averagesame thicknesstheyarecontinuous. and wedging out visMe In the medium dicker sands.thinner and medium the out wedging visMe Finer In over a lateral distanceol aover1.6m. lateral beds persistent. Thins out laterally Into an unNotm thickness olthickness7cm an unNotm Into bedslaterally persistent.out Thins its three subequal three units. Thicker bodsits havethree subequalThicker couple three of untootunits. upper oneupper is thicker. migration to & Iro towards H A 8. Lower beds show distinct wavy beds 8.distinct show A Lower H towards Iro to & migration packageot bedsot wedging showsdid lateral Overall interval. within baso, aroslondthining cuts. and of4-5Aggregate beds tfdcfcening beds. beds poorly developed. continuation of minor channel cuts (1,5m) along that bod. Within the cuts bod. Within channel (1,5m)along that of minor continuation setchannel developedpoorly x-beds, Oppositely directed accretion beds successively in olbrds bods group or Indicating overlying Lower beda thickens and thins due to erosional activity, Wed dueerosionalactivity, bedato thins thickens and Lower scaledeveloped and x-beddinglarger pinching above lateral (5-10cm) marked by discontinuous poorly ^avatoped base ^avatoped andtope. poorly discontinuous (5-10cm) by marked themod bed. tower In swakrtg parting. Deformed bedding present Deformed bedding parting. base.Erosional mod sectionbeds areaggregate* showing scouring of thinner thteklng up packagea crevasseup (1.4m) splay? thteklng affectconcaveand cuts. upwwd - Ldsrdbed isaggregdeot Every lamination. beds.. Wavy thin - beds.bed aggregate Is Here 2. ol thin Every of Unit Continuation -cuts top. Eroskmol the towards Increasing clasts up abundant Rip - beds. beds,olassociated Aggregate two thicker Parting with - beda ot Aggregatethin endot Unk4, marks boundary Upper - 3. etosiondOccasional concaveup Unit cuts. Largdscale cross - Aggregates ovetbank Supposedly rich. ot severalunto. Organic - dueof oiScropto possibly theerosion. upper Al Lack the bed* In 4MOR 4MOR SECTION E E S N 24 28 28 32 38 34 36 40 44 42 I E 0 T RSI BMPS BMPS 0 T A R u \ •ft', • t > 2 ! H j ------H \«^8+ },• j.j j,* •,» •,» j,* j.j },• I: x x x x x x x x x x x x xt I: ' • • v j.j 5 uxxxxxxxxxx: 7| [—clay ■ $ III vc mi vc DRAIN S I M

vttdnire

o t

present onwestern

but lace eastern In In 1.5 m. 1.5 ot ol erosion lacking erosion lacking ol Base ol Unit 1. The unit continues upto 25m. Base sharp, distinct distinct sharp, Base25m. upto continues unit The 1. BaseUnit ol Very fine sMstone beds are persistent, Smell cytfndrtcal burrows on burrows bedspersistent, cytfndrtcal Smell are sMstone fine Very Rip up clasts abundant Increasing towards the top. Erosional cuts Erosional top. the towards Increasing clastsabundant up Rip Every bed is aggregate ol thin beds.. Wavy lamination. Lateral Lateral lamination. beds.. Wavy thin ol aggregate bedis Every Dark spHnlery shale alternating with akty to very fina sandy fina tovery akty with shale alternating spHnlery Dark average same thickness and they are continuous. distance alateral over rite sidedevelopment. bedding planesbedding the stty arein tine present layers. Specks present in sSty shale layers which oocasionaRy has pyrite and nodular and has pyrite oocasionaRy shale which present sSty layers in (ace(subtle). present Lower parts aggregate ol several beds. Very small shale small beds.severalpresent Very ol aggregateparts Lower x-laminat(on. developed poorly clasts, very evidence wedging out visible in the medium thicker sands. Finer and thinner andthinner sands. Finer thicker medium the in visible out wedging 7cm ot thickness an uniform into laterally out bedsThins persistent. (62-Mmicfo mythin bedded to laminated. Bedding in abate and sidy and abate in (62-Mmicfo beddedBedding to laminated. mythin lamination, Sfkyfcandy layers 2-3cm thick. Overalsectionis 2-3cmthick. layers Sfkyfcandy lamination, Burrows are very linevery aretype), (B2 pyrite,coal, Burrows lealiound. impressions shale duepresent todisturbed but burrowing. SandstonesMftstones showsdeveloped indpiently wavy predominated by shale wNh thin layer* olbetween. layer* in sNtstones thin byshale predominated wNh 12

t V t r - - » » V t m r n i V

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Burrows are not 3cmthick, (max. aocm). Characterized 1 Wavy bedding, i l sand,io - Lower top surface Is a majorsurface along which lowermost beds Very fine sandstonewith abundant horizontal burrows (planotiles). ripple cross lamination. Occasional interbeds 5-10cmthick getting as nonuniformly ripple laminated.Organlc rich sWy/shaly partings rippple to x-laminated, scale increasing. Organics demarcatethe shale/organics. Sedimentarystructure characterized by wavy to Organics common throughout. veneers. From 18m up the beds look massive but internallystill thin as 1-2cmcharacterized bydark skty shale with very fine sand as of upper units pinches upwardout and from5-7cm wedges atthe base out.and Overall then progressively even thickening thickerto thickening 0.6m*1m.and Grainslse Fu in tothe 20-21cm inupperthe parl.IntemaWy thelower part beds is IL are less bioturbated and reactivation surfaces.From Top 18mbedding burrows are surfaces practically nonsurface.existent. are The NnguoidProbablya bedsrippled. arethen mi^or practically tabular with total exclusion of of very fine sand.Upper surfacerippling. of finesands wavy indicating bywavy discontinuousparting with orwithout thin veneerof discontinuous lenses (pinch and swell) exhibiting lenticular bedding. Scalarituba burrowscommon butvery fine scale. common (<0.5-2cmthick).ln many instances sqeezed out tothin any shalypartings. Qrainsizejumps fromtofL fU. Overall thickness increasing (max. 0.6m). Occasional 0.5-2cmthick Bedsets somewhat thin upward. only concentrated oncurrent the bedding ripple laminated, planes. but was Overall difficult to measurenonuniformly due to poor veryfine sand/sHtstone partings with hbundant micas andorganics. outcrop condition. - Uptil 15 meters sandstone,fU tabularbedded (av, 15-20cm). \ 0.6m is fine sand interbedded sHtstonewith veryfine stringers of n 8 • 8 16 18 14

12- 10 'T Ol 22- -20

: : , ; a x: x: xlT, xlT, xxx: xxx: ...... xxmx

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x x x x :o - •: x x x x x x x x x x x x xxxx xxxx: xxxx xxxxx xxxxiTnrTTx’. •: x x x x x x x x x x x x x

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APPENDIX B

Data from clay mineralogic analyses, X-ray diffractogram from smear slides and electron microprobe backscatter images with spot analysis spectra, are presented in this appendix. For details refer to Chapter 4. X-ray diffractograms from samples TG4MOR2, TGWFH9, TGNFH2, and TG2MR3 are presented in sequence followed by backscatter images and associated spot analysis spectra from samples 3MOR2, EFH5, SFH3, and 2MOR5. For details refer to Chapter 4.

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00‘8QS B^unoo 00*0

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0 0 0 '*-

X T O O 'i

►00'B u in i **m a til 00*998 s^unoo 00*0

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8)tino3

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p j OOOfi

-in

y- 000

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AI i I Si

Mg Fe

M A VMJ^ ItA u,

t M t l T tkCVI

Si At

Mg Fe

AMS K ' • * I

iV5 i.O 4.9 9.0 4.9 10.9 (p.»:jy itavi

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2MOR5 B

:#r

tj«

• j

A1 Si

**U»ryu*.

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APPENDIX C

IRMA grain-fabric data. Data from distributary mouthbar, delta front, and crevasse splays are presented in order. Refer t to Chapter 4 for details.

D elta F ro n t Sam ples Sam ple K m ax K int Kmin (D/I)max (D/I)min Anisotropy % L F SFH95.U 12739 122.06 114.87 2873/-13.9 58.2/-69.4 10.47 1.05 1.06 SFH16t 151.13 141.48 137.13 4 2 .7 /-1.4 4.9/+88.2 9.77 1.07 1.03 NFH3 197.62 187.99 183.43 121.9/-2.7 196.8/+79.8 7.48 1.05 1.02 N FH 3a 198.17 18838 184.10 1163/+03 208.1/+73.7 739 1.05 1.02 SFH3b.95 10733 105.11 98.78 298.4Z-7.8 65.0/-77.1 8.24 1.02 1.06 SFH 3aa95 108.18 100.93 9937 288.7/-18.1 40.9/-493 8.36 1.07 1.01 NFH95.1a 224.03 215.78 209.17 45.9/+10.8 296.0/+60.8 6.87 1.04 1.03 SFH 16a 136.19 13037 1 2 334 39.9/-1.6 116.0/+833 9.75 1.04 1.06 SFH 16i 335.84 3 2 8 3 9 296.92 52.8/+9.0 241.9/+80.9 12.15 1.02 l.ll SFH95.1 104.97 103.44 97.41 251.4/-14.6 45.6Z-73.9 7.42 1.01 1.06 SFH1.95 90.45 89.02 84.38 254.9/-25.1 50.8/-62.9 6.9 1.02 1.05 NFHlp.96 200.17 190.41 18034 214.7/-43 300.6/+423 10.3 1.05 1.05 NFH1.96 150.07 14831 144.03 238.9/-273 50.3/-62.6 5.43 1.03 1.03 NFHlc.95 133.17 127.76 12539 2 7 6 .1 /-2 1 3 4 9.7/-603 5.87 1.04 1.02 NFH2.96 168.88 162.73 16135 284.1/-24.4 156.2/-53.6 4.46 1.04 1.01 SFH3pl.95 119.43 114.67 109.98 280.8/-10.7 66.1/-77.0 8.24 1.04 1.04 SFH 3s.95 114.76 11338 10534 3123/+2.1 35.6/-733 837 1.01 1.08 Distributary Mouth Bar Samples WFH1 172.02 166.38 163.20 29.4/+83 253.8/+78.2 5.27 1.03 1.02 EFH 3b 2 0 6 3 4 198.79 162.36 352.3/+7.4 1 99.1/+81.7 23.35 1.04 1.22 EFH3d 120.82 117.56 101.83 20.7/+0.1 112.6/+863 16.75 1.03 1.15 WFH2.95 225.17 209.18 157.63 5 2 3 /+ 1 .9 260.3/+87.8 34.23 1.08 1.33 WFH2a.95 22138 208.13 186.94 70.9/-13 4.1/+86.7 16.85 1.06 1.11 EFH 5a 125.74 124.69 12338 9531+9.1 2053/+62.6 1.9 1.01 1.01 E F H lb 164.60 15136 120.00 131.0/-1.7 168.8/+87.8 30.68 1.09 1.26 W FH 7y 170.16 14235 101.31 65.6/+23 277.4/+87.1 49.88 1.19 1.41 W FH 7a 162.30 139.20 127.82 3 3 5 3 /+ 8 .4 192.0/+793 24.09 1.17 1.09 EFH 3a 192.23 17538 146.62 350.8/+2.1 239.1/+84.4 26.60 1.09 1.19 EEH2c. _ 256.82 .250.88 -J.8Q.L3- 3S 2 3 /+ 6 .7 _ . I85AL+82J __ . 3 3 .4 5 . 1 02 table con’d.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. W FH7b 140.83 129.40 120.19 292.61+2.2 1 9 7 3 /+ 6 6 3 15.85 1.09 1.08 EFH6 150.12 143.25 140.26 62.6/+9.7 322.2/+46.7 6.8 1.05 1.02 EFH 2b.95 80.65 76.25 76.16 118.6/+2.8 31.3/-433 5.83 1.06 1.00 EFH2c.95 9739 93.69 93.16 113.9/-33 2I3.8/-70.2 4.67 1.04 1.00 W FH7x 76.95 73.84 69.42 183/+1L2 257.7Z+68.6 1 0 3 6 1.04 1.06 Crevasse Splay Samples 3MOR953a 8934 8634 82.62 301.7/-3.4 202.3/-69.8 7.8 1.03 1.05 3MOR95.4a 86.80 82.72 81.18 281.6/-0.9 I883/-74.3 6.7 1.05 1.02 3M OR95.4 64.55 63.31 60.81 263.6/+11.7 254.1/-78.1 5.95 1.02 1.04 3M OR95.3 164.29 15431 13539 328.6Z+25.4 329.7/-643 19.16 1.06 1.14 3M O R95.2a 87.87 8 3 3 2 78.32 8 1 3 /+ 1 6 .0 2238.6Z+72.8 11.47 1.05 1.07 3MOR4a 133.18 1124.0 101.68 3223/+U 206.6^+873 2634 1.07 1.22 3M OR4 99.56 94.98 82.83 108.2/+ 03 207.7/+87.0 18.10 1.05 1.15 3MOR2a.96 138.14 125.89 120.69 30I.0/-13.9 176.2/-663 13.61 1.10 1.04 3M OR3.96 83.90 7 9 3 7 7 4 3 8 281.4/-7.9 171.4/-68.3 10.80 1.04 1.07 3MOR3a.96 8533 80.15 75.79 278.0/-4.4 354.8/+713 12.10 1.07 1.06 3M OR2.96 117.96 111.84 107.82 278.6/-2.6 1 7 3 3 /-8 0 3 9.01 1.05 1.04 3M OR4.96 11835 11135 100.60 3 2 4 3 /+ 0 .2 235.1/-78.9 16.29 1.06 l.ll 3MOR4a.96 99.69 97.06 9030 298.2/-3.8 17.2/+70.7 939 1.03 1.07

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX D

Plate 1 Preservation stages of phytoclasts (infested, amorphous structured, amorphous non-structured, opaques, and fluffy material) and spores (Lycospora sp., Laevigatosporites sp., and Florinites sp.). Magnification of each element is 800X.

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Preservation PHYTOCLASTS Stages

Infested Amorphous Structured

Amorphous Non-Structured O paques

PALYNOMORPH

Fluffy Material Spores

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Tanwi Gangopadhyay was bom on October 16,1967, to Namita and Santosh

Ganguly. She completed her elementary schooling from Nava Nalanda School and

secondary and higher secondary schooling from Multipurpose Government Girls

School in Calcutta. Mrs. Gangopadhyay earned a bachelor of science degree with

honors in Geology from Jogmaya Devi College under the University of Calcutta, in

1988. She completed a master of science degree in Geology from the Ballygunj

Science College also under the University of Calcutta, in 1991. Her master’s thesis

was on sedimentology and stratigraphy of Gondawana aged deposits from the Kulti

area in the state of West Bengal. Dr. A.K. Baneijee served as her supervisor for the

master’s project

Mrs. Gangopadhyay embarked on her doctor of philosophy program at

Louisiana State University in August 1992. She has worked in association with Dr.

Arnold H. Bouma who served as her supervisor. She has gained valuable research and

academic experience during her stay at Louisiana State University, part of which was

spent teaching introductory classes. During her stay in the United States she has

travelled extensively and learned a lot about people and places. She expects to receive

the doctor of philosophy degree from Louisiana State University in August 1997. Mrs.

Gangopadhyay married Debnath Basu, who is also a geologist, in May, 1992. She

expects to return to India in the summer of 1997.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DOCTORAL EXAMINATION AND DISSERTATION REPORT

Tanwi Gangopadhyay

Major Mold: Geology

Title of Dissertation: High Resolution Facies Analysis of Deltaic Subenvironments of the Pennsylvanian Atoka Formation, Arkansas

Approved:

and

Iraduate school

IINING COMMITTEE:

Date of Kraei nation:

May 2, 1997______

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