Stratigraphic architecture of a shallow-water delta

STRATIGRAPHIC ARCHITECTURE OF A SHALOW-WATER DELTA
DEPOSITED IN A COASTAL-PLAIN SETTING: NESLEN
FORMATION, FLOY CANYON, UTAH
by
Kimber C. O’Brien
A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines
in partial fulfillment of the requirements for the degree of Master of Science (Geology).
Golden, Colorado
Date___________________________
Signed: _______________________________
Kimber O’Brien
Signed: _______________________________
Dr. David Pyles
Co-Advisor
Signed: _______________________________
Dr. John Humphrey
Co-Advisor
Golden, Colorado
Date __________________________
Signed: ____________________
Dr. Paul Santi
Professor and Head
Department of Geology and Geological Engineering
ii
ABSTRACT
Shallow-water, low-accommodation, coastal-plain deltas are deposited in wetlands
detached from the coeval shoreline associated with broad, low-gradient settings and are an
underappreciated architectural style in marginal marine systems. The goal of this study is to
document the stratigraphic architecture and evolution of a shallow-water delta in the Neslen
Formation, Floy Canyon, Utah and compare it to modern analogs deposited in a similar water
depth. A further goal is to contrast the shallow-water delta documented to a comparatively
deeper-water delta to test the implications of the numerical model developed by Edmonds et al.,
(2011).
The delta analyzed in this study was deposited in a ~ 4 m deep wetland that was located
about 65 kilometers landward of the coeval shoreline. The outcrop of the delta is highly rugose
and exposes, in three dimensions, a large portion of the delta: from the feeder channel to the
delta front. Upward and lateral facies variations of the delta stratigraphy are evident. The feeder
channels contain fine-grained, cross-stratified sandstone. These channels transfer longitudinally
to the axial parts of mouth bars that are composed of amalgamated fine-grained, upwardcoarsening and thickening beds that contain predominantly ripple laminations. The beds deamalgamate and become thinner and finer-grained towards the lateral and distal margins of the
delta. Stratigraphically adjacent elements in the Neslen delta stack highly compensationally.
Within the elements, stratigraphically adjacent stories stack laterally and rotate relative to one
another to form a radially dispersive pattern. Younger stories stack progradationally downcurrent.
The shallow-water delta documented in this study has similar longitudinal lithofacies
trends and stacking patterns to modern analog deltas deposited in similar water depths (such as
iii
the Okavango and Wax Lake deltas). When compared to a relatively deeper-water delta, such as
the delta documented by Matthew Andresen in the Iles Formation, the lithofacies trends and
stacking patterns are different. For example, the shallow-water delta (Neslen delta) has bottomset
aggradation only at the distal part of the system, the clinoform angles are shallow (1 to 3
degrees), the foresets were deposited primarily by tractional processes, and the mouth bars stack
more compensationally. Conversely, the comparatively deeper-water delta has bottomset
aggradation throughout the deposit, the clinoform angles are steeper, the foresets were deposited
primarily by gravitational processes, and the mouth bars stack less compensationally.
iv
ORIENTATION
This thesis is half of a paired study, the goal of which is to test the prediction that the
initial water depth influences the stratigraphic architecture and lithofacies distribution of riverdominated deltas. Chapters 1 through 4 document and analyze a shallow-water, river-dominated
delta deposited on a coastal plain in the Neslen Formation. A companion study by Matthew
Andresen, another Masters student at the Colorado School of Mines, documents a riverdominated delta deposited in a deeper-water setting in the age-equivalent Iles Formation.
Chapter 5 is co-written by Matthew Andresen and me. Chapter 5 summarizes and analyzes the
properties of the Neslen and Iles deltas, thus accomplishing the goals of the paired study.
v
TABLE OF CONTENTS
ABSTRACT ................................................................................................................................... iii
ORIENTATION ............................................................................................................................. v
LIST OF FIGURES ..................................................................................................................... viii
LIST OF TABLES .......................................................................................................................... x
ACKNOWLEDGEMENTS ........................................................................................................... xi
CHAPTER 1
INTRODUCTION AND BACKGROUND ......................................................... 1
1.1
Introduction ........................................................................................................... 1
1.2
Geologic Setting ............................................................................................................. 3
1.3
Data ....................................................................................................................... 4
1.4
Heirarchical Framework for Deltaic Strata .......................................................... 5
1.5
Lithofacies ............................................................................................................ 7
CHAPTER 2
RESULTS .............................................................................................................. 8
2.1
Stories ................................................................................................................... 8
2.2
Elements.............................................................................................................. 10
2.3
Evolution of the Complex ................................................................................... 11
CHAPTER 3
DISCUSSION ...................................................................................................... 13
3.1
Compensational Stacking............................................................................................ 14
3.2
Water Depth ........................................................................................................ 15
3.3
Brackish-Water Influence ................................................................................... 15
CHAPTER 4
MODERN ANALOGS ....................................................................................... 17
4.1
Okavango Delta, Botswana................................................................................. 17
4.2
Wax Lake Delta, Louisiana, USA .............................................................................18
vi
CHAPTER 5
COMPARISON OF COEVAL DELTAS .......................................................... 21
CHAPTER 6
APPLICATION ................................................................................................. 25
CHAPTER 7
CONCLUSIONS ............................................................................................... 26
FIGURES ...................................................................................................................................... 28
TABLES ....................................................................................................................................... 50
REFERENCES ............................................................................................................................. 58
APPENDIX A ............................................................................................................................... 62
A-1
Stratigraphic Columns ....................................................................................... 62
vii
LIST OF FIGURES
Figure 1
Cross section of delta stratigraphy……………………………………………….28
Figure 2
Modified cross sections of delta stratigraphy……………………………………28
Figure 3
Aerial photographs of Okavango and Mississippi deltas………………………...29
Figure 4
Location and geologic maps of the study area……………………………….......30
Figure 5
Stratigraphy of the Book Cliffs…………………………………………………..31
Figure 6
Paleogeographic map of the Book Cliffs…………………………………….......32
Figure 7
Detailed Neslen Formation Stratigraphy……………………………………...…33
Figure 8
Hierarchical framework for deltaic stratigraphy…………………………………34
Figure 9
Photographic examples of lithofacies……………………………………………35
Figure 10
Interpreted photopanels…………………………………………………………..36
Figure 11
Common trace fossils…………………………………………………………….37
Figure 12
Diagram of ripple-laminated sandstone……………………………………….....38
Figure 13
Chart of amalgamation ratios…………………………………………………….39
Figure 14
Isopach map of sand thickness………………………………………………...…40
Figure 15
Paleogeographic reconstruction……………………………………………...…..41
Figure 16
Photograph of coastal-plain stratigraphy…………………………………...……42
Figure 17
Diagram summarizing story characteristics…………………………………..….43
Figure 18
Diagram of compensationally stacking mouth bars……………………………...44
Figure 19
Diagram of brackish-water trace fossils…………………………………………45
Figure 20
Diagram comparing deltas deposited in similar water depths………………..….46
Figure 21
Diagram comparing Neslen delta and Iles delta…………………………………47
Figure 22
Vertical transect comparison of Neslen delta and Iles delta……………………..48
viii
Figure 23
Story-scale comparison of Neslen delta and Iles delta…………………………..49
ix
LIST OF TABLES
Table 1
Comparison of hierarchical classifications………………………………………50
Table 2
Lithofacies descriptions………………………………………………………….51
Table 3
Bioturbation index……………………………………………………………….52
Table 4
Story dimensions…………………………………………………………………53
Table 5
Measurements of the overlap of stratigraphically adjacent stories………………54
Table 6
Relative abundance of sand-rich lithofacies to silt-rich lithofacies……………...55
Table 7
Element dimensions……………………………………………………………...55
Table 8
Measurements of the overlap of stratigraphically adjacent elements……………56
Table 9
Summary of characteristics of modern deltas to the Neslen delta.........................56
Table 10
Comparison of the Iles delta to the Neslen delta………………………………...57
x
ACKNOWLEDGEMENTS
The work necessary to complete this thesis, including data collected in the field, data
analysis, data interpretations, writing, and editing was a collective effort by my colleagues and
me. I cannot take full credit for the vast amounts of time and energy it took to produce this
thesis. I would like to thank the following people and organizations for their contributions, work,
and support:

My thesis committee: co-advisors Dr. David R. Pyles and Dr. John Humphrey, and
committee members Mark Kirschbaum and Dr. Wendy Zhou for their mentoring. I would
like to particularly thank Mark Kirschbaum and David Pyles for taking extra time out of
their lives to help edit this thesis and discuss ideas.

My fellow students in the Chevron Center of Research Excellence: Matthew Andresen,
for his assistance in the field and co-writing a section of our theses together, Jesse Pisel,
for his assistance with various programs, and Bradley Nuse and Alyssa Charsky for their
assistance in the field.

A special thanks to our program manager, Cathy Van Tassel, for helping with travel
arrangements, paperwork, and encouragement.

The Chevron Center of Research excellence for their support and funding as well as their
encouragement and assistance throughout the duration of my graduate education.

Funding was also provided by an AAPG Graduate Student Grant in Aid, a BP
scholarship, and from the Gulf Coast Association of Geological Studies (GCAGS).

Finally, I would like to thank my family for their patience and support. My boyfriend,
Morgan, has also been incredibly supportive even though graduate school meant two
years apart from one another.
xi
CHAPTER 1
INTRODUCTION AND BACKGROUND
This chapter discusses the importance of this study and introduces the problems and
objectives that are addressed. Important background information, including the components of
river-dominated deltas and current models for deltaic stratigraphy, are discussed, followed by an
overview of the data and methodology necessary to conduct the study.
1.1
Introduction
Deltas are defined as a mass of sediment that forms at the mouth of a river where the flow
enters a standing body of water (Barrell, 1912). Deltas can be divided into different end member
types on the basis of the dominant process that deposit the sediment, i.e. the river, tides, and
waves (Galloway, 1975). High river energy relative to the energy from tides and waves results
in a river-dominated system where the behavior of the river effluent is determined by the outflow
inertia, bed friction, the flux of water and sediment, and the outflow buoyancy (Wright, 1977).
Deltas have three components: topset, foreset, and bottomset. These components are alternately
known as the delta plain, delta front, and prodelta, respectively. The topset consists of
distributary channels and interdistributary muds, whereas foresets are composed of dominantly
mouth bars (Edmonds et al., 2011). The bottomset is composed of distal prodelta muds and sands
(Figure 1A).
Edmonds (2011) categorized river-dominated deltas on the basis of initial water depth
and angle of the lower bounding surface, in which deeper-water deltas contain a higher fraction
of foreset strata than their shallow-water counterparts (Figure 2). Whereas shallow- and deepwater deltas appear morphologically similar in plan-view (Figure 3); Edmonds geometric model
proposes that the stratigraphic architecture is different and varies longitudinally (Edmonds et al.,
1
2011) in contrast to previous studies that did not show longitudinal variations in architecture
(Bhattacharya, 2006) (Figure 1B). We modified Edmonds (2011) model to include bottomset
strata to account for the aggradation of the sediment-water interface, which reduces the gradient
through deposition (Figure 2). Deep-water, foreset-dominated deltas have been studied in the
greater detail than shallow-water deltas because they are easily recognizable systems at the
shoreline with numerable modern analogs (McGowen, 1971; Le Blanc, 1975; Xue, 1993;
Overeem, 2003; Stefani et al., 2005; Fielding et al., 2005; Fielding et al., 2006; Olariu et al.,
2006; Wellner et al., 2006; Olariu et al., 2012; Shaw et al., 2013).
Key insights of the model developed by Edmonds (2011) are the following. First, the
stratigraphic architecture of deltas varies longitudinally and with water depth (Figure 1A and
Figure 2). Second, the gradient of the sediment-water interface influences the distance at which
the delta transfers from a topset-dominated to foreset-dominated delta (Figure 2). Thirdly, the
novel characteristic of topset-dominated deltas is the distributary channels commonly incise into
pre-deltaic material (Figure 2). Lastly, shallow-water deltas have a higher proportion of topset
strata whereas deeper-water deltas have a higher proportion of foreset strata. Edmond’s model
provides new insight into delta architecture; however, it has not yet been tested in a natural
system, nor does it consider any small-scale characteristics, such as lithofacies distributions and
internal stacking patterns (i.e. compensational stacking).
To test Edmond’s model, this study uses an exceptional three dimensional exposure of an
ancient river delta in the Neslen Formation in Floy Canyon, Utah (Figure 4) in order to document
the stratigraphy and evolution of a shallow-water, end-member delta. Results of this study are
compared to those of modern delta analogs (the Okavango delta and the Wax Lake delta) and to
a companion study (conducted by Matthew Andresen, of a deeper-water delta in the age-
2
equivalent Iles Formation in Hunter Canyon, Colorado). The key conclusion is that while the
plan-view morphologic expression of river-dominated deltas appears fractal the stratigraphy
stacks hierarchically. A further conclusion is that while the plan-view morphologies of deltas
deposited in different water depths are similar (Figure 3), their cross sectional geometries,
stacking patterns, and lithofacies distributions are notably different.
1.2
Geologic Setting
This study is focused on an outcrop of the Neslen Formation, in which a shallow-water
delta is exposed in three dimensions (Figures 4 and 5). The study area is located northeast of
Green River, Utah, in Floy Canyon, within the southern portion of Uinta basin (39°01’18.84”N,
109°51’32.97”W). The delta documented in this study has a regional structural dip of 1 degree to
the northeast but the paloeflow of the delta is from west to east. The depositional gradient is 0.4°.
A corresponding study of a comparatively deeper-water delta by Matthew Andresen was
conducted 65 kilometers east (downdip) in the age-equivalent Iles Formation at Hunter Canyon,
near Fruita, Colorado (Figure 5).
Fluvial systems transported sediment eastward from the Sevier highlands to the coast,
and coal-forming wetlands developed in the coastal plain during the Late Cretaceous (Hettinger
and Kirschbaum, 2002) (Figure 6). The Mesaverde group (Figure 6) is a clastic wedge of
sediment that prograded eastward into the Western Interior basin, a foreland basin that developed
on the east side of the Sevier Orogenic belt as a consequence of the Sevier Orogeny. The Sevier
Orogenic belt was active between 140 Ma to 50 Ma during which time the oceanic Farallon Plate
subducted under the North American Plate (DeCelles, 2004). The younger Laramide orogeny
fragmented the Western Interior basin into several smaller basins, including the Piceance and the
3
Uinta basins, and accounts for the structure present in the area today (DeCelles, 2004) (See inset
map in Figure 5).
The Campanian Neslen Formation was first named in the 1930’s and the nomenclature is
still used today (Fisher, 1936). The Neslen is a coal-bearing formation of alternating sandstone
and slope-forming mudstone packages (Fisher, 1936). Hettinger and Kirschbaum (2002)
document the regional stratigraphy of the Book Cliffs (Figure 5). The Neslen and the
longitudinally equivalent Iles Formations were interpreted to be coastal plain and shoreface delta
environments, respectively (Roberts and Kirschbaum, 1995; Hettinger and Kirschbaum, 2002)
(Figure 6). The location documented in this study is interpreted to be 65 kilometers away from
the shoreline during the deposition of the delta in the Neslen Formation. Kirschbaum and
Hettinger (2004) interpret the Neslen Formation in Floy Canyon to consist of alternating
estuarine complexes with bay head deltas that prograded into an incised valley (Figure 5 and 7).
Shiers et al., (2014) documents tidally influenced fluvial successions in the Neslen Formation,
but not the deltas that formed on the coastal-plain. The Neslen Formation also has been
interpreted to consist of crevasse splay bodies near Rangely, Colorado (Anderson, 2005),
however, this study identifies the deposit in the Neslen Formation deposits in Floy Canyon on
the opposite side of the basin as coastal-plain deltas because the channels become dispersive into
other lithofacies and there is no large channel that scales to the body of the delta that bypasses
the area.
1.3
Data
The data used to address the goals of this study include: 1) stratigraphic columns (Figure
4 and Appendix), 2) high resolution Gigapan photos, 3) a geologic map with GPS locations
(Figure 4), and 4) sediment transport directions. Detailed stratigraphic columns document
4
centimeter-scale stratigraphic characteristics such as physical and biogenic sedimentary
structures, textures, grain size, sorting, bedding styles, type of fossils present, and paleocurrent
measurements. The locations of the 30 stratigraphic columns collected in this study are orange
dots in Figure 4. High resolution photopanels provide mapping templates for mapping lithofacies
distributions and trends, stratal boundaries, and paleocurrents. GPS measurements were used to
constrain the locations of stratigraphic columns and the locations of notable lithofacies
transitions cross referenced with the Gigapan photos (Figure 4). These data were collectively
used to map and correlate the stratigraphy across the field area, and measure the dimensions of
depositional units. Dimensions of stories were collected at an orientation perpendicular to the
sediment transport direction. Individual mouth-bar and channel stories were mapped in 3D based
on their contacts on each face of the outcrop and drawn based on their dominant flow direction.
The distance between the thickest part of each story and the point at which the story thins to a
thickness of zero was measured and extrapolated to help constrain the dimensions of each story.
These raw data were used to create a secondary dataset, which includes: 1) cross sections
of each exposed face, 2) measurements of amalgamation ratios (the thickness of amalgamated
sand divided by the total thickness of the depositional unit), 3) an isopach map of the delta, 4)
lithofacies, 5) lithofacies proportions and distributions, 6) dimensional data for stories and
elements, 7) measurements of overlap between stratigraphically adjacent stories and elements,
and 8) the ratios of sand-rich lithofacies to mud-rich lithofacies per story.
1.4
Hierarchical Framework for Deltaic Strata
This study uses a hierarchical framework that overlaps with Campbell’s (1967)
classification and is modified from Pyles (2007) (submarine fans), Pyles et al. (2010) (deepwater channels), Straub and Pyles (2012) (submarine fans), and Ford and Pyles (2014) (fluvial
5
systems). The approach used in this study combines aspects from deep-water and fluvial regimes
and applies them to marginal marine regimes to provide a common mode of comparison and
classification of different depositional environments. Table 1 compares the hierarchical
classifications of this study to selected other studies. A five-level hierarchy is used. From
smallest to largest the hierarchical components are: bed, bedset, story, element, and complex. A
demonstration of this approach is depicted on a stratigraphic column and photo in Figure 8.
Each hierarchical level represents different depositional units based on the time span of
existence, size of stratal packages, crosscutting relationships, and number of superimposed
stratigraphic units (Pyles, 2007). A bed refers to a single depositional event (Campbell, 1967). A
bedset is a genetically related group of beds that have similar lithofacies, sediment transport
directions, and geometries (Campbell, 1967). Beds and bedsets amalgamate and de-amalgamate
depending on their location relative to the axis of the units. Stacked beds and bedsets have
coarsening and thickening upward patterns with similar lithofacies and paleoflow directions
separated by relatively small-scale erosional surfaces and comparatively minor offset of the
location of the axis of deposition. These stacked beds and bedsets are referred to as stories
(Straub and Pyles, 2012). A story is a genetically related group of bedsets containing similar
lithofacies and sediment transport directions where story boundaries record abandonment and
small-scale avulsion of the river system. An element is a group of genetically related stories
containing similar lithofacies and sediment transport directions characterized by its external
shape in depositional-strike view that forms the fundamental building blocks for larger
stratigraphic units (Pyles, 2007 p. 5). The boundaries between adjacent elements records
significant offset in the locations of the axes of stratigraphically adjacent elements (Straub and
Pyles, 2012). Genetically related elements group to form complexes.
6
Stratigraphic columns document detailed, bed-scale vertical stratigraphic trends and high
resolution photopanels document bed-sets, stories, and elements in three dimensions and are
discussed in the following sections. The stratigraphic architecture of the delta is documented in
three dimensions on the photopanels.
1.5
Lithofacies
This study utilizes the definition of facies as stated by Cross and Homewood (1997 p.
1620) translation of Amanz Gressly (1838) which is, “those observable physical, chemical, and
biological properties of rocks that collectively permit objective description, as well as
distinctions among rocks of different types”. The term lithofacies is used descriptively and does
not indicate the environment of deposition and is not specific to time. Rather, lithofacies give
insight into the hydrologic processes active during deposition. Lithofacies can change laterally
along discreet beds (Cross and Homewood, 1997). Seven lithofacies are identified in this study;
four are composed of sandstone, two are mudstone and siltstone, and one is mudstone.
Photographs of each lithofacies are documented in Figure 9. For brevity, the physical
characteristics, sedimentary structures, lithofacies names, and interpreted hydrodynamic
processes are in Table 2. Table 3 lists the criteria used to determine the bioturbation index (BI)
for each lithofacies. Detailed descriptions of lithofacies are documented in the appendix. The
lithofacies are mapped on photopanels A-D across the outcrop in three dimensions (Figure 10).
The un-interpreted photopanels and their locations on the topographic map are labeled in Figure
4.
7
CHAPTER 2
RESULTS
This chapter discusses the results documented in this study, including the characteristics
of the different types of stories and elements, and the paleogeographic evolution of the complex.
2.1
Stories
The delta in the Floy Canyon study area is composed of channel stories and mouth-bar
stories. The story types are differentiated based on 1) external morphology, 2) lithofacies
associations, 3) sediment transport directions relative to the morphology, and 4) interpreted
energy of deposition.
Channel stories have erosional lower bounding surfaces that incise into older mouth-bar
stories (Story 3H in Figure 10A and 10B). The upper bounding surfaces are both erosional and
conformable. When viewed along depositional strike in cross-sectional view, the external form is
concave up at the base, flat at the top (Story 3H in Figure 10B), and elongate in the down-current
direction with aspect ratios (width/thickness) ranging from 11 to 25 (Table 4). Channel stories
are thickest at their axis and thin towards their margins. The most abundant lithofacies in channel
stories are rippled sandstone (L4) and low-angle cross-stratified sandstone (L6) (Table in Figure
10A). Lithofacies transfer vertically from larger grain sizes to smaller grain sizes. Channel
stories transfer down-current to mouth-bar stories.
These stories are interpreted as channels. This interpretation is based on collective
observations of a lenticular external shape and low aspect ratios (Table 4) characteristic of
channels, internal lithofacies associations that shows decreasing upward flow strength, the
position of the story relative to older stories, and mainly unidirectional sediment-transport
directions.
8
The characteristics of mouth-bar stories are documented based on the surfaces mapped on
the photopanels (Figure 10B-D). Mouth-bar stories have erosional lower bounding surfaces at
their axes and have conformable lower bounding surfaces at their margins (Figure 10B). The
upper bounding surface is either gradational into silt, marked by a sharp wave-rippled surface
(Figure 10B see SC20 and SC2), marked by a burrowed layer with Teredolites and Skolithos
(Figure 11), or rarely erosional (Figure 10B see SC 7). The cross-sectional form, both in
depositional strike and dip -view, is a sigmoidal shape (Figure 10B; stories 3H-3I in Figure 10C).
Mouth-bar stories are lobate in plan-view with aspect ratios (width/thickness) ranging from 50 to
250 (Table 4). The stories are thickest at their axis and thin toward their lateral and distal edges
where sand deamalgamates and transfers to mud. The most abundant lithofacies in mouth-bar
stories is rippled sandstone (L4) (Figure 12). There is proportionally more interbedded mud and
silt (L2) relative to ripple-laminated sandstone (L4) towards the margin and distal edge of
mouth-bar stories (Tables in Figure 10B-D). The overlap between stratigraphically adjacent
mouth-bar stories averages 54 percent, meaning mouth bars are stacking compensationally as
opposed to vertically stacking (Table 5). On average, mouth-bar stories are composed of 82
percent sand-rich amalgamated lithofacies and 18 percent de-amalgamated mud-rich lithofacies
(Table 6). The oldest stories in Element 3 (3A-3D) are more amalgamated than their younger
counterparts (3E-3F) (Figure 13).
These stories are interpreted as mouth-bars. This interpretation is based on the collective
observations of sigmoidal external shape and elongate plan-view shape characteristic of mouth
bars, internal lithofacies associations showing upward coarsening and thickening of beds, high
aspect ratios relative to channels, the position of the axis of stories relative to older stories, and
radially dispersive sediment-transport directions.
9
2.2
Elements
Elements are composed of genetically related channel stories and mouth-bar stories
composed of similar lithofacies trends. Most of the following discussion focuses on Element 3,
which is composed of nine mouth bar stories (Figure 10A-D stories 3A-3I). The lower bounding
surface of Element 3 is erosional near the axis and conformable near the north east margin and
erosional near the south west margin (Figure 10B). The upper bounding surface is marked by a
wave rippled surface that is overlain by interbedded mud and silt (Figure 10A-D see complex
boundary). The cross-sectional shape in strike view is sigmoidal shaped. The base and top of
elements are relatively flat. All three elements are thickest near the axis and thin near the margin.
The plan view morphology of the elements is elongate in the up-current direction and lobate
down-current. The most abundant lithofacies is rippled sandstone (L4), interbedded mud, and silt
(L2), and amalgamated ripple-laminated sandstone with rip-up clasts (L3). The aspect ratios
(width/thickness) range from 117-178 (Table 7). The measurement of the overlap of
stratigraphically adjacent elements averages 15 percent, meaning the elements stack more
compensationally than mouth-bar stories (Table 8). The thickest part of the delta complex is
where the maximum overlap and the maximum incision into pre-deltaic strata coincide. As a
result, the thickest parts of the delta are located from the feeder channel to slightly off-axis of
Element 3, where it overlaps with Element 1 according to the isopach map of sand thickness
(Figure 14) and where the amalgamation ratios of mouth-bar stories are high (Figure 13).
These elements are interpreted to make up part of a delta complex that is not fully
exposed. One fully exposed element (Element 3) and two partially exposed elements (Elements 1
and 2) form the basis for the interpretation of the evolution of a delta lobe deposited in a coastalplain setting.
10
2.3
Evolution of the Complex
The purpose of this section is to place stories and elements into a larger context by
documenting the paleogeographic evolution of the delta complex within a shallow-water,
coastal-plain setting. Paleogeographic interpretations of the Neslen delta, including elements 1-3,
which collectively comprise a channel-mouth bar complex, and stories 3A-3I are presented in
Figure 15. The maps represent a plausible interpretation of the outcrop based on the synthesis of
the collected data, including mapped photopanels, sediment transport directions, and measured
thicknesses. These data allow for the documentation of the evolution of a partial delta complex.
The presence of mudstone and carbonaceous shale (L1) in every stratigraphic column
throughout the study interval indicates that a fresh-water coastal plain was present prior to the
deposition of the delta (Figure 10 and Figure 16) (Kirschbaum and Hettinger, 2004, their Figure
6 on column 2 at 100 m). The accommodation space that allows for the deposition of the delta
could have been created by flooded incised valleys, or rising water table during backstepping of
the shoreline, but the cause is beyond the scope of this investigation.
The erosional contact between elements 1-3 and the underlying coastal plain, the height
of the mouth bars, and the presence of brackish-to-fresh water trace fossils indicates the delta in
the studied interval prograded into a shallow body of water, probably initiated by avulsion.
Channel stories in the northwest portion of the study area transition down-current to the mouthbar stories of Elements 1-3 based on cross-cutting relationships and superposition (Figures 10A,
and 15). Elements 1 and 2 were deposited first and Element 3 was deposited in the topographic
low between the older elements (Figures 10B and 15). The six oldest mouth-bar stories in
Element 3 (3A-3F) stack laterally away from Element 1 to the North East (stories 3A through 3F
in Figure 10B; Figure 15). The average paleocurrent direction for stories 3A-3D is 150° and for
11
stories 3E and 3F is 130°. Following the deposition of the laterally stacked stories younger
mouth bars (3H-3I) are deposited in the down-current direction, indicating a transition from
lateral stacking to progradational stacking, and are associated with small channels that incise into
the older mouth-bar stories (stories 3I-3H in Figure 10C). Story 3H is the most fully exposed
mouth bar in the study area. Following the deposition of mouth-bar story 3I marks the final
deposition of the delta based on vertical and down-current lithofacies transitions to mudstone.
12
CHAPTER 3
DISCUSSION
The purpose of this section is to document the stratigraphic architecture and the water
depth of the delta. The paleogeographic reconstruction of the complex shows that stratigraphic
architecture varies laterally and longitudinally, as predicted by Edmonds (2011) model (Figure
2). An observation shared in other studies, for example, by Olariu and Bhattacharya (2006) and
Gani and Bhattacharya (2007), which is at odds with Waltherian Stratigraphy (Figure 1B).
Walter’s Law states that vertically adjacent lithofacies also accumulated adjacent to one another
laterally given there are no unconformities (Middleton, 1973). This study documents high
variability in vertical transects throughout one genetically related delta complex, and lithofacies
deposited vertically were not necessarily deposited laterally (Figure 10). In this study, the
shallow initial water depth limits vertical accommodation space and results in a greater tendency
for the shallow-water delta in this study to stack compensationally, and increases lateral
variability.
An idealized model of a story (Figure 17) is constrained from observations of Story 3H,
and documents the plan-view shape of mouth bars documented in this study. The characteristics
summarized in Figure 17, including the dominant lithofacies per story, the maximum
amalgamated sand thickness, and the relative abundance of sand-rich lithofacies to silt-rich
lithofacies along a longitudinal transect, indicates the delta was dominated by tractive deposition.
The down-current and lateral thinning of mouth-bar stories and the down-current transition to
mud indicates waning flow, because the energy of deposition decreases as the average grain size
decreases, as the distance from the apex of the feeder channel increases (Figure 17) (Wellner et
al., 2006).
13
3.1
Compensational Stacking
Compensational stacking is the tendency of sediment to fill in topographic lows resulting
from avulsion of the sediment into areas where the relief is greatest (Mutti and Normark, 1987).
Compensational stacking patterns have been recognized and quantified in previous studies in
channelized sedimentary deposits (Straub et al., 2009) and submarine fans (Pyles, 2007; Straub
and Pyles, 2012). The cross sections in Figure 10 shows the delta in Floy Canyon stacks
compensationally, as demonstrated by the down-lapping surfaces of the mouth-bar stories. The
compensational stacking is quantified on the basis of the measurements of average percent
overlap between stratigraphically adjacent depositional bodies; 54% for stories and 18% for
elements (Tables 5 and 8). The overlap of stratigraphically adjacent bodies is used as a proxy for
the amount of compensation in the delta; a high percent overlap indicates low compensation
while a low percent overlap indicates more compensation. The stories have a higher average
percent overlap between stratigraphically adjacent units than the elements because they represent
smaller-scale avulsions over a shorter distance relative to elements. Element 3 stacks
compensationally into a topographic low between Elements 1 and 2, resulting in stratigraphic
architecture that is partially dependent on the existing topography. Similarly, the stories within
Element 3 stack laterally away from Element 1 and rotate relative to older stories. Element 3 has
an erosional contact with Element 1 and a conformable contact with Element 2. This difference
in contacts is attributed to local differences in flow that resulted in a greater degree of erosion to
the south. Compensational stacking is also evident at the bedset scale; increasing lateral
heterogeneity within stories, as evidenced by the bedset attacking patterns in the heterolithic
strata northwest of the study area (Figure 18).
14
The stacking style of architectural elements and stories in the Floy Canyon delta
demonstrate the lateral and longitudinal variability of the delta. The plan-view morphologic
expression of river-dominated deltas in shallow and deeper-water settings appears fractal, but, as
demonstrated by this study, the stratigraphy stacks hierarchically, indicating that plan-view
morphology does not necessarily reflect the stratigraphy.
3.2
Water Depth
The initial water depth can be estimated by observing the maximum height of the mouth
bars because mouth-bar stories can only build to the top of the water column. The maximum
relief of elements documented in this study is 4 m, indicating the maximum water depth was 4 m
at the end of deposition. The water level likely fluctuated during deposition due to seasonal
fluctuations in the flow of the river, but the delta was subaqueous until the end of deposition
based on the lack of rootlets, mud cracks, or other subaerial exposure indicators at the upper
contact between stories documented by the lithofacies (Table 2). The limited vertical
accommodation and the tendency for sediment to fill topographic lows results in the sigmoidal
cross-sectional shape of mouth-bar stories documented in this study (Figure 10).
3.3
Brackish-Water Influence
The three elements are interpreted to be deposited in a fresh-water to brackish-water
environment. The brackish water influence in the Neslen Formation was interpreted by Fisher
(1936) and later confirmed by Kirschbaum and Hettinger (2004) based on documentation of both
freshwater and brackish water fossils in the Neslen Formation. This study documents similar
brackish-water trace fossil assemblages (Figure 11 and Figure 19). The low diversity of trace
fossils and Skolithos is an indicator of a brackish water or marginal marine environment
(Pemberton, 1992).
15
Brackish-water influence can result from four different possible scenarios: 1) proximity
to the shore, 2) salt water intrusion, 3) storm surge from the coeval shoreline, and 4)
transgressive estuary valley fill (Kirschbaum and Hettinger, 2004). The proximity to the
shoreline is not likely to be the cause of the brackish water because the shoreline during the
Campanian has been documented as approximately 65 kilometers from the location of the study
area (Figure 6). The salt water intrusion hypothesis would provide salt for a brackish water
influence but it does not account for the marine to brackish trace fossil assemblage. The
preferred hypothesis is that a combination of storm surge and transgressive estuary valley fill is
responsible for the brackish water influence. Storm surge refers to the upper surface of a water
body that is elevated from the low barometric pressure and high speed, landward directed wind,
such as that from a hurricane or a low pressure system. Storm surges occur when the water level
is greater than high spring tides. If storm surges are combined with high tides, known as storm
tides, the water level is even higher and the water can be pushed farther back on the coast, such
as observed in modern Hurricanes like Hurricane Rita in 2005, which flooded 80 kilometers
inland, effectively inundating the coastal-plain with marine water (Berenbrock et al., 2009;
Williams, 2009).
16
CHAPTER 4
MODERN ANALOGS
In an effort to understand the role of water depth on delta architecture, the study interval
analyzed within the Neslen Formation is compared to modern analogs deposited in similar initial
water depths (3-6m) (Table 9 and Figure 20); the Okavango delta in Botswana, and the Wax
Lake Delta in the Atchafalaya Bay in Louisiana.
4.1
Okavango Delta, Botswana
The Okavango delta and the Neslen delta share several similarities. The first similarity
between these deltas is the depositional water depth. The Okavango Delta, located in Northern
Botswana, is a shallow-water, inland delta with a maximum water depth of 3 m (Table 10 and
Figure 20); similar to the water depth documented in this study (~4 m). The second similarity is
the depositional gradient. The Okavango Delta covers an aerial extent of 25,000 square
kilometers from the meander belt feeder channel to the termination of the distributary system and
a low gradient (0.02% or 1:35,000) (McCarthy et al., 1988; McCarthy et al; 1991; Stanistreet and
McCarthy, 1993). The delta documented in this study has a dip of 0.4°, but it is on a smaller
scale than the Okavango delta, covering 1 square kilometer. The third similarity is the climatic
control on deposition. The sediment influx of the Okavango delta is controlled by seasonal
climatic fluctuations and dominated by meandering and low sinuosity channels (McCarthy,
1993). Similarly, the Neslen delta is interpreted to be dominated by climatic changes that
influence the flux of the river because of the Teredolites mats of wood that form on the tops of
beds that are buried during the deposition of subsequent stories. The fourth similarity is the
dominant flow process building mouth bars. Neither delta has mouth bars built by sediment
gravity flows. Bedload is the dominant mode of sediment transport and traction is the mode of
17
sedimentation for both the Okavango delta and the Neslen delta (McCarthy et al., 2002). The
Okavango delta is completely detached from the oceanic shoreline and has no marine influence;
creating a virtually closed system (McCarthy et al., 2002), whereas the Neslen delta had fresh-tobrackish water influence. The Okavango River feeds large areas of permanent swamps and the
thick vegetation invades channels and stabilizes channel banks (McCarthy et al., 1992). The
roots of the vegetation in the swamp and the presence of permeable organic material mixed with
mud allow the water to leak through the levees into the adjacent swamps (McCarthy et al., 1991).
The distributary channels in the Okavango and Neslen deltas have similar down-current patterns.
The similar water depth, gradient, climatic control, and flow processes between the
Okavango delta and the Neslen delta indicate the stratigraphic architecture of both deltas should
be similar since they are deposited in similar water depths (Edmonds et al., 2011) (Figure 20),
however the Okavango delta is dominated by channel stories and adjacent swamps while the
Neslen delta is dominated by mouth-bar stories (Figure 15). This difference can be attributed to
the scale of the studied intervals. The Okavango delta is at the complex scale whereas a partial
complex is documented in the Neslen delta. The vegetation on the Okavango delta stabilizes
channels and limits dispersive deposition of mouth bars. It is possible that the Neslen delta is
also amid peat swamps due to the evidence of coal in the Neslen, but a larger study interval
would be needed to make that conclusion.
4.2
Wax Lake Delta, Louisiana, USA
Several similarities exist between the Wax Lake delta in the Atchafalaya Bay in
Louisiana and the Neslen delta (Table 9 and Figure 20). The first similarity is the initial water
depth. The water depth of the Wax Lake delta is approximately 6 m and the aerial extent is 145
square kilometers whereas the Neslen delta was deposited in 4 m water depth and covers an area
18
of 1 square kilometer. The relative size of Wax Lake delta to the Neslen delta may be a more
relevant comparison based on the similar scales. The second similarity is the morphology of
mouth bars. Wax Lake delta is interpreted to be composed of morphologically similar mouth
bars (Wellner et al., 2006) with coarser sediment in the up-current direction that gradually fine
downstream from the apex of the river mouth. The mouth bars are elongate parallel to the
sediment transport direction with thicknesses of 1-3 m. The Neslen delta also has mouth bars
elongate parallel to the sediment transport direction with an average thickness of 2 m. The third
similarity is the relative proportion of topset and foreset strata along a longitudinal profile. The
Wax Lake delta and the Neslen delta have a ratio of channel depth to mouth bar height greater
than 1, meaning that the Wax Lake delta and the Neslen delta are dominated by topset
stratigraphy (i.e. interdistributary mud and distributary channels) (Edmonds et al., 2011). The
fourth similarity is the dominant flow process. The dominant flow process forming mouth bars
on the Wax Lake delta and the Neslen delta is traction based on the dominance of lithofacies
deposited by traction, i.e. ripple-laminated sandstone, cross-stratified sandstone, and planar
laminated sandstone.
The Wax Lake and Neslen deltas have the same dominant process building mouth bars,
but the dominant lithofacies for each delta is different. The mouth bars in the Wax Lake delta are
dominated by cross-stratified and laminated sand capped by ripples (Wellner et al., 2006)
whereas the mouth bars in the Neslen delta are composed of ripples; indicating the flow
processes building mouth bars are both dominated by traction, but the mouth bars in Wax Lake
delta have a mixed mode of sediment deposition: traction and fall-out from suspension, resulting
in the formation of turbidites (Wellner et al., 2006). Turbidite deposits are not found in the
mouth bars in the Neslen delta.
19
The distributary channels in the Wax Lake and Neslen deltas have similar down-current
patterns. The channels bifurcate and their cross-sectional area decreases after each bifurcation
(Olariu and Bhattacharya, 2006). Channels erode into the older deltaic material within the
bifurcating river network (Shaw et al., 2013), as observed in the Wax Lake and Neslen deltas.
One branch of the distributary channel network becomes dominant after bifurcation and diverts
the flow away from the abandoned channel (Shaw and Mohrig, 2014), resulting in subsequent
mouth bars building on top of the inactive channel, which can account for the overlap of stories
observed in both deltas. Channel erosion and seaward shallowing of channels has been predicted
by numerical modeling (Figure 1) (Edmonds et al., 2011). Multiple terminal distributary
channels are common in low accommodation settings, such as the Wax Lake delta (Figure 16)
(Olariu and Bhattacharya, 2006) and the Neslen delta, and have similar lobate plan-view
morphologies. The terminal distributary channels in the Wax Lake delta are 1-2 m deep and are
often below the mean sea level whereas proximal distributary channels are much deeper but do
not incise into pre-deltaic material. The Neslen delta has channels that shallow down-current and
do not incise into pre-deltaic material in the study area. The average thickness of distributary
channels in the Neslen delta is 2.5 m and the aspect ratios are 10-25. The Wax Lake delta is a
close modern analog for the Neslen delta based on the similar morphologies, dimensions of
mouth-bar and channel stories, and flow processes in both deltas.
20
CHAPTER 5
COMPARISON OF COEVAL DELTAS: NESLEN AND ILES FORMATIONS
The purpose of this section is to test the implications of Edmonds et al. (2011) model
based on the results from two outcrop studies in the Neslen and Iles Formations, conducted by
Kimber O’Brien and Matthew Andresen, respectively (Figures 5, 6 and 21). A stratigraphic
comparison between the Neslen delta and the Iles delta validates the prediction of Edmonds et al.
(2011) model that mouth bars have different architectures based on water depth. This comparison
suggests modifications that can increase the accuracy of the model. The result of this
stratigraphic comparison validates the prediction that water depth influences deltaic architecture
(Wright 1977; Edmonds et al., 2011). This comparison supports the assertion of Wright (1977)
that bed friction is relatively more significant in shallow-water and reduces the turbulence of the
outflow. Enhanced bed friction in shallow water results in tractive depositional conditions,
whereas reduced bed friction in deeper water results in a higher proportion deposition from
suspension. The following discussion compares and contrasts the two deltas.
The Neslen and Iles deltas are ideally-suited for this paired study. The deltas were
deposited in contrasting water depths of 4 m and 15 m, respectively (Table 10) with gradients of
0.4° and 0.6°, respectively. Within each partial delta complex, both deltas are river-dominated
and exhibit minimal influence or modification by waves or tides due to a relatively high
proportion of lithofacies deposited by tractive currents. Deposited in the Campanian, the deltas
are roughly coevally deposited shallow-marine systems that represent strata deposited along a
depositional profile from the coastal plain to open marine (Figure 5). Thus, the results are
indicative of the variability within a depositional landscape. Finally, the deltas are exposed
21
spectacularly in three dimensions so that the distinctive architectures can be studied from
multiple depositional perspectives.
Both studies use the same types of data and the same methodology to document the
stratigraphic architecture and spatiotemporal evolution of partial delta lobes. Data include: 1)
stratigraphic columns, 2) high-resolution photopanels, 3) sediment transport directions, and 4)
locations of deltaic features on a topographic map (Figure 21). Both studies use a five-tier
hierarchy modified from existing fluvial and deep-water hierarchies. From smallest to largest, the
hierarchical components are: bed, bedset, story, element, and complex (Figure 8).
The dominant lithofacies in the Neslen delta is composed of 57 percent rippled sandstone,
whereas the Iles delta is composed of 49 percent planar-laminated sandstone exhibiting partial
Bouma sequences (Figure 21). The Neslen delta also contains planar-laminated sandstone (11
percent), however, it is interpreted to be deposited by traction because the bedsets have erosional
boundaries without any gradational transition to ripple-laminated sandstone as opposed to the
planar-laminated sandstone in the Iles delta. The difference in dominant lithofacies is the result
of different flow processes building mouth-bars in each delta. The dominant flow processes for
the Neslen and Iles deltas are interpreted to be lower to upper flow regime traction plus
suspension deposition. However, there is no evidence of deposition purely by suspension in the
Neslen delta. Preserved graded beds and Bouma sequences in the Iles delta indicate that there is
preserved deposition from suspension and the system was more prone to the formation of
turbidites.
The lateral and longitudinal variations of lithofacies result in distinct architectural styles
in each delta. On average, the constituent beds within the Neslen delta are more vertically
amalgamated than the Iles delta (Figure 22) and more amalgamated throughout the body of the
22
delta (Figure 21). The Neslen delta has a higher proportion of sand-rich lithofacies along its
fractional length with comparatively fewer shale breaks than the Iles delta (As demonstrated by
the table in Figure 23). There is little bottomset aggradation in the Neslen delta whereas the Iles
delta has laterally persistent and aggradational bottomset (prodelta) strata (Figure 21). The
average element width is 540 m and 1000 m for the Neslen and Iles deltas, respectively. The
element aspect ratios (width/thickness) are greater in the Neslen delta than the Iles delta, 140 and
110, respectively. The overlap of stratigraphically-adjacent bodies averages 55% and ~90% for
stories in the Neslen and Iles deltas, respectively, and 15% and 57% for elements (Figure 23),
indicating that the Neslen delta stacks more compensationally. More overlap of mouth bars in the
Iles delta indicates the mouth bars stack more vertically than their shallow-water counterparts.
The Neslen delta stacks more compensationally because of the limited accommodation space
created by the shallow-water depth and the low gradient. The higher gradient in the Iles delta
results in higher accommodation space and thus a higher propensity for mouth bars to stack
vertically. The higher gradient in the Iles delta is also the likely cause for the presence of
turbidites because a higher gradient indicates a higher probability of slope failure.
According to this comparison, shallow-water deltas are dominated by tractive deposition,
have minimal bottomset aggradation, and stack highly compensationally (Table 10). The deltas
also share several important similarities. The stratigraphic architecture in the proximal portions
of the deltas is similar because they are deposited in similar water depths near the apex of the
river. For example, proximal portions of the deltas have similar proportions of topset strata and
similar compensational stacking patterns. These similarities are attributed to the similar water
depth of the deltas in the up-current part of the systems (Figure 2). It is hypothesized that if the
23
Neslen delta continued to prograde into deeper-water it would eventually become
stratigraphically similar to the Iles delta.
24
CHAPTER 6
APPLICATION
Results from this study, including lithofacies data, dimensions of stories and elements,
and the documented paleogeographic evolution can be used as an analog for hydrocarbon
exploration in subsurface reservoirs in similar low-accommodation depositional environments.
The measurements of the overlap of stratigraphically adjacent bodies can be used as a proxy for
reservoir connectivity for hydrocarbon reservoirs deposited in similar settings. Reservoir
connectivity is the length of sand-on-sand contacts between two stratigraphically adjacent
elements normalized by the total length (Funk et al., 2012, Moody et al., 2012). The data
collected from this study helps narrow the technology gap of the prediction of reservoir
connectivity at an intra-parasequence scale by documenting small-scale lithofacies changes in
three dimensions. The variability of lithofacies proportions, dimensions, and juxtapositions
influences how fluid will move through a reservoir and how those fluids can be produced
efficiently.
This study proposes a hierarchy to classify deltaic systems from the source of the river to
deep-water systems. The five tier hierarchy for deltaic systems can be used to promote objective
comparisons between fluvial, deltaic, and deep-water systems (Figure 8 and Table 1).
25
CHAPTER 7
CONCLUSIONS
1. The study interval documented in the Neslen Formation is identified as a shallow-water delta
deposited on a coastal plain with brackish-water influence. It is a river-dominated delta
composed of channel and mouth-bar stories that form the building blocks for channel and mouthbar elements. The lithofacies architecture, or lateral and vertical distributions of lithofacies,
reflects the depositional processes and the evolution of the complex.
2. Tractive deposition was the dominant depositional process in the Floy Canyon delta. Ripplelaminated sandstone is the dominant lithofacies, indicating the delta was deposited in a low flow
regime.
3. Architectural analysis of the three dimensional exposure documents that the delta stacked highly
compensationally. Element 3 is composed of nine mouth-bar stories that first stack laterally,
rotating relative to older stories, then progradationally. Three delta elements are identified in this
study and they collectively build a complex. The three elements are composed of channel and
mouth-bar stories.
4. The plan-view morphologic expression of river-dominated deltas appears fractal but the
stratigraphy stacks hierarchically.
5. Edmonds (2011) model predicts delta architecture to change along a longitudinal transect from
topset-dominated to foreset-dominated. This study confirms, through 3D characterization of a
shallow-water delta, the prediction to be plausible. The delta in the Floy Canyon study area has a
complicated architecture that varies longitudinally because the lithofacies proportions change
radially from the axis of the system to the margins.
26
6. Similarities between the Okavango delta, Wax Lake Delta, and the Neslen delta support the
conclusion that water depth influences the stratigraphic architecture of deltas.
7. The comparison of the delta in Floy Canyon delta with modern deltas, such as the Okavango
delta and the Wax Lake delta, indicates that shallow-water deltas (deposited in 2-6 m water
depth) share several architectural similarities including mouth bars built by traction,
compensational stacking, and high lateral and longitudinal variability due to high compensational
stacking and sediment dispersion.
8. The comparison of quantitative data from this study to a study of a deeper-water delta in Hunter
Canyon, Colorado documents that water depth influences the resultant stratigraphic architecture
of deltas. River-dominated deltas deposited in different water depths have similar plan-view
morphologic expressions, but the stratigraphic architectures are different. This study documents
several significant differences between the shallow-water delta in Floy Canyon (Neslen delta)
and the deeper-water delta in Hunter Canyon (Iles delta): the shallow-water delta has, 1) lower
clinoform angles, 2) a higher percentage of amalgamated sand, 3) bottomset aggradation only
near the terminations of mouth-bar stories, and 4) less overlap between stratigraphically adjacent
stories than their deeper-water counterparts. The critical conclusion is that shallow-water deltas
stack more compensationally than their deeper-water counterparts; resulting in more lateral
variability of sand bodies.
9. Shallow-water deltas are more amalgamated than their deeper-water counterparts because the
proportion of preserved inter-story mud is lower than in deeper-water deltas, thus resulting in a
higher proportions of amalgamated, erosional, sand-on-sand contacts in the shallow-water delta.
27
FIGURES
Figure 1: A) Diagram showing a dip-oriented cross section of the stratigraphy of delta from
Edmonds et al. (2011). B) Schematic diagram of a delta by Bhattacharya (2006) documenting the
previous interpretations of deltas. The key distinction between A and B is the thickness of the
topset in A is based on channel depth, which is highlighted with a dashed line, indicating that
topset thickness decreases along the depositional profile. The previous model (B) by
Bhattacharya (2006) does not account for longitudinal variations in stratigraphy.
Figure 2: Diagrams documenting the spatial variations of a shallow (A) and comparatively
deeper-water (B) delta based on Edmonds et al. (2011). Key predicted distinctions between
shallow-water and deeper-water deltas are: 1) shallow-water deltas have a higher proportion of
topset strata (i.e. distributary channels and interdistributary muds) whereas deeper-water deltas
have a higher proportion of mouth bars, and 2) the fractional length of topset-dominated strata is
greater in shallow-water deltas than deeper-water deltas.
28
Figure 3: Aerial photograph of the Okavango Delta, Botswana (left) and the Mississippi River
Delta, Louisiana (right). Both deltas are deposited in different water depths but the plan view
morphologies are similar. Despite the morphologic similarities Edmond’s model predicts them to
have different internal stratigraphic architecture (see Figure 2).
29
Figure 4: Location of the outcrop analyzed in this study. The study area is located in the Book
Cliffs, Grand County, Utah, S31 T20S R19E. A) The location of the study on a topographic map.
B) A geologic map of Floy Canyon, which is composed of the Cretaceous Mesaverde Group.
The specific study interval is in the Neslen Formation.
30
Figure 5: Stratigraphic columns and inset map showing the stratigraphy of the Book Cliffs of Colorado and Utah (modified from
Hettinger and Kirschbaum, 2002). The Neslen and Iles Formations are time equivalent stratigraphic units. See Figure 7 for a detailed
cross section of the field area.
31
Figure 6: Paleogeographic map of the Neslen and Iles Formations. The locations of this study in
Floy Canyon, Utah and the companion study in Hunter Canyon, Colorado are labeled. The Floy
Canyon study area was deposited at the interface between fluvial and coastal plain paleoenvironments (personal communication, Mark Kirschbaum).
32
Figure 7: Detailed cross section of stratigraphic columns measured in the Floy Canyon area. The
center column was measured along the transect labeled in Figure 4 (personal communication,
Mark Kirschbaum).
33
Figure 8: Annotated stratigraphic column (SC 18), lithofacies, and photograph demonstrating the
hierarchical framework used in this study. The location of the stratigraphic column (SC) is
labeled on the map in Figure 4.
34
Figure 9: Photos of the seven lithofacies identified in this study. The colored square in the upper
left hand corner of each photo represents the color of each lithofacies mapped on the photopanels
(Figure 10 and Table 2).
35
Figure 10: Interpreted photopanels A-D (at the same scale) with stories, elements, sediment transport directions, and lithofacies proportions labeled.
36
Figure 11: Common trace fossils identified in the study area. See Figure 4 for the location of SC
4.
37
Figure 12: Ripple laminations present in lithofacies 4: ripple-laminated sandstone.
38
Figure 13: Chart of intra-story amalgamation ratios in each mouth-bar story in Element 3. The
amalgamation ratio is the vertical thickness of amalgamated sand divided by the total vertical
thickness of the story and was calculated from stratigraphic columns.
39
Figure 14: Isopach map of the sand thickness of the genetic unit. The maximum measured sand
thickness is 4.85 m.
40
Figure 15: A) Paleogeographic reconstruction of Elements 1-3. B) Early stage of the
paleogeographic reconstruction of the stories within Elements 3 where stories 3A-3D stack
laterally. C) Middle stage of the paleogeographic reconstruction of Element 3 where stories 3E
and 3F stack laterally and are rotated relative to the older stories in A. D) Late stage of the
paleogeographic reconstruction of Element 3 where stories 3H-3I were deposited via incision
and down-current progradation.
41
Figure 16: Coastal-plain stratigraphy overlain by deltaic deposit.
42
Figure 17: Diagram summarizing aerial and cross-sectional characteristics of an idealized story
(3H) identified in this study.
43
Figure 18: Diagram of compensational stacking of flow expansion bars in the north western updip portion of the study area.
44
Figure 19: Examples of trace fossils. A) Bergaueria, B) crocodile claw marks, and C)
Rhizocorallium.
45
Figure 20: A) Diagram comparing modern deltas deposited in different water depths to the delta analyzed in this study. B) Cross
sections showing the stratigraphy of shallow-water deltas and deeper-water deltas. The deltas have a similar plan view shape and are
scale invariant. See Table 9 for a detailed comparison of the deltas listed above.
46
Figure 21: Diagram comparing the lithofacies and stratigraphic architecture of the Neslen delta
and the Iles delta. For the location of the study areas see Figures 5 and 6. The inset map show the
exposure used to document the lithofacies.
47
Figure 22: Diagram comparing the vertical transects observed in the Neslen delta and the Iles
delta. The beds in the Neslen delta are more vertically amalgamated than the Iles delta. The
stratigraphic column for the Neslen Delta is SC 16 in Figure 4 and is represented by a black
vertical line in Figure 21.
48
Figure 23: Diagram comparing the story-scale characteristics for the Neslen delta (left) and the
Iles delta (right) at the same scale. The mouth-bar stories in the Neslen delta are more sand-rich,
smaller, and have lower aspect ratios than mouth bars in the Iles delta.
49
TABLES
Table 1: Table comparing the hierarchical classification used in this study to selected previous
studies.
This Study
(Neslen
Delta)
Campbell (1967)
Pyles (2007)
Deepwater
Hierarchy
NA
NA
Bed
Bedset
Lamina
Lamina set
Bed
Bedset
Lamina
Lamina set
Bed
Bedset
Story
NA
NA
Element
NA
Architectural
element
Complex
NA
NA
Systems tract
Depositional
sequence
Straub and
Pyles (2012)
Deepwater
Hierarchy
NA
NA
Bed to bedsets
lobe and channel
beds)
Story (lobe and
channel stories)
Element (lobe
and channel
elements)
Complex (lobe
and channel
complexes)
50
Ford and Pyles
(2013)
Fluvial
Hierarchy
Lamina
Lamina set
Bed
Bedset
Story
Element
Archetype
Table 2: Descriptions of the seven lithofacies identified in this study.
L
1
Lithofacies
Name
Lithofacies
Subdivision
Description and Sedimentary
Structures
Organic-rich
mudstone
and
carbonaceous
shale
Laminated
carbonaceous
mudstone
Laminated carbonaceous mudstone and
silt with disseminated plant material. BI=0
NA
Laminated
mudstone
Convoluted
mudstone
NA
Laminated mudstone with interbedded silt
BI=0 rootlets, disseminated plant material
Light grey convoluted mudstone with a
rounded weathering texture. BI=5
1-5 cm beds of clay, silt and very finegrained sand that lack organic material
composed of unidirectional ripples and
lenticular beds. Trace fossils include
crocodile or turtle tracks, Bergaueria,
Skolithos, Planolites, and Rhizocorallium.
BI=3
Clay
Clay,
silt, and
very
finegrained
sand
Lower flow regime
with tractive
deposition and
periods of
suspension and
fall-out; low
energy
NA
Rippled to structureless sandstone with
mud clasts and wood clasts. Wood mats
at the top of beds are burrowed by
Teredolites. BI=3
Finegrained
sand
Lower flow regime
with tractive
deposition;
moderate energy
Amalgamated
faintly ripplelaminated
sandstone
Deamalgamated
faintly ripplelaminated
sandstone
Distinctly
ripplelaminated
sandstone
Contorted
ripple
sandstone
NA
Obscured bed boundaries and structures.
Ripples 1-4 cm in relief. Unidirectional
asymmetric ripples. The dominant trace
fossil is Skolithos. BI=4
1-20 cm beds with interbedded silt and
mud and unidirectional asymmetrical
ripples. The dominant trace fossil is
Skolithos. BI=3
Finegrained
sand
Lower flow regime
to suspension fallout; low to
moderate energy
10-30 cm discontinuous ripple-laminated
sandstone beds with 3D ripples,
unidirectional symmetrical ripples, and
water escape structures. BI=0-1
Contorted ripple beds with faint remnant
structures. The dominant trace fossil is
Skolithos. BI=2
20-30 cm bed of gradational planarlaminated sandstone composed of
coarsening-up laminations with minor
burrowing. BI=1
Low angle cross-stratified sandstone with
distinct beds and mudstone rip-up clasts
from eroded underlying material BI=1
Finegrained
sand
Contorted cross-stratified beds. Structures
are obscure or distinct. The dominant
trace fossil is Skolithos. BI=2
Finegrained
sand
Mudstone rip-up clasts in a matrix of silt
with unidirectional ripples. BI=0
Clay to
silt
L
2
Interbedded
mudstone,
sandstone,
and siltstone
L
3
Amalgamated
ripplelaminated
sandstone
with wood
clasts and ripup clasts
Ripplelaminated
sandstone
L
4
L
5
Planarlaminated
sandstone
L
6
Crossstratified
sandstone
L
7
Silt with
mudstone
clasts
Low-angle
crossstratified
sandstone
contorted
crossstratified
sandstone
NA
51
Grain
Size
Mode of
sediment
transport/ mode
of deposition
Lower flow regime
and fall-out from
suspension.
Clay
Silt to
finegrained
sand
Finegrained
sand
Finegrained
sand
Finegrained
sand
Upper flow regime
with tractive
deposition; high
energy
Lower flow regime
with tractive
deposition;
moderate to high
energy
Lower flow regime
with tractive
deposition; low
energy
Table 3: Table of the bioturbation index used in this study.
Index Number
0
1
2
3
4
5
Bioturbation Index (BI)
Degree of Bioturbation (%)
Less than 50
Greater than 50
52
Description
No burrows
A few discrete trace fossils
Multiple overlapping trace fossils
Distinct laminations
Laminations faint
No laminations or very indistinct
laminations
Table 4: Dimensions of mouth-bar and channel stories.
Story Dimensions
MouthBar
Stories
Channel
Stories
Story
Maximum
Thickness (m)
Width (m)
Aspect ratio
(width/thickness)
1A
3.85
300
77.9
Comments
NA
Laterally stacking mouth
bar
Laterally stacking mouth
bar
Laterally stacking mouth
bar
Laterally stacking mouth
bar
Laterally stacking mouth
bar
Laterally stacking mouth
bar
Laterally stacking mouth
bar
Laterally stacking mouth
bar
Incision and down-current
progradation
Incision and down-current
progradation
2A
1.45
85
58.6
2B
1.75
85
48.6
3A
2.37
150
63.3
3B
2.6
210
80.8
3C
1.4
360
257.1
3D
2.08
280
134.6
3E
1.63
220
135.0
3F
0.97
205
211.3
3G
2.48
295
119.0
3H
2.07
305
147.3
3I
1.15
215
187.0
Incision and down-current
progradation
1A
2.8
48
17.1
NA
2A
2
50
25.0
NA
2B
2
50
25.0
NA
3A
NA
NA
NA
NA
3B
NA
NA
NA
NA
3C
NA
NA
NA
NA
3D
NA
NA
NA
NA
3E
NA
NA
NA
NA
3F
NA
NA
NA
NA
3G
2.5
37.5
15
3H
2.5
27
10.8
3I
NA
NA
NA
53
Incision into older mouth
bars
Incision into older mouth
bars
individual channel unclear
Table 5: Table documenting the percent overlap between stratigraphically adjacent stories. The
maximum length of overlap is measured between two stratigraphically adjacent stories.
Percent Overlap
Story
Maximum
Length of
Overlap
Width of Story
Percent
Overlap
1A and 3A
80
300
26.7
1A and 3C
30
300
10.0
2A and 3F
37
85
43.5
2B and 3F
37
85
43.5
3A and 1A
80
150
53.3
3A and 3B
35
150
23.3
3A and 3C
100
150
66.7
3B and 3A
35
210
16.7
3B and 3C
210
210
100.0
3B and 3D
162
210
77.1
3B and 3E
125
210
59.5
3B and 3F
60
210
28.6
3B and 3G
100
210
47.6
3B and 3H
75
210
35.7
3C and 1A
30
210
14.3
3C and 3A
100
280
35.7
3C and 3B
210
280
75.0
3C and 3D
260
280
92.9
3C and 3E
145
280
51.8
3C and 3F
75
280
26.8
3C and 3G
150
280
53.6
3C and 3H
120
280
42.9
3D and 3B
262
280
93.6
3D and 3C
260
280
92.9
3D and 3E
275
280
98.2
3D and 3F
145
280
51.8
3D and 3H
112
280
40.0
3E and 3B
125
220
56.8
3E and 3C
145
220
65.9
3E and 3D
175
220
79.5
3E and 3F
135
220
61.4
3F and 3B
60
205
29.3
3F and 3C
75
205
36.6
3F and 3D
145
205
70.7
3F and 3E
135
205
65.9
3F and 2A
37
205
18.0
54
Table 5: Continued
3F and 2B
3G and 3B
3G and 3C
3G and 3H
3G and 3I
3H and 3B
3H and 3C
3H and 3D
3H and 3G
3H and 3I
3I and 3G
3I and 3H
AVERAGE
37
100
150
237
190
75
120
112
237
215
190
215
129.6
205
295
295
295
295
305
305
305
305
305
215
215
239.7
18.0
33.9
50.8
80.3
64.4
24.6
39.3
36.7
77.7
70.5
88.4
100.0
53.6
Table 6: Table documenting the relative abundance of sand-rich lithofacies (L3-L6) to silt-rich
lithofacies (L2), or axis to margin, respectively.
Story
margin
width/axis
width
margin
width/total
width
axis
width/total
width
3A
0
0
1
3B
0.40
0.29
0.71
3C
0.28
0.22
0.78
3D
0.10
0.09
0.91
3E
0.22
0.18
0.82
3F
0.46
0.32
0.68
Table 7: Table of dimensions of the elements identified in this study.
Element Dimensions
Element
Maximum
Thickness
(m)
Width (m)
Aspect ratio
(width/thickness)
1
3.8
490
128.9
2
3.2
570
178.1
3
4.6
540
117.4
Comments
Interpreted
width. Limited
preservation
Interpreted
width. Limited
preservation
Best exposed
55
Table 8: Table documenting the percent overlap between stratigraphically adjacent elements
identified in this study.
Percent Overlap between Stratigraphically Adjacent
Elements
Element
Length of
Overlap (m)
3 on 1
70
Width of
Element
(m)
490
3 with 1 and 2
150
570
26.3
3 on 2
80
540
14.8
Percent
Overlap
14.3
Table 9: Table documenting characteristics of modern deltas compared to the delta documented
in this study.
Water depth
and aerial
extent
Environment
of deposition
Flow
processes
Vertical
stratigraphic
trends
Channel
Characteristics
Mouth Bar
Characteristics
Okavango Delta,
Botswana
25,000 km^2, 2-4 m
water depth (McCarthy et
al., 1991)
Inland delta (McCarthy et
al., 1992; Stanistreet and
McCarthy, 1992).
Bedload/Traction
(McCarthy et al., 1992)
Neslen Formation Delta, Floy
Canyon, Utah
4 m water depth, 1 km^2
exposed aerial extent
Wax Lake Delta, Louisiana
Low gradient coastal plain
Open marine
Bedload/Traction
Upward transition from
layers of organic material
to upward-fining channel
deposits (McCarthy et al.,
1988).
1-3 m thick aggradational
meandering levee bound
channels (Stanistreet and
McCarthy, 1993).
Low angle, vegetation
stabilized mouth bars
formed by dispersive flow.
(McCarthy et al., 1988;
McCarthy et al., 1991).
Upward thickening and upward
coarsening of genetic units.
Sharp based contact with
coastal plain carbonaceous
shale.
1-3 m thick erosional channels
composed of fine-grained
sand.
Bedload/Traction, and fall-out
from suspension (Wright, 1977;
Wellner et al., 2006).
Upward thickening and
coarsening of genetic units.
(Wellner et al., 2006).
3 elements that are 3-5 m
thick. Mouth bars are elongate
parallel to the sediment
transport direction. The
dominant lithofacies is ripplelaminated sandstone.
56
4-6 m water depth, 145 km^2
1-2 m thick erosional channels.
(Edmonds et al., 2011; Shaw et
al., 2013; Shaw and Mohrig,
2014)
1-3 m thick mouth bars that are
elongate parallel to the
sediment transport direction.
The dominant lithofacies are
trough-cross bedded
sandstone. (Wellner et al.,
2006).
Table 10: Table documenting the properties of a shallow-water delta (conducted by Kimber
O’Brien) compared to a deeper-water delta (conducted by Matthew Andresen).
Shallow-Water Delta:
Kimber O'Brien
Deeper-Water Delta: Matthew
Andresen
Formation
Neslen Formation
Upper Sego Member, Iles
Formation
Age
Campanian
Campanian
Locality
Floy Canyon, UT
Hunter Canyon, CO
Elements
MB 1,2, 3
MB 1, 2, 3
Clinoform angle
(°)
0.5-3
<1 - 5
Water depth (m)
4
15+
Element
thickness (m)
4
9
540
1000
135
110
15%
57%
Story percent
overlap
55%
~90%
Mode of
sediment
transport/ mode
of deposition
bedload/traction
Suspension/traction
Element width
(m)
Element Aspect
ratio
(width/thickness)
Amount of
overlap of
stratigraphically
adjacent
elements
Dominant facies
Foreset: RippleForeset: Planar-laminated
laminated sandstone and
sandstone, interbedded mud and
cross-stratified
silt
sandstone
Wright (1977)
classification
Friction dominated
Inertia dominated
Top-truncated?
No
Yes
57
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61
APPENDIX A
A-1
Stratigraphic Columns
Figure A-1: Stratigraphic columns SC1-SC6. Locations are documented in Figure 4.
62
Figure A-2: Stratigraphic columns SC7-SC12. Locations are documented in Figure 4.
63
Figure A-3: Stratigraphic columns SC13-SC18. Locations are documented in Figure 4.
64
Figure A-4: Stratigraphic columns SC19-SC24. Locations are documented in Figure 4.
65
Figure A-5: Stratigraphic columns SC25-SC30. Locations are documented in Figure 4.
66