stratigraphic architecture and avulsion deposits...
TRANSCRIPT
STRATIGRAPHIC ARCHITECTURE AND AVULSION DEPOSITS OF A LOW NET-
SAND CONTENT FLUVIAL SUCCESSION: LOWER WASATCH
FORMATION, UINTA BASIN
by
Kassandra L. Sendziak
ii
A thesis submitted to the Faculty and the 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: ____________________________ Kassandra L. Sendziak
Signed: ____________________________ Dr. David R. Pyles
Thesis Advisor Golden, Colorado
Date _____________
Signed: ____________________________ Dr. John D. Humphrey
Professor and Head Department of Geology and Geological Engineering
iii
ABSTRACT
This study documents the stratigraphic architecture of channel and floodplain strata of
low net-sand content fluvial deposits in outcrops of the lower Wasatch Formation, Desolation
Canyon, Uinta Basin, Utah. The lower Wasatch Formation has a net sand-content of 0.27 and
contains predominantly floodplain strata (79% in the field area). Three types of crevasse splays
are recognized in this field area based on their physical relationship to adjacent channel-belt
strata. Associated coeval splays are laterally adjacent and are physically connected to a
channel-belt element, indicating that the crevasse splay was deposited coeval with the channel
fill. Unassociated splays are spatially isolated from channel-belt elements and are interpreted to
represent a failed avulsion. Associated non-coeval splays underlie the channel-belt element
and are interpreted to be genetically related to the overlying channel-belt element, and therefore
are a record of a successful avulsion.
Three distinct types of associated non-coeval splays are identified in this study area
based on physical, observable characteristics: type I, type II, and type III. A conceptual model is
proposed that describes longitudinal changes in associated non-coeval splay deposits where
type I, type II, and type III splay units represent proximal, medial, and distal positions in splay
deposits relative to the source channel, respectively. Decreases in the following characteristics
of splays occur with increased distance from the source channel: (1) thickness of splay unit, (2)
thickness and abundance of splay beds, (3) net-sand content, (4) grain size, and (5) erosion.
The occurrence of floodplain and channel-belt strata in a vertical transect through the
outcrop are evaluated to: (1) determine whether the dominant avulsion style is aggradational or
incisional, and (2) relate channel story type (i.e. downstream versus lateral accreting) to
avulsion style. The correlation between the abundance of splay beds and the abundance of
overlying channel-belt elements is interpreted to indicate that the succession resulted from
predominantly aggradational avulsion processes. The occurrence of splay beds below channel-
iv
belts containing predominantly downstream-accreting stories is interpreted to indicate that these
channels resulted from predominantly aggradational avulsion processes. The lack of splay
beds below channel-belts containing predominantly lateral-accreting stories is interpreted to
indicate that these channels resulted from predominantly incisional avulsion processes.
v
TABLE OF CONTENTS
ABSTRACT ................................................................................................................................III
LIST OF FIGURES ................................................................................................................... VI
LIST OF TABLES ..................................................................................................................... VII
ACKNOWLEDGEMENTS ....................................................................................................... VIII
CHAPTER 1 INTRODUCTION ................................................................................................ 1
3.1 Lithofacies........................................................................................................... 9
3.2 Architectural and Hierarchical Fluvial Classification ............................................ 9
CHAPTER 4 ARCHITECTURE OF THE LOWER WASATCH FORMATION ..........................17
4.1 Channel-Belt Architecture ................................................................................. 17
4.1.1 Channel-Belt Elements ..................................................................................... 17
4.1.2 Channel-Belt Stories ......................................................................................... 18
4.2 Floodplain-Belt Architecture .............................................................................. 20
4.2.1 Floodplain-Belt Elements ...................................................................................20
4.2.2 Floodplain-Belt Stories .......................................................................................20
CHAPTER 5 CREVASSE SPLAY TYPES ..............................................................................26
CHAPTER 6 DISCUSSION ....................................................................................................31
6.1 Spatially Varying Characteristics of Associated Non-Coeval Splays (i.e. Avulsion
Complexes) ....................................................................................................... 31
6.2 Upward Trends: Relationship between Floodplain and Channel-Belt Elements 33
CHAPTER 7 APPLICATIONS .................................................................................................40
CHAPTER 8 FUTURE WORK ................................................................................................43
CHAPTER 9 CONCLUSIONS.................................................................................................44
REFERENCES CITED ..............................................................................................................46
SUPPLEMENTAL FILES ..........................................................................................................52
vi
LIST OF FIGURES
Figure 2.1 Location map of the Uinta Basin and chronostratigraphic chart of basin fill ......... 6
Figure 2.2 Topographic map of the study area ..................................................................... 7
Figure 2.3 Photographs of the study area ............................................................................ 8
Figure 3.1 Quantitative data of the lower Wasatch Formation in the field area ....................13
Figure 3.2 Lithofacies photographs .....................................................................................14
Figure 3.3 Three-level hierarchical classification scheme ...................................................15
Figure 4.1 Examples of downstream accreting channel-belt stories and elements ..............22
Figure 4.2 Examples of laterally accreting channel-belt stories and elements .....................23
Figure 4.3 Example of crevasse channel story ....................................................................24
Figure 4.4 Examples of crevasse-splays and floodplain-fine deposits .................................25
Figure 5.1 Field example and schematic diagram of an associated coeval splay ................28
Figure 5.2 Field examples and schematic diagram of associated non-coeval splays. .........29
Figure 5.3 Field examples and schematic diagram of unassociated splays. ........................30
Figure 6.1 Field example and schematic diagram of type I splays ......................................35
Figure 6.2 Field example and schematic diagram of type II splays......................................36
Figure 6.3 Field example and schematic diagram of type III splay s ....................................37
Figure 6.4 Schematic of spatially varying characteristics of associated non-coeval splays .38
Figure 6.5 Upward trends of the lower Wasatch Formation .................................................39
Figure 7.1 Gamma Ray signatures of associated non-coeval spaly types ...........................42
vii
LIST OF TABLES
Table 3.1 Lithofacies descriptions ......................................................................................14
viii
ACKNOWLEDGEMENTS
There are many people with whom this project would not have been possible. I am
particularly grateful for the guidance of my advisor Dr. David Pyles. The knowledge and
enthusiasm he shares for the scientific method have made me a better scientist, geologist, and
critical thinker. I would also like to thank other members of my committee: Dr. Rick Sarg,
committee-chair, for his support and guidance regarding coursework, research, and my future
as a geologist; Dr. Bryan Bracken, for his time in and out of the field, mentoring, and support
throughout this project; and Dr. Matthew Pranter, for his guidance and encouragement.
I would like to express my greatest gratitude to Grace Ford. This project would not have
started nor been completed without her encouragement and extraordinary mentoring in and out
of the field. I wish to acknowledge Chelsea Philippe for her assistance in getting us safely down
the river and through many days in the field, I might not have survived without her. Additional
field assistance from Neil Sharp is also greatly appreciated. I would like to thank other
members of the CoRE team for their support throughout this project: Jane Stammer, Jeremiah
Moody; Greg Gordon; Charlie Rourke, Linda Martin, and Cathy Van Tassel. Finally, I would like
to thank my parents, Linda and Walter Sendziak, most of all. Without their support I would have
never made it through these countless years of school.
The majority of funding for this research was provided by the Chevron Center of
Research Excellence. Additional funding was provided by the Timothy & Barbara Bartshe
Fellowship, Robert L. Burch Fellowship, Devon Scholarship, and Colorado School of Mines
Department of Geology and Geologic Engineering.
1
CHAPTER 1
INTRODUCTION
Fluvial deposits host significant amounts of hydrocarbons, but characterization of these
reservoirs (i.e. sandstone geometry, distribution, and connectivity) is challenging (Leeder, 1978;
Allen, 1978; Bridge and Leeder, 1979; Mackey and Bridge, 1995; Heller and Paola, 1996,
Pranter and Sommer, 2011). Reservoir characterization is especially difficult in low net-sand
content fluvial successions where the discontinuity of the channel-belts results in internally
heterogeneous reservoirs (Pranter et al, 2009; Pranter and Sommer, 2011). Knowledge of the
distribution and dimensions of channel-belt strata is crucial to characterizing connectivity in low
net-sand content fluvial reservoirs (Pranter et al., 2009). The term ‘channel-belt’ is defined
herein as deposits associated with channel processes including channel bar and channel fill
deposits. Coarse-grained floodplain deposits, such as crevasse-splay deposits, also impact
connectivity of channel-belts (Pranter et al., in press). Despite the abundance of floodplain
strata in low net-sand content fluvial systems and the potential role that floodplain strata has in
sandstone-body connectivity, research has focused primarily on the architecture of channel-
belts, the exceptions are Bown and Kraus (1987), Kraus and Aslan (1993), Smith (1990, 1993),
and Willis and Behrensmeyer (1994). Even fewer studies describe the relationship between
floodplain strata and the associated channel-belt strata (e.g. Kraus and Gwinn, 1997; Kraus and
Wells, 1999; Mohrig et al., 2000; Jones and Hajek, 2007).
Avulsion processes influence the distribution of sediment on the floodplain and the
resulting floodplain deposits (Allen, 1965; Bridge and Leeder, 1979; Mackey and Bridge, 1995;
Miall, 1996). Avulsion processes also impact channel-belt stratigraphy on a larger scale,
particularly the stacking patterns and distribution of channel-belts (Smith et al., 1989; Mackey
and Bridge, 1995; Heller and Paola, 1996). Avulsion is the ‘process by which flow diverts out of
an established river channel into a new permanent course on the adjacent floodplain’
2
(Slingerland and Smith, 2004, p. 259). Studies of the modern Saskatchewan River by Smith et
al. (1998) and Smith and Perez-Arlucea (1994) document that during avulsion, flow from the
parent channel diverts to a laterally adjacent network of crevasse channels and splays. This
avulsion complex receives water and sediment during all stages of flow (Smith et al., 1989). As
avulsion progresses, flow within the avulsion complex becomes concentrated into a smaller
number of channels, until a single channel is sustained, and the avulsion process is complete.
Relatively few studies describe ancient avulsion deposits (e.g. Kraus and Gwinn, 1997;
Kraus and Wells, 1999; Mohrig et al., 2000; Jones and Hajek, 2007). Kraus and Wells (1998)
build upon the model of Smith et al. (1998) by documenting ancient avulsion deposits termed
‘heterolithic avulsion deposits’ that underlie paleochannels in the Willwood and Fort Union
Formations. Mohrig et al. (2000) document paleochannels in the Guadalope-Matarranya
system in Spain and the Wasatch Formation in western Colorado that incise directly into the
floodplain, and lack underlying heterolithic avulsion deposits. To address these contrasting
stratigraphic expressions of avulsion, Mohrig et al. (2000) proposes two end-member styles of
avulsion. The first is termed aggradational avulsion, in which a network of crevasse splays is
developed in the adjacent floodplain that is subsequently followed by the new channel (i.e. fill
then cut), a model similar to that proposed by Smith et al. (1989). The second is termed
incisional avulsion, in which the channel cuts directly into the fine-grained strata in the floodplain
and is subsequently filled with channel-belt strata (i.e. cut-then-fill model). Jones and Hajeck
(2007) also document two end-members of avulsion stratigraphy based on their observations in
the Willwood and Ferris Formations in Wyoming. The first is termed stratigraphically
transitional, where crevasse splays and other non-overbank deposits (i.e. crevasse channel and
splay deposits) are overlain by paleochannels, these are similar to deposits that result from
aggradational avulsion (sensu Mohrig et al., 2000). The second is termed stratigraphically
abrupt, where the main paleochannel stratigraphically overlies fine-grained overbank-floodplain
3
deposits, these are similar to deposits that result from incisional avulsion (sensu Mohrig et al.,
2000). Although the various stratigraphic manifestations of avulsion styles have been
documented, no studies describe the spatial variability within these deposits.
This study uses exceptionally well-exposed outcrops of the low net-sand content (net-
sand content of 0.27, floodplain-to-total of 0.79; calculated herein) fluvial strata of the lower
Wasatch Formation to document, for the first time, how stratigraphic architecture of crevasse-
splay deposits vary with increased distance from the source channel. Furthermore, this study
relates upward patterns in the floodplain to upward patterns in the channel-belt, documenting an
association between floodplain strata, channel-belt strata, and channel-belt style. Understanding
the evolution of a fluvial succession and the relationship between floodplain and channel-belt
strata within a fluvial succession has important implications for reservoir characterization.
4
CHAPTER 2
GEOLOGIC SETTING AND FIELD AREA
The Uinta Basin is a foreland basin located in northeastern Utah (Figure 2.1 A) that
encompasses a total area of ~2,000 km2 (Montgomery and Morgan, 1998). The basin is
bounded to the north by the Uinta uplift, to the east by the Douglas Creek Arch, to the south by
the San Rafael Swell, and to the west by the Sevier thrust belt (Figure 2.1 B). The basin
developed in the Latest Cretaceous Period through the Early Oligocene Epoch (Fouch, 1975;
Fouch et al., 1994a) (Figure 2.1 C).
The Uinta Basin contains up to 5,000 m of siliciclastic and carbonate strata (Fouch et al.,
1994a). These strata are interpreted to have been deposited in a range of environments
including open lacustrine, marginal lacustrine, and fluvial (Fouch, 1975). These strata
unconformably overlie Campanian strata of the Mesaverde Group (Figure 2.1 C). Formations of
the Uinta Basin fill succession are the North Horn, Wasatch, and Green River Formations
(Figure 2.1 C). The Wasatch Formation unconformably overlies the Flagstaff Formation in most
areas (Figure 2.1 C). Where it does not, Fouch (1976) referred to it as the Colton Formation.
The Colton and Wasatch Formations are time-equivalent units and the spatial extent of the
underlying Flagstaff Formation is unknown. Therefore, the term Wasatch is used herein (Ford,
2012). The Green River Formation conformably overlies the Wasatch Formation (Fouch, 1976)
(Figure 2.1 C).
The Wasatch Formation is composed of variegated shale and sandstone interpreted to
have been deposited in a fluvial environment (Spieker, 1946). The Wasatch Formation is
informally divided in to the lower, middle, and upper members based on upward changes in
lithofacies, depositional style, and net-sand content (Ford, 2012). Each of these members are
separated by regionally extensive red, compound paleosols. This study focuses on the lower
5
member of the Wasatch Formation, which has a net-sand content of 0.27 (calculated herein)
and contains single-story and multi-story channel-bar deposits, channel-fill deposits, crevasse-
channel deposits, crevasse-splay deposits, floodplain-fine deposits, and paleosols. The lower
member is interpreted to have been deposited in a floodplain-dominated fluvial succession. The
middle Wasatch Formation has a net-sand content greater than 0.75 and contains amalgamated
multi-story channel-fill deposits, crevasse-splay deposits, and floodplain-fine deposits (Ford,
2012). The upper Wasatch Formation has a net-sand content of 0.5 and contains amalgamated
multi-story channel-fill deposits, crevasse-splay deposits, and floodplain-fine deposits (Ford,
2012).
The study area is located in Desolation Canyon along the southern margin of the Uinta
Basin, where Joe Hutch Canyon and Rain Canyon intersect with the modern Green River
(Figure 2.2). The study area is 1.5 km2 and contains a complete exposure of the lower Wasatch
Formation (Figure 2.2). The depositional strike of the strata is approximately east-west, and the
dip is close to zero degrees. Vegetation is sparse and strata are exceptionally well exposed
and accessible (Figure 2.3).
Figure 3: A, Map of Uinta Basin major structural features. B, Geologic map of field area. The Upper, Middle, and Lower Wasatch are mapped as Wasatch Formation (undifferentiated) east of the river and Colton Formation west of the river. 3A modified from Chidsey (1980). 3B modified from Witkind (1988).
0 5 mi
0 5 km
44
QalQal
TcTcQfQf
TwTw
QalQal
QalQalQfQf
QcfQcf
QfQf
TwTw
TcTc
QalQal
QalQalQfQf
TfnTfn
TgTg
KpruKpru KtKt
TwTw
Legend
Outcrop Location
KpruKpru Upper part of Price River Formation
KtKt Tuscher Formation
QalQal Alluvium
QcfQcf Coalesced alluvial fan deposit
QfQf Alluvial fan deposit
TcTc Colton Formation
TfnTfnFlagstaff Member of Green River Formation and North Horn Formation
TgTg Green River Formation
TwTw Wasatch Formation (undifferentiated)
water
Figure 2.1: (A) Location of the Uinta Basin in northeastern Utah (Ford, 2012). (B) Map of the Uinta Basin documenting the location of the study area and major structural features (Ford, 2012). (C) Chronostratigraphic chart of Lower Cretaceous and Upper Tertiary strata in the Uinta Basin (modified from Fouch et al., 1994a; Johnson and Roberts, 2003; Ford, 2012).
CA
B
E
E
E
L
M
L
L
30
35
40
25
45
50
55
60
65
70
75
80Mesaverde Group
North Horn Fm
Flagstaff Member
Wasatch Fm
Green River Fm
Duchesne River& Uinta Fms
CEN
OZO
ICTE
RTIA
RYPA
LEO
GEN
EEO
CEN
EO
LIG
OCE
NE
PALE
OCE
NE
MES
OZO
ICCR
ETAC
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SL.
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TACE
OU
SCA
MPA
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NM
AAS.
uppermiddlelower
Ma
Uin
ta B
asin
Fill
Suc
cess
ion
Sevier Thrust Belt
Uinta Mountains
Wyoming
Colo
rado
Oil Fields
GasFields
Vernal
Green River
San
Rafa
el S
wel
l
Uncompahgre
Uplift
0
0
10
10 20
20
30
30
40
miles
kilometers
Study Area
500 kmN
Uinta Basin
UTAH
6
Uinta Basin Syncline
Uinta Basin
Dou
glas
Cre
ek A
rch
¯Outcrop of Wasa
tch Fm
Study Area(Figure 2.2
and 2.3)Price
111°
110°39°
40°
110°
40°
7
n = 25
x = 16°
Rose Diagram of SedimentTransport Directions
N
middle WasatchFormation
lower WasatchFormation
lower WasatchFormation
Alluvium
Alluvium
0 52.5
0 321 mi
km
¯lower Wasatch Formation
Alluvium
middle Wasatch Formation
Figure 2.2: Topographic and geologic map of the field area showing location of outcrops studied in this research. Sediment transport direction and the locations of photopanels in Figure 2.3 are also shown.
Joe Hutch Canyon
Rain Canyon
Green River
Main Field Area Figure 2.3 BSupplemental Files
A-1 & A-2
Figure 2.3 D
Figure 2.3 C
8
A
B
C
Figure 2.3: (A) Photograph of the exposures of the study area documenting the contact between the lower and middle members of the Wasatch Formation. (B) Photograph of the main study area. (C) Photograph of the southern part of the study area. (D) Photograph of the northern part of the study area. See Figure 2.2 for locations of photographs in B, C, and D.
SW NE
SW NEMiddle Wasatch
Lower Wasatch
250
m
250
m
150
m
250
m
NWSENW SE D
Middle Wasatch
Lower Wasatch
Middle Wasatch
Lower WasatchLower Wasatch
9
CHAPTER 3
DATA AND METHODOLOGY
In order to address the goals of this study, the following data were collected:
1) forty-eight stratigraphic columns totaling 1,040 m that record grain size and
physical and biogenic sedimentary structures at a centimeter-scale resolution
(Supplemental File A-1);
2) paleocurrent measurements (n = 25) collected from channel margins, flutes, 3D
trough cross-beds, and 3D ripple laminations (Figure 2.2 and Supplemental File A-
2);
3) high-resolution Gigapan photopanels which are used to document stratal
boundaries (Figure 2.3 and Supplemental Files A-2); and
4) thickness measurements (n=98) of stratal units, widths cannot be defined as the
orientation of the outcrop is parallel to paleoflow of most units (Figure 3.1, and
Supplemental File A-3).
3.1 Lithofacies
This study uses Gressley’s (1938) definition of lithofacies as those observable, physical,
chemical, and biological properties of rock that collectively permit objective description (Cross
and Homewood, 1997). Ten lithofacies are documented in the field area. A detailed description
and interpretation of each is included in Table 3.1. Photographic examples of each lithofacies
are included in Figure 3.2.
3.2 Architectural and Hierarchical Fluvial Classification
There are two prevailing methods for describing and classifying fluvial strata: Allen’s (1983)
hierarchical surfaced-based approach, and Miall’s (1985) architectural-element analysis.
Building upon the work of McKee and Weir (1953) and Brookfield (1977), Allen (1983) proposes
10
a method in which first-, second-, and third-order bounding surfaces, which are determined by
stratal terminations and superposition, are used to define a hierarchy of strata. This method has
many strengths; however, it does not account for the internal or external geometry of stratal
units or sedimentary processes. For example, variations in channel-belt architecture (e.g.
lateral-accreting bars and downstream-accreting bars) and sedimentary process (e.g.
suspension versus tractive deposition) are not considered. These characteristics are important
because they have implications for continuity of sandstone units within channel-belts,
connectivity between stratigraphically adjacent channel-belts, porosity, and permeability.
Additionally, this method is difficult to apply to floodplain strata. For example, crevasse-splay
sandstone beds have approximately parallel bounding surfaces. As a result each stratal
boundary is a first-order surface, regardless of the size or timespan of deposition of the units.
Furthermore, bedding in floodplain strata is difficult to correlate due to poor exposure and
bioturbation, rendering this technique inadequate.
Building upon the work of Fisk (1944), Beerbower (1964), and Allen (1983), Miall’s (1985)
method for describing and classifying fluvial strata is based on the division of fluvial strata into
eight architectural elements: (1) channels, (2) gravel bars and bedforms, (3) sandy bedforms,
(4) foreset macroforms, (5) lateral-accretion deposits, (6) sediment-gravity-flow deposits, (7)
laminated-sand sheets, and (8) overbank fines. These elements are differentiated by the nature
of their bounding surfaces, external and internal geometry, and scale (i.e. width and thickness).
This method does not integrate sedimentary processes, timespan of deposition, or temporal
context. For example, channel elements are not distinguished by their fill type (e.g. mud versus
gravel) or sedimentary process (e.g. suspension versus tractive deposition). Furthermore,
lateral-accretion elements, for example, are deposited over a longer time scale than sandy
bedforms. Lastly, lateral-accretion elements can be composed of sandy or gravelly bedforms,
allowing one element to be composed of others, thus nullifying the hierarchical concept.
11
Although the fluvial classification methods of Allen (1983) and Miall (1985) provide a good
foundation for describing fluvial successions, they do not adequately incorporate characteristics
that are essential in understanding reservoir properties.
This study utilizes the method proposed by Ford (2012), which builds upon components of
the surface-based hierarchical approach of Allen (1983) and the architectural-element approach
of Miall (1985) (Figure 3.3). Ford’s (2012) methodology for describing fluvial successions is
based on physical, observable characteristics and is constrained by lithofacies associations,
external geometry, nature of bounding surfaces, and cross-cutting relationships. In this method,
fluvial strata are divided into a three-level hierarchy (stories, elements, and archetype) that
incorporates channel-belt and floodplain strata (Figure 3.3).
Stories are the lowest hierarchical level and the fundamental building blocks of the
classification scheme (Figure 3.3). Building upon the work of Feofilova (1954), Potter (1967),
Jackson (1975), and Friend (1983), Ford (2012) defines stories as meso-scale strata formed
from genetically related beds or bedsets produced by the migration, fill, or overbank discharge
of a single fluvial channel. The thickness of stories scales to bank-full discharge and flood-
stage water depth. Stories associated with channels are: (1) downstream accreting, (2) lateral
accreting, (3) erosionally-based fine-grained fill, and (4) fine-grained fill associated with lateral
accretion. Stories associated with floodplains are: (1) crevasse channels, (2) crevasse splays,
and (3) floodplain fines. Stories stack to build elements, the second hierarchical level (Figure
3.3).
Ford (2012) defines an element as a macro-scale lithosome produced by channel
migration and overbank discharge of a single fluvial channel. Elements are separated by
floodplain fines and/or paleosols, except when eroded by younger elements. Elements are
divided into two classes (Figure 3.3): channel-belt and floodplain-belt. Elements that are
12
composed of one story are referred to as single-story elements whereas those composed of
more than one story are referred to as multi-story elements, following the terminology of
Feofilova (1954), Potter (1967), and Gibling (2006). Elements stack to build an archetype, the
largest hierarchical level (Figure 3.3).
Ford (2012) defines an archetype as a macro-scale feature consisting of a channel-belt
element and the genetically related floodplain-belt element. Archetypes are divided into two
classes based on the predominant channel-belt architectural style: those composed
predominantly of downstream accreting stories (referred to as braided archetypes) and those
composed predominantly of lateral accreting stories (referred to as meandering archetypes).
The boundaries between stratigraphically adjacent archetypes document abandonment and an
abrupt shift in the location of the axis of deposition, which is interpreted as a record of avulsion
(Figure 3.3).
13
150
0.2
0.4
0.6
0.8
1
0 5 10 20
150
0.2
0.4
0.6
0.8
1
0 5 10 20
0.6
150
0.2
0.4
0.8
1
0 5 10 20 150
0.2
0.4
0.6
0.8
0 5 10 20
Mul
ti-st
ory
Chan
nel-B
elt E
lem
ent
Sing
le-s
tory
Cha
nnel
-Bel
t Ele
men
t
150
0.2
0.4
0.6
0.8
1
0 5 10 20
Lateral AccretingDownstream Accreting
Elem
ent
Stor
y
f/n
f/n
f/n
f/n
f/n
f/n
T
T
T
T
T
T
F3 (10 %)
F4 (27 %)
F5 (20 %)
F6 (2 %)
F7 (35 %)
F8 (5 %)
F10 (1 %)F9 (1 %)
F3 (11 %)
F4 (13 %)
F5 (18 %)
F6 (3 %)
F7 (48 %)
F8 (5 %) F2 (<1%)
F4 (17 %)
F5 (19 %)
F6 (2 %)
F7 (44 %)
F4 (13 %)
F5 (22 %)
F6 (5 %)
F7 (51 %)
F8 (4 %)F3 (4 %)F10 (1 %)
F4 (21 %)
F5 (9 %)
F6 (15 %)
F7 (47 %)
F8 (4 %)F3 (2%)F10 (2 %)
F4 (11 %)
F5 (25 %)
F6 (2 %)
F7 (52 %)
F8 (5 %)F3 (5 %)F10 (1 %)
F9 (1 %) F10 (<1 %)
F3 (11 %)F8 (5 %)F2 (<1 %)
F9 (1 %) F10 (<1 %)DownstreamAccreting
Lateral Accreting Erosionally-Based Fine-Grained Fill
Fine-Grained Fill Associatedwith Lateral Accretion
x = 11 mn = 5
m = 10-15 m
x = 8 mn = 19
m = 5-10 m
x = 13 mn = 3
m = 5-20 m
x = 10 mn = 2
m = 5-15 m
x = 7 mn = 36
m = 0-5 m
x = 7 mn = 20
m = 5-10 m
150
0.2
0.4
0.6
0.8
1
0 5 2010
x = 3 mn = 1
m = 3.2 m
f/n
T 150
0.2
0.4
0.6
0.8
1
0 5 2010
x = 4 mn = 12
m = 0-5 m
f/n
T 150
0.2
0.4
0.6
0.8
1
0 5 2010
A
B
low
er W
asat
ch F
m
C
Channel-Belt21 %
Floodplain-Belt79 %
DownstreamAccreting
66 %
LateralAccreting
32 %
Fine-GrainedFill Assoiated
with Lateral Accretion< 1 %
Erosinally-BasedFine-grained Fill
2 %
Crevasse Channels < 1 %
Splay Beds6 %
Paleosols1 %
Floodplain Fines93 %
Sandstone27 %
Mudstone73 %
Figure 3.1: (A) Thickness and lithofacies percentage of channel-belt stories. (B) Thickness and lithofacies percentage of channel-belt elements. (C) Percentage of various rock types and architectural components in the lower Wasatch Formation in this study area. (n = number of occurrences; x = average thickness; m = mode; f = frequency; T = thickness)
14
0.5 m 0.25 m 0.2 m
0.25 m
0.2 m0.25 m
0.1 m0.4 m0.5 m
0.5 m
F1: Green or Gray Mottled Mudstone
F5: Tabular Cross-Stratifed Sandstone
F9: Trough Cross-Stratifed Conglomerate F10: Chert-Clast Conglomerate
F4: Laminated Sandstone
F8: Horizontally-Strati�ed Conglomerate
F3: Burrowed SandstoneF2: Red Mottled Mudstone
F6: Trough Cross-Strati�ed Sandstone F7: Massive Sandstone
Figure 3.2: Photographic examples of the ten lithofacies of the lower Wasatch Formation in the study area. Descriptions and interpretations of the lithofacies are summarized in Tale 3.1.
Figure 3.3: Schematic chart of the three-level hierarchical classification used to describe the fluvial architecture of the lower Wasatch Formation (Ford, 2012).
15
16
Facies # Facies Name Grain Size Description Interpretation
F1Green or Gray Mottled Mudstone
mud Non-distinct bedding with minor burrows and granular ped structures; rare desiccation cracks; gradational lower contact. Gradational to sharp upper contact.
Post depositional bioturbation resulting in deformation of sedimentary structures.
F2Red Mottled Mudstone mud Non-distinct bedding with minor burrows, glaebules, and granular
ped structures; rare desiccation cracks.; gradational to sharp lower and upper contacts.
Post depositional bioturbation resulting in deformation of sedimentary structures.
F3Burrowed Sandstone very fine- to fine-grained
sandstoneNon-distinct bedding; horizontal and vertical burrows, vertical roots; rare ripple laminations; sharp to erosive lower contact; sharp upper contact.
Post depositional bioturbation resulting in deformation of sedimentary structures.
F4Laminated Sandstone fine- to medium-grained
sandstoneAsymmetric unidirectional ripples; planar to wavy laminations; climbing ripples rare; gradational to sharp lower and upper contacts.
Lower to upper flow-regime, tractive deposition; low to high energy.
F5Tabular Cross-Stratified Sandstone
fine- to medium-grained sandstone
Tabular laminations, typically <25°; gradational to sharp lower and upper contacts.
Lower flow-regime, tractive deposition; high energy.
F6Trough Cross-Stratified Sandstone
fine- to medium-grained sandstone
Trough laminations; undulatory lower contact; gradational to sharp upper contact.
Lower flow-regime, tractive deposition; high energy.
F7Massive Sandstone fine- to medium-grained
sandstoneNA NA
F8
Horizontally-Stratified Conglomerate
granule to pebble clasts in fine- to medium-grained sandstone matrix
Horizontally-stratified mud-clast laminae alternating with sandstone matrix laminations; clasts are commonly imbricated; sharp to erosive lower contact; gradational to sharp upper contact.
Upper flow-regime, tractive deposition; lower flow regime with high sedimentfall out rates; very high energy.
F9Trough Cross-Stratified Conglomerate
granule to pebble clasts in fine- to medium-grained sandstone matrix
Trough cross-stratified mud-clast laminae alternating with sandstone matrix laminations; clasts are commonly imbricated; sharp to erosive lower contact; gradational to sharp upper contact.
Upper flow-regime, tractive deposition; very high energy.
F10Chert-Clast Conglomerate granule to cobble clasts
in fine- to medium-grained sandstone matrix
Non-distinct bedding; chert clasts within sandstone matrix; sharp to erosive lower contact; gradational to sharp upper contact.
Cohesive flow and/or bank collapsesettling.
Table 3.1: Table listing descriptive characteristics and interpretation of the ten lithofacies recognized in the lower Wasatch Formation in this study area. Photographic examples of each lithofacies are documented in Figure 3.2.
17
CHAPTER 4
ARCHITECTURE OF THE LOWER WASATCH FORMATION
4.1 Channel-Belt Architecture
The lower Wasatch Formation, in the study area, channel-belt strata predominantly
contains downstream accreting stories (Figure 3.1 and Figure 4.1). The channel-belt strata
consists of 66% downstream-accreting stories, 32% lateral-accreting stories, 2% erosionally-
based fine-grained fill stories, and less than 1% fine-grained fill associated with lateral accretion
stories (Figure 3.1).
4.1.1 Channel-Belt Elements
The average thickness of multi-story elements containing predominantly downstream-
accreting stories is 11 m (Figure 3.1). The most abundant lithofacies in these elements is
massive sandstone (Facies 7) followed by laminated sandstone (Facies 4) and tabular-cross
stratified sandstone (Facies 5) (Figure 3.1). The average thickness of single-story elements
containing predominately downstream-accreting stories is 8 m (Figure 3.1). The most abundant
lithofacies in these elements is massive sandstone (Facies 7) followed by tabular-cross stratified
sandstone (Facies 5) and laminated sandstone (Facies 4) (Figure 3.1).
The average thickness of multi-story elements containing predominantly lateral-accreting
stories is 13 m (Figure 3.1). The most abundant lithofacies in these elements is massive
sandstone (Facies 7) followed by tabular-cross stratified sandstone (Facies 5) and laminated
sandstone (Facies 4) (Figure 3.1). The average thickness of single-story elements containing
predominantly lateral-accreting stories is 10 m (Figure 3.1). The most abundant lithofacies in
these elements is massive sandstone (Facies 7) followed by laminated sandstone (Facies 4)
and trough-cross stratified sandstone (Facies 6) (Figure 3.1).
18
Channel-belt elements have an asymmetrical bowl shape when observed in depositional
strike view that is thickest in its axis and thins towards its lateral margins. In dip view, the
elements have an elongate wedge or tabular shape. The lower bounding surface and lateral
margins of channel-belt elements are erosional. The upper bounding surface of the elements is
undulatory and conformable, except where younger strata erode into it. Channel-belt elements
evolve from the migration, fill and/or abandonment of a single fluvial channel (Ford, 2012).
4.1.2 Channel-Belt Stories
Downstream-accreting stories are the most common channel-fill component in the lower
Wasatch Formation, in the study area. The most abundant lithofacies in these stories is
massive sandstone (Facies 7) followed by tabular-cross stratified sandstone (Facies 5) and
laminated sandstone (Facies 4) (Figure 3.1). Lenses of mud-clast or chert conglomerates occur
at the base of these stories (Figure 4.1). These stories generally maintain a consistent grain
size in an upward transect (i.e. blocky profile) and have an average thickness of 7 m (Figure
4.1). These stories have an asymmetrical lens shape when observed in depositional strike view
that is thickest and sandiest in its axis and thins towards its lateral margins. In dip view, the
stories have an elongate wedge shape that tapers in the downstream direction. The lower
bounding surface of the stories is convex upward and erosional. The upper bounding surface of
the stories is slightly undulatory and conformable, except where younger strata erode into it.
Sediment-transport direction is parallel to forset migration. Stories predominantly stack
aggradationally (i.e. on top of one another) and/or nonsequentially (i.e. adjacent stories were not
deposited in chronological order; e.g. story 1 can share a contact with story 3) (Figure 4.1). The
stories are interpreted as mid-channel bars with accretion dominantly in the downstream
direction (Ford, 2012).
The most abundant lithofacies in lateral-accreting stories is massive sandstone (Facies
7) followed by tabular-cross stratified sandstone (Facies 5) and laminated sandstone (Facies 4)
19
(Figure 3.1). These stories generally fine upwards and have an average thickness of 7 m
(Figure 4.2). These stories have a sigmoidal shape when observed in depositional strike view
that is thickest in its axis and thins towards its lateral margins. In dip view, the stories have an
elongate wedge shape that tapers in the upstream and downstream directions. The lower
bounding surface of the stories is erosional. The upper bounding surface of the stories is
undulatory and conformable, except where younger strata erode into it. Sediment-transport
direction is perpendicular to forset migration. Bed and bedsets within lateral accreting stories
predominantly stack sequentially (i.e. adjacent stories were deposited in chronological order;
e.g. story 1 shares a contact with story 2 but not story 3; Figure 4.2). The stories are interpreted
as side-attached bars with accretion dominantly in the lateral direction (Ford, 2012).
Erosionally-based, fine-grained fill stories consist of red mottled mudstone (Facies 2)
and burrowed sandstone (Facies 3). The average thickness of these stories is 4 m (Figure 3.1).
These stories have an asymmetrical bowl shape when observed in depositional strike view that
is thickest in its axis and thins towards its lateral margins (Figure 4.1 and 4.2). In dip view, the
stories have an elongate wedge shape that tapers in the downstream direction. The lower
bounding surface of the stories is convex upward and erosional. The upper bounding surface of
the stories is conformable, except where younger strata erode into it. The stories are
associated with downstream accreting and lateral accreting stories. The stories are interpreted
as channel-fill deposits where avulsion of the channel occurred prior to deposition of the
associated channel bars (Ford, 2012).
Fine-grained fill associated with lateral accretion stories consists of red mottled
mudstone (Facies 2). The average thickness of these stories is 3 m (Figure 3.1). These stories
have a bowl shape when observed in depositional strike view that is thickest in its axis and thins
towards its lateral margins (see Supplemental File A-2 for examples). In dip view, the sotires
have an elongate wedge shape that tapers in the downstream direction. The lower bounding
20
surface of the stories is conformable along the margin that is adjacent to a lateral-accretion
story and erosional on the opposite margin. The upper bounding surface of the stories is
conformable. The stories are interpreted as channel fill deposits associated with lateral
migration of a channel bar, informally referred to as mud plugs (Ford, 2012).
4.2 Floodplain-Belt Architecture
The lower Wasatch Formation containing predominantly floodplain strata (79 %) (Figure
3.1). The floodplain strata consists of 93% floodplain fines, 6% crevasse-splay beds, 1%
paleosols, and less than 1% crevasse-channel stories (Figure 3.1).
4.2.1 Floodplain-Belt Elements
Floodplain-belt elements have a wedge shape that thins away from the adjacent
associated channel-belt element. When observed in dip view, the elements have a tabular
shape. The lower bounding surface of the elements is conformable to erosional. The upper
bounding surface of the elements is conformable and undulatory, except where younger strata
erode into it. Floodplain-belt elements are built by floodplain-fill stories created from the
overbank discharge and migration of a single fluvial channel (Ford, 2012).
4.2.2 Floodplain-Belt Stories
The most abundant lithofacies in crevasse-channel stories is burrowed sandstone
(Facies 3). The stories have a symmetrical bowl shape when observed in depositional strike
view that is thickest in its axis and thins towards its lateral margins (Figure 4.3). In dip view, the
stories have a wedge shape that tapers in the downstream direction. The lower bounding
surface of the stories is unconformable. The upper bounding surface of the stories is
conformable and undulatory, except where younger strata erode into it. The stories are
interpreted as crevasse channels (Ford, 2012).
21
The most abundant lithofacies in crevasse-splay stories is burrowed sandstone (Facies
3). The stories have a thin tabular or lobe shape when observed in depositional strike view
(Figure 4.4). In dip view, the stories have a wedge shape that tapers in the downstream
direction. The lower bounding surface of the stories is conformable to erosional. The upper
bounding surface of the stories is conformable and undulatory to planar, except where younger
strata erode into it. The stories are interpreted as crevasse splays (Ford, 2012).
The most abundant lithofacies in the stories are red mottled sandstone (Facies 2) and
gray mottled mudstone (Facies 1). The stories have a planar to wedge shape when observed
in depositional strike and dip view (Figure 4.4). The lower and upper bounding surfaces of the
stories are conformable. The stories are interpreted as floodplain fines deposited during
flooding events (Ford, 2012).
22
23a-D
23b-D
23c-D
23d-D23e-D
23f-D24a-L24b-L24c-L
25-F
Mud or chert clasts
Dessication cracks
Trough cross-strati�cation
Tabular cross-strati�cation
Facies number
Planar laminations
Ripple laminations
Bioturbation
B
C
A
C
C
F6: T
roug
h Cro
ss-S
tratifi
edSa
ndsto
ne
F7: M
assiv
e San
dston
e
F8: H
orizo
ntally
-Stra
tified
Cong
lomer
ate
Dess
icatio
n Cra
cks
20 m
20 m
2 m
50 c
m
10 c
m 50 c
m
50 c
m
Lateral AccretingDownstream Accreting Erosionally-Based Fine-Grained FillElement Boundary Story Boundary Bedding Surface Truncation
0
5
10
15
CMCVc SiF VFSandGR
23-D
23d-D
23e-D
23b-D
23c-D
23a-D
F10F10F7
F7
F7
F7F4
F4
F4F3
F3
F4
F4
F4
F4
F7
F7
F7
F10
F8
F9
F8
F6
F6
F5
F5
F8F10
Figure 4.1: Example of a channel-belt element (23-D) containing predominantly downstream-accreting stories. (A) Uninterpreted photopanel. (B) Line drawing based on photopanel in A (See Supplemental File A-2 for location). (C) Measured section (location shown in A and B) for downstream-accreting story with photographs of lithofacies. See Supplemental File A-2 for explanation of the naming convention.
36a-L
36b-L 36c-L 36d-L
L36-e 36f-L 36g-L
36h-L
36i-L36j-L
36k-L39-F 38-F
37-F
0
5
10
CMCVc SiF VFSandGR
36-L
F10
F5
F3
F7
F5F8F4F8
F5
F8
F7
F4
F4
F7
F4F7F6
F10
36c-L
36b-L
36a-L
mud or chert clasts
Trough cross-strati�cation
Tabular cross-strati�cation
Facies number
Planar laminations
Ripple laminations
Bioturbation
B
C
A
C
C
F4: L
amina
ted S
ands
tone
F10:
Cher
t-Clas
t Con
glome
rate
F5: T
abula
r Cro
ss-S
tratifi
edSa
ndsto
ne
F6: T
roug
h Cro
ss-S
tratifi
edSa
ndsto
neF3
: Bur
rowe
d San
dston
e
F8: H
orizo
ntally
-Stra
tified
Cong
lomer
ate
20 m
20 m
20 m
20 m
20 m
10 c
m
10 c
m 10 c
m
10 c
m
10 c
m 5 cm
Lateral Accreting Erosionally-Based Fine-Grained FillElement Boundary Story Boundary TruncationBedding Surface
Figure 4.2: Example of a channel-belt element (36-L) containing predominantly lateral-accreting stories. (A) Uninterpreted photopanels. (B) Line drawing based on photopanel in A (See Supplemental File A-2 for location). (C) Measured section (location shown in A and B) for laterally accreting story with photographs of lithofacies. See Supplemental File A-2 for explanation of the naming convention.
23
24
B
CA
Story Boundary Bedding Surface
Figure 4.3: Example of a crevasse channel story in this study area. (A) Uninterpreted photopanel. See Supplemental File A-2 for location (below and to the right of element 14-D). (B) Line drawing based on photopanel in A. (C) Measured section (location shown in A and B ) for crevasse channel story.
Crevasse Channel Story
CMCVc SiF VFSandGR
0
5
F2
F4F4
F4
F7
F3
F6
F6F8
Mud or chert clasts
Trough cross-strati�cation
Ripple laminations
Bioturbation
5 m
5 m
Facies numberF8
C
C
25
Mud or chert clasts
Dessication cracks
Bioturbation
Rhizolith
White Horizon
Facies Number
BA C
B
F2: R
hizoli
th
F2: F
loodp
lain F
ines
Splay
Bed
sSp
lay B
eds
0
5
10
15
20
25
30
CMCVc SiF VFSandGR
F3F2
F3
F3
F3F3
F3
F3F3
F3
F3
F3
F3
F3
F3
F3
F3
F3F2
F1
F1
F1
F1
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F3
F3
F3
F3
F3
F3
F3
F2
Figure 4.4: Example of crevasse-splay and floodplain-fine stories in the study area. (A) Photographic examples of crevasse-splay and floodplain-fine stories. (B) Measured section for crevasse-splay and floodplain-fine stories (location shown in A). See Supplemental File A-2 for location (below element 23a-D). (C) Photographic examples of crevasse-splay and floodplain-fine stories and lithofacies.
25 m 1
m1
m
10 c
m
50 c
m
1
2
3 4
1
2
F3
26
CHAPTER 5
CREVASSE SPLAY TYPES
Ford (2012) identifies three types of crevasse-splay deposits based on the physical
relationship between crevasse-splay(s) and adjacent channel-belt element (Figure 5.1, 5.2, and
5.3): (1) associated coeval splays, (2) associated non-coeval splays, and (3) unassociated
splays. These same types of crevasse-splay deposits are identified in this study area. A
description of each is included below.
Associated coeval splays are located laterally adjacent to and are physically connected
to a channel-belt element, indicating that the crevasse splay was deposited coeval with the
channel fill (Figure 5.1) (Ford, 2012). The widths of these crevasse splays are generally wider
than that of the associated channel-belt. Associated non-coeval splay units are comprised of a
succession of crevasse-splay beds that underlie the channel-belt element (Figure 5.2). The
axis, or thickest part of the crevasse-splay unit, is located near the axis of the overlying channel-
belt. Within an associated non-coeval splay unit, crevasse-splay beds generally thicken from
one to the next in an upward succession and are commonly separated by paleosols, indicating
that the crevasse splay unit was deposited over multiple flooding events. The width of
associated non-coeval splay units exceeds the width of the overlying channel-belt element.
Associated non-coeval splay units are interpreted to be genetically related to the overlying
channel-belt element, and therefore are a record of a successful avulsion (Ford, 2012). Smith
et al. (1998), Kraus and Wells (1999), and Jones and Hajeck (2007) refer to these deposits as
avulsion complexes, heterolithic avulsion deposits, and stratigraphically transitional deposits,
respectively. Unassociated splay units are comprised of a succession of crevasse-splay beds
that are spatially isolated from channel-belt elements (Figure 5). Within an unassociated splay
unit, crevasse-splay beds generally thin from one to the next in an upward transect and are
27
commonly separated by paleosols. Unassociated splays are interpreted to represent a failed
avulsion (sensu Strouthamer, 2001).
28
a a’
A
B
Figure 5.1: (A) Uninterpreted and interpreted photographic example of associated coeval splay from the field area. The example is from the right margin of element 14-D (see Supplemental File A-2 for location). (B) Schematic diagram of associated coeval splay. Units are not drawn to scale.
10 m
10 m
Cross-Section
Splay
Channel bars
Channel �ll
SplayChannel bars14-D
Plan View
Splay
Channel barsChannel �ll
Floodplain �nes
a
a’
Floodplain �nes
29
Splays
Channel bars Channel �llb b’
Time 2
1 23
C
A
B
Figure 5.2: (A) Photographic examples of associated non-coeval splays from the field area. Red line indicates division between channel-belt strata (above) and floodplain strata (below). (B) Simplified Supplemental File A-2 with location of each photographic example (A 1-3). (C) Sche-matic diagram of associated non-coeval splays. Units are not drawn to scale.
1
10 m
2
10 m
3
15 m
Plan View
Cross-Section
Channel
Splays
Splays
Splays
Channel
Channel
7-L
14-D
16-D 15-D
Floodplain �nesPlan View
Splay
Channel barsChannel �ll
b
b’
Time 1
Floodplain �nes
30
A
Figure 5.3: (A) Photographic examples of unassociated splays from the field area. (B) Simplified Supplemental File A-2 with location of each photographic example (A 1-4). (C) Schematic diagram of unassociated splays. Units are not drawn to scale.
2
50 m
4
25 m
3
25 m1
25 m
C
c c’
4
32
1
Cross-Section
Splay
B
Floodplain �nes
Splays
SplaysSplays
Splays
Time 2
Plan ViewPlan View
Splay
Channel barsChannel �ll
Floodplain �nes
c
c’
Time 1
c
c’
31
CHAPTER 6
DISCUSSION
6.1 Spatially Varying Characteristics of Associated Non-Coeval Splays (i.e. Avulsion Complexes)
Associated non-coeval splay units (i.e. avulsion complexes, heterolithic avulsion
deposits, and stratigraphically transitional deposits) are interpreted to be the record of a
successful avulsion (Ford, 2012). Three distinct types of associated non-coeval splays are
identified in this study area based on physical, observable characteristics (Figures 6.1, 6.2, and
6.3): type I, type II, and type III.
The thickness of type I splay units scale to the thickness of the overlying channel, and
range from 10 to 20 m (Figure 6.1). The thickness of individual sandstone beds range from 0.1
to 3 m. The number of sandstone beds in these units ranges from 10 to 20 beds. Type I splay
units have a net-sand content greater than 0.7, and grain size ranges from mud to fine-grained
sand. Erosion surfaces are abundant. The amount of erosion scales to the thickness of splay
beds, and range from less than 0.1 to 3 m.
The thickness of type II splay units are less than the thickness of the overlying channel,
and range from 5 to 10 m (Figure 6.2). The thickness of individual sandstone beds ranges from
0.1 to 1 m. The number of sandstone beds in these splay units ranges from 5 to 15 beds. Type
II splay units have a net-sand content between 0.4 and 0.6, and grain size range from mud to
very fine-grained sand. Erosion is not evident.
The thickness of type III splay units are less than the thickness of the overlying channel,
and range from 1 to 3 m (Figure 6.3). The thickness of individual sandstone beds within these
units is less than 0.5 m. The number of sandstone beds in these units ranges from 1 to 3 beds.
32
Type III splay units have a net-sand content less than 0.15, and grain size range from mud to
very fine-grained sand. Erosion is not evident.
Five systematic differences are noted between type I, type II, and type III splays. First,
the thickness of the splay units and individual sandstone beds within type I splays are thicker
than those of type II, which are thicker than those of type III splays. Second, there is a broad
decrease in grain size from type I, to type II, to type III splays. Third, type I splays have a
greater net-sand content than type II, which have a greater net-sand content than type III
splays. Fourth, there is more erosion in type I splays than in type II and type III splays. Fifth,
the number of sandstone beds decreases from type I, to type II, to type III. Following this trend
a fourth type of associated non-coeval splay type could exist, one in which no sandstone beds
underlie the channel and the unit consists only of floodplain-fine deposits. However, this type IV
splay is not exposed in this study area.
Experimental studies and studies of modern floodplains document important spatial
trends in floodplain deposits. First, experimental and studies of modern floodplain document
that flow is fastest in the main channel, and flow velocity and shear stress diminish with
increased distance from the source channel (Sellin, 1964; Ghosh and Kar, 1975; Knight and
Shino, 1990; Shino and Knight, 1991; Willetts and Hardwick, 1993; Naish and Sellin, 1996;
Willetts and Rameshwaran, 1996; Wormleaton, 1996; Nicholas and McLelland, 1999; Patra and
Kar, 2000; and Knight and Brown, 2001). As a result, sediment transport capacity decreases
and grains are deposited in order of decreasing size away from the main channel (Stoke, 1851;
Rouse, 1950). Second, O’Brien and Wells’ (1986) study on the Clarence and Timbarra River
system documents gradual thinning of crevasse splays with increased distance from the source
channel. Finally, Allen (1970) proposes a concept in which erosion at the base of crevasse-
splay deposits is focused to areas near the source channel.
33
The concepts listed above are combined with the general characteristics observed in
type I, type II, and type III splays to develop a conceptual model that describes longitudinal
changes in the stratigraphic characteristics of crevasse-splay deposits. In this model, type I,
type II, and type III splay units are interpreted to represent proximal, medial, and distal positions
in crevasse-splay deposits relative to the source channel, respectively (Figure 6.4). Type IV
units represent the most distal deposits and are located beyond the avulsion complex (sensu
Smith et al. 1998). Figure 6.4 describes how stratal characteristics change spatially, with
decreases in the following characteristics away from the source channel: (1) thickness of splay
unit, (2) thickness and abundance of splay beds, (3) net-sand content, (4) grain size, and (5)
erosion.
6.2 Upward Trends: Relationship between Floodplain and Channel-Belt Elements
Incisional avulsion (sensu Mohrig et al., 2000) consists of channel-belt strata that lack
underlying crevasse-splay deposits. In contrast, aggradational avulsion (sensu Mohrig et al.,
2000) implies a genetic relationship between channel-belt elements and the underlying
crevasse-splay deposits via avulsion processes. In an effort to document the avulsion style of
the lower Wasatch Formation, upward profiles are evaluated to identify correlations between
channel-belt strata and underlying crevasse-splay deposits.
The upward profiles show two intervals where channel-belt elements increase in
abundance upward (Figure 6.5 A). Each interval is underlain by intervals where crevasse-splay
beds increase in abundance, indicating an association between the abundance of channel-belt
elements and abundance of the underlying crevasse-splay beds (Figure 6.5 A). This
association is interpreted to indicate that the succession resulted from predominantly
aggradational avulsion (sensu Mohrig et al., 2000).
34
The relationship is strongest with channel-belt elements containing predominantly
downstream-accreting stories as opposed to those containing predominantly lateral-accreting
stories (Figure 6.5 A). The lack of crevasse-splay deposits beneath lateral-accreting deposits
is interpreted in two ways. First, that the floodplain strata underlying these channels are
stratigraphically abrupt (sensu Jones and Hajek, 2007), indicating that the channel-belt
elements containing predominantly lateral-accreting stories are associated with incisional
avulsion (sensu Mohrig et al., 2000). Second, floodplain-fine deposits underlying channel-belt
elements containing predominantly lateral-accreting stories are the most distal expressions of
associated non-coeval splays (i.e. Type IV). In this case the stratigraphically abrupt splays
(sensu Jones and Hajek, 2007) are simply the distal expression of stratigraphically transitional
splays (sensu Jones and Hajek, 2007). In other words, the cross-section of the exposed
channel is located beyond the limits of the sand-rich splays that are proximal to the source
channel. This interpretation implies that channel-belt elements containing predominantly
downstream-accreting stories are associated with aggradational avulsion longitudinally transfer
to channel-belt elements containing predominantly lateral-accreting stories. A regional study is
needed to test this interpretation.
35
Figure 6.1: (A) Measured section of type I splays from study area (location shown in C). (B) Photograph of type I splays (location shown in C). (C) Simplified Supplemental File A2 with location of A and B. (D) Schematic cross-section of type I splays.
A B
D
B
A
Mud or chert clasts
Dessication cracks
Bioturbation
White Horizon
Green Horizon
Facies Number
0
5
10
15
CMCVc SiF VFSandGR
29-D
20
spla
y un
it
3 m
F3
F3
F1
F2
F3
F3
F2
F2
F1
F3
F3F1
F2F3
F1F2
F3
C
Channel Channel
Spla
y un
it
Splays
Channel bars Channel �ll
Cross-Section Floodplain �nes
36
Figure 6.2: (A) Photograph of type II splays (location shown in C). (B) Measured section of type II splays from study area (location shown in C). (C) Simplified Supplemental File A2 with location of A and B. (D) Schematic cross-section of type II splays.
B
Tabular cross-strati�cation
Floodplain �nes
A
D
0
5
10
CMCVc SiF VFSandGR
23-D
spla
y un
it
10 m
F3
F3
F5F3
F3
F3
F3
F3
F1
F3F2
F3
F2
Dessication cracks
Bioturbation
Facies NumberF5
AD
C
Splay
Channel bars Channel �ll
Cross-Section
Channel
Splay unit
7-L
37
Figure 6.3: (A) Photograph of type III splays (location shown in C). (B) Measured section of type III splays from study area (location shown in C). (C) Simplified Supplemental File A2 with location of A and B. (D) Schematic cross-section of type III splays.
Dessication cracks
Bioturbation
A
B
D
0
5
CMCVc SiF VFSandGR
35-D
spla
y un
it AB
White Horizon
Facies Number
F2
F3
F2
F8
F2
F1
F2
F3
F8
C
Splays
Channel bars
Cross-Section
Channel
Floodplain �nes
Channel
Floodplain �nes
5 m
10 m
35-D
35-D
Floodplain �nes
Channel �ll
38
Proximal Splay (Type I)
Medial Splay (Type II)
Decr
easin
g: (1
) thic
knes
s of s
play u
nit, (
2) th
ickne
ss a
nd a
bund
ance
of
splay
bed
s, (3
) net
-san
d-co
nten
t, (4
) gra
in-siz
e, a
nd (5
) ero
sion.
Incre
asing
dist
ance
from
the s
ource
chan
nel.
d d’
e e’
f f’
Figure 6.4: Schematic diagram of spatially varying characteristics of associated non-coeval splays.
Distal Splay (Type III)
Beyond Avulsion Complex (Type IV)g g’
Plan
Vie
wCr
oss-
Sect
ion
Time 2
Splay
Channel barsChannel �ll
Floodplain �nes
Time 1
d
f’
d’
e’e f
gg’
Floodplain-�nes
0
40
80
120
160
200
220
0
40
80
120
160
200
220
0
40
80
120
160
200
220Channel-belt to Floodplain Downstream Accreting Storiesto Lateral Accreting Stories
DownstreamChannel-BeltElement
Floodplain-BeltElementLateral Floodplain-
Fines
0 15105
ChannelThickness
Figure 6.5 Upward trends of architectural components in the lower Wasatch Formation. Data points represent the vertical average over a 40 m thick by 600 m wide interval.
1006040200 801006040200 801006040200 801006040200 80
Splay Beds to Crevasse ChannelStories to Floodplain-Fine
StoriesNet Sand Content
Channel-belt
Floodplain
Channel Thickness (m)
Sandstone %
0
40
80
120
160
200
220
Downstream AccretingStories
Lateral AccretingStories
Floodplain-Fine Stories
Splay beds
Splay Beds
0 100604020 80 100604020 801006040200 801006040200 80
Vert
ical
Pos
itio
n (m
)Ve
rtic
al P
osit
ion
(m)
Channel-Belt Floodplain- Belt
50
0
100
150
200
Crevasse Channel Stories
1006040200 80 1006040200 80
intervals ofincreasing channel-belt
intervals of increasing downstreamchannel-belt intervals of increasing
lateral channel-beltintervals of increasing
splay beds
39
A
B
Percent (%) Percent (%) Percent (%) Percent (%) Percent (%) Percent (%)
Percent (%) Percent (%) Percent (%) Percent (%)
40
CHAPTER 7
APPLICATIONS
This thesis proposes a model that describes longitudinal changes in splay deposits. Type I, II,
and II splays can be differentiated in well logs and core by their relative high net-sand content,
sandstone bed thickness, and grain size (Figure 7.1). There is a high potential for connectivity
between adjacent sandstone beds in type I splays due to amalgamation and erosion between
the sandstone beds (Figure 6.1 and 7.1). Whereas in type II and III splays, the sandstone beds
are more likely to be isolated from one another by mud (Figure 6.2, 6.3, and 7.1). Additionally,
connectivity of associated non-coeval splays with overlying channel-belt strata may enhance
reservoirs where hydrocarbons are present. However, it is important to note that although the
sandstone beds within type I, II, and II splays are laterally extensive, desiccation cracks are
abundant. Baffles or barriers may exist within these splay beds if the desiccation cracks are
filled with fine-grained sediment. Finally, the spatially varying characteristics and architecture
documented in this thesis can be incorporated into reservoir models and considered when
choosing exploration and development wells.
This thesis documents that in the lower Wasatch Formation, channel-belt elements
containing predominantly downstream-accreting stories are associated with aggradational
avulsion (sensu Mohrig et al., 2000) based on the presence of associated non-coeval splays
(Figure 6.5). Whereas channel-belt elements containing predominantly lateral-accreting stories
are associated with incisional avulsion (sensu Mohrig et al., 2000) or they are the distal
expression of aggradational avulsion based on the lack of associated non-coeval splays (Figure
6.5). This relationship between channel story type (i.e. downstream or lateral accreting) and
splays has implications for predictability of stratigraphic architecture of floodplain strata
underlying channel-belt strata. In areas where subsurface data is limited regarding floodplain
strata (e.g. low-resolution seismic or well log and/or core data are limited to the large reservoir
41
sandstone bodies) but channel story type is known (e.g. identify distinct upward patterns in grain
size changes in downstream- and laterally-accreting stories in well log data), the architecture of
the floodplain strata can be appropriately modeled based on the relationship between channel
story type and splay type. However, identifying channel story type in the subsurface can be
difficult (Keeton, 2012).
Finally, the outcrop of the lower Wasatch Formation, documented in this study, has a
net-sand content of 0.27 and contains predominantly floodplain strata (79%) making it an
excellent analog for low net-sand content or floodplain-dominated fluvial successions. This
thesis provides quantitative data (i.e. story and element proportions, thicknesses, and facies
proportions; Figure 3.1) of architectural components that can be directly applied to reservoirs in
lower Wasatch Formation in the Uinta Basin (Fouch et al. 1994b). Additionally, the lower
Wasatch Formation is a good analog to other low net-sand content fluvial successions including
the lower Williams Fork Formation in the Piceance Basin (Pranter and Sommer, 2011) and the
Mungaroo Formation of the northwest shelf of Australia (Stoner, 2010).
42
Figure 7.1: Diagrammatic example of pseudo gamma ray signature of associated non-coeval splay types.
Increasing splay: (1) net sand content, (2) sandstone bed thickness, (3) grain-size, (4) connectivity between sandstone beds
Type I Type II Type III Type IV (no splays)
Associated Non-Coeval Splayspseudo gamma ray pseudo gamma ray pseudo gamma ray pseudo gamma ray
43
CHAPTER 8
FUTURE WORK
Recommendations for future work include:
1. A regional study in the lower Wasatch Formation and/or other low net-sand content
fluvial successions such as the lower Williams Fork that document vertical trends in
channel-belt and floodplain strata. This documentation can be used to determine if
the correlation between downstream-accreting stories and aggradational avulsion
style is limited to the lower Wasatch Formation documented in this study or if it can
be applied to the entire lower Wasatch Formation and other low net-sand content
fluvial successions.
2. A regional study in the lower Wasatch Formation that maps associated non-coeval
splays and the source channel along a longitudinal profile. This documentation
would test the spatial variability of splay deposits and could also provide additional
information such as the scale and transition between type I, II, and III splays.
3. A regional study to evaluate white paleosol horizons (facies 1) and their association
with channel-belt strata (Supplemental File A-2) to test if the abundance and location
of horizons relate to the location of channel-belt strata and more specifically channel
story type (i.e. downstream versus lateral accreting).
44
CHAPTER 9
CONCLUSIONS
Documentation of well-exposed outcrop from the lower Wasatch Formation in the Uinta
Basin, Utah was used to determine the following:
1. Three types of crevasse splays are recognized in this field area based on their
physical relationship to adjacent channel-belt strata. Associated coeval splays are
laterally adjacent and are physically connected to a channel-belt element, indicating
that the crevasse splay was deposited coeval with the channel fill. Unassociated
splays are spatially isolated from channel-belt elements and are interpreted to
represent a failed avulsion. Associated non-coeval splays underlie the channel-belt
element and are interpreted to be genetically related to the overlying channel-belt
element, and therefore are a record of a successful avulsion.
2. Associated non-coeval splays, interpreted to be the stratigraphic record of
aggradational avulsion processes, have physical, observable characteristics that
vary spatially along a proximal-to-distal transect relative to the source channel.
Decreases in the following characteristics of splays occur with increased distance
from the source channel: (1) thickness of splay unit, (2) thickness and abundance of
splay beds, (3) net-sand content, (4) grain size, and (5) erosion.
3. Vertical trends in floodplain and channel-belt strata can be used to: (1) determine
whether the dominant avulsion style is aggradational or incisional, and (2) relate
channel story type (i.e. downstream versus lateral accreting) to avulsion style.
4. The occurrence of splay beds below channel-belt strata, documented in the vertical
profile, is interpreted to indicate that the lower Wasatch Formation resulted from
predominantly aggradational avulsion processes.
45
5. The occurrence of splay beds below channel-belt elements containing predominantly
downstream-accreting stories is interpreted to indicate that these channels resulted
from predominantly aggradational avulsion processes.
6. The lack of splay beds below channel-belt elements containing predominantly by
lateral-accreting stories is interpreted to indicate that these channels resulted from
predominantly incisional avulsion processes.
46
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52
SUPPLEMENTAL FILES
The supplemental files include oversized figures that could not be included in the main
thesis and include: photopanels, maps, measured sections, and data tables. The supplemental
files are in the order in which they were referred to in the thesis.
A-1: Measured Sections 48 stratigraphic columns totaling 1,040 m that record grain size and physical and biogenic sedimentary structures at a centimeter-scale resolution
1.1_Reference Photopan.pdf Reference photopan indicating location of measured sections and legend for measured sections
1.2_MS Legend.pdf Legend for measured sections 1.3_MS-1.pdf Measured section 1 1.4_MS-2.pdf Measured section 2 1.5_MS-3.pdf Measured section 3 1.6_MS-4.pdf Measured section 4 1.7_MS-5.pdf Measured section 5 1.8_MS-6.pdf Measured section 6 1.9_MS-7.pdf Measured section 7
1.10_MS-8.pdf Measured section 8 1.11_MS-9.pdf Measured section 9
1.12_MS-10.pdf Measured section 10 1.13_MS-11.pdf Measured section 11 1.14_MS-12.pdf Measured section 12 1.15_MS-13.pdf Measured section 13 1.16_MS-14.pdf Measured section 14 1.17_MS-15.pdf Measured section 15 1.18_MS-16.pdf Measured section 16 1.19_MS-17.pdf Measured section 17 1.20_MS-18.pdf Measured section 18 1.21_MS-19.pdf Measured section 19 1.22_MS-20.pdf Measured section 20 1.23_MS-21.pdf Measured section 21 1.24_MS-22.pdf Measured section 22 1.25_MS-23.pdf Measured section 23 1.26_MS-24.pdf Measured section 24 1.27_MS-25.pdf Measured section 25 1.28_MS-26.pdf Measured section 26
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1.29_MS-27.pdf Measured section 27 1.30_MS-28.pdf Measured section 28 1.31_MS-29.pdf Measured section 29 1.32_MS-30.pdf Measured section 30 1.33_MS-31.pdf Measured section 31 1.34_MS-32.pdf Measured section 32 1.35_MS-33.pdf Measured section 33 1.36_MS-34.pdf Measured section 34 1.37_MS-35.pdf Measured section 35 1.38_MS-36.pdf Measured section 36 1.39_MS-37.pdf Measured section 37 1.40_MS-38.pdf Measured section 38 1.41_MS-39.pdf Measured section 39 1.42_MS-40.pdf Measured section 40 1.43_MS-41.pdf Measured section 41 1.44_MS-42.pdf Measured section 42 1.45_MS-43.pdf Measured section 43 1.46_MS-44.pdf Measured section 44 1.47_MS-45.pdf Measured section 45 1.48_MS-46.pdf Measured section 46 1.49_MS-47.pdf Measured section 47 1.50_MS-48.pdf Measured section 48
A-2: Interpreted Architecture of lower Wasatch Formation
Interpreted photopanel of the main field area
2.1_Main_Photopan.pdf A-3: Data Tables
Includes (1) Raw data associated with each architectural story and element: thickness, associated measured sections; geographic coordinates, paleocurrent information, width, facies (2) Thickness data summarized for each story and element type (3) Facies proportions of each story and element type (4) vertical trends through succession. Sand Body B5 (14-D)
3.1_Data_Tables.xlsx