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ABSTRACT
This study documents the effects of changinglake level (limnostasy), tectonics, sediment yield,and basin physiography on the facies architecture,sequence stratigraphy, and reservoir quality ofthree fine-grained deltas deposited along the east-ern margin of late Pleistocene Lake Bonneville.Analysis of facies architecture indicates that theWeber and Spanish Fork deltas were strongly wave-modified because they were situated along openlyexposed portions of the shoreline. The Bear Riverdelta, nestled in a relatively isolated northeast armof the lake, records both fluvial and wave process-es. These fine-grained deltas were fed by low-gradi-ent rivers that drained large regions that weresparsely glaciated, whereas other contemporane-ous, coarse-grained “Gilbert” deltas were fed bysteep-gradient rivers that drained local source areasthat were strongly influenced by glaciers.
Limnostasy, tectonics, and sediment yield weresimilar for all three fine-grained deltas, implyingthat the most inf luential forcing parameter onsequence stratigraphy is basin physiography. Basinphysiography (specifically ramp length and accom-modation) most strongly controlled the externaland internal geometry of the lowstand systemstract of each delta.
Potential hydrocarbon reservoir quality (e.g.,grain size and sorting) is largely a product of
635AAPG Bulletin, V. 83, No. 4 (April 1999), P. 635–665.
©Copyright 1999. The American Association of Petroleum Geologists. Allrights reserved.
1Manuscript received October 28, 1996; revised manuscript receivedDecember 11, 1997; final acceptance July 27, 1998.
2Exxon Exploration Company, P.O. Box 4478, Houston, Texas 77210-4478; e-mail: drlemons@juno.com
3Department of Geology and Geophysics, University of Utah, Salt LakeCity, Utah 84112.
This project was funded by NSF grant EAR 9303519. A research grantfrom the Geological Society of America supported radiocarbon dating. Wethank Mark Milligan, Kerry Barker, and Matt Rees for field assistance.Constructive criticisms by W. B. Harris, K. Kelts, R. G. Stanley, and KevinBohacs much improved the original manuscript.
Facies Architecture and Sequence Stratigraphy of Fine-Grained Lacustrine Deltas Along the Eastern Margin of Late Pleistocene Lake Bonneville,Northern Utah and Southern Idaho1
David R. Lemons2 and Marjorie A. Chan3
drainage basin size and stream gradient. Relativedifferences in ramp lengths seemed to determinethickness and lateral continuity of delta frontdeposits. Relative differences in accommodationappeared to determine the internal geometry ofthe delta front deposits, especially in the low-stand systems tract. These deltas can serve asanalogs for lacustrine exploration and productionwhere many forcing parameters typically areunknown.
INTRODUCTION
Lake Bonneville (Figure 1) was the largest of the94 Pleistocene lakes that occupied the Basin andRange physiographic province of the westernUnited States (Williams and Bedinger, 1984;Grayson, 1993). The deposits left by this lake offerthe opportunity to examine the dynamics of rapidlake level change, tectonics, physiography, and sed-iment flux as expressed in the stratigraphy of theWeber, Bear, and Spanish Fork deltas. The purposeof this paper is to evaluate the stratigraphy of thesefine-grained deltas in the context of facies architec-ture and nonmarine sequence stratigraphy. Ourdiscussion focuses on description of lithofacies,architectural elements, and depositional styles,especially as they relate to hydrocarbon reservoirsize, shape, and quality.
The cornerstone of previous work on LakeBonneville is G. K. Gilbert’s (1890) classic work,which includes his description of coarse-grained or“Gilbert” deltas, as well as his earlier associatedpaper (Gilbert, 1885) on topographic features oflake shores. Following these works was a 50 yrperiod of research on Lake Bonneville related land-form evolution and glacial geomorphology (e.g.,Atwood, 1909; Blackwelder, 1931; Pack, 1939).Subsequent works on Lake Bonneville focused ondetailed geologic mapping and stratigraphic studies(e.g., Hunt et al., 1953; Williams, 1962; Morrison,
1965a, b, c). More recent studies documented LakeBonneville stratigraphy, chronology, climate histo-ry, paleolimnology, paleohydrology, and geomor-phology (e.g., McCoy, 1981; Scott et al., 1983;Currey and Oviatt, 1985). Previous studies of thefine-grained deltas (Feth, 1955; Bissell, 1963;Bright, 1963; Feth et al., 1966) were made prior tothe advent of process-response sedimentary mod-els, facies architecture, and sequence stratigraphy;therefore, these studies did not place thesedeposits into a comprehensive depositional frame-work in the context of lake level. The well-con-strained age, limnostatic (lake level change) con-trol, basin physiography, tectonics, climate, andsediment yield of these Bonneville deposits providea model for hydrocarbon reservoir properties oflacustrine subsurface deposits where many of thesecontrolling parameters are typically unknown orpoorly understood.
GEOLOGIC SETTING
Lake Bonneville
Late Pleistocene Lake Bonneville occupied atopographically closed basin at the eastern marginof the Great Basin in the Basin and Range physio-graphic province. Both fine- and coarse-graineddeltas developed along the lake’s margin (Milliganand Lemons, 1998) (Figures 1, 2).
Lake Bonneville deposits are packaged into theBonneville alloformation. The Fielding geosol
(∼40–25 ka), where present, separates Bonnevilledeposits from the underlying lacustral and intrala-custral sediments of the Cutler Dam alloformation(Oviatt et al., 1987). The Weber and Spanish Forkdeltas were deposited on previous lake depositsand the Bear River delta was deposited, at leastpartially, on the Tertiary Salt Lake Group. Depth tobedrock is up to 1500 m deep in the Salt LakeValley (Radkins, 1990). Unconsolidated sedimentsare up to 2100 m thick near Preston, Idaho(Stanley, 1972).
Wasatch and East Cache Fault Zones
The Weber and Spanish Fork deltas are situat-ed along the Wasatch fault zone, which formsthe western structural margin of the WasatchMountains (Figure 2). The Wasatch fault zone isa long (∼340 km), active, normal fault compris-ing ten seismical ly independent segments(Machette et al., 1991). Uplift along the Wasatchfault zone began between 17 and 12 Ma andlocally has produced at least 3–11 km of verticaloffset (Zoback, 1983; Parry and Bruhn, 1987).The displacement rate averaged between 0.4 and0.67 mm/yr for this time period (Naeser et al.,1983; Parry and Bruhn, 1986); however, morerecent slip rates along the Wasatch fault zone arehigher (M. Machette, 1996, personal communi-cation). The Bear River delta lies along the west-ern margin of the Bear River Range, which isbounded by high-angle normal faulting along theEast Cache fault zone (Figure 2). This fault zonehas been inactive since the late Pleistocene(McCalpin, 1994).
Tectonically caused topographic relief betweenthe Bonneville basin and the Wasatch and BearRiver ranges, as well the Uinta Mountains (wherethe Weber and Bear rivers originate), controllederosional relief and river gradient. These factorsplayed a significant role in determining sedimentgrain size for deltas deposited along the easternmargin of Lake Bonneville. The lower gradients ofthe Weber, Bear, and Spanish Fork rivers resulted infine-grained deltas (Figure 3; Table 1). This con-trasts with locally sourced rivers, such as theAmerican Fork, Big Cottonwood, and Box ElderCreek (Figure 2), which have higher gradients andsupplied the coarse-grained material formingGilbert deltas (Milligan, 1995).
Glaciation and Paleoclimate
During the existence of Lake Bonneville, boththe Weber and Bear River drainages underwentglaciation of their uppermost reaches in the Uinta
636 Lake Bonneville Deltas
Figure 1—Location of late Pleistocene Lake Bonneville atits highest (Bonneville) shoreline (Oviatt et al., 1992)and study area containing the three deltas.
0 300
km
N
IDAHO
WYOMING
UTAH
COLORADO
NEVADA
Lake Bonneville
Great SaltLake
ARIZONA
2
1
3
1. Weber River delta2. Bear River delta3. Spanish Fork delta
Mountains during Pinedale glaciation (Atwood,1909; Richmond, 1965). In addition, the BearRiver drainage underwent glaciation in the upperportions of the Bear River Range in northern Utahand southern Idaho (DeGraff, 1976), as well asminor glaciation in southeastern Wyoming.Areally, less than 5% of either drainage basinunderwent glaciation during Pinedale deposition.Limited study on glaciation in Spanish Fork Canyonsuggests little glaciation occurred during Pinedaledeposition (Atwood, 1909; Rawson, 1957).Compared with other drainages that sourcedcoarse-grained Gilbert deltas (e.g., American Forkand Big Cottonwood deltas, whose drainage areaswere glaciated up to approximately 25%), a rela-tively minor amount of coarse-grained glacial debrisexisted in the Weber, Bear, and Spanish Forkdrainages (Milligan, 1995). The relative lack ofglaciation and subsequent coarse-grained glacialdebris is partially responsible for the fine-grainednature of the Weber, Bear, and Spanish Fork deltas.
Climate, especially precipitation, may have anoverriding effect on sediment yield rates in moun-tainous terrain (Hicks et al., 1990). Late Pleistoceneprecipitation rates for the approximate highstandperiod of Lake Bonneville were estimated based onsediment yields for the fine-grained Weber Riverdelta and the coarse-grained American Fork delta (aGilbert delta located about 40 km south of Salt LakeCity). Both deltas’ sediment yields indicate maxi-mum paleoprecipitation rates as much as 33% high-er than present-day rates in the Wasatch and Uintamountains (Lemons et al., 1996). Basin-floor meanannual paleotemperatures for the same time periodare estimated at about 13°C cooler than present(McCoy, 1981; Mears, 1981; Lemons et al., 1996).
FACIES ARCHITECTURE
Facies architecture is based on a hierarchy ofinternal bounding surfaces separating depositional
Lemons and Chan 637
Figure 2—Index mapshowing study localities of fine-grained and coarse-grained deltasexposed along the WasatchMountains and Bear RiverRange (modified fromMachette, 1988).
IDAHO
UTAHLAKE
AREAOF
FIGURE
GREATSALTLAKE
Salt LakeCity
Ogden
0 10 20 30 40 50
km
N
BEARLAKE
IDAHO
UTAH
WYOMING
AREA OFFIGURE
WYOMING
UTAH
2
UINTAMTNS.
WASATCHMOUNTAINS
1
mountains
water
normal fault (ball ondownthrown side)
3
41o
40o
42o
112o
BEARRIVER
RANGE
UTAHLAKE
112O
WASATCHMOUNTAINS
111O
Provo
1
Wasatch Fault Zone
Bear River
Ogden River
Weber River
JordanRiver Provo
River
Spanish ForkRiver
1. Weber River delta
3. Spanish Fork delta
2. Bear River delta
AmericanFork
Provo
Big CottonwoodCreek
Box ElderCreek
PRESTON
Wasatch faultzone East Cache
fault zone
modern river drainages
coarse-grained deltas
units of different temporal and spatial scales (e.g.,small-scale ripples deposited in minutes or hours toentire depositional systems deposited in hundredsor thousands of years or more). These boundingunits separate lithofacies and assemblages of litho-facies (architectural elements) that represent a par-ticular process or suite of processes occurringwithin a depositional system (Campbell, 1967;Miall, 1988a, b, 1990, 1991, 1994; Walker, 1990).
We used facies architecture concepts to system-atically describe and interpret the lithostratigraphyof the Weber, Bear, and Spanish Fork deltas. In thisstudy, a modified version of the bounding surfacehierarchy proposed by Miall (1988a, b) is used(Figure 4; Table 2). This hierarchy comprises sixscales of bounding surfaces, with each progressivebounding surface (in descending order) encom-passing a spatially and temporally larger deposit.We determined the facies architecture through (1)measured sections, (2) photomosaic overlays, and(3) interpreting these data to form a systematicseries of lithofacies and architectural elements usedto describe the entire delta.
Architectural Element Description andInterpretation
Thirteen, seven, and eight lithofacies are identi-fied in the Weber, Bear, and Spanish Fork deltas,
respectively, and five, three, and four architecturalelements are similarly recognized (Tables 3–8;Figures 5–7). The architectural elements consist ofassemblages of the various lithofacies. Horizontaland vertical scales of the architectural elementsrange from meters to kilometers. The horizontaland vertical extents of individual lithofacies withinthese elements occur on a smaller scale of centime-ters to meters.
Delta-front sheet sand, lacustrine clay, and beachgravel architectural elements are volumetricallygreatest in all three deltas. In addition, delta-marginand f luvial channel elements are present in theWeber River delta, and fluvial gravel and loess ele-ments exist in the Spanish Fork delta.
SEQUENCE STRATIGRAPHY
The concepts and principles of sequence stratig-raphy developed in the marine realm (e.g., Vail,1987; Posamentier and Vail, 1988; Van Wagoner etal., 1988, 1990; Posamentier et al., 1992) can beapplied in a straightforward manner to the lacus-trine delta exposures of the Weber, Bear andSpanish Fork deltas. The use of sequence stratigra-phy rather than lithostratigraphy to correlate genet-ic packages of sediments may be more appropriatein lacustrine basins, where similar lithologies maybe produced by several lake cycles (Oviatt et al.,1994). Interpretations of sequence stratigraphy inthis study are enhanced by the well-establishedhydrograph (Figure 8) with which to relate the sed-imentary packages.
All three deltas exhibit systems tracts ofmarkedly different character (especially the low-stand), as well as unique assemblages of lithofa-cies and architectural elements; however, thereare features common to all three deltas. (1) Strike
638 Lake Bonneville Deltas
Table 1. Drainage Basin Characteristics of LakeBonneville Deltas
Size atBonneville
Delta ShorelineRiver System Type Gradient (km2)
Weber River Fine grained 0.01* 3328**Bear River Fine grained 0.003–0.004* 11,225**Spanish Fork Fine grained 0.02* 886**American Fork Coarse grained 0.06† 160†
Big Cottonwood Coarse grained 0.06† 135†
Box Elder Creek Coarse grained 0.09† 98†
(Brigham City)
*Computed from U.S. Geological Survey 30 × 60° quadrangle maps(scale: 1:100,000).
**Linearly interpolated between gauging stations (ReMillard et al., 1993).†From Milligan (1995).
10
100
1000
104
105
fine-grained deltas
coarse-grained deltas
Gradient
0.001 0.01 0.1
2
1
3
456
Dra
inag
e B
asin
Siz
e (k
m2 )
Figure 3—Cross-plot of river gradient vs. drainage basinsize for streams that deposited deltas along the easternmargin of late Pleistocene Lake Bonneville. Fine-graineddeltas were sourced by low-gradient rivers with largedrainage basins that underwent only minor glaciation.Coarse-grained deltas (“Gilbert” deltas) were sourced byhigh-gradient rivers with small drainage basins thatwere strongly affected by glaciation (numbers refer todeltas listed in Table 1).
vs. dip variability in stratal stacking patterns (e.g.,Martinsen and Helland-Hansen, 1995) in the trans-gressive and highstand systems tracts is low, indi-cating minimal effects of local subsidence trendsand strike-variable sediment supply. (2) Healing-phase deposits (relatively sand-poor, early trans-gressive deposits primarily eroded from preexistingdelta plain/coastal plain sediments) of Posamentierand Allen (1993) are not recognized. (3) Fluvialincision into the transgressive and highstand sys-tems tracts does occur, unlike the development ofthe lowstand systems tract in most ramp physio-graphic settings (Posamentier and Allen, 1993). (4)No turbidites are recognized in the lowstand sys-tems tract. (5) As a consequence of lowstand ero-sion, the lowstand systems tract is not physicallyattached (e.g., Ainsworth and Pattison, 1994) to thehighstand systems tract.
Forcing Parameters
Previous work has established important bound-ary conditions for interpreting forcing parameterson deposition: (1) lake level change vs. time forLake Bonneville, (2) tectonic uplift rates along theWasatch Mountains and Bear River Range, (3)resulting physiography due to uplift and lake levelchange, and (4) sediment yield rates along the east-ern margin of Lake Bonneville. The relative impactof the four forcing parameters (Posamentier andJames, 1993) is assessed for each delta (Table 9).
LimnostasyThe term “limnostasy” is suggested for basin-
wide changes in lake level. This definition contrastswith that for “eustasy,” which refers to global
Lemons and Chan 639
Figure 4—Hierarchy ofbounding surfaces (numbered 1–6 in ascending order) used in developing the faciesarchitecture for theWeber, Bear, and SpanishFork river deltas (modifiedfrom Miall, 1988a) (Table2). TST = transgressive systems tract, HST = highstand systems tract,LST = lowstand systemstract.
clay
clay
4
4
3
2intraclasts
10s m
1—10
s m
geosol
6
5 4
10s km
100s
mWest East
TST/HST
LSTWasatch/BearRiver range
reactivationsurface
1
changes in sea level (e.g., Dott, 1992). In dealingwith nonmarine strata, this distinction seemsappropriate. Unlike most marine settings, LakeBonneville has a preexisting well-established hydro-graph of base level (lake level) change vs. time(Figure 8); therefore, packages of sedimentsequences can be definitively tied to the hydro-graph on a smaller scale than in most marinesequence stratigraphic studies. The Weber, Bear,and Spanish Fork deltas were deposited on the timescale (∼20 k.y.) of a fifth-order high-frequency cycle(Mitchum and Van Wagoner, 1991).
TectonicsEstimated ranges of subsidence values (dip sepa-
ration along the Wasatch and East Cache faultzones) for all three deltas are shown in Table 9.Maximum subsidence values for the exposed por-tions of all three deltas range from 0 to 25 m.Similarly, minimum subsidence values range from 0to 12 m. Contemporaneous lake level fluctuations
were on the order of 300 m; therefore, accommo-dation (space made available for potential sedimentaccumulation) (Posamentier et al., 1988) was large-ly controlled by lake level. Slip rates along theWasatch fault over the last 15 k.y. may be muchhigher than longer term slip rates (M. Machette,1996, personal communication); consequently,subsidence values during delta formation (∼28–10ka) may be less than suggested.
Sediment Yield RatesThe sediment yield rate for the Weber River delta
is estimated to be 705 m3/km2/yr based on deltavolume, drainage size, and time involved in deltadeposition (Lemons et al., 1996). This value isabout two times higher than modern basins of simi-lar size (Gregory and Walling, 1973), although it isdifficult to find direct analogs in terms of climate,relief, and bedrock geology. Paleoprecipitation esti-mates based on this sediment yield rate suggest thatmaximum annual paleoprecipitation rates during
640 Lake Bonneville Deltas
Table 2. Bounding Surface Hierarchy for the Weber, Bear, and Spanish Fork Deltas
Bounding Surfaces and Attributes
1. First-Order Surfaces (same as Miall, 1988a)A. Cross-bed set bounding surfacesB. Little or no erosionC. Represent virtually continuous sedimentation of a train of similar bed forms; therefore, lithofacies and
sedimentary structures directly overlying surface are the same (can include reactivation surfaces)D. Bounds microform deposits
2. Second-Order Surfaces (same as Miall, 1988a)A. Coset bounding surfacesB. Change in lithofacies due to change in flow conditions or a change in flow directionsC. Lithofacies and bed forms above and below surface are differentD. No significant time breakE. Bounds mesoform deposits
3. Third-Order Surfaces (modified from Miall, 1988a)A. Erosional features within macroforms (not necessarily crosscutting erosion)B. Intraclast breccia commonly overlies surface
4. Fourth-Order Surfaces (same as Miall, 1988a)A. Upper bounding surfaces of macroformsB. Change in lithosome due to change in flow conditionC. Bounds architectural elements
5. Fifth-Order Surfaces (modified from Miall, 1988a)A. Bounds deposits of major rapid lake level fluctuations (transgression/regression), e.g., surface between
Bonneville and Provo level deltas (which separates the highstand systems tract and lowstand systems tract)B. Also includes surfaces resulting from smaller oscillations of lake level (e.g., Keg Mountain oscillation)
6. Sixth-Order Surfaces (modified from Miall, 1988a)A. Bounds mappable sediment packages of a single lake cycle (e.g., Fielding geosol at base of Bonneville
alloformation)B. Alloformation boundaries
Lemons and Chan 641
Table 3. Weber River Delta Lithofacies Classification
Facies Lithofacies Sedimentary Structures Interpretation
Gravel, Gravel, clast- Typically upward-fining, Beach or shoreline massive to supported, sandy matrix; crude horizontal bedding deposits sourcedcrudely sometimes matrix supported from local canyonsbedded gravelly sand
Gravel, Gravel or granules, Horizontal to low-angle Associated with wave-subhorizontal usually occurs as bedding (<10°) influenced deposits, usually
discontinuous lenses; found in delta-frontvery fine-grained to medium- sands depositedgrained, sandy matrix near mountain front
Gravel, Gravel or granules in a Angular to concave Current ripplescross- fine to coarse-grained foresets with dips up formed in fluvial channelbedded sandy matrix to 30°; commonly have deposits; associated
erosive basis with Sxb† lithofacies
Sand, Very fine to medium Wave-ripples with ripple, Wave influencedwave grained sand, may be index of 5–10 and in shallow waterrippled silty; contains higher ripple symmetry index of ~1; (~0.5–1 m), usually
silt and clay content may be intricately woven, delta-front andwhen developed in some soft-sediment delta-margin depositsdelta margin deposits, deformation and sometimes interstratified intraclast brecciawith subhorizontal sand
Sand, Very fine to fine grained Thin clay laminations Restricted to deltaflaser silty sand, smaller draped over wave margin depositsbedding amounts of clay ripples
Sand, Very fine to medium Angular to concave Current ripples cross- grained sand, may foresets with dips up formed in fluvialbedded contain granules to 30°; commonly have channel deposits or
and small pebbles erosive basis beach gravels
Sand, Very fine to medium Horizontal to low-angle Usually found insubhorizontal grained sand, may be bedding (<10°), some delta-front and
silty; contains higher soft-sediment deformation, delta-margin depositspercentage of silt and clay some intraclast brecciawhen developed indelta-margin environments;can contain granules andsmall pebbles whendeveloped in fluvial channeldeposits; sometimesinterstratified with wave-rippledsand
Sand, Very fine grained sand, Most hummocks have Associated withhummocky silt, and some clay amplitudes of about 6–8 distal portionscross- cm, whereas internal of delta-margin deposits;stratification laminations have rarely found in delta-front
amplitudes of about 2–3 depositscm; hummocks havelateral spacings from 1–3 m
(Continued on next page)
the approximate highstand of Lake Bonneville mayhave been up to 33% higher than present rates(Lemons et al., 1996). Decreasing precipitation asLake Bonneville waned apparently resulted inlower sediment yields. Sediment yield data are cur-rently not available for the Bear and Spanish Forkrivers; however, the close proximity of all threedrainage basins and the apparent importance ofprecipitation suggest that sediment yield rates aresimilar.
PhysiographyThe physiography of the surface on which each
delta was deposited resembles a ramp. The ramplength and accommodation for each delta areshown in Table 9. There is some uncertaintyregarding when the Bear River headwaters werediverted into the Bonneville basin from the SnakeRiver basin; however, the Bear River was flowinginto the Bonneville basin prior to the initiation ofLake Bonneville (Bright, 1963; McCoy, 1987;Hochberg, 1995).
The Weber and Spanish Fork deltas have shortramp lengths (tens of kilometers), whereas the
Bear River delta has a long ramp length (>100km). The Spanish Fork delta has approximately90 m less potential accommodation (∼180 m)than either the Weber or Bear River delta (∼270m). The Weber and Spanish Fork deltas are situ-ated along portions of former Lake Bonnevillewhere they were openly exposed to waves andcurrents. In contrast, the Bear River delta is situ-ated in a relatively isolated arm in the northeast-ern portion of former Lake Bonneville over 100km from the present-day Great Salt Lake; there-fore, the Bear River delta has a depositional sur-face with a lower gradient than the other twodeltas. Even though all three deltas have ramp-like physiography with low depositional gradi-ents, variations in accommodation and downdipramp length lead to markedly different sequenceexpressions, particularly in the lowstand sys-tems tract.
An important distinction needs to be madebetween ramp length and ramp gradient withregards to marine vs. nonmarine depositional set-tings. In marine sequence stratigraphy, the shore-line is assumed to be more or less linear. Thus, in amarine ramp setting, ramp gradient can vary, but
642 Lake Bonneville Deltas
Table 3. Continued
Facies Lithofacies Sedimentary Structures Interpretation
Sand, Fine to medium Current ripples with RSI* Only recognized incurrent grained sand, may up to 4.5 and RI** up to 14 delta-margin deposits;rippled be silty associated with local
canyons contributingunusually large amounts ofsediment; rarely found
Sand Very fine to medium Trough cross-beds up to Restricted to fluvialtrough grained sand, may contain 0.3 m across channel environmentscross- granules or small pebbles along more distalbedded portions of delta
adjacent to mountains;rarely found
Fines, Silt and clay, smaller Horizontal to low-angle Usually found insubhorizontal amounts of very fine bedding (<10°), some delta-front and delta-
to fine grained sand soft-sediment deformation margin deposits
Fines, Clay Massive to Lacustrine depositsclay laminated
Marl, Friable marl containing None recognized Fauna suggestswhite abundant ostracods, deposition in a
gastropods, bivalves wetland environmentepiphytic diatoms, fish marginal to thebones, and plant remains delta (R. Forester, 1996,
personal communication)
*RSI = ripple symmetry index = length of horizontal projection of stoss side/length of horizontal projection of lee side (Reineck and Singh, 1980).**RI = ripple index = ripple length/ripple height (Reineck and Singh, 1980).†Sxb = sand, cross-bedded.
Lemons and Chan 643
Tab
le 4
. W
eber
Riv
er D
elta
Arc
hit
ectu
ral
Ele
men
ts
Pri
nci
pal
Lith
ofa
cies
Geo
met
ry a
nd
Elem
ent
Ass
emb
lage
*R
elat
ion
sIn
terp
reta
tio
n
Del
ta-fr
on
t sh
eet
san
dSw
r, S
sh, F
sh,
Shee
t, b
lan
ket
(up
to
~10
0M
ost
ab
un
dan
t el
emen
t; w
ave
rip
ple
s, d
elta
elo
nga
tio
n t
o s
ou
th, a
nd
Gsh
, Sh
csm
th
ick
×10
00s
m w
ide
and
lack
of
pre
serv
ed d
elta
-pla
in d
epo
sits
ind
icat
e h
igh
wav
e ac
tivi
tylo
ng)
; in
terf
inge
rs b
asin
war
d(W
righ
t, 1
977;
Ort
on
an
d R
ead
ing,
199
3) g
ener
ally
dir
ecte
d f
rom
th
ew
ith
lacu
stri
ne
clay
an
dn
ort
hw
est
(Will
iam
s, 1
994)
; em
pir
ical
eq
uat
ion
s d
eriv
ed b
y T
ann
ersh
ore
war
d w
ith
all
oth
er(1
971)
an
d A
sple
r et
al.
(199
4) in
dic
ate
dep
osi
tio
n in
elem
ents
pal
eow
ater
dep
th o
f 0.
5–1
m
Del
ta-m
argi
n d
epo
sits
Sfb
, Sh
cs,
Wed
ge (
10s
m t
hic
k ×
Rec
ord
s en
viro
nm
ents
ran
gin
g fr
om
sto
rm-g
ener
ated
hu
mm
ock
ySs
h, F
sh, S
wr,
100s
to
100
0s m
wid
e);
cro
ss-s
trat
ific
atio
n a
lon
g th
e so
uth
ern
mar
gin
s o
f th
e d
elta
to
wh
ite
Mw
dep
osi
ted
alo
ng
mar
gin
sm
arl d
epo
site
d in
wet
lan
d e
nvi
ron
men
ts (
R. F
ore
ster
, 199
6, p
erso
nal
of
del
ta a
nd
sh
ore
war
dco
mm
un
icat
ion
) to
cu
rren
t-ri
pp
led
san
ds
alo
ng
loca
l can
yon
so
f d
elta
-fro
nt
shee
t sa
nd
s;co
ntr
ibu
tin
g u
nu
sual
ly h
igh
am
ou
nts
of
fin
e-gr
ain
ed s
and
to
pre
vale
nt
wit
h in
crea
sin
gd
epo
siti
on
in w
ave-
infl
uen
ced
mu
dfl
at a
nd
san
dfl
at e
nvi
ron
men
tsd
ista
nce
alo
ng
stri
ke f
rom
(Alle
n a
nd
Co
llin
son
, 198
6); a
lon
g so
uth
ern
po
rtio
ns
of
del
ta,
rive
r m
ou
th; l
arge
-sca
leth
is e
lem
ent
has
un
der
gon
e m
ajo
r p
ost
-Bo
nn
evill
e d
epo
siti
on
slu
mp
ing
alo
ng
the
sou
ther
nla
tera
l sp
read
ing
(Van
Ho
rn, 1
975)
mar
gin
of
the
del
ta
Flu
vial
ch
ann
el d
epo
sits
Sxb
, Gx
b, S
tx,
Len
ticu
lar
[met
ers
thic
k ×
Rel
ativ
ely
rare
; gen
eral
ly c
oar
se g
rain
ed (
smal
l gra
vel,
gran
ule
s, a
nd
Ssh
, Gsh
10s(
?) m
lon
g]; t
ypic
ally
san
d)
and
cro
ss-b
edd
ed, a
nd
can
ex
hib
it e
rosi
ve b
ases
; so
urc
ed f
rom
pre
serv
ed n
ear
mo
un
tain
loca
l sm
alle
r ca
nyo
ns;
rep
rese
nt
eph
emer
al, s
edim
ent-
char
ged
fro
nt,
ass
oci
ated
wit
hst
ream
s p
rovi
din
g lo
caliz
ed c
last
ic in
flu
x t
o t
he
sho
relin
ed
elta
-mar
gin
dep
osi
ts
Bea
ch g
rave
lsG
mc,
Gsh
, Ssh
,R
ecta
ngu
lar
wed
ge (
10s
mLo
caliz
ed a
dja
cen
t to
th
e W
asat
ch M
ou
nta
ins
and
fo
rm a
lin
ear
Swr
thic
k ×
1000
s m
wid
e);
stra
nd
of
bea
ch o
r sh
ore
line
dep
osi
ts (
Nel
son
an
d P
erso
niu
s, 1
993)
;ca
ps
del
ta a
t B
on
nev
ille
rep
rese
nt
coar
se-g
rain
ed s
edim
ent
sou
rced
by
loca
l sm
alle
rsh
ore
line;
pre
sum
ably
can
yon
s w
ith
ste
ep g
rad
ien
tsp
rese
nt
in lo
wer
par
ts o
fd
elta
alo
ng
mo
un
tain
fro
nt,
bu
t n
ot
exp
ose
d; s
ou
rced
fro
mlo
cal c
anyo
ns
Lacu
stri
ne
clay
FcSh
eet,
bla
nke
t (u
p t
o 1
0sP
rese
nt
bas
inw
ard
of
del
ta-fr
on
t sh
eet
san
ds;
th
inly
lam
inat
ed w
ith
m t
hic
k ×
1000
s m
wid
e an
dlig
ht
tan
an
d r
edd
ish
bro
wn
alt
ern
atin
g b
and
s ~
0.5–
1.0
cm t
hic
k,lo
ng)
; typ
ical
ly in
terf
inge
rssu
gges
tin
g an
ox
ic b
ott
om
wat
ers
and
so
me
typ
e o
f se
aso
nal
ity
insh
ore
war
d w
ith
del
ta-fr
on
tse
dim
ent
infl
ux
(A
nd
erso
n a
nd
Dea
n, 1
988;
Sm
oo
t, 1
993)
; tw
osh
eet
san
ds;
per
sist
sla
rge-
scal
e in
terv
als
(~10
–20
m)
of
lacu
stri
ne
clay
dep
osi
tio
nd
ow
nd
ip in
to d
eep
erp
arts
of
the
bas
in
Fsh
= f
ines
, su
bhor
izon
tal,
Gm
c =
gra
vel,
mas
sive
to
crud
ely
bedd
ed,
Gsb
= g
rave
l, cr
oss=
bedd
ed,
Gsh
= g
rave
l, su
bhor
izon
tal,
Mw
= m
arl,
whi
te,
Sfb
= s
and,
fla
ser
bedd
ed,
Shc
s =
san
d, h
umm
ocky
cros
s-st
ratif
ied,
Ssh
= s
and,
sub
horiz
onta
l, S
tx =
san
d tr
ough
, cro
ss-b
edde
d, S
wr
= s
and,
wav
e rip
pled
, Sxb
= s
and,
cro
ss-b
edde
d.
ramp length measured from the shoreline general-ly does not. In the Bonneville basin, all three deltashave a low depositional gradient; however, due tobasin and range physiography, the ramp length forthese deltas varies from tens of kilometers to over100 km. Both ramp length and gradient contributeto accommodation; however, unlike marine set-tings, where ramp gradient is a more importantcontributor to accommodation, ramp length is moreimportant in the Bonneville basin.
Weber River Delta
The sequence stratigraphy of the Weber Riverdelta closely resembles that of a passive continentalmargin (Vail, 1987; Van Wagoner et al., 1988, 1990;Steckler et al., 1993), even though it is situated inan intracratonic setting. The transgressive systemstract consists of a retrogradational parasequence
set with a minimum thickness of about 130 m. Thisis a minimum value because the base of theBonneville alloformation is not exposed. The trans-gressive systems tract contains two parasequences(labeled 1 and 2 on Figure 10) composed primarilyof fine-grained delta-front sheet sands overlain bylacustrine f looding surfaces or their shorewardequivalent (Figures 9–11). This setting suggeststhat lake level rise was punctuated, allowing fortwo cycles of progradation of delta-front sedi-ments (i .e. , sedimentation rates equal to orgreater than accommodation created by risinglake level). These parasequences were subse-quently transgressed during rapid phases of lakelevel rise where sedimentation rates were exceed-ed by rising lake level.
A detailed look at the pinchout of the lacustrineclays into delta-front sheet sands suggests some typeof seasonality or cyclicity. This is evidenced by arepetitive pattern of lacustrine clay, subhorizontal
644 Lake Bonneville Deltas
Table 5. Bear River Delta Lithofacies Classification
Facies Lithofacies Sedimentary Structures Interpretation
Gravel, Gravel, clast- Typically upward-coarsening, Beach or shorelinemassive to supported, usually crude horizontal bedding deposits, sourced fromcrudely sandy at base of local drainages,bedded upward-coarsening recognized in outcrop at
cycle Bonneville shoreline andProvo and post-Provodeposition sediments(Bright, 1963)
Sand, Very fine to Wave ripples may be Wave-influencedwave medium grained intricately woven, delta-front depositsrippled sand, may be silty some soft-sediment
deformation
Sand, Very fine to Horizontal to low-angle Delta-front depositssubhorizontal medium grained bedding (<10°), some
sand, may be silty soft-sediment deformation
Sand, Very fine to Current (unidirectional) Fluvially influencedcurrent medium grained ripples, some soft- delta-front depositsrippled sand, may be silty sediment deformation
Sand, Very fine to Climbing current ripples, Fluvially influencedclimbing medium grained usually preserved as delta-front depositsrippled sand with lee side ripple laminae-in-drift
preserved, stossside sometimespreserved
Fines, Silt and clay, Horizontal to low-angle Delta-front depositssubhorizontal smaller amounts bedding (<10°), some
of very fine to fine soft-sediment deformationgrained sand
Fines, clay Clay Massive to laminated Lacustrine deposits
Lemons and Chan 645
Tab
le 6
. B
ear
Riv
er D
elta
Arc
hit
ectu
ral
Ele
men
ts
Pri
nci
pal
Lith
ofa
cies
Elem
ent
Ass
emb
lage
*G
eom
etry
an
d R
elat
ion
sIn
terp
reta
tio
n
Del
ta-fr
on
t sh
eet
san
ds
Scr,
Sw
r, S
sh,
Shee
t, b
lan
ket
(up
to
~10
0 m
Mo
st a
bu
nd
ant
elem
ent;
wav
e ri
pp
les
and
cu
rren
tSc
l, Fs
hth
ick
×10
00s
m(?
) w
ide
and
and
clim
bin
g cu
rren
t ri
pp
les
ind
icat
e co
mb
inat
ion
of
lon
g); l
ater
ally
co
nti
nu
ou
s in
w
ave
and
flu
vial
infl
uen
ce; p
hys
iogr
aph
ical
ly lo
cate
do
utc
rop
ove
r at
leas
t 2
km in
dip
in is
ola
ted
no
rth
east
arm
of
lake
[C
ach
e V
alle
y b
ayd
irec
tio
n, i
nte
rbed
s b
asin
war
d
of
Gilb
ert
(189
0)];
alt
ho
ugh
del
ta e
xp
erie
nce
d le
ssw
ith
lacu
stri
ne
clay
wav
e ac
tivi
ty t
han
th
e W
eber
Riv
er d
elta
, ab
sen
ce o
fd
elta
-pla
in d
epo
sits
(B
righ
t, 1
963)
ind
icat
es t
her
e w
asst
ill a
hig
h d
egre
e o
f re
wo
rkin
g
Bea
ch g
rave
lsG
mc
Len
s-sh
ape
or
po
ssib
lyG
rave
l pro
ven
ance
ap
pea
rs t
o b
e p
rin
cip
ally
fro
m t
he
late
rally
co
nti
nu
ou
s o
ver
sho
rtM
ioce
ne–
Plio
cen
e Sa
lt L
ake
Gro
up
an
d n
earb
yd
ista
nce
s [m
eter
s th
ick
×10
s–10
0sP
aleo
zoic
fo
rmat
ion
sm
(?)
wid
e]; c
aps
del
ta a
tB
on
nev
ille
sho
relin
e; a
pp
aren
tly
sou
rced
by
loca
l, st
eep
er g
rad
ien
td
rain
ages
an
d c
anyo
ns
Lacu
stri
ne
clay
FcSh
eet,
bla
nke
t [1
s–10
s o
f m
P
rese
nt
bas
inw
ard
of
del
ta-fr
on
t sh
eet
san
ds
inth
ick
×10
00s
m(?
) w
ide
and
an
inte
rfin
geri
ng
rela
tio
nsh
ip; m
ost
of
the
clay
lon
g]; l
ater
ally
co
nti
nu
ou
s in
is
th
inly
lam
inat
ed w
ith
ligh
t ta
n a
nd
red
dis
ho
utc
rop
ove
r at
leas
t 2
km in
dip
bro
wn
alt
ern
atin
g b
and
s su
gges
tin
g an
ox
icd
irec
tio
n; i
nte
rfin
gers
sh
ore
war
db
ott
om
wat
ers
and
so
me
typ
e o
f se
aso
nal
ity
wit
h d
elta
-fro
nt
shee
tin
sed
imen
t in
flu
x; s
ever
al s
mal
l alt
ern
atin
g cy
cles
san
ds;
pre
sum
ably
per
sist
s o
f d
elta
-fro
nt
shee
t p
rogr
adat
ion
an
d t
ran
sgre
ssiv
ed
ow
nd
ip b
asin
war
dla
cust
rin
e cl
ay d
epo
siti
on
wit
hin
on
ela
rge-
scal
e p
acka
ge o
f la
cust
rin
e cl
ay (
corr
elat
ive
wit
h u
pp
erm
ost
lacu
stri
ne
clay
in t
he
Web
er R
iver
del
ta);
low
er c
lay
may
be
pre
sen
t, b
ut
was
no
t re
cogn
ized
in o
utc
rop
or
pu
blic
ly a
vaila
ble
dri
llers
’ lo
gs
*Fc
= fi
nes,
cla
y, F
sh =
fine
s, s
ubho
rizon
tal,
Gm
c =
gra
vel,
mas
sive
to c
rude
ly b
edde
d, S
cl =
san
d, c
limbi
ng r
ippl
ed, S
cr =
san
d, c
urre
nt r
ippl
ed, S
sh =
san
d, s
ubho
rizon
tal,
Sw
r =
san
d, w
ave
rippl
ed.
646 Lake Bonneville Deltas
Table 7. Spanish Fork Delta Lithofacies Classification
Facies Lithofacies Sedimentary Structures Interpretation
Gravel, Gravel, clast Vague upward-fining and Can occur as beach gravelmassive to supported; pebbly to upward-coarsening cycles, deposits preserved in narrow crudely sandy matrix; can contain crude horizontal bedding, corridor along Wasatchbedded subhorizontal fine- to dips usually <10° Mountains or as fluvial gravel
coarse-grained sand deposits that are gradedstringers and lenses; to the Provo shorelinerarely has small-scale and cap delta front depositschanneling
Gravel, Gravel, clast supported, Beds that dip >20°; Fluvial gravels deposits that arecross- very fine to fine grained can have erosive base when graded to Provo shorelinebedded matrix, may contain sand associated with channeling and cap delta-front deposits
stringers with dips >20°; into underlying delta-frontassociated with massive depositsto crudely bedded gravels
Sand, Very fine to medium Wave-ripples with RI* Wave influenced in shallowwave grained sand, may be of 5–12 and RSI** water (~1–2 m); onlyrippled silty; commonly of ~1; may be intricately found in delta front deposits
interbedded on 1s–10s woven; paleocurrentcm scale with subhorizontal measurements indicatefines predominant wave
orientation from thenorth–northwest
Sand, Very fine to medium Horizontal to low-angle Usually found in delta-frontsubhorizontal grained sand, may be bedding (<10°), may deposits; less
silty; commonly contain intraclast breccia; frequently seen atinterbedded on centimeter some soft-sediment transition fromscale with subhorizontal fines deformation; rarely has fluvial gravels to loessindicating some type of erosive base whenseasonality; rarely stratigraphically overlyingcontains thin gravel fluvial gravels (massive tostringers crudely bedded gravels or
cross-bedded gravel)
Sand, Medium to coarse Angular to tangential Finer grained fluvialcross-bedded grained sand, can contain foresets with dips of deposit that occurs
granules or small pebbles 10–15°; can have erosive at transition from delta-frontbase; sometimes to fluvial gravels elementshas lower angle dips and at fluvial gravels-to-loess(<10°); can have soft- element transition;sediment deformation that rarely seen in thin sandaffects underlying deposits stringers in massive to crudely
bedded gravels
Sand, Very fine to coarse Angular to concave foresets Rarely found in bothplanar grained sand, may be dipping from 5–20°; delta-front deposits and incross-bedded silty, commonly relatively rare; sometimes sand stringers in massive to
interbedded on cm grades laterally into crudely bedded gravelsscale with subhorizontal fines wave-rippled sand
Sand, Very fine to coarse Trough cross-beds up Recognized only intrough grained sand; may to 1 m across delta-front deposits of thecross-bedded contain granules transgressive systems tract
Lemons and Chan 647
Table 7. Continued
Facies Lithofacies Sedimentary Structures Interpretation
Fines, Silt with smaller Horizontal to low-angle Found in delta-front elementsubhorizontal amounts of very fine bedding (<10°), some or as loess capping
grained sand and clay; soft-sediment deformation delta stratigraphicallycalcareous when occurs and slumping; sometimes above fluvial gravelsas loess forms a drape over
underlying, unconsolidatedwave-rippled sand; sometimescontains intercalated wave-rippledsand lenses
*RI = ripple index = ripple length/ripple height (Reineck and Singh, 1980).**RSI = ripple symmetry index = length of horizontal projection of stoss side/length of horizontal projection of lee side (Reineck and Singh, 1980).
sand, and wave-rippled sand that repeats on a 10–30cm scale (Figure 5F). Landward of delta-front sheetsand deposition, contemporaneous delta-margin, flu-vial channel, and beach gravels were deposited.
The lacustrine flooding surfaces could correspondto periods of rapid subsidence (faulting) that loweredthe depositional surface and produced “instanta-neous” lacustrine flooding (Plint and Browne, 1994);however, the vertical displacements associated withmovement on the Wasatch fault along the WeberRiver delta are too low (usually in the range of 1.5 to2.5 m), making this subsidence scenario unlikely(Machette et al., 1991).
The subsequent highstand systems tractdeposits are exposed directly adjacent to theWasatch Mountains (Figures 10, 11). A paucity oflacustrine clay indicates that coarse clastic sedi-mentation kept up with lake level rise, at leastnear the Wasatch Mountains, during highstanddeposition. The maximum thickness of the high-stand systems tract is approximately 85 m. Part ofthe highstand systems tract was eroded when thewater level in Lake Bonneville fell catastrophical-ly to the Provo level. The bulk of the preservedhighstand systems tract deposits consist of delta-margin, f luvial channel, and beach gravel sedi-ments deposited landward of delta-front sheetsands. The limited exposures of the highstandsystems tract (Figure 11) show the deposits to besubhorizontal. Thus, it is impossible to recognizethe maximum f looding surface by downlap.Presumably, the highstand systems tract resultedin aggradational to progradational parasequencesets (Van Wagoner et al., 1988, 1990).
Lowstand systems tract sediments consist of alter-nating delta-front sheet sands and lacustrine clays(Figure 5) recording a punctuated post-Provo regres-sion of the lake. Minimum thickness of the lowstandsystems tract is on the order of tens of meters. Fluvialincision resulted in steep, unstable clay slopes alongthe margins of the river, and a series of landslides has
occurred along these slopes (Blackett, 1979). Theoverlying lowstand systems tract consists partly ofcannibalized highstand and transgressive systemstract sediments that were deposited at the toe of theWeber River delta during a forced regression or rela-tive lake level lowering (Posamentier et al., 1992;Posamentier and Morris, 1996) of Lake Bonnevillethat formed the subaerially exposed sequenceboundary (Posamentier and Vail, 1988).
Bear River Delta
The sequence stratigraphy of the Bear Riverdelta was heavily inf luenced by its long, low-gradient ramp geometry. Outcrop exposure of thetransgressive systems tract is incomplete, themaximum exposed thickness being about 50 m.Transgressive systems tract deposits formed dur-ing the early stages of Lake Bonneville as it trans-gressed northeastward are not exposed. Theexposed portion of the transgressive systemstract (situated at the mouth of the OneidaNarrows; Figure 12) consists of very fine grainedprogradational delta-front sheet sands capped bya lacustrine f looding clay unit that forms oneparasequence (Figures 6E, 13). Because the BearRiver delta was situated in a relatively isolatednortheastern arm of Lake Bonneville (Figure 1)and because the prevailing winds were from thenorthwest there was limited wave activity; conse-quently, the delta-front sheet sands display morefluvial inf luence than those of the Weber Riverdelta. The relatively rapid rise of Lake Bonneville,however, resulted in wave-reworking of any delta-plain sediments (e.g., distributary channel depositsand organics) into delta-front deposits. Landwardof delta-front sheet sand sedimentation, beach grav-els were contemporaneously deposited.
As a result of the Bear River delta’s low gradi-ent ramp, there is more large-scale interfingering
648 Lake Bonneville DeltasT
able
8. Sp
anis
h F
ork
Del
ta A
rch
itec
tura
l E
lem
ents
Pri
nci
pal
Lith
ofa
cies
Geo
met
ry a
nd
Elem
ent
Ass
emb
lage
*R
elat
ion
sIn
terp
reta
tio
n
Del
ta-fr
on
t sh
eet
san
dSw
r, S
sh, S
px
b,
Shee
t o
r b
lan
ket
(at
leas
tR
ipp
le s
pac
ing
and
gra
in s
ize
ind
icat
e d
epo
siti
on
Stx
, Fsh
seve
ral 1
0s m
th
ick
×in
wat
er d
epth
s o
f 1–
2 m
(T
ann
er, 1
971;
Asp
ler
et10
00s
m w
ide
and
lon
gal
., 19
94);
ab
sen
ce o
f d
elta
-pla
in d
epo
sits
, ab
un
dan
cein
low
stan
d s
yste
ms
trac
t);
of
wav
e ri
pp
les,
an
d r
ipp
le o
rien
tati
on
s su
gges
t h
igh
geo
met
ry in
tra
nsg
ress
ive
wav
e ac
tivi
ty d
irec
ted
fro
m t
he
no
rth
wes
t; la
ke le
vel
syst
ems
trac
t an
d h
igh
stan
do
scill
atio
ns
reco
gniz
ed in
fo
rese
ts a
s an
gula
r re
lati
on
ssy
stem
s tr
act
no
t kn
ow
n;
bet
wee
n d
ipp
ing
fore
sets
an
d s
ub
ho
rizo
nta
l bed
s;fo
rms
bas
inw
ard
-dip
pin
gsy
nd
epo
siti
on
al s
lum
pin
g m
ay r
epre
sen
tfo
rese
ts w
ith
dip
s o
f 25
–p
aleo
seis
mic
ity
alo
ng
the
Was
atch
fau
lt (
D.
35°
wh
en p
art
of
low
stan
dC
urr
ey, 1
996,
per
son
al c
om
mu
nic
atio
n);
sea
son
alit
y in
syst
ems
trac
t (f
allin
g la
ked
epo
siti
on
is s
ugg
este
d b
y re
pea
ted
cyc
les
of
fin
e- t
ole
vel)
; su
bh
ori
zon
tal b
eds
med
ium
-gra
ined
san
d a
nd
th
inn
er b
eds
of
clay
ey s
iltw
hen
par
t o
f tr
ansg
ress
ive
syst
ems
trac
t o
r h
igh
stan
dsy
stem
s tr
act
(ris
ing
lake
leve
l)
Loes
sFs
hSh
eet
or
bla
nke
t (m
eter
sLo
ess
dep
osi
tio
n p
rob
ably
sta
rted
ab
ou
t 12
.5 k
a, b
ut
thic
k ×
1000
s m
wid
ese
ems
to h
ave
bee
n g
reat
est
du
rin
g th
e ea
rly
and
lon
g); o
verl
ies
grav
els
and
mid
dle
Ho
loce
ne
(Mac
het
te a
nd
Lu
nd
, 198
7)(m
assi
ve t
o c
rud
ely
bed
ded
grav
el o
r cr
oss
-bed
ded
gra
vel)
or
san
ds
(su
bh
ori
zon
tal s
and
s)o
f fl
uvi
al g
rave
ls in
th
e ce
ntr
alp
ort
ion
of
the
del
ta; s
ilty
Bea
ch g
rave
lsG
mc
Rec
tan
gula
r ri
bb
on
(10
s m
Co
arse
-gra
ined
sed
imen
t so
urc
ed b
y lo
cal,
stee
per
thic
k ×
1000
s m
wid
e)gr
adie
nt
stre
ams
and
rew
ork
ed b
y w
aves
pre
serv
ed in
a n
arro
wco
rrid
or
alo
ng
the
Was
atch
Mo
un
tain
s ab
ove
th
e P
rovo
sho
relin
e; p
resu
mab
lyp
rese
nt
in lo
wer
po
rtio
ns
of
del
ta a
lon
g th
e m
ou
nta
infr
on
t, b
ut
no
t ex
po
sed
;so
urc
ed f
rom
loca
l can
yon
s;er
osi
on
of
this
ele
men
taf
ter
the
cata
stro
ph
ic f
all
of
Lake
Bo
nn
evill
e to
th
eP
rovo
sh
ore
line
pro
vid
edso
urc
e fo
r fl
uvi
al g
rave
ls
Lemons and Chan 649
Tab
le 8
. C
on
tin
ued
Pri
nci
pal
Lith
ofa
cies
Geo
met
ry a
nd
Elem
ent
Ass
emb
lage
*R
elat
ion
sIn
terp
reta
tio
n
Flu
vial
gra
vels
Gm
c, G
xb
, Ssh
Elo
nga
te w
edge
th
at t
hin
sFo
rms
a to
pse
t u
nit
th
at d
irec
tly
ove
rlie
s b
asin
war
d-
Sxb
bas
inw
ard
an
d is
gra
ded
dip
pin
g d
elta
-fro
nt
shee
t sa
nd
s; c
on
tact
wit
hto
th
e P
rovo
sh
ore
line
un
der
lyin
g d
elta
-fro
nt
shee
t sa
nd
s is
so
met
imes
ero
sive
(~10
m t
hic
k ×
1000
s(e
.g.,
loca
l sco
urs
on
th
e o
rder
of
a fe
w m
eter
s); f
luvi
alm
wid
e an
d lo
ng)
;gr
avel
s re
pre
sen
t re
wo
rked
bea
ch g
rave
ls o
rigi
nal
lyo
verl
ies
del
ta-fr
on
t el
emen
t;d
epo
site
d n
ear
the
Was
atch
Mo
un
tain
s (M
ach
ette
,ca
n h
ave
ero
sio
nal
bas
e19
92);
co
arse
-gra
ined
nat
ure
an
d c
ross
-bed
ded
bar
fo
rms
in s
hal
low
-wat
er g
eom
etri
es in
dic
ate
dep
osi
tio
n b
y b
raid
ed s
trea
ms
as L
ake
Bo
nn
evill
ere
gres
sed
fro
m t
he
Pro
vo s
ho
relin
e
*Fsh
= f
ines
, su
bhor
izon
tal,
Gm
c =
gra
vel,
mas
sive
to
crud
ely
bedd
ed,
Gxb
= g
rave
l, cr
oss-
bedd
ed,
Spx
b =
san
d, p
lana
r cr
oss=
bedd
ed,
Ssh
= s
and,
sub
horiz
onta
l,Stx
= s
and
trou
gh,
cros
s-be
dded
,S
wr
= s
and,
wav
e rip
pled
, Sxb
= s
and,
cro
ss-b
edde
d.
Figure 5—Architectural elements for the WeberRiver delta. Major boundingsurfaces labeled using thebounding surface hierarchy are shown inTable 2. Black-and-whitestaff divisions are in 15 cmintervals. (A) Lacustrineclay (LC) draped over delta-front (DF)wave-rippled sand. Two zones of intraclastbreccia form third-orderbounding surfaces. (B) Two sequences of delta-margin (DM) flaser-bedded sand overlain by fluvial channeldeposits (FC). The fluvialchannel exhibits an erosive base with cross-beds prograding to the left (west), and apebbly lag that gradesupward into fine to medium grained sand. (C) Beach gravels consisting of massive tocrudely bedded gravel andsubhorizontal sand. (D) Delta-front (DF)deposits overlain by lacustrine clay (LC). At thebase of some lacustrineclay sequences, thinly bedded alternatingsequences of delta-frontand lacustrine sediments(commonly draped overwave ripples) suggestepisodic transition to deeper water lacustrinedeposition or episodicstorm (and other)sand deposition. (E) Thin-bedded lacustrineclay. (F) Lacustrine clay(LC) interfingering in acyclic manner with updipdelta-front (DF) sands. A cycle typically consists ofdelta-front subhorizontalsand overlain by delta-front wave-rippledsand subsequently overlainby darker colored lacustrine clay. (G) Abruptchange from lacustrineclay (LC) into delta-front(DF) deposition indicatingrapid delta-front progradation and the for-mation of a fourth-orderbounding surface. Note the gravels (Gsh) containedin the delta-front deposits(arrow).
DM
FC
DM
4
4
(E)
(B)
(F)
0
1/2mLC
DF
0
1m
(C)
(G)
DF
LC
4
Gsh
(D)
LC
4DF
3
3
4
LCDF
(A)
(more than 20–30 m vertically) between thedelta-front and lacustrine clay deposits in com-parison to the Weber River delta. Some workershave subdivided this interfingering into a greaternumber of parasequences (Anderson and Link,1998); however, we interpreted these to be localto the Bear River delta (not present basinwide in theother deltas) and thus grouped these deposits into asingle parasequence. The exposed lacustrine clayappears to be correlative to the uppermost lacustrineclay of the Weber River transgressive systems tract;however, the updip termination of the clay is approx-imately 25–30 m lower in elevation in the Bear River
delta than in the Weber River delta. This discrepancyprobably results from isostatic rebound differencesbetween the Weber and Bear River deltas. The iso-statically adjusted Bonneville shoreline is 1552 m(Currey and Oviatt, 1985). The current Bonnevilleshoreline ranges from 1555 ±2 m south of Preston,Idaho, to 1584 ±1 m near Salt Lake City (Crittenden,1963; Currey, 1982; Bills et al., 1994). Thus, isostaticrebound resulting from the desiccation of LakeBonneville has resulted in approximately 25–30 m ofrebound in the Weber River delta, with only a fewmeters of rebound in the Bear River delta; therefore,intrabasinal correlation of f looding surfaces and
Lemons and Chan 651
Figure 6—Architectural elements for the Bear Riverdelta. Major bounding surfaces are labeled usingthe bounding surface hierarchy shown in Table 2. (A) Delta-frontsheet sands deposited during regression of Lake Bonneville. Arrowpoints to the base of a wave-rippled, low-angle clinoform set. (B) Similar to(A), but note high-angleclimbing-ripples (Scl) with transport direction to left (west). From the base of the outcrop, thiserroneously appears to beangular eastward-dippingcross-beds suggesting transport direction to the right. (C) Crudely bedded beach gravels. (D) Lacustrine clay (view to the north).
(C)
(D)
0
2m
3
(A)
(B)
Scl
3
0
2m
0
2m
other sequence components based solely on eleva-tion, even in relatively undisturbed Quaternarydeposits, may be misleading.
Highstand systems tract deposits are poorlyexposed. Exposures south of the Oneida Narrows arecomposed of delta-front sheet sands and beach grav-els. Small exposures north of the mouth of the OneidaNarrows show a gentle gradient for the upper deltasurface (less than 0.1°) from the delta origin over adownstream distance of over 16 km (Bright, 1963).The highstand systems tract deposits are subhorizon-tal and show no evidence of downlap onto the maxi-mum flooding surface. Maximum thickness of thehighstand systems tract is on the order of 85 m.
Lowstand systems tract deposits consist of poorlyexposed, basinally isolated (patchy deposits in a bas-inward position relative to previous shoreline),downstepping shoreline deposits reworked fromhighstand systems tract and transgressive systems
tract deposits. These are associated with the forcedregression of Lake Bonneville, which produced thesubaerially exposed sequence boundary as the BearRiver entrenched into the soft sediments of its owndelta when the water level of Lake Bonneville fellcatastrophically from the Bonneville to the Provoshoreline. Fluvial incision created steep, unstableslopes along the margin of the river, resulting in theBear River landslide complex (Mahoney et al., 1987).Due to lower wave energy in the Bear River delta,there was not much Provo-level erosion of highstandsystems tract deposits. The downstream distancefrom the Bear River delta to the present-day GreatSalt Lake is much larger than the distance from theWeber River delta to the lake (approximately 125 vs.20 km), which means that the lowstand systems tractis spread out over a much larger area. Some lowstandsystems tract deposits are preserved on the sides ofthe valley incised by the Bear River. These deposits
652 Lake Bonneville Deltas
(A)
(C)
(B)
5
(D)
(E)
L 4
FG
0
2m
0
2m
Figure 7—Architectural elements for the SpanishFork delta. Black-and-whitestaff divisions are in 15 cmintervals. Major boundingsurfaces labeled using thebounding surface hierarchy shown in Table 2. (A) Transgressivedelta-front sheet sands. (B) Regressive, wave-rippled, delta-frontsheet sands consisting of basinward-dipping (to theright) foresets. Angularcontact with overlying subhorizontal beds indicates a major lake level oscillation and formsa fifth-order bounding surface. (C) Loess (L) capping fluvial gravels(FG). (D) Beach gravels. (E) Fluvial gravels containing cross-beddedbar forms (dipping to the left or south) deposited by braidedstreams.
include low-angle, wave-rippled clinoforms (Figure6A, B). Maximum thickness of the lowstand systemstract is on the order of tens of meters.
Spanish Fork Delta
The sequence stratigraphy of the Spanish Forkdelta was strongly affected by lack of accommo-dation and the smaller drainage-basin size of theSpanish Fork River. Outcrop exposure of thetransgressive systems tract is poor and consists ofa few exposures of subhorizontal delta-front
sheet sands preserved adjacent to the WasatchMountains below the Provo shoreline (Figures 7,14–16). Lacustrine f looding clays that typicallyserve as parasequence boundaries in the Weberand Bear River deltas may exist, but are notexposed. Shoreward of delta-front sheet sand sedi-mentation, beach gravels were contemporaneouslydeposited. Maximum thickness of the transgressivesystems tract is unknown.
Highstand systems tract deposits are also poorlyexposed. Part of the highstand systems tract was can-nibalized when the water level in Lake Bonneville fellcatastrophically to the Provo level. The Spanish Fork
Lemons and Chan 653
(Madsen and Currey, 1979)
Lake level, altitudes adjustedfor net isostatic rebound(Currey and Oviatt, 1985)
Provo
Stansburyoscillation
oscillation
Bonneville
shoreline
Gilbert
Bonnevilleflood
AGE (103 yr)
1015202530
4100 1250
4200
4300
4400
4500
4600
1300
1350
1400
4700
4800
4900
5000
5100
1450
1500
1550
ALTITUDE
Glacial maxima, Pinedale deposition
shoreline
shoreline
ft m
Keg Mountain
Figure 8—Reconstructed Lake Bonneville lake levels (Currey and Oviatt, 1985) with glacial maxima during Pinedaledeposition shown in shaded area. Altitudes are adjusted for isostatic rebound and faulting. At its maximum heightof 1552 m above sea level, the highstand of Lake Bonneville was slightly later than the glacial maxima duringPinedale deposition (Madsen and Currey, 1979). Lake Bonneville began to rise about 28 ka and continued totransgress until the Stansbury oscillation (22–20 ka), when it formed the Stansbury shoreline (1372 m). The lakethen continued to rise, with minor fluctuations, to its highest level (1552 m), where it formed the Bonneville shore-line sometime shortly after 15.3 ka (Oviatt et al., 1992). At the Bonneville shoreline, the lake overflowed intermit-tently near Red Rock Pass into the Snake River drainage of southeastern Idaho (Currey et al., 1984). The overflowwaters eventually caused hydraulic failure of relatively unconsolidated sediments forming the basin rim near RedRock Pass, scoured a channel down to well-indurated materials, and released the catastrophic Bonneville flood atapproximately 14.5 ka (Jarrett and Malde, 1987). This flood lowered lake level approximately 100 m to the Provoshoreline, where the lake overflowed intermittently, until the post-Provo regression (14–12 ka) ended the Bon-neville deep-lake cycle (Oviatt et al., 1992). The Great Salt Lake remains today as a much-evolved descendant of theformer Lake Bonneville.
654 Lake Bonneville DeltasT
able
9. Su
mm
ary o
f Fo
rcin
g P
aram
eter
s
Ram
p C
har
acte
rist
ics*
*Le
ngt
hA
cco
mm
od
atio
nSe
dim
ent
Del
taT
ecto
nic
s*P
hys
iogr
aph
y(k
m)
(m)
Yie
ld†
Lim
no
stas
y††
Web
erLa
te P
leis
toce
ne
and
Ho
loce
ne
slip
rat
es ~
0.9–
Sho
rt r
amp
~20
~27
0~
705
m3 /
km2 /
yrSa
me
for
all
Riv
er1.
9 m
m/y
r fo
r la
st 1
5 k.
y. (
Nel
son
an
dth
ree
del
tas
del
taP
erso
niu
s, 1
993)
; tim
e re
pre
sen
ted
in e
xp
ose
dd
elta
is ~
12–1
3 k.
y.; t
her
efo
re, m
axim
um
sub
sid
ence
wo
uld
hav
e b
een
~12
–25
m
Bea
rP
re-la
te Q
uat
ern
ary
slip
rat
es a
lon
g Ea
st C
ach
eLo
ng
ram
p~
100
~27
0C
om
par
able
to
Sam
e fo
r al
lR
iver
fau
lt z
on
e n
ear
del
ta a
re ~
0.05
–0.1
0 m
m/y
r;W
eber
Riv
er d
elta
thre
e d
elta
sd
elta
ho
wev
er, n
o a
pp
aren
t m
ove
men
t in
late
Qu
ater
nar
y (M
cCal
pin
, 198
8, 1
994;
McC
alp
inan
d F
orm
an, 1
991)
; acc
om
mo
dat
ion
sp
ace
likel
y a
fun
ctio
n o
f la
kele
vel o
nly
Span
ish
Slip
rat
es a
lon
g th
e W
asat
ch f
ault
zo
ne
nea
rSh
ort
ram
p~
17~
180
Co
mp
arab
le t
oSa
me
for
all
Fork
Span
ish
Fo
rk a
re m
ore
dif
ficu
lt t
o d
eter
min
eW
eber
Riv
er d
elta
thre
e d
elta
sd
elta
du
e to
co
mp
lex
ity
of
the
fau
lt a
s it
fo
rms
a m
ajo
rco
nca
ve-t
o-t
he-
wes
t b
end
(M
ach
ette
, 199
2);
ho
wev
er, l
ate
Ple
isto
cen
e sl
ip r
ates
of
0.5–
3.0
mm
/yr
seem
rea
son
able
(M
. Mac
het
te, 1
996,
per
son
al c
om
mu
nic
atio
n);
tim
e re
pre
sen
ted
inex
po
sed
del
ta is
~7
k.y.
; th
eref
ore
, max
imu
msu
bsi
den
ce d
uri
ng
that
tim
e w
ou
ld h
ave
bee
n
~3.
5–21
m
*Sub
side
nce
alon
g W
asat
ch o
r E
ast C
ache
faul
t zon
e.**
Ram
p le
ngth
is m
easu
red
from
mou
th o
f riv
er (
i.e.,
at th
e W
asat
ch M
ount
ains
or
One
ida
Nar
row
s) to
sho
relin
e of
pre
sent
-day
Gre
at S
alt L
ake.
Acc
omm
odat
ion
is m
easu
red
from
Bon
nevi
lle s
hore
line
(i.e.
, hig
hest
lake
leve
l) to
the
pres
ent-
day
valle
y flo
or im
med
iate
ly b
asin
war
d of
eac
h de
lta.
† Fro
m L
emon
s et
al.
(199
6).
††La
ke B
onne
ville
hyd
rogr
aph
from
Cur
rey
and
Ovi
att (
1985
).
Lemons and Chan 655
I-84
Weber River
Ogden RiverMiddle ForkWeber River
Bonnevilleshoreline
Wasatchfault
Great Salt Lake
North
89
A’
B
I-15
WasatchMountains
North ForkWeber River
South ForkWeber River
21
1 20km
km outcrop localitydriller’s log
normal fault(ball on downthrown side)
core description
Figure 9—Base map of the WeberRiver delta.
TST
LST
truncated section
TST
HST
Great Salt Lake
WasatchMountains
~ 300 m
East
~ 20 km
West
delta-front sheet sands
lacustrine clay
delta-margin, fluvial channel,and beach gravel deposits
delta-front sheet sands with interbedded clayMFS
sequenceboundary
correlativeconformity
Wasatch fault
Fielding geosol
2
1
Provoshoreline
Bonnevilleshoreline
Pre-Bonneville sediments
Figure 10—Simplified two-dimensional model of the sequence stratigraphy of the Weber River delta. TST = trans-gressive systems tract, HST = highstand systems tract, LST = lowstand systems tract, MFS = maximum flooding sur-face; 1 and 2 are parasequences.
River has a smaller drainage-basin size than the Weberand Bear rivers, resulting in a small delta. Wave erosionassociated with the Provo level of Lake Bonnevilleremoved a large portion of the highstand systemstract. Preserved highstand systems tract deposits exist
in a narrow corridor directly adjacent to the WasatchMountains and consist of delta-front sheet sands andbeach gravels. Because the boundary between thetransgressive and highstand systems tracts is notexposed, maximum thickness is unknown.
656 Lake Bonneville Deltas
NS
E
W
Bonneville shoreline
Provo shoreline
TST
HST
LST
NS
E
W
NS
E
W
lacustrine clay delta-front sheet sand
fluvial channel, delta-margin,and beach gravel deposits
delta-front sheet sandand clay
WasatchMountains
WasatchMountains
WasatchMountains
Bonnevilleshoreline
100s
m10
0s m
100s
m
10s km10s km
10s km10s km
10s km10s km
(A)
(B)
(C)
Figure 11—Block diagrams illustrating the progressive development of the sequencestratigraphy of the Weber Riverdelta. The development of theBear River delta is similar to thatof the Weber River delta, exceptthat the lowstand systems tract is spread out over a much largerarea and exists as a series of basinally isolated shorelinedeposits.
The lowstand systems tract of the Spanish Forkdelta is quite different than that of the Weber andBear River deltas. This systems tract mainly consistsof basinward-dipping foresets composed of fine-to medium-grained delta-front sheet sands cappedby f luvial gravels and loess (Figure 7C, E). Theforced regression of Lake Bonneville formed a sub-aerially exposed sequence boundary and led towidespread erosion of the transgressive and high-stand systems tract. Accommodation for theSpanish Fork delta is approximately 90 m less thanthat for the Weber and Bear River deltas; thisreduced accommodation caused reworked trans-gressive and highstand systems tract finer grainedsediments to be deposited as strongly progradation-al, wave-rippled delta-front clinoforms. Acceleratormass spectrometer radiocarbon dates of two gastro-pod shells found in these deposits indicate thereworked (18,700 ±70 yr ago, deposited during laketransgression; Beta-94278; Figure 8) and regressive(14,740 ±60 yr ago, deposited after fall of lake toProvo level; Beta-94279; Figure 8) nature of these
deposits. The oblique (vs. sigmoidal) nature of the cli-noforms indicates rapid progradation with slow aggra-dation (Helland-Hansen, 1993). Wave-ripple spacingsand grain size suggest deposition in water depths of1–2 m (Tanner, 1971; Aspler et al., 1994). Angularcontacts between basinward-dipping foresets andoverlying subhorizontal beds record at least twomajor oscillations of the post-Provo regression of LakeBonneville (Figure 7B). Reworked coarse-grainedtransgressive and highstand systems tract sediments(e.g., beach gravels) were later deposited as a veneerof fluvial gravels that thins basinward, graded to theProvo shoreline. Following deposition of the fluvialgravels, a thin veneer of loess was deposited along thecentral portions of the delta.
DISCUSSION
The forcing parameters (in the sense ofPosamentier and Allen, 1993) governing the devel-opment of depositional sequences are limnostasy,
Lemons and Chan 657
TreasuretonOneidaNarrows
Preston
Banida
Riverdale
BearRiver
34 36
0 1km 1/2
1/2km
1 N
outcrop locality driller’s log
normal fault(ball on downthrown side)
91
34
EastCachefault zone
Bonnevilleshoreline
91
8991
34
91
km
km
0 5 10
510 N
I-15
Logan
BrighamCity
Great Salt Lake
Idaho
Utah
Bonnevilleshoreline
BearRiver
36
Bea
r R
iver
Ran
ge
WasatchMountains
Preston
Bonnevilleshoreline
Bonnevilleshoreline
Whitney
Figure 12—Base map of the Bear River delta.
658 Lake Bonneville Deltas
0
1
2
21
outcrop localitydriller’s lognormal fault(ball on downthrown side)
km
km
Payson Salem
SpanishFork
Springville
I-15
50691
Bonnevilleshoreline
Bonnevilleshoreline
Bonnevilleshoreline
Wasatchfault
8950
650
SpanishForkRiver
ProvoBayUtah Lake
North
Figure 13—Simplified two-dimensional model of the sequence stratigraphy of the Bear River delta.
Figure 14—Base map of the Spanish Fork delta.
Pre-Bonneville sediments
Fielding geosol
LST
~300 m
Bear River Range
portion of delta exposednear Preston, Idaho
truncated section
Great Salt Lake
?
?
~125 km
delta-front sheet sand(with interbedded clay)
delta-frontsheet sand
lacustrine clay beach gravels
HST
TST
East Cachefault zone
sequence boundary
MFS
tectonics, sediment yield rates, and physiography(Table 9). The change in the water level of LakeBonneville was similar for all three deltas. Tectonicsubsidence ranged from maximum of 25 m to mini-mum of 0 m; however, compared to changing lakelevel, which was on the order of 300 m, subsidencewas relatively insignificant (Oviatt et al., 1994).Sediment yield rates have been determined only forthe Weber River (Lemons et al., 1996), but areassumed to be similar for all three rivers. The oneunique parameter for each delta is physiography;therefore, the variability in the stratal architectureof each delta, especially within the lowstand sys-tems tract, appears to be chief ly the result ofdifferences in ramp length and accommodation(Figure 17). This finding is consistent with the sug-gestion of Posamentier and Allen (1993) that sedi-ment flux and physiography determine the stratalarchitecture between bounding surfaces, whereaseustasy and tectonics determine the timing of the
sequence boundary surface. As the result of thecatastrophic fall of Lake Bonneville, all three deltashave synchronous sequence boundaries.
As an alternative to the traditional sequencestratigraphic model (e.g., Posamentier et al., 1988;Van Wagoner et al., 1988), the lowstand systemstract identified in these three deltas could be classi-fied as the forced regressive wedge systems tract(e.g., Hunt and Tucker, 1992; Helland-Hansen andGjelberg, 1994; Mellere and Steele, 1995). Theforced regressive wedge systems tract forms duringfalling base level and is bounded above by thesequence boundary, representing the lowestpoint in base level. After sequence boundary for-mation, the lowstand prograding wedge systemstract develops as relative base level begins to rise.Using this model, the present-day Great Salt Lakewould represent lowest base level, the sequenceboundary would be currently forming, and thepreviously defined lowstand systems tracts would
Lemons and Chan 659
?
?
Dimple Dell geosolWasatch fault
WasatchMountains
truncatedsection
sequenceboundary
~ 200 m
~ 17 km
LST
TST & HST
delta-front sheetsand (TST and HST)
fluvial andbeach gravels
delta-frontforesets (LST)
loess
UtahLake
West East
lacustrine clay ?
Figure 15—Simplified two-dimensional model of the sequence stratigraphy of the Spanish Fork delta.
be packaged into the forced regressive wedge sys-tems tract.
These lacustrine deltas can serve as analogs forhydrocarbon exploration and production in similarbasins. For all three deltas, potential hydrocarbonreservoir quality (e.g., grain size and sorting) islargely a product of drainage-basin size and streamgradient, whereas size and shape of the delta-frontdeposits is more a product of accommodation andramp length of the preexisting depositional surface(Tables 4, 6, 8). Compaction, diagenesis, and othersimilar processes also would be important withtime and burial. The relatively small drainage-basinsize and higher stream gradient of the Weber andSpanish Fork rivers provided coarser grainedsands (fine to medium grained sand) and less siltand clay than the Bear River (very fine grained
sand). As a consequence, the delta-front sheetsands of the Weber River delta (and presumablythe Spanish Fork delta) have higher permeabili-ties than similar deposits of the Bear River delta(Lemons, 1997).
Relative differences in ramp lengths influencedthickness and lateral continuity of delta-frontdeposits. The relatively short ramp length of theWeber River and Spanish Fork deltas resulted inthicker, more laterally continuous delta-frontdeposits. The relatively long ramp length of theBear River delta resulted in thin, patchy delta-front deposits (spread out over a much largerarea), especially in the lowstand systems tract.Relative accommodation differences influencedthe internal geometry of the delta-front deposits,especially in the lowstand systems tract. Lake
660 Lake Bonneville Deltas
NS
E
W
Bonneville shoreline
Provo shoreline
(B) LST
WasatchMountains
NS
E
W
WasatchMountains
(A) TST and HST
delta-front foresetsfluvial and beachgravel depositsdelta-front sheet sand
?
??
regressingshoreline
??
?
100s
m10
0s m
Dimple Dellgeosol
Dimple Dellgeosol
10s km10s km
10s km10s km
Figure 16—Block diagrams illustrating the progressive development of thesequence stratigraphy of the Spanish Fork delta.
level rise was so rapid that accommodation wasnot a significant factor until Lake Bonnevillecatastrophically fell to the Provo level. Relativelylow accommodation in the Spanish Fork deltayielded a series of strongly progradational clino-forms in the lowstand systems tract. Relativelyhigher accommodation in the Weber River deltaresulted in subhorizontal lowstand systems tractdeposits. The lowstand systems tract of the BearRiver delta contains both subhorizontal depositsand low-angle clinoforms.
CONCLUSIONS
(1) Three fine-grained deltas on the eastern marginof Lake Bonneville exhibit distinctive facies architec-ture and sequence stratigraphic packages. Significantglaciation and steep river gradients contributed to
the formation of coarse-grained Gilbert deltas alonglocally sourced rivers, whereas minor glaciation andlower river gradients contributed to contemporane-ous fine-grained deltas fed by larger rivers.
(2) The Weber River delta is a wave-influenced deltacomposed of five architectural elements: delta-frontsheet sands, delta-margin deposits, fluvial channeldeposits, beach gravels, and lacustrine clays. Thesequence stratigraphy of the Weber River delta closelyresembles that of a passive margin. The transgressivesystems tract consists of two parasequences forming aretrogradational parasequence set. The parasequencesare composed of lacustrine flooding clays overlain byprograding delta-front sheet sands. The highstand sys-tems tract was partially removed by wave erosion. Thelowstand systems tract consists partly of cannibalizedtransgressive and highstand systems tract sedimentsdeposited in the delta-front environment at the toe ofthe delta during a forced regression of the lake.
Lemons and Chan 661
Figure 17—Effects of accommodation and ramp lengthon the sequence stratigraphy of the Weber, Bear, and Spanish Forkdeltas (WRD, BRD, and SFD, respectively). Physiography (asexpressed in accommodation andramp length) is mainly responsiblefor the variability in stratal architecture of each delta, especially in the lowstand systemstract. The high accommodationand short ramp length in theWeber River delta resulted in asequence stratigraphy similar tothat of a passive margin. The highaccommodation and long ramplength of the Bear River delta led to a lowstand systems tract consisting of poorly exposed, basinally isolated patches of downstepping shoreline deposits.The low accommodation andshort ramp length of the SpanishFork delta yielded a lowstand systems tract consisting of basinward-dipping clinoforms.
accommodation highlow
long
shor
tra
mp
len
gth
BRD
WRDSFDLST
LST
LST
delta-front sheetsand (TST and HST)
fluvial andbeach gravels
delta-frontforesets (LST)
loess
delta-front sheet sandand interbedded clay
delta-front sheet sand
lacustrine clay
delta-margin, fluvial channel,and beach gravel deposits
delta-front sheet sand
delta-front sheet sand(and interbedded clay)
lacustrine clay
beach gravels
(3) The Bear River delta was influenced by bothwave and fluvial processes. This delta is composedof three architectural elements: delta-front sheetsands, beach gravels, and lacustrine clays. Thesequence stratigraphy of the Bear River delta washeavily influenced by its long, low-gradient rampgeometry. Outcrop exposure of the transgressivesystems tract shows one parasequence composedof lacustrine flooding clays overlain by progradingdelta-front sheet sands. The highstand systems tracthas similar poor exposures with basinally isolateddownstepping shoreline deposits formed during aforced regression of the lake.
(4) The Spanish Fork delta is a wave-influenceddelta composed of four architectural elements:delta-front sheet sands, beach gravels, fluvial grav-els, and loess. The sequence stratigraphy of theSpanish Fork delta was strongly affected by lack ofaccommodation and the smaller size of the delta.The transgressive systems tract is poorly exposedand partially consists of subhorizontal delta-frontsheet sands. The highstand systems tract was most-ly removed by wave erosion. The lowstand systemstract consists mainly of strongly progradational,basinward-dipping foresets formed in the delta-front environment.
(5) Limnostasy, tectonics, and sediment yieldwere similar for all three fine-grained deltas; there-fore, by process of elimination, the forcing parame-ter that exerted the greatest inf luence on thesequence stratigraphy of all three deltas appears tobe physiography. This physiographic control ismost pronounced in the lowstand systems tract ofeach delta.
(6) Potential hydrocarbon reservoir quality (e.g.,grain size and sorting) is largely a product ofdrainage-basin size and stream gradient. Relativedifferences in ramp lengths affected thickness andlateral continuity of delta-front deposits. Relativedifferences in accommodation determined theinternal geometry of the delta-front deposits, espe-cially in the lowstand systems tract.
(7) The well-documented age, limnostasy, basinphysiography, tectonics, climate, and sedimentyield of these Bonneville deposits make an excel-lent analog for understanding lacustrine subsurfacehydrocarbon reservoirs where stratal surfaces maybe preserved but forcing parameters typically areunknown.
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David R. Lemons
David R. Lemons is a geologistwith Exxon Exploration Company.He received his Ph.D. in geologyfrom the University of Utah in 1997.His work focused on the stratigra-phy, permeability distribution, andpaleoclimatic implications of fine-grained lacustrine deltas in theBonneville basin. He completed hisB.S. and M.S. degrees in geology atBaylor University in 1984 and 1987,respectively. After obtaining his M.S. degree, he workedfor Oryx Energy as a production and exploration geologist.
Marjorie Chan
Marjorie Chan received her Ph.D.from the University of Wisconsin–Madison in 1982. She has been atthe University of Utah for 15 yearsand currently is a full professor. Sheand her students have worked on awide range of clastic sedimentologyand stratigraphy problems in thewestern United States, ranging fromthe Precambrian up through thePleistocene.
ABOUT THE AUTHORS
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