Age and paleoclimatic significance of the Stansbury shoreline of Lake Bonneville, Northeastern Great Basin

QUATERNARY

RESEARCH

33, 291-305 (1990)

Age and Paleoclimatic Significance of the Stansbury Shoreline of Lake Bonneville, Northeastern Great Basin CHARLES G. OVIATT,*

DONALD R. CunnnY,t

AND DAVID M. MILLER*

*Department of Geology, Kansas State University, Manhattan, Kansas 66506; fDepartment of Geography, University of Utah, Salt Lake City, Utah 84112; and W.S. Geological Survey, M.S. 975, 345 Middlefield Road, Menlo Park, California 94025 Received January 23, 1989 The Stansbury shoreline, one of the conspicuous late Pleistocene shorelines of Lake Bonneville, consists of tufa-cemented gravel and barrier beaches within a vertical zone of about 45 m, the lower limit of which is 70 m above the modem average level of Great Salt Lake. Stratigraphic evidence at a number of localities, including new evidence from Crater Island on the west side of the Great Salt Lake Desert, shows that the Stansbury shoreline formed during the transgressive phase of late Pleistocene Lake Bonneville (sometime between about 22,008 and 20,000 yr B.P.). Tufa-cemented gravel and barrier beaches were deposited in the Stansbury shorezone during one or more fluctuations in water level with a maximum total amplitude of 45 m. We refer to the fluctuations as the Stansbury oscillation. The Stansbury oscillation cannot have been caused by basin-hypsometric factors, such as stabilization of lake level at an external overtlow threshold or by expansion into an interior subbasin, or by changes in drainage basin size. Therefore, changes in climate must have caused the lake level to reverse its general rise, to drop about 45 m in altitude (reducing its surface area by about 18%, 5000 km2), and later to resume its rise. If the sizes of Great Basin lakes are controlled by the mean position of storm tracks and the jetstream, which as recently postulated may be controlled by the size of the continental ice sheets, the Stansbury oscillation may have been caused by a shift in the jetstream during a major interstade of the Laurentide ice sheet. Q1990 University

of Washington.

INTRODUCTION

help to define the Stansbury problem. These are reviewed below. Although the Stansbury shoreline can be mapped throughout the Great Salt Lake basin at altitudes between about 1347 and 1378 m (Fig. l), it is poorly preserved in many places (Gilbert, 1890; Currey, 1980, 1982). This lack of clear definition on the landscape led some workers to suggest that the Stansbury shoreline dated from an earlier lake cycle (Antevs, 1948; Marsell and Jones, 1955). The Stansbury shoreline ranks second to the Provo shoreline (Table 1) in volume of tufa (Gilbert, 1890, p. 134). Currey (1980, pp. 75-76) noted that tufa-cemented gravel and tufa-encrusted bedrock at the Stansbury shoreline are common on steep rocky headlands, whereas larger depositional features, such as barrier beaches, are more common in bays. Tufa-cemented gravel is

G. K. Gilbert (1890) recognized the Stansbury shoreline of Lake Bonneville’ as one of the prominent geomorphic features of the lake basin, but it was not until after concluding his field work that he realized its importance in Lake Bonneville history (Gilbert, 1890, p. 187). Gilbert posed an unsolved question of the cause of a stillstand at the Stansbury shoreline, a question referred to by Eardley et al. (1957) as the “Stansbury problem.” The Stansbury shoreline is one of the four prominent shorelines of Lake Bonneville (Table l), and it has several unique characteristics compared to the other major shorelines that ’ In this paper we refer to the last major lake cycle in the Bonneville basin as “Lake Bonneville” following the usage of Oviatt and Currey (1987). 291

0033-5894190 $3.00 Copyright All rights

0 1990 by the University of Washington. of reproduction in any form reserved.

292

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MILLER

1. MAJOR SHORELINES OF LAKE BONNEVILLE

Shoreline

Altitude (4

Approximate age (lo3 yr)

Stansbury

1347-1378

Bonneville Provo Gilbert

1.552-1626 1444-1503 1311-1293

Sometime between 22 and 20 -15.5-14.5 -14.5-14.2 -11-10

LIFrom Currey (1980) and Currey (1982); shoreline altitudes are reported as ranges because the shorelines have been isostatically deformed.

thickest and most common near the lower altitudinal limit of the Stansbury shorezone, and barrier beaches are most common in the upper parts of the zone (Currey, 1980, p. 75). The Stansbury shoreline does not coincide in altitude with an external threshold or with any internal threshold between subbasins of the Bonneville basin (Eardley er al., 1957, pp. 1161-1163). Therefore, the Stansbury shoreline did not form as the result of lake-level stabilization due to overflow or to a disproportionately rapid increase in surface area with expansion into a subbasin. The lake-level stabilization that resulted in the Stansbury shoreline had to be caused by some other hydrologic factor, probably climate (Eardley et al., 1957). With the few exceptions noted above, the Stansbury shoreline was regarded for many years as a regressive-phase shoreline of Lake Bonneville, formed after the development of the Provo shoreline late in the history of the lake (Ives, 1951; Gvosdetsky and Hawkes, 1956; Eardley et al., 1957; Broecker and Orr, 1958; Morrison, 1965a, 1966; Goode, 1975). The interpretation that the Stansbury shoreline is post-Provo in age was attractive because the Stansbury is approximately midway in altitude between the Provo shoreline and 20th century levels of Great Salt Lake. Gilbert (1890, pp. 134135, 167, 186187) was vague concerning the relative age of the shoreline, but one of his statements suggests that he thought it was post-Provo in age (Gilbert, 1890, p.

FIG. 1. Map of Lake Bonneville showing the Stansbury shoreline (heavy line), Bonneville shoreline (light line), and localities discussed in the text. Stippled areas are above the Bonneville shoreline. CI, Crater Island; BRC, Bear River City; SI, Stansbury Island; ORB, Old River Bed; FSF, Fish Springs Flat; SP, collection site of cores containing Stansbury (?) mirabilite along Southern Pacific causeway; GSL, Great Salt Lake; UL, Utah Lake; SL, Sevier Lake.

135). Gilbert’s field notes record only one locality where he observed stratigraphic relationships at the Stansbury shoreline, but he did not interpret those observations in his notes (Hunt, 1982, p. 54). The most recent assertions that the Stansbury shoreline formed during the regressive phase are by

LAKE BONNEVILLE

Hunt (1986, p. 122; 1987, p. 16). Hunt (1982, p. 5) also suggests that the Stansbury shoreline is “a convenient field marker for the Pleistocene-Holocene boundary.” Recent observations at the type locality of the Stansbury shoreline on Stansbury Island (Fig. 1; Currey et al., 1983a, 1983b; Green and Currey, 1988) do not support the hypothesis that the shoreline formed during the regressive phase. Instead, stratigraphic and geomorphic relationships suggest that the Stansbury shoreline formed during the transgressive phase of Lake Bonneville. Based on field studies of stratigraphic sections and shorelines primarily at Stansbury Island and the Old River Bed, but also at other localities, the Stansbury oscillation is interpreted as one or more changes in lake level with a maximum altitudinal range of 45 m (Oviatt, 1984, 1987; Currey and Oviatt, 1985). Spencer (1983) and Spencer et al. (1984) noted geochemical and mineralogical changes in Lake Bonneville sediments in cores from Great Salt Lake that suggested to them that Lake Bonneville went through a series of low-amplitude fluctuations during its transgressive phase

SHORELINE

293

at about the level of the Stansbury shoreline. Recently obtained radiocarbon ages from one of the cores indicate that the geochemical changes noted by Spencer et al. (1984) are younger than the Stansbury shoreline, although other data from the cores (discussed below) are now interpreted as evidence for the Stansbury oscillation (R. M. Forester, personal communication, 1989). In this paper, we present new data on the geomorphology, stratigraphic position, and age of the Stansbury shoreline. In addition, we offer a paleoclimatic hypothesis to explain the Stansbury oscillation. CRATER ISLAND LOCALITY

Crater Island is the northern extension of the Silver Island Mountains on the western margin of the Great Salt Lake Desert (Fig. 1). Crater Island is a low mountain range surrounded on three sides by extensive alluvial plains and playas. The Bonneville, Provo, Stansbury, and Gilbert shorelines are well formed at Crater Island (Fig. 2; Currey, 1982; Miller, in press; Miller et al., in press). Below we describe exposures of

FIG. 2. Map of the Crater Island locality (T. 5 N., R. 17 W., sec. 34). Profiles AA’ and BB’ are shown in Fig. 3. CI, Crater Island; SIM, Silver Island Mountains; PR, Pilot Range; NM, Newfoundland Mountains; GSLD, Great Salt Lake Desert; PVP, Pilot Valley Playa; Qla, Quatemary lacustrine and alluvial deposits; S, Stansbury shoreline; P, Provo shoreline, which appears double because it loops around both sides of a narrow ridge. Stippled areas are underlain by Paleozoic bedrock. Locations of larger-scale map areas of this figure are shown in the inset maps in the lower left comer.

294

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deposits in the Stansbury shorezone along two ephemeral washes on the northwest end of Crater Island (Figs. 2 and 3). Some of these exposures were noted by W. L. Anderson (in Schaeffer, 1960). The washes drain arid, steep slopes underlain by Paleozoic carbonate rocks and sandstone that are locally mantled by alluvium, talus, and lacustrine gravel. Description The main features of the profiles exposed along the washes at Crater Island are summarized in Fig. 3. The profiles were constructed from measured sections of wash exposures tied together by hand-level and rod surveys. Profile AA’ includes exposures with two marl units separated by gravel, strongly indicating an oscillation in lake level. Profile BB’ includes lesscomplete exposures, but allows better definition of the altitudinal range of the oscillation. Two marl stratigraphic units, lower

marl (LM) and upper marl (UM), are separated by a lacustrine gravel unit, which we call the middle gravel (MG). The stratigraphic units in the two sections are correlated based on lithology, similarity of stratigrallhic sequence, and similarity of weathering and geomorphic expression, especially of the marl units. Although correlation errors between the sections are possible, we consider them unlikely. We interpret the two marl units as representing offshore and relatively deep-water deposition, and the gravel units as representing shorezone deposition. Therefore, the sequences at Crater Island indicate that the lake initially transgressed or rose to a relatively high level and deposited the lower marl, then it dropped and deposited the middle gravel. The lake subsequently transgressed again, deposited the upper marl, and finally dropped to very low levels. The evidence for this is described in detail below. We consider the stratigraphic

A

NIS

5 a

i I

6’

FIG. 3. Profiles AA’ and BB’ at the Crater Island locality. See Fig. 2 for locations. See text for descriptions of lithologic units. LG, lower gravel; MG, middle gravel; UG, upper gravel; TG, transgressive graveI; BF, Bonneville flood marker bed. Vertical dashed lines locate changes between north and south sides of washes. Localities numbered in A and B are discussed in the text.

LAKE

BONNEVILLE

sections described here to belong to a single lake cycle because there is no evidence for major unconformities between the lower marl and the middle gravel, or between the middle gravel and the upper marl. In addition, there are no buried weathering profiles in the sequence. The lower marl (LM) overlies lacustrine gravel (lower gravel LG), and in most places consists of basal marly sand that grades upward into laminated marl containing fine plant-stem or algal-filament impressions. In complete sections, the laminated marl grades upward into marly sand, which is overlain abruptly by lacustrine gravel. Thin sand or gravel lenses are locally interbedded within the lower marl. In profile BB’, the lower marl contains pebbles that we interpret as dropstones at locality 1 (Fig. 3B). The lower marl in section BB’ grades laterally upslope from laminated marl (locality 1) into sandy marl at higher exposures (locality 2), and then into marly sand where it pinches out at its upper end (locality 3) between two gravel units, the lower and middle gravels (LG and MG). The upper (higher) altitudinal limit of the lower marl in the two sections is about 1368 m. Because the lower marl is not found higher than this, and because of the lateral facies change toward higher elastic content offshore from the large gravel barrier (LG, between 3 and 4; Fig. 3B), we interpret the pinch-out of the lower marl at 3 (Fig. 3B) to be close to its original upper (altitudinal) depositional limit. The upper marl (UM; Fig. 3) is discontinuously exposed along both washes. Its lower part consists of silty to sandy marl with abundant ostracodes, and its upper part consists of sandy marl or calcareous sand (Fig. 3B) with gastropods (Lymnaea, Amnicofa). The upper marl has been eroded from the steep slopes in much of the area, but is preserved locally beneath a cover of alluvial and/or lacustrine gravel. It can be traced to the Provo shoreline in this area where it inter-fingers with Provo-age deposits. Above the Provo shoreline no marl is preserved on the steep slopes. A

SHORELINE

295

thin sandy bed, which contains abundant ostracodes and which we interpret as a marker bed indicating the Bonneville flood (Oviatt, 1987), can be identified locally within the upper marl. These relationships show that the upper marl was deposited during the deepest-water phase of Lake Bonneville. Below and above the lower marl are lacustrine gravel units referred to here as the lower gravel (LG) and middle gravel (MG). Both of these units consist of well-rounded pebbles and cobbles of locally derived Paleozoic sedimentary rocks and minor Jurassic igneous rocks. At locality 1 in section AA’ (Fig. 3A), the middle gravel consists of subrounded to subangular cobbles and boulders derived from the adjacent steep slope. Elsewhere, both the lower and middle gravels contain generally smaller, morerounded clasts, and they are locally thickened into buried barrier-beach ridges. Two barrier-beach ridges in the lower gravel are apparent in section BB’. The crest of the lower barrier beach (between 1 and 2; Fig. 3B) is at an altitude of about 1366 m, and the truncated crest of the higher barrier (locality 5; Fig. 3B) is at about 1383 m (the lower gravel shown at locality 5 in Figure 3B is part of the same large barrier as at 4, but is on the south side of the wash). Both barrier beaches once enclosed lagoons, but the higher lagoon was much deeper; its fill of the upper marl and calcareous sand is partly preserved. The upper marl in the lagoon just east of locality 4 (Fig. 3B), which can be traced upstream along the wash to the Provo shoreline, lies conformably on the 43”-dipping shoreward face of the tufa-cemented gravel barrier. The lower lagoon-fill (at locality 2; Fig. 3B) includes the lower marl and the middle gravel. The middle gravel represents a drop in lake level following deposition of the lower marl. A single barrier-beach deposit in the middle gravel in section AA’ has a crestal altitude of about 1370 m and a lagoon filled with the upper marl (Fig. 3A; MG at localities 2 and 3 represents the same barrier on

296

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two sides of the wash). The Bonnevilleflood marker bed is exposed in the upper marl in this lagoon (Fig. 3A, locality 4). Of special note in the middle gravel is its tufa cement, which is especially abundant and well indurated near the lower altitudinal limit of the gravel (Fig. 3A, locality 1; Fig. 3B, locality 1). It is the middle gravel and its tufa cement that forms the prominent Stansbury shoreline on the adjacent mountain flanks between altitudes of about 1359 and 1365 m (Table 2). A gravel unit overlying the lower gravel in the higher parts of section BB’ is referred to here as “transgressive” gravel (TG, locality 5; Fig. 3B). It lies disconformably above the lower gravel higher than the upper limit of the middle gravel, and can be traced to altitudes higher than section BB’ where it overlies tufa-encrusted bedrock and is overlain by the upper marl. We interpret this “transgressive” gravel to represent the beginning of the second transgression to altitudes above the Stansbury shorezone. The upper gravel (UG) in the sections is relatively poorly sorted, poorly bedded, and generally contains more angular clasts than do the lower and middle gravels. It also contains some rounded clasts of tufa and Paleozoic rocks that apparently have been reworked from older lacustrine deposits. We interpret the upper gravel as largely post-Bonneville alluvium, although part of it could be lacustrine gravel that represents the final regression of the lake. TABLE

The bedforms and clasts of the middle gravel may provide clues to the regressiontransgression cycle, although local geomorphic factors may have strongly affected the gravel deposition. In profile AA’ (only partly shown in Fig. 3A, locality 2), the lower marl is in some places overlain by sand, which in turn is abruptly overlain by gravel that can be divided into three parts. Cross strata in the basal fine gravel bed (1) dip gently, in contrast to steeper cross strata in the thick overlying coarse gravel (2). The coarse gravel is composed of boulder beds with interbedded cobbles and pebbles; clasts are heterolithic, requiring at this site an alluvial source. Overlying the coarse gravel is a zone of monolithic pebbles and small cobbles (3), cemented by abundant tufa. This upper interval of the middle gravel is capped by tufa heads and by the upper marl. In profile BB’ (near locality 1; Fig. 3B), the middle gravel consists of a lower cross-stratified gravel that truncates underlying marl beds, and an upper, tufacemented cobble gravel that overlies the cross-stratified gravel and truncates it with an angular contact. We interpret the coarsening-upward sequence in the middle gravel, from lower marl to fine gravel to coarse gravel, as the result of lake regression. The gravelto-marl sequence at the top of the middle gravel must broadly represent the subsequent transgression. The change from regression to transgression is marked by the gravel/upper-marl boundary, where tufa de-

2. ALTITUDINAL

LIMITS~ OF THE STANSBURY OSCILLATION DETERMINED GEOMORPHICBTRATIGRAPHIC EVIDENCE AT FOUR LOCALITIES

Locality

Maximumb

Minimum’

Water-level change

Stansbury Island Crater Island Old River Bed Fish Springs Flat

>1365 21383 -1385 -1387

-1361 Gl353 -1340 -1340

>4 330 -45 -47

FROM

Bonneville shoreline” 1615 1600 1591 1591

D All values in meters are above sea level; except, values for water-level change are in meters. b Local altitude of the upper limit of the fast transgression. c Local altitude of the lower limit of the Stansbury regression(s). d Local altitude of the Bonneville shoreline (after Currey, 1982). e Local altitudinal range of tufa-cemented gravel forming the Stansbury shoreline.

Stansbury shoreline’ 1363-1372 1359-1365 135e-1360 1358-1365

LAKE

BONNEVILLE

posited in the shorezone was apparently rapidly submerged by a rising lake level and was buried by marl. The lo-m-high barrier (Fig. 3A, localities 2 and 3) built of reworked alluvial materials during the regression may indicate a relatively slow or oscillatory decline in water level, as compared to the subsequent transgression through this interval during which little or no elastic material was deposited. Interpretation of the Crater Island Sections We interpret the two sections at Crater Island as follows. The lower gravel represents a lake transgression to an altitude of at least 1383 m, which is the truncated crest of the higher barrier-beach ridge (locality 4, Fig. 3B). This first transgression could have exceeded 1383 m, but we have no direct evidence for its maximum extent at this locality. During this transgression, the lower marl was deposited in offshore to neat-shore environments up to at least 1368 m, or a minimum of I5 m below the crest of the higher barrier. The middle gravel represents a drop in lake level from its previous high of 2 1383 m to at least 1353 m, the lowest extent of tufaencrusted gravel. The lake could have dropped lower than 1353 m, but we have no direct evidence that it did so. Evidence at other localities (Table 2) indicates the total range of the oscillation was closer to 45 m. We interpret the tufa that cements the middle gravel as precipitates, which probably involved the growth of algae, in the shorezone of the closed-basin lake that was becoming more concentrated in dissolved solids as it desiccated (Oviatt, 1987). Only a slight concentration of dissolved solids would be necessary to encourage tufa growth, as suggested by the observation that Lake Bonneville precipitated at least some tufa at almost all levels, and the greatest volume of tufa was laid down at the Provo shoreline during a period of overflow. Following deposition of the middle gravel and tufa, the lake transgressed again. This

297

SHORELINE

second transgression may have been more rapid than the first because no new barrier beaches were formed in the Stansbury shorezone. The gravel unit labeled TG in section BB’ was deposited over and behind the higher barrier (LG) during the second transgression. The upper limit of the second transgression is beyond the limit of our sections, but by tracing the upper marl upslope to the Provo shoreline, and by identifying the Bonneville-flood marker bed in the upper marl, we suggest that the lake transgressed all the way to the Bonneville shoreline (the highest level of Lake Bonneville). Thus, the upper marl was deposited during the second transgression and during the development of the Bonneville and Provo shorelines. The upper gravel (UG) represents the final regression of the lake and deposition of Holocene alluvium. SYNTHESIS

Regional

Evidence

The Crater Island locality is one of several in the Bonneville basin at which stratigraphic evidence for the relative age of the Stansbury shoreline is exposed (Table 2). Two other localities described in the literature are Stansbury Island (Currey et al., 1983b; Green and Currey, 1988) and the Old River Bed (Oviatt, 1984, 1987). At Stansbury Island (Fig. 1) two marl units, equivalent to the lower and upper marls at Crater Island, are separated by a coarse-gravel wedge at the tufa-cemented Stansbury shoreline. A radiocarbon age of 20,710 ? 310 yr B.P. (number 8, Table 3) on shells collected at the lower end of the gravel wedge indicates the age of the low point of the Stansbury regression. At the Old River Bed (Fig. l), the Stansbury oscillation is represented by fluvial entrenchment of the fine-grained delta or underflow fan of the paleo-Sevier River (Oviatt, 1984, 1987). Altitudinal limits of the Stansbury oscillation at the Old River Bed are determined by tracing the unconformities within the underflow-fan deposits (yellow clay). A third area in which exposures in Stans-

B-5038

W-4898

W-5269

B-27190

W-4897

L-775N

B-5566

W-816

SI-4124

B-21810 ETH-3 114 B-21809 w-941

B-18271

L-6725

w-4421

W-5272

W-5326

2

3

4

5

6

I

8

9

10

11

14

15

16

17

18

12 13

B-8343

Lab number

1

Number (Fig. 4)

3.

19,620 ” 300

19,700 2 300

19.700 2 200

19,800 2 400

19,920 + 230

20,370 t 260 20.300 2 300

20,390 * 240

20,500 2 200

20,600 2 500

20,710 2 310’

20,800 + 300

20,908 k 250

20,930 r 230

22,300 f 400

22,500 2 300

23,190 + 1360’

24,870 f 4106

Sample material

AGES

RELATED

wood

shells

shells

Wood

Picea

wood

Snail shells

Amnicola

Tufa Wood

Wood

Wood

Wood

Amnicola

Two small branches of wood Laminated algae-rich sediments Abies or Juniperus wood Wood

Picea

Aragonitic marl (< 10 pm fraction) Sphaerium shells

RADIOCARBON

Radiocarbon age (yr B.P.)

TABLE Stratigraphic setting

STANSBURY

OSCILLATION

Post-Stansbury transgressive shorezone Post-Stansbury transgressive shorezone Transgressive lagoon/bar complex Transgressive lagoon/bar complex Transgressive lagoon/bar complex

Stansbury tufa caprock Transgressive shorezone

Transgressive lagoon/bar complex Stansbury deltaic silt

Lower end of Stansbury gravel wedge Transgressive shorezone

Transgressive lagoon/bar complex Transgressive shorezone

Transgressive offshore zone Transgressive nearshore zone Transgressive gravel-bar complex Lagoonal mud at Stansbury shoreline Deep-water muds

TO THE

M

M

E

K

N

J, G

F

F

I

H

D

G

E

F

M

E

B, C, D

A

Reference”

(PLOTTED

4) Location

CPC gravel pit, north Salt Lake, Utah CPC gravel pit, north Salt Lake, utah CPC gravel pit, north Salt Lake, Utah

Moffat Bros. gravel pit, Keams, Utah CPC gravel pit, north Salt Lake, Utah Cutbank along Bear River, Bear River City, Utah Gravel pit north of Jordan Narrows, Utah Little Valley, Promontory Mountains, Utah Stansbury Gulch, south end Stansbury Island Little Valley, Promontory Mountains, Utah CPC gravel pit, north Salt Lake, Utah Excavation on Capitol grounds, Salt Lake City Inactive borrow pit, Sandy, Utah Little Valley, Promontory Mountains, Utah Borrow pit near north shore of Sevier Lake Old River Bed at the Shutoff

Stansbury Gulch, south end Stansbury Island Old River Bed near Shutoff

IN FIG.

s

LAKE

cl cr.7 L

E

w

2

F;

M

8

BONNEVILLE

SHORELINE

299

bury shorezone deposits are known is Fish Springs Flat (Fig. 1; Oviatt, 1989b). Evidence for the Stansbury oscillation in this area consists of tufa caprock, thick deposits of carbonate-coated sand and charophyte debris, gravel barrier beaches, and gravel wedges stratigraphically between marl units. The lower altitudinal limit of the Stansbury oscillation in this area is estimated by the altitude of the lower end of a wedge of carbonate-coated sand where it pinches out and interfingers with overlying and underlying marl units. The upper limit of the Stansbury oscillation is estimated by the altitude of a barrier beach that is overlain disconformably by carbonate-coated sand that grades upward into marl. The carbonate-coated sand grades laterally into tufa-cemented gravel and deposits of charophyte debris. The four localities listed in Table 2 display similar stratigraphic sequences. However, the locally observed altitudinal limits of the components of the Stansbury oscillation differ for four reasons: (1) a complete sedimentary record is not exposed through the entire altitudinal range of the Stansbury oscillation at any of the localities; (2) it is difficult to determine precise water levels from observations of stratigraphic sections and inferences about depositional environments; (3) the depositional environments, or geomorphology, were different at each locality, and the depositional processes at each locality changed through time as the water level changed; and (4) the relative altitudes of the different localities were affected by differential isostatic rebound during and following the removal of the Lake Bonneville water load. For example, the altitude of the Stansbury oscillation minimum varies directly with the local altitude of the Bonneville shoreline (Table 2); the localities close to the center of the basin rebounded more than peripheral localities. Figure 4 synthesizes our current knowledge of the Stansbury oscillation. The data come from the four localities in Table 2 and from other localities where reconnaissance

300

OVIATT,

CURREY,

AND

MILLER

16 IO

-

7G3 -a-e-ez

‘-

IS

-ii?-

1400-

: 2

; : 0

3 -pi-

2_

,” y1 : 2 ‘35o I

-

I I. “,. ‘, ,,nc 11 ..:I . . 11 II1 :II I I B--

I5

: -

20

7-4 11 I -a-- I I I Jq B

A

1300 -

1

I

I

25

24

23

5 -c 22 AGE

(IO’yrB

21

I

1

20

19

IS

P)

FIG. 4. Summary hydrograph showing the early transgressive phase of Lake Bonneville and the Stansbury oscillation. Solid circles are radiocarbon ages on organic carbon; open circles are radiocarbon ages on carbonate carbon (see Table 3). The horizontal bars represent one standard deviation statistical errors. A, informally named “Thiokol” basaltic ash (Oviatt and Nash, 1989), which is exposed as high as the altitude shown (its age is not precisely known, but is close to 25,000 yr B.P.); B, time-altitude rectangle, during which the Stansbury oscillation probably occurred; C, possible secondary fluctuation during Stansbury oscillation as suggested by Oviatt (1987). Altitudes are adjusted for isostatic rebound; radiocarbon ages are plotted at their estimated depositional altitudes (see

text).

studies have been conducted. Figure 4 is constructed as follows. Radiocarbon ages (Table 3) are plotted at their calculated depositional altitude using the empirical formula of Currey and Oviatt (1985). The lakelevel changes directly associated with the Stansbury oscillation itself are plotted relative to the depositional altitude of radiocarbon sample p-5566 (Table 3, number 8), which represents the Stansbury oscillation minimum at Stansbury Island. The upper limit of the first transgression (as determined at the Old River Bed and Fish Springs Flat) is taken as 45 m above the depositional altitude of p-5566. Radiocarbon ages 15 and 20, from deposits that represent the post-Stansbury transgression at about the level of the upper limit of the first

transgression (as determined from stratigraphic evidence), are plotted at that altitude (1383 m) rather than at their calculated depositional altitudes (1367 and 1371 m, respectively). This is done because the empirical formula for calculating depositional altitudes (Currey and Oviatt, 1985) cannot be used to predict both the upper and lower limits of the oscillation at one locality. Although the altitudinal limits of the Stansbury oscillation are fairly well known from stratigraphic and geomorphic evidence, the precise timing still needs to be refined. Therefore, in Fig. 4 a rectangle is drawn with the upper and lower altitudinal limits of the Stansbury oscillation as solid lines, and the older and younger limits as currently understood as dashed lines (la-

LAKE

BONNEVILLE

beled B in Fig. 4). The Stansbury oscillation probably occurred within the limits of this time-altitude rectangle. A secondary transgression during the Stansbury oscillation is also plotted in Fig. 4 (dashed line labeled C). Stratigraphic evidence for this secondary transgression comes from the Old River Bed (Oviatt, 1987), but has not been found at the other localities. The Old River Bed locality may have been geomorphically more sensitive to rapid water-level changes because it was in a deltaic setting (Oviatt, 1987). The other localities are in wave-dominated, steep shorezone settings in which several short-duration transgressions and regressions could have taken place and not be detected in the coarse gravel facies. To express our uncertainty, the secondary transgression is indicated with a dashed line. In Fig. 4, radiocarbon ages of wood samples are generally interpreted as maximumlimiting ages of the transgression, and ages of shell and tufa samples are generally interpreted as minimum-limiting ages. However, because of uncertainties both in the ages (largely due to poorly evaluated contamination problems) and in stratigraphic interpretations, the lake-level curve cannot be drawn with a high degree of precision. Instead it is shown as a broad band with diffise boundaries. We show one possible interpretation of the timing of the Stansbury oscillation within the time-altitude rectangle in Fig. 4. Between 1400 and 1420 m there is a cluster of radiocarbon ages on wood associated with pre-Bonneville soils or with transgressive-phase shorezone deposits. We have drawn the transgression of the lake as younger than most of these ages. However, another possible interpretation would show the Stansbury oscillation as slightly earlier and a smaller fluctuation or fluctuations unrelated to the Stansbury between about 1400 and 1420 m, and 21,000 and 20,000 yr B.P., as was shown in Currey and Oviatt (1985, Fig. 2). In addition, other small-scale fluctuations, on the order of 10 to 20 m, are

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probable along the lake-level curve, but at present there is no well-documented stratigraphic evidence for any of these. Offshore sediments believed to have been deposited during the Stansbury oscillation have been identified at several localities. Radiocarbon age 5 (Fig. 4 and Table 3) is on organic-rich sediment collected along the Bear River at Bear River City, Utah (Fig. 1). The sample consisted of pureblack laminated muds containing abundant well-preserved algal filaments identified as Cladophora (?) sp. (J. P. Bradbury, personal communication, 1988). The laminated algal muds contain ostracodes that allow correlation (R. M. Forester, personal communication, 1984, 1988) with the sediment cores of Spencer (1983) and Spencer et al. (1984). Stratigraphic correlations based on ostracodes, a radiocarbon age, and the “Thiokol” basaltic ash (Oviatt and Nash, 1989) collected from another exposure about 2 km upstream along the Bear River (Oviatt, 1986) suggest that the laminated algal muds were deposited during the Stansbury oscillation. We suggest that during the Stansbury oscillation, the lake waters became enriched in nutrients and other dissolved solids, thus causing a trend toward eutrophication. Algae would have grown prolifically in the shallow lake water near shore, and floated around the lake as dense mats. Filaments from the mats would have settled to the bottom, mixed with fineelastic sediment, and formed laminated sediment. Cladophora glomerata has been observed in free-floating masses in the Great Lakes (Herbst, 1969). An identical algal-laminated unit containing a similar ostracode fauna was encountered in cores from Great Salt Lake (unit IIIe of Spencer et al., 1984), and is now interpreted as having been deposited during the Stansbury oscillation (R. M. Forester, personal communication, 1989). An accelerator radiocarbon age of 21,050 t 320 yr B.P. on the unit-IIIe algal-laminated sediment from core C of Spencer et al. (1984) has been obtained by R. S. Thomp-

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son (personal communication, 1989) and is the Bonneville basin is accompanied by a statistically identical (within one a) to our surface-area change of about 5000 km2 and a volume change of about 1000 km3, which age No. 5 (Fig. 4 and Table 3). An alternaare 18 and 50% changes, respectively. Postive interpretation of the algal-laminated sediment is that the benthic Cladophora sible causes for surface-area and volume lived in the photic zone on the lake bottom changes of these magnitudes are (1) a during the low stand of the Stansbury os- change in the size or shape of the lake’s cillation. However, our data on the minidrainage basin; (2) expansion of the lake mum depth of the lake during the oscillation into a subbasin or outflow to an external do not support this interpretation. At the drainage followed by downcutting of the lowest point of the oscillation the water threshold; or (3) a change in one or more would have been about 100 m deep at the climatic/hydrologic variables. The first poslocation of core C, and it seems unlikely sible cause can be eliminated because there that benthic algae would have grown at this were no known river diversions in the Bonneville basin between 22,000 and 20,000 yr depth. Offshore sediments apparently associ- B.P., and because tectonic changes in the ated with the Stansbury oscillation have size or shape of the basin during this time also been observed about 37 m below the interval would have been slight. The secGreat Salt Lake sediment-water interface ond possible cause has been rejected by in boreholes drilled in the area of deepest Eardley er al. (1957) on the basis of basin hypsometry, and we agree with their conwater along the Southern Pacific causeway, about 10 km west of Promontory Point (SP, clusions. Therefore, a change in climate is Fig. 1). In these cores, a 1.3-m-thick bed of the most likely cause of the Stansbury osmirabilite (sodium sulfate), which Currey cillation. In a closed-basin lake with a stable water (1988, Table 1) refers to as salt horizon 3, level, a delicate balance exists between the has an estimated age of 20,900 yr B.P. input of water by precipitation and the outbased on radiocarbon ages of carbonate carbon and organic carbon from underlying put by evaporation (e.g., Benson and and overlying units. Salt horizon 3 may rep- Thompson, 1987a). Changes in the balance resent saturation of hypolimnion deep brine are reflected in volume changes, which are with respect to sodium sulfate during one or recorded as changes in water level. Theremore significant regressions of the Stans- fore, some major shift in an external clibury oscillation. To produce a saturated matic variable probably caused Lake Bondeep brine in the concentrated water, the neville to stop its rise and to drop about 45 mirabilite probably crystallized during the m sometime between 22,000 and 20,000 winter at the lake surface, where temperayr B.P. tures would have been lowest; the crystals Antevs (1948), Scott et al. (1983), Benson would have then settled to the lake bottom and Thompson (1987b), and Lao and Benand been preserved by causing saturation son (1988) have suggested that the levels of of the bottom-most brine (G. I. Smith, per- Great Basin lakes are directly associated sonal communication, 1989). with the mean positions of storm tracks, which are related to the position of the jetDiscussion stream over North America. COHMAP If our interpretations shown in Fig. 4 are Members (1988) and Lao and Benson (1988) correct, the Stansbury oscillation involved presented evidence that the position of the water-level changes with a maximum am- jetstream in ice-age North America was plitude of about 45 m sometime between controlled by the height and extent of the about 22,000 and 20,000 yr B.P. A water- continental ice sheets. The major late Pleislevel change of about 45 m at this level in tocene lake cycles in the Bonneville basin

LAKE BONNEVILLE

seem to correlate in a general way with changes in global ice volume (Scott et al., 1983; Oviatt et al., 1987), although the lake maxima lagged somewhat behind the glacier maxima (Scott et al., 1983). There is some suggestion that older lake cycles in the Bonneville basin may also have been correlated with major glaciations (Oviatt and Currey, 1987), although the middle and early Pleistocene pattern at Searles Lake, California, is more complex (Smith, 1984). If the fluctuations of continental ice sheets and resulting changes in the position of the jetstream control the timing and magnitude of major lake cycles, they may also control smaller-scale lake-level changes, such as the Stansbury oscillation. If so, the record from Lake Bonneville should show some similarity to the record of advances and retreats of the Laurentide ice sheet. One possible correlative event is the Connersville interstade of the Laurentide ice sheet, which occurred between about 21,000 and 20,000 yr B.P. (Mickelson et al., 1983, pp. 17-18). Although the Connersville interstade, as its age is presently known, falls within the estimated age range of the Stansbury oscillation, our data are insufficient to determine whether the two were causally linked. The orbitally tuned oxygen-isotope data of Martinson et al. (1987, Table 2) suggest that global ice volume decreased significantly between about 21,940 and 20,860 yr B.P., a period which is within the estimated age of the Stansbury oscillation. However, we point this out with caution because the stacked, normalized, and orbitally tuned oxygen-isotope data from deep-sea sediments are not directly comparable to the Lake Bonneville evidence. CONCLUSIONS

Stratigraphic and geochronologic evidence demonstrates that the Stansbury shoreline was formed during the early transgressive phase of Lake Bonneville. Although the lake dropped through the Stansbury shorezone during its final regression, we are aware of no evidence that the prom-

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inent Stansbury shoreline formed during the regressive phase, as has been traditionally believed. Nor is there any evidence that it formed during a previous lake cycle. The Stansbury shoreline formed in response to a change in climate, the significance of which is still unknown, but which may be linked to a change in global atmospheric circulation in response to fluctuations of Northern Hemisphere ice sheets. Recognition of the Stansbury oscillation and its geomorphic and stratigraphic record is important in studies of basin sedimentation, isostatic rebound, and paleo-hydrochemistry. Finally, the geomorphic and physical stratigraphic records of Bonneville and other closed-basin lakes may provide clues to important paleoenvironmental changes that may not be detected in other less-sensitive records. ACKNOWLEDGMENTS This research was partially funded by the Utah Geological and Mineral Survey, the U.S. Geological Survey, and NASA Contract NASS-28753. Acknowledgment is also made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. Ted Burr, Rose Ditley, Doug Ekart, Susan Green, and Bill Mulvey helped in the field. We are grateful to Platt Bradbury, who helped identify a sample of algae, to Rick Forester for ostracode identifications and helpful discussions, to Bob Thompson for sharing unpublished data, and to Bruce Bills and David McConnell for helpful comments on an earlier draft of the manuscript. We thank R. M. Forester, W. E. Scott, and G. I. Smith for critical and constructive reviews of the manuscript.

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