New insights into the outlet conditions of late Pleistocene Lake Bonneville, southeastern Idaho, USA Susanne U. Janecke* and Robert Q. Oaks, Jr.* Department of Geology, 4505 Old Main Hill, Utah State University, Logan, Utah 84322-4505, USA ABSTRACT variations in energy (Chan and Archer, 2003). and continuing for a time as it evaporated to its The great Bonneville fl ood in the northeast present level at the Great Salt Lake (~1000 ft Geologic, geomorphic, and geophysical Great Basin, roughly 17,400 calendar years ago, [305 m] below the highest Bonneville shore- analy ses of landforms, sediments, and struc- is a well-known event of this type that affected line), tilted the highest shorelines of the Bonne- tures in northern Cache Valley, USA, docu- one of the best known pluvial lakes of North ville lake cycle gently northward in Cache ment a revised history of fl ooding and recession America (Gilbert, 1880, 1890; Malde. 1968; Valley. The altitude drops from 5165 ± 10 ft of Lake Bonneville, the world’s premier plu- Currey , 1990; Oviatt et al., 1992; O’Connor, (~1575 m) at the south end of Cache Valley to vial lake. Crosscutting relationships suggest 1993; calibration to calendar years in part from 5107 ± 1 ft (~1557 m) at Franklin, Idaho; 5110 ± that the Riverdale fault produced a surface- Guido et al. [2007] and Hart et al. [2004]). At the 10 ft (~1558 m) slightly north of Red Rock Pass; rupturing earthquake ~25 km southeast of time of this fl ood, the shoreline in the Bonne ville and 5090 ± 10 ft (~1552 m) at the southern Zenda shortly before, during, or after the basin dropped 335 ft (~102 m), and 380 mi3 end of Oneida Narrows where the Bear River Bonneville fl ood, as well as possible younger (~4750 km3) of water drained northward into enters Cache Valley in the northeast (Gilbert, surface ruptures. Thus fl uctuating stresses and the Snake River with a maximum discharge of 1880, 1890; Williams, 1962a, 1962b; Critten- pore pressure induced by changing lake levels about a million cubic meters per second (Figs. den, 1963; McCalpin, 1989, 1994). Locally this may have triggered a large earthquake that, in 1–4; Malde, 1968; O’Connor, 1993). Roughly regional tilting was complicated by downward turn, triggered the Bonneville fl ood. The fl ood half the water in the basin exited during the defl ections near small Gilbert-type deltas and by scoured ~25 km of Cache and Marsh Valleys fl ood (O’Connor, 1993). Holocene faulting (McCalpin, 1994; Black et al., and activated landslides during its ~100 m inci- Surprisingly little is known about the geologic 1999, 2000). Shorelines of the Provo occupation sion to a new outlet near Swan Lake, Idaho. record of this fl ood and its aftermath near its of the lake are much less tilted than the Bonne- One to two thousand years of steady outfl ow threshold in northern Cache Valley (Figs. 1, 3, ville shoreline in Cache Valley (see below). produced the main ~4775 ± 10 ft (1455 ± 3 m) and 4). The subsequent events of Lake Bonne- Provo shoreline, ~10 m above the commonly ville at the Provo shoreline in this critical region Previous Interpretations of Drainage accepted altitude. Later Lake Bonneville oscil- are also poorly documented. The confl icting Divides, Outlets, and the Bonneville Flood lated below the main Provo shoreline, incised interpretations of these events are published the Swan Lake outlet, rose and paused briefl y mostly in abstracts, guidebook articles, geologic Gilbert (1880, 1890) interpreted the geologic at a new lower Provo sill (4745 ± 10 ft [1446 ± maps, and theses, and most in-depth analyses record near the current drainage divide between 3 m], P9?) 23 km south of Zenda, before revert- were based on examining landforms in the cen- northern Cache Valley and southern Marsh Val- ing to a closed-basin condition. Correlation to tral part of the Bonneville basin, particularly in ley (Red Rock Pass) and provided the seminal the Blue Lake chronology of Benson et al. sug- mid-lake locations (cf. Currey et al., 1984; Oviatt , interpretation of hydrology of the highest gests that aridity during the Heinrich 1 event 1987, 1997; Burr and Currey, 1988; Currey and Bonne ville lake level, the subsequent Bonne- activated the lower Provo sill ~15.9 ka. An Burr, 1988; Oviatt et al., 1992) and cores (Spencer ville fl ood, and the Provo shorelines (Figs. 1, 3, abandoned, meandering riverbed, north of et al., 1987; Oviatt et al., 1999; Benson et al., 4, 5, and 6). Gilbert (1880, 1890) interpreted the lower Provo sill, records a large northward 2011; McGee et al., 2011); or focused primar- both the Bonneville and Provo shorelines as fl owing river. The Great Basin’s modern divide ily on fl ood deposits farther north (Malde, 1968; resulting from an open, overfl owing lake, and at Red Rock Pass formed in the Holocene O’Connor, 1993). New mapping and analysis, he identifi ed the threshold at the time of the when a small alluvial fan fi lled the dry bed and new shoreline-elevation data, in conjunction Bonneville fl ood at Zenda, Idaho (Figs. 1–5). of this paleoriver. with previous and new geologic observations He concluded that the outlet of Lake Bonne- and inferences, support a new synthesis of the ville then shifted south at least 12 km to a new INTRODUCTION AND SETTING shifting outlets of Lake Bonneville, and clarify northern lake margin, at the Provo shoreline the likely sequence of events. Reservoir-induced “…between Swan Lake and the Round Valley General Statement seismicity and/or landsliding along the newly Marsh” (Gilbert, 1890, p. 178), and that “…the identifi ed Riverdale structure are implicated as point of outlet was at the edge of what is called Singular high-energy events in geologic his- a possible cause of the Bonneville fl ood. This Round Valley Marsh …, nine miles south of Red tory often leave disproportionate geologic records raises the possibility that tectonic processes were Rock (Pass)” (Gilbert, 1880, p. 348). He gave because they represent unusual amounts and more important than climatic forcing in trigger- the name “Bonneville River” to the large river ing the great Bonneville fl ood. that fl owed northward from Lake Bonneville. *Emails: Janecke: [email protected]; Oaks: Isostatic rebound, after Lake Bonneville Gilbert’s argument for a large southward [email protected]. dropped to the two successive Provo shorelines shift of the outlet was forgotten, and subsequent Geosphere; December 2011; v. 7; no. 6; p. 1369–1391; doi:10.1130/GES00587.1; 11 fi gures; 1 supplemental fi le; 1 supplemental table. For permission to copy, contact [email protected] 1369 © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/6/1369/3341874/1369.pdf by guest on 02 October 2021 Janecke and Oaks P o r tn That Marsh 112° e Dune-covered subdeltas, Fig. 5 u cher, Gem, Ge after Provo time Valley f R a Provo level of Lake Bonneville Fig. 2 n and 4775 ft (1446 m) shoreline g south of deltas e ntile Composite Provo delta of the Red Rock Pass Bear River below both the Zenda sill v Ba alleys 4775 ft (1455 m) and 4745 ft (1446 m) shoreline n s Lake Bonneville at its w n Swan Lake sill highest shoreline oc Swan Lake scour channel kRan Oxford and flood path R in Marsh Valley N Oneida Narro Was F reek ink C Bonneville delta of the g a U M Bear River and Mink Creek t ch fault e Clifton D Clifton sill City Bonneville delta Basin and Range fault, dotted where concealed RN Provo Preston F delta Cub River N Highest sapping-related Weston landforms of Marsh Creek C fan-pediment l Franklin ID a ID r 42° UT k UT Sill for a level of Lake Lewis s Bonneville t B o Fig. 4 e t n on fault a W Trenton r Bedrock high transverse to est Cache fault z R Cache Valley i Richmond v Present course of Bear River e r Flute or scour from the lake R Bonneville flood bottom a n Smithfield Meander belt in Round Valley g one e Fig. 1 Wel Logan anyon Mendon Cache Logan C l sv Valley East Cache fault i l l e Wasatch fault Hyrum R a Wellsville n g e Paradise 10 km Figure 1. Map of lake levels within Cache Valley. Deltas of the Bear River are differentiated from other lacustrine deposits. Provo shoreline is simplifi ed west of Cache Valley. RNF—Riverdale normal fault (+landslide?). Inset map shows location within the Great Basin (modifi ed from Morrison, 1991). 1370 Geosphere, December 2011 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/6/1369/3341874/1369.pdf by guest on 02 October 2021 Lake Bonneville at outlet Flow direction from Provo shoreline Abandoned meander belt of Bonneville River Portneuf Range 1 Bonneville deposits and 112° shoreline, except at delta Provo delta south of Twin 4 Red Rock Pass Lakes Sill and drainage divides Bannock Range Sw Bonneville delta an L ak e Basement ridge sc o u r c from mapping h a n n and gravity data e l Flood-related 2 scours and outflow channels Swan Lake Gravity low Figure 2. Key features of north- Fig. 9 ern Cache Valley and southern Swan Lake horst Marsh Valley. Numbered sills 111.875° and drainage divides controlled lake levels and outlets of Lake 42.25° Bonneville, from 1 (oldest) to N Round U 4 (youngest). Riverdale fault Valley Bonneville delta (+landslide?) zone is shown D R ive with a lineament symbol, and rd ale U dotted at its inferred locations.