Pleistocene glacial and lacustrine features of the Wasatch Front

A field trip guide for the Paleoglaciers Workshop

Salt Lake City, Utah

November 11, 2023

Locations of field trip stops are included in the Google Map below.

Introduction

The field trip accompanying the Paleoglaciers Workshop explores the Pleistocene geomorphology of the Wasatch Front south of Salt Lake City, Utah. The trip is designed to provide workshop attendees the opportunity to see shorezone features of Lake Bonneville, glacial features at Little Cottonwood and Bells Canyons, and the Wasatch Fault.

The trip partially follows a GSA field trip planned by Oviatt et al. (2021) for a Rocky Mountain GSA meeting in Provo, UT, which was postponed to May 2025 due to the pandemic. Attendees are referred to the guidebook for detailed information about Pleistocene Lake Bonneville and mountain glaciation of the western Wasatch Range.

Lake Bonneville was the largest of numerous Pleistocene paleolakes that expanded in the Great Basin during marine oxygen-isotope stage 2 (Fig. 1). With a surface area of roughly 51,000 sq. km, the lake was much larger than its modern successor, Great Salt Lake. Geomorphic and sedimentary evidence of the lake was first documented in the well-known monograph by G.K. Gilbert (1890), which includes numerous observations of shoreline/nearshore features along the Wasatch Front and elsewhere in the lake basin, most of which have been deemed correct by subsequent geological studies. Such studies include radiocarbon dating of shoreline deposits of Lake Bonneville spanning the lake transgression, overflow, and regression. We will discuss the timing of the Lake Bonneville transgression and overflow relative to mountain glaciation in the Wasatch and implications for Late Pleistocene climate. The history of Lake Bonneville is described in a comprehensive review by Oviatt (2015) and in numerous papers compiled in Lake Bonneville: A Scientific Update, edited by Oviatt and Shroder (2016).

Mountain glaciation in the western Wasatch Range was also described by G.K. Gilbert (1890), including the observation that the highest shoreline of Lake Bonneville intersected glacial features near the mouths of Little Cottonwood and Bells Canyons, which we will visit today. Whether the lake and glaciers coexisted was (somewhat calmly) debated among geomorphologists in the 1970s and 1980s, due in part to the lack of numerical ages of glacial features relative to the abundance of radiocarbon age limits on shorelines of Lake Bonneville. Some maintained the idea that ice retreat in the mountains bordering Lake Bonneville contributed meltwater to the lake, thereby filling the the lake to its highstand. Others proposed that direct precipitation to the lake was needed to sustain it along with return flow from valleys within its watershed (Mifflin and Wheat, 1979). This field trip visits two glacial valleys where cosmogenic Be-10 chronologies of glacial deposits help to reveal the relative timing of mountain glaciation the Lake Bonneville highstand. Laabs and Munroe (2016) report cosmogenic Be-10 exposure ages of terminal and recessional moraines at Little Cottonwood and Bells Canyons, and Quirk et al. (2020) report cosmogenic ages from other glacial valleys in the western Wasatch Range. These age relationships are discussed on the field trip along with inferred paleoclimate from considerations of both lacustrine and glacial features.

Starting point

Meet at the south entrance to the Salt Lake Plaza Hotel at 122 W S Temple St. Follow the Google Map to proceed to Stop 1 near the mouth of Big Cottonwood Canyon.

Stop 1: Delta deposits at Big Cottonwood Canyon

Park on Gun Club Road near the gate north of the gravel pit. The gravel pit has mined delta sands and gravels deposited by Big Cottonwood Creek when it flowed into Lake Bonneville. Outcrops of such delta deposits here and elsewhere along the Wasatch Front were described in the famous monograph by G.K. Gilbert (1890), which correctly identified shoreline/nearshore geomorphic features of Lake Bonneville.

Stop 1: Delta deposits at Big Cottonwood Canyon

Here, two delta surfaces are preserved, one grading to the Bonneville shoreline, the lake highstand, and one to the Provo shoreline, roughly 100 meters lower. Shoreline features at these two elevations, ca. 1550 m and 1450 m asl, are commonly observed along the Wasatch Front and elsewhere in the lake basin, representing two phases of the lake. The hydrograph of Lake Bonneville (Fig. 2 below) includes the slow rise of Lake Bonneville during its transgressive phase (T), followed by the abrupt decline of the lake resulting from on outburst flood at the lake threshold where it spilled into the Snake River. The lake dropped to a new bedrock controlled threshold where it overflowed more or less continuously from ca. 18-15 ka, and subsequently declined to become the Great Salt Lake.

Figure 2. Hydrograph of Lake Bonneville from Oviatt (2015).

The brief interval of overflow at the Bonneville shoreline (1550 m asl) is somewhat inconsistent with the preservation of such continuous and abundant shoreline features, which likely formed over periods of centuries or longer. Oviatt (2016) notes that most shoreline features near the Bonneville level are constructional; that it, they developed during the prolonged transgressive phase of the lake and consist of shoreline deposits on an erosional slope. Wave-cut platforms, which would imply a prolonged occupation of the Bonneville shoreline, are not found at the elevation of the Bonneville shoreline. The model for constructional platform development is shown in Fig. 3 below.

Figure 3. Diagrams of constructional shorelines (A) and erosional platforms (B), from Oviatt et al. (2021).

Stop 2: G.K. Gilbert Geologic View Park

From Stop 1, follow the Google Map to G.K. Gilbert Geologic View Park, parking at the southwest side of the park. This brief stop affords a view directly upvalley at Little Cottonwood Canyon, the moraines at the mouth of the canyon, and the Wasatch Fault. Here, the field trip discusses these features as they were originally observed by G.K. Gilbert and the relationship between Lake Bonneville and mountain glaciation.

Stop 3: Faulted moraines at Little Cottonwood and Bells Canyons.

From Stop 2, follow the Google Map to the Bell Canyon Granite Trailhead. Restrooms are available here. Hike along the trail to the left lateral moraine at the mouth of the Little Cottonwood Canyon and the terminal loop at Bells Canyon. The trail intersects the faulted portions of the moraine in multiple locations. Above the fault are numerous erratic boulders sampled for cosmogenic dating by Laabs et al. (2011) at Little Cottonwood Canyon and Laabs and Munroe (2016) at Bells Canyon. The field trip will discuss the cosmogenic chronologies of the moraines, implications for paleoclimate, and the Wasatch Fault.

Figure 4. Faulted moraines at Little Cottonwood and Bells Canyons (Source: Oviatt et al., 2021). More detailed mapping is displayed by Adam McKean during the field trip.

Fig. 4 displays a lidar-based shaded-relief image of the moraines at Little Cottonwood and Bells Canyons, where cosmogenic chronologies reveal two phases of glaciation, each representing maximum and near-maximum glacier lengths. The first phase, represented by the age of the left lateral moraine in Little Cottonwood and terminal moraine in Bells Canyon, suggests that these moraines were deposited during the Last Glacial Maximum at ca. 21-22 ka. The age of the right lateral moraine in Little Cottonwood and a recessional moraine in Bells Canyon indicate that the two glaciers were near their maximum lengths at ca. 16-17 ka while Lake Bonneville overflowed. Additional chronologies from Quirk et al. (2020) indicate that glaciers in neighboring valleys of the western Wasatch were also near their maximum extent during the time when Lake Bonneville overflowed. This is consistent with stratigraphic observations of interbedded lacustrine and glacial deposits near the Lake Bonneville shoreline and the presence of monzonite dropstones in Lake Bonneville sediments to the southwest.

The observation of terminal moraines corresponding to the latter part of the global Last Glacial Maximum (22-19 ka) and downvalley recessional moraines corresponding to 18-15 ka is observed elsewhere in the Great Basin and Rocky Mountains (Laabs et al., 2020). For the western Wasatch Mountains, Quirk et al. (2020) adopt the Pinedale 1 and Pinedale 2 phases of the last Pleistocene glaciation (originally described by Licciardi and Pierce (2018)), which include a glacial maximum prior to the Lake Bonneville highstand at 18 ka and a persistance or readvance to near-maximum glacier lengths while the lake overflowed (Fig. 5).

Figure 5. Cosmogenic ages of terminal and recessional moraines in the Great Basin (Oviatt et al., 2021).

Faulting of lateral and terminal moraines at Little Cottonwood and Bells Canyons is easily visible and forms complex patterns across the mouths of the two canyons. Detailed mapping by McKean and Solomon (2018) displays multiple prominent normal fault scarps with some antithetic scarps forming grabens (Fig. 6).

Figure 6. Slope shade map displaying fault scarps cutting across moraines at Little Cottonwood and Bells Canyons from McKean and Solomon (2018).

Stop 4: Provo shoreline spit

From Stop 3, follow the Google Map to the Walmart parking lot in Sandy. North of the parking lot is a prominent spit formed at the Provo shoreline elevation. Time permitting, the field trip discusses the formation of shoreline features in Lake Bonneville.

After this stop, return to the Salt Lake Plaza hotel.

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