IMPLICATIONS OF NEW DATES FOR SAN JUAN RIVER TERRACES NEAR BLUFF, UTAH

Five optically stimulated luminescence dates and one radiocarbon date were obtained for alluvium from selected terraces within the broad valley 5-7 km west of Bluff and 9-11 km upstream from the bedrock-controlled knickpoint at the head of the canyon through the Monument uplift. Terrace alluvium consists of higher-energy sandy cobble gravel that is overlain by lower-energy sands and silts. To avoid bioturbation, OSL samples were collected from bedded sands either just above or within the upper gravels. Fossil snails occur within muds of one terrace.

Dates (OSL and 2σ unless otherwise stated) and tread heights (without overlying colluvium and eolian deposits) are: 60.81 ± 9.82 ka, ≈60 m; 41.59 ± 6.36 ka, ≈30 m; 29.09 ± 4.14 ka and conventional ¹⁴C 30.52 ± 0.19 ka, ≥14 m; 20.68 ± 2.62 ka, ≥11 m; and 0.47 ± 0.15 ka, ~4 m. These dates may represent most levels below ≈60 m but higher, undated levels are present. The earliest nearby date is 1.36 +0.20/-0.15 Ma on a ≈140-m terrace remnant at Bluff (Wolkowinsky & Granger, 2004; cosmogenic burial, 1σ). In comparison, the highest remnant at 210 m may be older than 2 Ma.

The ~61-ka date supports an early Wisconsinan (MIS4) glacial advance in the Rocky Mountains (Pierce, 2004), and is comparable to ages for terrace M3 in the eastern Grand Canyon (Anders et al., 2005). However the San Juan near Bluff has incised below its late-Pleistocene deposits, unlike the Colorado in that area.

Average incision rates were estimated for periods between the formation of specific sub-alluvial straths. With large uncertainties, rates are moderate between the ≈30-m terrace and modern channel, very rapid betweem the ≈30-m and ≈60-m terraces, and slow between the ≈140-m and ≈60-m terraces. These variations suggest that the wave of transient incision identified in the eastern Grand Canyon and Glen Canyon (Darling et al., 2012) has already passed through this area.