AGE OF THE STANSBURY SHORELINE

In 1890 G.K. Gilbert named three major shorelines of Lake Bonneville: Bonneville, Provo, and Stansbury. Gilbert (1890, USGS Monograph 1) did not specifically say that the Stansbury shoreline formed after the development of the Provo shoreline, but he hinted at that interpretation (he didn’t spend much time on the question) and most subsequent workers thought of the shoreline as having formed during the regressive phase, apparently based on its topographic position lower than the Provo shoreline. However, in the early 1980s Don Currey found outcrops in Stansbury Gulch on Stansbury Island that showed stratigraphically that the Stansbury shoreline formed during the transgressive phase. Currey dated gastropod shells from sediments in Stansbury Gulch that were interpreted as having been deposited as the shoreline was forming at about 25 cal ka, an age that was accepted for many years. Over a period of almost 30 years beginning in the mid 1990s, studies led by Vicki Pedone (Pedone et al., 2023, JQS) of a microbialite (tufa) deposit near Lakeside, UT, yielded stratigraphic interpretations and radiocarbon and uranium-thorium dates that indicated Lake Bonneville stayed at a relatively low elevation (fluctuating at levels a few meters higher than the modern average level of Great Salt Lake) until after about 24 cal ka. The Stansbury shoreline formed after about 24 cal ka, possibly about 23 cal ka (Oviatt and Pedone, 2024; QR). Perhaps the snail shells that Currey dated were reworked from older deposits as the lake transgressed across the pre-Bonneville landscape, or maybe a local hard-water effect influenced the radiocarbon age of the snails(?) — more work is needed. The long-term rate of lake-level rise during the transgressive phase between about 24 cal ka and 17.5 cal ka (the age of the Bonneville flood) was roughly 340 m in 6.5 ka in areas of maximum isostatic rebound and about 270 m in 6.5 ka in areas of minimal isostatic rebound. The Stansbury oscillations were two or more of the many transgressive-phase oscillations of Lake Bonneville, probably caused by changes in climate, during the period characterized by a general rise of the lake (the transgressive phase).