Figure 1. Bidahochi Basin oblique terrain model overlooking northeastern Arizona and western New Mexico, including simulated 300-kilometer-long Hopi Lake rising through elevation 1770 meters (5,800 feet). Rendered by Nate Loper as a 3D terrain model using Blender software and posted on 3D hosting platform Sketchfab (Loper, 2022a). have engendered their own legacy concerning spilling lakes. During the last 50 years the consensus creationist explanation has been that the 300-kilometer-long Hopi Lake spilled over the Kaibab Upwarp to erode Grand Canyon (Austin et al., 2020). What was the elevation of the paleolake? A likely elevation of the paleolake is 1860 meters (6100 feet) as recognized from tufa deposits just east of Grand Canyon and similar tufa near St. Johns, Arizona (Scarborough et al., 1998; Scarborough 2001; Austin et al., 2020). Current models of 1860-meter lake level assume post-lake tectonic tilting of Kaibab Plateau (Austin et al., 2020). A higher elevation of the putative paleolake, without plateau tilting, was suggested by Douglass et al. (2020). The Bidahochi Basin terrain model (Figure 1) depicts a stage in filling when the paleolake was at elevation 1770 meters (5800 feet) as terraces formed at Wagon Box Draw. We believe the lake continued to rise to ~1860 meters (~6100 feet). Figure 1 shows the eastern Grand Canyon and Kaibab Upwarp in the upper left. Eastward of Grand Canyon is Echo Cliffs Monocline crossing Marble Canyon at Lees Ferry, Arizona. On the horizon in the top middle of Figure 1 is the upper Colorado River basin in Utah and Colorado. In the upper right of Figure 1 is the Defiance Upwarp forming the highlands on the eastern end of Bidahochi Basin. Mogollon Rim is the highland in the foreground marking the southwestern edge of the Colorado Plateau. Through the years three geologic evidences have been offered by spillover advocates indicating the Bidahochi Basin was a depression filled by a lake. First, sedimentary deposits in the upper half of the Bidahochi Formation, along with an ecosystem of lake fossils, argue strongly for the paleolake (Douglass et al. 2020). Second, near 1770 meters elevation, there is no tectonic confinement within the middle of Bidahochi Basin allowing western extension and filling with water against the Kaibab Upwarp (Scarborough, 1989; Holm, 2001; Dallegge et al. 2003; He and Kapp, 2021). Third, widely distributed shoreline tufa deposits above 1860 meters elevation make the paleolake credible (Harris et al., 1998; Scarborough, 2001; Austin et al, 2020). A fourth geologic evidence for the lake might be offered in terracelike landforms at the shore. However, no publication yet concerns Hopi Lake shoreline terraces. The present authors here seek to explore this possible shoreline evidence. Before proposing a search for shoreline terraces of Hopi Lake, we must ask if our motives for such a search are geologically reasonable. Slopes along the shoreline of putative Hopi Lake just below 1860 meters (6100 feet) elevation are typically 0.02 (topographic “rise” divided by horizontal “run,” sometimes depicted as 1:50). Slope expressed as 0.02 (rise over run) is equivalent to slope angle 1.1 degree (the arctangent of 0.02). Ice Age Lake Bonneville has the most intensely studied Pleistocene shorelines (Gilbert 1890, Chen and Maloof 2017, Oviatt 2020). If the comparison of Ice Age Lake Bonneville to Hopi Lake is proper, then shoreline terraces of ancient glacial Lake Bonneville should be evident on slopes of 0.02 or less. However, Lake Bonneville shorelines are typically found on slopes averaging 0.1 (rise over run 1:10), equivalent to 5.7 degrees. We have been searching Bonneville slopes of 0.02 (rise over run), equivalent to 1 degree for shoreline erosional terraces, but we have not found good examples. The expression of former Lake Bonneville shorelines on different slopes is perhaps best illustrated at Brown Knoll, a volcanic lava-dome complex in the southern basin of Lake Bonneville, Utah (lat./long., +38.722, -113.350). Figure 2 is the oblique profile aerial photograph of the western dome at Brown Knoll that was completely transgressed by filling to the highest level of Lake Bonneville. Brown Knoll West displays excellent transgressive shoreline terraces on slopes of 0.1 (6 degrees) near the base of the volcanic dome structure. However, at slopes of 0.02 (1 degree) at the top of the western dome, shoreline terraces do not occur. The same conclusion could be made about AUSTIN, HOLROYD, FOLKS, AND LOPER Shoreline Transgressive Terraces 2023 ICC 347
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