Among the geomorphologists who have so marvelously documented the highest shorelines of Lake Bonneville during the last 130 years, the consensus is that these highest Bonneville shorelines are “transgressive depositional terraces” (Chen and Maloof, 2017; Oviatt and Jewell, 2016). These most intensively studied Bonneville terraces are typically imprinted on slopes of 0.1 (rise over run 1:10), but we are unaware of reported examples for slopes of 0.02 (rise over run 1:50). These high Bonneville shorelines are widely recognized to be transgressive because they culminate with the highest shoreline and the catastrophic flood and rapid regression of the lake by over 100 meters. Figure 20 is our adaptation of the “transgressive depositional terrace” model of Chen and Maloof (2017) to the lower slopes of Hopi Lake within Bidahochi Basin. Observations of Bonneville high shorelines allowed Chen and Maloof (2017) to derive their model of a “transgressive depositional terrace” assuming a steep and steady slope over uniform bedrock. Our adaptation of their model assumes a low-sloping surface with a dip slope having alternating resistant and friable beds of thin-bedded Harrisburg Limestone. Chen and Maloof need little vertical exaggeration in their Bonneville terrace model. We used “extreme vertical exaggeration” of at least 25 times vertical in Figure 20 to display the Hopi shoreline terrace on a perspective block diagram. Water waves are widely appreciated to have declining water velocity (and shear stress on bedrock surfaces) with increasing depth. Recreational snorkel divers and SCUBA divers are experienced with this fact. Declining velocity with depth is caused by the circular motion of molecules within the water surface wave that die out with depth. Figure 20 (upper right) displays our model of the velocitydepth profile as water waves impact the shore parallel to the limestone dip slope. Figure 20 shows the initial lake shoreline level (S-1) where shallow water waves with higher velocity and shear stress have already begun to bevel a new surface called the “terrace erosional platform” into the “resistant” limestone dip slope. The down-slope side of “terrace erosional platform” begins to accumulate coarse gravel in the lower right as the platform continues to be eroded by waves. The erosion process occurs fastest at the “notch” on the growing platform as the “friable” limestone bed within the dip slope is intersected. Even though the water becomes deeper with time, the erosional terrace continues to extend shoreward forming the “notch” into the dip slope with erosion of the underlying “friable” limestone bed. In Figure 20 at lake level S-2, the level of the lake has risen enough to reach a critical system threshold. At the increased water depth at the “notch,” there is no longer enough shear stress to erode the “resistant” limestone underlying the “friable” layer. Therefore, the “erosional platform” stops being eroded, and the erosive power of waves is expended on the “erosional scarp” above the “notch.” That “erosional scarp” is a concave-upward surface where water waves pluck the limestone substrate. The debris plucked is accumulated downslope of the “inflection line” where a convex-upward mound of coarse gravel builds up. The threshold at lake level S-2 causes the excavated “erosional platform” (sculpted at S-1) to be overlain by the “gravel-covered platform” (deposited at S-2). In Figure 20 after lake level S-2, the lake continues to rise even more reaching another critical system threshold. At S-3 there is no longer enough shear stress within the deeper water to pluck limestone from the “erosional scarp.” Instead, deeper water allows gravel to be deposited over the “notch” and “erosional scarp.” The newer, elevated water level at S-3 is, again, on the upper surface of the “resistant” limestone dip slope. The erosive power of shallow waves is, again, able to start forming another “erosional platform.” This process initiates the formation of the next “transgressive depositional terrace.” While the second depositional terrace is forming, the first depositional terrace’s erosional scarp receives silt and sand which fills most of the concavity. Then, on shallow, quiet, sunlit, cool, offshore beveled limestone when the lake level has risen above S-4, lacustrine microbial carbonate encrustation form “tufa.” The second depositional terrace block diagram (top of Figure 20) displays the structure after Hopi Lake has drained and after soil has occupied the new landform. The final condition of the terrace landform (top of Figure 20) shows the “berm,” “face,” and “flat” as we describe them in the field. DISCUSSION We interpret these Wagon Box Draw landforms to be old shoreline depositional terraces carved within the limestone dip slope as Hopi Lake rose to fill Bidahochi Basin. Could other explanations be offered instead of shoreline erosion and deposition on terraces? Here we note four processes that could be alternates to shoreline erosion and we respond briefly. The four alternate processes are: (1) pattern from multiple flat-lying strata eroding differentially within the slope, (2) pattern from erosion of strata that possess strong joint expression, (3) pattern from erosion of hummocky bedforms, dunes or boudinage within the Harrisburg Limestone, and (4) pattern of terrace-like steps caused by surficial gravity deformation through soil creep or solifluction. A compelling case can be made that Nate’s Hill is a dip slope, not an outcrop erosion pattern of numerous, level and successive strata (see Figure 14). The dip slope is expressing just a few strata at its surface, not multiple successive strata. Landforms on the dip slope are not controlled by vertical limestone joints, because landform alignments deviate greatly from steady northwestward joint orientation on and around Nate’s Hill. Could hummocky, dunelike or boudinage bedding structure within the limestone cause regularly spaced ridges (resembling terraces) to be expressed on the dip slope? We have not found hummocks in the uppermost Harrisburg Member with 20-meter spacing. Figure 13 shows remarkable planar beds, not hummocks, within the upper Harrisburg. Reflect upon the improbability that crests of ancient limestone hummocks, dunes or boudins would follow present topographic contour. Coarse gravel deposits on bedrock terraces would hardly be regarded as susceptible to soil creep and solifluction. Remarkably, linear landforms strongly parallel the 2-meter topographic contour overlay except where the limestone is deformed by drag adjacent to faults. We observed a berm structure that could be traced one kilometer along the limestone dip slope of Nate’s Hill, yet the berm’s elevation appears to vary by less than a meter. The limestone dip slope at Nate’s Hill is terminated on its northeastern edge by the graben structure hosting Buffalo Range Road, with the associated landforms northeast of the graben having 15-meter lower elevation compared to the southwestern side. Thus, these landforms correspond marvelously with the bedrock structural geology and dip slope stratigraphy. These observations support shoreline terraces. Critics of Hopi Lake often ask why shorelines have not been found at Hopi Lake. Michael Oard (2010, 2016, 2021) described successions of erosional benches that he understands to be shoreline terraces associated with steep slopes adjacent to glacial Lake Bonneville. Oard shows transgressive shoreline terraces for Lake Bonneville’s highstand in Utah. These glacial lake terraces are imprinted on slopes steeper than 0.1 (rise over run steeper than 1:10). The Bonneville example of Oard include transgressive shoreline terraces as defined by Chen and Maloof (2017) along the tectonically active Wasatch AUSTIN, HOLROYD, FOLKS, AND LOPER Shoreline Transgressive Terraces 2023 ICC 358
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