within Diablo Canyon has exposed the full, 23-meter-thick strata sequence within the Harrisburg Member of the uppermost Kaibab Formation. The Harrisburg Member is slope-forming brownish-gray to reddish-gray interbedded, thin-bedded limestone, sandstone, and siltstone. That Figure 13 displays the complete 23 meters thickness of the Harrisburg Member is confirmed in Diablo Canyon by contact with the overlying Moenkopi Formation (reddish and brownish friable siltstone, sandstone and mudstone) which usually weathers and crumbles markedly to form the ramp’s surface at the top of Harrisburg Member. Also, within Diablo Canyon, is the cliff outcrop of the underlying Fossil Mountain Member of the Kaibab Formation (light-gray cherty, thick-bedded limestone that is extremely resistant to erosion). Within and around the structural ramp of the Wagon Box Draw research tract, Billingsley et al. (2013) mapped several small erosional outliers of Moenkopi Formation sitting on Harrisburg Member. Thus, we conclude that the ramp is expressed with the beds of the upper Harrisburg Member. We identified informally the uppermost six meters of Harrisburg Member in Figure 13 (slope forming, thin-bedded limestone, with thin bedded sandy and silty limestone) as the “platform carbonate unit” that we regard as marker beds that can be recognized widely on the ramp’s surface. An excellent exposure of these ramp limestone strata has been created by Interstate Highway roadbuilders who quarried surficial gravel from the rectangular borrow pit on the north side of Highway 40 at “Interstate Highway Arch” (Figure 9). Here the landform terrace erosional surface and its overlying terrace deposit has been quarried. That the dip slope of the “platform carbonate unit” is the bedrock host for the linear, terracelike landforms on Nate’s Hill is demonstrated, we believe conclusively, by geometric analysis of oblique photography. Figure 14 is a compelling oblique image of Nate’s Hill looking westward. Linear landforms on the southern slope of Nate’s Hill are seen extending eastward to the scarp of the prominent listric normal fault where the limestone strata of the “platform carbonate unit” are displayed in the scarp beneath the linear landforms. Obviously, the landforms in Figure 14 are inscribed on the dip slope of limestone. In the field we were impressed by the horizontality of the linear landforms. Through one-kilometer distance across Nate’s Hill we were able to follow a linear landform ridge that appears to vary in elevation by less than one meter. We observed this extraordinary horizontality on Figure 11’s DEM overlay and on the outcrop in the field using the level bubble on a Brunton compass. Even more remarkable, however, is how the landforms deviate from horizontality at both east and west ends of Nate’s Hill. Notice on Figure 11 how the landforms turn down slope as the listric normal faults are approached on both the east and west ends of Nate’s Hill. These deviations from horizontality, we believe, are caused by drag on the faults. Figure 12 even shows the effect of fault drag on the landforms on the opposite side of the large graben at what we call Graben Hill. Overall, the Buffalo Range Road graben offsets landforms vertically by 15 meters, down on the eastern side. These observations of deviations from horizontality persuade us, by their cross-cutting relationship, that nearly level terracelike landforms on the dip slope are older than the normal faults that produced the graben. Thus, we understand the Bidahochi Basin has been deepened tectonically by listric faulting since the time that Hopi Lake formed. OBSERVATIONS—OVERALL APPEARANCE OF LANDFORMS Landforms were studied in the field to obtain a better appreciation of relationships to bedrock stratigraphy and structure. Oblique aerial video (Loper, 2022b) shows linear landforms on Nate’s Hill that typically light-brown ridges of rock separate darker brown depressions with sandy soil and xerophytic plants. Of the rangeland plants, bushes on the flats are the most visible from the aerial video. Bushes typically are half-meter diameter, so that scale indicates that light-brown ridges of rock are typically spaced 20 (+/- 5) meters. When walking in the upslope direction across the sandy low areas, one first encounters the linear rocky ridge with a noteworthy steeper, inclined rocky surface element that we call the “face” of the landform. Sometimes the “face” shows evidence of concave upward curvature and the strike the limestone. Upslope from the “face” is a convex rock surface with the highpoint being the crest of the landform. We call this convex rock surface the “berm” of the landform. In the upslope direction, the “berm” passes into the adjacent “flat” usually expressed as gravel or sand. The overall height of the sloping “face” and the upslope convex “berm” is typically 0.4 (+/- 0.2) meter. The width of the rocky mound over the ground surface is usually less than 5 meters. These rocky mounds do not display soil and, therefore, Figure 14. Oblique aerial view of Nate’s Hill looking west over the fault scarp. Terrace landforms typically have 20-meter spacing. This oblique view, we believe, makes a compelling case for the successive terraces being inscribed on the dip slope exposing a single, thinner limestone bed at the top of the Harrisburg Member. This view disputes the notion that the successive landforms on the slope are outcrop expressions of a series of thinner limestone beds. Figure 15. Chevron Hill displays the Harrisburg Member outcrop ridge (berm and face) landform that divides two flat terrace landforms. The entire limestone outcrop is encrusted with thinly laminated, cool-water carbonate (“tufa”). We saw (1) abundant bladelike limestone clasts, (2) positioning of bladelike clasts on the downslope side of an outcropping bedrock ridge, and (3) encrustation with calcareous tufa. We noted the similarity to Lake Bonneville tufa and Lake Bonneville transgressive shoreline terraces (Chen and Maloof 2017) and we asked, “Is this a beach deposit?” AUSTIN, HOLROYD, FOLKS, AND LOPER Shoreline Transgressive Terraces 2023 ICC 354
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