do not promote growth of bushes, the prominent desert plant, on these rocky mounds. Contrast this with soil-covered surface of the “flat” that has darker color and distinctive texture caused by bushes and aeolian volcanic deposition. These contrasting colors and textures accentuate the linearity of landforms seen, especially, in the oblique fly-over in the aerial video (Loper, 2022b). Slopes are poorly appreciated on aerial video because video hardly ever shows the horizon. Field observations allow us to state that landforms are expressed upon gentle slopes of 0.02 to 0.08 (rise over run 1:50 to 1:12). Before addressing details in the different elements of these landforms, we must recognize and describe the carbonate coating that frequently covers Harrisburg Member limestone, especially near the tops of hills. The carbonate-rock coating is widespread with the landforms, but it is not universal. Figures 15 to 18 help characterize this carbonate coating. Figure 15 shows the widespread distribution of the carbonate-encrusted Harrisburg Member near the top of Chevron Hill. The limestone exposure is almost completely encrusted with yellowish-brown, spongy carbonate. As is typical of these outcrops, the coating conforms to angular and broken corners of limestone boulders and cobbles. Figure 16 shows a larger cobble encrusted with typical, yellowish-brown, spongy carbonate. The yellowish-brown coating in Figure 16 is thin where the surface of the cobble is broken exposing gray Harrisburg Limestone that is the substrate for the coating. The background in Figure 16 accentuates the story: the cobble was moved from the rocky ridge to the adjacent flat to contrast its color with the darker, reddish-brown soil in the flat. Figure 17 displays a typical cross-sectional view of the encrusting coating which possesses pillarlike botryoidal columns and faintly laminated structure within an open spongy texture. Figure 17 shows the encrustation in its common 2 centimeters thickness. We examined the encrustation with hand lens and microscope. Figure 18 shows the encrustation by a “flashlight microscope” inserted into the open spongy texture. Figure 18 displays the encrustation dominated by silt-size calcite crystals that express botryoidal masses that have grown into a hole between pillars. What name should we assign to this laminated carbonate encrusting rock? One can visualize calcite crystals being precipitated while immersed in fresh water, at Earth’s normal surface temperature, beneath an alkaline organic slime layer that temporarily covered the rock surface (Ford and Pedley, 1996; Pedley et al., 2008). Our visualized calcite precipitation process differs markedly from the present desert slope, near the top of a dry hill, where we find the carbonate encrustation today! The encrustation is surficial, not associated with joints or faulting. No evidence was found for hydrothermal process. The encrustation does not compel us to imagine a thermal spring deposit (travertine) as we have studied at Yellowstone. Furthermore, this surficial feature is unlike a cave deposit (speleothem). The spongy encrustation reminds us of the rock called “tufa” that we have seen along the high shoreline of Ice Age Lake Bonneville (Hart et al., 2004; Nelson et al., 2005; Felton et al., 2006). We have found the rock we call calcareous tufa to be Figure 16. Gray, very hard, Harrisburg Limestone cobble is thinly encrusted with friable, laminated, yellow-brown, porous tufa. Figure 17. Cross-sectional view of the encrusted Harrisburg Limestone surface. Tufa shows internal lamination parallel to the limestone surface and displays internally pillarlike calcite structures with abundant porosity. Figure 18. Flashlight microscope view of encrustation on limestone surface. The open structure of pillars, with abundant holes between, displays botryoidal masses composed of silt-size calcite bound together by microcrystalline calcite. These types of carbonate structures form in modern shallow lake shorelines, at normal surface temperature, where microbial films coat surfaces creating a sticky and alkaline microenvironment, inducing trapping of silt-size particles, and cementation with microcrystalline calcite. This thin encrustation, we propose, should be called tufa. AUSTIN, HOLROYD, FOLKS, AND LOPER Shoreline Transgressive Terraces 2023 ICC 355
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