OCR Text |
Show A contour map of the thickness of the low density rock that would produce the residual gravity anomaly in Heber and Rhodes Valleys ( fig. 22) was perpared by making an iterative three- dimensional solution with a digital computer ( Cordell and Henderson, 1968). In this analysis the computer input is the residual Bouguer gravity data and a density contrast of 0.5 g per cm3, which was assumed to exist between the low density material underlying the valleys and the more dense pre- Tertiary rocks in the bordering mountains, and the requirement that the low density mass extend to the surface of observation. The solution is a mass distribution that will produce the measured gravity anomalies. In figure 22 some minor adjustments of the lines have been made to make the low density mass distribution consistent with exposures of bedrock. An irregular thickness of low density material underlies Heber Valley. Three areas of closure, where the maximum thickness is 800 feet or more, are indicated. In the western half of the valley there is a bedrock ridge which plunges to the south between the towns of Midway and Heber City. This ridge is exposed at the surface near the northeast corner of Midway. A shallow trough of low density material extends northward up the Provo River for about 3 miles. The computed model is generally consistent with the sediment thickness data revealed by drill holes in Heber Valley indicating that the assumed density contrast is approximately correct. The computed model indicates that the northern half of Rhodes Valley is underlain by low density material with a miximum thickness of about 1,600 feet. The model shows the bedrock bottom of the valley sloping steeply towards the center in all directions. A trough of low density material 300- 400 feet thick extends south beneath the town of Francis. No drill hole data are available in Rhodes Valley to confirm the computer model. An aeromagnetic map ( fig. 23) of the area was taken from a survey flown in 1965 ( Meuschke and Kirby, 1966). Total intensity magnetic measurements were made with a fluxgate magnetometer mounted in a tailboom on a Convair aircraft. Flight lines were north- south, 2 miles apart and at 11,000 feet barometric elevation. No magnetic evidence of igneous rock underlying Heber Valley is apparent; however, a small but significant magnetic anomaly was observed over northern Rhodes Valley and is strong evidence that igneous rock underlies this part of the valley. The magnetic anomaly in the area of the thickest low density material indicates that part of the low density material is volcanic rock. Tertiary extrusive rocks are present to the west ( Stokes, 1964), and may be the source of the anomaly in the valley. Because the density of the volcanic rock is probably higher than that of the sediments producing the gravity low in Heber Valley and the southern part of Rhodes Valley, the actual thickness of low density material in northern Rhodes Valleyis probably greater than the thickness indicated on the model. The high amplitude magnetic positive anomaly in the mountains northwest of Heber Valley is related to Tertiary granitoid rocks ( Stokes, 1964). The magnetic data suggests an eastward extension of the anomaly along the north side of the valley. The gradient along the south side of the extension corresponds with the north edge of the gravity low and may define the northern limit of the valley. 57 |