Thursday, May 19, 2011

The Bonelli Deposit

The Bonelli Deposit – the Bonelli salt deposit is in the NW1/4 of Sec 4, T. 20 R. 68E and near the east bank of the Virgin River.

The salt exposure is confined to the western extremity of a minor spur where it appears as the crest of a small, sharp, anticlinal fold of which the axis strikes about 50 degrees east. The salt mass, as seen in section in the open cut shown in the illustration, is about 20 feet high and 40 feet wide. It is directly overlain by about 70 feet of brown silt containing lenses of very impure gypsum. Overlying this brown silt is about 10 feet of impure gypsum which forms the summit of the spur. In a small ravine immediately north of the salt deposit, crystalline rocks, presumably of pre-Cambrian age, are exposed. The salt-bearing beds were either deposited against a steep slope of these rocks or have been faulted down against them. 

In either vase the salt bed is definitely limited in that direction. To the east and south the salt mass dips steeply under the overlying silts of the Muddy Creek formation. On the west, the salt bed has been cut away by the Virgin River. Some of the higher hills north of the deposit are capped with basalt indicating that the salt probably lies in the same general stratigraphic zone as the other deposits, namely, a few hundred feet below the basalt flow of the former lacustrine period.

A tunnel, shown in figure 105, to the right of the open cut, has been driven into the fill for about 100 feet, penetrating brown silt. Between the tunnel and the open pit there is exposed a little very impure salt, apparently the result of the impregnation of the silt by salt carried in solution from the main salt mass. It is reported that some salt was secured from this deposit about 1895 to 1900, but apparently no mining has been done since that time.

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