| Description |
The Precambrian Mineral Fork Formation of the central Wasatch Mountains, Utah, consists of about 3,200 feet of dark grey conglomeratic sandy siltstone (tilloid), dark grey non-bedded siltstone, I dark grey very thin-bedded. siltstone, dark to light grey sandstone, and. light grey poorly sorted, pebble conglomerate. These lithologies were deposited in basins eroded into the underlying quartzites and phyllites of the Precambrian Big Cottonwood Series. Most· previous workers concluded that the r·1ineral Fork Formation is a tillite, but recently it was suggested that it may be a turbidite or a glaciomarine deposit. The problem in interpretation is a common one in studies of possible ancient glacial formations, and arises because of an afferent lack of characteristics diagnostic of a specific environment of deposition. Detailed stratigraphic and petrographic analyses conducted during the present study show that dark grey conglomeratic sandy siltst.one (tilloid) comprises about 70 percent of the formation. This ffi.3.terial has 18 genetically significant characteristics in cluing presence of angular clasts in all size ranges, extreme range of grain sizes, interbeds of other lithologies, .erosional contacts, lack of alteration in large clasts (in the type section), lack of bedding, Lack of vertical size gradation, lack of sorting, and several others. Thus, the conglomeratic sandy siltstone part of the formation is concluded to be a. true tillite. This conclusion can be safely stated even though common glacial characteristics such as striated bedrock floors and boulder pavements are not present. All other lithologies are interbedded in the tillite, and ware formed in closely associated glaciofluvial and glacial lacustrine depositional environments. On the basis of the characteristics of the tillite and the associated lithologies, it is concluded that the Mineral Fork Formation was deposited in a glacial to near glacial environment, and thus represents deposits formed during a Late Precambrian glaciation in north central Utah. |
