THE FRENCHMAN MOUNTAIN DOLOSTONE: A NEW FORMATION WITHIN THE TONTO GROUP OF GRAND CANYON

The Cambrian formations of Grand Canyon (Tapeats Ss, Bright Angel Shale, Muav Ls) comprise the Tonto Group. Conformably overlying the Muav Limestone is an interval of unfossiliferous dolomite which E. D. McKee, in his classic 1945 monograph “Cambrian History of the Grand Canyon Region,” separated from the Muav as “Undifferentiated Dolomites.” These strata have remained unnamed and undated since 1945.

In order to sort out the age and correlation of McKee’s “undifferentiated dolomites,” we measured sections in Grand Canyon and across the Lake Mead Basin to Frenchman Mountain at Las Vegas. Using sequence stratigraphy and lithostratigraphy, we were able to correlate the cratonal “undifferentiated dolomites” with roughly the upper two-thirds of the Banded Mountain Member of the Bonanza King Formation in the Basin-and-Range Province. We identified seven third-order depositional sequences within this interval, each of which is represented by an upward-shallowing succession of facies that is bounded at the top by a zone of maximum exposure.

The “undifferentiated dolomites” range in thickness from 30 m in the eastern Grand Canyon to 371 m at Frenchman Mountain. Thicknesses increase sharply west of the Grand Wash Cliffs, which suggests the presence of a Cambrian hinge line at approximately this position. In all Grand Canyon sections these dolomites are unconformably overlain by the Devonian Temple Butte Formation. However, at Frenchman Mountain the “undifferentiated dolomites” are conformably overlain by the well-dated, trilobite-bearing, Furongian-age (Cambrian Series 4) Nopah Formation. Thus we can say that McKee’s “undifferentiated dolomites” are Miaolingian (Cambrian Series 3) in age. Due to pre-Temple Butte erosion, the top of this interval becomes progressively older toward the east. We propose formally naming this interval the Frenchman Mountain Dolostone, with its type section at Frenchman Mountain.