MARKAGUNT MEGABRECCIA (MIOCENE) AND MOUNT DUTTON FORMATION DEBRIS FLOW DEPOSITIONAL CONTACTS, FIVEMILE RIDGE QUADRANGLE AND VICINITY, HIGH PLATEAUS, UTAH: ONE HEAD OF THE MYSTICAL CREATURE

The Mount Dutton Formation (MD) is a thick (45-2000 m), mostly volcaniclastic debris flow deposit that blankets large areas of the High Plateaus, Utah. It constitutes the latest major regional depositional event on the High Plateaus and its areal extent overlaps with that of the underlying Markagunt Megabreccia (MM), an allochthonous mass of Oligocene rock on the Markagunt Plateau and vicinity. Evidence from the contacts of these units indicates that deposition of the MD was in part coeval with that of the MM and that a massive MD debris flow was the cause and agent of transportation of the MM.

The MD appears to be absolutely pristine at many sites and its contacts with large, fractured, allochthonous blocks of the MM are sharp, uneven, depositional contacts, not faults. Lack of an intervening layer of sediment between the two masses of rock and steeply dipping contacts further indicate that the contacts are depositional in nature and not an eroded, pre-MD substrate. Therefore, the allochthonous blocks of the MM must have been transported as clasts in the suspended or traction load of a massive MD debris flow. The upper limit of clast size in the MD is unknown, but may range up to the allochthonous Isom Formation of Haycock Mountain.

The MD debris flow at other sites exhibits a pervasive, penetrative fabric of closely-spaced, cm- to dm-scale shear planes combined with brecciation, thrust shears and folds in underlying MM rocks, all of which are indicative of high lateral compressive stress. Contacts in some of these cases exhibit slickensides and, at a few sites, pseudotachylyte(?) along the contacts and in fractures which emanate from them. However, the contacts and associated fill material typically lack the shear fabric of the MD. The fact that all of these features are at relatively shallow stratigraphic and structural levels is evidence that the shear force originated at the surface, then propagated downward, as the MD debris flow impacted and overrode the substrate.

The contacts between the MD and rocks involved in the MM, the overlapping areal distribution, and the overall size, shape and geometry of these formations suggest that the MM is the result of a massive bulking debris flow and distal runout, with deeper décollement failure triggered by an enormous advancing surge head or by cumulative MD depositional loading of the landscape.