Interim Geologic Map of the West Part of - Utah Geological Survey

to the north, the main mass of the Markagunt megabreccia rests on Leach CanyonFormation. Because the southern margin of the megabreccia (and underlyingregional ash-flow tuffs) is erosional in nature, we do not know their southerndepositional limit. However, because debris from the Leach Canyon Formation ismissing in areas mapped as QTbx in the northern part of Cedar Breaks NationalMonument and at and near Blowhard Mountain, it seems likely that the LeachCanyon did not extend much further south than its present-day outcrop. Becausethere is no evidence for a post-Leach Canyon (but pre-Markagunt megabreccia)unconformity that cuts out strata southward across the west edge of theMarkagunt Plateau, lead-author Biek interprets these large Isom blocks to belandslide remnants that are at a lower structural level than the main mass of theMarkagunt megabreccia, inferring that the blocks reflect late Tertiary andQuaternary northward retreat of the erosional escarpment that stretches fromBrian Head peak eastward to Haycock Mountain. Thus in this view, similar tothat suggested by Moore (1992) and Sable and Maldonado (1997a), the blocks areremobilized parts of the Markagunt megabreccia, the southern extent of whichwas emplaced on a paleohigh of Brian Head strata that served to constrain thesouthern limit of the Isom and Leach regional ash-flow tuffs; the megabreccia hassince been let down to its present position principally by landsliding in lateTertiary and Quaternary time, with smectitic clays of the Brian Head Formationproviding the weak shear surface for downslope movement of the blocks.Hatfield and others (2003), Moore and others (2004), and Rowley and others (inpreparation), however, interpreted the blocks as a bona fide part of the MioceneMarkagunt megabreccia, which thus must have been emplaced as far south asBlowhard Mountain over a significant Miocene unconformity; subsequentweathering and sapping of the megabreccia and underlying Claron Formation thenspread debris southward to the area beyond State Route 14. Our differences ininterpretation reflect this uncertainty.QTaf High-level fan alluvium (Pleistocene and Pliocene?) – Poorly to moderatelysorted, non-stratified, subangular to subrounded, boulder- to clay-size sediment;mapped in the Sevier Valley southeast of Panguitch, where it forms deeplydissected surfaces; deposited principally as debris flows and debris floods;exposed thickness is about 100 feet (30 m).QTap High-level pediment alluvium (Pleistocene and Pliocene?) – Moderately sorted,subrounded to rounded pebble to boulder gravel and sand that forms a gently eastdipping,locally resistant cap over the Limerock Canyon Formation (Tl) and upperTertiary fan alluvium (Taf) near the east margin of the Markagunt Plateau; surfaceof deposit typically covered by veneer of pebble and cobble residuum; divisibleinto two different levels (Moore and Straub, 1995), but undivided here due to mapscale; deposited principally as debris flows, debris floods, and in ephemeralstream channels; probably less than 20 feet (