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

Sevier Plateau was uplifted along the Sevier fault zone at least 6000 feet (2000m). Anderson (1987) provided evidence that basin-fill deposits once filled theancestral Sevier Valley to a depth at least 1000 feet (300 m) above the modernriver where it cuts through Circleville Canyon, immediately north of the map area,showing that Circleville Canyon was cut by superposition of the Sevier Riveracross the resistant Spry intrusion and vent-facies rocks of the Mount DuttonFormation.TvfUpper Tertiary fine-grained basin fill (Miocene) – Light-brown, pinkish-gray,and white tuffaceous mudstone, siltstone, fine-grained sandstone, and localdiatomite; moderately to poorly consolidated; laminated to thick beds, locallywith small gastropods; contains few thin beds of peloidal micritic limestone;exposed along Sevier Valley southeast of Panguitch; likely deposited in smalllake basins and floodplains (Moore and Straub, 1995); exposed thickness about100 feet (30 m).unconformityMarkagunt megabreccia (lower Miocene) – Structurally chaotic assemblage ofMiocene and Oligocene regional ash-flow tuff, local volcanic rock, and lessersedimentary strata that covers much of the central and northern Markagunt Plateau;mapping and stratigraphic studies during the 1970s to 1990s show how understanding ofthis complex unit has evolved and continues to be controversial, as summarized byMaldonado and others (1992), Anderson (1993), Moore and Nealey (1993), Sable andMaldonado (1997a), Hatfield and others (2003, 2004), Moore and others (2004), andRowley and others (in preparation). Sable and Maldonado (1997a) noted that fourseparate rock units have been termed the megabreccia, including (1) primary volcanicmudflow deposits, (2) megabreccia that resulted from collapse of high-angle fault scarps,(3) megabreccia associated with the Red Hills shear zone (Maldonado and others, 1989,1992; Maldonado, 1995), and (4) the principal mass of the Markagunt megabreccia thatcovers much of the central and northern Markagunt Plateau; to this we add a fifth unit,namely unconsolidated megabreccia rubble. Sable and Maldonado (1997a) restricted theterm to unit (4), with which we concur, noting that unit 1 consists of primary volcanosedimentarybreccia, likely the alluvial facies of the 22-32 Ma Mount Dutton Formation;that unit 2 is now known to be a large, modern landslide complex below Black Ledge(Maldonado and others, 1997; Rowley and others, in preparation); and that unit 3 isgeographically separate from, but may be a dismembered part of, the main mass of theMarkagunt megabreccia. We interpret the unconsolidated rubble breccia (the fifth unit),located south of the main mass of the Markagunt megabreccia, to be simply theweathering product of the Markagunt megabreccia – residuum, colluvium, landslide andcollapse material, and alluvium, here collectively mapped as QTbx – that is commonlypresent at a lower structural level along its distal southern margin (such rubble is alsopresent on the main mass of megabreccia, but it is not practical to differentiate such lateTertiary and Quaternary weathering products where they overlie the rubble of theMiocene megabreccia itself). Hatfield and others (2003, 2004), Moore and others (2004),27