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

and Rowley and others (in preparation) mapped or called this rubble (here mapped asQTbx) “Markagunt megabreccia,” although they noted its unconsolidated nature.Most reports describe the megabreccia as consisting of house-size to city-blocksizeblocks, or even blocks that are as much as one square mile (2.5 km 2 ) in size, but inthis map area (which covers only the southern part of the megabreccia outcrop belt), wesee the megabreccia principally as a large sheet, tens of square miles in extent, of mostlyintact Isom Formation and comparatively minor amounts of thin underlying Wah WahSprings and Brian Head Formations and overlying mafic block and ash-flow tuff, BearValley Formation, and Mount Dutton Formation that has moved more or less as acoherent mass and remained in proper stratigraphic order. Exposures of the megabrecciaare limited so it is difficult to ascertain attitudes of individual units, but outcrop patternssuggest that most of the strata within the megabreccia, and the megabreccia as a whole,dips gently east following the regional dip of the plateau. Only in a few locations in themap area, as northwest of Castle Valley and north of Bunker Creek in the SW1/4 section35, T. 35 S., R. 8 W., are strata seen to dip moderately 20° to 25° northeast. Clearly theremust be faults within the megabreccia that bound isolated tilted blocks such as these, butthey are not readily discernable on aerial photographs and are thus impractical to map at1:100,000 scale.The basal slip surface of the Markagunt megabreccia generally dips gently east(mimicking the regional dip of the plateau because it was tilted with underlying stratafollowing its emplacement) and south (because the inferred source of the megabrecciawas to the north; Sable and Maldonado, 1997a; Anderson, 2001), but at HaycockMountain the basal slip surface dips north. The northward-dipping Isom Formation (caprock of Haycock Mountain) was interpreted by Anderson (1993) and Sable andMaldonado (1997a) as autochthonous, and they also interpreted autochthonous IsomFormation at the type area of the megabreccia along Highway 143 east of PanguitchLake. However, we identified a previously unreported basal conglomerate and associatedclastic dikes exposed at the base of the megabreccia on the south side of HaycockMountain (figures 1a, 1b; 2a, 2b, and 2c). These exposures show that the entire Isomsection is likely part of the gravity slide. If true, the northward dip likely reflectsthrusting and folding in the toe of the gravity slide, not post-megabreccia tilting andfolding. Moderately northeast-dipping blocks near Castle Valley and Bunker Creek,described above, may also reflect thrusting and folding in the toe area of the Markaguntmegabreccia gravity slide. Just south of Panguitch Lake, Claron and Brian Head stratadip moderately to the northwest, and this may reflect folding above a structurally deeperlevel of the gravity slide. Several previous workers reported slickenlines on the basal slipsurface of the megabreccia, as well as roche-moutonnée-like features and tilted beds, thatcollectively suggest southward transport. Slickenlines at the base of the megabrecciaexposed on the south side of Haycock Mountain, as well as clastic dikes, alsodemonstrate south-southeast transport, as well as catastrophic emplacement by gravitysliding.Among the authors of this map, there remains disagreement as to the age ofemplacement of the Markagunt megabreccia; Anderson (2001) described the key pointsof this disagreement, and an additional complication is described below. The resolutionof this problem involves, among other issues, the Haycock Mountain Tuff in the type areaof the Markagunt megabreccia, first described in detail by Anderson (1993). He reasoned28