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

that the Haycock Mountain Tuff (22.75 ± 0.12 Ma, Ed Sable, U.S. Geological Survey,unpublished data, 1996) and underlying alluvial gravels are unconformable on and thuspostdate the Markagunt megabreccia. Sable and Maldonado (1997a) interpreted theHaycock Mountain Tuff to be part of the upper plate of the Markagunt megabreccia, as adistal facies of the Leach Canyon Formation. Mapping in the Panguitch Lakequadrangle, described below (see description of the Leach Canyon Formation), however,now confirms that the 23.8 Ma Leach Canyon Formation and 22.8 Ma HaycockMountain Tuff are different units of slightly different age. Thus, the interpretation ofAnderson (1993, 2001) that the Haycock Mountain Tuff represents a post-Markaguntmegabrecciatuff that partly filled a stream channel eroded into the megabreccia appearseminently reasonable. The fact that the Haycock Mountain Tuff at its type location isundeformed was used by Anderson (1993) as evidence that it postdates emplacement ofthe megabreccia. However, it should be noted that the caprock of Haycock Mountain,although composed of resistant Isom Formation that is part of the megabreccia, alsoappears undeformed as little as 30 feet (10 m) above the basal gravity-slide plane.Furthermore, on the south side of Haycock Mountain, the basal slip surface of themegabreccia is in strata we tentatively identify as the 20 to 21 Ma Limerock CanyonFormation (it is possible that these beds are in the pre-30 Ma Brian Head Formation, or inthe 24 Ma Bear Valley Formation, or that the Limerock Canyon has a wider age rangethan we can now demonstrate; this awaits further geochemical and radiometric agecontrol). One final complication involves exposures in Parowan Canyon that wereinterpreted by Maldonado and Moore (1995) as Harmony Hills Tuff in normal faultcontact against Isom Formation, but that lead-author Biek reinterprets as Isom Formationthat is part of the Markagunt megabreccia that unconformably overlies the 22.03 MaHarmony Hills Tuff (i.e., the two units are separated by a gently southeast-dippinggravity-slide fault, not a steeply west-dipping normal fault). If this interpretation iscorrect, it suggests that the Markagunt megabreccia was emplaced sometime after 22.03Ma. To summarize, all workers agree that catastrophic emplacement of the Markaguntmegabreccia postdates the 23.8 Ma Leach Canyon Formation; most of us now agree thatit also postdates the 22.8 Ma Haycock Mountain Tuff. Lead-author Biek tentativelyinterprets the megabreccia as post-dating the 22.03 Ma Harmony Hills Tuff, suggestingthat it may be somewhat younger than most workers previously envisioned; ongoingmapping and pending 40 Ar/ 39 Ar age analyses may resolve timing of Markaguntmegabreccia emplacement.The Markagunt megabreccia was interpreted by Maldonado (1995) and Sable andMaldonado (1997a) to have formed by either gravity sliding off the Iron Peak laccolithand associated large shallow intrusive bodies or by low-angle, thinned-skinned thrustingaway from the intrusions about 20 to 22 million years ago. Anderson (2001), however,noted that the 20 to 21 Ma Iron Peak laccolith may be too young and too small to producea dome large enough to produce the megabreccia. Rather, Anderson (2001) suggestedthat the megabreccia originated from southward failure off the backslope of westnorthwest-strikingMiocene fault blocks.Interestingly, on-trend with the south margin of the Markagunt megabreccia in theBryce Canyon area, south-vergent thrust faults involving Upper Cretaceous andPaleocene-Eocene Claron Formation strata are well documented. These thrust faults havebeen interpreted to represent gravitational loading and collapse of the Marysvale volcanic29