Holocene faulting in the western Grand Canyon, Arizona PETER W. HUNTOON Department of Geology and Water Resources Research Institute, University of Wyoming, Laramie, Wyoming 82071 ABSTRACT pre-1964 literature on events associated with the entire Hurricane fault zone of Utah and Arizona appears in Averitt (1964). The The Toroweap and Hurricane faults and a subsidiary fault in the findings of these workers and others substantiate that the southern western Grand Canyon exhibit evidence of Holocene movement. Colorado Plateau was experiencing compressional stress through- This evidence includes scarps in alluvium and sediments ponded out the Laramide orogeny, during which folds, usually east-dipping against a fault on the downthrown block. These displacements are monoclines, were formed (Huntoon, 1974). By Miocene time the the latest in a well-exposed record of recurrent movements along region was under tension, and a system of high-angle normal faults the major faults in the region. developed, many along the trends of the earlier Laramide folds. The tensional environment has persisted to the present. Reverse drag, INTRODUCTION defined as downfolding along the downthrown side of a fault an- tithetic to the displacement, developed along many of the faults in In this paper I document newly discovered examples of Holocene the region contemporaneously with Cenozoic movements faulting in the western Grand Canyon and place this faulting in (Hamblin, 1965). perspective as part of the record of recurrent movements along the major faults in the area. RECENT FAULTING The arid climate, topographic relief, and presence of successive lava flows and young alluvial sediments in the western Grand Can- A detailed examination of the literature and field work convinces yon combine to offer classic exposures in which recurrent move- me that our ability to distinguish additional Cenozoic movements ments along several of the major faults can be readily documented along faults such as the Hurricane or Toroweap is limited only by (see Fig. 1). A partial record of recurrent movements along the To- the existing number of successively young lava flows and other dat- roweap and Hurricane faults in and immediately adjacent to the able strata that cross the faults. It is clear, as stated by Hamblin Grand Canyon now exists through the efforts of Dutton (1882), (1974, p. 169), that movement along these faults has been relatively Davis (1903), Huntington and Goldthwait (1903), Gardner (1941), continuous, at least from Miocene to Holocene time. McKee and Schenk (1942), Koons (1945), Kurie (1966), Hamblin William Morris Davis (1903, p. 20) made the startling discovery (1970a), Young (1970), and Lovejoy (1973). A summary of the that there were at least 6 m of Holocene movement along the To- roweap fault that resulted in displacement of alluvium in Prospect Valley immediately south of the Colorado River (Fig. 2). Such a fresh scarp was most unusual for the region, and 1 am not aware of a discovery of such a feature in the southwestern part of the Col- orado Plateau previous to Davis's work. Although no one would deny the existence of Holocene faulting in the southwestern part of the Colorado Plateau, evidence to document such features is 0 5 10 Kilometers E3HE5CE5 I Figure 2. Holocene fault scarp in alluvium and lava along Toroweap Figure 1, Location of the faults described. Shaded parts of fault traces fault in Prospect Valley. Notice that lava flow to right of cone is displaced are those that exhibit evidence of Holocene movement. more than alluvium to left of cone. View is toward southeast. Geological Society of America Bulletin, v. 88, p. 1619-1622, 4 figs., November 1977, Doc. no. 71108. 1619 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/11/1619/3429262/i0016-7606-88-11-1619.pdf by guest on 27 September 2021 1620 P. W. HUNTOON difficult to establish. This is primarily because rocks younger than Mesozoic in age are largely stripped from the area. For example, the youngest rocks composing the land surface in the vicinity of the eastern Grand Canyon are Permian limestones. Koons (1945, p. 160) observed that the same scarp recorded by Davis was present on the eastern side of Vulcan's Throne, a small volcano situated on the Toroweap fault on the north edge of the gorge of the Colorado River. Hamblin (1970a, p. 12) identified a Holocene 15-m scarp in lava rocks along the Hurricane fault a few kilometres north of the Colorado River. Koons (1964, p. 104, 106, 107) documented Holocene faulting along major faults on the Hualapai Indian Reservation within 50 km of the Colorado River and east of the Hurricane fault. His observations were of faults that have not been proven to be as recent as those observed by Davis or Hamblin, but they do displace Quaternary lavas and gravels. Av- eritt (1964) summarized data from Townley and Allen (1939), Heck (1947), and the U.S. Coast and Geodetic Survey (1928- Figure 3. Holocene fault scarps in alluvium along Hurricane fault south 1951), in which are listed 15 minor tremors attributable to move- of the mouth of Whitmore Wash. Notice adjustment of stream channel in ment along the Hurricane fault between 1881 and 1951. Sturgul lower right corner. Inner gorge of Colorado River at top. View is toward and Irwin (1971) updated earthquake data for the region through the east. 1966. Field mapping conducted by myself, George Billingsley, and James Sears led to the discovery of additional evidence for HURRICANE FAULT Holocene faulting along several kilometres of the Toroweap fault, the Hurricane fault, and a subsidiary fault west of the Hurricane The Hurricane fault is a high-angle normal fault that marks the fault. In the cases of the Hurricane and Toroweap faults, we have boundary between the Colorado Plateau and Basin and Range located 1.5- to 6-m scarps in alluvium which are becoming partially provinces in Utah; however, the Grand Wash fault is defined as the eroded and rounded. The recency of faulting along the subsidiary province boundary 55 km to the west of Hurricane fault in the fault is revealed by locally ponded sediments on the downthrown western Grand Canyon region of Arizona. The strike of the Hur- block, which are now partially dissected by a small drainage ricane fault in the vicinity of the Grand Canyon is sinuous but channel. maintains a northerly regional trend. The west side is displaced down, and the fault is characterized by reverse drag north of the TOROWEAP FAULT Colorado River. The 10-km-wide block of Paleozoic rocks that separates the Hurricane and Toroweap faults in the Grand Canyon The Toroweap fault is an extensive, north-trending, high-angle is broken by numerous high-angle normal faults that trend predom- normal fault that has displaced the Paleozoic rocks along the Col- inantly toward the northwest. orado River down to the west a total of 193 m (McKee and Schenk, Rocks younger than Paleozoic are virtually missing along the 1942, p. 262). A 1.2 ± 0.6-m.y.-old lava flow (Damon, 1965, p. Hurricane fault in the Grand Canyon south of Whitmore Wash. 42) that occurs near river level has been displaced 44.5 m by the However, the mouth of Whitmore Wash provides classic exposures fault. Older lava flows that fill Prospect Valley have been displaced in which are recorded evidence for at least three periods of recur- 46 m 2.5 km south of the river (Hamblin, 1970a, p. 17). North of rent movement. The Paleozoic section is displaced on the order of Volcan's Throne, Koons (1945, p. 160) described various flows 300 m by the fault at this site. Basalt flows that fill an old course of that are displaced between 11 and 46 m along the fault. These flows Whitmore Wash to a depth of about 300 m are displaced 23 m by are classified by Koons as stage Ilia and were reclassified by the fault. These lavas are similar to the older basalts that fill To- Hamblin (1970b) as stage IV. Koons (1945, p. 160) correlated the roweap Valley and are no younger than early stage III flows flows with others in the vicinity that have been dated in the (Hamblin, 1974, p. 156). Stage IV flows that crossed the fault and 25,000-to 30,000—yrB.P. range (McKee and others, 1967, p. 44). cascaded into Whitmore Wash are displaced about 15 m, a rela- The fresh scarp in the alluvium of Prospect Valley described by tionship cited by Hamblin (1970a, p. 12). This scarp is Holocene in Davis (1903) can be traced about 5.5 km to the south of the Col- age. orado River. As shown in Figure 2, both alluvium and a cinder cone During the course of this investigation, an apparently younger and associated flow are displaced along the scarp. The cinder cone Holocene scarp, coincident with Hamblin's and which alternately lies 4 km south of the Colorado River and is in the stage IV group displaced lavas and alluvium, was traced 14 km northward along of Hamblin (1970b), which suggests a date of eruption that is Whitmore Wash from the Colorado River. The scarp is less than probably more recent than 30,000 yr B.P. The alluvium is offset 4.5 m high and is rounded by erosion. North of the mouth of about 6 m in the vicinity of the cone; however, the lava from the Whitmore Wash, the scarp is a single break that occurs at eleva- cone is displaced below the surface on the downthrown western tions between 800 and 1,300 m. As shown in Figure 3, a short block, which requires a vertical throw of at least 15 m.