Arizona Geological Society Digest, Volume XII, 1980 177 The Muddy Creek Formation at Colorado River in Grand Wash: The Dilemma of the Immovable Object by Earl M. P. Lovejoy1 Abstract It has previously been noted that the Colorado River could not have flowed west through Grand Wash contemporaneously with deposition of the Muddy Creek fanglomerates. This paper suggests that the Colorado River turned sharply south in Grand Wash and flowed into Red Lake Valley in "Muddy Creek time." Sandstones in Grand Wash at the mouth of Grand Canyon are considered here to be gravel-deficient river sands, gradational with the contemporaneous fanglomerate laterally and with the younger and higher Hualapai limestone vertically . If this is correct, then the earlier course of the Colorado River could have been through Grand Canyon to Grand Wash throughout Cenozoic time, following its Paleogene origin as an obsequent stream; thus, the Muddy Creek Formation in Grand Wash need not be considered the "immovable object" that Hunt considered it to represent, and the evi­ dence upstream of an early origin for the river system, Hunt's "irresistible force," becomes compatible with the evidence at Grand Wash. Introduction Solution of this problem would throw a flood of light on regional Tertiary events Hunt (1956) summarized evidence from the and relationships that are now obscure Colorado Plateau that indicates a great span [po 817]. [And] it is not possible to out­ for the period of post-uplift erosion, Hunt's line a full history of the Colorado River "irresistible force." The Muddy Creek Forma­ without resort to speculation [p. 834]. tion, a Pliocene fanglomerate, lies athwart the course of the Colorado River at Grand Wash Neither Blackwelder (1934) nor Longwell (Fig. 1) in a position believed by Blackwelder (1946) believed that the Colorado River could (1934), Longwell (1946), and Lucchitta (1966) have flowed through Grand Wash before or dur­ to imply that the river could not have flowed ing deposition of the Muddy Creek Formation. through Grand Wash prior to or during deposi­ Lucchitta (1966, 1967, 1972) and Goetz and oth­ tion of the Muddy Creek fanglomerates, the "im­ ers (1975) have accepted their interpretations movable object" of Hunt (1956). The dilemma of and have presented a hypothesis for an alterna­ the "irresistible force" and the "immovable object" tive course for the Colorado River north of its has led many to seek either alternative courses present position. Shoemaker (1975) has ex­ for the Colorado River, either to the south tended this concept and hypothesized a course through the Peach Springs and Truxton Val­ all the way to the Pacific Ocean across Nevada leys, or to the north near St. George, Utah, prior to development of basin-and-range struc­ or alternatives to the fluvial process, prior to ture. Thus, the hypothesis conceived by deposition of the Muddy Creek sediments Blackwelder (1934) and fortified by Longwell (Hunt, 1969; Goetz and others, 1975). ( 1946) remains with us. Longwell (1946) wrote: Nevertheless, on the baSis of regional studies in the Basin and Range and Colorado One of the major unresolved problems of Plateau provinces, I believe that an alternative the region is the date of the origin of hypothesis based on the available evidence the Colorado River itself as a through­ might provide a reasonable and alternative an­ flowing stream in its present course swer to some of the "major unsolved problems" of the region . If the alternative interpretation of the evidence at Grand Wash offered here is IDepartment of Geological Sciences, The reasonable, the Grand Wash dilemma may not University of Texas at El Paso, El Paso, Texas exist because the "immovable object," the Mud­ 79968. dy Creek Formation at Grand Wash, would then 178 Earl M. P. Lovejoy NEVA DA UTA H 0 20 40 60 80 /00 A R / Z 0 NA - *Till 0 /0 20 30 40 50 0 laJ 2(X) 300 ill) $CALE SC ALE km UTA H W E 8 A SIN and COLORADO S RAN G £ N E V A D A Uinkarflf Plaff/au PLATEAU aNA Fig. 1. Index map of study area. be shown not to have been "immovable" after Time of Uplift of the all. Colorado Plateau As Blackwelder (1934, p. 564) noted: Solution of the problem at Grand Wash de ­ "Science advances not only by the discovery pends greatly on the determination of the period of facts but also by the proposal and consider­ of relative uplift of the Colorado Plateau with ation of hypothesis . II respect to the Basin and Range province along Earl M. P. Lovejoy 179 its great boundary faults, the Grand Wash and The Muddy Creek Formation fills a deposi­ Hurricane faults. Longwell (1945) believed that tional trough west of the Grand Wash Cliffs. the Grand Wash fault was of Laramide age. The plateau block was uplifted with respect to Shoemaker (1975) is the latest in a long list of the Basin and Range province to the west along workers beginning with Gardner (1941) who the Grand Wash fault prior to "Muddy Creek have presented substantial and cogent evidence time," because as Longwell (1945) showed, the that the Colorado Plateau uplift began as late fault lies buried beneath the unfaulted Muddy as Miocene time. Certainly, a Pliocene uplift Creek Formation in Grand Wash. Along the (McKee and others, 1967) after deposition of west side of Grand Wash is Wheeler Ridge, the Muddy Creek deposits at Grand Wash is not composed of Paleozoic strata, which dip up to possible because the Muddy Creek Formation 70° E. in an excellent example of reverse drag has not been displaced in Grand Wash along (cL Longwell, 1945). On the west side of the Grand Wash fault (cL McKee and McKee, Wheeler Ridge is Wheeler fault, which has dis­ 1972, and Lovejoy, 1973b). Indeed, Anderson placed Muddy Creek strata down on the west and Mehnert (1979) have concluded that most side only 500 to 1,200 feet (150 to 360 m) if not all of the Colorado Plateau uplift on the (Lucchitta, 1966, p. 140-142), several kilo­ Hurricane fault occurred in the Quaternary. meters west of the buried Grand Wash fault. However, there is excellent evidence along Stratigraphic separation on Grand Wash fault the only place in the Hurricane fault zone where is unknown, although Longwell (1945, p. 114) the date of faulting can be clearly demonstrated suggested that it might be as much as "20,000 that 85 percent of the stratigraphic separation ft" (6,100 m). Thus, at Grand Wash, post­ on the Hurricane fault occurred in pre-Miocene Muddy Creek throw represents less than 6 per­ time and that faulting had begun in Paleocene cent of the Cenozoic fault throw . time (Lovejoy, 1964, 1973a, 1978b). Further, evidence along the northern extension of the The pertinent late Neogene stratigraphy in Grand Wash fault (Moore, 1972), which Dobbin Grand Wash includes, among others, three (1939) termed the "Cedar Pocket Canyon fault," units important in this analysis; (1) the fan­ indicates a pre-Miocene period for most of the glomerate facies of the Muddy Creek Formation stratigraphic separation on that fault (Lovejoy , noted by Blackwelder (1934), (2) "the sand­ 1976a). Evidence supplied by Young and Bren­ stone-siltstone facies" of the Muddy Creek nan (1974) is interpreted to indicate major fault Formation noted by Lucchitta (1966, p. 99-105), movement· prior to 17 m. y. ago . and (3) the Hualapai lacustrine limestone noted by Longwell (1946) . I suggest that a Laramide age for the Colo­ rado Plateau uplift ought to be considered as The Fanglomerate Facies of a basis for further geomorphic analysis . the Muddy Creek Formation Therefore, alternative explanations for the genesis of the Colorado River, tied to the tec­ The fanglomerate facies or piedmont depos­ tonics of the region, as noted by Longwell its described by Blackwelder (1934) and Long­ (1946), perhaps might be considered in addi­ well (1946) consist of a sequence of fanglomer­ tion to those of McKee and others (1968) ; but ates up to 90 m thick, which contains clasts of compare Hunt (1968), Lucchitta (1972), and Rapakivi granite and schists derived from the Hamblin (1970) and other contributors to the west. The bulk of the deposit consists of cob­ literature on the subject. bles and small boulders of schist and granite derived from the Precambrian terrane 8- 10 km west of Grand Wash. Lake Mead now locally covers its base in Grand Wash. Where it lies Late Neogene Stratigraphy on the Kaibab Formation east of Wheeler Ridge at Grand Wash local accumulations of Paleozoic carbonate cob­ bles and boulders occur, but nowhere are they Blackwelder (1934) and Longwell (1946) con­ stratigraphically higher than 10 m above their Sidered the Muddy Creek Formation to repre­ contact with the limestone; these cobbles were sent a great amount of boulder gravel trans­ locally derived from the Kaibab formation of ported east from the vicinity of Bonelli Peak Wheeler Ridge as it was being buried by Muddy (Figs. 2 and 3) to the foot of the Grand Wash Creek boulders from the west. The Rapakivi Cliffs. Boulders up to 6 m long of Rapakivi granite boulders were probably carried in mud granite (Volborth, 1962) traceable to Bonelli flows down the old (now deeply dissected) pedi­ Peak moved east down a piedmont slope . They ment surface between Bonelli Peak and Grand­ lie on both sides of the west-flowing Colorado Grapevine Wash . This facies can be followed River east of Wheeler Ridge; the surface lag south 40 km from the Colorado River along the boulders represent a disproportionately large west flank of Grapevine Wash. Thus, condi­ part of the clasts on the present surface (Figs .