Unconformity at the Cardenas-Nankoweap contact (Precambrian), Grand Canyon Supergroup, northern Arizona DONALD P. ELSTON I U.S. Geological Survey, Flagstaff, Arizona 86001 G. ROBERT SCOTT* ' ABSTRACT sedimentary and volcanic rocks about Group (Noble, 1914) and overlying strata 4,000 m thick overlies crystalline basement of the Chuar Group (Ford and Breed, Red-bed strata of the Nankoweap For- rocks (—1.7 b.y. old; Pasteels and Silver, 1973). Because the Nankoweap had not mation unconformably overlie the 1965). These strata, in turn, are overlain by been subdivided into named formations, —l,100-m.y.-old Cardenas Lavas of the sandstone of Early and Middle Cambrian Maxson (1961) reduced the Nankoweap to Unkar Group in the eastern Grand Canyon. age. Near the middle of the series, a 300- formational rank and renamed it the Nan- An unconformity and an apparent discon- m-thick section of basaltic lava flows is un- koweap Formation, a usage herein adopted formity are present. At most places the conformably overlain by about 100 m of (Art. 9a, 15c, American Commission on upper member of the Nankoweap overlies red-brown and purplish sandstone and Stratigraphic Nomenclature, 1961). The the Cardenas, and locally an angular dis- siltstone. The lavas — named the Cardenas Grand Canyon Series of Walcott (1894) cordance can be recognized that reflects the Lavas or Cardenas Lava Series by Keyes thus consists of three distinct major units; truncation of 60 m of Cardenas. This un- (1938), and adopted as the Cardenas Lavas they are, in ascending order, the Unkar conformity also underlies a newly recog- by Ford and others (1972) — and the over- Group, Nankoweap Formation, and Chuar nized ferruginous sandstone of probable lying red sandstone were included in the Group; this series is here redesignated the local extent that underlies the upper upper part of Walcott's (1894) Precam- Grand Canyon Supergroup, as current member and that herein is called the fer- brian Unkar terrane. Van Gundy (1934) stratigraphic practice dictates (Art. 9e, ruginous member of the Nankoweap. named the red sandstone the Nankoweap American Commission on Stratigraphic Truncation of the Cardenas beneath the Group and reported (1951) unconformities Nomenclature, 1961). Appendix 1 sum- ferruginous and upper members locally at its lower and upper contacts. Maxson marizes the lithologie characteristics and may have been as much as 300 m. Basal (1961) subsequently called this unit the thicknesses of the formations and the conglomeratic sandstone of the upper Nankoweap Formation, but he showed no stratigraphic nomenclature for the Grand member locally overlies the ferruginous unconformity at its base in the explanation Canyon Supergroup. member with apparent disconformity, for his geologic map (1967), and he in- reflecting a probable hiatus in deposition. cluded the Nankoweap as the uppermost GEOLOGIC SETTING Stratigraphic and structural relationships formation in the Unkar Group. indicate that a ferruginous weathered zone Information that sheds light on this The Grand Canyon Supergroup is well was developed on an erosionally truncated stratigraphic problem has been obtained exposed in the eastern Grand Canyon (Fig. section of the Cardenas before, and perhaps during paleomagnetic investigations that 1). The beds dip commonly about 8° or less during, the time of deposition of the fer- are being carried out on stratified Precam- and are cut by a few normal faults (see, for ruginous member. Erosion of the ferrugi- brian rocks of the Grand Canyon. Results example, Maxson, 1967). The strata dip nous weathered zone provided material for of the initial investigation near Tanner generally to the east and overlie crystalline the ferruginous member of the Nankoweap. Canyon have already been summarized basement. The ferruginous weathered zone locally was (Elston and Scott, 1973). Key exposures in two areas record criti- faulted against unweathered Cardenas be- cal details of the contact relations between fore deposition of the upper member of the Stratigraphic Nomenclature the Cardenas Lavas and Nankoweap For- Nankoweap. mation. One area is in the graben im- The Nankoweap Formation is discon- New field observations indicate that a mediately north of the Tanner Canyon formably overlain by marine strata of the significant interval of time elapsed between rapids, and the other area is on the south Chuar Group. Three distinct units sepa- deposition of the Cardenas Lavas and side of the Colorado River in the unnamed rated by unconformities (the Unkar Group, deposition of the lower and upper members canyon 0.5 km west of Tanner Canyon Nankoweap Formation, and Chuar Group) of the overlying Nankoweap Formation. (Fig. 2). thus are recognized in the Grand Canyon The earlier part of this interval locally was Series of Walcott. Following current marked by the erosional truncation of a CARDENAS LAVAS stratigraphic practice, the Grand Canyon complete section of the Cardenas and the Series is herein redesignated the Grand chemical weathering of the exposed lava Canyon Supergroup. flows. The later part of this interval was The Cardenas Lavas are about 290 m marked by faulting of the weathered zone thick in the Basalt Canyon—Tanner Canyon INTRODUCTION against unweathered flows, followed by area (Figs. 1, 2; Appendix 1). The forma- erosion and then deposition of the upper tion consists of a lower rubbly weathering, In the eastern Grand Canyon (Fig. 1), a member of the Nankoweap. The evidence slope-forming unit that is about 90 m thick section of little-disturbed Precambrian supports Van Gundy's (1951) conclusion and of an overlying cliff-forming unit ~200 * Present address: Division of Geosciences, Uni- that his Nankoweap Group is a unit sepa- m thick, which includes seven or more versity of Dallas, Dallas, Texas 75320. rate from underlying strata of the Unkar flows separated by layers of silicified Geological Society of America Bulletin, v. 87, p. 1763-1772, 4 figs., December 1976, Doc. no. 61210. 1763 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/12/1763/3418460/i0016-7606-87-12-1763.pdf by guest on 25 September 2021 EXPLANATION Cambrian rocks Gbt, Bright Angel Shale and underlying Tapeats Sandstone Gbt Gt Gt, Tapeats Sandstone Unconformi ty Precambrian rocks Grand Canyon Supergroup S Chuar Group, undivided Unconformity Nankoweap Formation pGnu Upper member Di sconformity pGnfj Ferruginous member Unconformity p€wz Ferruginous weathered zone on figure 2 Cardenas Lavas Unkar Group » Cardenas Lavas (C) pGd Dox Sandstone (D) pGs Shinumo Quartzite Unconformi ty pGh Hakatai Shale 2MILES Bass Limestone and underlying pGb Hotauta Conglomerate 2 KILOMETERS Unconformi ty pGv Vishnu Schist Figure 1. Geologic map of Precambrian strata, eastern Grand Canyon, Arizona. Geology generalized and modified from Maxson (1967). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/12/1763/3418460/i0016-7606-87-12-1763.pdf by guest on 25 September 2021 UNCONFORMITY AT THE CARDENAS-NANKOWEAP CONTACT 1765 sandstone that range from <1 to ~6 m in structures. The Cardenas thus gives every either at high temperatures during thickness. The lowest flow of the Cardenas indication of having been emplaced sub- emplacement and cooling of the individual is highly vesiculated and discontinuous, aerially. flows or at low temperature after emplace- and it intertongues with the Dox Sandstone Mineralogically and chemically, the ment of the flows. This disseminated hema- (Figs. 2, 3B). Most of the flows display flows are basalt and basaltic andesite (Ap- tite is older than the hematite of the fer- characteristics attributable to subaerial ex- pendix 2; Hendricks and Lucchitta, 1974, ruginous weathered zone. trusion. Vesiculated and rubbly tops for in- Table 1). The dark-gray flow rock com- A whole-rock Rb-Sr isochron has re- dividual flows are common, particularly in monly has a decided reddish cast on fresh vealed an age of 1,090 ± 70 m.y. for the the cliff-forming upper unit. A few small surfaces, caused by disseminated hematite Cardenas Lavas (McKee and Noble, 1974). (~0.3 m) spheroidal structures occur in ex- (see description of intermediate-level flow, K-Ar ages that have been reported for the posures in Basalt Canyon. They are inter- Appendix 2). Apparently hematite was a lavas commonly are 200 m.y. or more preted as weathering features, not pillow ubiquitous oxidation product, forming younger, and presumably reflect loss of EXPLANATION Figure 2. Reconnaissance geologic map of Precambrian and basal Cambrian strata in Tanner Canyon-Basalt Canyon area, eastern Grand E Canyon, Arizona. Geology modified from Maxson (1967). Alluvium Talus and landslide deposits O P. Unconformity n 3 -et i Tapeats Sandstone Unconformity ¡H Chuar Group Unconformity p€nu [ p€nf Nankoweap Formation p6nu, upper member Disconformi ty p£nf, ferruginous member Unconformity pCwz Ferruginous weathered zone developed on Cardenas Lavas tí < Cardenas Lavas p€cu, upper part; cliff- and ledge- forming basaltic flows pGcl, lower part; rubbly-weathering basaltic flows. West of Tanner Canyon includes thin ledge-forming flow at base, beneath thin red sandstone (dotted) that is laterally traceable into the Dox Sandstone pGc, Cardenas Lavas, undivided p-€d Dox Sandstone Contact. Dashed where approximately located. Dotted where concealed. High-angle fault. Dashed where approximately located. TT" Dotted where concealed. Bar and ball on downthrown side. 1,2 indicate sequence of faulting. Syncline, showing plunge. "Upper axis of monocline. Strike and dip of beds. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/12/1763/3418460/i0016-7606-87-12-1763.pdf by guest on 25 September 2021 1766 ELSTON AND SCOTT argon caused by deep burial with attendant weathering was developed across the sec- erosion in a locally less resistant part in the rise in temperature. tion, apparently before deposition of the lower unit of the Cardenas apparently pro- upper member of the Nankoweap Forma- vided a site for the deposition and sub- Unconformities tion. The ferruginous weathered zone west sequent preservation of the ferruginous of Tanner Canyon has a rather uniform sandstone beds of the Nankoweap.