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
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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)
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).
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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.
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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. Unconformable relations between the thickness of about 10 m. Stratification in Microscopic analysis reveals that much Cardenas Lavas and the Nankoweap For- the Cardenas can be traced from unaltered of the original texture and structure and mation can be observed at three places: (1) beds across the zone of alteration to the some of the mineralogy of the unaltered in the western wall of Basalt Canyon, about erosion surface (Fig. 3B). The erosion sur- flow rock are retained in the altered rock 0.5 km northwest of the area shown in Fig- face is nearly planar across the resistant (Appendix 2). However, original ilmenite ure 2, (2) in the graben immediately north upper part of the Cardenas. Differential has been altered to specular hematite and of the Tanner Canyon rapids, and (3) on the east wall of the unnamed canyon 0.5 North km west of Tanner Canyon. Details of the South geologic relationships in the latter two Graben north of areas are shown in Figures 2 and 3. In West of Tonner Canyon Basalt Canyon the upper member of the Nankoweap overlies the Cardenas. A ba- saltic flow at the top of the Cardenas is truncated at a low angle beneath the Nan- / \ koweap in a distance (north to south) of about 0.5 km. Hendricks and Lucchitta (1974) reported the truncation of 60 m of section. This most likely is the uncon- formity beneath the Nankoweap that was observed, but not described, by Van Gundy (1951). In the graben at the Tanner Canyon rapids, the Cardenas Lavas are discon- formably overlain by the lower, ferruginous member of the Nankoweap Formation (Fig. 3A). Resistant strata of the lower two- thirds of the ferruginous member cap the sheer cliff that overlooks the rapids, and the contact occurs as much as 10 m below the top of the cliff. The contact of the Cardenas with the ferruginous member can be exam- ined closely at only one locality, adjacent to a small unmapped fault that lies west of the horst in the graben (Fig. 2). The contact is sharp, and there is no appreciable zone of weathering on the Cardenas or stratum of reworked Cardenas at the base of the fer- ruginous member. The ferruginous member in the graben displays locally abrupt changes in thickness, particularly near minor unmapped faults that cut the Car- denas and the lower part of the Nan- koweap. Displacements on these faults ap- pear to be smaller in the uppermost beds of Figure 3A. Stratigraphie relationships in ferruginous and upper members of Nankoweap Forma- the ferruginous member and in the lower- tion, Tanner Canyon rapids graben and canyon west of Tanner Canyon. most few metres of the upper member of the Nankoweap than in the underlying Cardenas. These relations suggest that some faulting predated deposition of the sw NE ferruginous member, producing an irregu- Figure 3B. Rela- lar surface at the top of the Cardenas, and tionships exposed in that minor renewed faulting occurred after east wall of canyon deposition of the ferruginous member and west of Tanner Can- the lowermost few metres of the upper yon. View is to member of the Nankoweap. northwest from be- hind outcrop. See Figure 2 for areal Ferruginous Weathered Zone geology and explan- ation of symbols. The entire section of the Cardenas Lavas was erosionally truncated west of Tanner Canyon, and a hematite-rich zone of
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leucoxene, and parts of the groundmass (Appendix 2) is a coarse-grained and poorly north of the fault (Fig. 3D). The fault itself have been altered to red earthy hematite sorted and stratified sandstone whose al- is sharp and clean and can be traced for and siderite. Detrital angular fragments of tered detritus appears to have been derived about 1 km to the west (Fig. 2). Where it in- specular hematite are interbedded with from the adjacent weathered zone. tersects the upper member of the Nan- chemically precipitated laminae of red 2. The ferruginous weathered zone, at its koweap about 300 m to the west, hematite in the laminated ferruginous southernmost exposure, is in fault contact downward-to-the-south displacement is es- sandstone that directly overlies altered flow with unaltered Cardenas that is directly timated to be about 5 m, somewhat less rock. Variously altered mineral grains and overlain by the upper member of the Nan- than the displacement seen in the area of rock fragments, also derived from the sub- koweap (Fig. 3D); the faulting entirely pre- Figure 3D. jacent flows, occur in the ferruginous dates deposition of the Tapeats Sandstone. 3. A thin (=sl m) iron-rich fossil soil sandstone. Good exposures exist here, and no infer- (Appendix 2) is developed on the inlier of Lithologic and structural relationships ences or assumptions as to the geometric re- strata assigned to the ferruginous member indicate that the ferruginous alteration of lationships need be made. Two periods of of the Nankoweap and also on the adjacent the flows was the product of weathering on faulting having opposite senses of dis- ferruginous weathered zone (Fig. 3C). The an erosion surface that at least partly pre- placement appear to be required to explain soil is overlain by the Tapeats. Details dated, but that could have in part accom- the observed relationships. The earlier summarized in Appendix 2 suggest that the panied, deposition of the ferruginous faulting was marked by downward-on- soil is a residual regolith formed after de- sandstone beds. Details of the stratigraphic the-north displacement of the Cardenas and velopment of the ferruginous weathered and structural relationships are shown in the preservation of the ferruginous weath- zone and deposition of the ferruginous Figures 3B, 3C, and 3D. At first glance it is ered zone against unweathered Cardenas member of the Nankoweap. With the ex- tempting to relate development of the on the south. Downward-to-the-north ception of some inclusions above the fer- hematite-rich zone to an interval of weath- dragging of the base of the ferruginous ruginous member, the characteristics of the ering that preceded deposition of the weathered zone is seen adjacent to the fault underlying materials incorporated in the Tapeats Sandstone (Cambrian). However, and presumably reflects the earlier faulting. soil have been destroyed by the intense the details listed below interfere with such a This was followed by truncation that pre- weathering. straightforward solution. ceded deposition of the upper member of 4. The Tapeats Sandstone overlies trun- 1. Strata assigned to the ferruginous the Nankoweap and then by deposition of cated sections of the Cardenas Lavas south member of the Nankoweap west of Tanner the upper member on unaltered flow rock of Comanche Creek and north of the Unkar Canyon rest on the lower part of the Car- south of the fault. (The upper member rapids (Fig. 1). At neither place is a fer- denas and overlie the ferruginous weath- presumably also was deposited on the fer- ruginous weathered zone on the Cardenas ered zone (Figs. 3B, 3C). The beds of the ruginous weathered zone north of the fault, apparent beneath the Tapeats. This ferruginous member contain material re- but direct evidence for this has been re- suggests that weathering after deposition of worked from the weathered Cardenas (Ap- moved by pre-Tapeats erosion.) The second the Chuar Group but before deposition of pendix 2). The lower of two beds (Appen- period of faulting was marked by the Tapeats Sandstone did not cause ap- dix 2) contains altered Cardenas detritus downward-to-the-south displacement and preciable alteration of the Cardenas. and depositional laminae of hematite. On the juxtaposition of unaltered Cardenas lithologic grounds, the lower bed correlates and overlying beds of the upper member of NANKOWEAP FORMATION with ferruginous laminated sandstone that the Nankoweap against the ferruginous overlies the Cardenas and underlies the weathered zone. This resulted in a The Nankoweap Formation is redefined upper member of the Nankoweap in the downward-to-the-south deflection of the to include a newly recognized ferruginous Tanner Rapids graben to the north (Fig. ferruginous weathered zone in a compara- sandstone unit at the base, here called the 3A). The upper of the two sandstone beds tively broad zone of fracturing that lies ferruginous member. Overlying, previously
Figure 3C. Exposure of ferruginous member of Nankoweap Formation west of Tanner Canyon as viewed at outcrop. View is to south.
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recognized strata of the Nankoweap are as- Our studies of the Unkar Group indicate markedly on the west side. Thinning ap- signed here to the upper member (Appendix that these strata correlate with the lower pears to result from a combination of thin- 1). Strata of the upper member have been part of the Dox Sandstone, and they are ning by rise of the base, internal thinning, described by Van Gundy (1951), who mea- shown here as Dox. This has the effect of and truncation beneath the upper member. sured the type section in the northern part reversing the sense of Precambrian dis- The section shown in Figure 3A has been of Basalt Canyon. placement. Stratigraphic considerations in- pieced together from exposures near small The distribution of exposures of the dicate that the fault that underlies the faults west of the horst that lies within the Nankoweap Formation is shown in Figure Cedar Mesa monocline had about 1,200 m graben. Finely laminated, very fine grained, 1. One additional exposure occurs in Nan- of downward-to-the-south displacement in light red-brown sandstone characterizes the koweap Valley on the axis of the Chuar Precambrian time. member. Fine depositional laminae of syncline, about 15 km north of the area hematite are separated by very thin layers shown in Figure 1. Strata along the Col- Ferruginous Member of detritus derived from the lava flows, orado River north of a fault that coincides producing a varved appearance. The de- with the Cedar Mesa monocline (northeast The type section of the ferruginous scription of the lower laminated sandstone part of area shown in Fig. 1) were mapped member is in the graben at the Tanner Can- bed in the exposure west of Tanner Canyon as Nankoweap by Van Gundy (1951) and yon rapids (Figs. 2, 3 A). The unit appears (Appendix 2) applies generally to the fer- by Maxson (1967), although McKee (1945, to be restricted to the area of the graben, ruginous sandstone in the graben. This dis- p. 142) recognized the sandstone as Dox. pinching out on the east and thinning tinctively laminated sandstone has not been observed anywhere else in the section. The lower two-thirds of the member in the graben is very well cemented by hema- tite, resists erosion, and caps the sheer cliffs (Fig. 3A). The well-cemented part ranges from about 3 to 9 m in thickness, and at places it appears to vary abruptly in thick- ness, principally near small unmapped faults in the graben. The upper one-third of the member, as much as 4 m thick, is lithologically similar to the underlying beds but is less well cemented and commonly is eroded to a narrow bench above the sheer cliffs. Two 2- to 4-cm-thick beds of fine- grained purple-red sandstone were ob- served at one locality in the uppermost 1 m. They are lithologically similar to quartzose sandstone of the upper member of the Nankoweap and record the first appearance Approximate scale of material derived from a source other than the Cardenas Lavas. These beds ap- pear to be a precursor to depositional con- ditions that characterized the upper member. Very locally the uppermost 0.3 m Northeast Southwest is comparatively poorly stratified, sorted, and cemented, and may be somewhat weathered. In stratification and sorting it resembles the upper bed at the exposure west of Tanner Canyon (Appendix 2; Figs. Approximate 2, 3B), with which it could be laterally Scale equivalent (Fig. 3A). This soft and granular
25 m weathering interval is overlain by essen- tially nonstratified and noncemented dark purple-red sand and silt, which characteris- - 20 tically contain irregularly distributed, smooth, and irregularly shaped masses of 15 white to yellowish chert that range from small pebble to cobble size. This is the - 10 basalmost unit of the upper member. It is not everywhere present, but locally it is as - 5 much as 0.3 m thick. Above this is a light red-brown to pale purple, finely con- -0 glomeratic cross-bedded sandstone that is found in most places at, or very near, the View is to southeast base of the upper member. In Basalt Can- yon and in Comanche Creek (Fig. 1), west Figure 3D. Geologic sketch map and cross section of Cardenas-Nankoweap-Tapeats relationships and east of the Tanner Canyon rapids grab- at southwesternmost exposure of ferruginous weathered zone, west of Tanner Canyon. Numbers in en, the noncemented chert-bearing unit and brackets are amount of vertical displacements along faults. See Figure 2 for explanation of symbols. the basal finely conglomeratic sandstone
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rest directly on little-altered or unaltered Tanner Canyon is shown in Figure 4. It is Cardenas has been downdropped to the flows of the Cardenas. The presence of the based on the removal of the effects of south against the Dox Sandstone along this basal conglomeratic units of the upper Phanerozoic (post-Mesozoic) and Precam- fault. Northwest of the area shown in Fig- member above the ferruginous member in brian (after deposition of the Chuar Group) ure 2, a basaltic dike, which on paleo- the graben indicates that the unconformity folding. However, the effects of faulting are magnetic grounds is related to intrusion of beneath the upper member observed away only partly understood, and the area Precambrian diabase sills during the time of from the graben includes a probable discon- shown in the right-hand or northeast side of deposition of the Dox, occurs in the Dox formity between the ferruginous and upper the diagram is terminated at a northwest- along this fault. This indicates that the fault members that is seen only in the graben. trending Precambrian fault (Fig. 2). The came into existence before the time of de- A second exposure of the ferruginous member occurs west of Tanner Canyon SOUTHWEST NORTHEAST (Fig. 2). The ferruginous member here over- lies a ferruginous weathered zone that is developed across an erosionally truncated section of the Cardenas Lavas (Figs. 3B, 3C). The lower sandstone bed (Appendix 2) is lithologically identical with the laminated ferruginous sandstone in the graben to the p-€cu north. The overlying bed is a poorly strat- ified, thin-bedded but massive-weathering sandstone whose fine- to coarse-grained detritus apparently was derived entirely from the adjacent ferruginous weathered zone. From its stratigraphic position, the upper bed could be laterally equivalent to poorly stratified material preserved locally €ba •Cba in the uppermost part of the ferruginous member in the graben (Fig. 3A). The fer- €t -1 p*-WT p-Pnf ruginous member appears to owe its exist- ence to the formation and erosion of the ferruginous weathered zone on the Car- p-€cu denas. Both may owe their origin and preservation to their position near faults as- p€cl sociated with the throughgoing Butte fault p€d (Fig. 2), which has had a long and not com- B pletely understood history of Precambrian and Phanerozoic movements. The ferruginous weathered zone was de- veloped subaerially following local (?) ero- sional truncation of the Cardenas. The present westerly dips of the Cardenas and the ferruginous member are similar (Figs. 2, 3B), which suggests that only little tilting of the Cardenas occurred before deposition of p-Ccu the laminated ferruginous sandstone on the weathered zone. This is supported by the apparently concordant attitudes of strata of the Cardenas and the ferruginous and upper members of the Nankoweap in the graben at the Tanner Canyon rapids. Thus, an east-facing erosional escarpment having a height about as great as the thickness of 0 200 400 600 800 METERS the Cardenas is inferred to have existed in the area west of Tanner Canyon. Figure 4. Diagrammatic sections showing development of stratigraphic and structural relations west of Tanner Canyon. View is to northwest. No vertical exaggeration. Cambrian: Cambrian sym- Deposition of the laminated ferruginous bols Cba = Bright Angel Shale, Ct = Tapeats Sandstone. Precambrian: pCch = Chuar Group; pCnu sandstone occurred in quiet, presumably = upper member of Nankoweap Formation; pCnf = ferruginous member of Nankoweap Formation; standing water that allowed regular alter- pCwz = ferruginous weathered zone on Cardenas Lavas; pCcu = upper part of Cardenas Lavas; pCcl nations between chemical precipitation of = lower part of Cardenas Lavas; includes Dox-like sandstone and ledge-forming lava flow at base; hematite laminae and deposition of thin pCd = Dox Sandstone. Dotted beds = sandstone. A, Precambrian time: After deposition of Cardenas layers of very fine grained sand. The depo- Lavas, a prominent east-facing erosional escarpment, perhaps fault controlled, was developed across sitional environment may have been a pond lava series. Flows were horizontal, or they dipped no more than 5°W. Following development of fer- ruginous zone of weathering and alteration on escarpment, beds of ferruginous member of Nan- or lake of interior drainage, trapped in a koweap were deposited on lower part of scarp, followed by deposition of red beds of upper member. structural depression near the Butte fault. B, Cambrian time: Folding after deposition of Chuar Group was followed by erosion and removal of The hematite and very fine grained hemati- Chuar Group and part of Nankoweap Formation. Cambrian Tapeats Sandstone then was deposited tic detritus were derived from weathered across irregular Ep-Algonkian erosion surface; it is thinnest above resistant sandstone strata in upper Cardenas (Appendix 2). member of Nankoweap. C, Present time: Post-Mesozoic folding led to development of east-facing A reconstruction for the area west of Butte or East Kaibab monocline; part of upper limb is shown on right side of the diagram.
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position of the Cardenas. Later Precam- member almost certainly was deposited on weathering before the deposition of overly- brian faulting that dropped the Cardenas the ferruginous weathered zone north of the ing argillite. Moreover, a bed of hematite against the Dox may have occurred shortly fault, but evidence for this was removed by occurs in the argillite 3 m above the flow. after deposition of the Cardenas. This pre-Tapeats erosion. A second period of The flow and the sedimentary hematite would be in accord with other evidence that faulting, of opposite sense, dropped unal- mark the first, and stratigraphically lowest, has been reviewed here for structural ad- tered Cardenas and the lowermost beds of occurrences of each in the Precambrian justments before and during deposition of the upper member against the ferruginous Apache Group of central Arizona. Sedimen- the lower parts of the Nankoweap For- weathered zone, preserving evidence for the tary iron deposits occur elsewhere at this mation. pre—upper member age of the weathered stratigraphic horizon, which immediately If the upper limit of deposition of the fer- zone. follows the first appearance of basaltic ruginous member as shown in Figure 4a is The unconformity at the base of the flows in the section. Precambrian diabase represented by the preserved beds, then the Nankoweap recognized by Van Gundy sills of northern and central Arizona, which ferruginous member locally may have been (1951) in Basalt Canyon is an unconformity are about the same age as the flows, were as much as 80 m thick. The overlying inter- marked by appreciable local stratigraphic not exposed subaerially during Precamb- val, then, presumably was once occupied by discordance. The discordance amounts to rian time, and they lack hematitic dissemi- strata of the lower part of the upper ~60 m in Basalt Canyon and apparently nations. Thus, in both northern and central member of the Nankoweap. If the recon- ~300 m west of Tanner Canyon where the Arizona, there appears to be a spatial and struction is valid, the upper member of the unconformity underlies the ferruginous temporal relationship between the subaerial Nankoweap locally was as much as 270 m member of the Nankoweap. However, the exposure and hematitic alteration of basal- thick, and the entire Nankoweap was about basal conglomeratic beds of the upper tic flows and the subsequent deposition of 350 m thick. member overlie the ferruginous member in hematite-rich strata. the graben at the Tanner Canyon rapids, SUMMARY and the ferruginous member displays local ACKNOWLEDGMENTS variations in thickness that appear attribu- An erosional escarpment, perhaps struc- table both to deposition on an irregular We thank the National Park Service for turally controlled, was locally (?) developed faulted surface on the Cardenas and to permission to sample in the Grand Canyon. across the Cardenas Lavas near faults as- truncation beneath the upper member. For Notes and samples obtained from the fer- sociated with the Butte fault, west of Tan- this reason, the contact between the mem- ruginous units west of Tanner Canyon by ner Canyon. A 10-m-thick hematite-rich bers is classed as a disconformity and could J. P. Schafer augmented our observations zone of weathering was developed on the represent a significant hiatus in deposition. and data, and we gratefully acknowledge Cardenas across the escarpment but for The unconformity recognized by Van his review of this manuscript. Key expo- some reason was not developed (or Gundy in Basalt Canyon thus apparently sures in the graben at the Tanner Canyon preserved?) on the plateau areas that were includes two intervals of time marked by rapids and at the southernmost exposure of to become covered by the upper member of erosion or nondeposition before deposition the ferruginous weathered zone on the Car- the Nankoweap. Ferruginous laminated of the upper member of the Nankoweap. denas Lavas were examined in the company sandstone that is assigned to the ferrugin- Because the Nankoweap Formation is of Dana Gebel. Walter S. White reviewed ous member of the Nankoweap Formation bounded at the base and top by uncon- an early version of this manuscript. was deposited against the lower part of the formities, it here is removed from the Unkar escarpment. Ferruginous laminated Group and established as a separate unit in APPENDIX 1. sandstone also was deposited on unaltered the Grand Canyon Series of Walcott GENERALIZED DESCRIPTION OF flow rock on an irregular, apparently (1894). The recognition of three major GRAND CANYON SUPERGROUP, faulted surface at the top of the Cardenas in units, the Unkar Group, Nankoweap For- NORTHERN ARIZONA the graben north of the Tanner Canyon mation, and Chuar Group, allows the rapids. The Cardenas here apparently either Grand Canyon Series to be redesignated the The Precambrian Grand Canyon Supergroup was not subjected to protracted subaerial Grand Canyon Supergroup. (Grand Canon Group of Powell, 1876; Grand weathering or the weathered zone was re- Canyon Series of Walcott, 1894) is 3,743 to Although the exposures in the graben at 4,320 m thick and comprises the strata listed be- moved by erosion. The hematite in the the Tanner Canyon rapids and west of low. laminated sandstone in part consists of Tanner Canyon have shed light on strati- Chuar Group (Chuar Group of Walcott, 1883; chemically precipitated laminae and in part graphic relationships in the Grand Canyon Maxson, 1961, 1967; Wilson, 1962; Ford and hematitic altered volcanic detritus. The fer- Supergroup, they also have posed problems Breed, 1973; Chuar terrane of Walcott, 1894); ruginous weathered zone nearby concerning the character of the Precam- 2,010 m thick. Gray shale; subordinate, locally presumably served as the source. Deposi- brian environment. The extensive but ap- algal-bearing gray limestone, buff dolomite, and tion of the ferruginous member occurred in parently local alteration of the Cardenas variegated sandstone; red sucrosic dolomite at quiet, standing water, perhaps in a struc- Lavas and the formation of hematite and base and red sandstone and chert-bearing breccia at top. turally controlled pond or small lake of locally siderite and iron-rich magnesite re- interior drainage that lay near the Butte Unconformity (Walcott, 1894; Van Gundy, quire rather special chemical conditions 1951). fault. and imply an environment that was sig- Nankoweap Formation (Nankoweap Group Before deposition of the upper member nificantly different from the present. Al- of Van Gundy, 1934, 1951; Nankoweap Forma- of the Nankoweap Formation, the fer- though commonly not so intense, the alter- tion of Maxson, 1961, 1967); 113 m thick ruginous weathered zone, at its southwest- ation process must have been general and (thickness by reconstruction, 350 m). ern limit of preservation, was faulted related to subaerial exposure. All Precamb- Upper member (Nankoweap of Van Gundy, against unweathered Cardenas along an rian basaltic flows that we have examined 1951); 100 m thick (thickness by reconstruction, east-trending fault. The faulting was fol- in northern and central Arizona contain 270 m). Sandstone, mainly fine grained and in pervasive hematitic disseminations. At one part quartzitic; shaly and silty sandstone and lowed by erosion and then by deposition of shale; red-brown and purple, locally white; well the upper member of the Nankoweap, locality in central Arizona, fist-size nodules stratified; thin to medium bedded with cross- which directly overlies unweathered Car- of specular hematite occur in the upper part beds, ripple marks, mud cracks, and locally salt denas Lavas south of the fault. The upper of a basaltic flow that had undergone casts; wormlike trails abundant at places; base
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commonly marked by thin, sparsely conglomera- thick. Lower half: 65+ m, red-brown siltstone quartz-grain aggregates are only slightly impreg- tic sandstone that contains chert fragments, loc- and silty sandstone. Overlying part: 25+ m, nated with hematite. The relations described ally underlain by noncemented purple sand and orange-red silty sandstone. Upper one-third: above suggest that the inclusions acquired their silt that contains irregular masses of chert. 40+ m, fine- to medium-grained cross-bedded hematite before incorporation in the matrix, and Disconformity. Van Gundy (1951) recognized lavender sandstone; forms slopes and minor they may have been derived from tiny quartz vein- an unconformity beneath the upper member of ledges; distinctive thin orange silty sandstone lets seen in the underlying sandstone. Quartzose the Nankoweap truncating a flow in the upper present locally at base. inclusions are sparse where the unit directly over- part of the Cardenas Lavas in Basalt Canyon. Bass Limestone and underlying Hotauta Con- lies altered lava of the Cardenas. The matrix is The unconformity in Basalt Canyon includes the glomerate (Noble, 1914); 37 to 104 m thick. made up of about equal proportions of finely di- unconformity beneath the ferruginous member Dolomite and subordinate limestone, gray to red vided, very dark red hematite and clay minerals. and the disconformity between the ferruginous gray; locally algal bearing; interbedded with The lower contact of the unit is gradational in 1 and upper members of the Nankoweap near the sandstone, conglomerate, and siltstone, purple- to 2 cm with the ferruginous member of the Butte fault. brown to dark red and reddish brown; well- Nankoweap and the ferruginous weathered zone Ferruginous member; 13 m thick (thickness by rounded conglomeratic detritus derived from developed on the Cardenas. The unit is sharply reconstruction, 80 m). Sandstone, very fine crystalline terrane; forms cliffs, ledges, and steep and disconformably overlain by the Tapeats grained and silty, containing hematite laminae slopes. where it underlies the basalmost grayish red- and fine to locally coarse grains of altered vol- purple unit of the Tapeats that is preserved in a canic rock; mostly finely laminated; reddish local depression. Origin: ferruginous fossil soil brown to reddish orange; hematite and magne- APPENDIX 2. — a residual regolith derived from weathering of site cement abundant; lower part very well DESCRIPTION OF BASAL CAMBRIAN underlying ferruginous materials. cemented; occurs in graben at Tanner Canyon AND SELECTED PRECAMBRIAN Grand Canyon Supergroup. The Grand Can- rapids and locally west of Tanner Canyon. STRATA EXPOSED IN CANYON WEST yon Supergroup comprises the strata listed be-1 Unconformity. Ferruginous weathered zone OF TANNER CANYON, EASTERN low. on Cardenas Lavas; 10 m thick. Deeply GRAND CANYON, ARIZONA Nankoweap Formation. Ferruginous member: weathered basalt and basaltic andesite; resistant lens of sandstone comprising two beds, preserved to erosion; original textures preserved but Cambrian in a pocket about 4 m deep and somewhat more groundmass pervasively stained and altered to than 18 m wide, eroded into comparatively soft earthy hematite and siderite. Developed across Tapeats Sandstone. Fine- to coarse-grained interval in upper part of lower unit of the Car- erosionally truncated section of Cardenas Lavas sandstone with scattered very coarse grains of denas. Principal attitude is strike 205°, dip 10°W west of Tanner Canyon. grayish-orange (10 YR 7/4) quartz; local quartz (5° less than general westward dip of underlying Unkar Group (Unkar terrane of Walcott, pebble (as much as 15 cm) conglomerate at base Cardenas); on east side of exposure, beds strike 1894; Unkar Group of Noble, 1914); 1,620 to and scattered quartz granules above. Promi- 200°, dip 7°W; on west side, beds appear to turn 2,197 m thick. Comprises strata listed below. nently cross-bedded. Basalmost beds in a local up slightly, and attitude is strike 295°, dip 5°NE. Cardenas Lavas (lava series of Walcott, 1894; depression, 9 m wide and 0.6 m deep, are grayish Configuration, after 10° to 15° rotation of sec- Cardenas Lava Series of Keyes, 1938; Rama red-purple (5 RP 4/2), parallel bedded, and con- tion, suggests unit was deposited on a terrace cut Formation of Maxson, 1961, 1967; Cardenas tain intergranular detritus apparently derived in the Cardenas. Lower contact sharp and uncon- formable with local relief of ~0.3 m. Lavas of Ford and others, 1972); 244 to 450 m from subjacent weathered zone(s), particularly thick. Basalt and basaltic andesite, dark gray to grayish red-purple Ep-Algonkian soil. Overlain Upper bed: sandstone, very fine to coarse medium green; faint to moderate reddish cast on by 6 to 7 m of pale-red (10 R 6/2 to 5 R 6/2) grained, 2.5 m thick, grayish red (5 R 4/2 and 10 fresh surfaces from disseminated hematite. sandstone, becoming grayish orange-pink (5 YR R 4/2) to pale red (10 R 6/2) and laced (particu- Lower part about 90 m thick in Basalt Canyon, 7/2) at top and grading upward to very pale larly in uppermost part) with myriad thin weathers to a crumbly slope and contains several orange (10 YR 8/2) and nearly white in overlying moderate reddish-orange (10 R 6/6) limonite- thin discontinuous sandstone beds; slope- beds. Just east of outcrop of ferruginous member stained veins of carbonate; clay and carbonate forming unit is capped by ledge-forming of Nankoweap, very planar beds of lower (magnesite) cement common; poorly sorted; sandstone about 5 m thick. Upper part about 2O0 Tapeats overlie a very flat unconformity and dip grains and rock fragments subangular; abundant m thick in Basalt Canyon; forms cliff and con- gently southeast (strike 045°, dip 8°SE). altered volcanic rock fragments display pilotaxi- sists of seven or more flows commonly separated Unconformity (Ep-Algonkian surface). tic texture, with plagioclase as only original min- by thin layers of parallel-bedded and cross- eral still present; other rock-forming minerals re- bedded sandstone 1 to 3 m thick. West of Tanner Precambrian placed by magnesite and hematite. Mont- Canyon, a discontinuous ledge-forming flow, ~5 morillonite and kaolinite are irregularly distrib- m thick, underlies the lower slope-forming unit uted. Opaque minerals include red hematite and Ferruginous soil. Ferruginous silty claystone, and is separated from it by ~2 m of red detrital specular hematite in very fine to medium, arenaceous, 0.25 to
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single bed characterized by subdued, faintly dis- discrete small grains. Lowest flow of upper cliff- mate, Pt. 1 — Regional studies: Geol. Soc. tinguishable parallel and cross laminations. Loc- forming part of lava series; 30 m thick. America Rocky Mountain Sec. guidebook, ally, basal 0.06 to 0.18 m consists of medium- to Sandstone: Very fine grained sandstone, with 27th ann. mtg., Flagstaff, Arizona, p. coarse-grained crumbly sandstone derived from occasional medium-grained layers; 2 m thick, 65-86. subjacent altered Cardenas and is similar in ap- grayish red (5 R 4/2) to pale brown (5 YR 5/2); Keyes, Charles, 1938, Basement complex of the pearance to material of overlying upper arkosic sublitharenite, laminated (~ 2.0 to 0.5 Grand Canyon: Pan Am. Geologist, v. 20, sandstone bed. Contact on altered basalt is mm) to very thin bedded; 80 percent quartz, 8 no. 2, p. 91-116. sharp. percent feldspar (mainly in coarser layers), 7 per- Maxson, J. H., 1961, Geologic history of the Unconformity. cent dark minerals (mafic and black opaque), 5 Bright Angel quadrangle, in Geologic map Ferruginous weathered zone on Cardenas percent muscovite; moderately well sorted; of the Bright Angel quadrangle: Grand Lavas (unit 3 in App. Table 1). Altered basalt grains subangular to subrounded; cement is Canyon Natural History Association, scale and basaltic andesite, fine grained, 9 to 10 m quartz and hematite. Somewhat more resistant 1:24,000 (3d ed., rev., 1968). thick, grayish red (5 R 4/2 to 10 R 4/2), pale red- than overlying flows of slope-forming interval of 1967, Preliminary geologic map of the dish brown (10 R 5/4), and moderate reddish lava series. Directly overlies basal flow; traceable Grand Canyon and vicinity, Arizona, east- orange (10 R 6/6); plagioclase, in pilotaxitic tex- laterally into Dox Sandstone. Strike 200°; dip ern section: Grand Canyon Natural History ture, is abundant; matrix is pervasively stained 15°W. Association, scale 1:62,500. with hematite and commonly altered to siderite Basal flow (unit 1 in App. Table 1): Medium- McKee, E. D., 1945, Cambrian history of the and local concentrations of earthy hematite; grained basalt, 4.5 m thick, medium olive-gray (5 Grand Canyon region: Carnegie Inst. specular hematite is abundant and is in various Y 5/1) to brownish gray (5 YR 4/1); plagioclase Washington Pub. 563, pt. 1, p. 1-170. stages of replacing leucoxene; high carbonate laths moderately large, producing felted, pilotax- McKee, Edwin H., and Noble, Donald C., 1974, and iron contents suggest abundant disseminated itic texture that approaches ophitic texture; 65 Rb-Sr age of the Cardenas Lavas, Grand siderite; fractures filled with secondary quartz percent plagioclase, 5 percent olivine, 8 percent Canyon, Arizona, in Karlstrom, T.N.V., and serpentine. secondary serpentine, 15 percent opaques (in- Swann, G. A., and Eastwood, R. L., eds., Unkar Group. Cardenas Lavas, intermediate- cluding hematite), 5 percent chalcedony, 2 per- Geology of northern Arizona, with notes on level flow (unit 2 in App. Table 1): Basaltic ande- cent quartz. Plagioclase displays albite twinning archaeology on paleoclimate, Pt. 1 — Re- site, fine grained, olive gray (5 Y 4/1), pilotaxitic and occurs as lathlike crystals; most is altered to gional studies: Geol. Soc. America Rocky texture, with groundmass replaced by brown to seriate. Mafic minerals include olivine, bleached Mountain Sec. guidebook, 27th ann. mtg., green serpentine; 70 percent plagioclase; 20 per- crystals (0.5 mm) of iddingsite, limonite, and Flagstaff, Arizona, p. 87-96. cent groundmass; 8 percent opaques (ilmenite, hematite. Gray, highly reflective opaque mineral Noble, L. F., 1914, The Shinumo quadrangle, hematite, others), 2 percent mafic minerals lines cavities and fills microfractures. Hematite is Grand Canyon Distria, Arizona: U.S. Geol. (clinopyroxene, amphibole); plagioclase displays common as an alteration product and staining. Survey Bull. 549, 100 p. albite twinning and is partly altered to seriate Upper part vuggy. Weathers as a distinct ledge; Pasteels, P., and Silver, L. T., 1965, Geo- with fracture fillings of hematite; ilmenite is pinches out abruptly at places to the north and chronologic investigations in the crystalline common, displaying hexagonal crystal form, west. rocks of the Grand Canyon: Geol. Soc. slight anisotropism, and twinning; magnetite is America Abs. for 1965, Spec. Paper 87, sparse and occurs in fine lamellae with ilmenite, p. 124. as thin partial rims on ilmenite crystals, and as REFERENCES CITED Powell, J. W., 1876, Report on the geology of the eastern portion of the Uinta Mountains and a region of country adjacent thereto: U.S. APPENDIX TABLE 1. CHEMICAL American Commission on Stratigraphie Geol. and Geog. Survey of the Territories ANALYSES OF SAMPLES FROM Nomenclature, 1961, Code of stratigraphie (Powell), vii, 218 p. SOME GRAND CANYON nomenclature: Am. Assoc. Petroleum Van Gundy, C. E., 1934, Some observations of SUPERGROUP UNITS Geologists Bull., v. 45, p. 645-665. the Unkar Group of the Grand Canyon AI- Daneker, Thomas, M., 1974, Sedimentology of gonkian: Grand Canyon Nature Notes, Rock unit" the Precambrian Shinumo Quartzite, Grand v. 9, no. 18, p. 338-349. 3 2 1 Canyon, Arizona: Geol. Soc. America Abs. 1951, Nankoweap group of the Grand Can- with Programs, v. 5, p. 438. 54.1 46.7 yon Algonkian of Arizona: Geol. Soc. Si02 46.1 Elston, D. P., and Scott, G. R., 1973, Paleomag- 15.9 America Bull., v. 62, p. 953-959. AI2O3 11.6 13.0 netism of some Precambrian basaltic flows Walcott, C. D., 1883, Pre-Carboniferous strata Fe203 13.9 10.2 9.4 and red beds, eastern Grand Canyon, in the Grand Canyon of the Colorado, FeO 4.01 4.49 2.72 Arizona: Earth and Planetary Sci. Letters, Arizona: Am. Jour. Sci., v. 26, p. 437-442. MgO 3.5 3.2 10.4 v. 18, p. 253-265. 2.4 0.4 1894, Pre-Cambrian igneous rocks of the CaO 1.9 Ford, T. D., and Breed, W. J., 1973, Late Pre- 0.1 2.6 0.9 Unkar terrane, Grand Canyon of the Col- Na20 cambrian Chuar Group, Grand Canyon, 4.2 6.9 orado, Arizona, with notes on the petro- K2O 7.1 Arizona: Geol. Soc. America Bull., v. 84, graphic character of the lavas by J. P. Id- Ti02 1.48 1.52 0.98 p. 1243-1260. 0.22 0.18 dings: U.S. Geol. Survey Ann. Rept. 14, pt. P2O5 0.27 Ford, T. D., Breed, W. J., and Mitchell, J. W., 0.11 0.13 0.08 2, p. 497-524. MnO 1972, Name and age of the upper Precam- + 2.09 2.16 4.75 Wilson, Eldred D., 1962, A resumé of the geol- H2O brian basalts in the eastern Grand Canyon: 1.12 0.50 0.93 ogy of Arizona: Arizona Bur. Mines Bull. H2O- Geol. Soc. America Bull., v. 83, p. 223- 6.47 1.10 0.30 171, 140 p. co2 226. Total 99.8 99.8 100.5 Hendricks, John D., and Lucchitta, Ivo, 1974, Upper Precambrian igneous rocks of the MANUSCRIPT RECEIVED BY THE SOCIETY OC- Note: D. J. Emmons, analyst. Grand Canyon, Arizona, in Karlstrom, TOBER 1, 1975 * Unit 3 is ferruginous weathered zone on T. N. V., Swann, G. A., and Eastwood, REVISED MANUSCRIPT RECEIVED MARCH 8, Cardenas Lavas, 2 is intermediate-level flow, and R. L., eds., Geology of northern Arizona, 1976 1 is basal flow of Cardenas Lavas. with notes on archaeology and paleocli- MANUSCRIPT ACCEPTED APRIL 6, 1976
Printed in U.S.A.
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