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Age of the Brushy Basin Member of the Morrison Formation, Colorado Plateau, western USA
Cretoceous Research (1991) 12, 483-493
Age of the Brushy Basin Member Colorado Plateau, western USA *Bart J. Kowallis,
*Eric H. Christiansen
of the Morrison
Formation,
and l-Alan L. Deino
‘Department of Geology, Brigham Young University, Pruvo, UT 84602, U.S.A. li;eochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley,
CA 94709, U.S.A
Received 14 Februar?, 1990 and accepted 10 October 1990
New single-crystal, laser-fusion 40Ar/39Ar ages on plagioclase and alkali feldspar from the Brushy Basin Member of the Morrison Formation range from 153 to 145 ( f about l-2) Ma or Late Jurassic in age (mostly Tithonian based upon Palmer, 1983). All but one of these new ages come from a section of Brushy Basin Member near Montezuma Creek, Utah, USA. The other, which is the oldest sample at 153 Ma, comes from Dinosaur National Monument, from a bentonite previously dated at 135.2 f 5.5 Ma (K-Ar biotite age) by Bowman et al. (1986). The new feldspar ages are essentially concordant with previously reported fission track ages from Notom, Utah, with ages ranging from 144 to 135 ( f about 15) Ma in the lower two-thirds of the Brushy Basin Member, but contrast with fission track ages from the upper one-third (about 15 m) of the Notom section, which ranged from 123 to 99 (f about 13) Ma (Kowallis & Heaton, 1987). However, none of the new ages comes from the upper part of the Brushy Basin Member; the uppermost sample dated from the Montezuma Creek section is more than 20 m below the top of the Morrison Formation. Precise ages from the uppermost part of the Brushy Basin Member are still needed to determine if the Jurassic-Cretaceous boundary lies within the uppermost Brushy Basin Member. KEY WORDS:
WAr/39Ar ages; Upper Jurassic; Morrison Formation; USA
1. Introduction
The Morrison Formation in the western United States has produced the most spectacular, diverse and abundant dinosaur fossils of any formation in the world. Its age, however, has long been a topic of debate, having been labelled as Jurassic, Cretaceous, or Jurassic-Cretaceous over the years (e.g. Emmons et al., 1896; Darton, 1922; Simpson, 1926; Stokes, 1944; Imlay, 1952; Imlay, 1980). The controversy extends back to the earliest formal descriptions of the formation when Emmons et al. (1896) wrote: “As both molluscan and plant remains found at this horizon have too wide a range to be of value in the determination of age of the enclosing beds, this determination has been based exclusively upon the vertebrates. Although the latter have some affinities with the European Wealden or Lower Cretaceous, to which Professor Marsh was at first inclined to assign the horizon, he found the evidence in favor of late Jurassic age to be so much stronger that he assigned to this period not only these but other beds with a similar fauna, notably the Potomac beds of the East, which on other grounds had been considered early Cretaceous and which present many structural analogies with the Morrison beds. From the point of view of the stratigrapher, the assignment of the Morrison beds to the Lower Cretaceous rather than to the Upper Jurassic is much more desirable, not only because it accords better with the sequence of sedimentation thus far disclosed in the adjoining regions of Kansas and Texas, but because it places the physical break whose effects are recognized over the whole continent between these two great time divisions rather than in the midst of one of them.” 0195-6671/91/050483
+ I1 $03.00/O
@ 1991 Academic Press Lunited
484
B. J. Kowallis ct al.
More recently, papers presented during a special session dedicated to the Morrison Formation at the Fourth North American Paleontological Convention in 1986, demonstrated that the problems have not been solved. Data presented there, would allow the age of the Morrison to be as old as pre-Kimmeridgian (Hotton, 1986) and/or as young as Neocomian (Bowman et al., 1986; Kowallis, 1986). The new single crystal 40Ar/39Ar ages presented here do not solve all the problems of Morrison Formation chronostratigraphy, but they improve the picture. Previous papers by Kowallis et al. (1986) and Kowallis & Heaton (1987) gave fission track ages for a section of the Morrison outside of Capitol Reef National Park
1 X0 km
I
+ NTM
UTAH Figure 1. Index map of Utah showing the location of the three sites discussed in the text, the Notom section (NTM), the Montezuma Creek section (MC), and the Dinosaur National Monument section (DINO).
Brushy Basin Member of the Morrison Formation
485
near Notom, Utah (Figure 1). The 15 fission track ages obtained there from the Brushy Basin Member suggested that the lower two-thirds of this member were about 140 f 10 Ma and probably uppermost Jurassic, but possibly Lower Creraceous, depending on the time scale used. The upper third (about 15 m of section) at Notom was younger, ranging in age from about 125 to 100 f 10 Ma, definitely Cretaceous in age. However, several problems exist with the Notom section. The Brushy Basin Member there is unusually thin (only 45 m as opposed to about 100 m elsewhere); also, its top is not as clearly defined as in other sections because of the lack of a well-developed basal conglomerate (Buckhorn Conglomerate Member) in the overlying Cedar Mountain Formation; and, finally, although the fission track ages are useful in providing some constraints on the age of the Brushy Basin Member, they possess errors that are too large to be useful in more precise correlations. In order to constrain better the age of the formation, we have obtained new radiometric ages using 40Ar/39Ar laser-probe dating techniques on individual plagioclase and sanidine grains from altered volcanic ash layers. 2. Sample localities The samples dated in this study come from three localities (Figure 1). Ages from one of the sections (Notom) have been previously reported (see Section 1), but will be re-examined here in light of the new data. The second section (Montezuma Creek) was chosen because it has a relatively thick Brushy Basin Member and contains numerous, closely spaced altered ash beds. The third locality was at Dinosaur National Monument, where only three samples were collected. 2.1. Notom section The lithostratigraphy of this section was presented by Petersen & Roylance (1982). It contains a very bentonitic section of the Brushy Basin Member about 41.5 m thick. The section is located about 5 km east of Capitol Reef National Park at 111”06’W longitude and 38” 14’N latitude. Nineteen samples from separate bentonitic horizons were collected, of which 11 yielded enough zircon for fission track dating as reported in Kowallis & Heaton (1987). 2.2. Montezuma Creek section The Montezuma Creek section, Utah, is located at 109” 19’W, 37” 19’N and there the Brushy Basin Member is about 105 m thick. It was deposited within the boundaries of Lake T’oo’dichi’, a saline, alkaline lake that covered a large area in the Colorado Plateau region during this interval of time (Bell, 1986; Peterson & TumerPeterson, 1987; Turner-Peterson & Fishman, 1988). The playa lake environment, we felt, would be a more favorable environment for preservation of ash beds than the fluvially dominated section at Notom. From the Montezuma Creek section we collected samples from 40 altered ash layers. About one-fourth of the altered ashes contained enough large (~60 mesh) feldspar for single crystal 40Ar/39Ar dating. We chose five of these samples for age determination, including the lowermost datable sample from about 30 m above the base of the Brushy Basin Member and the uppermost one from 20 m below its top. The samples were labelled in stratigraphic sequence with numbers increasing toward the top of the section. 2.3. Dinosaur National Monunmt
section
Three bentonitic samples were collected near the visitor’s center at Dinosaur National Monument, Utah. The entire section of Brushy Basin Member where the
486
B. J. Kowallis er ul
samples were collected is about 125 m thick (Bilbey et al., 1974). Two of the samples (DINO-I and DINO-3) came from about 10 m above the main dinosaurbearing layer. These two samples were collected about 0.5 km apart in what was thought to be the same bentonitic layer. Zircon morphologies of these samples, seem to confirm that they are indeed from the same ash layer. Individual zircon grains from these samples were examined on scanning electron microscope (SEM) photos at magnifications of x 100-200. Each grain was classified using the system developed by Pupin (1980), which is based upon the relative size and development of the (100) and (110) prism faces and the (101) and (211) pyramid faces. This classification reduces the morphological data to two indices; an A index, which is determined from the pyramid faces, and a T index, which is determined from the prism faces. Kowallis et al. (1989) and Kowallis & Christiansen (1989) have shown that average Pupin indices of zircons are consistent for an individual ash bed even over several hundred kilometers. DINO-1 and -3 have average A indices of 400 f 10 and 405 f 8, respectively, and average T indices of 435 f 11 and 426 f 10, respectively. DINO-2, which came from a different layer located immediately below the dinosaur-bearing layer, has an average A index of 376 f 11 and an average T index of 508 f 14, quite different from the other two samples. 3. Sample collection and preparation In the Notom and Montezuma Creek sections, every bentonitic horizon in the Brushy Basin Member was sampled for possible dating. Samples were about 10 kg each. Clays were removed by soaking the samples in water and then washing them over a wilfley table. The residue from the washed samples was examined with a binocular microscope to determine the phenocrysts present and to look for detrital contaminants. Then the samples were further processed through heavy liquids and a magnetic separator to concentrate heavy minerals and separate feldspars. Most of the samples contained some obviously rounded, probably detrital grains. The most common detrital grains were zircon (in a variety of colors), clear apatite, dark yellowish-brown tourmaline, pink garnet, quartz and microcline. These were more common in the Notom section and at Dinosaur National Monument than in the Montezuma Creek section. The heavy mineral suite is essentially the same as that described by Hansley (1986) from fine- to medium-grained sandstones of the Morrison Formation in the Grants area of New Mexico. Many of the samples also contained gypsum (or anhydrite) and barite. From the final mineral separates, euhedral zircons and angular feldspars were hand picked for dating. 4. 40~/39Atgeochronology Feldspar phenocrysts from five samples from the Montezuma Creek section and one sample from Dinosaur National Monument were dated by the single-crystal, laser-fusion 40Ar/39Ar method. The details of the preparation, irradiation, and analysis of these samples follows closely those of Deino & Potts (1990). The analytical accuracy of the plagioclase age determinations is not strongly compromised by the necessity to correct the argon isotope analyses for 36Ar and 39Ar produced from Ca during neutron bombardment in the reactor. By far the more important of these two corrections is 36Ar,-;a.We have determined the production ratio 36Ar,-a/37ArCaof the reactor through repeat analyses of optical grade fluorite, with a precision of about 2% (Table 1). This translates to an uncertainty in the final
Brushy Basin Member of the Morrison Formation
Table 1. Lab ID# Irrad. MGJMB-57 K-feldspar: 2322-01 27A 2322-13 27A
_7
0.01769 0.01769
37Ar/39Ar
0.0077 0.485 1
487
40Ar/39Ar analytical data.
36Ar/39Ar wAr*/39Ar o/d36Ar/39Ar),,t
0.00013 0.00023
5:
4.865 4.878
9/,“OAr* Moles 40Ar Age(Ma) f 1a§
4.2 E-13 6.2 E-13
99.2 99.3
148.9 149.3
0.5
149.2
0.4
147.8 144.8 143.1 149.6 147.7 147.1
3.3 1.9 2.7 1.2 1.0 1.6
147.6
0.8
149.9 148.0 150.1 152.0 146.7 142.9 146.8 149.3 146.5 144.4 145.4
2.5 1.3 2.3 2.8 1.2 2.8 1.5 1.5 1.4 1.3 2.0
147.0
0.6
99.3
146.3
0.5
98.0 99.0 96.5 98.8 98.7 100.3
143.7 144.0 143.0 148.2 149.7 149.9 145.2
1.0 1.0 1.2 1.4 1.6 2.3 1.2
Weighted mean age = Plagioclase: 2322-03 2322-06 2322-07 2322-09 2322-15 2322-16
27A 27A 27A 27A 27A 27A
0.01768 0.01769 0.01769 0.01769 0.01769 0.01769
2.9629 3.0941 8.2339 3.4613 1.7891 6.2516
0 Xi069 0.00110 0.00275 0.00162 0.00123 0.00193
100 73 77 55 38 84
4.827 4.726 4.668 4.887 4.823 4.802
5.6 E-14 6.2 E-14 4.3 E-14 1.0 E-13 1.3 E-13 8.0 E-14
100.4 98.1 96.1 95.7 95.5 98.1
Weighted mean age = MC-JhUS-53.5 Plagioclase: 22E 1943-11 1943-12 22E 22E 1943-15 1943-16 22E 1943-17 22E 1943-18 22E 1943-19 22E 22E 1943-20 1943-21 22E 1943-22 22E 1943-23 22E
0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148
2.3692 2.7349 3.4339 2.4185 3.6001 6.7687 1.8843 2.1812 2.4674 2.2974 6.3250
0.00058 0.00077 0.00119 0.00018 0.00140 0.00291 0.00035 0.00071 0.00079 0.00097 0.00168
100 91 75 100 66 60 100 79 81 61 97
7.545 7.445 7.557 7.657 7.376 7.181 7.384 7.513 7.365 7.258 7.312
5.6 E-15 1.3 E-14 7.2 E-15 6.3 E-15 1.5 E-14 5.3 E-15 1.1 E-14 1.1 E-14 1.2 E-14 1.3 E-14 8.4 E-15
100.1 99.7 98.8 101.7 98.1 95.4 100.5 99.4 99.4 98.5 99.8
Weighted mean age =
0.4
MC-JMB-48 K-feldspar: 22A 1901-11
0.01168
0.4171
0.00027
40
Plagioclase: 1901-02 1901-05 1901-06 1901-07 1901-10 1901-12
22A 22A 22A 22A 22A 22A
0.01168 0.01168 0.01168 0.01168 0.01168 0.01168
3.4279 4.6625 7.6420 6.8106 3.7527 7.6464
0.00136 0.00145 0.00283 0.00205 0.00130 0.00190
65 83 70 86 75 100
MGJMB-43 K-feldspar: 1941-16 22E
0.01148
0.0283
0.00011
6
7.564
1.6 E-13
99.6
150.3
0.4
Plagioclase: 1941-01 1941-02 1941-04 1941-05 1941-07 1941-08 1941-09 1941-10 1941-11 1941-12 1941-13 1941-14 1941-17 1941-18
0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01148 0.01148 0.01148 0.01148 0.01148
6.4156 7.8919 7.1404 6.7468 7.2838 9.3295 9.4860 7.2508 9.9020 1.9889 6.3036 6.5352 9.7044 6.6031
0.00219 0.00304 0.00254 0.00243 0.00286 0.00321 0.00247 0.00238 0.00325 0.00085 0.00234 0.00196 0.00290 0.00234
76 67 73 72 66 75 99 79 79 60 70 86 86 73
7.360 7.281 7.282 7.283 7.090 7.356 7.518 7.451 7.269 7.546 7.472 7.457 7.486 7.253
1.9 E-14 2.2 E-14 1.9 E-14 2.2 E-14 1.1 E-14 1.3E-14 2.0 E-14 2.3 E-14 1.8 E-14 7.1 E-14 2.3 E-14 1.3 E-14 2.2 E-14 1.2E-14
97.9 96.1 97.2 97.3 96.0 96.8 99.9 98.0 97.2 98.7 97.2 98.9 98.4 97.4
146.9 145.4 145.4 145.4 141.7 146.8 149.9 148.7 145.2 149.9 148.5 148.2 148.8 144.3
1.0 0.9 0.9 0.8 1.5 1.3 0.9 0.9 1.1 0.5 0.9 1.2 0.9 1.4
147.7
0.6
22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E
7.229
4.1 E-14
7.099 1.7 E-14 7.113 1.7 E-14 7.059 1.4 E-14 7.329 1.2 E-14 7.408 1.1 E-14 7.415 7.9 E-15 Weighted mean age =
Weighted mean age =
B. J. Kowallis et al.
488
Table 1. Lab lD# Irrad. MC-JhIB-17.75 K-feldspar: 2314-08 27A 2314-13 27A
J
0.01769 0.01769
37Ar/39Ar
0.0197 0.0129
(Contd.)
)=Ar/=Ar ‘0Ar*/3”Ar %(3bAr/39Ar)C,t
0.00017 0.00005
3 7
4.894 4.893
%*Ar* Moles “‘Ar
4.8 E-13 1.0 E-12
Age(Ma) f 105
98.9 99.7
Weighted mean age = Plagioclase: 1947-01 1947-04 2314-04 2314-06 2314-09 2314-10 2314-15
22E 22E 27A 27A 27A 27A 27A
0.01153 0.01153 0.01769 0.01769 0.01769 0.01769 0.01769
7.0721 8.8515 2.2134 4.3115 2.7884 2.1687 1.9666
0.00208 0.00286 0.00112 0.00185 0.00109 0.00079 0.00077
88 80 51 60 66 71 66
7.477 7.528 4.887 4.801 4.848 4.977 4.866
8.2 E-15 5.5 E-15 5.9 E-14 6.3 E-14 6.0 E-14 8.6 E-14 1.0 E-13
99.0 97.8 96.8 95.6 97.8 98.6 98.4
Weighted mean age = DINO-3 Plagioclase: 1902-01 1902-02 1902-04 1902-06 1902-12
22A 22A 22A 22A 22A
0.01168 0.01168 0.01168 0.01168 0.01168
2.7622 4.1840 4.2009 3.9396 2.8513
0.00088 0.00186 0.00210 0.00179 00.162
81 58 52 57 45
7.621 7.180 7.570 7.516 7.285
2.0 5.3 1.0 5.7 5.2
E-14 E-15 E-14 E-15 E-15
99.3 96.9 96.2 97.0 96.5
Weighted mean age = I,~w.radiopenic reiecta (<95% MGJhIBZ7 . 2322-05 27A 0.01769 0.01769 2322-08 27A 0.01769 2322-14 27A MC-JMIk-48 1901-01 22A 0.01168 1901-03 22A 0.01168 0.01168 1901-04 22A 1901-09 22A 0.01168 MC-JhIB-43 0.01153 1941-03 22E 0.01153 1941-06 22E 1941-1s 22E 0.01148 MC-JMB-17.75 1947-05 22E 0.01153 0.01153 1947-06 22E 1947-08 22E 0.01153 0.01769 2314-01 27A 0.01769 2314-02 27A 0.01769 2314-03 27A 2314-05 27A 0.01769 2314-07 27A 0.01769 0.01769 2314-12 27A 2314-14 27A 0.01769 2314-16 27A 0.01769 DINO-3 0.01168 1902-03 22A 0.01168 1902-05 22A
149.8 149.8
0.4 0.4
149.8
0.3
149.2 150.1 149.6 147.1 148.4 152.2 149.0
2.1 3.0 2.1 1.8 1.9 1.5 1.1
149.4
0.7
153.9 145.3 152.9 151.8 147.3
0.9 3.2 1.9 3.2 3.5
152.9
1.2
radioaenic Ark 7.2504 7.5874 7.6292
0.00296 0.00278 0.00621
63 70 32
4.812 4.612 4.382
1.1 E-13 2.6 E-14 1.3 E-14
93.7 94.9 77.7
147.4 141.5 134.7
1.4 3.7 9.7
2.2127 11.5427 4.1499 6.4243
0.00205 0.00481 0.00479 0.00338
28 62 22 49
7.037 6.716 7.213 6.941
1.6 1.4 3.0 7.9
E-14 E-14 E-14 E-15
94.1 92.5 86.7 93.1
142.5 136.3 146.0 140.7
1.2 1.3 0.8 2.3
7.5959 10.3118 6.6798
0.00369 0.00704 0.00374
53 38 46
7.084 7.175 7.452
1.5 E-14 2.5 E-14 1.5 E-14
93.2 84.6 92.6
141.6 143.3 148.1
1.3 0.9 1.4
9.9339 5.3069 8.1592 6.0514 10.0519 5.7797 10.6748 2.2163 2.1796 6.4129 6.6245
0.00448 0.00273 0.00357 0.00400 0.01020 0.00440 0.00380 0.00150 0.00159 0.00430 0.00990 0.00338 0.00810
57 so 59 39 25 34 72 38 35 39 17
7.024 7.500 7.116 4.653 4.142 4.319 4.988 4.640 4.814 4.316 4.250
5.3 E-13 1.0 E-14 6.9 E-15 2.6 E-14 5.7 E-14 1.4 E-13 3.4 E-14 7.5 E-14 9.1 E-14 5.5 E-14 8.2 E-14
92.5 94.9 94.2 86.5 64.7 83.3 94.1 94.4 94.0 84.6 63.6
140.5 149.6 142.2 142.7 127.6 132.8 152.6 142.3 147.5 132.7 130.8
3.2 1.9 2.5 4.9 2.8 1.2 3.8 1.6 1.3 2.3 2.3
25 9
6.879 7.596
5.1 E-15 1.0 E-14
90.1 77.7
139.4 153.4
3.8 2.3
1 3
6.069 6.009
2.5 E-12 9.3 E-13
99.3 99.5
184.0 182.3
0.4 0.5
2
9.249
3.6 E-13
99.7
185.1
0.3
3.2798 2.9230
Infer& detrital contaminants, K-feldspar: MC-JhIB-57 2322-04 27A 0.01769 0.0074 o.ooo13 2322-10 27A 0.01769 0.0097 0.00010 MC-JMB-48 1901-08 22A 0.01168 0.0065 0.00010
489
Brushy Basin Member of the Morrison Formation
Table 1. Lab ID#
J Irrad.
MC-JhIB-17.75 2314-11 27A DINO-3 1902-10 22A
37Ar/39Ar
(Co&.)
)36Ar/39Ar WAr*/39Ar %@AP o/<36Ar/39Ar),,? Moles 40Ar Age(Ma) i 1a8
0.01769
0.0105
0.00013
2
6.056
4.0 E-13
99.3
183.6
0.6
0.01168
0.0069
0.00004
4
10.756
1.4 E-13
99.9
213.5
0.4
0.00117 0.00073
81 63
5.885 6.006
3.3 E-13 1.2 E-13
98.8 98.6
178.7 182.2
0.6 1.2
Inferred detrital contaminants,
plagioclase:
MC-JMB-57 2322-11 27A 2322-12 27A
3.6460 1.7779
0.01769 0.01769
Low-volume rejects (<5 X lo-l5 moles MAr): MGJMB-57 2322-02 27A 0.01769 7.4013 0.00164 MGJMB-53.5 1943-09 22E 0.01153 0.0000 0.07320 1943-13 22E 0.01148 9.6119 0.00328 1943-14 22E 0.01148 12.3776 0.00521 MC-JMB-17.75 1947-02 22E 0.01153 3.1209 0.14019 1947-07 22E 0.01153 11.8847 0.00296 1947-09 22E 0.1153 1.9367 0.00053 DINOJ 1902-07 22A 0.01168 3.3402 0.00330 1902-08 22A 0.01168 0.0771 -0.05617 1902-09 22A 0.01168 0.0671 0.00228 1902-11 22A 0.01168 4.3464 -0.04645
100
4.996
2.5 E-15
101.6
152.8
2.1
0
-9.016 7.599 7.719
7.8 E-17 2.5 E-15 2.3 E-15
-71.5 96.9 92.8
-198.0 150.9 153.2
424.7 6.3 7.3
1 -16.263 100 7.630 94 11.386
7.2E-16 3.9 E-15 1.1 E-16
-65.0 100.4 99.9
-374.5 152.1 222.5
398.7 4.5 259.0
4.8 E-15 9.4E-17 l.OE-IS 3.3 E-16
90.6 214.2 72.7 187.1
141.8 559.7 37.2 546.3
3.8 1271 6.4 171.7
76 61
26 0 1 0
6.998 31.138 1.785 30.277
I Errors in age quoted for individual runs are la analytical uncertainty (errors in age quoted for weighted means are 1u standard error of the mean); 7 percent of 36Ar derived from calcium; MAr* = radiogenic argon; moles JOAr= estimated total moles of 4”Ar released during fusion based on spectrometer sensitivity considerations; A = 5.543 x lo-” y-l; J is the neutron flux parameter. Isotopic interference corrections: (36Ar/37Ar),z,= 2.58 x lo- ’ f 6 x 10e6, (39Ar/37Ar),, = 6.7 x 10m4f 3 x lo-‘, (mAr/39Ar)K = 8 x IO-” f 7 x 10e4.
age results for these plagioclase analyses of about 0.1 Ma. Postulating even an uncertainty of 10% in 36Arca/37Arca leads to a maximum uncertainty of only about 0.6 Ma in the end result. Analytical data are provided in Table 1. Before assessing the meaning of these results it is necessary to filter out several types of analyses that are considered unreliable for either geological or analytical reasons. Those analyses with too small a volume of gas to measure reliably were eliminated from the data set and are not included on Table 1; the cutoff here is taken as 5 X lo-” moles of total 40Ar. Grains with low values of radiogenic argon (O/04’Ar*)are likely to have been altered and typically give young ages; here an arbitrary cutoff of 950h4’Ar* is used even though some grains with less than 95% may still give valid results. Also, grains that are much older than the dominant age component, and clearly represent detrital contaminants, are also set aside from the calculation of mean eruptive ages, although we will return to interpret their significance later. Rejected analyses are listed separately in Table 1. The great majority of the phenocrysts dated were plagioclase (based on their 37Ar/39Ar ratio, which provides a direct measure of Ca/K content), although the rare alkali feldspar grains encountered provide an informative comparison. Table 2 summarizes the dating results on plagioclase. Within analytical error most
490
B. J. Kowallis
Table 2. Sample number MC-JMB-57 MC-JMB-53.5 MC-JMB-48 MC- JMB-43 MC-JMB-17.75 DINO-3
Summary
of “‘Ar/“Ar
rt ul.
dating
of plagioclase
grains
Weighted mean age (Ma)
f lo Std. error of the mean
% Std. error of the mean
147.6 147.0 145.2 147.8 149.4 152.9
0.X 0.6 1.2 0.6 0.7 1.2
0.6”/;, 0.4%1 0.80/u 0.40/u 0.5”/0 0.8%
of the samples are indistinguishable in apparent age; however, a general agreement with the expected decrease in age with increasing stratigraphic height in the Montezuma Creek section is observed from the mean ages. Although MC-JMB-48 appears out of sequence, it should be noted that this sample has about twice the error of the other ages from the Montezuma Creek section. The one sample from Dinosaur National Monument (DINO-3) gives a slightly older apparent age than the samples from Montezuma Creek, but again the error on this sample is about twice the size of most of the Montezuma Creek samples. Two precise analyses of alkali feldspar from the uppermost bentonite dated (MC-JMB-57) in the Montezuma Creek section give a weighted mean age of 149.2 f 0.4 Ma, while two grains from the lowermost tuff dated (MC-JMB-17.75) give 149.8 f 0.4 Ma. Two other alkali feldspars from altered ashes toward the middle of the section give 146.3 f0.5 Ma (MC-JMB-48) and 150.3 f 0.4 (MCJMB-43). We hesitate to place too much importance on these few alkali feldspar analyses, but in general they support the plagioclase results of about 147-150 Ma for the age of the dated Morrison strata. Obvious contaminant grains, characterized by anomalously older ages but analytically sound measurements, were encountered in four of the six dated samples. In the Montezuma Creek section all such grains, which included both alkali feldspar and plagioclase, fell within a narrow age range of 179-185 Ma (Middle Jurassic following Palmer, 1983). These ages are probably representative of the wall rock through which the ashes erupted. The Dinosaur National Monument sample bore a much older alkali feldspar, dated at 213.5 Ma, hinting at differences in bedrock geology either in the paleo-drainage areas or in the vent areas for the eruptions. 5. Discussion of Morrison Geochronology Only a few isotopic ages were previously available from the Morrison Formation, none of which employed feldspars. Lee & Brookins (1978) reported a 139 f 12 Ma Rb-Sr minimum age for the formation from unaltered chlorite grains thought to have formed during early diagenesis; J. D. Obradovich (1984, pers. comm.) reported a 134 Ma K-Ar biotite age from western Colorado and a 152 Ma 40Ar/39Ar biotite total gas age from just below the Cleveland-Lloyd dinosaur quarry in eastern Utah; and Bowman et al. (1986) reported K-Ar biotite ages of 147.2 f 1.0 and 146.8 f 1.OMa from the Cleveland-Lloyd quarry from about 1 m above the quarry, and a 135.2 f 5.5 Ma biotite age from Dinosaur National Monument from about 10 m above the quarry in the same bentonite as our DINO-3 sample. Thus, previous dating efforts suggested that the Morrison Formation was deposited between about 134-152 Ma, compared to the narrower age range of 145- 153 Ma suggested by the 40Ar/39Ar ages on feldspar presented here.
491
Brushy Basin Member of the Morrison Formation
Top of Montezum,l
Creek
Sectlon
100
80
T
5
h0
5
E
Y
.‘r? g
Top of Notom r\
30
Section
f-”
‘0
A
”
Base of Brushy
0
r-
I 100
r\
Basin
I 120
I 140
I 160
Age (Ma) Figure 2. Plot of radiometric age versus stratigraphic thickness for the 40Ar/39Ar ages from the Montezuma Creek section (plagioclase ages = dark circles, k-feldspar ages = open-squares) and for the fission track ages from the Notom section (open circles). Error bars represent two standard errors of the mean. The base of the Brushy Basin Member is shown at 0 m. The tops of the two sections are also shown.
A comparison of the new 40Ar/39Ar ages on plagioclase and k-feldspar with fission track ages from the Notom section (Kowallis & Heaton, 1987) is shown in Figure 2. Given their sizeable uncertainties, fission track ages in the lower two-thirds of the Notom section are compatible with the 40Ar/39Ar ages from the Montezuma Creek section, although they average about 10 Ma less in age. The fission track ages in the upper third (about 15 m of section), however, are much younger than any of the
492
B. J. Kowallis et ul
40Ar/3yAr ages from Montezuma Creek (125-100 Ma versus 153-145 Ma). We were not able to obtain age data from the upper 20 m of the Montezuma Creek section (the one sample collected in this ash-poor interval did not yield feldspars), but it may be that younger sediments exist in this part of the section. Perhaps a more likely explanation, however, is that the upper third of the Notom section sits unconformably on the lower part of that section. Kowallis & Heaton (1987) noted that the Buckhorn Conglomerate, usually found at the base of the Cedar Mountain Formation which overlies the Brushy Basin Member of the Morrison Formation throughout this part of the Colorado Plateau, is apparently absent at Notom. The fact that the Notom section is relatively thin, has no Buckhorn Conglomerate, and exhibits a striking change in age in the upper part of the section, not found in the Montezuma Creek section, all suggest that part of the section is missing at Notom. Peterson (1986) indicates that the upper part of the Brushy Basin member and much of the Cedar Mountain Formation was eroded from a large part of southeastern Utah that includes the Notom section. 6. Conclusions New 40Ar/39Ar radiometric age data reported here confirm that the Brushy Basin Member of the Morrison Formation is Late Jurassic in age (late Kimmeridgian to Tithonian, time scale of Palmer, 1983), and that most of the member was probably deposited between 153 to 145 Ma. The lack of new age data from the uppermost part of the Brushy Basin Member leaves open the possibility that some of the upper beds could be Early Cretaceous in age. Additional ages are needed from the upper part of the formation in order to determine if it is entirely Jurassic, or in part Cretaceous. In addition, dates from several different sections are needed to determine if the Morrison Formation is time-transgressive or time-synchronous. Acknowledgments We wish to thank Fred Peterson, Christine Turner-Peterson and Dan Chure for their assistance in collecting samples used in this report. We greatly appreciate the help in sample preparation provided by Dave Tingey and his staff. SEM photos of zircon for morphological classification were provided by Sarah Robinson. The research was supported mainly from NSF grant EAR-8720651, with additional support from Dinosaur National Monument, and from the Department of Geology and the College of Physical and Mathematical Sciences at Brigham Young University. Thoughtful reviews of this manuscript were provided by M. G. Best, F. Peterson and J. D. Obradovich. References Bell, T. E. 1986. Deposition and diagenesis of the Brushy Basin Member and upper part of the Westwater Canyon Member of the Morrison Formation, San Juan Basin, New Mexico. American Association of Pktroleum Geologists Studies in Geo&y
22,77-91.
Bilbey, S. A., Kerns, R. L. & Bowman, J. T. 1974. Petrology of Morrison Formation, Dinosaur Quarry Quadrangle, Utah. Utah Geological and Miwel Survey Special Studies 48, 15 pp. Bowman. S. A. B.. Bowman. 1. T. & Drake. R. E. 1986. Internretation of the Morrison Formation as a time-transgressive unit. ‘4ik Nortk A&an Paleontologicul’Convention, Boukler, Colorado, Abstracts with Programs, A5. Darton, N. H. 1922. Geologic structure of parts of New Mexico. U.S. Geological Suruq Bulletin 726, 173-275.
Brushy Basin Member of the Morrison Formation
493
Deino, A. & Potts, R. 1990. Single-crystal 40Ar/39Ar dating of the Olorgesailie Formation, southern Kenva Rift. 7oumul ofGeobhvsica1 Research 95. 8453-8470. Emmons,*S. F., Cross, W. & &ridge, G. H. 1896. Geology of the Denver Basin in Colorado. U.S. Geological Survey Monograph 27, 527 pp. Hansley, P. L. 1986. Relationship of detrital, nonopaque heavy minerals to diagenesis and provenance of the Morrison Formation, southwestern San Juan Basin, New Mexico. American Association of Petroleum Geologists Studies in Geology 22, 257-276.
Hotton,
C. L. 1986. Palynology of the Morrison
Formation.
4th North Atian
Paleontological
Convention, Boukkr Colorado, Abstracts with Programs, A20.
Imlay, R. W. 1952. Correlation of the Jurassic formations of North America exclusive of Canada. Atian
Association of Petroleum Geologists Bulletin 63,952-992.
Imlay, R. W. 1980. Jurassic paleobiogeography of the conterminous United States in its continental setting. U.S. Geological Survey Professional Paper 1062, 134 pp. Kowallis, B. J. 1986. Fission track dating of bentonites and bentonitic mudstones from the Morrison Formation, Utah and Colorado. 4th North American Paleontological Convention, Boulder, Colorado, Abstracts with Programs, A26.
Kowallis, B. J. & Christiansen, E. H. 1989. Applications of zircon morphology: correlation of pyroclastic rocks and petrogenetic inferences. Geological Society of America Abstracts with Program 21, A244. Kowallis, B. J. & Heaton, J. S. 1987. Fission-track dating of bentonites and bentonitic mudstones from the Morrison Formation in central Utah. Geology 15, 1138-1142. Kowallis, B. J., Christiansen, E. H. & Deino, A. 1989. Multi-characteristic correlation of Upper Cretaceous volcanic ash beds from southwestern Utah to central Colorado. Utah Geological and Mineral Survq, Miscellaneous Publication 89-5, 22 pp. Kowallis, B. J., Heaton, J. S. & Bringhurst, K. 1986. Fission-track dating of volcanically derived sedimentary rocks. Geology 14, 19-22. Lee, M. J. & Brookins, D. G. 1978. Rubidium-strontium minimum ages of sedimentation, uranium mineralization, and provenance, Morrison Formation (Upper Jurassic), Grants Mineral Belt, New Mexico. American Association of Petroleum Geologists Bulletin 62, 1673-1683. Palmer, A. R., 1983. The Decade of North American Geology 1983 Geologic Time Scale. Geology 11, 503-504. Petersen, L. M. & Roylance, M. M. 1982. Stratigraphy and depositional environments of the Upper Jurassic Morrison Formation near Capitol Reef National Park, Utah. Brigham Young University Geology Studies 29 (2), l-12.
Peterson, F. 1986. Jurassic paleotectonics in the west-central part of the Colorado Plateau, Utah and Arizona. American Association of Petroleum Geologists Memoir 41, 563-596. Peterson, F. & Turner-Peterson, C. E. 1987. The Morrison Formation of the Colorado Plateau-recent advances in sedimentology, stratigraphy, and paleotectonics. Hunter& 2 (1), l-18. Pupin, J. P. 1980. Zircon and granite petrology. Ccmtributions to Mineralogy ana’ Petrology 73, 207-220. Simpson, G. G. 1926. The age of the Morrison Formation. AmericanJournal ofScience 12, 198-216. Stokes, W. L. 1944. Morrison and related deposits in and adjacent to the Colorado Plateau. Geological Society of America Bulletin 55, 951-992.
Turner-Peterson, C. E. & Fishman, N. S. 1988. Origin and distribution of albite, illite/smectite, and chlorite in Jurassic Lake T’oo’dichi’: consequence of early diagenesis in a saline, alkaline lake, Geological Society of America Abstracts with Programs 20, A5 1.
Age of the Brushy Basin Member Colorado Plateau, western USA *Bart J. Kowallis,
*Eric H. Christiansen
of the Morrison
Formation,
and l-Alan L. Deino
‘Department of Geology, Brigham Young University, Pruvo, UT 84602, U.S.A. li;eochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley,
CA 94709, U.S.A
Received 14 Februar?, 1990 and accepted 10 October 1990
New single-crystal, laser-fusion 40Ar/39Ar ages on plagioclase and alkali feldspar from the Brushy Basin Member of the Morrison Formation range from 153 to 145 ( f about l-2) Ma or Late Jurassic in age (mostly Tithonian based upon Palmer, 1983). All but one of these new ages come from a section of Brushy Basin Member near Montezuma Creek, Utah, USA. The other, which is the oldest sample at 153 Ma, comes from Dinosaur National Monument, from a bentonite previously dated at 135.2 f 5.5 Ma (K-Ar biotite age) by Bowman et al. (1986). The new feldspar ages are essentially concordant with previously reported fission track ages from Notom, Utah, with ages ranging from 144 to 135 ( f about 15) Ma in the lower two-thirds of the Brushy Basin Member, but contrast with fission track ages from the upper one-third (about 15 m) of the Notom section, which ranged from 123 to 99 (f about 13) Ma (Kowallis & Heaton, 1987). However, none of the new ages comes from the upper part of the Brushy Basin Member; the uppermost sample dated from the Montezuma Creek section is more than 20 m below the top of the Morrison Formation. Precise ages from the uppermost part of the Brushy Basin Member are still needed to determine if the Jurassic-Cretaceous boundary lies within the uppermost Brushy Basin Member. KEY WORDS:
WAr/39Ar ages; Upper Jurassic; Morrison Formation; USA
1. Introduction
The Morrison Formation in the western United States has produced the most spectacular, diverse and abundant dinosaur fossils of any formation in the world. Its age, however, has long been a topic of debate, having been labelled as Jurassic, Cretaceous, or Jurassic-Cretaceous over the years (e.g. Emmons et al., 1896; Darton, 1922; Simpson, 1926; Stokes, 1944; Imlay, 1952; Imlay, 1980). The controversy extends back to the earliest formal descriptions of the formation when Emmons et al. (1896) wrote: “As both molluscan and plant remains found at this horizon have too wide a range to be of value in the determination of age of the enclosing beds, this determination has been based exclusively upon the vertebrates. Although the latter have some affinities with the European Wealden or Lower Cretaceous, to which Professor Marsh was at first inclined to assign the horizon, he found the evidence in favor of late Jurassic age to be so much stronger that he assigned to this period not only these but other beds with a similar fauna, notably the Potomac beds of the East, which on other grounds had been considered early Cretaceous and which present many structural analogies with the Morrison beds. From the point of view of the stratigrapher, the assignment of the Morrison beds to the Lower Cretaceous rather than to the Upper Jurassic is much more desirable, not only because it accords better with the sequence of sedimentation thus far disclosed in the adjoining regions of Kansas and Texas, but because it places the physical break whose effects are recognized over the whole continent between these two great time divisions rather than in the midst of one of them.” 0195-6671/91/050483
+ I1 $03.00/O
@ 1991 Academic Press Lunited
484
B. J. Kowallis ct al.
More recently, papers presented during a special session dedicated to the Morrison Formation at the Fourth North American Paleontological Convention in 1986, demonstrated that the problems have not been solved. Data presented there, would allow the age of the Morrison to be as old as pre-Kimmeridgian (Hotton, 1986) and/or as young as Neocomian (Bowman et al., 1986; Kowallis, 1986). The new single crystal 40Ar/39Ar ages presented here do not solve all the problems of Morrison Formation chronostratigraphy, but they improve the picture. Previous papers by Kowallis et al. (1986) and Kowallis & Heaton (1987) gave fission track ages for a section of the Morrison outside of Capitol Reef National Park
1 X0 km
I
+ NTM
UTAH Figure 1. Index map of Utah showing the location of the three sites discussed in the text, the Notom section (NTM), the Montezuma Creek section (MC), and the Dinosaur National Monument section (DINO).
Brushy Basin Member of the Morrison Formation
485
near Notom, Utah (Figure 1). The 15 fission track ages obtained there from the Brushy Basin Member suggested that the lower two-thirds of this member were about 140 f 10 Ma and probably uppermost Jurassic, but possibly Lower Creraceous, depending on the time scale used. The upper third (about 15 m of section) at Notom was younger, ranging in age from about 125 to 100 f 10 Ma, definitely Cretaceous in age. However, several problems exist with the Notom section. The Brushy Basin Member there is unusually thin (only 45 m as opposed to about 100 m elsewhere); also, its top is not as clearly defined as in other sections because of the lack of a well-developed basal conglomerate (Buckhorn Conglomerate Member) in the overlying Cedar Mountain Formation; and, finally, although the fission track ages are useful in providing some constraints on the age of the Brushy Basin Member, they possess errors that are too large to be useful in more precise correlations. In order to constrain better the age of the formation, we have obtained new radiometric ages using 40Ar/39Ar laser-probe dating techniques on individual plagioclase and sanidine grains from altered volcanic ash layers. 2. Sample localities The samples dated in this study come from three localities (Figure 1). Ages from one of the sections (Notom) have been previously reported (see Section 1), but will be re-examined here in light of the new data. The second section (Montezuma Creek) was chosen because it has a relatively thick Brushy Basin Member and contains numerous, closely spaced altered ash beds. The third locality was at Dinosaur National Monument, where only three samples were collected. 2.1. Notom section The lithostratigraphy of this section was presented by Petersen & Roylance (1982). It contains a very bentonitic section of the Brushy Basin Member about 41.5 m thick. The section is located about 5 km east of Capitol Reef National Park at 111”06’W longitude and 38” 14’N latitude. Nineteen samples from separate bentonitic horizons were collected, of which 11 yielded enough zircon for fission track dating as reported in Kowallis & Heaton (1987). 2.2. Montezuma Creek section The Montezuma Creek section, Utah, is located at 109” 19’W, 37” 19’N and there the Brushy Basin Member is about 105 m thick. It was deposited within the boundaries of Lake T’oo’dichi’, a saline, alkaline lake that covered a large area in the Colorado Plateau region during this interval of time (Bell, 1986; Peterson & TumerPeterson, 1987; Turner-Peterson & Fishman, 1988). The playa lake environment, we felt, would be a more favorable environment for preservation of ash beds than the fluvially dominated section at Notom. From the Montezuma Creek section we collected samples from 40 altered ash layers. About one-fourth of the altered ashes contained enough large (~60 mesh) feldspar for single crystal 40Ar/39Ar dating. We chose five of these samples for age determination, including the lowermost datable sample from about 30 m above the base of the Brushy Basin Member and the uppermost one from 20 m below its top. The samples were labelled in stratigraphic sequence with numbers increasing toward the top of the section. 2.3. Dinosaur National Monunmt
section
Three bentonitic samples were collected near the visitor’s center at Dinosaur National Monument, Utah. The entire section of Brushy Basin Member where the
486
B. J. Kowallis er ul
samples were collected is about 125 m thick (Bilbey et al., 1974). Two of the samples (DINO-I and DINO-3) came from about 10 m above the main dinosaurbearing layer. These two samples were collected about 0.5 km apart in what was thought to be the same bentonitic layer. Zircon morphologies of these samples, seem to confirm that they are indeed from the same ash layer. Individual zircon grains from these samples were examined on scanning electron microscope (SEM) photos at magnifications of x 100-200. Each grain was classified using the system developed by Pupin (1980), which is based upon the relative size and development of the (100) and (110) prism faces and the (101) and (211) pyramid faces. This classification reduces the morphological data to two indices; an A index, which is determined from the pyramid faces, and a T index, which is determined from the prism faces. Kowallis et al. (1989) and Kowallis & Christiansen (1989) have shown that average Pupin indices of zircons are consistent for an individual ash bed even over several hundred kilometers. DINO-1 and -3 have average A indices of 400 f 10 and 405 f 8, respectively, and average T indices of 435 f 11 and 426 f 10, respectively. DINO-2, which came from a different layer located immediately below the dinosaur-bearing layer, has an average A index of 376 f 11 and an average T index of 508 f 14, quite different from the other two samples. 3. Sample collection and preparation In the Notom and Montezuma Creek sections, every bentonitic horizon in the Brushy Basin Member was sampled for possible dating. Samples were about 10 kg each. Clays were removed by soaking the samples in water and then washing them over a wilfley table. The residue from the washed samples was examined with a binocular microscope to determine the phenocrysts present and to look for detrital contaminants. Then the samples were further processed through heavy liquids and a magnetic separator to concentrate heavy minerals and separate feldspars. Most of the samples contained some obviously rounded, probably detrital grains. The most common detrital grains were zircon (in a variety of colors), clear apatite, dark yellowish-brown tourmaline, pink garnet, quartz and microcline. These were more common in the Notom section and at Dinosaur National Monument than in the Montezuma Creek section. The heavy mineral suite is essentially the same as that described by Hansley (1986) from fine- to medium-grained sandstones of the Morrison Formation in the Grants area of New Mexico. Many of the samples also contained gypsum (or anhydrite) and barite. From the final mineral separates, euhedral zircons and angular feldspars were hand picked for dating. 4. 40~/39Atgeochronology Feldspar phenocrysts from five samples from the Montezuma Creek section and one sample from Dinosaur National Monument were dated by the single-crystal, laser-fusion 40Ar/39Ar method. The details of the preparation, irradiation, and analysis of these samples follows closely those of Deino & Potts (1990). The analytical accuracy of the plagioclase age determinations is not strongly compromised by the necessity to correct the argon isotope analyses for 36Ar and 39Ar produced from Ca during neutron bombardment in the reactor. By far the more important of these two corrections is 36Ar,-;a.We have determined the production ratio 36Ar,-a/37ArCaof the reactor through repeat analyses of optical grade fluorite, with a precision of about 2% (Table 1). This translates to an uncertainty in the final
Brushy Basin Member of the Morrison Formation
Table 1. Lab ID# Irrad. MGJMB-57 K-feldspar: 2322-01 27A 2322-13 27A
_7
0.01769 0.01769
37Ar/39Ar
0.0077 0.485 1
487
40Ar/39Ar analytical data.
36Ar/39Ar wAr*/39Ar o/d36Ar/39Ar),,t
0.00013 0.00023
5:
4.865 4.878
9/,“OAr* Moles 40Ar Age(Ma) f 1a§
4.2 E-13 6.2 E-13
99.2 99.3
148.9 149.3
0.5
149.2
0.4
147.8 144.8 143.1 149.6 147.7 147.1
3.3 1.9 2.7 1.2 1.0 1.6
147.6
0.8
149.9 148.0 150.1 152.0 146.7 142.9 146.8 149.3 146.5 144.4 145.4
2.5 1.3 2.3 2.8 1.2 2.8 1.5 1.5 1.4 1.3 2.0
147.0
0.6
99.3
146.3
0.5
98.0 99.0 96.5 98.8 98.7 100.3
143.7 144.0 143.0 148.2 149.7 149.9 145.2
1.0 1.0 1.2 1.4 1.6 2.3 1.2
Weighted mean age = Plagioclase: 2322-03 2322-06 2322-07 2322-09 2322-15 2322-16
27A 27A 27A 27A 27A 27A
0.01768 0.01769 0.01769 0.01769 0.01769 0.01769
2.9629 3.0941 8.2339 3.4613 1.7891 6.2516
0 Xi069 0.00110 0.00275 0.00162 0.00123 0.00193
100 73 77 55 38 84
4.827 4.726 4.668 4.887 4.823 4.802
5.6 E-14 6.2 E-14 4.3 E-14 1.0 E-13 1.3 E-13 8.0 E-14
100.4 98.1 96.1 95.7 95.5 98.1
Weighted mean age = MC-JhUS-53.5 Plagioclase: 22E 1943-11 1943-12 22E 22E 1943-15 1943-16 22E 1943-17 22E 1943-18 22E 1943-19 22E 22E 1943-20 1943-21 22E 1943-22 22E 1943-23 22E
0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148 0.01148
2.3692 2.7349 3.4339 2.4185 3.6001 6.7687 1.8843 2.1812 2.4674 2.2974 6.3250
0.00058 0.00077 0.00119 0.00018 0.00140 0.00291 0.00035 0.00071 0.00079 0.00097 0.00168
100 91 75 100 66 60 100 79 81 61 97
7.545 7.445 7.557 7.657 7.376 7.181 7.384 7.513 7.365 7.258 7.312
5.6 E-15 1.3 E-14 7.2 E-15 6.3 E-15 1.5 E-14 5.3 E-15 1.1 E-14 1.1 E-14 1.2 E-14 1.3 E-14 8.4 E-15
100.1 99.7 98.8 101.7 98.1 95.4 100.5 99.4 99.4 98.5 99.8
Weighted mean age =
0.4
MC-JMB-48 K-feldspar: 22A 1901-11
0.01168
0.4171
0.00027
40
Plagioclase: 1901-02 1901-05 1901-06 1901-07 1901-10 1901-12
22A 22A 22A 22A 22A 22A
0.01168 0.01168 0.01168 0.01168 0.01168 0.01168
3.4279 4.6625 7.6420 6.8106 3.7527 7.6464
0.00136 0.00145 0.00283 0.00205 0.00130 0.00190
65 83 70 86 75 100
MGJMB-43 K-feldspar: 1941-16 22E
0.01148
0.0283
0.00011
6
7.564
1.6 E-13
99.6
150.3
0.4
Plagioclase: 1941-01 1941-02 1941-04 1941-05 1941-07 1941-08 1941-09 1941-10 1941-11 1941-12 1941-13 1941-14 1941-17 1941-18
0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01153 0.01148 0.01148 0.01148 0.01148 0.01148
6.4156 7.8919 7.1404 6.7468 7.2838 9.3295 9.4860 7.2508 9.9020 1.9889 6.3036 6.5352 9.7044 6.6031
0.00219 0.00304 0.00254 0.00243 0.00286 0.00321 0.00247 0.00238 0.00325 0.00085 0.00234 0.00196 0.00290 0.00234
76 67 73 72 66 75 99 79 79 60 70 86 86 73
7.360 7.281 7.282 7.283 7.090 7.356 7.518 7.451 7.269 7.546 7.472 7.457 7.486 7.253
1.9 E-14 2.2 E-14 1.9 E-14 2.2 E-14 1.1 E-14 1.3E-14 2.0 E-14 2.3 E-14 1.8 E-14 7.1 E-14 2.3 E-14 1.3 E-14 2.2 E-14 1.2E-14
97.9 96.1 97.2 97.3 96.0 96.8 99.9 98.0 97.2 98.7 97.2 98.9 98.4 97.4
146.9 145.4 145.4 145.4 141.7 146.8 149.9 148.7 145.2 149.9 148.5 148.2 148.8 144.3
1.0 0.9 0.9 0.8 1.5 1.3 0.9 0.9 1.1 0.5 0.9 1.2 0.9 1.4
147.7
0.6
22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E 22E
7.229
4.1 E-14
7.099 1.7 E-14 7.113 1.7 E-14 7.059 1.4 E-14 7.329 1.2 E-14 7.408 1.1 E-14 7.415 7.9 E-15 Weighted mean age =
Weighted mean age =
B. J. Kowallis et al.
488
Table 1. Lab lD# Irrad. MC-JhIB-17.75 K-feldspar: 2314-08 27A 2314-13 27A
J
0.01769 0.01769
37Ar/39Ar
0.0197 0.0129
(Contd.)
)=Ar/=Ar ‘0Ar*/3”Ar %(3bAr/39Ar)C,t
0.00017 0.00005
3 7
4.894 4.893
%*Ar* Moles “‘Ar
4.8 E-13 1.0 E-12
Age(Ma) f 105
98.9 99.7
Weighted mean age = Plagioclase: 1947-01 1947-04 2314-04 2314-06 2314-09 2314-10 2314-15
22E 22E 27A 27A 27A 27A 27A
0.01153 0.01153 0.01769 0.01769 0.01769 0.01769 0.01769
7.0721 8.8515 2.2134 4.3115 2.7884 2.1687 1.9666
0.00208 0.00286 0.00112 0.00185 0.00109 0.00079 0.00077
88 80 51 60 66 71 66
7.477 7.528 4.887 4.801 4.848 4.977 4.866
8.2 E-15 5.5 E-15 5.9 E-14 6.3 E-14 6.0 E-14 8.6 E-14 1.0 E-13
99.0 97.8 96.8 95.6 97.8 98.6 98.4
Weighted mean age = DINO-3 Plagioclase: 1902-01 1902-02 1902-04 1902-06 1902-12
22A 22A 22A 22A 22A
0.01168 0.01168 0.01168 0.01168 0.01168
2.7622 4.1840 4.2009 3.9396 2.8513
0.00088 0.00186 0.00210 0.00179 00.162
81 58 52 57 45
7.621 7.180 7.570 7.516 7.285
2.0 5.3 1.0 5.7 5.2
E-14 E-15 E-14 E-15 E-15
99.3 96.9 96.2 97.0 96.5
Weighted mean age = I,~w.radiopenic reiecta (<95% MGJhIBZ7 . 2322-05 27A 0.01769 0.01769 2322-08 27A 0.01769 2322-14 27A MC-JMIk-48 1901-01 22A 0.01168 1901-03 22A 0.01168 0.01168 1901-04 22A 1901-09 22A 0.01168 MC-JhIB-43 0.01153 1941-03 22E 0.01153 1941-06 22E 1941-1s 22E 0.01148 MC-JMB-17.75 1947-05 22E 0.01153 0.01153 1947-06 22E 1947-08 22E 0.01153 0.01769 2314-01 27A 0.01769 2314-02 27A 0.01769 2314-03 27A 2314-05 27A 0.01769 2314-07 27A 0.01769 0.01769 2314-12 27A 2314-14 27A 0.01769 2314-16 27A 0.01769 DINO-3 0.01168 1902-03 22A 0.01168 1902-05 22A
149.8 149.8
0.4 0.4
149.8
0.3
149.2 150.1 149.6 147.1 148.4 152.2 149.0
2.1 3.0 2.1 1.8 1.9 1.5 1.1
149.4
0.7
153.9 145.3 152.9 151.8 147.3
0.9 3.2 1.9 3.2 3.5
152.9
1.2
radioaenic Ark 7.2504 7.5874 7.6292
0.00296 0.00278 0.00621
63 70 32
4.812 4.612 4.382
1.1 E-13 2.6 E-14 1.3 E-14
93.7 94.9 77.7
147.4 141.5 134.7
1.4 3.7 9.7
2.2127 11.5427 4.1499 6.4243
0.00205 0.00481 0.00479 0.00338
28 62 22 49
7.037 6.716 7.213 6.941
1.6 1.4 3.0 7.9
E-14 E-14 E-14 E-15
94.1 92.5 86.7 93.1
142.5 136.3 146.0 140.7
1.2 1.3 0.8 2.3
7.5959 10.3118 6.6798
0.00369 0.00704 0.00374
53 38 46
7.084 7.175 7.452
1.5 E-14 2.5 E-14 1.5 E-14
93.2 84.6 92.6
141.6 143.3 148.1
1.3 0.9 1.4
9.9339 5.3069 8.1592 6.0514 10.0519 5.7797 10.6748 2.2163 2.1796 6.4129 6.6245
0.00448 0.00273 0.00357 0.00400 0.01020 0.00440 0.00380 0.00150 0.00159 0.00430 0.00990 0.00338 0.00810
57 so 59 39 25 34 72 38 35 39 17
7.024 7.500 7.116 4.653 4.142 4.319 4.988 4.640 4.814 4.316 4.250
5.3 E-13 1.0 E-14 6.9 E-15 2.6 E-14 5.7 E-14 1.4 E-13 3.4 E-14 7.5 E-14 9.1 E-14 5.5 E-14 8.2 E-14
92.5 94.9 94.2 86.5 64.7 83.3 94.1 94.4 94.0 84.6 63.6
140.5 149.6 142.2 142.7 127.6 132.8 152.6 142.3 147.5 132.7 130.8
3.2 1.9 2.5 4.9 2.8 1.2 3.8 1.6 1.3 2.3 2.3
25 9
6.879 7.596
5.1 E-15 1.0 E-14
90.1 77.7
139.4 153.4
3.8 2.3
1 3
6.069 6.009
2.5 E-12 9.3 E-13
99.3 99.5
184.0 182.3
0.4 0.5
2
9.249
3.6 E-13
99.7
185.1
0.3
3.2798 2.9230
Infer& detrital contaminants, K-feldspar: MC-JhIB-57 2322-04 27A 0.01769 0.0074 o.ooo13 2322-10 27A 0.01769 0.0097 0.00010 MC-JMB-48 1901-08 22A 0.01168 0.0065 0.00010
489
Brushy Basin Member of the Morrison Formation
Table 1. Lab ID#
J Irrad.
MC-JhIB-17.75 2314-11 27A DINO-3 1902-10 22A
37Ar/39Ar
(Co&.)
)36Ar/39Ar WAr*/39Ar %@AP o/<36Ar/39Ar),,? Moles 40Ar Age(Ma) i 1a8
0.01769
0.0105
0.00013
2
6.056
4.0 E-13
99.3
183.6
0.6
0.01168
0.0069
0.00004
4
10.756
1.4 E-13
99.9
213.5
0.4
0.00117 0.00073
81 63
5.885 6.006
3.3 E-13 1.2 E-13
98.8 98.6
178.7 182.2
0.6 1.2
Inferred detrital contaminants,
plagioclase:
MC-JMB-57 2322-11 27A 2322-12 27A
3.6460 1.7779
0.01769 0.01769
Low-volume rejects (<5 X lo-l5 moles MAr): MGJMB-57 2322-02 27A 0.01769 7.4013 0.00164 MGJMB-53.5 1943-09 22E 0.01153 0.0000 0.07320 1943-13 22E 0.01148 9.6119 0.00328 1943-14 22E 0.01148 12.3776 0.00521 MC-JMB-17.75 1947-02 22E 0.01153 3.1209 0.14019 1947-07 22E 0.01153 11.8847 0.00296 1947-09 22E 0.1153 1.9367 0.00053 DINOJ 1902-07 22A 0.01168 3.3402 0.00330 1902-08 22A 0.01168 0.0771 -0.05617 1902-09 22A 0.01168 0.0671 0.00228 1902-11 22A 0.01168 4.3464 -0.04645
100
4.996
2.5 E-15
101.6
152.8
2.1
0
-9.016 7.599 7.719
7.8 E-17 2.5 E-15 2.3 E-15
-71.5 96.9 92.8
-198.0 150.9 153.2
424.7 6.3 7.3
1 -16.263 100 7.630 94 11.386
7.2E-16 3.9 E-15 1.1 E-16
-65.0 100.4 99.9
-374.5 152.1 222.5
398.7 4.5 259.0
4.8 E-15 9.4E-17 l.OE-IS 3.3 E-16
90.6 214.2 72.7 187.1
141.8 559.7 37.2 546.3
3.8 1271 6.4 171.7
76 61
26 0 1 0
6.998 31.138 1.785 30.277
I Errors in age quoted for individual runs are la analytical uncertainty (errors in age quoted for weighted means are 1u standard error of the mean); 7 percent of 36Ar derived from calcium; MAr* = radiogenic argon; moles JOAr= estimated total moles of 4”Ar released during fusion based on spectrometer sensitivity considerations; A = 5.543 x lo-” y-l; J is the neutron flux parameter. Isotopic interference corrections: (36Ar/37Ar),z,= 2.58 x lo- ’ f 6 x 10e6, (39Ar/37Ar),, = 6.7 x 10m4f 3 x lo-‘, (mAr/39Ar)K = 8 x IO-” f 7 x 10e4.
age results for these plagioclase analyses of about 0.1 Ma. Postulating even an uncertainty of 10% in 36Arca/37Arca leads to a maximum uncertainty of only about 0.6 Ma in the end result. Analytical data are provided in Table 1. Before assessing the meaning of these results it is necessary to filter out several types of analyses that are considered unreliable for either geological or analytical reasons. Those analyses with too small a volume of gas to measure reliably were eliminated from the data set and are not included on Table 1; the cutoff here is taken as 5 X lo-” moles of total 40Ar. Grains with low values of radiogenic argon (O/04’Ar*)are likely to have been altered and typically give young ages; here an arbitrary cutoff of 950h4’Ar* is used even though some grains with less than 95% may still give valid results. Also, grains that are much older than the dominant age component, and clearly represent detrital contaminants, are also set aside from the calculation of mean eruptive ages, although we will return to interpret their significance later. Rejected analyses are listed separately in Table 1. The great majority of the phenocrysts dated were plagioclase (based on their 37Ar/39Ar ratio, which provides a direct measure of Ca/K content), although the rare alkali feldspar grains encountered provide an informative comparison. Table 2 summarizes the dating results on plagioclase. Within analytical error most
490
B. J. Kowallis
Table 2. Sample number MC-JMB-57 MC-JMB-53.5 MC-JMB-48 MC- JMB-43 MC-JMB-17.75 DINO-3
Summary
of “‘Ar/“Ar
rt ul.
dating
of plagioclase
grains
Weighted mean age (Ma)
f lo Std. error of the mean
% Std. error of the mean
147.6 147.0 145.2 147.8 149.4 152.9
0.X 0.6 1.2 0.6 0.7 1.2
0.6”/;, 0.4%1 0.80/u 0.40/u 0.5”/0 0.8%
of the samples are indistinguishable in apparent age; however, a general agreement with the expected decrease in age with increasing stratigraphic height in the Montezuma Creek section is observed from the mean ages. Although MC-JMB-48 appears out of sequence, it should be noted that this sample has about twice the error of the other ages from the Montezuma Creek section. The one sample from Dinosaur National Monument (DINO-3) gives a slightly older apparent age than the samples from Montezuma Creek, but again the error on this sample is about twice the size of most of the Montezuma Creek samples. Two precise analyses of alkali feldspar from the uppermost bentonite dated (MC-JMB-57) in the Montezuma Creek section give a weighted mean age of 149.2 f 0.4 Ma, while two grains from the lowermost tuff dated (MC-JMB-17.75) give 149.8 f 0.4 Ma. Two other alkali feldspars from altered ashes toward the middle of the section give 146.3 f0.5 Ma (MC-JMB-48) and 150.3 f 0.4 (MCJMB-43). We hesitate to place too much importance on these few alkali feldspar analyses, but in general they support the plagioclase results of about 147-150 Ma for the age of the dated Morrison strata. Obvious contaminant grains, characterized by anomalously older ages but analytically sound measurements, were encountered in four of the six dated samples. In the Montezuma Creek section all such grains, which included both alkali feldspar and plagioclase, fell within a narrow age range of 179-185 Ma (Middle Jurassic following Palmer, 1983). These ages are probably representative of the wall rock through which the ashes erupted. The Dinosaur National Monument sample bore a much older alkali feldspar, dated at 213.5 Ma, hinting at differences in bedrock geology either in the paleo-drainage areas or in the vent areas for the eruptions. 5. Discussion of Morrison Geochronology Only a few isotopic ages were previously available from the Morrison Formation, none of which employed feldspars. Lee & Brookins (1978) reported a 139 f 12 Ma Rb-Sr minimum age for the formation from unaltered chlorite grains thought to have formed during early diagenesis; J. D. Obradovich (1984, pers. comm.) reported a 134 Ma K-Ar biotite age from western Colorado and a 152 Ma 40Ar/39Ar biotite total gas age from just below the Cleveland-Lloyd dinosaur quarry in eastern Utah; and Bowman et al. (1986) reported K-Ar biotite ages of 147.2 f 1.0 and 146.8 f 1.OMa from the Cleveland-Lloyd quarry from about 1 m above the quarry, and a 135.2 f 5.5 Ma biotite age from Dinosaur National Monument from about 10 m above the quarry in the same bentonite as our DINO-3 sample. Thus, previous dating efforts suggested that the Morrison Formation was deposited between about 134-152 Ma, compared to the narrower age range of 145- 153 Ma suggested by the 40Ar/39Ar ages on feldspar presented here.
491
Brushy Basin Member of the Morrison Formation
Top of Montezum,l
Creek
Sectlon
100
80
T
5
h0
5
E
Y
.‘r? g
Top of Notom r\
30
Section
f-”
‘0
A
”
Base of Brushy
0
r-
I 100
r\
Basin
I 120
I 140
I 160
Age (Ma) Figure 2. Plot of radiometric age versus stratigraphic thickness for the 40Ar/39Ar ages from the Montezuma Creek section (plagioclase ages = dark circles, k-feldspar ages = open-squares) and for the fission track ages from the Notom section (open circles). Error bars represent two standard errors of the mean. The base of the Brushy Basin Member is shown at 0 m. The tops of the two sections are also shown.
A comparison of the new 40Ar/39Ar ages on plagioclase and k-feldspar with fission track ages from the Notom section (Kowallis & Heaton, 1987) is shown in Figure 2. Given their sizeable uncertainties, fission track ages in the lower two-thirds of the Notom section are compatible with the 40Ar/39Ar ages from the Montezuma Creek section, although they average about 10 Ma less in age. The fission track ages in the upper third (about 15 m of section), however, are much younger than any of the
492
B. J. Kowallis et ul
40Ar/3yAr ages from Montezuma Creek (125-100 Ma versus 153-145 Ma). We were not able to obtain age data from the upper 20 m of the Montezuma Creek section (the one sample collected in this ash-poor interval did not yield feldspars), but it may be that younger sediments exist in this part of the section. Perhaps a more likely explanation, however, is that the upper third of the Notom section sits unconformably on the lower part of that section. Kowallis & Heaton (1987) noted that the Buckhorn Conglomerate, usually found at the base of the Cedar Mountain Formation which overlies the Brushy Basin Member of the Morrison Formation throughout this part of the Colorado Plateau, is apparently absent at Notom. The fact that the Notom section is relatively thin, has no Buckhorn Conglomerate, and exhibits a striking change in age in the upper part of the section, not found in the Montezuma Creek section, all suggest that part of the section is missing at Notom. Peterson (1986) indicates that the upper part of the Brushy Basin member and much of the Cedar Mountain Formation was eroded from a large part of southeastern Utah that includes the Notom section. 6. Conclusions New 40Ar/39Ar radiometric age data reported here confirm that the Brushy Basin Member of the Morrison Formation is Late Jurassic in age (late Kimmeridgian to Tithonian, time scale of Palmer, 1983), and that most of the member was probably deposited between 153 to 145 Ma. The lack of new age data from the uppermost part of the Brushy Basin Member leaves open the possibility that some of the upper beds could be Early Cretaceous in age. Additional ages are needed from the upper part of the formation in order to determine if it is entirely Jurassic, or in part Cretaceous. In addition, dates from several different sections are needed to determine if the Morrison Formation is time-transgressive or time-synchronous. Acknowledgments We wish to thank Fred Peterson, Christine Turner-Peterson and Dan Chure for their assistance in collecting samples used in this report. We greatly appreciate the help in sample preparation provided by Dave Tingey and his staff. SEM photos of zircon for morphological classification were provided by Sarah Robinson. The research was supported mainly from NSF grant EAR-8720651, with additional support from Dinosaur National Monument, and from the Department of Geology and the College of Physical and Mathematical Sciences at Brigham Young University. Thoughtful reviews of this manuscript were provided by M. G. Best, F. Peterson and J. D. Obradovich. References Bell, T. E. 1986. Deposition and diagenesis of the Brushy Basin Member and upper part of the Westwater Canyon Member of the Morrison Formation, San Juan Basin, New Mexico. American Association of Pktroleum Geologists Studies in Geo&y
22,77-91.
Bilbey, S. A., Kerns, R. L. & Bowman, J. T. 1974. Petrology of Morrison Formation, Dinosaur Quarry Quadrangle, Utah. Utah Geological and Miwel Survey Special Studies 48, 15 pp. Bowman. S. A. B.. Bowman. 1. T. & Drake. R. E. 1986. Internretation of the Morrison Formation as a time-transgressive unit. ‘4ik Nortk A&an Paleontologicul’Convention, Boukler, Colorado, Abstracts with Programs, A5. Darton, N. H. 1922. Geologic structure of parts of New Mexico. U.S. Geological Suruq Bulletin 726, 173-275.
Brushy Basin Member of the Morrison Formation
493
Deino, A. & Potts, R. 1990. Single-crystal 40Ar/39Ar dating of the Olorgesailie Formation, southern Kenva Rift. 7oumul ofGeobhvsica1 Research 95. 8453-8470. Emmons,*S. F., Cross, W. & &ridge, G. H. 1896. Geology of the Denver Basin in Colorado. U.S. Geological Survey Monograph 27, 527 pp. Hansley, P. L. 1986. Relationship of detrital, nonopaque heavy minerals to diagenesis and provenance of the Morrison Formation, southwestern San Juan Basin, New Mexico. American Association of Petroleum Geologists Studies in Geology 22, 257-276.
Hotton,
C. L. 1986. Palynology of the Morrison
Formation.
4th North Atian
Paleontological
Convention, Boukkr Colorado, Abstracts with Programs, A20.
Imlay, R. W. 1952. Correlation of the Jurassic formations of North America exclusive of Canada. Atian
Association of Petroleum Geologists Bulletin 63,952-992.
Imlay, R. W. 1980. Jurassic paleobiogeography of the conterminous United States in its continental setting. U.S. Geological Survey Professional Paper 1062, 134 pp. Kowallis, B. J. 1986. Fission track dating of bentonites and bentonitic mudstones from the Morrison Formation, Utah and Colorado. 4th North American Paleontological Convention, Boulder, Colorado, Abstracts with Programs, A26.
Kowallis, B. J. & Christiansen, E. H. 1989. Applications of zircon morphology: correlation of pyroclastic rocks and petrogenetic inferences. Geological Society of America Abstracts with Program 21, A244. Kowallis, B. J. & Heaton, J. S. 1987. Fission-track dating of bentonites and bentonitic mudstones from the Morrison Formation in central Utah. Geology 15, 1138-1142. Kowallis, B. J., Christiansen, E. H. & Deino, A. 1989. Multi-characteristic correlation of Upper Cretaceous volcanic ash beds from southwestern Utah to central Colorado. Utah Geological and Mineral Survq, Miscellaneous Publication 89-5, 22 pp. Kowallis, B. J., Heaton, J. S. & Bringhurst, K. 1986. Fission-track dating of volcanically derived sedimentary rocks. Geology 14, 19-22. Lee, M. J. & Brookins, D. G. 1978. Rubidium-strontium minimum ages of sedimentation, uranium mineralization, and provenance, Morrison Formation (Upper Jurassic), Grants Mineral Belt, New Mexico. American Association of Petroleum Geologists Bulletin 62, 1673-1683. Palmer, A. R., 1983. The Decade of North American Geology 1983 Geologic Time Scale. Geology 11, 503-504. Petersen, L. M. & Roylance, M. M. 1982. Stratigraphy and depositional environments of the Upper Jurassic Morrison Formation near Capitol Reef National Park, Utah. Brigham Young University Geology Studies 29 (2), l-12.
Peterson, F. 1986. Jurassic paleotectonics in the west-central part of the Colorado Plateau, Utah and Arizona. American Association of Petroleum Geologists Memoir 41, 563-596. Peterson, F. & Turner-Peterson, C. E. 1987. The Morrison Formation of the Colorado Plateau-recent advances in sedimentology, stratigraphy, and paleotectonics. Hunter& 2 (1), l-18. Pupin, J. P. 1980. Zircon and granite petrology. Ccmtributions to Mineralogy ana’ Petrology 73, 207-220. Simpson, G. G. 1926. The age of the Morrison Formation. AmericanJournal ofScience 12, 198-216. Stokes, W. L. 1944. Morrison and related deposits in and adjacent to the Colorado Plateau. Geological Society of America Bulletin 55, 951-992.
Turner-Peterson, C. E. & Fishman, N. S. 1988. Origin and distribution of albite, illite/smectite, and chlorite in Jurassic Lake T’oo’dichi’: consequence of early diagenesis in a saline, alkaline lake, Geological Society of America Abstracts with Programs 20, A5 1.