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The Age of the Morrison Formation

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THE AGE OF THE MORRISON FORMATION BART J. KOWALLIS ",*, ERIC H. CHRISTIANSEN ", ALAN L. DEINO b, FRED PETERSONC,CHRISTINE E. TURNER", MICHAEL J. KUNK and JOHN D OBRADOVICH ' " Dcyartnlent of Geology, Briglzunz Young University, Proijo, UT 84602, USA; Berkelej~Ceocl~ronolog~~ Center., 2455 Ridge Road, Berkeley, CA 94709, USA; US Geologiciil Surve.~,Feilercrl Cmter, Denver, CO 80225, USA; US Grological Sclr11ej,,981 Natior~alCenter, Reston, VA 22092, USA IReceii~erlin fi~l~Ifvi.i??4 April 1997) The Morrison Formation in Utah and Colorado contams many volcanic ash layers and ashy beds, now altered rnostly to bentonite, that have yielded isotopic ages. The Brushy Basin Member. at the top of the formation, gives single-crystal. laser-fusion and step-heating, pla- teau 40~r/3'~rages on sanidine that range systematically between 148.1 50.5 (1 std. error of mean) at the top of the nlember to 150.3 rt 0.3 Ma near the bottom. The Tidwell Member, at the base of the Morrison Formation, contaills one ash bed about 3 m above the J-5 unconfor- lnity that occurs in at least two widely separated sections. This ash has been dated by "~r/'~Ardating of saliidi~leand gives ages of 154.75 rt 0.54 Ma (Deino NTM sample, laser- fusion), 154.82 31 0.58 Ma (Demo RAIN sample, laser-fusion), 154.87 !C 0.52 (Kunk NTM sample, plateau). and 154.8 !C 1.4 Ma (Obradovich NTM sample, laser-fusion). The Morrison Formation, therefore, ranges in age fron: about 148 to 155 Ma and, based upon fossll evi- dence appears to be entirely Late Jurassic, including the latest Oxfordian(?). Kimmeridgian, and early Tithonian Ages. Kej.,t'or.rf.~:Morrison Formation; lsotoplc ages; Latest Oxfordian('?); Kimmeridgian; Early Tithonian One of the significant unresolved problems related to the Morrison For- niation is its age. both ~11ronostratigrapl1icallyand biostratigraphically. The for~nationhas been labeled as Jurassic. Cretaceous, and Jurassic- Cretaceous over the years (e.g., Eninions and others, 1896; Darton, 1922; * Corresponding autlior 236 B.J. KOWALI.IS ri tri. Simpsoa, 1926; Stokes, 1944; Imlay, 1952; 1980; Bilbey-Bowman and others; 1986; Hotton, 1986; Kowallis anci others, 1986; Kowallis and Heaton, 1987). In one of the lllost recent papers to address this problem, Kowallis and others (1991) observed that, "Precise ages from the ~~ppermostpart of the Brushy Basin Member are still needed to deteriiiitle if the Jurassic- Cretaceous boundary lies within the uppermost Brushy Basin Meruber." The authors might also have added that very little is known about the age of the lower members of the Morrison Formation. The age data reported in this paper, in addition to those ages reported earlier in Kowallis and others (1991), and in conjuilctio~lwith the biostratigraphic informa- tion reported by Schudack and others (this volume) and Litwin and others (this volume), provide a solid framework for determining the age, both iso- topic and biostratigraphic, of the Morrison Formation. PREVIOUS AGE ASSIGNMENTS Table I lists studies reporting isotopic ages froni the Morrison Formation (most of the previously published ages came fi-om the Brushy Basin Mem- ber), excluding the new ages reported here. These radiometric ages span the Cretaceous-Jurassic bou~idaryaccording to published time scales (e.g., the Jurassic-Cretaceous boundary is given as 130Ma by Kennedy and Odin, 1982; 145.6 Ma by Haslalid and others, 1990; 141.1 Ma by Bralower and others, 1990; and 142Ma by Obradovicli, 1993), and do not resolve longstanding debates concerning the age of the formation (i.e., is the for- mation Jurassic, Cretaceous, or a bit of both?). These previously published isotopic ages have helped to better define the age of the formation, but they all have inlierent problems. The RbbSr age by Lee and Brookiiis (1978) comes fro111 a~~thigenicclay and thus tilust be viewed as a minin~urnage. The K Ar biotite ages are questionable because biotite is aliliost u~iiversally altered to sotile extent in bentonites, and potassium in biotite can be redistributed during alteration of the volcanic ash beds without wffectiag the appearance of the grains. Turner and Fishman (1991) and Christiansen and others (1994) have show11 that potassium was quite mobile during alteration of the Brushy Basin Member ash beds. The fission-track data have 1;rsge errors. include ages on some detrital material, and give several ages that appear to be too young when compared with other methods. The best set of previously published data are the 40~r/"~slases-I'~~si~n plagioclase ages (Kowallis and others. 1991). but plirgioclase has fairly low TABLE I lsotoplc ages from the Morrison For~natioll P Rclc>i'ciic,c, 4go & Ei-1.0,-( i 0) Mnrerinl A4erhod Loculioii & Mernher * 2 Lec and BI-ookiiis. 1978 148+9Ma Clay Rb-Sr New Mexlco (BB) vn Obi-ado\ ich. 19x4 pers. comm. 134i 1.2Ma Biotlte K-Ar Colorado (BB) 3 Bilhrq. 1992 147.2+ 1.0Ma Biotite K-Ar Cleveland-Lloyd Q. (BB) Billx>. 1992 146.8 + 1.OMa Biotite K- Ar Cleveland-Lloyd Q. (BB) 78 Bilbsq. 1992 135.23~5.5 Ma Biotite K-Ar D~nosaurNatl. Moil. (BB) 2 Ko\valli\ and othzrs. 1986 99- 144 + 8 Ma Zircon Fission Track Notom, Utah (BB) KO\\ allis ;\lid Hcaton. 1987 142- 195 + 9 Ma Zii-con Fission Track Notom, Utah (SW) Z Kounllis and Heaton. 1987 157+7Ma Zircon Fission Track Notom. Utah (Ti ";? Ko~allisand Heaton. 1987 145k 13Ma Apatite Fissioii Track Noiom. Utah (BB) 7 Ko\\.allis ~indHeatoil. 1957 132+ IOMa Apatite Fission Track Kotom. Utah (SW) Kowallia and others. 1991 147.6 ?L 0.8 Ma Plagioclase Ar--Ar laser-fusioil Montezuma Creek (BB) F Plagioclase Ar-Ar laser-fusion Moiltezun~aCreek (BBI r! KO\\allis and others. I99 I 147.0 + 0.6 Ma 0 Ko\valIis :\lid ot11e1.c. 199 1 145.2i- 1.2 Ma Plagioclase Ar-Ar laser-fusion Montezuma Creek (BB) 2 I<onaIlis and others. 1991 147.8 * 0.6 Ma Plagioclase Ar-Ar laser-fusioil Montezuina Creek (BB) Ko\~allisand others. 199 1 149.4 & 0.7 Ma Plagioclase Ar--Ar laser-fusion Montezuma Creek (BB) KO~LIIIISiind otllel-s. 1991 152.9 + 1.2 Ma Plagioclase Ar--Ar laser-fusion Dinosaur Nail. Moil. (BB) DeCellr and Burden, 1992 129 f 14 Ma Zircon Fission Track Central W) oming (BB'?) fvlsiilhci-\ .ire RB - Brn\li) Basin. SW = Salt Wash. T= Tlduell 738 B.J. KOWALLIS ei cil levels of potassium, increasing the errors in these ages. In addition, plagio- clase apl.'ears to be more susceptible to alteration in the Morriso~~'ish beds than sanidine. The paleontological data base has also been a source of shifting age as- sign~nentsfor the Morrison Formation. Table 11 lists sigllificant previously published references related to the biostratigraphic age of the Morrison Formation. We have attempted to compile a list of references that is repre- sentative and fairly complete, but we have undoubtedly nlissed some refer- ences because the Morrison literature is quite extensive. These references show that early ia this century the formation was thought to be Jurassic, Cretaceous, or Jurassic-Cretaceous; during the mid-1900s the forination was thought to be definitely Jurassic; whereas most recently the age has again been questioned, with some scientists proposing a Jurassic Cretac- eous age. In addition to the isotopic and paleontological age determinations listed in Tables I and 11, a few papers have been published on the paleomag~letic reversal sequence in the Morrison Formation. Steiner and Helsley (1975a,b), Steiner (1980), and Steiiler and others (1994) observed that the reversal sequence seen in the Morrison Formation correlates "well with the reversal sequence of about the same age [Kimmeridgian], derived from the oldest observed magnetic anomalies in the sea floor." Swierc and Johilson (1990) examined the reversal pattern in the Morrison and Cloverly formations in the Bighorn Basin of Wyoming and inferred a ~naxin~un~of 7 million years of deposition for the Morrison Formation during the Late Jurassic. They also estimated a niinimunl of 15 million years of nondeposi- tion between the Morrison and Cloverly formations. However, the nature and position of the contact between the Morrison and Cloverly forlnations in Wyoming is problenlatic and no reliable isotopic ages have been obtained there. One zircon fission-track age of 129f 14Ma has been reported from central Wyoining by DeCelles and Burden (1992) from sedi- ments they believe to be Cloverly Formation, but which have been pre- viously assigned to the Morrison. Until these probleins are resolved, a chronologic interpretatioil of the Wyoming paleon~agneticdata will be imprecise STRATIGRAPHY The Morrison Formation in the Colorado Plateau consists largely of intcr- bedded conglomerate, sandstone, mudstone, and altered volcanic ash with relatively small amounts of marlstone, limestone, and claystone. The ash TABLE I1 Uiostrat~gi-aphicage information for the Morrison Formation Ref~frrerlce Age Tvpc of E~~i~ii~rrce Emmons and others, Vertebrates s~rn~larto Jurassic forms 1896 elsewhere Marsh, 1896 Jurassic Dmosaurs are like those of European Jurassic Logan, 1900 Jurassic Illvertebrates similar to Jurassic Wealden of Europe Wdrd, 1900 Jurassic Cycadeoid flora is like that of the .Iurassic IZigg\, 1902 Jurass~c Dinosaurs are like thosc of European Jurassic W~lllston,1905 Cretaceous Vertebrates indicate a Cretaceous age Wa~d,1906 Jurassic Cycads like Jurassic forms Lull, I91 I Cret:iceous Fauna like that of Cretaceous Aru~idel Formation, Maryland Berry, 1915 Cretaceous Flora similar to Cretaceous Potomac Group Lee, 1915 Cretaceous Morrison fauna needed more time to evolve than Jurassic Lull. 1915 Cretaceous D~liosaurssilnilar to African (Tendaguru Fm.) Cretaceous forms Osborn, 1915 Cretaceous-Jurassic Kimmeridgian dinosaurs are like Morrison's, but could be Cret.
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  • Paleopathological Analysis of a Sub-Adult Allosaurus Fragilis (MOR

    Paleopathological Analysis of a Sub-Adult Allosaurus Fragilis (MOR

    Paleopathological analysis of a sub-adult Allosaurus fragilis (MOR 693) from the Upper Jurassic Morrison Formation with multiple injuries and infections by Rebecca Rochelle Laws A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Earth Sciences Montana State University © Copyright by Rebecca Rochelle Laws (1996) Abstract: A sub-adult Allosaurus fragilis (Museum of the Rockies specimen number 693 or MOR 693; "Big Al") with nineteen abnormal skeletal elements was discovered in 1991 in the Upper Jurassic Morrison Formation in Big Horn County, Wyoming at what became known as the "Big Al" site. This site is 300 meters northeast of the Howe Quarry, excavated in 1934 by Barnum Brown. The opisthotonic position of the allosaur indicated that rigor mortis occurred before burial. Although the skeleton was found within a fluvially-deposited sandstone, the presence of mud chips in the sandstone matrix and virtual completeness of the skeleton showed that the skeleton was not transported very far, if at all. The specific goals of this study are to: 1) provide a complete description and analysis of the abnormal bones of the sub-adult, male, A. fragilis, 2) develop a better understanding of how the bones of this allosaur reacted to infection and trauma, and 3) contribute to the pathological bone database so that future comparative studies are possible, and the hypothesis that certain abnormalities characterize taxa may be evaluated. The morphology of each of the 19 abnormal bones is described and each disfigurement is classified as to its cause: 5 trauma-induced; 2 infection-induced; 1 trauma- and infection-induced; 4 trauma-induced or aberrant, specific origin unknown; 4 aberrant; and 3 aberrant, specific origin unknown.