new stratigraphic subdivision, depositional environment

Copyright q American Museum of Natural History 2005 ISSN 0003-0082

P U B L I S H E D B Y T H E A M E R I C A N M U S E U M O F N AT U R A L H I S T O RY

CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024

Number 3498, 31 pp., 14 figures, 3 maps, 2 tables November 29, 2005

New Stratigraphic Subdivision, DepositionalEnvironment, and Age Estimate for the Upper

Cretaceous Djadokhta Formation, Southern UlanNur Basin, Mongolia

DEMBERELYIN DASHZEVEG,1 LOWELL DINGUS,2 DAVID B. LOOPE,3

CARL C. SWISHER III,4 TOGTOKH DULAM,5 AND MARK R. SWEENEY6

ABSTRACT

Studies of key and newly discovered sections of the Upper Cretaceous Djadokhta Formationalong the southern margin of the Ulan Nur Basin allow a new subdivision based on lithology.The formation and its members were mapped at both Bayn Dzak, an area that includes theFlaming Cliffs, and Tugrugyin Shireh, an area about 50 km to the northwest of Bayn Dzak.Stratigraphic sections at both localities were remeasured. The considerably enlarged formationcomprises a lower Bayn Dzak Member, dominated by moderate reddish orange sands withsubordinate mudstone units, and an upper Tugrugyin Member, composed of pale orange tolight gray sands. Investigations of key sections at Tsonzh and Alag Teer demonstrate thepresence of transitional mudstone lenses between these members within the Djadokhta For-mation. Two distinct, sandy, sedimentologic facies are recognized in both members. Cross-bedded intervals, occasionally exhibiting wind-ripple cross lamination, document the presence

1 The Geological Institute of the Mongolian Academy of Sciences, Peace Street 63, Ulaanbaatar-51, Mongolia([email protected]).

2 Division of Paleontology, American Museum of Natural History ([email protected]).3 Department of Geosciences, University of Nebraska, Lincoln, Nebraska 68588-0340 ([email protected]).4 Department of Geology, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854

([email protected]).5 Mineral Resources Authority of Mongolia, Mongolian Geological Survey, ‘‘Gurvan Talst’’ Company Ltd., Ulaan-

baatar, Mongolia.6 Desert Research Institute, University of Nevada, 2215 Raggio Parkway, Reno, NV 89512

([email protected]).

2 NO. 3498AMERICAN MUSEUM NOVITATES

of a Cretaceous dunefield in the Ulan Nur Basin. Structureless intervals are interpreted torepresent wet sandy fluvial deposits and debris flows that moved down the dune faces. In theBayn Dzak Member, lenses of brownish mudstone are interpreted to represent interdune de-position in shallow ponds by fluvial action. Fluvial action is also represented in the Bayn DzakMember by beds of caliche, which contain conglomerate at the base but fine upward intolimestone.

The vertebrate fauna from the Djadokhta Formation is summarized. Although the BaynDzak fauna lived somewhat earlier than that from Tugrugyin Shireh based on the superpositionof the members, it is not clear how much earlier. The fauna from the Djadokhta Formationhas previously been assigned ages from Cenomanian to earliest Maastrichtian. New magne-tostratigraphic data document a sequence of normal and reversed magnetozones through theBayn Dzak Member up into the basal Tugrugyin Member. The presence of reversed magne-tozones establishes that the sediments containing the faunas were probably deposited afterC34n. The quick stratigraphic succession of normal and reversed magnetozones suggests, butdoes not clearly establish, that the sediments may have been deposited during the rapid se-quence of polarity changes in the late part of the Campanian between about 75 to 71 Ma.

INTRODUCTION

In 1922–1923 and 1925 the Central Asi-atic Expedition (CAE) of the American Mu-seum of Natural History worked in the UlanNur (5 Ulaan Nuur of Benton, 2000) Basin(Berkey and Morris, 1927; Andrews, 1932;Morris, 1936). Bayn Dzak (5 Bayan Zag ofBenton, 2000) was the main locality to bestudied by the CAE under the name of Sha-barakh Usu (5 Flaming Cliffs). There,Americans found dinosaur eggs and nests,Cretaceous mammals, complete skeletons ofProtoceratops, and three forms of theropoddinosaurs, including Velociraptor, Saurorni-thoides, and Oviraptor (Osborn, 1924). Ber-key and Morris first referred the recognizedDjadokhta Formation to the Lower Creta-ceous and then to the lower part of the UpperCretaceous. Gradzinski et al. (1977) providedthe lists of fossils known at that time fromthe Djadokhta, younger Barun Goyot (5 Ba-ruungoyot of Benton, 2000), and NemegtFormations, and estimated the age of theDjadokhta Formation as ?upper Santonianand/or ?lower Campanian.

In 1946 the Bayn Dzak locality was rein-vestigated by the Mongolian PaleontologicalExpedition (MPE) of the Academy of Scienc-es of the USSR, which was headed by Efre-mov (1949, 1954). The investigations yieldedless significant results, including a completepostcranial skeleton of the armored dinosaurPinacosaurus grangeri Gilmore (5 Syr-mosaurus viminicaudus) (Maleev 1952,1954). Additional results regarding this spec-

imen were discussed by Maryanska (1971), ina paper not associated with the MPE.

In 1963–1971 the Polish–Mongolian Pa-leontological Expedition (PMPE) searchedfor Cretaceous mammals and dinosaurs atBayn Dzak. The main result was the discov-ery of abundant fossils of multituberculates,eutherian and deltatheroidan mammals in thelocality’s concretionary sandstones (Kielan-Jaworowska, 1969, 1970; Kielan-Jaworow-ska and Dovchin, 1969; Kielan-Jaworowskaand Barsbold, 1972), as well as numerouslizards (see Alifanov, 2000, for review), anda peculiar long-necked crocodile (Osmolska,1972).

In 1971 a spectacular specimen termed the‘‘fighting dinosaurs’’ was found by PMPE atTugrugyin Shireh (5 Togrogiin Shiree ofBenton, 2000). These articulated and asso-ciated skeletons of Velociraptor and Proto-ceratops are discussed by Kielan-Jaworow-ska and Barsbold (1972) and Osmolska(1993). Another interesting finding of thePMPE at Tugrugyin Shireh was skeletons ofProtoceratops andrewsi preserved in a ver-tical position (Jerzykiewicz et al., 1993).

In 1969–1970 the joint Soviet–MongolianPaleontological Expedition (SMPE) foundskeletons of Protoceratops andrewsi andother fossils at Tugrugyin Shireh about 50km northwest of Bayn Dzak. Other results ofboth the SMPE and its successor, the Rus-sian–Mongolian Expeditions are reviewed byKurochkin and Barsbold (2000).

From 1992 to the present, the Japan–Mon-golia Joint Paleontological Expedition

2005 3DASHZEVEG ET AL.: STRATIGRAPHY OF DJADOKHTA FORMATION

(JMJPE) has conducted extensive field sea-sons throughout the Gobi Desert. A recentaccount of their activities at both Bayn Dzakand Tugrugyin Shireh is presented by Ishii etal. (2000, 2004).

In 1991 the Mongolian Academy of Sci-ence–American Museum of Natural HistoryExpedition (MAE) began field investigationsbased on the Agreement on Scientific Co-operation between those two institutions. In1991–1992, 1994–1996, and 1998 the MAEvisited Bayn Dzak and Tugrugyin Shireh.Significant remains of multituberculates andplacental mammals were collected. Manyspecimens in these collections are now beingstudied. The MAE investigated all previouslystudied reference sections and describedsome new key sections from the DjadokhtaFormation, allowing a stratigraphic reassess-ment of this formation throughout the region.Major lithostratigraphic units and sedimen-tary facies were mapped, and samples for pa-leomagnetic analysis were collected fromdifferent sites and stratigraphic levels. It isnoteworthy that despite extensive descrip-tions of fossil faunas, no regional stratigraph-ic scheme for correlating the sections andfaunas of these Upper Cretaceous deposits inthe Djadokhta Formation has been previous-ly proposed. Investigations of the MAE re-sulted in a refined subdivision of these de-posits in the Ulan Nur Basin that allow thestratigraphic relationship between the depos-its at Bayn Dzak and Tugrugyin Shireh to beclarified.

SUBDIVISION OF THE UPPERCRETACEOUS DJADOKHTA

FORMATION ALONG THE SOUTHERNBORDER OF THE ULAN NUR BASIN,

MONGOLIA

The Djadokhta Formation is representedby moderate reddish orange and pale orangeto light gray, medium- to fine-grained sandsand sandstones containing subordinate layersof calcareous concretions and orangishbrown silty clay. Total exposed thickness ofthe formation in the Ulan Nur Basin, a post-Oligocene tectonic depression (Gradzinski etal., 1977) is at least 80 m. Highly fossilif-erous, the formation can be divided into twomembers and correlated to analogous depos-

its in adjacent regions. The Djadokhta For-mation was initially considered to consistonly of reddish orange sandstones at BaynDzak. This understanding of its stratigraphicextent and lithologic composition held untilrecently. Discussions of earlier geologicstudies in the region include Gradzinski et al.(1969, 1977); Kielan-Jaworowska and Dov-chin (1969); Gradzinski (1970); Gradzinskiand Jerzykiewicz (1972); Kielan-Jaworows-ka and Barsbold (1972); Gradzinski and Jer-zykiewicz (1974); Osmolska (1980); Jerzyk-iewicz and Russell (1991); Jerzykiewicz etal. (1993); Kielan-Jaworowska et al. (2003);Benton et al. (2000) and chapters therein, in-cluding Benton (2000), Kurochkin and Bars-bold (2000), Shuvalov (2000), and Jerzyk-iewicz (2000). Based on new stratigraphicand paleontologic data, however, the UpperCretaceous Djadokhta Formation of the UlanNur Basin can now be subdivided into a low-er Bayn Dzak Member and an upper Turgru-gyin Member.

THE BAYN DZAK MEMBER

The Bayn Dzak Member forms a range ofmoderate reddish orange sandstones andmoderate reddish brown mudstones, whichoccur only along the southern edge of thebasin. The most complete and fossiliferousexposures of the member are exposed atBayn Dzak, including the famous outcrops ofthe Flaming Cliffs (fig. 1; Berkey and Mor-ris, 1927; Andrews, 1932; Gradzinski et al.,1969, 1977; Lefeld, 1971; Novacek et al.,1994). The type section was recorded at theFlaming Cliffs (Gradzinski et al., 1969: fig.30). This section consists of strongly dis-sected scarps composed predominantly ofmoderate reddish orange sandstone. The cen-tral scarps form a large remnant named theFlaming Cliffs by the CAE, in which thecombined strata rise more than 30 m abovethe floor of the local basin. The Bayn DzakMember as exposed in the Flaming Cliffssection contains the following lithologicunits (see fig. 2):

1. Well-sorted, unbedded, medium-grainedsand of moderate reddish orange color (Ks).This unit varies between 2 m and 14 m inthickness along the main escarpment andcontains the greatest number of dinosaur fos-


Fig. 1. View looking west at the eastern face of the main escarpment of the Flaming Cliffs. Theperson in the left-center foreground is walking just above the prominent Kcal Unit 4 in the FlamingCliffs section (fig. 2), which overlies the prominent cliff-forming sands of Unit 3.

sils. Skeletons and skeletal fragments of Pro-toceratops andrewsi, Pinacosaurus grangeri,Velociraptor mongoliensis, Oviraptor philo-ceratops, and Saurornithoides mongoliensiswere collected from this unit. Our investi-gation was unable to distinguish two, litho-logically distinct, structureless sandstoneunits bearing calcareous nodules at the baseof the Flaming Cliffs section. Thus, Unit 1of this study combines Beds 1 and 2 of Grad-zinski et al. (1969).

2. Well-cemented, reddish brown sand-stones sometimes exhibiting large-scalecrossbeds (Kcon), 5–7 m thick (fig. 3). Fieldinvestigations of the MAE in 1994–1995documented that concretions in these depos-its contain abundant remains of mammals,lizards, and other vertebrates. In 1964 thePMPE found a multituberculate cranium insitu (Gradzinski et al., 1969, 1977). The re-mains of Cretaceous mammals and othersmall vertebrates are often derived fromthese sandstones at the base of the FlamingCliffs scarp. Thus, Unit 2 of this study,which contains mammalian remains, is lo-cated above the dinosaur-bearing Unit 1 at

the base of the Flaming Cliffs sequence, andUnit 2 of this study is equivalent to Bed 3of Gradzinski et al. (1969).

3. Unbedded, moderate reddish orangesands with irregular sandstone concretionsand calcareous nodules (Ks). This unit com-prises about 12 m of structureless, cliff-form-ing, generally poorly consolidated sandstone(fig. 4). It is equivalent to Unit 4 of Grad-zinski et al. (1969).

4. Grayish white conglomerates with smallrounded calcareous nodules and pebbles ofcrystalline rock at the base (fig. 5), finingupward into calcareous caliche at the top ofthe unit (Kcal). Unit 4 of this study equalsBed 5 of Gradzinski et al., 1969).

5. Unconsolidated moderate reddish or-ange sands with small sandy irregular con-cretions (5 Bed 6 of Gradzinski et al., 1969).This unit was mapped as part of the Kcalcomplex on the Facies Map (map 2, pp.12–13).

6. Light gray calcareous caliche conglom-erate with caliche pebbles at base (fig. 5) (5Bed 7 of Gradzinski et al., 1969). This unit


Fig. 2. Stratigraphic section at the Flaming Cliffs proper. Unit designations for this paper are at thefar left. Unit designations for Gradzinski et al. (1969) are to the immediate right. Colors are based onthe GSA color chart. The position of the paleomagnetic samples is to the right of the section. Thepolarity of the samples is to the right of the sample number. The polarity zonation is at the far right.


Fig. 3. Resistant, reddish brown, concretionary sandstone of Unit 2 (Kcon) exposed to the east andnear the base of the main escarpment of the Flaming Cliffs.

is mapped as part of the Kcal complex onmap 2, pp. 12–13.

7. Moderate reddish orange, structurelesssandstone with occasional irregular, roundedconcretions (5 Bed 8 of Gradzinski et al.,1969). Thickness is 4 m. This unit is mappedas part of the Kcal complex on map 2, pp.12–13.

8. Moderate reddish orange sands withrounded sandstone concretions (5 Bed 9 ofGradzinski et al., 1969). This unit is mappedas part of the Kcal complex on map 2, pp.12–13.

9. Moderate reddish orange cementedsands and sandstone (5 Bed 10 of Gradzin-ski et al., 1969). In 1994 the MAE collecteda skull of the lizard Carusia intermedia alsoknown from the Cretaceous deposits ofKhermin Tsav and Ukhaa Tolgod from thisunit, as well as Chelonia remains from thislocality. This unit is mapped as part of theKcal complex on map 2, pp. 12–13.

The upper part of the Bayn Dzak Memberat the Flaming Cliffs has been eroded. Qua-ternary pediment sits atop the described sec-tion. The PMPE studied and described the

uppermost bed of the moderate reddishbrown sequence of the Bayn Dzak Memberlocated to the northwest of the Flaming Cliffs(Lefeld, 1971). See the subsequent descrip-tion of Khren Tsav for details.

GEOLOGIC MAP OF MAJORLITHOSTRATIGRAPHIC UNITS AT

BAYN DZAK

Exposures of the Upper Cretaceous Dja-dokhta Formation are intermixed with thoseof Quaternary sediments across the northerntwo-thirds of the mapped region on map 1.The Djadokhta Formation represents the old-est rocks exposed in the mapped area. TheFlaming Cliffs exposures of the DjadokhtaFormation stand out in the northern quarterof the map as a southeast to northwest trend-ing arc. To the south of the southeastern endof the Flaming Cliffs exposures, the Dja-dokhta Formation crops out at lower eleva-tions near the bottom of the major drainagecalled Khatato Gol.

Overlying the Djadokhta Formation aresediments of the Paleocene Hashaat suite.


Fig. 4. Structureless, cliff-forming sandstone (Unit 3) exposed near the east end of the main es-carpment of the Flaming Cliffs. Unit 4 can be seen on top of Unit 3.

These exposures rim those of the DjadokhtaFormation in the central region of themapped area on either side of the KhatatoGol.

Overlying the Hashaat suite are Eocenesediments. These are primarily exposed inthe southernmost part of the mapped area,rimming the exposures of the Hashaat suite.

The youngest rocks in the mapped areaconsist of Quaternary alluvium, colluvium,and eolian sands. These are distributedthroughout the mapped area.

FACIES MAP OF BAYN DZAKMEMBER AT BAYN DZAK WITH

ENVIRONMENTAL INTERPRETATIONS

The Bayn Dzak Member at Bayn Dzak iscomposed of four different lithologic facies,shown on map 2, pp. 12–13. Very similarfacies were recognized by Jerzykiewicz et al.(1993) and Eberth (1993) in exposures of theDjadokhta Formation at Bayan Mandahu in

northern China. Topographic and distancereferences are based on the topographic mapfor the Bayn Dzak area constructed by L. B.Roberts, F. B. Butler, and H. O. Robinson in1925, which was printed in Gradzinski et al.(1969: fig. 29). The four lithologic facies in-clude the following:

1. Structureless sands, usually poorly con-solidated and commonly containing irregularcalcareous nodules, dominate Units 1 and 3of this study. In situ dinosaur nests and skel-etal remains are common in Unit 1. Calcar-eous nodules occurring as float on this unitoccasionally contain fossils of mammals andlizards. Trace fossils, often mud filled, arecommon in unit 3. The fossils are identifiedas Ophiomorpha by Loope and Dingus(1999). These structureless sands are inter-preted to represent fluvial deposits and aremapped as Ks. Other units of structurelesssand are similar to the structureless sands de-scribed from Ukhaa Tolgod by Loope et al.


Fig. 5. View looking east at the western face of the main escarpment of the Flaming Cliffs, showingthe two beds of light gray conglomerate (Units 4 and 6) near the top of the exposures.

(1998). There, the structureless units containpebbles and cobbles floating in the sandymatrix that were much too large and heavyto be carried by the wind. This and the fieldrelations of the structureless sand bodies,which were shingled in between cross-bed-ded sand bodies typical of eolian dunes, pro-vided evidence that the structureless sandswere deposited as sandslides at the base ofthe dunes (Dingus et al., in review). This in-terpretation differs from that of Eberth(1993), who interpreted structureless sand-stone sheets at Bayan Mandahu to representvertically agrading, eolian sands that accu-mulated during very high energy windstorms, possibly in zibars.

Structureless sands that comprise Units 5,7, 8, and 9 of the present study are moredifficult to interpret environmentally. Theyare interbedded with caliche pebble con-glomerates (see facies 3 below) that clearlyrepresent a fluvial environment. At least oneof the structureless sandstone units (#9) con-tains fossils of lizards and turtles. These

structureless sandstone units probably alsorepresent fluvial deposits.

2. Concretionary sheets of sandstoneforming large-scale crossbeds dominate Unit2 of this study. These sheets consistently dipwith an azimuth to the northeast at 208–308.Rare chunks of these concretionary layerscontain in situ fossils of mammals. Morecommonly, nodules occuring as float onthese outcrops contain fossils of mammalsand lizards. Outcrops where these concre-tionary sheets dip with an azimuth to thenortheast are interpreted to represent eoliancrossbeds of Cretaceous dune deposits.These units are mapped as Kcon. Similarconcretionary calcite underlying now burieddune lee slopes has recently been describedfrom Quaternary eolian deposits in westernChina by Yang et al. (2003). Rhizoliths (cal-cite-replaced plant roots) are common withinthis facies. A similar facies, lacking rhizo-liths, is present at Ukhaa Tolgod (E-2 faciesof Dingus et al., in review).

3. Relatively thin beds of white calcareous


caliche containing basal pebbles of calicheand subordinate clasts of crystalline rockdominate Units 4 and 6 of this study. Theseunits are unfossiliferous. They are interpretedto represent fining upward beds deposited byfluvial action. They are mapped as Kcal, andone can be traced across the Flaming Cliffsexposures for more than 1 km.

4. Beds of siltstone and mudstone, mappedas Km on map 2, occur as lenses interbeddedwith the structureless sands of facies Ks.These units are not laterally continuous formore than 100 m, and they are not fossilif-erous. They are interpreted to represent fine-grained interdune units deposited in shallowponds and depressions by fluvial action.

These facies can be mapped across theBayn Dzak region, as was done using airphotos taken in August of 1996. The faciesare exposed on and to the north of a 7-km-long scarp that separates a plateau capped byQuaternary pediment to the south and a basinfilled with Quaternary colluvium, alluvium,and eolian dune deposits to the north. Theeastern leg of the scarp trends nearly east towest, beginning just west of the Khatato Goland extending to the main exposures of theFlaming Cliffs. The western leg of the scarptrends more northwest and extends past theRuins to the Volcano.

Most of the eastern leg of the scarp isformed by Ks. Isolated outcrops of Kconprotrude out of the Quaternary colluviumthat forms the floor of the basin to the northof the scarp. Many of these outcrops areformed by concretionary sheets representingUnit 2. Fourteen attitudes were measured onthese sheets. The azimuths of dip directionson these sheets interpreted as large-scale eo-lian crossbeds range from N 258E to N 858E.Near the top of the scarp, a prominent bedof Kcal can be traced along the western quar-ter of the eastern leg. It sits immediately ontop of Unit 3, which represents the structure-less sands that comprise most of the expo-sures that form the Flaming Cliffs proper.

The Flaming Cliffs themselves, capped byQp, jut northwest out from the plateau intothe basin to the north. A small northeast–southwest trending fault cuts across thesouthern end of the Flaming Cliffs. Offset isbetween 1 m and 2 m, with the northwestblock containing the Cliffs being down-

dropped in relation to the southeast blockthat forms the main plateau capped by Qp.Another small fault trending nearly east towest cuts across the northern end of theCliffs. A remnant of the Cliffs exposures sitsto the north of this fault. The base of theremnant is formed by Kcon of Unit 2, butthe higher parts of the remnant are cappedby Ks of Unit 3. The remnant appears to rep-resent a block of the Flaming Cliffs exposurethat has dropped 3–5 m along the fault inrelation to the main units that form the extantCliffs. The prominent Unit 4 of Kcal can betraced completely around the top of the mainCliffs exposures immediately on top of theprominent cliff-forming Ks of Unit 3. TheKcal of Unit 4 is extremely well cementedand resistant. It clearly protects the poorlyconsolidated sands of Unit 3 from more ac-celerated erosion and allowed the distinctivegeomorphologic relief of the Flaming Cliffsto develop.

At the base of the northwest corner of theFlaming Cliffs lies a distinctive set of con-cretionary sandstone sheets composing Kconof Unit 2. Ten attitudes recorded on thesesheets reveal dip directions with azimuthsranging from S 68E to N 108E, with mostattitudes between N 258E and N 508E. Again,these sheets are thought to represent large-scale eolian crossbeds.

A prominent northwest–southeast trendingdrainage lies immediately west of the mainexposures of the Flaming Cliffs (fig. 5). Inthe exposures along the southwest side of thedrainage, the prominent Kcal of Unit 4 canbe traced for about 600 m to the northwestwhere it pinches out. In the southernmost ex-posures of the scarp along the southwest sideof the drainage, isolated outcrops of Kconappear to be shingled in between outcrops ofKs. Rhizoliths oriented parallel to the cross-bedding are prominently preserved on thetops of the sheets (Loope and Dingus, 1999).This sequence represents one of the clearestexamples of the field relations between theeolian crossbedded sheets of Kcon and thestructureless sandslides of Ks exposed in theBayn Dzak region. Six attitudes were record-ed on the concretionary sheets in this area.The azimuths of dip directions range from N208E to N 358E.

Just northwest of these exposures a small

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Map 1. Geologic map of major lithologic units at Bayn Dzak. Compiled by T. Dulam in July 1998.1. Holocene eolian sands vQiv; deluvid sands dpQiv; lake clay and sand lQiv; proluvial sand and debris pQiv.2. Pleistocene deluvial-prodeluvial sands and debris, partly with limy matrix.3. Eocene; small grained shingles composed of less rounded and less sorted sands, and thin layers of white limy

marl (1–2 cm).4. Paleocene; Hashaat Suite: coarse-grained sands of brownish yellow, red, and greenish colors with thin layers of

shingled red and reddish brown clay, and greenish gray aleurite, marl.5. Upper Cretaceous; Bayn Dzak Member: K0/2 lightly colored sandstone, sand, K1/2 reddish, weakly lithified,

small-medium-grained sand. In the lower part of the section, horizontally oriented sandstone nodules are common. K2/2 non-lithified red sands, rich with mud craters, footprints, and fewer sandstone nodules.

6. Reference horizons. a: white horizon with hard calcareous sandstone nodules. b: white conglomerates and layersof marl with reworked marl nodules. c: brecciated marl with lithic fragments.

7. Faults: a: known; b: inferred.8. Geologic boundaries: a: clear; b: unclear.9. Points of observation of fossil bone.10. Orientation of bedrock: a: inclined; b: horizontal.The entire undivided map may be printed from the AMNH library catalog at http://library.amnh.org/pubs.


Map 2. Facies map of Bayn Dzak Member at Bayn Dzak. The map has been divided into twohalves. The left half (this page) represents the northwest portion of the study area, which extends fromthe Volcano to the Ruins.


Map 2. Continued. The right half (this page) represents the southeast portion, which includes theMain Escarpment of the Flaming Cliffs. The line of section for fig. 2 is plotted just northwest of 44 0827 N, 103 44 06 E. The entire undivided map may be printed from the AMNH library catalog at http://library.amnh.org/pubs. The map was compiled by L. Dingus, D. Loope, and M. Sweeney in July 1998.


Fig. 6. View looking east from the western rim of the basin at Bayn Dzak at exposures of the Ruins.

northeast–southwest trending drainage isflanked on the northwest by a large modernsand dune (Qd). Our camp usually occupiesthe point on the Qp-capped plateau justnorthwest of this dune.

From our camp, the scarp trends morenorthwest toward the Ruins and the Volcano.About 750 m northwest of camp, three dis-tinctive mudstone units are exposed in thelower portion of the scarp. They crop out aslaterally discontinuous lenses and representsmall interdune deposits interbedded withstructureless sands of Unit 3 that lie abovethe concretionary sheets of Unit 2.

Two kilometers northwest of the FlamingCliffs lies the prominent scarp remnantnamed the Ruins by the PMPE (fig. 6), whichprovided skeletons of the long-necked croc-odile, Gobiosuchus kielanae (Osmolska,1972). An exposure of wind-rippled, cross-laminated sand occurs on the northern sideof the remnant. Concretionary sheets ofsandstone, presumably representing Unit 2,lie at the base of the remnant. The Ruins are

capped by structureless sands (Ks) contain-ing occasional dinosaur skeletons and abun-dant trace fossils. Presumably these sandsrepresent Unit 3.

About 700 m northwest of the Ruins liethe exposures of Khren Tsav (fig. 7). Thesection exposed here (fig. 8) is quite signif-icant. The basal 10 m are composed of mod-erate reddish orange, structureless sandstone(Ks), probably representing Unit 3 exposedat the Flaming cliffs. Vertebrate fossils, in-cluding a skull and skeletal fragments of Za-lambdalestes, were collected in situ. Tracefossils of Ophiomorpha are also abundant. Inthe main hill at the site, the structurelesssands, probably representing fluvial deposits,are immediately overlain by 5 m of orangishbrown mudstone (Km), representing fluvialpond deposition. The sequence is capped by2–3 m of moderate reddish orange structure-less sand. However, just west of the mainhill, at the stratigraphic level of the mud-stone, concretionary sheets of sandstone(Kcon) crop out. Four attitudes representing


Fig. 7. View looking west at the exposures of Khren Tsav showing the prominent unit of brownmudstone interpreted to represent interdune deposition by fluvial processes in ephemeral lakes or ponds.

dip directions on these sheets range from az-imuths of N 188E to N 308E. The contactbetween these units is either eroded or notexposed. However, three environmental fa-cies appear to be represented in close lateralproximity: eolian dune deposits representedby the crossbedded concretionary sheets, flu-vial sands represented by the structurelessand fossiliferous sandstone, and interdunefluvial pond deposits represented by themudstone.

Almost 2 km further northwest of the Ru-ins lies the locality named the Volcano bythe PMPE. Most of the conical remnant ofthe scarp is covered by colluvium; however,a few outcrops are exposed. A couple ofpatches of structureless concretionary sand-stone lie at the base. Nodules in these mas-sive concretions contain fossils of lizards,crocodiles, dinosaurs, and mammals. Abovenear the peak, a small patch of concretionarysandstone is overlain by a small outcrop ofstructureless sandstone containing abundantrhizoliths—among the best examples in theBayn Dzak area. It is unclear how these out-crops correlate with exposures to the south-east because of extensive intervening cov-ered areas.

STRATIGRAPHIC POSITION OFMAMMALIAN FOSSILS IN THE BAYN

DZAK MEMBER

In 1924 and 1925 the CAE discoveredCretaceous fossils of mammals and small di-nosaurs at the Flaming Cliffs. However, nei-ther the geographic position of the mamma-lian locality nor the stratigraphic position ofthe fossiliferous units in the Flaming Cliffssection were precisely reported (Efremov,1954). The fossils were merely reported tohave been found in small concretions. Themammalian fossils described from the for-mation are the following.

During fieldwork by the MPE, Novojilov(1954) studied the stratigraphic position ofmammalian fossils from Bayn Dzak. He sug-gested that the Cretaceous mammalian faunawas derived from a concretionary bed in theformation below the Paleocene fossiliferousexposures to the southeast discovered by theCAE called Hashaat.

Geological investigations at Bayn Dzakwere pursued in greater detail by the PMPE(Gradzinski et al., 1969, 1977; Lefeld, 1965,1971). The PMPE confirmed the occurrencesof Cretaceous deposits of the Djadokhta For-mation. Fossils were reported from three lo-


TABLE 1The Bayn Dzak Fauna

ReptiliaChelonia indet. Gilmore, 1931

Squamata

IguaniaIsodontosaurus gracilis Gilmore, 1943Mimeosaurus crassus Gilmore, 1943Priscagama gobiensis Borsuk-Bialynicka and

Moody, 1984

Scincomopha

Teiidae

Macrocephalosaurus ferrugenous Gilmore, 1943Adamisaurus magnidentatus Sulimski, 1972

Incertae sedis

Conicodontosaurus djadochtaensis Gilmore, 1943Globaura venusta Borsuk-Bialynicka, 1988Slavoia darevskyi Sulimski, 1984

Anguimorpha

Telmasaurus grangeri Gilmore, 1943Carusia intermedia Borsuk-Bialynicka, 1985Bainguis parvus Borsuk-Bialynicka, 1984Estesia mongoliensis Norell et al., 1992

Gekkota

Gekkonidae

Gobekko cretacicus Borsuk-Bialynicka, 1990

Archosauria

Crocodyliformes

Giobiosuchidae

Gobiosuchus kielanae Osmolska, 1972

Paralligatoridae

Shamosuchus djadochtaensis Mook, 1924

Dinosauria

Saurischia

Theropoda

Dromaeosauridae

Velociraptor mongoliensis Osborn, 1924

Troodontidae

Saurornithoides mongoliensis Osborn, 1924

Oviraptoridae

Oviraptor mongoliensis Osborn, 1924

Avialae

Mononykinae indet.

TABLE 1(Continued)

Ornithischia

Ankylosauria

Ankylosauridae

Pinacosaurus grangeri Gilmore, 1933Neoceratopsia

Protoceratopsidae

Protoceratops andrewsi Granger andGregory, 1923

Bainoceratops efremovi Terschenko andAlifanov, 2003

Mammalia

Multituberculata

Djadochtatherioidea

Sloanbaataridae

Sloanbaatar mariabilis Kielan-Jaworowska,1970

Djadochtatheriidae

Djadochtatherium matthewi, Simpson, 1925Kryptobaatar dashzevegi Kielan-Jaworowska,

1970Catopsbaatar catopsaloides Kielan-

Jaworowska, 1974

Incertae sedis

Kamptobaatar kuczynskii Kielan-Jaworowska,1970

Bulganbaatar memegtbaataroides Kielan-Jaworowska, 1974

Eutheria incertae sedis

Asioryctitheria

Asioryctidae

Kennalestes gobiensis Kielan-Jaworowska,1969

Zalambdalestidae

Zamlambdalestes lechei Gregory and Simpson,1926

Metatheria

Deltatheroidea

Deltatheriidae

Deltatheroides cretacicus Gregory andSimpson, 1926

Deltatheridium praetribuberculate Gregory andSimpson, 1926

Theria incertae sedis

Hyotherium dobsoni Gregory and Simpson, 1926


calities: (1) the Main Mammal Field, alongthe eastern margin at the base of the FlamingCliffs; (2) the Ruins, about 2400 m to thenorthwest of the Flaming Cliffs; and (3) theVolcano, about 2 km to the north of the Ru-ins, where the lower Kcon nodules producedremains of mammals and other vertebrates.According to Polish accounts (Kielan-Jawo-rowska, 1969, 1970; Gradzinski et al., 1977)these three localities yielded numerous skullsand jaws of mammals as well as abundantfossil remains of lizards, crocodiles, and oth-er vertebrates. Several mammal specimenshave been found in situ in all three abovementioned localities. So, restriction of themammalian fossils to the Upper CretaceousDjadokhta Formation was definitely estab-lished.

In 1991–1992, 1994–1996, and 1998 theBayn Dzak locality was reinvestigated by theMAE, which made collections of the Creta-ceous remains from the three PMPE locali-ties (above), as well as from two new local-ities in the Bayn Dzak region. (1) A craniumand associated skeletal fragments of Zalamb-dalestes were collected from the basal, mod-erate reddish orange sands of the Khren Tsavsection. (2) The cranium and jaws of placen-tal mammals were found in the moderate red-dish orange sandstones of Unit 5 along thescarp near the Flaming Cliffs. In 1971, amultituberculate cranium was also collectedthere by Dashzeveg.

BIOCHRONOLOGICINTERPRETATIONS FOR THE BAYN

DZAK FAUNA

The known vertebrate fauna from theBayn Dzak Member is listed in table 1. Rel-atively recent studies discussing the lizardfaunas from this area include Sulimski(1975), Borsuk-Bialynicka (1984, 1991a,1991b), Gao and Norell (2000), and Alifanov(2000). Relatively recent references for thearea’s archosaurian fauna include Maryanska(1971, 1977), Osmolska (1972, 1976), Mar-yanska and Osmolska (1975), Norell et al.(1994), Chiappe et al. (1998), and Benton etal. (2000). Relatively recent papers concern-ing the mammalian fauna for the area includeKielan-Jaworowska (1970, 1975, 1984), Kie-lan-Jaworowska and Hurum (1997, 2001),

Kielan-Jaworowska et al. (2000), Novacek etal. (1997), Rougier et al. (1977, 1996, 1998),Smith et al. (2001), and Wible and Rougier(2000). Recent studies focusing on dinosaurand other fossil eggs from the area includeMikhailov (1994, 1995, 1996), Mikhailov etal. (1994), Sabath (1991), and Norell et al.(1994).

There are no radioisotopic dates frombeds in the sequence that produce the BaynDzak fauna. According to different authors,its age has been reported to range from Cen-omanian to Early Maastrichtian. Berkey andMorris (1927) placed it in the early LateCretaceous. Polish authors variously as-signed it to a range between the Coniacianand the Santonian (Gradzinski et al., 1969;Kielan-Jaworowska, 1969, 1970, 1974; Kie-lan Jaworowska and Dovchin, 1969; Lefeld,1965, 1971). McKenna (1969) suggested aCenomanian age. Rozhdestvensky, in hisstudy of Mongolian dinosaurs (1971), in-ferred the probable age of the fauna to belate Santonian or possibly early Campanian.Fox (1978) suggested a Campanian agebased on comparisons between Mongolian,American, and Canadian dinosaurs from theLate Cretaceous. Lillegraven and McKenna(1986) considered the Bayn Dzak fauna tobe of Campanian-Early Maastrichtian age.Jerzykiewicz et al. (1993) and Eberth (1993)assign a Campanian age to the Djadokhtafaunas from both Bayan Mandahu and BaynDzak. Most recently, Averianov (1997) ten-tatively agreed with Gradzinski et al. (1977)that the fauna from the Djadokhta Forma-tion is early Campanian in age, based oncomparisons of mammalian and marine taxafrom the Darbasa Formation of Kazakhstan.So, although most recent workers have as-signed a Campanian age to the fauna fromthe Djadokhta, there has historically beenlittle consensus among biochronologic ageestimates and no evidence independent offaunal comparisons.

MAGNETOSTRATIGRAPHY OF THEBAYN DZAK MEMBER

Magnetostratigraphic samples were col-lected for analysis from both the FlamingCliffs and Khren Tsav sections (figs. 2, 8). Amore detailed account of all the magneto-


stratigraphic data for localities discussed inthis paper is being prepared for a separatepublication. The results are as follows.

At the Flaming Cliffs, Units 1 and 2 at thebase of the section described here occur in azone of normal polarity. Above, Unit 3,which comprises the principal cliff-formingsandstone, is reversed. The polarities ofUnits 4 and 5 are presently undetermined.Units 6 and 7 are of normal polarity, and thetop of the section represented by Unit 8 isreversed.

At Khren Tsav (fig. 8) two samples in thelower structureless sandstone are reversed,suggesting but not establishing a correlationto Unit 3 of the Flaming Cliffs section. Onesample in the overlying mudstone is also re-versed (M. C. McKenna, personal commun.).One sample from the uppermost sandstoneunit is also questionably reversed.

MAGNETOSTRATIGRAPHICIMPLICATIONS FOR THE AGE OF THE

BAYN DZAK FAUNA

The presence of reversed magnetozones inboth the Flaming Cliffs and Khren Tsav sec-tions means that most of these units must beyounger than Chron 34n—the upper bound-ary of which represents the boundary be-tween the Santonian and the Campanian atabout 83.5 Ma (Gradstein et al., 1995). Thequick stratigraphic succession of normal andreversed magnetozones in the Flaming Cliffssection suggests that the sediments were de-posited during the rapid sequence of polaritychanges in the late part of the Campanianincorporating the end of Chron 33 and Chron32 between about 75 and 71 Ma (Gradsteinet al., 1995). However, this tentative corre-lation to the Geomagnetic Polarity Timescalecannot yet be certainly established.

TRANSITIONAL DEPOSITS BETWEENTHE BAYN DZAK AND TUGRUGYIN

MEMBERS

Investigations by international paleonto-logic expeditions, which were conductedduring the past 40 years, have greatly en-hanced our knowledge of Cretaceous verte-brates from the Bayn Dzak and TurgrugyinShireh localities. It was discovered that, al-though color differences characterize the

Bayn Dzak and Tugrugyin Members, thesetwo localities produce many of the same di-nosaur and mammal taxa, including Proto-ceratops andrewsi, Saurornithoides mongo-liensis, Oviraptor philoceratops, Kryptobaa-tar dashzevegi, and Zalambdalestes leichei.The principal difficulty in establishing thegeochronological succession between theBayn Dzak and Tugrugyin deposits was thelack of sections containing transitional bedsthat document their superpositional relation-ships. In 1993–1996, the MAE investigatedtwo such key sections.

The Tsonzh section. Dashzeveg discoveredthis section in 1984, about 10 km to thesoutheast of the Flaming Cliffs. The lowscarps at Tsonzh face north and pass upwardinto an inclined plateau that extends to thesouth (fig. 9A). Both the Bayn Dzak andTugrugyin Members are exposed at Tsonzh,along with interbedded transitional lenses ofmudstone (fig. 10). The lower Bayn DzakMember is represented by well-sorted, mod-erate reddish orange sands. A tooth of Pro-toceratops andrewsi was found by Dashzev-eg near the middle of this unit. Transitionalbeds are represented by lenses of compactbrown mudstone, which rests in some placeson the eroded underlying Bayn Dzak Mem-ber. The overlying Tugrugyin Member is rep-resented primarily by yellowish gray to verylight gray sands and sandstones (fig. 9). Thetransitional clay lenses are wedged into thesands of the Tugrugyin Member in someplaces. In 1993, the MAE collected scatteredremains of a large theropod in the TugrugyinMember. A paleomagnetic sample takenfrom the transitional reddish brown clay isreversed in polarity (M. C. McKenna, per-sonal commun.)

The Alag Teer section. Dashzeveg discov-ered the Alag Teer (5 Alag Teeg of Benton,2000) section in 1964 (fig. 11). It lies 3.5 kmfrom the main eastern scarp of TugrugyinShireh. The succession of beds is recorded infigure 12, and includes moderate reddishbrown sandy siltstone at the base, overlainby interbedded moderate reddish orangesandstone and very pale orange sandstone.Above these lies yellowish gray sandy silt-stone and interbedded moderated reddish or-ange sandstone and very pale orange sand-


Fig. 8. Stratigraphic section at Khren Tsav. Paleomagnetic samples are listed to the right of thesection. The lithologic key is on the right. The polarity of samples is listed to the right of the samplenumbers.

stone. Very pale orange sandstone caps thesection.

Like Tsonzh, this section is noteworthy fordemonstrating the superpositional relation-ship between moderate reddish orange sand-stones and moderate reddish brown mud-

stone typical of the Bayn Dzak Member inthe lower portion of the section and very paleorange sandstones typical of the TugrugyinMember in the higher portion. The interplaybetween units typical of Bayn Dzak and Tug-rugyin lithology, as well as the fact that typ-


Fig. 9. A. Overview looking east at the eastern exposures of Tsonzh showing brown mudstone,similar to that exposed near the top of the section at Khren Tsav, wedged between underlying, struc-tureless, reddish orange sandstone of the Bayn Dzak Member and overlying pale orange to light graysandstone of the Tugrugyin Member. B. View looking west at the western exposures at Tsonzh showingreddish orange sandstone of the Bayn Dzak Member underlying pale orange to light gray sandstone ofthe Tugrugyin Member.

ical Bayn Dzak units underlie the typicalTugrugyin units as at Tsonzh, suggest thatthis sequence documents a transition betweenthe Bayn Dzak and Tugrugyin Members.Unit 5 at Aleg Teer represents the base of thetypical Tugrugyin sands exposed in the main

scarp at Tugrugyin Shireh (see next section).So, the Alag Teer section is a direct lateralextension of the deposits at Tugrugyin Shi-reh; however, the stratigraphically lowertransitional units exposed at Alag Teer arenot exposed at Tugrugyin Shireh.


Fig. 10. Stratigraphic section at Tsonzh. The lithologic key is to the right of the magnetostratigraphicdata.

Dinosaurs from Aleg Teer are restricted toUnit 2, which contains the alternation of BaynDzak and Tugrugyin sandstones. Fossils ofhadrosaurs and Pinacosaurus were found inthe Tugrugyin sands in the main exposure atAlag Teer (Fastovsky et al., 1997; Watabe and

Suzuki, 2000). Plant remains occur commonlyin the reddish brown mudstones.

Four magnetostratigraphic samples werecollected and analyzed (fig. 12). The fact thatthe sandy siltstones at the base of the AlegTeer section are of normal polarity indicates


Fig. 11. View looking south at the exposures at Aleg Teer, just north of the exposures at TugrugyinShireh, showing reddish orange sandstone of the Bayn Dzak Member underlying pale orange to lightgray sandstone of the Tugrugyin Member, identical to the superpositional relationships seen at Tsonzh.

that they formed at a different time than thetransitional mudstones of reversed polarity atKhren Tsav and Tsonzh to the east.

THE TUGRUGYIN MEMBER

The key stratigraphic section is exposed atthe Tugrugyin Shireh locality, 23 km to thenorthwest of the administrative center at Bul-gan. The section was discovered by Dash-zeveg in 1963. Two years later, Nikoloff andDashzeveg revisited it (Nikoloff and Huene,1966). The Tugrugyin Shireh locality is ascarp about 2.5 km long capped by a plateauof Quaternary pediment (fig. 13A,B). Theexposures of the scarp are monotonous lightgray sands and sandstones. The TugrugyinShireh scarp faces south, and the plateau isinclined to the north.

The locality was investigated by thePMPE and by the SMPE in 1969–1974 (Be-lyaeva et al., 1974). This locality producedthe interlocked specimens of Velociraptorand Protoceratops referred to as the ‘‘fight-ing dinosaurs’’ (Kielan-Jaworowska andBarsbold, 1972; Osmolska, 1993). Most re-

cently, the locality was reviewed by Fastov-sky et al. (1997) and results of the JMJPEare summarized in Watabe and Suzuki(2000). The lizard fauna, in particular, wasreviewed by Gao and Norell (2000), and theanatomy of a dromaeosaurid skeleton wasdescribed by Norell and Makovicky (1997).

Fastovsky et al. (1997) document threestratigraphic sections across the scarp total-ing 30 m. The sandstones of the TugrugyinMember are described as thickly bedded,with large-scale, high-angle crossbeds dis-tinctive of eolian dune slip faces. The cross-beds dip with an azimuth to the east-north-east, indicating predominantly westerlywinds in the region during the Cretaceous.Three types of trace fossils are described andattributed to burrowing, probably by arthro-pods. Articulated skeletons of Protoceratopsare common and often oriented parallel to theeast-northeast dip direction of the crossbeds.Fastovsky et al. (1997) attribute their deathto both sandstorms and less dramatic causes.The paleoenvironment is thought to representa distal, basinward setting on a nearby braid-


Fig. 12. Stratigraphic section at Alag Teer. The lithologic key is to the right. Unit numbers of thispaper are listed to the far left of the section. Colors are based on the GSA color chart. The position ofsamples is listed to the immediate right of the colors. The polarity of samples is shown to the right ofthe sample numbers. Polarity zonation is at the far right.

plain-to-eolian suite of depositional environ-ments. This paleoenvironmental interpreta-tion is in accordance with that suggested forDjadokhta Formation exposures at BayanMandahu in northern China studied byEberth (1993).

THE TUGRUGYIN SHIREH SECTION

Our section documents 52 m of light gray,crossbedded, and structureless sands and

sandstones belonging to the Tugrugyin Mem-ber at Tugrugyin Shireh (fig. 14). Crossbed-ded units are rich in trace fossils, as notedby Fastovsky et al. (1997). Crossbeds are de-fined by thin concretionary sheets composedof iron oxide-cemented carbonate. Crossbed-ded units are laterally traceable for less thanseveral hundred meters. They are interbed-ded and laterally pass into massive units ofstructureless sands and sandstones.


Fig. 13. A. View looking northwest at the western end of the main escarpment at Tugrugyin Shirehshowing exposures of the Tugrugyin Member. B. View looking east at the eastern end of the mainescarpment at Tugrugyin Shireh showing exposures of the Tugrugyin Member.

No facies map of the small-scale units atTugrugyin Shireh is yet available; however,some preliminary observations can be notedas a basis for further testing during upcomingfield investigations. Two distinct sedimento-logic facies appear to be represented by thecrossbedded sands and the structureless

sands. We agree with Fastovsky et al. (1997)that the crossbedded sands represent sedi-ment deposited by eolian processes on the leefaces of dunes. The massive structurelesssands, however, exhibit no definitive sedi-mentary structures typical of eolian deposits.Instead, these could represent dune-derived


Fig. 14. Stratigraphic section at TugrugyinShireh. The lithologic key is to the right. The col-ors are based on the GSA color chart.

sandslides generated on the lee faces of eo-lian dunes.

The known vertebrate fauna collected atTugrugyin Shireh is listed in table 2. At leastmost of the large vertebrate skeletons andsmaller fossils contained in irregular iron car-bonate nodules are found in the structurelesssands and sandstones, as at Ukhaa Tolgod.Vertebrate fossils collected by the MAE in-clude Protoceratops andrewsi, Velociraptormongoliensis, Shuvuuia deserti, Zalambda-lestes leichei, Kryptobaatar dashzevegi(Rougier et al., 1996), and abundant lizardfossils (Gao and Norell, 2000).

GEOLOGIC MAP OF MAJORLITHOSTRATIGRAPHIC UNITS AT

TUGRUGYIN SHIREH

The oldest rocks in the mapped area con-sist of Late Cretaceous sands, primarily rep-resenting the Tugrugyin Member of the Dja-dokhta Formation with subordinate outcropsof the Bayn Dzak Member (map 3). They areexposed throughout the northern half of themapped region in discontinuous patches,some of which contain outcrops of the mod-erate reddish orange sandstone of the BaynDzak Member at Aleg Teer. However in thesouthern half of the mapped area, sandstonesof the Tugrugyin Member form a prominentarc beginning in the southwest corner thatextends first to the east before turning northalong the eastern edge of the map. The mainescarpment of Tugrugyin Shireh forms thesouthernmost leg of this arc.

Overlying the Djadokhta Formation is amixture of Quaternary alluvium, colluvium,and eolian sands. These are exposed through-out the mapped region.

BIOCHRONOLOGICINTERPRETATIONS FOR THETUGRUGYIN SHIREH FAUNA

As noted in tables 1 and 2, numerous taxaare found in both the Bayn Dzak and Tug-rugyin Shireh faunas. Based on the strati-graphic relationships presented here it isclear that the Tugrugyin Member overlies theBayn Dzak Member, making it somewhatyounger. However, it is not yet possible toestimate the precise temporal difference.


TABLE 2The Tugrugyin Shireh Fauna

Reptilia

Squamata

Iguania

Isodontosaurus gracilis Gilmore, 1943Flavigama dzerzhinski Alifanov, 1989Mimeosaurus crassus Gilmore, 1943Zapsosaurus sceliphros Gao an Norell, 2000Temujinia ellisoni Gao and Norell, 2000

Teiidae

Adamisaurus magnidentatus Sulimski, 1972Dzhadochtosaurus giganteus Alifanov, 1993Gurvansaurus potissimus Alifanov, 1993

Varanidae

Cherminotus longifrons Borsuk-Bialynicka, 1984

Dinosauria

Saurischia

TheropodaDromaeosauridae

Velociraptor mongoliensis Osborn, 1924

Incertae sedis

Archaeornithoides deniosauriscus Elzanowskiand Wellnofer, 1992

Avialae

Shuvuuia deserti Chiappe et al., 1998

Enantiornithes

Euenantiornithes indet. Suzuki, Watabe, andTsogtbaatar, 1999

Mammalia

Multituberculata

DjadochtatherioideaDjadochtatheriidae

Kryptobaatar dashzevegi Kielan-Jaworowska,1970

Djadochtatherium matthewi Simpson, 1925

Eutheria incertae sedis

Asioryctitheria

Asioryctitheria

Kennalestes gobienisis Kielan-Jaworowska, 1969

Zalambdalestidae

Zalambdalestes lechei Gregory and Simpson,1926

MAGNETOSTRATIGRAPHICIMPLICATIONS FOR THE AGE OF THE

TUGRUGYIN SHIREH FAUNA

As noted in a previous section on the se-quence at Alag Teer, the basal, light graysands of the Tugrugyin Member in Units 4and 5 are reversed. This polarity correspondsto the uppermost beds of the Bayn DzakMember at the Flaming Cliffs, the mudstoneat Khren Tsav, and the mudstone at Tsonzh.If the zones of reversed polarity at these lo-calities correlate, it means that these unitsmust be younger than Chron 34n—the upperboundary of which represents the boundarybetween the Santonian and the Campanian atabout 83.5 Ma (Gradstein et al., 1995). Thequick stratigraphic succession of normal andreversed magnetozones in the Flaming Cliffssection suggests, but does not clearly estab-lish, that the sediments were deposited dur-ing the rapid sequence of polarity changes inthe late part of the Campanian incorporatingthe end of Chron 33 and Chron 32 betweenabout 75 and 71 Ma (Gradstein et al., 1995).

CONCLUSIONS

Two new members can be broken outwithin the Djadokhta Formation based on ex-posures at Bayn Dzak, Tugruyin Shireh,Tsonzh, and Alag Teer. The lower member,named the Bayn Dzak, is composed of mod-erate reddish orange, crossbedded, and struc-tureless sandstones, with subordinate depos-its of brown siltstone and mudstone. Thecrossbedded and structureless sandstones re-spectively represent eolian dunes and fluvialdeposits or sandslides that occurred on thedune faces. The mudstone and siltstone rep-resents an interdune facies deposited by flu-vial action. The upper member is named theTugrugyin Member and consists of very paleorange to light gray sands that also representa crossbedded dune facies and a structurelesssandslide facies. Transitional mudstone andsiltstone units separate the two members atTsonzh and Alag Teer.

Vertebrate fossils occur almost exclusivelyin the structureless sandstones of both mem-bers. Although the fauna from the BaynDzak Member is somewhat older than thatfrom the Tugrugyin Member, based on thesuperpositional relationships, many of the


Map 3. Geologic map of major lithologic units at Tugrugyin Shireh. Compiled by T. Dulam in July1998. 1. Holocene: eolian sands, vQiv; deluvid sands, dpQiv; lake clay and sand, lQiv; proluvial sandand debris, pQiv. 2. Pleistocene: deluvial–proluvial sand and debris, partly with limy matrix. 3. UpperCretaceous: Tugrugyin Member: K1/2 yellowish, weakly lithified, small-medium-grained sand. Whitesand slice. K2/2 unlithified, yellowish sands containing concretionary white sandstone nodules. K3/2less concretionary white sand. K5/2 white sand. K6/2 white sand. Bayn Dzak Member: K4/2 red sandwith light spots. 4. Unlithified, white sand, rich with mud craters, footprints, and less sandstone nodules.5. Faults: a: known; b: inferred. 6. Geologic boundaries: a: clear; b: unclear. 7. Points of observationof fossil bone. 8. Orientation of bedrock: a: inclined; b: horizontal.

same taxa have been collected from bothmembers. It is not yet possible to estimateprecisely the temporal difference separatingthe faunas from the two members.

New magnetostratigraphic samplesthroughout the Bayn Dzak Member and atthe base of the Tugrugyin Member documentthe presence of a sequence of normal andreversed magnetozones. Previous biochron-ologic age estimates taken in conjunction

with this new magnetostratigraphic data sug-gest that the fossiliferous sediments of theDjadokhta Formation were most likely de-posited during the quick succession of nor-mal and reversed intervals in the late Cam-panian between 71 and 75 Ma.

ACKNOWLEDGMENTS

The authors wish to thank the MongolianAcademy of Sciences as well as the field


crews from the American Museum of Natu-ral History. We also wish to thank Christo-pher Bennett, Alexander Kellner, and ZofiaKielan-Jaworowska for their extensive re-views of this manuscript, without implyingthat they concur with all the statements andconclusions presented herein. Financial sup-port for this study was provided by the FrickEndowment to the Division of Paleontologyat the American Museum of Natural History,the InfoQuest Foundation, the Ann and Gor-don Getty Foundation, and the Charlotte andWalter Kohler Charitable Trust.

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new stratigraphic subdivision, depositional environment

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