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Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
Stratigraphy and paleoenvironmental evolution of the dinosaur-rich
Baruungoyot-Nemegt succession (Upper Cretaceous), Nemegt Basin,
southern Mongolia
T
David A. Eberth
Royal Tyrrell Museum of Palaeontology, Box 7500, Drumheller, Alberta T0J 0Y0, Canada
A B S T R A C T
Portions of the dinosaur-rich Upper Cretaceous Baruungoyot-Nemegt succession are exposed in four geographic
areas of the Nemegt Basin, Mongolia: Nemegt, Altan Uul, Bügiin Tsav, and Hermiin Tsav. Lithostratigraphic
correlations of these areas employ marker beds and non-random stratigraphic patterns of lithology, grain-size
variation, and inferred paleoenvironments. Correlations suggest a minimum thickness of 350 m for the succession. Exposures at Nemegt, Altan Uul, and Bügiin Tsav form a 100 km east-west transect along the northern
margin of the basin, exposing 265 m of section. From east to west the transect exposes higher portions of the
succession, extending from the uppermost Baruungoyot Formation and encompassing an almost complete section of the Nemegt Formation, which has a minimum thickness of 235 m and can be consistently divided into
three informal members: lower, middle, and upper. The lower Nemegt is well exposed at Nemegt and Altan Uul
and is dominated by ﬂuvial deposits. The middle and upper Nemegt are well exposed at Altan Uul 2 and Bügiin
Tsav and consist of alluvial plain, paludal, lacustrine, and ﬂuvial deposits. As conﬁrmed by earlier studies, the
Baruungoyot and Nemegt formations exhibit an interﬁngering transition at Nemegt. The Hermiin Tsav section is
a geographic outlier, tens of kilometers west-southwest of the northern transect. It exposes about 150 m of
section that overlaps with, but is mostly lower, stratigraphically, than the northern transect. The uppermost
Hermiin Tsav section correlates with the lowermost Nemegt section, and also exhibits an interﬁngering contact
between the Baruungoyot and Nemegt formations. The remaining section at Hermiin Tsav comprises sediments
of the Baruungoyot Formation (135 m) that were deposited in alluvial fan, ﬂuvial, paludal, lacustrine, and eolian
environments, and can be divided into two informal members designated as lower and upper. The distribution of
facies through the composite section records changes in paleoenvironments during the Late Cretaceous that may
provide insights into the origins of the dinosaurian biostratigraphy.
1. Introduction
Portions of the dinosaur-rich Upper Cretaceous Baruungoyot and
Nemegt formations are exposed in at least four discrete geographic
areas of the Nemegt Basin in south-central Mongolia: Nemegt, Altan
Uul, Bügiin Tsav, and Hermiin Tsav (Fig. 1). Each of the four areas
consists of multiple sub-areas or localities that are recognized primarily
on the basis of fossil and trace assemblages dominated by dinosaurs. For
example, the Nemegt area includes the localities of Nemegt and
Khulsan, the Altan Uul area includes Altan Uul 1, 2, 3, and 4 (east to
west; designated I, II, III, and IV by earlier workers), the Bügiin Tsav
area includes Bügiin Tsav and Gurlin Tsav, and Hermiin Tsav includes
Hermiin Tsav 1 and 2 (previously designated I and II by Gradzinski and
colleagues). Although lithologies and stratigraphic patterns have been
described from some of these areas (Gradzinski et al., 1968; Gradzinski,
1970; Gradzinski and Jerzykiewicz, 1974a, 1974b; Gradzinski et al.,
1977; Eberth et al., 2009a) a complete composite stratigraphic section
for the Baruungoyot-Nemegt interval has heretofore been lacking,
compromising an understanding of (1) how vertebrate fossil assemblages from these diﬀerent localities relate to one another, and (2)
stratigraphic and paleogeographic patterns of paleoenvironmental
change.
Fortunately, three of the four areas (Nemegt, Altan Uul, and Bügiin
Tsav) are distributed in patches close to the north margin of the Nemegt
Basin—a linear distance of approximately 100 km—and can be correlated to build a stratigraphic transect, oriented southeast-northwest,
through a signiﬁcant portion of the basin’s Upper Cretaceous strata.
From southeast to northwest, the transect exposes successively higher
portions of the Upper Cretaceous succession, and includes (1) the uppermost portion of the Baruungoyot Formation and (2) an almost
E-mail address: david.eberth@gov.ab.ca.
https://doi.org/10.1016/j.palaeo.2017.11.018
Received 26 May 2017; Received in revised form 3 November 2017; Accepted 7 November 2017
Available online 08 November 2017
0031-0182/ Crown Copyright © 2017 Published by Elsevier B.V. All rights reserved.
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
44.00°N
44.00°N
100°E
Mongolia
Ulaanbaatar
Fig. 1. Google Earth (2010) location of ﬁeld areas:
Nemegt, Altan Uul, Bügiin Tsav, and Hermiin Tsav in
southern Mongolia. Inset shows ﬁeld area in Mongolia.
Dotted lines show Gobi-Altai uplifts in the regional basinand-range setting. Tick marks show latitude and longitude.
Abbreviations: km, kilometers; E, east; N, north.
Bügiin Tsav
101°E
Altan Uul
4 3 2
Nemeg
t Basin
Altan N
emegt
1
Nemegt
Altan Uul
Hermiin Tsav
43.45°N
Nemeg
t
Basin
43.45°N
20 km
100°E
101°E
sedimentology of the Baruungoyot and Nemegt formations, and documenting the stratigraphic transition between these units at Nemegt, a
small area of canyonlands along the southeastern ﬂank of Altan
Nemegt, a Tertiary-age uplift (Fig. 1; Gradzinski, 1970; Gradzinski and
Jerzykiewicz, 1974a; Gradzinski et al., 1977; Eberth et al., 2009a; Fanti
et al., 2017, this volume). This study began in that area in 2006 and,
subsequently, data were added from localities to the west (Altan Uul,
Bügiin Tsav, and Hermiin Tsav) from 2007 to 2010 as part of the
Korean Mongolian International Dinosaur Project.
Upper Cretaceous strata at Nemegt dip about 3° to the southwest,
and even though outcrops of the Nemegt and Baruungoyot formations
can be traced in patches in the canyon system that dissects the pediment of the Altan Nemegt, the amount and direction of structural dip in
the bedrock is similar to the modern pediment gradient, resulting in
bedrock exposures with limited stratigraphic thicknesses. Similar
structural patterns and challenges were encountered at all other sites
due to the fact that bedrock exposures are generally present only along
the ﬂanks of Tertiary age uplifts in the region (Fig. 1).
On the basin scale, the age and nature of the boundary between the
Baruungoyot Formation and overlying Nemegt Formation is poorly
understood (Gradzinski et al., 1977; Jerzykiewicz and Russell, 1991).
Conventional practice is to regard both units as chronostratigraphically
distinct, with the Nemegt Formation and its fauna regarded as younger
than the Baruungoyot Formation and its fauna (e.g., Jerzykiewicz and
Russell, 1991; Osmólska, 1997a, 1997b; Jerzykiewicz, 2000; Shuvalov,
2000). In biostratigraphic terms, this inferred temporal dichotomy is
supported by the distinctiveness of the faunas from the two units, even
though there is some faunal overlap (Gradzinski et al., 1977; Osmólska,
1997a, 1997b; Fanti et al., 2012; Currie, 2016).
complete section of the Nemegt Formation. In contrast, exposures at
Hermiin Tsav comprise an outlier section but still can be correlated
lithostratigraphically to the other areas using the shared presence of a
transitional interval between the Baruungoyot and Nemegt formations
(Eberth et al., 2009a).
Here, measured sections from the four areas are presented and
correlated, and a lithostratigraphy and paleoenvironmental-zonationscheme are proposed. Spellings of Mongolian localities and formations
in the Nemegt Basin follow those listed and standardized by Benton
(2000).
2. Location, geologic setting, methods
The areas described here, as well as the geological setting, sedimentology, and biostratigraphic importance of the Upper Cretaceous
Baruungoyot and Nemegt formations have been presented and discussed by Gradzinski and colleagues in a succession of papers that
present the results of the Polish-Mongolian Palaeontological
Expeditions (e.g., Gradzinski et al., 1968; Gradzinski, 1970; Gradzinski
and Jerzykiewicz, 1974a, 1974b; Gradzinski et al., 1977; Jerzykiewicz,
2000). Exposures of the Baruungoyot and Nemegt formations are exposed patchily around the margins of the Nemegt Basin (Fig. 1), a
narrow and elongate sub-basin of the larger Gobi-Altai basin-and-range
tectonic setting that formed and ﬁlled grabens and half-grabens during
episodic Late Cenozoic strike-slip transpression in response to the IndoEurasia collision (Jerzykiewicz and Russell, 1991; Jerzykiewicz, 2000;
Cunningham, 2010). Because of the absence of microfossil biostratigraphic schemes and datable volcanics in Cretaceous deposits across
southern Mongolia (cf. Osmólska, 1997b; Eberth et al., 2009a), it remains diﬃcult to determine the precise age of these stratigraphic units
and their fossils, or to temporally correlate exposures between geographically dispersed areas. The age of the Nemegt Formation is widely
regarded as Maastrichtian (e.g., Khand et al., 2000) but more precise
ages for its boundaries and subdivisions are still lacking.
Standard methods were used for mapping, section and transect
documentation, and sedimentary geology data-gathering. Previous
stratigraphic/sedimentologic studies across northern China (Hendrix
et al., 1992; Eberth et al., 2001) have shown that a sequence stratigraphic approach (e.g., Miall, 1991; Catuneanu, 2003) can be used to
correlate between localities in this setting, and to infer the temporal
signiﬁcance of surfaces and formational boundaries. Thus, the focus
here is on identifying unique stratigraphic patterns and interpreting
sedimentologic changes—in both stratigraphic sections and along
transects—as a means for correlating sections.
Previous lithostratigraphic work has focused on describing the
3. Nemegt area
Five stratigraphic sections measured at Nemegt in 2006 were presented and correlated by Eberth et al. (2009a), and conﬁrmed many
sedimentological descriptions and interpretations of Gradzinski and his
colleagues (see references above). Additional deposits and sections
were examined in 2009 and 2010, and those data are included here to
extend and reﬁne the correlations and interpretations of Eberth et al.
(2009a) (Fig. 2; see also Fanti et al., 2017, this volume).
The Baruungoyot Formation comprises a stacked succession of
tabular redbeds deposited in alluvial, lacustrine, and eolian environments by sheetﬂoods, meandering channels, ﬂoodplains, eolian megadunes, and as deﬂation lags (Fig. 3). Baruungoyot sediments are
dominated by sandstones and siltstones, with only rare occurrences of
claystones (Gradzinski and Jerzykiewicz, 1974a). Sandstones are ﬁne to
30
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
N
1.0 km
0.7 km
1.1 km
m
m
40
65
60
m
35
Formation
40
Nemegt
45
40
30
m
45
44
55
50
CAMP
08-18-06
S
2.4 km
AVIMIMUS
08-19-06
m
27
some
conglomeratic
sheet flood
RECON ‘A’
08-19-06
20
30
?
15
40
sandy
alluvial
Y
Y
Y
Y
Y
Y
Y
10
20
RECON ‘B’
08-19-06
30
5
0
m
cl
10
18
sandy
alluvial
15
20
5
20
0
“Big Red”
“Big Red”
“Big Red”
5
sandy
alluvial 10
10
fss m c
ss
“Big Red”
10
20
si
muddy alluvial
08-28-09 (2)
30
25
DISTAL
08-20-06
cl
si
fss m c cong 25
ss
Baruungoyot
Formation
23 08-28-09 (1)
20
20
15
15
0
5
5
0
0
cl
si
fss m c cong
ss
15
cl
10
si
fss m c cong
ss
cl
si
fss m c cong
ss
10
10
5
5
sandy
eolian
5
muddy alluvial
sandy
eolian
0
0
0
cl
si
fss m c cong
ss
cl
si
fss m c cong
ss
A
caliche nodules, clasts
granules
5YR
pebbles
sandstone laminae
255º
shale/clay laminae
GSA color
paleocurrent indicator/direction
articulated dinosaurs
inclined bedding
vertebrate bones
trough cross beds
bivalve shells
planar cross beds
eolian cross beds
dinosaur tracks/traces
current ripples
invertebrate feeding traces
wave ripples
roots
soft sediment deformation
slickensides, peds
B
Fig. 2. Measured sections and correlations in the Nemegt area. A) Seven stratigraphic sections measured and correlated in 2006 and 2009. Modiﬁed from Eberth et al. (2009a).
Correlations indicate a basinward (southward) progradation of the Nemegt Formation as it interﬁngers with and over-rides the Baruungoyot Formation. B) Symbol key for stratigraphic
sections presented in this publication. Abbreviations: cl, claystone; cong, conglomerate; css, coarse grained sandstone; fss, ﬁne grained sandstone; GSA, Geological Society of America rock
color chart; km, kilometers; m, meters; mss, medium grained sandstone; N, north; S, southeast; si, siltstone.
very ﬁne grained, and thus notably ﬁner than the pebbly sandstone
deposits of the Nemegt Formation (see below). Invertebrate feeding
traces are common throughout the Baruungoyot sediments (Fig. 3D, E),
and well deﬁned to indistinct dinosaur tracks are locally abundant,
especially in overbank mudstones and channel abandonment mudstones that are overlain by silty-to-sandy channel ﬁll deposits.
In contrast, the Nemegt Formation comprises a stacked succession of
light grey to tan colored alluvial channel, splay, and sheetﬂood deposits
(Fig. 4). The alluvial channel deposits are overwhelmingly dominated
by stacked successions of large-scale inclined beds (Fig. 4A), which are
reasonably interpreted as lateral accretion deposits in meandering
rivers (Gradzinski et al., 1977; Eberth et al., 2009a). Large-scale inclined surfaces bound sets of trough cross-beds and current ripples
(Fig. 4B), which are here interpreted as the preserved remains of subaqueous dunes and ripples that migrated along submerged point bar
surfaces and channel thalwegs. In-situ caliches are present (Fig. 4C) but
not abundant, whereas reworked caliches (Fig. 4D) are ubiquitous,
occurring commonly as carbonate-cemented lag deposits at the base of
upward-ﬁning paleochannel successions. As in the Baruungoyot Formation, simple tubular invertebrate traces are common throughout
deposits of ﬁne grained sandstone and siltstone.
Dinosaur tracks in mudstone are particularly well preserved due to
inﬁlling by carbonate-cemented sandstones (Fig. 4F; Currie et al., 2003;
Nakajima et al., 2017, this volume; Stettner et al., 2017, this volume).
The uppermost one-third of the Nemegt exposures in the Nemegt area
exhibits increased occurrences of conglomerate beds that contain
31
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
Fig. 3. Photographs showing typical deposits of the
Baruungoyot Formation at the Nemegt area. A) Inclined
beds in a four-m-thick meandering stream deposit.
Localized contact with the Nemegt Formation overlies the
inclined beds. B) Horizontally bedded mudstones and ﬁne
grained sandstones. C) High-relief inclined surface separating giant eolian cross-beds (below) from ﬂuvial crossbedded strata (above). Exposed interval is approximately
eight meters thick. D, E) Typical examples of simple tubular
burrow ﬁlls interpreted as nonmarine invertebrate traces.
Scale bars are 10 cm. Abbreviations as in Fig. 2.
~4 m
Baruungoyot inclined (lateral accretion) beds
A
~6m
B
Baruungoyot mixed eolian-alluvial cross beds
~8 m
Baruungoyot eolian cross beds
C
E
feeding traces
D
and sandy-to-pebbly sheet-ﬂood deposits of probable distal alluvial-fan
origin.
The composite 60 m stratigraphic section at Nemegt exhibits a 23 m
interﬁngering stratigraphic relationship with the Baruungoyot and
Nemegt formations, which is expressed across a distance of more than
ﬁve kilometers from north to south (Fig. 5A). Eberth et al. (2009a)
described prograding and basinward growth of Nemegt clastics from a
reworked-caliches. At the very top of the southern-most measured
sections (Avimimus and Distal sections; Fig. 2) CaCO3-cemented sandy
sheets containing well-rounded to sub-angular metamorphic granules
and pebbles are ubiquitous (Fig. 4E). Unlike the Baruungoyot Formation where very ﬁne grained sandstone and mudstone comprise more
than three-quarters of any section (see Gradzinski et al., 1977), the
Nemegt Formation is dominated (> 80%) by channel-ﬁll sandstones
32
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
~5 m
A
B
C
D
E
F
Fig. 4. Photographs showing typical facies in the lower Nemegt Formation in the Nemegt area (modiﬁed from Eberth et al., 2009a). A) Sandy lateral accretion beds in a ﬁve-m-thick
meandering stream deposit. Person for scale is circled in the upper right. The paleochannel succession was used to reconstruct the dimensions of one of the larger meandering channels in
the Nemegt Formation. Bankful depth is estimated at 6 m (precompacted vertical relief); bankful width is calculated at 75 m. Calculations based on methods of Ethridge and Schumm
(1978). B) Meter-thick trough cross-bedded sandstone. Hammer is ~ 30 cm long. C) In-situ caliche glaebules and concretions. D) Reworked caliche clasts in a normal- and reverse-graded
sandstone deposit. E) Granule-stone at the top of the Avimimus section. F) Well-cemented track-ﬁll of a hadrosaur dinosaur; originally located at the base of a cliﬀ-forming sandstone at
meter 30 in the Camp section. Scale bars in CeF are 10 cm. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure, the reader is referred to the web version
of this article.)
33
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
Nemegt fluvial deposits
Nemegt tongue
“Big Red”
Baruungoyot
Fm
(~45 m thick)
A
caliche conglomerates
Nemegt Fm
fluvial deposits
(~ 6 m thick)
Nemegt Fm fluvial deposits
(~25 m thick)
“Big Red”
Baruungoyot Fm
C
3.25 m
B
28º
incising basal
surface of
megadune sandstone
D
Fig. 5. Interﬁngering contact between the Baruungoyot and Nemegt formations in the Nemegt area. A) Looking east-northeast at the contact. Altan Nemegt (Fig. 1) in the background.
Paleoﬂow direction is out of the picture toward the southwest. The right-dipping lateral accretion surfaces and blunt termination of the “Nemegt tongue” indicate meandering channelbelt abandonment. B) Part of the Recon ‘A’ section (see Fig. 2). Note the presence of “Big Red,” a prominent redbed tongue of the Baruungoyot Formation in this area. C) Sharp contact of
the Nemegt Formation on the Baruungoyot Formation close to the Camp section. D) Close-up of a set of eolian cross strata (annotated) in the upper Baruungoyot Formation at Nemegt.
Note the steeply dipping foresets, typical of eolian cross-beds, and the deep erosional scour-and-ﬁll on underlying sediments. Abbreviations as in Fig. 2.
4. Altan Uul area
northern source area. Additional data gathered during 2009 and 2010
conﬁrm this interpretation.
Because outcrops of the Baruungoyot become patchier and more
isolated in distal areas (to the south), it remains challenging to correlate
speciﬁc occurrences of eolian cross-bedding with more proximal alluvial deposits. Nonetheless, it was observed that although Baruungoyot
alluvial deposits are replaced by sets of eolian megadune and interdune
deposits in a basinward (southern) direction (Figs. 2, 5), alluvial deposits prograde over eolian megadune and interdune deposits in that
same direction (see inferred stratigraphic relationships of sections 0828-09 and 08-28-09 with the 2006 sections in Fig. 2). This arrangement
indicates paleogeographic variation in paleoenvironments across the
Baruungoyot landscape, with alluvial deposits dominating toward
source area (north) and eolian deposits dominating downdip (south). It
also supports an interpretation of climatic variability through time,
with eolian facies dominating in older deposits of the Baruungoyot
Formation, and alluvial facies dominating in younger deposits of the
Baruungoyot and Nemegt formations.
Geographic variations in depositional environments across hundreds of meters to a few kilometers are common at the margins of
modern intracratonic basins in northern China and southern Mongolia
(Eberth, 1993), and have been observed in the Upper Cretaceous Djadokhta Formation localities of northern China (e.g., Eberth, 1993,
Fig. 19), in the middle Jurassic Shishugou Formation and its stratigraphic equivalents around the margins of the Junggar basin (Eberth
et al., 2001), and in the Cretaceous Javkhlant Formation of southeastern Mongolia (Eberth et al., 2009b).
Sections were measured at Altan Uul 2, 3, and 4 in 2009, and at
Altan Uul 2 and 4 in 2010 (Figs. 1, 6). The outcrops at Altan Uul 1 were
deemed too limited stratigraphically to be of value to this study, and
were not examined.
Facies and facies assemblages—especially in the lower Nemegt
Formation at Altan Uul (Figs. 6, 7A)—are identical to those in the
Nemegt Formation at Nemegt, and are dominated by light red to tan
colored sandy alluvial channel, splay, and sheetﬂood deposits (Eberth
et al., 2009a; Fig. 7). The lowest exposures include calcium-carbonatecemented sandstone sheets containing reworked caliches and wellrounded to sub-angular metamorphic granules and pebbles that are
similar to those at the top of the section at Nemegt (Fig. 7B). Channel
deposits are dominated by inclined sets of trough cross-beds and current ripples, which are interpreted as lateral accretion deposits in
meandering rivers (Eberth et al., 2009a, 2009b; Fig. 7A). Fine grained
deposits (Fig. 7D, E) consist of tabular sets of red-brown, sandy mudstone, in-situ caliches, slickensides, root traces, tubular invertebrate
feeding traces, and dinosaur tracks where sandstones overlie and ﬁll
tracks in underlying mudstones (Fig. 7E). Many of these features indicate the presence of subareally-exposed surfaces and paleosols. Upper
Nemegt sandy deposits at Altan Uul are light colored (10YR 7/4–6/6;
GSA, 1991) and diamictic, containing “ﬂoating” granules and small
pebbles. The massive nature of these poorly sorted deposits (Fig. 8)
supports an interpretation that they were deposited as mass sediment
ﬂows.
34
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
Stratigraphy
upper Nemegt
175
150
Altan Uul 2
Marker Beds
(white-grey granulitic sandstones)
m
202
200
m
08-26-2010
135
Grey-white beds
130
125
120
115
Grey-white beds
110
105
100
95
90
Grey-white beds
85
paleosol
MnO
80
Paludal/
Paleosols
75
middle Nemegt
100
Nemegt Formation
125
(abundant mudstones)
70
65
60
55
Paludal/Paleosols
Turtles
Dragon’s Tomb Horizon
40
Altan Uul 4
Altan Uul 3
08-20+21-2009
08-21-2009
Paludal/
Paleosols
20
75
20
70
15
65
10
60
5
psol
25
ostracods
MnO
paleosol
Deinocheirus
type
CaCO3
lower Nemegt
30
m
30
25
80
prominent
red mdst
15
10
5
Fe
0
CaCO3
cl si fss m c cong
ss
CaCO3
0
55
50
cl si fss m c cong
ss
tapering
tree roots
45
40
35
30
25
CaCO3
CaCO3
Fe
CaCO3
Tarbosaurus 53
paleosol
15
5
extrabasinal
clasts
cl si fss m c cong (marker)
t
Altan
Uul 4
CaCO3
paleosol-slick
CaCO3 Polish/Mongolian
dino carving
CaCO3
0
t
dino b
carving
CaCO3
CaCO3
20
10
0
50
45
m
85
(stacked sandstones; minor mudstones)
25
paleosol
35
75
50
paleosol
Deinocheirus
type
Dragon’s
Tomb
Altan
Uul 3
b
Altan
Uul 2
2 km
ss
Fig. 6. Three measured sections, marker beds, and correlations at Altan Uul. Annotated Google Earth photograph at lower right indicates locations of sections, two fossil occurrences
(Dragon’s Tomb, Deinocheirus type location), and a geographic marker (“dino carving”). Abbreviations as in Fig. 2 except: b, section base; CaCO3, calcium carbonate concretion; Fe, iron
staining; mdst, mudstone; MnO, manganese oxide staining; psol, paleosol; slick, slickensides; t, section top.
35
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
B
A
~12 m
thick
12 m
thick
C
D
tracks
1.5 m
thick
F
E
Fig. 7. Lower Nemegt Formation ﬂuvial sandstones (A–C) and middle Nemegt Formation mudstone-rich exposures (D–F) at Altan Uul. A) Typical stacked paleochannel sandstones of the
lower Nemegt Formation at Altan Uul 4. B) Typical diamictic sandstone with granule and small pebble “ﬂoat”. C) Structurally deformed lower Nemegt Formation strata on the ﬂanks of
the Altan Uul Tertiary uplift. D) Prominent recessive interval dominated by alluvial, paludal, and lacustrine mudstones in the middle Nemegt Formation at Altan Uul 2. E, F) Planarbedded ﬁne sandstones and sandstone/mudstone heteroliths in the middle Nemegt Formation at Altan Uul 2. Note the sandstone-ﬁlled dinosaur tracks and soft-sediment deformed strata
near the top of E. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure, the reader is referred to the web version of this article.)
36
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
~8 m
thick
B
50 m
thick
A
~3 m
thick
D
C
F
G
E
Fig. 8. Upper Nemegt Formation deposits at Altan Uul. A) Transition from middle to upper Nemegt Formation (double-headed arrow). B) An approximately eight-m-thick exposure of
stacked sheet sandstones. C) Typical well-rounded quartz and feldspar granules. D) Rare occurrence of inclined bedded strata of lateral accretion origin. Outcrop is approximately three
meters thick. E) Upper Nemegt Formation light-colored diamict. F) Modern diamict present along the canyon edges at Altan Uul 2. G) Heavily bioturbated sandstone with invertebrate
traces. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)
mudstone of alluvial, paludal, and lacustrine origin in the middle 70 m
at Altan Uul 2; and (3) the transition to the upper Nemegt light colored
sediments (dominated by coarse grained sheet ﬂood diamicts and rarer
meandering channel deposits) in the uppermost 50 m of the section at
Altan Uul 2.
The composite Altan Uul section is 202 m thick (Fig. 6). Altan Uul 4
Study at Altan Uul focused on stratigraphic correlation of the sections (Fig. 6), and correlating the exposures with those at Nemegt (to
the east) and Bügiin Tsav (to the northwest). Correlations were
achieved by matching meter-to-decameter-thick stratigraphic intervals
dominated by (1) extrabasinal clasts and granulitic alluvial sandstones
in the lowest exposed beds at Altan Uul 4; (2) thick deposits of dark-red
37
Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50
D.A. Eberth
outcrops studied here (Fig. 8F).
Decimeter thick beds of tan-to-red-brown mudstone are also present, but represent < 10% of the measured section in the upper Nemegt. These include in-situ caliche glaebules and concretions, and thick caliche hardpans that are absent in the middle and lower Nemegt Formation portions of the Altan Uul section. Tracks and traces include rhizoliths, a variety of unidentiﬁed burrows (Fig. 8G)—likely made by invertebrates—and sandstone-ﬁlled dinosaur tracks on the upper surfaces of the mudstones. Local yellowto-orange red staining of sandstones is present through the section and but may simply indicate groundwater-related diagenesis due to proximity to the mid-Cenozoic Altan Uul uplift. The gradual upsection increase in the abundance light colored sediments coupled with the presence of dinosaur tracks throughout the upper interval indicate that this light-colored upper Nemegt interval is Cretaceous in age, and rests conformably above the more typical Nemegt sandstone-mudstone succession that underlies it. exposes the lowest beds in the section, and Altan Uul 2 contains the highest beds. Exposures at Altan Uul 3, although more limited in stratigraphic extent (~ 30 m), overlap those at Altan Uul 2 and 4, thus providing additional conﬁrmation for the correlation proposed here. The Baruungoyot Formation is not exposed at Altan Uul, but is presumed to occur in the sub-surface. The lower one-half of the exposures at Altan Uul contain sandy, tancolored alluvial deposits indistinguishable from Nemegt Formation deposits at Nemegt (compare Figs. 4A, B and 7A). Extrabasinal pebbles and granules at the top of the Nemegt Formation section at Nemegt and the base of the Nemegt section at Altan Uul 4 (Figs. 4E and 7B) suggest that the base of the Altan Uul section may be more or less lithostratigraphically equivalent to the top of the Nemegt section at Nemegt. Although elevation data suggest a potential displacement of ~ 100 m between the top of the Nemegt and the base of the Altan Uul sections—the Altan Uul 4 section base is approximately 1584 m above sea level [masl] and the top of the Nemegt section occurs at 1486 masl—structurally warped strata and fault displacements are ubiquitous throughout the region wherever Nemegt Formation strata abut mid-Cenozoic-age uplifts, such as at Altan Nemegt and Altan Uul (Fig. 7C). Thus, elevation data are a less reliable means of establishing or testing stratigraphic correlation between areas. In this context, correlating the base of exposures at Altan Uul with the top of the exposures at Nemegt, as proposed above, remains the best and most conservative means of correlating the exposures between these areas, and provides an estimate for the minimum thickness of the lower Nemegt Formation in the region (~ 130 m). The middle 70 m of the composite section at Altan Uul includes numerous beds of red-colored, caliche-rich sandy-mudstone that is recessive in outcrop (Fig. 7D–F). These deposits are interpreted as interﬂuve paleosols, and paludal and lacustrine sediments deposited and modiﬁed under seasonally arid conditions (Gradzinski et al., 1977; Eberth et al., 2009a). This interval also includes a paleochannel-hosted monodominant bonebed known as the “Dragon's Tomb” (e.g., Dewale et al., 2015; Bell et al., 2017, this volume), which hosts the skeletal remains of the hadrosaurid dinosaur Saurolophus angustirostris. Sandy portions of this mudstone dominated interval exhibit cross-bedded sandstones of ﬂuvial origin (similar to the lower Nemegt section exposed at Altan Uul 4), and ﬁnely laminated, horizontally bedded sandstones and mudstones deposited under lower energy conditions, likely in ﬂood-basin marshes or small ponds (Fig. 7E, F). The transition to the 50-m-thick upper Nemegt Formation at Altan Uul is marked by increasing abundances of light colored granulitic and pebbly sandstones (Fig. 8A–C), which begin to dominate the section at 86 m in the Altan Uul 2 section (Fig. 6). This light-colored upper Nemegt interval is also exposed in the northeastern-most exposures at Bügiin Tsav (see below). Sandstones in the upper Nemegt at Altan Uul contain abundant light colored felsic (quartz and feldspar) sandstone grains, granules, and pebbles, which impart overall light colors (white, grey and tan; 10YR 6/6 and lighter) to the deposits and their draped and weathered surfaces (Fig. 8C). These colors stand out in sharp contrast to the darker red-brown hues (5R, 10R, and 5YR) of the underlying middle and lower Nemegt intervals (Fig. 7). Intra-clast redbrown mudstone conglomerates are present locally in lag deposits. Although sandy lateral-accretion deposits are present (Fig. 8D), coarse grained ﬂuvial units dominate this upper interval. These are preserved as upward ﬁning, tabular-to-wedge-shaped sets consisting of meso-scale cross-bedding, ripple lamination, and local occurrences of climbing ripples and mega-ripples. These facies are all suggestive of seasonally variable sheetﬂood and low-sinuosity stream deposition in a distal-fan and alluvial plain setting. Diamicts, containing granule and pebble “ﬂoat” are also common, and likely reﬂect deposition by masssediment ﬂows occurring in a mid-to-distal fan setting (Fig. 8E). This interpretation is conﬁrmed by the presence of modern sheetﬂood diamict deposits that have nearly identical fabrics and are exposed along the margins of some of the modern canyons that were used to access the 5. Bügiin Tsav area Ten stratigraphic sections were measured in the Bügiin Tsav area in 2009 and 2010, and eight were used here to build a composite section and explore lateral variations in facies relationships (Fig. 9). Bedrock at Bügiin Tsav is exposed along an escarpment and in patches southwest of the escarpment (Fig. 10A, B). Open areas of badlands-style topography provide three-dimensional access to exhumed paleochannel, paludal, and lacustrine deposits (Figs. 10–12), some of which are quite rich in fossil resources, and all of which exhibit evidence of loading and deformation by dinosaur trampling (Figs. 10G–I and 11B, C). Because exposures are patchy across the ﬁeld area, section measurement routinely required frequent shifts in location accomplished by tracing laterally continuous beds. In total, there is approximately 80 m of exposed section at Bügiin Tsav (Fig. 9) comprising a lower interval with sub-equal amounts of red-brown mudstone and sandstone (55 m; Fig. 10A, E, F) and an upper grey-white granulitic interval dominated by alluvial paleochannels and sheet sandstones (25 m; Figs. 10A, 12). This two-fold stratigraphic pattern is identical in lithology, facies and facies thicknesses, and sandstone/mudstone ratios to the middle and upper Nemegt Formation beds documented at Altan Uul. Accordingly, the Bügiin Tsav exposures are correlated lithostratigraphically with those intervals at Altan Uul, and the same stratigraphic terminology is applied to the Bügiin Tsav section (Fig. 9). 5.1. Middle Nemegt Formation The lowest 55 m of the Nemegt Formation section at Bügiin Tsav includes grey-to-brown lenticular paleochannel sandstones, alluvial sheet sandstones, and planar laminated red-brown to grey-green mudstones and ﬁne-grained sandstones of paludal and lacustrine origin. Soft sediment deformation is abundant, and all alluvial deposits are rich in fossils consisting of wood, vertebrates, gastropods, bivalves, and invertebrate traces. Lacustrine deposits, which are quite limited stratigraphically and geographically, yield conchostracans and ostracods. Coarse-grained deposits are similar to those described above from the lower and middle Nemegt beds at Altan Uul, and include tabular sheets of granulitic sandstones and 1.0–2.5 m-thick upward-ﬁning sandy paleochannel rhythms with decimeter-thick pebbly lags overlain by 1.0–2.0 m-thick deposits of inclined strata of point-bar origin (Fig. 10C). Inclined strata include decimeter scale sets of crossbedding and thick deposits of ripple laminae. Inclined strata are commonly heterolithic, and exhibit a range of lithologies from mudstones to sandstones deposited under varying ﬂow rates (Fig. 10B–D). A few local occurrences of decameter-wide lenticular paleochannel sandstones were also noted but are relatively rare compared to the meandering channel deposits. The tops of paleochannel successions are marked by 38 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth Bugin Tsav 2a m 08-16-09 Bugin Tsav North 80 m m m 08-19-2009 upper Nemegt 70 65 60 CaCO3 CaCO3 paleosol/slick 45 Fe 60 CaCO3 55 5 onset of white beds and gravels 0 15 middle Nemegt 20 Nemegt Formation 25 5 0 cl si ss c 15 40 turtle 15 35 30 lacust base 10 25 si Bugin Tsav fss 0 CaCO3 5 5YR 6/4 conch; ostracod; plant frags CaCO3 CaCO3 10R 6/6 5Y 7/6 0 cl si fss mss css cong slick 5 cl psol-slick 10YR 5/4 lake/paludal deposits CaCO3 Mongolchelys BB 0 psol-slick 10 CaCO3 5 CaCO3 psol lacust top cl si fss m ss c m 20 Sauropod tracksite 08-17-09 (2) 10 08-23-09 15 Bugin Tsav 2b Hayashibara camp area 08-17-09 CaCO3 m sauropod track zone cl 5 si 10R 7/4 125° CaCO3 fss m c ss 08-17-09(03) 5 Hayashibara Camp CaCO3 5 0 Bügiin Tsav Saurolophus 10 marker 10 0 08-19-09 10R 6/6 CaCO3 5 10 m 10R 6/6-4/6 10YR 6/6 25 20 15 marl CaCO3 10 HCS HCS fss m c ss Deinocheirus Quarry si fss 20 45 Y 30 m 30 25 cl 30 Gallimimus tracksite Bugin Tsav, 08-17-09(3) Deinocheirus Q. 08-23-2009 m marl plant frags CaCO3 Fe CaCO3 Teichichnus CaCO3 35 9 35 10 CaCO3 CaCO3 40 15 65 40 Bugin Tsav CaCO3 45 CaCO3 CaCO3 Y 35 m 08-29-2010 50 20 70 “Big White” meandering channel 50 40 08-24-2009 25 74 50 55 45 Bugin Tsav Camp 2010 55 75 50 Bugin Tsav Camp 2009 CaCO3 10R 6/6 10R 7/4 0 cl si 08-16-09 0 cl si fss m c ss 08-17-09(02) 08-17-09(01) 08-24-09 fss m c cong ss 2 km Camp 2009 08-29-10 Fig. 9. Eight measured sections through the middle and upper Nemegt Formation at Bügiin Tsav, and their correlation using a variety of local marker beds. Annotated Google Earth photograph at lower right indicates location of sections, as well as the 2009 camping sites of the Korean-Mongolian International Dinosaur Project and Hayashibara Expedition. Abbreviations as in Fig. 2, except: BB, bonebed; conch, conchostracan fossils; frags, fragments; HCS, apparent hummocky/swaley cross-beds; lacust, lacustrine; Q, dinosaur quarry. (For interpretation of the references to color in this ﬁgure, the reader is referred to the web version of this article.) bedded claystone and siltstone, which exhibit high-ﬁdelity dinosaur tracks and partial dinosaur skeletal remains (Fig. 11B, C; see also Lee et al., 2017, this volume); 2) massive red-brown claystones containing assemblages of conchostracans and ostracods (Fig. 11D) along with the more ubiquitous occurrences of gastropods; and (3) apparent hummocky/swaley cross-stratiﬁed and inclined beds of very-ﬁne-grained sandstones of possible wave origin (Fig. 11A). centimeter-to-decimeter-thick tabular sets of inclined-to-planar siltstone and sandstone that are heavily rooted, bioturbated, and trampled. Because of the extensive, ﬂat-lying modern landscape at Bügiin Tsav, there are many places where the tops of resistant, carbonate-cemented lateral accretion beds have been exhumed, thus marking the location and movements of ancient point bars within meander belts (Fig. 10B). In such instances, channel widths were estimated as ranging up to 50 m. Vertebrate fossils, including ﬁsh debris, turtle shells, and articulated, associated, and isolated dinosaur skeletal remains are common in the lags and along the surfaces of lateral accretion deposits. Red-brown-to-grey variegated sandy mudstones dominate sections between paleochannel deposits and represent deposition by interﬂuve ﬂood waters (Fig. 10E, F, H). These deposits sometimes display a diamict fabric characterized by “ﬂoating” grains of coarse sand and granules in ﬁner sandstone or siltstone, thus indicating proximity to local relief and the occurrence of mass-sediment ﬂows. These deposits are also biogenically and diagenetically modiﬁed, and commonly contain scattered, in-situ caliche glaebules, nodules, and hardpans (Fig. 10G, H), as well as root traces, rhizoliths, and slickensides. Dinosaur trample/load marks and well-deﬁned tracks are common within these deposits, and are particularly well preserved where they form depressions that are ﬁlled by overlying sandstones (Fig. 10H, I). Such sandstone-ﬁlled load marks and tracks typically show vertical relief of < 30 cm, exhibit grey-green (reduced) colors, and also exhibit abundant burrows and feeding traces of invertebrates, probably freshwater crustaceans and insects (e.g., Buatois and Mangano, 2011). Red-brown-to-grey caliche-rich recessive mudstones are abundant, and are identical to those recorded in the middle Nemegt Formation at Altan Uul (Fig. 10E–H). Locally, the presence of paludal and lacustrine paleoenvironments are conﬁrmed by: 1) planar beds of rhythmically 5.2. Upper Nemegt Formation The upper Nemegt Formation is 25 m thick and dominated by coarse-grained, light-grey-to-tan colored siltstones, sandstones, and granular beds (Figs. 9, 10A, 12). Similar to the upper Nemegt Formation at Altan Uul, felsic grains dominate (Fig. 12C) giving the interval a light color. A diamict fabric, characterized by “ﬂoating” sand grains, granules, and small pebbles in silty ﬁne-grained sandstone is common (Fig. 12C–E), indicating frequent mass sediment ﬂows. Locally, diamicts contain frosted and well-rounded grains and granules suggesting occasional reworking from eolian sources. Invertebrate traces are abundant and range from simple burrows to complex networks (Fig. 12D, F). Decimeter-thick, red-brown and lighter-colored mudstones with caliches, dinosaur tracks, and rich assemblages of invertebrate traces also exhibit diamict fabrics, but represent a very small part of the exposed section (Fig. 9). Tabular-to-wedge shaped deposits of poorly-sorted, pebbly, granulitic sandstone and siltstone with local diamict fabrics are common in the lower portions of the interval, and likely represent deposition by sheet-ﬂoods, or ﬂows with more concentrated sediments (possibly hyperconcentrated to mass sediment ﬂows) across unconﬁned landscapes and within broad but shallow paleochannels. Meter-thick packages of 39 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth ~50 m thick ~50 m A B D C ~7 m thick E F G H I Fig. 10. Middle Nemegt Formation at Bügiin Tsav. A) Overall setting at Bügiin Tsav with light-colored upper Nemegt strata in the distance. B) Exhumed and cemented topsets of a meandering paleochannel deposit. C, D) Typical meso-scale lateral accretion deposits; abundant calcium carbonate cement is present in C, whereas recessive sandstone/mudstone heteroliths are present in D. E) Planar bedded mudstones and minor sandstones. F) Freshly exposed mudstones exhibiting color variegation and mottling associated with water table ﬂuctuation. G) In-situ caliche glaebules and nodules typical of paleosol development in a seasonally dry setting, and soft sediment deformation likely caused by dinosaur trampling. H) Isolated load structure likely caused by dinosaur trampling. I) Load structure with high-ﬁdelity dinosaur track to the right of the pick axe. Planar laminated mudstones suggest a paludal/ lacustrine origin for the underlying sediments. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 40 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth A 50-60 cm diameter B C D Fig. 11. Paludal/lacustrine deposits in the middle Nemegt Formation at Bügiin Tsav. A) Hummocky-swaley cross-stratiﬁed ﬁne-grained sandstone suggestive of wave-inﬂuenced deposition in a lacustrine setting. B) Very ﬁnely interlaminated mudstones and sandstones of shallow lacustrine origin. In this outcrop, load structures are directly associated with tracks and fossilized, vertically oriented, hind-limbs of bipedal, small-theropod dinosaurs. C) An exhumed, sauropod-track-assemblage in the middle Nemegt Formation. D) Close-up of a fossil conchostracan from a laminated mudstone. Scale bars in A and B are 10 cm. Scale bar in D is 1 cm. Abbreviations as in Fig. 2, except: cm, centimeters. by sheet sandstones containing granulitic-to-pebble strings and diamicts, and minor occurrences of sandy pedogenic/bioturbated mudstones; (2) upper Baruungoyot Formation—a 45-m-thick interval of redbrown strata containing interbedded eolian dune deposits, sheet sandstones, paleochannel ﬁlls, and tabular, sandy mudstones; (3) transitional Nemegt-Baruungoyot—a 15-m-thick interval exhibiting interbedded red-brown and tan strata identical in color and facies to the transitional interval of Baruungoyot to Nemegt strata at the Nemegt area; and (4) lower Nemegt Formation—a 20-m-thick interval (erosionally truncated at the top) that is identical in color, lithology and facies composition to lower Nemegt Formation strata along the northern transect at the Nemegt and Altan Uul areas. Because the uppermost 40 m of the 150-m-thick Hermiin Tsav section is identical in color (hues in the 5YR, 5R, and 10R range), composition, facies composition, and vertebrate fauna to the lowermost 40 m of exposure at the nearby Nemegt area, we hypothesize that it is lithologically correlative, and thus likely time-equivalent, to the complete exposed section at Nemegt, but older than the sections exposed at both Altan Uul and Bügiin Tsav. This interpretation suggests that most of the Baruungoyot section at Hermiin Tsav is older than exposures at the other three areas. upward-ﬁning granulitic sandstones with lenticular relief, basal lags, cross-beds, and planar lamination are common throughout the section and represent deposition in paleochannels of approximately the same size as those in the underlying intervals (< 100 m bankful width). However, in the middle of the interval, there are local occurrences of oﬄapping, very-large-scale inclined beds (≤ 11° dip) of sandstone, forming packages up to 15 m thick that exhibit local relief of a meter or more (Fig. 12G). These are interpreted as lateral accretion beds in large paleochannels with reconstructed widths of approximately 150 m, and bankful depths of up to 15 m. Such a large increase in the inferred size of paleochannels underscores an increase in discharge rates, likely resulting from an increase in seasonal wetness (Knighton, 2014; Charlton, 2008). 6. Hermiin Tsav area Four stratigraphic sections measured at Hermiin Tsav in 2007 (Fig. 13) document an approximately 150-m-thick exposed section comprising four distinct lithostratigraphic intervals. In ascending stratigraphic order these are: (1) lower Baruungoyot Formation—a 75-mthick red-brown-to-tan colored interval within the lowest exposures of formation. It comprises a lower interval dominated by stacked paleochannel sandstones and the remaining section, which is dominated 41 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth ~30 m thick A B C D E F G Fig. 12. Upper Nemegt Formation deposits at Bügiin Tsav. A) Typical exposure of light-colored sandstone, identical to that in the upper Nemegt Formation at Altan Uul. Exposures are approximately 30 m thick. B) Stacked succession of granulitic, tabular to low-angle-inclined beds of sandstone. Red circle indicates 1.5 m Jacob's Staﬀ. Exposure is approximately 10 m thick. C) Trace fossil reworked granulitic-to-pebbly sandstone. Light-colored lithic grains are dominated by quartz and feldspar. D) Heavily bioturbated, granulitic sandstone overlain by granulitic sandstone with a diamictic fabric suggests mass-sediment ﬂow and proximity to source. E) Pebble diamict in calcareous, coarse-grained sandstone. Scale units are 1 cm each. F) Complex and deep invertebrate burrow network. G) Localized inclined bedded strata of lateral accretion origin. Red circle indicates 1.5 m Jacob's Staﬀ. Exposure is approximately 12 m thick. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) decimeter-scale sets of tabular-to-large-scale trough cross-bedding, as well as climbing ripples and planar-to-inclined lamination and are occasionally separated by thin lenses of red-brown mudstone of interﬂuve origin. Root traces occur locally at the top of paleochannel deposits, and soft-sediment deformation is locally abundant suggesting both tectonic inﬂuences, and trampling and loading by large dinosaurs. Stacked paleochannel deposits are rich in fossil vertebrate bones and bone fragments. 6.1. Lower Baruungoyot Formation The lowest 25 m of this 75 m thick interval is dominated by crossbedded ﬂuvial channel-ﬁll deposits and is also characterized by angular, dark-colored (derived from a variety of greenschist grade metamorphics) extraformational clasts, up to 10 cm in diameter, that occur as lags, graded deposits, and diamicts hosted by ﬁne to coarse grained sandstone sheets (Fig. 14A, B). Paleochannel sandstones consist of 42 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth Nemegt Cliff 09-15-2007 m m 30 155 25 150 145 lower Nemegt Fm 140 (fluvial) Megadune m 135 130 125 Camp transitional BaruunNemegt goyot m 130 90 125 15 1 80 115 75 110 70 105 105 100 upper Baruungoyot Fm (eolian, alluvial fan, paludal, lacustrine) 65 CaCO3 CaCO3 CaCO3 CaCO3 CaCO3 90 55 90 85 50 40 CaCO3 35 65 30 25 55 20 50 15 29-30° Eolian dune deposits CaCO3 20 15 CaCO3 CaCO3 10 CaCO3 CaCO3 5 CaCO3 0 cl si 80 fss m c cong ss 75 upper Bgt 70 CaCO3 CaCO3 65 CaCO3 lower Bgt CaCO3 CaCO3 60 cl si fss m c cong ss CaCO3 CaCO3 25 CaCO3 CaCO3 CaCO3 70 CaCO3 30 CaCO3 85 45 75 35 CaCO3 CaCO3 80 CaCO3 CaCO3 95 CaCO3 0 40 100 60 95 10 5 CaCO3 120 85 CaCO3 45 Nemegt equiv. CaCO3 15 50 09/12-13/2007 95 09-12-2007 55 09/13-14/2007 135 120 10 1 m Main CaCO3 20 60 ~ 30 m thick CaCO3 55 CaCO3 50 CaCO3 10 45 40 lower Baruungoyot Fm 5 40 35 (alluvial fan, fluvial, paludal) 0 45 CaCO3 CaCO3 35 cl si fss m c cong ss 30 30 25 25 20 20 15 15 10 10 5 5 0 0 CaCO3 Camp Megadune Main Nemegt Cliffs 2 km cl si fss m c cong ss Fig. 13. Four sections measured at Hermiin Tsav in 2007 and their correlation using a variety of local marker beds in the lower and upper Baruungoyot Formation, and lower Nemegt Formation. Annotated Google Earth photograph at lower right shows locations of sections. Center photograph shows overall setting indicating the boundary (dashed line) between the lower and upper Baruungoyot Formation (Bgt) in the distance. Other abbreviations as in Fig. 2. massive, diamictic beds of ﬁne-to-medium grained sandstone hosting “ﬂoating” granules and pebbles, and in-situ-to-reworked caliche glaebules (Fig. 14C–F). Invertebrate burrows and vertebrate fossils are present locally. Cross-bedded sandstones are rarely observed in this interval. The abundance of sandstones with a diamictic fabric suggest repeated occurrences of mass sediment ﬂow in a seasonally dry, distalfan setting. The facies in these lower beds suggest ephemeral ﬂow conditions and deposition by mass sediment ﬂows, high-energy sheet ﬂoods and channelized ﬂows in a mid-to-distal alluvial-fan setting, but close to an exposed metamorphic source. Seven paleocurrent measurements on meso-scale cross-beds indicate ﬂow toward the west and northwest (Fig. 13; “Main” section, 10–20 m), and a southerly source area. In contrast, the upper 50 m of the lower Baruungoyot Formation consists almost exclusively of red-brown (10R7/4, 10R4/6, 10R6/6, 5YR5/6) 43 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth A B ~8 m thick C D F E Fig. 14. Lower and upper Baruungoyot deposits at Hermiin Tsav. A) Stacked channel and alluvial sheet sandstones showing typical dominance of sandstones in the lower Baruungoyot Formation. Red circle indicates 1.5 m Jacob's Staﬀ. B) typical cross-bedded light-colored sandstones in lower Baruungoyot Formation containing granule lenses and strings, and exhibiting localized, soft-sediment deformation of likely tectonic origin. C) Tabular beds and strings of interbedded gravel and sandstone in the lower Baruungoyot Formation. D) Lowest portion of the upper Baruungoyot Formation. Note trend toward subequal sandstone-mudstone occurrences, as well as the localized, grey-colored conglomeratic channel-sandstone that incises red sandy siltstones (arrow). E) Localized occurrence of a heavily bioturbated and soft-sediment deformed mudstone and sandstone succession in the upper Baruungoyot Formation. F) Exhumed surface exhibiting a rich assemblage of feeding traces in the upper Baruungoyot Formation. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 44 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth A B C D ~5 m thick megadune deposit E G F H Fig. 15. Upper Baruungoyot Formation and lower Nemegt Formation deposits at Hermiin Tsav. A) Sub-equal sandstone-mudstone succession in the upper Baruungoyot Formation. Note heavily trampled and deformed strata in the upper one-third of the section. B) Exhumed sauropod track with vertically elongate scale-drag marks in the upper Baruungoyot Formation. C) Linear, asymmetric (current) ripples suggestive of low energy conditions, possibly wave generated. D) Mega-scale cross-bed of eolian origin in the upper Baruungoyot Formation. Red circle indicates 1.5 m Jacob's Staﬀ. E) Close-up of eolian cross beds. Light colors indicate sandier composition, whereas red colors indicate muddier composition. Sand-ﬂow lamination and soft-sediment deformation (above the Jacob's Staﬀ) is diagnostic of eolian dune deposits. F) Interbedded deposits of the Baruungoyot Formation (deeper red colored tabular deposits and eolian cross-beds) and Nemegt Formation (lighter colored sandstones with meso-scale cross-bedding). G) Close-up of meso-scale cross-bedded ﬂuvial sandstone sharply overlying a meandering-channel abandonment-ﬁll deposit in the lower Nemegt Formation. This is part of the transitional succession marked by interbedded Baruungoyot and Nemegt deposits. H) Heavily bioturbated sandstones of the lower Nemegt Formation. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 45 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth contact between the two formations, with the Nemegt Formation sharply overlying the Baruungoyot Formation (Fig. 16C). 6.2. Upper Baruungoyot Formation The upper Baruungoyot is marked by occurrences of eolian crossbeds, ﬁnely bedded, deep-red-colored sandy mudstones (10R6/6) of paludal/lacustrine origin, and wavy-laminated, ﬁne-grained sandstone, suggestive of very low amplitude wave ripples and hummocks in standing water (Fig. 15). The section also includes paleochannel deposits with cross-bedding and massive-to-diamictic sheet sandstones of the type observed in the lower Bruungoyot Formation. Thus, the upper Baruungoyot Formation can be reasonably interpreted as recording deposition at the distal margins of an alluvial fan system in an arid to semi-arid climate where mixed assemblages of alluvial, lacustrine/paludal and eolian deposits co-existed (cf. Eberth, 1993). Large-scale eolian cross-beds become more common toward the top of the upper Baruungoyot and, to the east, are particularly abundant in the “Megadune” section (Figs. 13, 15D, E). The tops of decimeter-thick tabular beds of sandy of red mudstone are often deeply trampled or exhibit discrete tracks that are inﬁlled with pebbly to granulitic sandstone (Fig. 15A). Some tracks are well preserved and exhibit high-ﬁdelity scale-drag marks that were generated when the feet of large dinosaurs (sauropods and hadrosaurs?) were lifted from the soft substrate (Fig. 15B). As in the lower Baruungoyot, in-situ and reworked caliche glaebules and nodules, root traces and rhizoconcretions, and invertebrate traces are common. 6.4. Lower Nemegt Formation Exposures of the lower Nemegt Formation are limited to the northcentral and northeastern portions of the Hermiin Tsav ﬁeld area (Nemegt Cliﬀ section, Fig. 13) where they are approximately 20 m thick. There, exposures comprise stacked successions of tan-colored Nemegt Formation ﬂuvial deposits, similar to those exposed at the Nemegt and Altan Uul areas (Fig. 16). Three paleocurrent measurements on meso-scale trough cross-beds in the “Nemegt Cliﬀ” section (Fig. 13) indicate a west-northwest direction of ﬂow, compatible with an interpretation of a southern source area. 7. Correlation of the Nemegt, Altan Uul, Bügiin Tsav, and Hermiin Tsav areas The shared, up-section transition of colors, lithologies, and facies at Nemegt, Altan Uul, Bügiin Tsav, and Hermiin Tsav provides an opportunity to correlate on a lithostratigraphic basis the exposed sections between those areas, and to interpret the stratigraphic pattern of paleoenvironmental change in the Nemegt Basin during the Late Cretaceous. Fig. 17A shows a composite stratigraphy based on the measured sections and correlations presented here. The unique transitional interval comprising interﬁngered Baruungoyot/Nemegt deposits at both Nemegt and Hermiin Tsav provides a basis for lithostratigraphic correlation between these two areas and suggests a basin-wide conformable transition between the Baruungoyot and Nemegt formations. The lower portion of the composite section, which is based on this correlation, indicates that the Baruungoyot formation is most fully exposed at Hermiin Tsav, is at least 135 m thick and is the lowest, and likely the oldest, interval in all the sections we examined. The shared occurrence of extrabasinal clasts at the base of the Altan Uul 4 section and the top of the Nemegt Formation at Nemegt provides a weak basis for correlating these horizons; additional horizons 6.3. Transitional Baruungoyot/Nemegt interval The up-section transition to the Nemegt Formation is characterized by beds of (1) red-brown, large-scale, eolian cross-beds, and massive, sandy siltstone diamicts both assignable to the Baruungoyot Formation, and (2) tan-colored, cross-bedded ﬂuvial deposits assignable to the Nemegt Formation (Fig. 15F, G). The transitional interval is approximately 15 m thick, and is identical in many respects to the transitional Baruungoyot and Nemegt interval that comprises most of the section at Nemegt. Interbeds of facies from both formations were documented only in the Hermiin Tsav Main section. To the east, at the Megadune and Nemegt Cliﬀ sections (Fig. 13), outcrops record a simple and sharp ~12 m thick ~5 m thick A B ~4 m thick C D Fig. 16. Lower Nemegt Formation exposures at Hermiin Tsav. A, B) Typical deeply weathered and patchily exposed lower Nemegt Formation exposures. C) Sharp contact (dashed white line) of the lower Nemegt Formation on the upper Baruungoyot Formation. Red circle indicates 1.5 m Jacob's Staﬀ. D) Close-up of boxed area (in C) highlighting an abandoned channel ﬁll. Abbreviations as in Fig. 2. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 46 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth m 360 Correlations Composite Stratigraphy and Paleoenvironments of Nemegt Basin Altan Uul 350 340 330 320 upper Nemegt Fm BügiinTsav Zone 7: fluvial upper Nemegt Fm upper Nemegt Fm 310 300 290 fluv/lac 280 270 260 middle Nemegt Fm Zone 6: middle Nemegt Fm mixed fluvial/ lacustrine/ paludal middle Nemegt Fm 250 240 230 220 210 200 190 180 fluvial; seasonally wet-dry 170 140 cong. lower Nemegt Fm lower Nemegt Fm l 130 120 l na tra n io sit na tra n io sit 110 100 90 80 Nemegt Hermiin Tsav 160 150 lower Nemegt Fm Zone 5: lower Nemegt Fm Zone 4: mixed fluvial/ eolian Zone 3: upper Baruungoyot Fm mixed fan/fluvial/ eolian/lacustrine/ paludal/ seasonally dry * l na itio s ran lower Nemegt Fm t transitional siti n tra al on upper Baruungoyot Fm upper Baruungoyot Fm 70 60 50 40 30 Zone 2: lower Baruungoyot Fm distal fan; seasonally dry 20 Zone 1: 10 mid fan; mesic lower Baruungoyot Fm 0 west 100 km east Fig. 17. Composite stratigraphy and succession of paleoenvironmental zones in the Baruungoyot and Nemegt formations of the Nemegt Basin, southern Mongolia. Correlations of the four ﬁeld areas are shown to the right, and are based on the measured sections and interpretations presented here and in Eberth et al. (2009a). Proposed subdivisions (lower, middle, and upper) of the Baruungoyot and Nemegt formations remain informal until more sections can be measured. Abbreviations as in Fig. 2, except: cong., conglomeratic sandstones; ﬂuv/lac, ﬂuvial/lacustrine; Fm, Formation. of two discrete stratigraphic intervals that exhibit unique facies and facies assemblages. The ﬁrst is a decameter thick interval dominated by ﬂuvial, lacustrine, paludal and ﬂoodplain paleosols that forms the middle Nemegt Formation and is paleoenvironmentally distinct from the ﬂuvially dominated, lower Nemegt Formation. Secondly, both areas exhibit a prominent, decameter thick white/grey-colored interval high in section (upper Nemegt Formation) that contains felsic sandstones, containing extrabasinal clasts may be present in the subsurface at the Altan Uul 4 location, or may have been erosionally removed at Nemegt. Thus, the 235-m-thick Nemegt Formation shown in Fig. 17 should be regarded as a representing the minimum thickness of that unit, as discussed above. The proposed lithostratigraphic correlation of the Nemegt sections at Altan Uul and Bügiin Tsav (Fig. 17) is well supported by the presence 47 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth granules, and pebbles and is associated with sheet ﬂood deposits and rarer occurrences of large-scale lateral accretion beds. The proposed correlations suggest that the entire composite section of Baruungoyot and Nemegt strata across the ﬁeld area is approximately 350 m thick. However, because neither the bottom of the Baruungoyot Formation, nor the top of the Nemegt Formation is exposed, and because of the varying thicknesses of both formations across the transitional interval that separates them, we assess the minimum thicknesses of the Baruungoyot and Nemegt formations as 135 m and 235 m, respectively. 8.3. Zone 3 (upper Baruungoyot Formation; Hermiin Tsav, Nemegt) The composite Baruungoyot-Nemegt section proposed here records evidence for paleoenvironmental changes during deposition of the Baruungoyot and Nemegt formations. These changes were likely expressed across the Nemegt Basin and may thus provide opportunities for tying other unexplored exposures into the composite section, and for assessing the origins of biostratigraphic patterns. Paleoenvironmental changes in the composite section most likely reﬂect simple paleogeographic shifts in the positions of alluvial fans, eolian dune landscapes, fan-margin wetlands, and channel belts and ﬂoodplains. In turn, these were likely driven by a combination of smallscale autogenic/autocyclic changes and on a more inclusive scale, basin-wide changes in tectonism and climate (cf. Hendrix et al., 1992; Jerzykiewicz and Russell, 1991; Eberth, 1993; Eberth et al., 2001). The paleoenvironmental interpretations presented in Fig. 17 are based on the data included here, and Gradzinski et al. (1968), Gradzinski (1970), Gradzinski and Jerzykiewicz (1974a, 1974b), Gradzinski et al. (1977), and Eberth et al. (2009a). Seven stratigraphically successive paleoenvironmental zones are recognized and described from bottom to top. Paleoenvironmental Zone 3 coincides with the upper Baruungoyot Formation at Hermiin Tsav and Nemegt, and records deposition at the distal margins of alluvial fans in an arid to semi-arid climate where alluvial, lacustrine/paludal, and eolian deposits co-existed. Eberth (1993) described a similar paleogeographic zone of mixed alluvial, lacustrine/paludal, and eolian facies in the Djadohkta Formation at Bayan Mandahu, Inner Mongolia, China. The shift from Zone 2 to Zone 3 at Hermiin Tsav suggests backstepping and withdrawal of the alluvial fan from this area with a commensurate expansion of lacustrine/paludal and eolian facies in the area. Such changing conditions would most likely result from increasing aridity (already underway in Zone 2) and an expansion of desert conditions. In such conditions, wetlands were likely localized at the margins (toes) of alluvial fans, and resulted from seepage of waters that inﬁltrated the upstream/proximal portions of the fan. In the modern Gobi landscapes of southern Mongolia and northern China localized wetlands and “border-seeps” (Bryan, 1919) are common at the distal margins of modern alluvial fans and eolian dune ﬁelds, thereby providing excellent analogues for the Baruungoyot setting. Although eolian dunes encroached into the Hermiin Tsav area during the latest stages of deposition in this stratigraphic interval, they were likely also present in more basinward locations during earlier phases of deposition in Zone 3 and throughout the time that Zone 2 was being deposited. This interpretation is supported by the presence of occasional diamict granules and coarse sand grains that are wellrounded, frosted, and pitted, and, thus, wind-blown. Tracks, dinosaur loading structures, and invertebrate burrows are uniquely abundant in Zone 3, and suggest that fan-margin wetlands were favored habitats for sauropods, large ornithischians, their predators, and an invertebrate infauna (see also Currie et al., 2017, this volume; Stettner et al., 2017, this volume; Nakajima et al., 2017, this volume). 8.1. Zone 1 (lower Baruungoyot Formation; Hermiin Tsav) 8.4. Zone 4 (upper Baruungoyot and lower Nemegt formations; Hermiin Tsav and Nemegt) 8. Paleoenvironmental changes in the Nemegt Basin during deposition of the Baruungoyot and Nemegt formations Paleoenvironmental Zone 1 comprises the lowest 25 m of the “Main” section at Hermiin Tsav, which represents the stratigraphically lowest exposures encountered during this study. The section is dominated by stacked occurrences of sandy, low-sinuosity paleochannels, and sheetﬂood and mass sediment ﬂow deposits and is characterized by a notable absence of interchannel ﬂoodplain deposits and the presence of abundant, poorly sorted and angular pebbles and cobbles mostly comprising metamorphic rock. Zone 1 is interpreted as a mid-alluvialfan setting close to an uplifted metamorphic terrain. Abundant largescale occurrences of convoluted and deformed cross-bedded sandstone suggest that there were frequent seismic events such as earthquakes. The abundance of channel sandstones in a fan setting suggest that sediments may have been deposited during slightly more mesic (warm and wet) climatic conditions compared to overlying zones (Charlton, 2008). Paleoenvironmental Zone 4 coincides with transitional stratigraphic interval at Hermiin Tsav and Nemegt where fan-margin paleoenvironments of the Baruungoyot Formation pass upward into the more uniform alluvial and ﬂuvial deposits of the lower Nemegt Formation. Accordingly, this interval records deposition of prograding distal fan deposits, the establishment of widespread alluvial and ﬂuvial conditions, and a reduction in eolian dune deposition, likely during a time of increasing wetness. This paleoenvironmental transition was described by Eberth et al. (2009a) in reference to the lowest part of the exposed section in the proximal-distal transect at Nemegt. The presence of insitu and reworked caliches, calcretes, and the co-occurrence of pebbly sheetﬂood and meandering channel deposits suggest a more complex climatic setting than those represented by the underlying zones, with a wide range of seasonally wet-and-dry conditions, and longer-term climatic shifts between wetter and drier conditions (Blissenbach, 1954; Charlton, 2008). 8.2. Zone 2 (lower Baruungoyot Formation; Hermiin Tsav) 8.5. Zone 5 (lower Nemegt Formation; Nemegt, Altan Uul, and Hermiin Tsav) The transition from Zone 1 to Zone 2 is marked by a sharp and prominent color change to darker red sediments (10R hue-range), and a shift to a facies assemblage that is dominated by diamictic sheet deposits, fewer ﬂuvial channel deposits, fewer and smaller extrabasinal clasts, and abundant reworked caliches and in-situ calcretes. The proposed setting of Zone 2 is a distal alluvial fan. The transition from Zone 1 to Zone 2 likely represents a shift to seasonally drier conditions and an increase in ephemeral ﬂow conditions as reﬂected by fewer channels, but more sheet-ﬂood and diamict deposits (Blissenbach, 1954). Paleoenvironmental Zone 5 corresponds to the lower one-half of the Nemegt Formation, which is exposed nearly completely at Altan Uul, but only in small part at Hermiin Tsav and Nemegt. In-situ and reworked caliches and calcretes are ubiquitous and paleochannels are small, measuring a few tens of meter wide and typically < 3 m deep. These features support an interpretation of deposition in a purely alluvial plain setting where ﬂow was characterized by channel meandering and sheet ﬂooding in a seasonally wet-dry climatic setting. 48 Palaeogeography, Palaeoclimatology, Palaeoecology 494 (2018) 29–50 D.A. Eberth this region has a minimum thickness of 350 m. 2. From east to west, the northern transect—formed by exposures at Nemegt, Altan Uul, and Bügiin Tsav—exposes successively higher portions of the stratigraphic section, extending from the uppermost Baruungoyot Formation and encompassing an almost complete section of the Nemegt Formation (approximately 235 m thick), which includes distinct lower, middle, and upper stratigraphic intervals, each characterized by unique lithologies and facies assemblages. 3. At Nemegt, Altan Uul, and Bügiin Tsav, sediment dispersal directions are broadly southward and westward, indicating a consistently northerly source area for the sediments exposed along the northern transect. In contrast, the Hermiin Tsav section remains an outlier with paleocurrent directions oriented broadly northward and westward, thus indicating a southern source area for those strata along the southern margin of the Cretaceous-age Nemegt Basin. Considering the paleocurrent diﬀerences between the northern transect versus Hermiin Tsav, the width of the Cretaceous Nemegt Basin in the ﬁeld area has an estimated minimum north-south width of 50 km. 4. The contact between the Baruungoyot and Nemegt formations is well exposed along opposite sides of the basin at Nemegt (north) and Hermiin Tsav (south). Regardless of which section is examined, an interﬁngered contact between the Baruungoyot and Nemegt formations is preserved, thus indicating a basin-wide control (tectonics and climate) on the transition. 5. Most of the 152 m thick composite section exposed at Hermiin Tsav is dominated by alluvial facies assignable to the Baruungoyot Formation (a total of 135 m), representing the oldest exposures of that formation in the ﬁeld area. 6. Diamict deposits are common and exhibit coarse sand, granule, and pebble “ﬂoat” indicating close proximity to source areas and recurring mass sediment ﬂows, identical to modern occurrences in Gobi Desert today. 7. Seven stratigraphically stacked, paleoenvironmental zones can be recognized in the composite section based on a range of paleoenvironments, including alluvial fan, eolian, and a range of alluvial settings. The stratigraphic distribution of paleoenvironmental zones allows for climatic inferences that suggest that the climate across Late Cretaceous southern Mongolia varied from mesic to seasonally wet-dry to arid, and then back to seasonally wet-dry during deposition of this succession. Although Eberth et al. (2009a) interpreted a pebbly interval at the top of the exposed section at Nemegt as tentative evidence for a prograding alluvial fan system, it is clear from examinations of sections at Altan Uul that there is no evidence above that horizon for alluvial fan facies. Instead, the lower Nemegt Formation comprises a repetitive assemblage of ﬂuvial facies, and thus, represents a long-term, proximalalluvial-plain-setting across which paleochannels meandered. 8.6. Zone 6 (middle Nemegt Formation; Altan Uul and Bügiin Tsav) Paleoenvironmental Zone 6 corresponds to the middle of the Nemegt Formation and is well exposed, but only at Altan Uul and Bügiin Tsav. It records continued deposition in an alluvial plain setting similar to that of Zone 5, but is characterized by the addition of lacustrine to paludal deposits that, locally, yield conchostracans and ostracods, and thick successions of red-brown paleosol-rich interﬂuve deposits. Articulated large-dinosaurs, dinosaur and turtle bonebeds, and a variety of dinosaur tracks and trackways are also abundant in this zone. The section at Altan Uul (Fig. 6) includes three stratigraphically discrete occurrences of the lacustrine/paludal/paleosol deposits, whereas the section at Bügiin Tsav includes at least two occurrences (Fig. 9). This paleoenvironmental zone can be interpreted as having developed in two alternative ways. First, it may represent a simple paleogeographic shift from an upper- to a middle/lower-alluvial-plain setting where lacustrine and paludal deposits are more common. In this interpretation, the middle Nemegt Formation mixed ﬂuvial-lacustrine deposits of Zone 6 expand their distribution into more up-dip locations that were previously occupied by lower Nemegt Formation ﬂuvial deposits (Zone 5). Alternatively, lacustrine and paludal deposits in paleoenvironmental Zone 6 may reﬂect a climatic increase in rainfall over paleoenvironmental Zone 5, and the wider establishment of lacustrine and paludal environments across the region. However, because there is no discernable diﬀerence in calcareous paleosol developmental stages between zones 5 and 6, the ﬁrst interpretation of a paleogeographic shift in facies remains more parsimonious and preferred over an interpretation of basin-wide climatic change. In this context, we propose that the basin likely became tectonically quiet, with reduced uplift and subsidence. During tectonic quiescence, alluvial fan deposits began to be reworked and fan slopes began to diminish due to erosion. As alluvium continued to ﬁll the basin under conditions of diminishing accommodation, depositional gradients were lowered creating opportunities for local ponding and lake development. Acknowledgments 8.7. Zone 7 (upper Nemegt Formation; Altan Uul 4 and Bügiin Tsav) I thank Philip J. Currie and Yuong-nam Lee for the many opportunities to work in southern Mongolia as part of the Nomadic Expeditions Tour in 2006 and the Korean-Mongolian International Dinosaur Project, 2007–2010. Thanks to all staﬀ of the Mongolian Academy of Sciences for sharing insights and providing a broad variety of support for this research; special recognition is due to Rinchen Barsbold, Demchig Badamgarav (deceased), Khishigav Tsogtbaatar, Ligden Barsbold, Bat Lkasuren (deceased). Nam-soo Kim, Yoshi Kobayashi, François Therrien, Darla Zelenitsky, Michael Ryan, David Evans, and Octavio Mateus all provided stratigraphic, sedimentological, paleontological insights. David Fastovsky and Mototaka Saneyoshi shared geological insights at Bügiin Tsav. I thank Federico Fanti and Philip Bell for the opportunity to participate in this special volume. Reviewers Brooks Britt and Ray Rogers, and editors Thomas Algeo and Federico Fanti are thanked for comments and critiques that improved the manuscript. 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