Age and Correlation of Fossiliferous Late Paleocene–Early Eocene Strata of the Erlian Basin, Inner Mongolia, China

2005·0.63 MB·English

by  Gabriel J. Bowen

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3474, 26 pp., 12 figures, 2 tables May 11, 2005
Age and Correlation of Fossiliferous Late
Paleocene–Early Eocene Strata of the
Erlian Basin, Inner Mongolia, China
GABRIEL J. BOWEN,1 PAUL L. KOCH,2 JIN MENG,3 JIE YE,4 AND
SUYIN TING5
ABSTRACT
The Asian continent preserves a rich and diverse record of Paleogene mammal faunas and
their evolution through time. The sequence of faunal succession is of key importance to our
understanding of the origin and diversification of modern mammal groups, as phylogenetic
data suggest that many major modern clades may be rooted in Asia. By calibrating theAsian
fauna sequence within a chronostratigraphic framework, we can begin to compare patterns of
succession on a global scale and constrain models for the originationanddispersalofmodern
mammal groups in the early Paleogene.
The Erlian Basin of Inner Mongolia preserves Early Paleogene strata and mammal fossils
assignable to the Gashatan, Bumbanian, and Irdin Manhan Asian Land Mammal Ages (AL-
MAs). We measured stratigraphic sections and analyzed the stable isotope composition of
paleosol carbonates and paleomagnetic directions of rocks at three localities in the Erlian
Basin. The data document patterns in lithology, carbon isotope composition, and magnetic
polarity that are consistent at all three localities and allow us to present two constrained
hypothesesforthecorrelationofthelocalstratigraphicsections.Withintheresultingcomposite
section,weareabletoidentifyaseculardecreaseinthecarbonisotopecompositionofpaleosol
carbonate that can be equated to a multimillion-year trend preserved in late Paleocene and
1Currentaddress:Department ofBiology, UniversityofUtah,SaltLakeCity,UT,84112([email protected].
edu).
2DepartmentofEarthSciences,UniversityofCalifornia,SantaCruz,CA 95064 ([email protected]).
3DivisionofPaleontology,AmericanMuseumofNaturalHistory([email protected]).
4InstituteofVertebratePaleontologyandPaleoanthropology,ChineseAcademyofSciences,P.O.Box643,Beijing
100044, P.R.China ([email protected]).
5MuseumofNaturalScience,LouisianaStateUniversity,Baton Rouge,LA70803 ([email protected]).
CopyrightqAmericanMuseumofNaturalHistory2005 ISSN0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3474
early Eocene terrestrial and marine records. Using this trend and previously documentedcon-
straints on the age of the Bumbanian ALMA, the composite section is shown to correlate
within the interval of time represented by chrons C26n–C24n of the Geomagnetic Polarity
Timescale(GPTS).Weoutlinethreepossiblecorrelationsofthesequenceofmagneticpolarity
zones in our composite section to the GPTS and explore the biostratigraphic implications of
these. All three possible correlations show that Gashatan faunas in Inner Mongolia occur
within chron C24R, and the preferred correlation suggests that the Gashatan taxa may have
persisted close to the Paleocene/Eocene boundary. If confirmedthroughfurthersampling,this
result would imply that the first appearance of the modern mammal orders Primates, Artio-
dactyla, and Perissodactyla in Asia at the base of the Bumbanian ALMA did notsignificantly
precede their first appearances in Europe and North America at the Paleocene/Eocenebound-
ary.FossilsitesintheErlianBasinpromisetobecentraltoresolvingthedebateaboutwhether
these clades lived and diversified in Asia before dispersing throughout the Northern Hemi-
sphere at the Paleocene/Eocene boundary.
INTRODUCTION pearance of these groups and lack of closely
allied fossil outgroups in early Paleogene
The transition between the Paleocene and
faunas led workers to conclude that the ap-
Eocene epochs was one of the most dynamic
pearances represent immigration fromanun-
intervals of the Cenozoic. Long-term trends
seen locus of evolution (e.g., McKenna,
in climate ushered in the warmest period of
1973). Phylogenetic analysis of recently dis-
the Cenozoic (Zachos etal.,2001).Through-
coveredPaleocenemammalfossilsfromChi-
out this gradual shift in climate, faunas and na, however, has led some workers to con-
floras evolved in a series of abrupt events clude that Asian faunas include taxa closely
leading to the establishment of the first truly allied to the new groups, and that the origins
modern terrestrial ecosystems evidenced in of the earliest Eocene debutants may be
the fossil record of the Holarctic continents. traced to a low-latitude Asian Eden (Beard,
Abrupt changes in climate and faunal and 1998; Beard and Dawson, 1999).
floral composition coincided at the Paleo- A simple test with the potential to verify
cene/Eoceneboundary,whentransientglobal Asia as a source of emigration at the Paleo-
warming (e.g., Kennett and Stott, 1991), dis- cene/Eocene boundary, if not as an evolu-
persal of mammal groups throughout the tionary locus, would be to evaluate the rel-
northern hemisphere (e.g., McKenna, 1973), ative age of the first appearances of clades
and shifts in floral composition (Harrington, on the three Holarctic continents.Iftheearly
2003) led to reorganization ofterrestrialeco- representativesofoneormoreofthemodern
systems on geologically short timescales mammal groups were found to have greater
(;100thousandyears(ky);Rohletal.,2000; antiquity in Asia than in Europe and North
Bowen et al., 2001; Farley and Eltgroth, America,thiswouldsupportAsiaasasource
2003). of evolution or emigration. Conversely,find-
One of the most profound changes in ter- ing that theAsian firstappearanceslaggedor
restrialecosystemsthatoccurrednearthePa- were synchronous with those on other con-
leocene/Eoceneboundarywastheabruptfirst tinentswouldconflictwithsuchahypothesis.
appearance of several modern groups of Recent work has significantly clarified the
mammals in faunas from Asia, Europe, and relative age of early Paleogene faunal turn-
North America (Gingerich, 1989; Hooker, over events on the Holarctic continents. Of
1998; Ting, 1998). The P/E first appearances key importanceis agloballycorrelatablecar-
included the first representativesofPrimates, bon isotope excursion documented in marine
Artiodactyla,andPerissodactyla,threeextant and terrestrial sedimentary rocks (Koch et
orders that went on to diversify and become al., 1992; Zachos et al., 1993) and recently
importantcomponentsofmodernfaunas,and chosen as the defining stratigraphic datum
Hyaenodontidae, a family of carnivorous for the Paleocene/Eocene boundary (Ouda,
mammalsthatsufferedextinctionintheMio- 2003). The base of the d13C excursion has
cene (e.g., Gingerich, 1989). The abrupt ap- been shown to correlate with the P/E first
2005 BOWEN ET AL.: ERLIAN BASIN STRATA 3
appearances to within ;10 ky in North lithological, biostratigraphic, paleomagnetic,
America(Bowenetal.,2001),wherethenew and isotopic data from three fossil-bearing
groups appear at the base of the Wasatchian stratigraphic sections in the Erlian Basin.
North American Land Mammal Age (NAL- These data allow refined stratigraphic corre-
MA;Gingerich,2001).WorkinsouthernEu- lation of rock units within the basin andsug-
ropehasshownthattheP/Efirstappearances gest correlations between these units and the
there are unlikely to predate those in North GPTS. We then examine the implications of
America(Cojanetal.,2000).InnorthernEu- these correlations for the potential antiquity
rope, taxa characteristic of the P/E first ap- of the Bumbanian ALMA and associated
pearance event first appear at the Dormaal modern mammal groups in Asia.
locality of Belgium (Smith, 2000). By cor-
relation to other early Paleogene localitiesin GEOLOGICAL SETTING
Belgium and northern France for which bulk
organic carbon isotope data are available, Fossil mammals of the Erlian Basin occur
this fauna is thought to occur within the CIE within a widely distributed package of sili-
and be time-equivalent to the earliest Was- ciclastic sedimentary rocks. These rocks are
atchian faunas (Steurbaut et al., 1999; Ma- largely undeformed and outcrop primarily in
gioncalda et al., 2004). widely separated low mesas and in shallow
In Asia, the first representative of Hyaen- washes. As a result of their poor local ex-
odontidae occurs in a fauna assigned to the posure and the relative isolation of outcrops
Gashatan Asian Land Mammal age (AL- in this region, stratigraphic correlation of
MAs; Meng et al., 1998), which has been rock units within the basin is difficultandno
shown to be Late Paleocene in age (Bowen clear regional stratigraphy has been devel-
et al., 2002). Other important appearance oped. Several stratigraphic units have been
events,however,includingthefirstAsianpri- defined based on a combination of litholog-
mates, perissodactyls, and perhaps artiodac- ical and faunal characteristics, and these
tyls (Tong and Wang, 1998) occur within the wererecentlyreviewedbyMengetal.(1998,
subsequent Bumbanian ALMA (Dashzeveg, 2004).
1988; Ting, 1998). Earlier work (Bowen et The rocks at our study sites include red
al., 2002; Ting et al., 2003) has shown that and tan mudstones, calcareous mudstones,
the base of the Bumbanian is no younger and siltstones, commonly preserving milli-
than the P/E boundary, at ;55 million years meter-scale laminations of probable lacus-
ago (Ma; Wing et al., 1999), and that Gash- trine origin, massive, brick red mudstone
atan index taxa persist at least until the end beds commonly preserving features indica-
of chron C25n of the geomagnetic polarity tive of paleosol development (see below),
timescale (GPTS) ;55.9 Ma (Cande and and laterally extensive sandstone and con-
Kent, 1995). These correlations leave an un- glomerate beds of fluvial origin. These rocks
constrained window of time some 900 ky have been previously assigned to the Nom-
long during which modern groups may have ogen, Arshanto, and Irdin Manha Forma-
evolved and diversified in Asia or, alterna- tions,althoughthedetailsoftheirassignment
tively, archaicgroups mayhavepersistedbe- has differed among studies (see Meng et al.,
fore the impending restructuring of their 2004). In many cases, these names refer to
communities. units recognized on the basis of combined
Here, our goal is to use magnetic and iso- lithostratigraphic and biostratigraphic char-
tope stratigraphy to determine the age of acteristics, and as a result it is unclear how
mammal faunas from the Erlian Basin of In- to assign many intervalsinourlocalsections
ner Mongolia (fig. 1). This region has pro- to these composite stratigraphic units. The
ducedseveraldiversecollectionsofearlyPa- stratigraphic nomenclature for the Erlian Ba-
leogene mammals, including several assem- sin is in need of revision, but because of the
blages assigned to the Gashatan ALMA, and limited stratigraphic and geographic cover-
new fossiliferous levels and important spec- age of the present study we will not attempt
imenscontinuetobediscovered(Mengetal., such a revision here. Instead, we will refer
2004; Smith et al., 2004). We present new throughout to informal lithostratigraphic
4 AMERICAN MUSEUM NOVITATES NO. 3474
Fig. 1. Locality map for fossiliferous Early Paleogene sites in the Erlian Basin (dashed line), Inner
Mongolia, China. Sites discussed in the text are Erden Obo (EO), Bayan Ulan (BU), and Huheboerhe
(HH). All localities are to the southwest of the boarder town of Erlianhot (square). Digital relief from
the GTOPO30 309 3 309 digital elevation model (U.S. Geological Survey, 1996).
units which we found to be useful in corre- traces, burrows, depletion channels, and car-
lating among our local sections, with the in- bonate nodules. Carbonate nodules werecol-
tention that our findings might be incorpo- lected frompaleosolbedsonfreshlyexposed
rated into a comprehensive revision of the rock surfaces where there was no evidence
stratigraphy of the Erlian Basin in thefuture. ofmodernsoilformation(e.g.,modernroots,
weathered textures).
METHODS Paleomagnetic analyses were conductedat
the University of California paleomagnetics
We collected lithological data, fossils, ori-
laboratory using a three-axis cryogenic su-
ented rock samples, and paleosol carbonate
perconducting magnetometer. Samples were
nodules from measuredstratigraphicsections
incrementally demagnetized in a shielded
at three localities (figs. 1, 2). Sections were
measured using a Jacob staff and described oven at temperatures between 0 and 6808C.
in the field from freshly exposed surfaces. Directions were fit to the stepwise demag-
Cores and blocks were collected for paleo- netizationdatausingleast-squaresanalysisof
magneticanalysisandorientedusingaBrun- vector component plots.
ton compass. Because exposure was poor, Samples of paleosol carbonate for stable
vertical horizination characteristic of soilde- carbon and oxygen isotope analysis were
velopment was not commonly observable in drilled from polished surfaces under magni-
the field. Paleosols were identified as fine- fication using a dental drill. Microcrystalline
grained siliciclastic rock units lacking inter- calcite was sampled exclusively; care was
nal stratification and containing features in- taken to avoid void-filling macrocrystalline
dicative of soil formation, including root phases which are likely to have formed dur-
2005 BOWEN ET AL.: ERLIAN BASIN STRATA 5
Fig. 2. Lithostratigraphic columns, key taxa, and faunal levels of the Erlian Basin sections. Bio-
stratigraphic affiliation of taxa is shown to the left of each section.
ing post-burial diagenesis (e.g., Koch et al., glomerate. Fragmentary fossil vertebrate re-
1995; Bao et al., 1998; Bowen et al., 2001). mains, including specimens of the anagalid
Samples were reacted with 100% H PO ina Pseudictops, arctostylopid Palaeostylops,
3 4
common acid bath at 908C, and the CO and multituberculates Lambdopsalis and
2
evolved was cryogenically purified in an au- Sphenopsalis,occurinthelower;5mofthe
tomatedpreparationdevice.ThepurifiedCO section (fig. 2). At the 21-m level there is a
2
was analyzed using a VG Optima or Prism transition to fine-grained rocks, and from 21
gas source mass spectrometer, and sample to 55 m there is a series of variegated red-
valuescorrectedrelativetoanalysesofanin- brown to gray-green unfossiliferous mud-
house calcite standard. Isotope values are re- rocks. These are capped by a thin (;0.5 m)
ported in per mil (‰) units and d notation medium-coarse-grained sandstone with a
relative to the V-PDB standard, where d 5 sharp, undulatory basal contact. The sand-
(R 2 R )/R * 1000, and R in- stone contains quartz pebbles lenses and
sample standard standard
dicates the ratio 13C/12C or 18O/16O. Measure- abundant fragmentary vertebrate fossils. Be-
ment precision was typically ,0.05‰ for tween 55.5 m and the top of the section re-
carbon and oxygen, based on repeated anal- ported here (78 m) is a series of interbedded
yses of the calcite standard. brown and tan siltstones and fine- to medi-
um-grained sandstones. These are typically
RESULTS ,3.5 m thick, and produce fossils at several
levels, including the tapiroid perissodactyl
LITHOSTRATIGRAPHY AND FAUNAL LEVELS Teleolophus at ;65 m.
At Erden Obo (428739N, 1118219E), the The sampled section at Bayan Ulan
basal ;21 m of the section consist of medi- (438089N, 1118359E), some 40 km NNE of
um to coarse-grained sandstone, soft, pink, ErdenObo(fig.1),consistsprimarilyofnon-
laminatedsiltstone,andlensesofpebblecon- bedded red, tan, and brown siltstones and
6 AMERICAN MUSEUM NOVITATES NO. 3474
Fig.3. Fieldphotographofmud-filledchannelsatBayanUlan.Thepatchilydistributedlight-colored
rockintheforegroundofthephotoissoft,white,siltymudstonecontainingpocketsofroundedclayball
conglomerate,whichfillsasinuouschannelincisedintothesurroundinghard,orange-redmudrock.The
top of this channel could not be located in the field area. An upper channel, ;5 m deep, can be seen
on the ridge (black arrows). This feature cuts perpendicular to the plane of the photograph and is filled
with white mudstone. A geologist is standing at the left of the photo (white arrow) in a plane between
the two channels, providing an approximate indication of the scale of these features.
mudstones. Within the lower 48 m of the level, where nonbedded red mudstones are
measured section, red-brown mudstone beds erosionally truncated by a stacked complex
up to 7.5 m thick are dominant and are in- of crossbedded to laminated, gray-white silt-
terbedded with thinner, white, tan and brown stone and fine-grained sandstone. Tapiroid
siltstone beds (fig. 2). Near the base of the fossils were collected from a bed just below
section these beds are fossiliferous, and the this transition.
Bayan Ulan fauna of Meng et al. (1998) was Within the mudstone interval comprising
collectedfromthebasal;3mofoursection. the basal 48 m of the Bayan Ulan sectionwe
The dominant taxon at this level is the mul- observed several scours and fine-grained
tituberculate Lambdopsalis. Other ‘‘index’’ sand or mud-filled channels (figs. 2, 3).
fossils from this level include Palaeostylops, These appear to cut several meters into the
the dinoceratid Prodinoceras, and Tribos- underlying strata, and add considerable
phenomys (a gliroid). During our fieldwork stratigraphic complexity to the section, and
of 2002, we found a fossiliferous level at 8– in some cases these beds may producemam-
9 m of the section (BU-01-01-04 was taken mal fossils that, without careful consider-
at 8 m; this is the fossil site 02–02), from ation for their stratigraphic context, might
which numerous specimens of small mam- lead to erroneous interpretation of the bio-
mals including Palaeostylops, Pseudictops, stratigraphy of the section. We found it to be
and a new basal gliroid (Meng et al., 2005) necessary to run our measured section
were collected. There is no specimen of through one mud-filled channel, and took
Lambdopsalis from this level, and this fossil note of its position. Although section meter
assemblage may represent a slightlyyounger levels reported for Bayan Ulan do not take
fauna than the previously described Bayan into account the depth of this erosional sur-
Ulan fauna (Meng et al., 2005). Slightly face, they do reflect the proper superposi-
abovethislevel,twocalcaneathataresimilar tional order of the sites considering their
to those of Gomphos were found (Meng et stratigraphic context.
al., 2004). The main lithologic transition in Our third section was measured at Huhe-
the Bayan Ulan section occurs at the 48 m boerhe (438229N, 1118459E), and is a revised
2005 BOWEN ET AL.: ERLIAN BASIN STRATA 7
version of the section of Meng et al. (2004) permost red mudstones. Particular care must
reflecting new observations made during the be taken in interpreting the provenance of
2004 field season. At Huheboerhe, the basal fossils collected from these mudstone beds.
24mofsectionconsistoflens-shaped,cross- The biostratigraphic affiliations of many
bedded, fine- to medium-grained sandstones, index taxa from the Erlian Basinsectionsare
soft, pink, laminated sandy siltstones, and shown in figure 2. Prodinoceras, Palaeosty-
brechiated to homogeneous, red-weathering lops, Tribosphenomys, and the multituber-
mudstones. Within this package, finer culates are all indicative of the Gashatan
grainedrocktypesbecomeincreasinglydom- ALMA, and most of the collections fromthe
inant up-section. The uppermost sandy silt- ErlianBasinsectionsthatbearthesetaxarep-
stone bed produced a specimen of Prodino- resent assemblages that are compositionally
ceras (23 m). The interval between 24 and similar to otherassemblagesofGashatanage
47.5 m is predominately mudstone. Beds (Ting, 1998). The only taxon in our sections
near the base of this interval are tan and that is considered diagnostic of the Bumban-
brown, with a transition to gray mudstone ian ALMA is G. elkema (Dashzeveg, 1988;
and a single, thick (7.5 m) red mudstone oc- Meng et al., 2004). Fossil tapirs are charac-
curring at the 40-m level. Specimens of the teristic of subsequent ALMAs, which have
basal gliroid Gomphos elkema (Meng et al., been termed the Arshantan and Irdin Man-
2004), a small perissodactyl, possibly Hom- han. These biostratigraphic units are weakly
ogalax, and a hyaenodontid smaller than defined based on a combination of litholog-
Prolimnocyon chowi (Meng et al., 1998) ical and faunal associations, and are in need
were recovered from this red bed during of review. Because the tapir specimens and
fieldwork in 2002; very large (up to 10 cm associated fossils referred to here were col-
diameter) paleosol carbonate nodules were lected from strata overlying the Bumbanian
also found near the base of the bed. Screen- andGashatanlevelsthathavebeendescribed
washing sediments from this bed in 2004 as the ‘‘Irdin Manha beds’’, we willdescribe
yielded teeth of at least two species of small them as representing the Irdin Manhan
rodents. These rodents are generally similar ALMA.
to those from the Bumban beds of Mongolia
(Shevyreva, 1989; Dashzeveg, 1990). How- PALEOMAGNETICS
ever, because the rodent specimens from the
Bumban beds are fragmentary and their tax- Samples from the Erlian Basinsections(N
onomicidentificationsarecurrentlyuncertain 5 183) displayed a wide array of magnetic
(Averianov, 1996), we do not provide pre- behaviors during stepwise demagnetization,
liminary identifications of the Huheboerhe but many siltstone samples and some mud-
specimensinthisstudytoavoidfurthercom- stones exhibited relatively stable behavior.
plication of the situation. Demagnetization of these samples typically
Lowerinthemudstoneinterval,at28.5m, revealed multiple directional components
pods of hard, brittle, carbonate grainstoneup (fig. 4). Most samples preserved a low-tem-
to;50cmindiameterwereobservedweath- perature component, which was removed at
ering out in a discrete horizon. These are temperatures below 3008C and exhibits an
comprised primarily of round, carbonate- average declination (D) of 3558 and inclina-
coated clayballs several millimeters in di- tion (I) of 758 (N 5 177; fig. 5A). We inter-
ameter, and are tentatively interpretedasfos- pret this component to be a modern field
sil cold spring deposits. At the top of the overprint. Stable samples also typically pre-
section, a coarse-grained sandstone and peb- served a second, characteristic direction,
ble conglomerate overlies the thick red mud- which was removed between 300 and 5808C
stone with a sharp basal contact. The con- (fig. 4). This component exhibits directions
glomerate contains abundant fragmentary indicativeofbothnormalandreversedfields,
vertebrate fossils, including tapiroid perisso- and theaverage directionsfornormalandre-
dactyls. Fragmentary fossils from the con- versedfieldsamplesareroughlyantipodal(D
glomeratebedhavewasheddownandcanbe 5 10.68, I 5 48.08, N 5 84, k 5 5.614, and
found on the surface of the underlying, up- D 5 191.58, I 5 242.78, N 5 58, k 5 6.215,
8 AMERICAN MUSEUM NOVITATES NO. 3474
Fig.4. VectorcomponentdiagramsforthethermaldemagnetizationofErlianBasinsamples.Crosses
show the horizontal magnetization component and circles the vertical component projected on the N–S
vertical plane. Labels indicate the thermal temperature step (8C) for each vertical plane point. Note the
relatively stable demagnetization behavior of samples with a wide range of remanence intensities. A,
BU020109A and B, BU020111C,samplescharacterizedbynormalfielddirections.C,BU020101Cand
D, HH020101E, samples preserving reversed field directions. All samples depicted here show the re-
moval of a normal field overprint at temperatures ,3008C and isolation of a characteristic component
between 300 and 5808C.
2005 BOWEN ET AL.: ERLIAN BASIN STRATA 9
Fig. 5. Equal-area projections of sample and site mean remanence directions. Mean directions for
each polarity are shown with stars, and circles represent a95. A, Low temperature component, isolated
below 3008C, for individual samples. The mean direction of this component (D 5 258, I 5 758) is
similartotheexpectedmodern-fielddirectionfortheErlianBasin(D508,I5628),withthesomewhat
steeper inclination likely reflecting the partial removal of primary reversed polarity magnetizationfrom
some samples at low temperature. B, Characteristic component, isolated between 300 and 5808 C. C,
Site mean directions, calculated from sample characteristic component directions. Mean normal and
reversedsitedirectionsaresimilartopreviouslyreporteddirectionsfromtheTertiaryofInnerMongolia
(Zhao et al., 1994).
respectively; fig. 5B). The unblocking tem- sites with multiple stable samples (table 1).
perature of this component suggests that it These were classed as alpha or beta sites ac-
was carried by magnetite, and we interpretit cording to data quality, with alpha sites hav-
to be depositional remanent magnetization ing four or more samples clustered at 99%
carried by detrital magnetite grains. confidence and beta sites having two sites
Site average directions werecalculatedfor clustered at 99% confidence or threeormore
10 AMERICAN MUSEUM NOVITATES NO. 3474
TABLE1
Paleomagnetic Data
Sitea Level(m) D siteb I siteb a95 N R k
ErdenObo
EO-01–27 67.5 12.1 261.8 1 1.00
EO-01-25 62 2.8 44.2 40.2 3 2.81 10.46
EO-01-23 57.5 44.8 69.4 1 1.00
EO-01-22 54.8 78.8 66 1 1.00
EO-01-18 43 88.6 44 28.6 4 3.73 11.30
EO-01-15 36.5 253.9 251.2 46.9 2 1.97 30.52
EO-01-07 24.5 237 236.6 11 5 4.92 49.57
EO-01-05 23.9 215.3 248.7 15.1 4 3.92 37.93
EO-01-04 17.2 193.8 269.7 180 2 1.68 3.13
EO-01-02 13.5 32 48.5 23.4 4 3.82 16.33
EO-01-01 9 354.8 56.1 117 3 2.11 2.26
Bayan Ulan
BU-02-01-13 48 20.2 17.8 16.2 5 4.83 23.33
BU-02-01-11 46.5 315.7 54.8 10.3 5 4.93 55.86
BU-02-01-10 46 126.6 58.8 41.9 4 3.48 5.77
BU-02-01-09 44 303.4 37.2 43.3 3 2.78 9.19
BU-02-02-01 42.5 1.3 59.1 8.3 5 4.95 86.97
BU-02-01-08 39.5 5.5 59.7 23.4 5 4.66 11.68
BU-02-01-07 34.5 23.5 72.6 16.7 5 4.82 21.90
BU-02-01-03 27 165.7 234.3 19.9 4 3.87 22.29
BU-02-01-04 25 179.6 229.1 180 2 1.77 4.32
BU-02-01-02 22 185.4 258.9 31.1 4 3.69 9.72
BU-02-01-01 15 195.3 264 17.6 5 4.80 19.83
BU-01-01-06 13 228 251.2 48 2 1.97 29.17
BU-01-01-04 8 234.5 23.3 13.4 5 4.88 33.62
BU-01-01-03 5 256.1 270.8 12.3 3 2.98 101.63
BU-01-01-02 3.2 219.8 254.1 33.9 5 4.34 6.06
BU-01-01-01 1.5 168.2 239 8.8 4 3.97 111.08
Huheboerhe
HH-02-01-10 53.8 191.1 215.4 40.4 3 2.81 10.36
HH-02-01-08 44.8 212 222.6 18.4 4 3.88 25.94
HH-02-01-05 40.3 181.4 223.4 21.9 5 4.70 13.17
HH-02-01-04 36.8 42.9 19.7 18.4 5 4.78 18.28
HH-02-01-03 35.3 202.1 230.1 26.7 4 3.77 12.82
HH-02-01-02 32.2 200.3 228.1 19.1 5 4.76 17.01
HH-02-01-01 30.3 167.2 218.6 13.3 5 4.88 34.04
aSites are classed as alpha (bold), beta (italics), or poorly clustered (normal font) according to data quality as
describedinthe text.
bSiteaveragedeclination(D)andinclination(I).
sites clustered at 95% confidence, according coherent polarity intervals in all three sec-
Watson’s test for randomness (Watson, tions (fig. 6). At Erden Obo, two polarity
1956). Site mean directions for normal and transitions are preserved.A basalnormalpo-
reversed sites are roughly antipodal (D 5 larity zone is indicated by data from a single
18.88, I 5 57.38, N 5 11, k 5 5.551, and D alpha site, but supported by data from two
5 202.48, I 5 241.78, N 5 17, k 5 8.108, other sites with poor clustering that suggest
respectively; fig. 5C). When plotted strati- thiszoneencompassesatleastthe9-to16.5-
graphically, the siteaveragedirectionsdefine mlevelsofoursection.Sitesfromthemiddle