Geology of an amber locality in the Hukawng Valley, Northern Myanmar

15 Pages·2003·0.68 MB·English

JournalofAsianEarthSciences21(2003)441–455
www.elsevier.com/locate/jseaes
Geology of an amber locality in the Hukawng Valley, Northern Myanmar
R.D. Cruickshanka,*, Ko Kob
aLeewardTigerLimited,#34,101Street,MTNT,Yangon,Burma
b8(A)MyaThiriLane,A1Compound,81/2Mile,PyayRoad,Yangon,Burma
Received16November2001;revised19April2002;accepted23April2002
Abstract
Amber(‘Burmite’)fromtheHukawngValleyofMyanmarhasbeenknownsinceatleastthe1stcenturyAD.Itiscurrentlybeingproduced
fromahillknownasNoijeBum,whichwasfirstdocumentedasasourceofamberin1836.
Severalgeologistsvisitedthelocalitybetween1892and1930.AllofthembelievedthatthehostrockstotheamberareTertiary(mostsaid
Eocene) in age, and this conclusion has been widely quoted in the literature. However, recent work indicates a Cretaceous age. Insect
inclusionsinamberareconsideredtobeTuronian–Cenomanian,andaspecimenoftheammoniteMortoniceras(ofMiddle-UpperAlbian
age)wasdiscoveredduringtheauthors’visit.Palynomorphsinsamplescollectedbytheauthorssuggestthattheamber-bearinghorizonis
Upper Albian to Lower Cenomanian. The preponderance of the evidence suggests that both rocks and amber are most probably Upper
Albian.Thisdeterminationissignificantforthestudyofinsectevolution,indicatingthattheoldestknowndefinitiveantshavebeenidentified
inthisamber[AmericanMuseumNovitates3361(2002)72].
ThissiteoccurswithintheHukawngBasin,whichiscomprisedoffoldedsedimentary(^volcanic)rocksofCretaceousandCenozoicage.
Themineexposesavarietyofclasticsedimentaryrocks,withthinlimestonebeds,andabundantcarbonaceousmaterial.Thesedimentswere
depositedinanearshoremarineenvironment,suchasabayorestuary.
Amber is found in a fine clastic facies, principally as disk shaped clasts, oriented parallel to bedding. A minority occurs as runnels
(stalactiteshaped),withconcentriclayeringcausedbyrecurringflowsofresin.
AnUpperAlbianageissimilartothatofOrbitolinalimestonesknownfromanumberoflocationsinnorthernMyanmar.Oneofthese,at
NamSakhaw,90kmSWofNoijeBum,hasalsobeenasourceofamber.
q2002PublishedbyElsevierScienceLtd.
Keywords:Hukawngvalley;SouthwestofMaingkwan;Lalawngvillage
1. Introduction earlier field geologists ascribed an Eocene age to the host
sediments, while insect inclusions in amber appear to be
Cretaceous.
Amber (Burmite) from the Hukawng Valley of
The authors of this work spent two days (April 29 and
northern Myanmar appears in most inventories of world
30, 2001) inspecting the current mining area. The
amber deposits, but there are few firsthand descriptions
objectives were to verify the source of the amber, and
of the production locality. There has been a recent
to obtain information on the geology and age of the host
resurgence of interest, with papers by Tin (1999),
rocks.
Zherikhin and Ross (2000), Levinson (2001) and
The Hukawng Valley is situated in Kachin State,
Grimaldi et al. (2002). However, none of these authors
northern Myanmar (Fig. 1). The principal town is Tanai,
visited the site in person, and the most recent account of
situatedonthe‘LedoRoad’(constructedduringWorldWar
a field visit is by Chhibber (1934). Zherikhin and Ross II).Thevalleyisaflatalluvialplainmeasuringabout80km
(2000) note an important geological problem, in that north–south by 50km east–west, surrounded on all sides
by hills. The amber mine occurs on the shoulder of a hill
* Corresponding author. Address: c/o J.W. Davis, Leeward Capital known as Noije Bum (‘Banyan Mountain’ in the Jingpaw
Corporation,#4,1922-9Ave.SE,Calgary,Alta.,CanadaT2G0V2.Tel.:
language),about20kmsouthwestofTanai(Fig.2).Thisis
þ1-95-1-200109;fax:þ1-95-1-252478.
thefirsthilltoriseabovetheplaininthatdirection,havinga
E-mail address: [email protected] (R.D. Cruickshank),
sofi[email protected](R.D.Cruickshank). reliefof about 250m.
1367-9120/03/$-seefrontmatterq2002PublishedbyElsevierScienceLtd.
PII: S1367-9120(02)00044-5
442 R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455
Fig.1.LocationoftheHukawngvalley.Thetraditionalphysiographic/geologicdivisionsofMyanmar,andtheSagaingtranscurrentfaultwithitssplays,are
alsoshown(seetextfordescriptions).
2. Historyof amber mininginthe HukawngValley by Fraquet (1987)). The first European to visit the amber
localitiesintheHukawngValleywasCapt.Hannayin1836,
2.1. Historyprior to1995 who returned accompanied by Griffith, in 1837. Griffith
described the location asarange oflowhills,southwest of
Adetailedhistoryisbeyondthescopeofthispaper,and Meinkhoon (probably Maingkwan, Fig. 2). The site they
the reader is referred to Zherikhin and Ross (2000) for an describeismostprobablythehillnowknownasNoijeBum.
excellent account. The following summary is taken partly Dr Noetling of the Geological Survey of India was the
from their work. For studies of the fossil inclusions in this first geologist to visit the area (in 1891–1892, Noetling,
amber, prior to2000, see Ross and York (2000). 1893). Some of his samples were examined by Otto Helm,
Ancient Chinese sources indicate that the Hukawng who considered the amber to be a new mineral species,
Valley of northern Myanmar has been a source of amber which he named ‘Burmite’. Noetling considered the host
sinceatleastthe1stcenturyAD(Laufer,1906,summarised rocks to be Miocene in age, because of lithological
R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455 443
Fig.2.LocalitiesintheHukawngValley.ComparisonwitholdreportsindicatesthattheNoijeBumhillrangehasbeenthesourceofHukawngValleyamber
foratleastthepast165years(seetextfordiscussion).
similarities with known Miocene formations (during this The best known description of the Hukawng Valley
excursionhefoundaloosepebblecontaininganammonite, amber mines is that of Chhibber (1934), based on an
but perhaps elsewhere in the Hukawng Valley). He inspection he made in 1930. He listed twelve production
describedtheamberlocationasalowhillrange,southwest sites, ten of which were near the northern end of Noije
of Maingkwan (and therefore likely in the vicinity of the Bum, with two others about 8km to the west. Khanjamaw
current mine). He was told that the principal mining area (Fig. 2), the principal mining site at that time, is now
wasatthesouthendoftherangenearLalawngvillage(Fig. overgrown with jungle. The Noijemaw site was ‘west of
2),buthedid notgo there. Noije Bum’ and may be near the current mine. He
Stuart (1923)wasthefirsttoproposeanEocene agefor reported that amber was produced from wells about one
the amber-bearing sediments. On the eastern flank of the metresquare,andupto15mdeep,notingthatamberfrom
NoijeBumhillrange,heobservedpitsdugforflintinathin shallower levels was of inferior quality. These diggings
limestonelayer.Inthespoilpileshefoundasinglepieceof and nearby stream sections exposed a sequence of
limestone containing ‘numerous specimens of Nummulites carbonaceous sandstones and shales, with minor limestone
biarritzensis’,whichherecognisedasbeingEoceneinage. and conglomerate beds. Amber was associated with very
His map and description show that amber was not known thin coal seams. Chhibber (1934) found Nummulites
fromthatlocation,butratherfromblueclay‘onthewestern fossils in situ in a stream exposure (perhaps on the east
portion of the hills’. He concluded that the amber-bearing flank of Noije Bum?), so concurred with the Eocene age
horizon underlies the Nummulites beds, but nonetheless proposed by Stuart (1923). However he apparently did not
forms partof anEocene succession. observe amber at that location.
444 R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455
SahniandSastri(1957)describeOrbitolinatheyfoundin (1934),andareoutdated.Forexample, hestatesthat‘from
samples collected by Stuart, from ‘Amber mines 268150N, the latter half of February, the local people who have then
968250E’(afterStuart(1923)).ExaminationofStuart(1923) gathered in their harvest, flock to the amber minesin great
shows that the longitude is an error resulting from the numbers’; this line is from Chhibber (1934), and is no
inaccuracy of his map, and the rocks must have been longer correct. In reality, the mining company has been
collected from somewhere on the Noije Bum range. Sahni granted exclusive rights to the area by the Ministry of
and Sastri (1957) concluded that these orbitolines were a Mines, and no one else is working there. Tin (1999)
previously undescribed species, which they named Orbito- accepted an Eocene age for these deposits.
linahukawngensis,andassigned‘aCenomanian,and,inany In their otherwise excellent review, Zherikhin and Ross
casenotolderthanAptian’age.TheycreditEamesasbeing (2000),incorrectlystatethataccesstothisareaisdifficultas
thefirsttodescribeOrbitolinafromthearea,buthebelieved it ‘remains controlled rather by the local clans and
that they were contained in derived clasts within Eocene insurrectionststhanbythecentralgovernmentinRangoon’.
sediments. In contrast, Sahni and Sastri (1957) concluded However, a peace agreement between the Myanmar
that a Cretaceous sequence with orbitolines occurs below government and the Kachin Independence Organisation
the Tertiary rocks of the Hukawng Valley. Despite this, (K.I.O.) came into effect in 1993. As a result, the national
subsequent authors continued to accept the Eocene age of governmentnowcontrolstheregion,incooperationwiththe
Chhibber (1934), and to overlook the possibility that the K.I.O.
rocks may beCretaceous. Subsequent to the 1993 peace agreement, mining
Zherikhin and Ross (2000) summarise studies of operations were undertaken in the period 1995–1997.
Myanmar amber from the collection of the British Natural This enterprise failed because the producers were unable
HistoryMuseum.Theynotetheidentificationof‘fiveinsect to locate reliable markets.
families or subfamilies that are not known later than In August 1999, the authors met some of the former
Cretaceous elsewhere’, although they do not rule out the miners,andpurchasedasmallquantityofamber,whichwas
possibility that the amber could be Tertiary. They suggest sent to Davis in Canada. He noted the occurrence of
thatifthesedimentsareEocene,thenCretaceousamberwas microscopicinsectswithinit,andforwardedthematerialto
recycledandredepositedinthem.Asevidence,theynotethe Dr Grimaldi in New York. Another local company
occurrence of rounded pits on the surface of some amber recommenced mining in 2000, after which more amber
pieces, which may have resulted from impacts during wasobtainedandsenttoDrGrimaldi.Inthesetwobatches
transport. of amber, he and his co-workers have found hundreds of
The Geological Survey of India reported amber pro- insect inclusions, and propose that they are Turonian–
ductionfortheyears1898throughto1940.Averageannual Cenomanian(Grimaldiet al., 2002).
production was about 1900kg, with a maximum of nearly Levinson (2001) briefly reports on the renewed com-
11,000kg in 1906. Recorded production stopped about mercial availability ofMyanmar amber.
1941.Thethrivingtradeinamber,andthemanufacturingof
jewellery have by now entirely disappeared, and the skills
havebeenlost.ThevillageofMaingkwan,reportedtobea 3.Regional geology
centre of the amber trade by Chhibber (1934), was
abandoned in 1967 when most of its population moved to 3.1.Synopsis of the geology ofMyanmar
the new townof Tanai.
Another amber locality in northern Myanmar is of Myanmarcanbedividedintofournorth–southtrending
interest. Ngaw (1964) reported that amber was produced physiographic regions, which have traditionally been
between 1948 and 1961, from a site near the Nam Sakhaw utilised for geological description as well. However there
stream, 90km southwest of Noije Bum (Fig. 3). This is no consensus on standard names for these belts. The
occurrencewasalsoknownincolonialtimes,asthenotation following summary (based on Bender (1983)) employs the
‘amber mines’ appears on the old 83-O topographic map. nomenclature shownon Fig. 1:
The amber is hosted by Cretaceous carbonaceous lime-
stones,bearingOrbitolina;suggestingasimilaragetoNoije 1. TheRakhineCoastalPlainisunderlainbydeformedLate
Bum (refer toSection5). Tertiary molasse sediments overlying Eocene to mid
Miocene flyschoid rocks, with local mafic to intermedi-
2.2. Historysince 1995 ate dykes and plugs.
2. TheWesternRangesconsistprincipallyofearlyTertiary
Theauthorsfeelobligedtocorrecterrorsintworecently flysch,deformedintoimbricatethrustzones.Theeastern
published works. marginoftherangesisunderlainbyTriassicturbidities,
Tin (1999) describes the history, mining methods, Cretaceous and Tertiary sedimentary rocks, meta-
geology, and other factors relating to Hukawng Valley morphic rocks, and ultramafic rocks (dismembered
amber. His descriptions are based largely on Chhibber ophiolites).
R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455 445
Fig.3.CretaceousgeologyofnorthernMyanmar.Otherunitsandlocalitiesmentionedinthetextarealsoshown.BasedonBender(1983),Mitchell(1993)and
ESCAP(1996),andothersources.
3. The Central Province comprises a series of Cenozoic Phanerozoic. Metamorphic, volcanic, and intrusive
sedimentary basins, and intervening uplift areas. Sedi- lithologies also appear, especially along the western
mentary fill of Eocene, Oligocene, and Miocene– margin (theMogok Belt).
Quaternary clastic rocks is underlain by Cretaceous,
and probably also older units. Basinal rocks are folded Mitchelletal.(2000)considerthatMyanmarconsistsof
and faulted. Uplift areas consist of older sediments and threegeologicalprovinces:(1)‘aWesternProvinceofmica
crystallinerocks.Thebeltisbisectedlongitudinallybya schists and overlying predominantly oceanic rocks’ (the
discontinuous line of Mesozoic and Cenozoic igneous Rakhine Coastal Plain, Western Ranges, and Central
rocks (in part, the ‘Inner Volcanic Arc’). Province); (2) the Mogok metamorphic belt, of marble,
4. The Eastern Province is underlainby sedimentaryrocks gneiss, and granitoids (the western margin of the Eastern
representing a broad interval of geological time, from Province); and (3) the Phanerozoic ‘Shan–Thai’ block to
at least latest Proterozoic, through much of the the east (bulk of the Eastern Province) (Fig. 3). They
446 R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455
postulate two separate orogenies during the Mesozoic: (1) northern continuation near the Ayeyawady confluence at
collisionofawestern,continental,blockwithanislandarc Myitson has been confirmed by the present authors. Chit
totheeastinearlyJurassic;and(2)collisionoftheresulting (2000)concludesthatlimestonesfromthisbelt(Taungbwet
complex with the Shan–Thai continentinmid-Cretaceous. Taung Formation) represent a shallow lagoonal facies, and
After the mid-Cretaceous orogeny, eastward subduction areAlbiantoCenomanianinage.Theseunitsareassociated
of oceanic crust continued (Mitchell et al., 2000). The with chert, basalt, and slate, tightly folded, and appear to
Cenozoic sedimentary sub-basins of the Central Province overlieophioliticultramaficrocks.Clegg(1941)considered
werefilledanddeformed.Thecurrentgeologicalsettingof that ‘in every locality where Cretaceous sediments are
Myanmarreflectsright-lateraldisplacementontheSagaing exposed,peridotites,orserpentinestheiralterationproduct,
Fault and its splays in northern Myanmar (Figs. 1 and 3). areinvariablyfoundwhilstdoleritesandvariouspyroclastic
TotaldisplacementonthefaultsinceearlyMiocenehasbeen rocks also occur’.
estimated by various workers to be from more than 300 to Discussing similar Orbitolina-bearing limestones near
460km,withatotalnorthwardmovementonthewestside Banmauk (Fig. 3),Chit (2000) concludes that there was an
ofperhaps1100kmsincelateCretaceous(Mitchell,1993). abrupt change from lagoonal to shallow marine facies in
Myanmar currently consists oftheAsia platetotheeast Cenomaniantime.
of the Sagaing Fault, and the Burma plate to the west. The Occurrences of Cretaceous limestones also occur in the
Indian plate is colliding with Asia to the north, and jade mines region and near Mt. Loi Mye (45km south of
subducting beneath the Burma plate to the east (Mitchell, Noije Bum, Fig. 3). The northernmost portion of the large
1993). Northward translation of the Burma plate is ultramaficophiolitebodyofthejademinesarea,andabody
continuing, as evidenced by recurrent seismicity on the ofgabbro,occurthere(Chhibber,1934),andvolcanicrocks
SagaingFault(Win, 1981). are also present.
The amber and Orbitolina-bearing limestones at Nam
3.2. Cretaceous geologyofnorthern Myanmar Sakhaw lie on the western margin of this district, where
Clegg (1941) observed associated calcareous sandstone,
SedimentaryrocksthathosttheHukawngValleyamber shale,andvolcanicrocks.AsattheAyeyawadydefiles,the
are now considered to be Cretaceous (refer to Section 5). carbonate units there are conspicuous: he recalled that the
Therefore the Cretaceous geology of northern Myanmar sheer cliff of Hpalamung Bum, 275m high, ‘when seen
willbebrieflyreviewedinmoredetail,andshownonFig.3. looming through the early morning mist from the low
Ingeneral,Cretaceousmarinesedimentaryrocksbecome ground tothe south is amostimpressive sight’.
progressivelyyoungerfromeasttowest,althoughoverlaps Jadeite-bearingultramaficrocksoccurinwesternKachin
occur. A skirt of Lower Cretaceous sedimentary rocks State. A longer belt of ultramafites is exposed along the
occurs along the western margin of the Eastern Province; Ayeyawady River to the east (Fig. 3), and they also
includingtheTithoniantoAptianPyinyaungBudaBedsof characterise the eastern margin of the Western Ranges.
Mitchell et al. (2000), and the Pan Laung Formation These are usually interpreted as dismembered ophiolites,
(describedasNecomianbyChit(2000);andmidJurassicto although a complete ophiolite succession has not been
midCretaceousbyMyint(2000)).Furtherwest,Albianand/ described. Mitchell et al. (2000) state that they were
or Cenomanian limestones, bearing several species of emplacedasnappesduringalowerJurassicorogeny.Maung
Orbitolina, occur in the Central Province, and the eastern (2000) believes that the jade mines bodies are Cretaceous,
partoftheWesternRanges.UpperCretaceous(Campanian whilethoseintheWesternRangesareTriassic.Incontrast,
to Maastrichtian) units are present along the western Hla (2000) believes that Western Ranges ophiolites were
boundaryoftheCentralProvince,andwidelyintheeastern emplaced as late as the Cretaceous, while those in the
part ofthe Western Ranges (e.g. Gramann,1974). CentralProvincemaybeCretaceoustoEocene.Thereforea
Limestones carrying an Orbitolina fauna have been consensus onthe age oftheseunits has notbeen achieved.
reportedfromvariouslocationsinnorthernMyanmar.They The majority of the granitoid bodies indicated on Fig. 3
occurinabeltfromsouthofBhamotonorthofMyitkyina, are described by Mitchell (1993) as being Late Cretaceous
inthevicinityofBanmauk,inthejademinesregion,inthe toearlyEocene.Morerecently,Barleyetal.(2000)reported
upper Chindwin area, and in parts of the Western Ranges agesof120–80MaforItypegranitoidsintheMogokBelt
(Fig. 3). Chit (2000) states that the rocks are Albian to and Western Myanmar. They recognise an up to 200km
Cenomanianinage.AccordingtoMitchelletal.(2000),the widemidCretaceousmagmaticbeltthatextendedalongthe
limestonesweredepositedinfrontofnappesresultingfrom entire continentalmargin fromTibet toSumatra.
an Aptian-midCretaceous orogeny. TheCretaceoussedimentaryrocksdescribedabovewere
Clegg (1941) described Orbitolina limestones from the depositedpriortoright-lateraldisplacementsontheSagaing
defilesoftheAyeyawady(northandwestofBhamo,Fig.3). Fault and its splays. If restored to their original position,
Henotestheoccurrenceofcalcareousgrits,sandstones,and they would form a narrower zone than at present, arrayed
shales; and of limestones bearing both foraminifera and along the former continental margin (western edge of the
large‘molluscs’(probablygastropods).Theoccurrenceofa Eastern Province). Maung (2000) concludes that the mid
R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455 447
Cretaceoussedimentswere laiddownbetweenamagmatic 4. Noije bum amber mine
arc to the east, and a trench basin to the west. Sahni and
Sastri (1957) note that a discontinuous belt of Orbitolina 4.1. Mining operation
limestones extends from Myanmar, across Tibet, to
Kashmir, northernmost Pakistan, and northernIran. The site resembles a small open pit mine, with all
excavation by manual methods. A work force of about 60
men was present during the time of the visit. They had
3.3. Tectonic setting ofthe Hukawng Basin
stripped overburden from an area measuring about
120£30m2 (Figs. 4–6), and were producing amber from
Bender(1983)considerstheHukawngBasintobeoneof
the unweathered rocks thus exposed. The site straddles a
the constituent sub-basins of the Central Province (his
ridge,withtheslopeonthenorthsideaveragingabout138.
‘Inner Burman Tertiary Basin’). He postulates that low
Deep shafts, as described by Chhibber (1934), are not
grade metamorphic rocks exposed to the south may
required, probably because on this steeper slope, the
represent basement, and states that aeromagnetic surveys
weathered layer is thinner. The current mining method
suggest5000mof overlying sediments.
producesgood,easilyaccessibleexposures,andtheauthors
Sedimentary (and lesser volcanic) rocks underlie hills
have probably had a more extensive view of unweathered
that surround the central alluvial plain of the Hukawng
bedrock than anyof the earlier workers.
Valley, with bedding trends and fold axes parallel to the
basin margins. The map of ESCAP (1996) shows the
4.2. Lithology andsedimentology
Hukawng Valley with areas of Eocene rocks around
the west, north, and east sides, and Miocene rocks on its
A variety of clastic sedimentary rocks, with thin
southwest and southeast margins. However, the amber-
limestone beds, and abundant coaly and carbonaceous
bearing sediments are Cretaceous, and not Eocene as
material, was recognised at the site. Chhibber (1934)
previously believed (this paper). In addition to the Noije
describes the rocks as being blue in colour, but in the
Bum area, Chhibber (1934) reported an amber locality on
authors’ opinion they are more nearly medium green,
the eastern margin of the valley, suggesting that a
greyish green, or rarely blue–green. Weathered rocks are
Cretaceous sequence may occur there as well. The authors
mainly tan brown with some shales being reddish. They
believe that more of the ‘Tertiary’ units may in fact be
have been subdivided into four or five units, as shown on
Cretaceous. These areasare indicatedon Fig. 3.
Fig. 4, andbriefly described below:
TheinterpretationofBender(1983),Fig.22)indicatesa
Thefineclasticfaciesconsistsoffineorveryfine-grained
NNE-plunging anticline at Noije Bum. He states that
sandstone(grainsusually0.1mmorless),withbedsoffiner
Cenomanian limestone occurs in the crest of this fold and
clastics (silt, shale), interbeds of grey micritic limestone a
anothertothewest,butdoesnotsayhowhearrivedatthis few centimetres thick, and coal laminations usually about
conclusion. He interprets the remaining rocks, including 1–2mm thick. The coal horizons, although thin, are
thosethathosttheamber,tobeofearlyTertiaryage.Alarge laterally persistent, and carbonaceous material is abundant
package offolded rocks occurs to the northwest, west, and inthisunit.Thisfaciesisalwaysthinbeddedorlaminated,
south of the amber locality on his map. and even parallel lamination is the predominant internal
AlongthenorthernmarginoftheHukawngBasin,andin sedimentary structure. The unit is usually about 1m thick.
the Western Ranges beyond, fold axes and bedding trends Theamber is associated with this facies.
have turned to an east–west orientation. Studies of Limestone beds, about 6–8cm thick, occur within the
landforms by Mitchell et al. (1978) and Bender (1983), fine clastic facies. This rock is medium grey in colour,
andthepresentauthors,suggestthatthenorthernboundary micritic, and typically of massive appearance. It often
of the basin may be a north vergent thrust fault exposed contains fine fragments or strands of coalified plant
along Gedu Hka (river). South of the river there is a material. Rounded coarse sand or granule-sized clasts are
remarkablecuesta-shapedridge,about60kmlong,withthe sometimes present at the base.
scarp face on the north side. The eastern end of this thrust Themediumclasticfaciesconsistslargelyofsandstone,
appearstobeconnectedtoasplayoftheSagaingFault(Fig. with grain sizes usually 0.4mm or less (fine to medium
3). Stuart (1923) reported a Cretaceous–Eocene unconfor- sand).Itoftenassumesa‘saltandpepper’appearanceunder
mity where his traverse passed Gedu Hka; he mentions no the hand lens. As mapped on Fig. 4, it is a somewhat
fault but his observation of serpentinite bodies below the heterogeneous unit,containing beds ofsiltstone,shale, and
contact suggests that one mustbe present. conglomeratethataretoothintobeshownseparately.Shale
DeformationoftheHukawngBasinmostprobablyresulted chips are sometimes observed within the sandstone.
from the continuing collision of India with Asia, and its Coalified plant fragments occur on bedding surfaces. The
subductionbeneaththeBurmaplate(initiatedinlatestEocene unit most commonly displays massive bedding or even
time). The Himalayan boundary is marked by southwest parallel lamination, but tabular cross beds were observed
vergentthruststothenortheastofthevalley(Fig.3). locally. Beds are usually 10–80cm thick. Locally,
448 R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455
Fig.4.GeologymapoftheNoijeBumambermine,asitappearedonApril29–30,2001.Seetextfordescriptions.
calcareousconcretionsarepresent,consistingoflimestone, micritefoundinthefineclasticfacies.Italsocarriespebbles
chert,andotherrocks,armouredbyprecipitatedcalcite.Fig. ofchert,mudstone,serpentinite,andvolcanicrocks,butno
7 shows an unusually thin-bedded (4–8cm) sandstone quartz, no plutonic, and few if any metamorphic clasts. In
sequence. someexamplesitisamatrix-richgrit,andinothersitisclast
Thin sections of medium sandstone indicate that lithic supported. Sorting tends to be poor, but the clasts are
clasts predominate, with lesser quantities of feldspar and rounded. The authors observed numerous small broken
quartz.Lithicclastsincludechert,andesite,basalt,quartzite, bivalve shells and a gastropod inthisbed.
micritic limestone, and serpentinite, with actinolite schist Rocks of the ‘channel facies’ occur in the southwest
notedinonespecimen.Plagioclaseoccursin0.25–0.4mm corner of the area, demonstrating distinctive sedimentary
grains, and quartz clasts are about 0.2mm in dimension. structures. Beds, about 75–125cm thick, noticeably fine
The texture is immature, being poorly sorted, with only upwards. Coarser portions of the beds are either massive
incipient rounding of clasts. The cement is coarsely bedded or display tabular cross beds, and finer ones are
crystalline calcite, and the rock effervesces vigorously in laminated.Troughcrossbeddingwasalsonoted.Lenticular
dilute hydrochloric acid. This is a calcareous lithic beds occur as medium sand lenses within much finer
sandstone. material. Channel scours are common, with layering in
Aconglomeratehorizonwasnotedinseveralexposures. underlying beds decisively truncated. These sediments are
Clast size generally decreases from south to north, ranging also carbonaceous, and coaly plant fragments on bedding
fromcobblesnearthefootpath,togranulesnearthenorthern surfaces are ubiquitous.
endoftheoutcrop.Thebedistypically1–2minthickness.
A thin conglomerate bed was also observed in the north- 4.3.Amber
ernmostpit,andlensesappearinthemediumclasticfacies
south of the footpath. The conglomerate carries clasts of a Amber is found within a narrow horizon in the fine
distinctive pale buff, pale grey, or pale green saccharoidal clasticfacies.However,oftwosuchbeds shownonFig.4,
limestone, carrying traces of pyrite, quite unlike the grey onlyoneproduced amber.
R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455 449
Fig.5.Aviewoftheopenpit.Minersintheforegroundareexcavatingthe Fig.6.Testshaft.Thedarkcolouredrocksatbottomaretheamberhorizon.
amberseam(darkcolouredrock).SeeFig.4forlocation. LocationshownonFig.4.
Mostamberoccursasdiscoidclasts,withawiderangein The amber is typically reddish brown in colour, with
sizes. These are thickest in the centre, tapering to rounded various shades of yellow, orange, and red also occurring.
edges.Thediametertothicknessratioofthedisksisusually These colours range from pale to dark, and it can vary
intherange2.4:1–3.0:1,withrarerflatexamplesupto5:1. from perfectly transparent, through translucent, to
Sizes range from small chips a few millimetres in opaque. Inclusions of organic matter (vegetation), are
dimension, to others several centimetres in diameter. They
common, but not always present. Insect fossils, which are
arenotperfectlysymmetrical,andirregularitiesareusually
mostly microscopic, occur at about 46 per kilogram of
present.Pittedsurfaces,asdescribedbyZherikhinandRoss
the current product (Grimaldi et al., 2000).
(2000)arenotubiquitous.Thedisksareorientedparallelto
Thinwhitecalciteveinlets,usually1mmorless,butup
bedding(Fig.8).
to4or5mminwidth,arecommonlyobservedintheamber.
A minor proportion of the amber occurs as runnels,
Their density varies considerably, with some examples
resembling small stalactites, with round cross-sections,
beingnearly freeofthem, andothers packedwith veinlets.
perhaps1cmindiameter,butsometimeslarger.Theseoften
They are a major factor in determining gem quality, and
showconcentriclayering,probablyresultingfromrepeated
manypieces are ruinedby their presence.
flows of resin. The shape of these ambers appears not to
Amongtheamberproducedbytheminersisoneexample
havebeenmodifiedbytransportation,exceptthattheywere
whichhasabivalveshellembeddedinitssurface.Thevalve
brokenintoshorterlengths.Fossilinsectsaremorecommon
measures18mmlong,and13mmwide.Itisorientedwith
in runnels than in disk-shaped amber clasts. Ross (1998)
explains the origin of this phenomenon: “The resin is the convex side embedded to its full height of 6mm in a
exudedasblobsorstalactites,whichdripandflowdownthe pieceofamber thatis50mminmaximumdimension. The
trunkofthetree.Often,asitexudes,insectsbecometrapped concave side of the shell faces away from the amber and
and engulfed in the sticky material. The resin eventually carriessandstonematrix.Itappearstohavebeenembedded
falls tothe ground and… fossilisesinto amber.” in the amber while the latter was in aplasticcondition.
450 R.D.Cruickshank,K.Ko/JournalofAsianEarthSciences21(2003)441–455
Fig.7.Aviewofthethinbeddednature,anddipoftherocks.LocatedonFig.4.
4.4. Structural geology no great displacement. Its attitude is 1688/608 NE.
Bedding has been contorted where it intersects the fine
Rocksatthissiteareorientedrightwayup,asevidenced clastic unit. The other fine clastic bed (near point ‘A’,
by channelscours, gradedbeds, andcrossbeds. Fig. 4) also exhibits contorted bedding and slickensides.
North of the ridge crest, bedding attitudes are quite Thin calcite veins occur not only in amber, but also
uniform, with NNE strikes, and dips of 50–708 to the E or within the sedimentary host rocks. Joints and fractures
SE(Fig.4).Southofthecrest,thestrikesturntotheSSEor hostingtheveinswouldhaveopenedafterconsolidationof
SE,anddipsflattentothe35–608range.Thissuggeststhat the rocks, in response to deformational or lithostatic
the site is on the northwest limb of a northeast-plunging stresses. Perhaps the brittle nature of the amber was
syncline.Chhibber(1934)reportedthatrocksinthisregion responsible for agreaterdensity offractures.
exhibit ‘tightly compressed anticlinal and synclinal folds’.
The relationship of this fold to the large anticline at Noije 4.5.Paleontology
Bum, interpreted by Bender (1983), was notdetermined.
A minor fault was noted in the central part of the site Severalmacrofossilswerelocatedbytheworkmeninthe
(Fig. 4). It has a conspicuous gouge zone, but apparently course of mining, and others were recovered during this
Fig.8.Amberinmatrix,withinthefine-grained,laminatedfacies.Thelargestamberdisk,indicatedbythepencil,measures27£10mm.Twosmallerpieces
appeartotheright.Notetheovalcross-sectionsofthediscoidamberclasts,andtheirorientationparalleltothelamination.Collectedfromthepitshown
inFig.6.