Reconnaissance rnagnetostratigraphy of the Precambrian- Cambrian boundary section at Meishucun, South west China

Transcription

1 Recnnaissance rnagnetstratigraphy f the Precambrian- Cambrian bundary sectin at Meishucun, Suth west China F. WU (1); R. VA DR V (1) and Q. 1 LIAG (2) (1) Department f Gelg cal Sc ences. The Un vershy fm ch gan. Ann AMr ML , USA (2) Yunnan Inst tute f Gelgical Sciences Kunm ng, Yunnan, Peple s Republ c f China ABSTRACT A sequence f platfrm carbnates ant! pitspitrites in sutitwest China, near Kunming, cntains tite Precambrian-Cambrian bundary and is well studied in terms f lititlgies and faunal cntent. Tite sequence, called tite Meishucun sectin, itas been prpsed as a glbal strattype fn tite bundary. In titis paper we reprt preliminary palemagnetie dala which reveal eigitt reversals. Tite data yield a paleple witich is unlike any paleples btained fr yunger Phanerzic rcks frm tite Suth China blck; titis ple falls at 68.8, , ant! is based n 57 5ampies (Decl./Incl. 4.2 li-7.1, k 9, a 9~ 6.6 ). A cmparisn f ur preliminary magnetstratigraphy with recrds btained frm Siberia, Australia ant! tite western USA shws dial alí sectins are citaracterized by frequent reversals, but tital detailed crrelatins are nt yet pssible. = = = ITRODUCTIO he Precambrian-Cambrian bundary is assciated witit tite first appearance f diverse shelly fssil assemblages ant! is titus a feature f primary imprtance in stratigrapity and earth histry. It is Lite subject f study by tite IUGS-IGCP Prject 29 Wrking Grup, in attempts t crrelate tite imprtant Precambrian-Cambrian sectins f Lite wrld, such as Litse in Siberia, Mnglia, Australia, ewfundland, tite western US and Canada, China and Mrcc (Cwie, 1984). In China, where tite Late Precambrian cmprises Lite Sinian system, an apparently cmplete and fssiliferus Sinian L Lwer Cambrian setin exists in Yunnan prvince. Abut 45 km suth-sutbwest f Kun-

2 . Wu, R. Van Der V and Q. 7. Liang 206 ming, t tite suth f Dianchi Lake, tite large Kunyang pitspitnite mine in Jinning Cunty expses a cmplete sequence ven a wide area (Hg. 1). Titis sequence is called tite Meishucun sectin (pnnunced May sh- Xiawa~Iushan Gadawan 5 4Dm C C,y 1y e,y 21A 2 e1q e, c less loo ver ISOm Fig. 1. Gelgical sketch map f the Kunyang mine arca (extending mre r the whle map area) in X unnan prvince, and schcmatic crss sectin f the sampled sectin (tp diagram: lwer Meishucun sectin, httm: upper Meishucun sectin; nly the lwer sectin yielded palemagnctic results). te the vertical exaggeratin f these crss-sectins. The inset map prvides the lcatin in suthwest China. The frmatin symbls are explaíned in the righthand clumn f Fig. 2. Figure adapted frm Lu Huilin et al. (1984).

3 Recnnaissance magnetsu atigraphy f the tsn), and we have btained preliminary buí prmising magnetstratigraphic data in a pilt study f tite uppermst Sinian and lwer Cambrian ( mmtian) frmatins. he Meisitucun sectin represenis a natinal strattype in China ant! has been prpsed as a Chinese candidate fr a glbal strattype sectin thai cntains ihe Precambrian-Cambnian bundary (Xing Yusiteng ant! Lu Huilin, 1984). hus tite imprtance f tite Meisitucun sectin lies in its significance fr glbal stratigraphic crrelatin f the first rder, even if it is eventually nt selected as a glbal strattype. A mngrapit has recently been publised (Lu Huilin et al,, 1984) with citapters devted t lithlgic descriptin, stratigraphic relatinships, palentlgy, istpie ages, and reginal l internatinal crnelatins. Despite titis wealtit f infrmatin nw accumulated abut tite Meishucun sectin, tite precise lcatin f tite Precambrian-Cambrian bundary is still a matter f sme dispute. Tite classical stratigraphic subdivisins f the sectin (Fig, 2) placed the bundany at tite base f the Xiawaitushan Member (e 1y3) f ihe Yuitucun Frmatin (psitin A in Fig. 2), but a secnd imprtaní fssil ccurrence (psitin B) near the tp f tite Zhngyicun Member (e 1y4) ftite Yuhucun Frmatin is currently preferred as marking tite Precambrian-Cambrian bundary, n tite basis f a vte by tite members f tite IUGS-IGCP Wrking Grup # 29. Tite reasns fr placing ihe bunt!ary higiten in tite sectin are partly based n istpic (Rb-Sr iscitrn) dating f tite verlying black shales frm the Badawan Member (e lql) f the Cambrian Qingzhusi Frmatin, witich gaye an age f ± 8.2 Ma, in additin L previusly repned ages f 587 ± 17, ± 15.2 ant! 588 ± 13 Ma fr similar strata in tite area Frmatin Qingzhusi Fm. Lwer Cambrian Member Symbl Yu anshan Me,,,. -~,q Badawan Mcm. Dahai Mcm. Zhngyicun Mcm, Yuhucun Fm. Upper Sinian 2 Xiawai[ushan Mcm, Bai>ranslia Meza, 4,y5 ~y,> 3 Z 2y Fig. 2 Late Precambrian (Sinian) t Cambrian stratigraphy f [he Meishucun area (frn-i Lu Huilin et al., 1984). Psitins A and B are pssible Precambnian-Cambrian bundary lcatins discussed iri lije tcxt.

4 . Wu, R. Van Der V and Q. 208 Z. Liang (Lu Huilin et al., 1984, p ). It is just abye this Badawan Member that the ldest trilbite in China (and penitaps the ldest in the wrld; Lu Huilin el al., 1984, p. 118) makes its appearance (Parahadiella yunnanens s). Hwever, the B psitin fr tite Precambnian-Cambnian bundary is underlain by the lwer Meishucun sectin (Zhngyicun ant! Xiawaitushan members, e ly3-4 in which several shelly fssil species are alreat!y abundant: titis Anabarites-Círctheca-Prthertz na (A-C-P) Zne f diverse shelly fssils wuld then be Precambrian, a chice which is smewhat dependent n what is preferred as the definitin f the Precambrian-Cambnian bundary. he tw citices fn the bundary (A and B) nw c-exist in the litenature ant! the figures in Lu Huilin et al. (1984) use ne n the then with little explanatin f tite unt!erlying natinale; see fn instance Fig. 3, which presents the mngraph s attempt at internatinal stratignaphic crrelatin f the Meishucun sectin with thse frm titer cuntnies. In this figure, the Precambrian-Cambnian bundary is placed at psitin A, belw tite A-C-P Zne. Our preliminary palemagnetic results frm tite Meishucun sectin suggest titat it may be pssible t establish an excellent neversal necrt! fn this sectin. bis wuld be f tite greatest imprtance fn a chice f Upper Simia Dengying ia an L~eerCambruan ~A B A. C.- P. JS. S P. S. U. 6%. Sa ~..~ Ruine <Li de Vi..) I Reed f)ímiíe (Árap Un0 105 Pu d Onarízile Map Unu II Uralauna Fa. Hrizn A mnsíetiar Stkiagbrd Fm. CLaispil Fm. Raudc,n Fu,. Map Unu, 12,6.13. ParachiIi a Fu,. M gi a 6umu]dar j3~~ laídabaaian) Tinulian [jeep Spriag Fm. Ubi a <Yun a ) SaIa,ygí m. Liukali Hartstili Fm. - Graná Bank Fm. Miguels Viet, Fm. Map Fu,. Upp r CLise catnes Y- S y 1. T,a,vf. Pe,írísve O lm. <Tummíiaa> valdai Gruup TaI win a, Ct arnian Síage > Qi,g h..si Fm. Tsagauulnm Fm P. P. vuhaena l~n,. Y d,a. 2Qiagchusian. ~rn~ S r em Meishucunias - Siberia, Pial frrn C M. urpea Fíatbern ~ Mruec U.. U.S.A. Prigu Fm. eufaadland ~ Sekwi Fa. Machenn a A[íOX Limeslune Australia Fig. 3 internatinal crrelatin f tije Precanibrian-Cambrian bundary sectin f varíus parts f thc wrld (frm Lu Huilin et al., 1984). Shelly fssil ines include: A.-C.-P., tije Annabarites-Circtheca-Prthertzina zne; P.-S. thc Paraglbrilus-Siphgnuchites zne; S.-., the Sinsachitesnvitatus zne; P., the Parabadiella (trilbite) zne; and. the redlichia zne. Trace fssil znes indude: 5., the Sellaulichnus meishucunensis zne; and P. 0w Plagigmus arcuatus zne.

5 Recnnaissance magnetstratigraphy fthe tite glbal strattype fr the bundary and fr internatinal crrelatins f seetins, because f tite glbal synchrneity f reversals ant! Lite pssibility L match «fingerprint»-type reversal recrds. Frm ur preliminary data, it appears that reversals are frequent ant! irregularly spaced, a prerequisite fr sucit crrelatin. GOLOGICAL STTIG AD PALOMAGTIC SAMPLIO Meishucun is lcated at 24.7, at the suthwestern margin f tite Yangtze Platfrm (5. China blck). Tite Sinian-Cambrian sequence cnsists f platfrm dlmites ant! pitspitrites in tite Baiyansha (Z2y2), Xiawaitushan (e ly3), Zhngyieun (e 1y4), ant! Dahai (e lys) members f tite Yuitucun Frmatin, and als in tite lwer part f the Hat!awan (e 1 ql) member f tite Qingzitusi Frmatin (Fig. 2). Tite verlying prtin f tite Qingzitusi Frmatin cnsists f sitales ant! silt- r sandstne. he strata dip gently t tite suth (abut 15 degrees) and are unmetamrpitsed. tite ttal tbickness f tite Yuitueun and Qingzhusi fnmatins is 332 m, f witich 139 m was sampled fr palemagnetie study. Parts nt sampled include tite lwermst 177 m f tite Yuitucun Frmatin ant! Lite uppermst 16 m f tite Qingzitusi Frmatin (in tite Yuanshan Member, e 1 q2). A ttal f 159 individually riented samples were cllected frm 112 hrizns, using a prtable gasline-pwered drilí and a Bruntn cmpass. Distances between the itrizns generally were n tite rder f 0.4 m in tite dlmites ant! phsphnites, witile in tite itighly fissile sitales f [he Badawan and Yuansitan members (e 1 ql -2) f Lite Qingzitusi Fnmatin, distances between itrizns ranged up t 18 m. We will cali the sampled dlmite-pitspitrite part f tite Yuhucun Frmatin (32 m titick) tite «lwer Meisitucun sectin»; this lwer sectin yielded 106 sampíes. Tite remaining part f tite sectin in tite Qingzitusi Frmatin (107 m thick) yielded nly 53 samples because f tite unfavrable lithlgies. PALOMAG IC AALYSIS Samples were cut int 2.2 cm higit specimens (2.5 cm diameter) in tite labnatry ant! stred in a magnetically sitielded rm befne ant! t!uring demagnetizatin treatment (restfield less titan 200 n ). Measurements were carried ut n a Sc crygenic magnetmeter, ant! alternating fleid (AF) ant! titermal t!emagnetizatins were perfrmed with Scitnstedt equipment. Cmpnents f magnetizatin were identified by visual inspectin f rtitgnal vectr diagrams (Zijderveld, 1967), and titeir direc-

6 . W u, R. Van Der Vn and Q. 210 Z. Liang tins were determined by principal cmpnent analysis (Kirschvink, 1980). atural Remanent Magnetizatin (RM) intensities ranged frm 0.1 t 1 ma/m fn alí rck types; 8 f the 159 samples had RM intensities that were t lw, and titese itave nt been used. he RM directins are tightly gruped arund tite present-day field directin (Fig. 4a), int!icating titat tite rcks have generally been heavily verprinted in recení times. Because in mst cases AF demagnetizatin did nt succeed in elimínating a significant part f tite RM (Fig. 5), titermal demagnetizatin was carried ut n alí samples. with sufficient RM intensities. henmal demagnetizatin f many samples islated nly ne cmpnent f magnetizatin (Fig. 6). he directins f Ihse 94 samples whicit shw nly ne cmpnent cnfrm t the present-day ficíd, indicating cmplete and recent remagnetizatin. Almst al] f tite 53 samples f tite clasties in tite upper sectin (Badawan ant! Yuanshan members) shw nly titis magnetizatin, called grup A; we cnclude that tite lititlgies f tite uppen sectin are nt suitable fr palemagnetism and magnetstratigraphy, because they have nt retained ancient magnetizatins. Many samples frm the lwen Meishucun sectin, hwever, reveal mre titan ne cmpnent f magnetizatin. Althugit a large part f the ttal remanence in these samples is remved by C with a diree[in cnfrming t titat f the present-day field, the directin f the magnetizatin remaining abye 300~ is clearly diffenent fnm tite gnup A dinectin. In tite samples f tite tp rw in Fig. 7, the higiten-tempenature magnetizatin is nrtherly and shallwly upward (grup B directins), wheneas in the samples f tite bttm rw f Fig. 7 tite directin is sutherly ant! shallwly dwnwart!s (grup C directins). Titene are 29 gnup b. a. C. ~ ~ ata. - Th~C~ Y O> ~4ti-~-~ y =159 ~. y 4lÑ~ÁjY>151 \. 5 - =57 Fig. 4 qual-area prjectins f (a) the atural Remanent Magnetizatin directins befare demagnetizatin, (b) tlíe characteristic directins f grups A, B, aud C cmbined, and (cl the characteristic directins f grups and C nly (alí withut crrectin fr the tilt f the strata). Pulí (pen) symbls represent prjectins ante Use lwer (upper) hemisphere. The star (in c) represents the directin f the present-day gemagnetic ficíd.

7 Recnnaissance magnetstratigraphy f the Y1184-A2 WiUp.1 ma/m 25 mt IDwn RM Fig. 5. Orthgnal vectr diagram (Zijderveld, 1967) f a sample treated witli alternating fields, iliustrating thai Ibis technique des nl succeed in remving Ihe remanence. Pltied pints represen[ he in-site endpints f the magnetizatin vectr measured after each treatment step (in mt); fulí (pen) symbls are prjectins nt the hrizn[al (vertical) plane; intensity f remanence is indicated alng the axes, B t!irectins and 28 grup C t!irectins in ur cllectin (f tite Grup B directins 26 are in tite lwer Meishucun sectin ant! nly 3 in tite uppen sectin; alí grup C dinectins are in the lwer sectin). In Fig. 4b alí t!irectins titus determined (grups A, B, and C) are shwn, witereas in Figure 4c nly tite 13 and C directins are pltted. he grup 13 ant! grup C directins are antipdal and represent neversals f an ancient magnetie fleld. In a subsequent sectin we will discuss tite planity recrd that can be cnstructed frm tite 13 and C directins. Because tite intensity f tite remanence abye treatment f 300 becmes ratiter lw ( ma/m) tite precisin f tite directin determinatin is frequently ratiter lw als. In sme samples it was nt pssible t determine [he directin precisely, althugb a determinatin f planity (grup 13 versus grup C directin) culd be made wititut itesitatin (Fig. 8). Such samples itave been included in tite plarity recrd, but they accunt fn sme f tite scatter in Fig. 4c.

8 Vii, R. Van Der V and (2. WUp Z. Liang Y1174 a -.- -_ WUp É4llZtZiii- Y 1275 W Up V1266 ~t. e >2~t 362 ~2R w IR ,, SRM> Dwn Y ~Dwr, Rae> SRM> ~ODwn Fig. 6. Orthgnal vectr diagrams, with the sanie cnventins as in Fig. 5, fr three sampies wich shaw nly ne cmpnení f magnelizatin; ibis directin f magnetizatin, cailed the grup A directin, cnfrms t the present-day gemagnetic fleld. Blcking temperatures falí between 200 ant! 500 fr grup 13 and C directins, s titat magnetite culd well be Lite canrier f Lite RM. Fig. 9a sitws tite acquisitin f Istitermal Remanent Magnetizatin (IRM) f five nepresentative samples. After an initial rapid increase in magnetizawup Y1203 Y 1206 U nt~ W Up WUp -4 suut 522.2t2..22, R22~ JflPRR> a~ ~ --t 3522<25263, 356 2~ 25R63ej - R6w2~ Ocaen DWn ODWfl SRM W Up WUP Y1216 VI 221.r WUp Y1199 2a2. s,u4> DDwfl DDwn SR > Fig. 7. Orthgnal vectr diagrams, with the same cnventins as in Fig. 5, fr six sampíes which shw tw cmpnents f rnagnetizatin. The tp rw shws three samples which cntain grup A and grup 8 magnetizatins, with grup 2C. Tite directins bitn mw being shws nrtherly linee 5amand pies with grup A ahye and grup C directins, the Iatter shaflwly upward temperalures f abui 200 being sutherly and shallwly dwnward ahye temperatures f abut 200 C. Grups B and C are the characteristie directins fr the Meishucun sectin and are antipdal t each ther.

9 Recnnaissance magnetstratigraphy ql/he tin acquired belw 50 mt, tite samples cntinue t increase titeir remanence up t fields f 1 T, int!icating titat a higiter-cercivity pitase is present, in additin t pssibly magnetite. A therm-magnetic analysis f a pwdered dlmite specimen, perfrmed witit a hrizntal Curie balance, is shwn in Fig. 9b. w Curie temperatures are revealed, n characteristic fr magnetite al abut 570 C and antiter with a Curie temperatune f 680 chanacteristic f hematite. DISCUSSIO AD PALOPOL POSITIO Almst alt f tite samples studiet! cntain tite grup A dinectin as an verprint ant! many samples cntained nly titis directin, interpreted as a recent remagnetizatin. Other cllectins frm China itave als yieldet! such widespread remagnetizatins (e.g., Kent et al., 1987). Hweven, a large prprtin f tite samples frm tite lwer sectin (54 ut f 106 sampíes) als revealed a significantly different magnetizatin. Althugh we have btained a dual plarity magnetizatin representative f an ancient magnetie field (since the gnup 13 ant! C directins are clearly different frm tite present-day fleid ant! tite grup A verprints), we can nt be a priri certain titat this magnetizatin is primary and f W Up Vil 94-A2 QCSrrA~ni 300 Bu IDwn RM Fig. 8. Orthgnal vectr diagrams, with the same cnventins as in Fig. 5, fr tw sampíes which shw a grup B r grup C directin (as in Fig. 7), 50 [hat the plarity f magnetizatin can be determined. Hwever, the directins f these samples are Iess well defíned because f the large variatins abye temperatures f abel 300 C.

10 17 Fu, R. Van Der V and ( a. Z Liang 300 a, c 200 a,c it a, 100 C c ID c Applied DC fisid (mt) b. 4- a 1.0 h 0 4- ) 0.5 a ) - «1 ) 4-6 tu ) : 200 Temperature 400 (c) 600 Fig. 9a. Acquisitin f Isthermal Remanent Magne[izatin f representative Meishucun sectin samples, indicating a lwer cercivity mineral (magnetite) as well as higher cercivity mineral such as hematite. Fig. 9b. Thermniagnetic analysis f a pwdered dlmite sample, shwing Curie temperatures al abut 570 and 680 C, indicating the c-existence f magnetite and hema[ite.

11 Recnnaissance rnagnetstraíigraphy f the O Q Fig. lo Paleple psitins frm the Yangtze Platfrm (Suth China blck). AII ples are frm Lin et al. (1985), excep[ where indicated therwise: Tmp, middle t Late Tertiary, Ku, Late Cretacenus, 1(1, arly Cretaceus, Ju, Late Jtirassic, Jm, Middle Jurassic, Trm, Middle Triassic, TrI, arly Triassic, Pu, Late Permian (Zha and Ce, 1987), Ci,, Late Carbniferes, Cl, arly Carbniferus, Sm, Middle Silurian (Opdyke e[ al., 1987), el, arly Cambrian, M, ple frm this study. Late Precambrian L Cambnian age. Untrtunately, Ibíd, cnglmerate r baked cntact tests are nt pssible with Lite lcal gelgical setting; mrever, flt!ing tk place during tite ertiary s titat tite fldtest wuld nt help much in cnstraining the age f magnetizatin. able 1 lists mean t!irectins in in-situ ant! tilt-crrected crdinates. Tw cnsideratins, hwever, may be itelpful in determining Lite age f magnetizatin. he first invlves tite directin ant! its paleple psilin in cmparisn witit tite Pitanerzic apparent plar wander patit. If tite 13 plus C paleple were t resemble ples determined fr yunger pe-

12 17. Fu, R. Van Der V and (2. 2? Liang 216 ríds, a stnng suspicin f nernagnetizatin wuld exist. he secnd cnsideratin, nt yet pssible with ur recnnaissance cllectin, examines whether tite reversals are layer-parallel: in sampling parallel sectins, ne can determine whether tite same reversal recrd exi;ts and if s, titis wuld strengthen tite argument that the magnetizatin is acquired during the time f depsitin. he paleple determined frm the cmbined grup B ant! C direetins (Table 1) is lcated at 68.8, ant! is pltted in Fig. 10 tgethen with tite Phanerzic paleples available fn tite Suth China blck. It appears that tite paleple frm titis study is different frm any Phanerzic paleples published thus fan (Mclhinny et a)., 1981; Chan et al., 1984; Lin et al., 1985; Chun et al., 1986; Kainian et al., 1986; Opdyke et al., 1986, 1987; Zita and Ce, 1987). Our paleple is als quite different frm tite arly Cambrian ple (Lin et al., 1985) derived frm rcks in Zitejiang ant! Hubei prvinces, whicit falís al 3.45, It is pssible that age differenccs between ur sectin ant! titse f Lin and c-wrkens during a time f appanent plar wanden can accunt fr tite difference in ple lcatin; the alternative f a relative rtatin f 80 dcgrees between tite tw arcas appears t be mucit less likely, given tite platfrm setting f tite sequences. he results f Lin et al. (1985) fr tite arly Cambnian are nt publisited in detail, s it is impssible t evaluate their results. Wc nte, itwever, that tite ple f Lin ant! clleagues falís veny clse t tite Silurian ple fn tite Suth China blck (Opdykc et al., 1987) ant! that a Silurian remagnetizatin can nt be precluded. MAG IC POLARITIS If we assume titat Lite magnetizatins f grups 13 ant! C are primary, a pneliminary magnetic plarity recrd can be cnstructed fn tite lwen ABL 1. Grup means f characteristic magnetizatin ín-s,tu Grup K Decl./lncl. A C B±C bedding-crrected « ± / / / 9.1 paleple 87.I, 134.O 60.8, , , 274W Decí/Incí /± ± / paleple 75.I, 68.8, 65.8, 68.S, 90.I I 2 70,7 is Uhe number f samples used t calculate the n,eans; Deel/Incí. are declinatin and inclinatin (in degrees); K and «95 are the statistical pararneters assciated with Uhe cneans; *grup C directins have beere inverted t calcutate the verail mean.

13 Recnnaissance magnetstratignaphy qfthe Meishucun sectin (Fig. II). In titis figure we can cmpare tite sample numbers and psitin, the identificatin f grup A, 13 r C directins, thc Virtual Gemagnetic Ple (VGP) latitudes (Lwrie et al., 1980) cnstructed frm grup 13 ant! C directins, ant! a magnetic plarity znatin with the lititlgies ant! members f tite frmatins. Als included is tite vcny preliminary magnetstratigrapity btained by Liang and c-wrkcrs (in Lu 1-luilin et al., 1984), wh btained many fcwcn grup C directins because f tite lw intensity f titeir samples measured with a spinncr magnetmeter ant! pssibly because f less detailed demagnetizatin. In tite clumn we have treated tite grup 13 dircetins as nrmal plarity (black) ant! the grup C directins as neversed. itis is at Litis time a rclatively arbitrary decisin, because titere is n indepent!cnt evidence t determine whctitcr a paleple in titis stut!y is a nrthple n a suthple. It may eventually turn ut titat tite plarity designatin used itere needs t be switched, but such a reinterpretatin wuld nt diminisit tite stratigraphic value f the reversal rccrd we have begun t dcument. On tite basis f ur dircetins, titere are al least eight reversed znes in the lwcr Meisitucun sectin. Wc nte titat tite titree neversed znes determined by Liang ant! c-wrkers have alí been cnfirmed by ur study. In tite lwermst part f ur sectin, un sampling density is lwer, ant! it may well be pssible titat several titer plarity znes itave yet t be discvered. Witat is remarkable is that after a penid f prcdminantly reversed (2) plarity in tite Xiawaitushan and lwcn Zhngyicun membcrs, titere is a perid with very frequent reversals in tite upper Zhngyicun ant! Daitai mcmbcrs. he A psitin fr tite Precambnian-Cambrian bundary wuld falí bclw tite pret!minantly reversed part f tite sectin, whercas the 13 psitin wuld be in the midt!le f tite zne witit frcqucnt neversals. It is wrtit strcssing at titis pint that ur magnetie plarity cnstructin is preliminany ant! in neet! f crrbratin. ven s, it is tempting t cmpare titis recrd witit recrt!s published fr titer parts f tite wrld (Fig. 12). he Amadeus Basin in Australia (Kirschvink, 1978) itas yielded a revensal stratigrapity witit predminantly nrmal planities in tite Late Precambrian, fllwed by a mixed intenval in whicit tite PrecambrianCambrian bundary is placed. hese Australian rcsults pass a flt!test ant! are thught t be primary n tite basis f paleple lcatin. he Lena River sectin iii Siberia (Kirscitvink ant! Rzanv, 1984) is mstly Tmmtian-Atdabanian in age and des nt include Lite Precambrian-Cambrian bundary; titis sectin als cntains frequent reversals. he Meisitucun, Siberian ant! Australian plarity znatins are nt readily cmpared t eacit titer, ant! tite Amadeus Basin ant! tite Lena River scctins have been interprcted t be f diffcrent age (Kirscitvink ant! Rzanv, 1984). Results frm Mnglia (Kirschvink el al., 1987) are unpublisited at titis time, but are als titught t be yunger titan the Precambrian-

14 17 Fu, R. Van Der V and ( sample sample -90 (M) magnet c p ar ty znatin VGP latitude psitin plarity 0 Z. Liang 900 AB gechrnlgy ,,g.>< O ~>< ~ ~ ~i~zzz 5 5 u O A ?, 1~.s ; 1184A 11b7 ~<~< x 1182 libia 1181 e 1180A,B A,B r ~ < x < 1177A 5 --~1176A >< 6<,c U 0.9Q0 nrmal dlrnite LII 90 reversed phsphrite shale Fig. 1 I. Magnetic plarity znatin derived frm [he characteristic grup B and C directins f this study, fr [he lwer Meishucun sectin. Frmatin names and psitins A and B fr thc Precambrian-Cambrian bundary are frm Lu Huilin et al. (1984) and cnfrm t thse in ñ Stratigraphic psitin (in m) is indicated n the left, fllwed by a clumn with ur sample numbers; [he sample plarity clumn cntains symbls fr grup A dircctins (crsscs, nt used), grup fl dircctins (dts) and grup C direc[ins (pen circíe); the Virtual Gemagnetic Ple (VGP) latitude is pltted accrding t the methd f Lwrie et al. (1980>. The Clurnn A rnagnetic plarity znatin is frm this study, whereas clumn Bis based n the very preliminary results f Liang el al. (1984).

15 Recnnaissance magnetstratigraphy f the... 1am 1cm u c u 1 u.0 u a LA RIVR a a.0 u u R ix> - A I Sin Orn MIS>*JCU Vi 1,2 <7 ½ R R VALLY DAM ROSS RlVR Fig. 12. Magnetsíratigraphic znatins frza Meishucun (tw plarity ptins shwn, given that we d nt knw which is reversed and what is nrmal), frm the Lena River sectin in Siberia, and frm tw sectins, Valley Daza and Rss Itiver, in Australia (Kirschvink, 1978 and Kirschvink and Itzanv, 1984). It appears that at this time it is n[ pssible t make any crrelatins between thcse sectins, indicating the need fr further wrk.

16 220. tu, R. Van Der V and (2. Z. Liang Cambrian bunt!any. he pssible presence f discnfrmities and hiatuses in the Amadeus Basin funher cmplicates a successful cmparisn. xtensive magnetstratigraphic wrk has been carried ut n Late Precambrian ant! Lwen t Middle Cambrian sectins in tite Desert Range f evada (Gillett ant! Van Alstine, 1979; Van Alstine ant! Gillett, 1979). Hwever, the Cambrian results are alí reversed ant! resemble results frm yunger rcks: it is likely that later Palezic remagnetizatins itave affeeted these ncks. he Late Precambrian Jhnnie Frmatin, n the ther hand, yielt!s a typical latest Precambrian paleple (1 O S, 1 62 ) ant! indudes fnequent neversals. he Jhnnie Frmatin is likely t be lder titan tite Siberian, Australian and Meishucun sectins, s that magnetstratigrapitie cmparisn is nt pssible. FUTUR WORK AD COCLUSIOS Wc intend t resample tite sectin and several parallel sectins in tite Kunyang mine in tite near future, cncentnating n tite lwer Meishucun sectin, in rder t establish a reliable magnetstratigrapity. It is pssible, althugh we d nt believe that it is likely, titat Lite grup B ant! C magnetizatins repned n abye are nt pnimany. Hwever, we believe titat il we btain a reversal stratigraphy that shws crrelatins between panallel sectins, we may be reasnably assured that tite magnetizatin (being layen-parallel) was acquired at tite Lime f depsitin r shntly thereafter. Witile we have in previus studies encuntered dualplarity remagnetizatins, we have never bserved Litese t be layerparallel. In cnclusin, we have btained a paleple (at 68.8>, ) unlike any Phanerzic paleples thus fan btained fr yunger rcks frm the Suth China blck. In the lwer Meisitucun sectin, eight reversals have been determined and tite dual-plarity directins are antipdal wititin the limits f errr. Witenever thcy itave been studiet!, tite Precambrian-Cambnian bundary sectins have yielded frequent reversals and titis study is n exceptin. Hwever, a detailed crrelatin between tite Sibenian, Australian, western USA ant! Citinese sectins is nt yet pssible at this time. Acknwledgements his study was supprted by grants fnm tite (lelgical Sciety f Amenica (MT. Stearns award t Fang Wu), tite Sctt Turner Fund f tite University f Michigan and tite Yunnan Institute f Gelgica] Sciences.

17 Recnnaissance magnetstratigraphy f the Tite labnatry wrk was supprted by tite Divisin f arth Sciences, Lite atinal Science Funt!atin, grant AR RFISRCS CHA, L. 5., WAG, C. Y., ant! WU, X. Y., Palemagne[ic results frm sme Permian-Triassic rcks frm suthwes[ern China, Gephys. Res. Let[., II, , CHU, L., ZHAO, X., CO,., ant! QIZHIG, L., Palemagnetic evidence: the anticlckwise rtatin f Qujing area during the Palezic (in Chinese), Scien[ia Gel. Sinica, 7, , COWI, J. W., Intrductin L papers n the Precambrian-Cambrian bundary, Gel. Magazine, 121, , GILLTT, 5. L., and VA ALSTI, O. It., Palemagnetism f Lwer ant! Middle Cambrian sedimentary rcks frm the Desert Range, evada, Jur. Gephys. Res., 84, , KAIIA, H., OPDYK,. Ii, KT, D. V., GUIZHIG, X., X., and RULOG, 51., Funher palemagnetie resulis frm Ihe Permian meishan basar in SW China, Kexue Tngba, 31, , KT, D. V., ZG, X-S., ZAG, W-Y., and OPDYK,. D., Widespread late Meszic [ Recent remagnetizatin f Palezic and lwer Triassic sedimentary rcks frm Suth China, Tectnphysics, 139, , KIRSCHV1K J. L., The Precambrian-Cambrian bundary prblem: Magnetstratigraphy f [he Amadeus Basin, Central Aus[ralia, Gel. Magazine, lis, , K1RSCHVIK, J. L., he least-squares line and plane and [he analysis f palemagnetic data, Gephys. J. R. astr. Sc., 62, , KIRSCHVIK, J. L., and ROZAOV, A. Yu., Magnetstratigraphy f lwer Cambrian strata frm [he Siberian Pla[frm: a palemagnetic ple and a preliminar> plarity time scale, Gel. Magazine, 121, , KIRSCHVIK, J. L, BUDY, C. J., ant! ZHURAVLV, A. Yu., Magne[s[ratigraphy f the Precambrian-Cambrian reference sectin near Salany-Gl, western Mnglia: cmparisn with [he Siberian Platfrm (abs.), Gel. Sc. America Abstraets with Prgrams, 19, p. 728, LIAG, Q. Z., and many thers, Iteprt n the reginal gelgical investigatin in [he Kunming Dis[rict, unpublished paper by the Yunnan Gelgical Bureau, LI, J., FULLR, M., ant! ZHAG, W-Y., Preliminar> Phanerzic plar wander paths fr the n[h and Su[h China blcks, ature, 313, , LOWRI, W., ALVARZ, W., PRMOLI SILVA, 1., and MOCHI, 5., Lwer Cretaceus magnetic s[ratigraphy in Umbrian pelagic carbnate rcks, Gephys. J. R. astr. Sc., 60, , LUO HUILI, and man> thers, Sinian-Cambrian bundary strattype sectin at Meishucun, Jinning, Yunnan, China, Peple s Publishing Huse, Yunnan, China, 22 plats, Pp. 154, McLHIY, M. W., MBLTO, B. J., MA, X. H., ant! ZHAG, Z. K., Fragmentatin f Asia in the Permian, ature, 293, , 1981.

18 222 F. Fu, R. Van Der 1/00 and (2. Z. Liang OPDYK,. D., HUAG, K., XU, O., ZHAG, W-Y., and KT, D. V., Palemagne[ic results frm the Triassic f [he Yangtze pla[frm, J. Gephys. Res., 91, , OPDYK,. D., HUAD, K., XU, G., ZHAG, W-Y., ant! KT, 1). V., Palemagnetic resul[s frm the Silurian f 4w Yangtze paraplatfrm. Tectnphysics, 139, , VA ALSTI, D. R., ant! GILLTT, 5. L., Palemagnetism f Upper Precambrian sedimentar> rcks frm the Desert Range, evada, Jur. Gephys. Res., , XIG, YUSHG, and LUO HUILI, Precambrian-Cambrian bundary candidate, Meishucun, Jinning, Yunnan, China, Gelgical Magazine, 121, , ZHAO, X. X., and CO, R. 5., Palemagne[ic cnstraints n the cllisin ant! r[atin f rth and Suth China, ature, 327, , ZIJDRVLD, J. D. A., AC demagnetizatin f rcks: analysis f results, in: Methds in Palemagne[ism, d. by 5. K. Ituncrn, K. M. Creer ant! ID. W. Cllinsn, lsevier, Amsterdam, p , Reccq ved 21 Dcc lccept- d 4 July 1988.