COMMONWEALTH OF AUSTRALIA.

DEPARTMENT OF NATIONAL DEVELOPMENT .

- , "

. BUREAU OF MINERAL RESOURCES

GEOLOGY AND GEOPHYSICS.

RECORDS.

1959/52

CORRELATION CHART OF CAMBRIAN AND ORDOVICIAN IN , .

AUSTRALIA

by

" A.A. Op1k

I . I. . ~.

I

Of

CORRELATION CHART OF CAMBRIAN AND ORDOVIC I AN

I N AUSTRALIA

RECORDS 1959/52

by

" A.A.Opi k

CONT;,NTS

Introduction

Meaning of correlation Geolog~cal or time - cor+elation Objects of time - correlati on ~eans of correlation Kinds of correlation

Notes on the Chart

References

INTRODUCTION

The present Correlation Chart summarizes the published information concerning the ~tratigraphy of the Cambrian and Ordo­vician of Australla as outlined in theltNotes tl a t the end of thj_s record . The Chart is incomplete, but it can be a mplified con­~-ren;ly with the publication of new information from the field . The m~in part of the chart is its IITirne-Scale", which a l so is subject to improvement and amplification . The main objective of forthcoming studies is a more detailed subdivision of the upper two- thirds of the Upper Cambrian and of the Ordovician. But even now t he scale is fu l ly spplicable for correlation of the deposits of central Australia (MacDonnell Ranges to western Toka Range region), as seen from fossil collections and field studies.

The author has had the opportunity to communicate orally to his colleagues his ideas concerning the meani ng, principles, and methods of geological correlation. These i deas are now summarized to give a brief outline of the philosophical background to t he present Chart .

ME.'JHNG OF CORRELATION

Geological or t1-me correlation ,

Geologi cal correlation as reflected in the present Chart; is a presentation of the distribution in time of formations and i :pterveni ng breaks. The ultimate goal of geological correlc.tj,...· is the discovery of contemporaneous geological events as recorded · i :: rocks', These events are deposition , as recorded by the formations) and e'rosion, deformation , and uplift, as recorded in the breaks. Time Irelationshi p alone is studied and presented, and causality , inte~dep.endence of events , reciproci ty , and any other IItrue cor­rela~ion" are not considered at all.

Established or estimated time relationships may, however , proviae clue s for the discovery of " true correlation!! as well. For example , uplift and erosion in the provenance areas on the one hand, and deposition of waste on t he other, are con currer; :: events ; but they may also be interrelated causally and therefore repre"sent !!true corre la tes II . Of course, contemporanei ty alone is not evidence that particular brea ks and formations are such cor-, relates.

These brief remarks indicate that "correlation" has a dual :meaning. For this reason terms like "geol ogical correlation " or "time-correlationll should be used to designate pure t ime relati or· · ship , and the term "correlatell (the noun) should not be used at a ll except in its correct lexical sense .

Objects of tim~-correlation

Time - co r rela tion operates with diverse objects. In the folloWing reasoning , for the sake of simplicity, the particular case of diverse f.ormations only is considered. So, diverse for­mations are recogni zed in the Chart by their diverse names and are arranged to demonstrate their distribution in time . Repeti t ion of one and the same formation name in several columns is not correla t :: ~ ,­because this repetition is not an indication of diversity, but a certificate of integrity.; and Single, integrate, objects cannot be correlated with themse~ves . Of course, time - correlation can be appl~ed between geographical ly separate and spatially discontinuou z members of a formation, because the idea of diversity can be app l ied to them .

"Extension of usage" of a formation name 1Iby correlation · cannot be accepted : when the term correlation (time and/or true correlation) is used, it is a statement of the existence of at least two diverse rock- units which should have different names .

•

.. >, .,

. '

- 2 -

Means of correlation

Means and methods of time- correlation have been repea'~edly discussed during the past century. Intuition, artistry, and science have been applied , and a review of the methods j.s unnecessary ir~ the pre sent brief note s. All means and methods are accep~ab le that lead to a result correct within limits, being based on observable facts . The foll owing discussion is restricted t o the fundamenta l principles of time- correlation .

A for@.at ioll is correlated when its posi ti en in a time scale is es t ablished; all forma tions or parts of formations in the same chart posj.tion e.re also ;imutual ly correlated II, Two or more formations (diverse rock- units) are mutually correlated when identical time,,'·'orkers are disccvered in these formations, even if these tirne -m~rkers cannot be r eferred t o a posit ion in a time ­scale, as , f or e xa mple, when a t i me- s cale is not available. SOt the principle ::. of ti!!le ~·correlation can be summarized as f oll ows : (J.) t,po or I!lor e (1iver:::a f ort'lat j ons arc the I",bj ec -:s of correlatio:J.j (2) within these diverse e!'ltities, identical time-markers should be present .

Fossils are the supreme and the only reliable time ­mar::ers . b1:)s·~ reliable are identical species , whereas genera a nd assemblages of ge nera are less reliable and introduce wider limi ts of uncert9.inty . The tire solving power" of fossils has limits within which the answer will remain uncertain .

Just n3 a the r mometer gives only its own temperature, which is used f or reasonable extrapolation , so a fossil determine s its oVin age, p.nd so does a IIf ossil band ll

• An extrapolation up or down from them becomec. r apidly unce::-tain and is a major source of error . Intr.J.:r:o.lat i on betwcen two fossil horizons is, however, m?fe and has been app J:ie-d wheJ:'e ·~eT ~HF' ,:::· i:'l . . :. L . .... ~10 ,re~: 'A,,": Chnl~: .

For th~ purpose of the Chart the l ower most and the uppermos t £oszil~ In a s ection and in each formation were most rewa rding . Of cour se , most of the correlated format ions are f ossiliferous in general.

~upe rpD sit i on is a~ important aid in correl~tion: a break or an unf"c.::.;;i liferous seq uence (formation) is corre l ated when the corre l ati on of beds ab ove and below is known .

Kinds of cc:crelation

Thr(":e kinds of time - corr elat ion can be listed here:

1. Correlation between two or more f ormations without referenco to a particula r time- scale. This i s uEua.~Lly of areal , local,or sub- regi onal significanc a . Whe;."! e. E: cale is not avai lable extrapolation over v:ider €e0g1~aphical regions rapidly becomes uncertain .

2 . Correlation by reference to a time - scale (the safest kind) j it is of regional significance and its accura:;y depends on the f'JE:sil record of the .formation and the accuracy of the sca le.

3 . Corre l ati on between diverse regional scales, which is of inte rregjonal and inte rc ontinental significance.

It sholJ. ld be underst ood tha t even wi thin a s ingle regi on several diyerse scales may have validity and are useful. They should be correlated mutually .

- 3 -

The present Chart is based on kind 2 , but t o construct the scale t~e other kinds ( 1 and 3) Wer.3 applied . The pre sent scal e leads toucorrelation with the Scandinavian, American, and Asian scales (Opik, in press) .

"Correlat i on H by similArity of lithology may be only a statement of integrity (di srupted continuity, identity), or a guessed time correlation . This tlcorrelation H is as uncertain as a statement of contemporaneity by reason of dissimilarity of lith­ologies . In the absence of f ossils other geological evidence (superposition , matching of sections, palaeogeography, unique marker beds in otherwise different sequences, etc.) should be considered, but even then the limits of uncertainty will rem~in very wide.

1.

2 .

3.

4 •

NOTZS ON THE CHART

The Chart is compiled from, and supplements,published material . The sources are the Australian Section of the Cambrian Symposium (6pik et al., 1957) and a forthcoming ~aper on the Cambrian and Ordovician geol ogy of Queensland (Opik , in press). These papers serve as exhaustive lIexplanatorj notes l1

, giving reasons for correlation, including overseas correlation, and locali ty maps .

The following regions are not included : South Australia , which refers mainly to Lower CAmbrian; Tasmania , whose correlation is already published (Opik et al . , 1957) ; central Australia (Ma cDonne ll Ranges to western Toko R~nge) , bec~use of the absence of published modern material, which is forthcoming .

Correlntion of the Victorian sequence is presented according to Opik (1956/57) and is at variance with Thomas & Singleton (1956/57). The c olumns for Cambridge Gulf, Ragged Range , and Negri are at variance with the correlation proposed by Traves (1956/57) . .

Ordovician is included, f irst, for its own sake, and, secondly, because of its lithological continuity with the Upper Cambrian. Reliable datum faunas have not yet been identifiea for the top of the Cambrian , nor for the b?se of the Ordovician .

5. "Pilpah Ra. II in the Chart should read II Ogilvie Ra. 11 (see Mt. I sa 4- mile sheet).

REFERENCES

" OPIK, A. A., and others, 1957 - The Cambrian geology of Austr alia. Bur.Min.Resour.Aust.Bull . 49 (Reprinted from CAMBRIAN 1l':iMPOSIUM , )art 2, 20th int.geo1.Congr ., Mex i co , 1956) .

" ~O=P~I~K~,~A~.~A~., in press - The Cambrian and Ordovician Geology of

THOMAS ,

Queensland. J .Geol . Soc . Aust .

D.E. , & SINGLETON, O.P., 1956/57 - The Cambrian stratigrap~~ of Victoria. CAMBRIAN SYMPOS I UM, part 2, p . 149.

TRAVES , D.M. , 1956/57 - Upper Proterozoic and Cambrian geology in north- western Australia . Ibid ., p . 75 .

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