124 by K. A. Pl umb New Precam bri an t i m e scal e: Repl y Dr . H. J . Ho f ma n n ' s c o mme n t s o n my p r e v i o u s a ti r c l e ( P l u mb, 1991 ) st ike r a t t he cor e of t he pr obl em of a Pr eca mbri an t i me s cal e: Equ a l - u ni t s u bd i v i s i on was c on s i de r e d a n d r e j e ct e d du r i ng t he ver y ea r l i es t del i ber at i ons of t he Subcommi ss i on ( J ames, 1973), and fi rstly, w het her a formal ti m e scale i s requi red at all and, secondl y, i t has been reconsi dered several t im es since. The f eedback t o t he what princi pl es and operat ional cri teri a such a ti m e scal e shoul d Subcom m i ssi on has been that equal- unit subdi vi si on "draws lit t le em brace. Unfor tunately, Dr. Hofm ann appears to have overlooked suppor t from t he geologi cal f rat er n ity at large: t he overwhelm i ng t he det ai l ed di s cus si ons of Pl umb and James (1 986). Pl umb ( 1991 ) i s pr ef er ence cl ear l y i s for t i me uni t s t hat a re s i gni f i cant i n t er ms of t he necessari ly only a brief announcem ent i ntended to form ali ze t he geol ogi c r ecor d" ( Pl umb and J ames, 1 986. p. 68). I ndeed, t he over - deci s i on by t he Subcommi s s i on on Pre ca mbr i an St r at i gr aphy ( SPS) of t he I nt e rnat i onal Uni on of Geol ogi cal Sci ences ( I UGS) and t o whe l mi n g ma j o r i t y o f wr it t e n c o mmen t s t o Pl u mb a nd J a me s ( 1 9 86 ) enabl e i ts i m m ediat e appli cati on to i nt ernati onal lit erat ure. A m ore accepted t hat "t he Precambri an ti m e scal e shall be di vi ded accordi ng t o t he chronom et ric subdi vi si on ..., i n whi ch t ime boundaries have com prehensi ve publi cati on t hat i s in preparati on wi l l detai l t he pri n- been sel ected so as t o encl ose or del im i t pri ncipal cycles of sedim en- ci pl es and developm ent of t he new t im e scal e and it s nom enclature. tat i on, orogeny. and m agm atism , but i n whi ch boundari es are de- Duri ng the infancy of m odem geology, correlation wi th a form al fined in years w i thout speci fi c reference to any bodi es of rock" chr onos t ra t i gr aphi c ( Pha ner ozoi c) t i me s cal e was t he onl y means ( Pl umb, 1 991, p. 1 39). For pr act i cal as wel l as s ci ent i fi c r easons , avail able t o determ i ne or descri be t he age of a rock. N ow , dir ect this has been achieved by placing boundari es sel ecti vel y at ti m es of chronometri c or isotopi c age i s t he si m plest way t o descri be t he age mi ni m al acti vi ty or gaps in t he know n geologic record, t he anti thesi s of an i ndi vi dual rock, parti cularl y i n the Precambrian. And cl early, of linking the defi ni ti on of uni ts di rect ly to t he rock record, as in a chronom etri c ages m ust be used i n the stat isti cal t reatm ent of rates of chronostrati graphic sense. The ti m e scale becom es t he means t o geol ogi c processes, i rrespecti ve of any subdi vi si on t hat m ay exi st. The di rect use of i sotopi c ages rem ains avail able t hroughout t he ti me cali brate the cri ti cal revi ew of Ear th's hist ory wit hout inhi bit ing the ob j e c t i vi t y of t ha t r e vi e w. s cal e ( Pr ecambr i an and Phaner ozoi c) and, i nde ed, i s encour aged wi t hi n t he new I UGS s cheme ( Pl umb, 1991). argum ent that the SPS and IUG S schem e i s not st ri ct ly chronom etri c. It foll ows that a formal ti m e scale or subdivi si on is not abso- The nature of a ti m e scal e, chronom etri c or chr onostrati graphi c, Dr. Hofm ann devotes consi derabl e di scussi on t o t he erroneous l ut ely necessary i n order t o depi ct rocks on large-scale geol ogi cal depends sol ely on how t he contai ned uni ts are defined, t hat i s, by m aps. The fundam ent al m ap uni t i n t hese cases i s generall y li thost rat- di rect reference t o an i sotopi c or "absolute" age or, i n t he m odern i graphi c, usual l y i ndi vi dua l f or mat i ons or t hei r equi val ent s ( f or ex- cont ext, by reference to a boundar y strat otype. Nowhere i s there any ampl e, tect onic, st ructural. met all ogeni c, sequence st ratri graphi c, requi rem ent, in i tsel f, t hat boundari es actually be sel ect ed i n any and pa l eogeogr aphi c uni t s) . The ages of t hes e uni t s coul d be cons i d- special w ay. For exam ple, boundar y strat ot ypes coul d be sel ected ered as sim ply anci ll ar y data i n the m ap l egend and coul d be pre- l egi t i mat el y at any ar bi t ra r y poi nt or at uni f orm ( t hi cknes s ) i nt erva l s sem ed as di rect i sot opi c ages, if t he data are avai lable. The real need f or af orm al subdi vision li es wi th t he need for ease ( f or exampl e, every 1, 000 m) t hr ough a ny ar bi t r ar i l y se l e ct ed s e- and effecti veness i n sci enti fi c com m uni cati on and t he need for t he have m axi m um corr el ati on potent ial and to be as near as practi cabl e groupi ng of rock uni ts on sm all -scale i nt erregional m aps, and herei n t o t h e b oun d a r i e s of t h e c l a s s i c t y pe s e c t i ons , t ha t i s . a t poi n t s j udg e d l ies t he desi rabi li ty for the t i m e scale to be signi fi cant in t erm s of t he to have m axim um geol ogic si gnifi cance or appl i cat ion. Di rect paral - geologi c record. In a disci pl ine i n whi ch t he use of nam ed subdi vi - sions i s t radi ti onal and deeply i ngrai ned, the Subcom missi on finds a cl ear preference for t he use of si m ple ti m e term s t hat are m ore expressi ve t han t hei r numerical count erpar ts. Regi onal m aps gener- qu e n c e. I n p r a c t i c e, t he y a r e b e i n g d e f i ne d f r om s e c t i on s j u dg ed t o l el s exist between t he sel ecti on, defi ni t ion, and appli cati on of t he Phanerozoi c chronost rati graphic and t he Precam brian chronom et ri c ti m e scal es. Pl umb and James (1 986) make a ver y cl ear di s t i nct i on bet ween ally requi re t hat t he fundam ent al li t host rati graphi c unit s be grouped the sel ecti on and defi ni ti on of t ime uni t s or boundari es. The form al i nt o larger uni t s. and, part icul arl y at conti nental and i nt erconti nent al scal e, t his grouping is com m onl y and t radit ionally m ade according t o de fi ni t i ons and nomencl at ur e as accept ed by SPS and I UGS met i cu- l ous l y avoi d a l l r ef er ences t o any s peci f i c bodi es of r ock or geol ogi c age. The need for ease and effecti veness of com m uni cati on requi res event s, and they defi ne boundari es st ri ctl y by chronom et ri c or iso- an agr eed upon and consi s t ent ( t ha t i s, f or ma l ) s ubdi vi si on and t opi c age onl y ( Pl umb, 1991 ). The us er can us e onl y t hos e chrono- nomencl at ure. me t r i c bo un da r i e s i n a pp l yi n g t he s c h e me . An y po s s i bl e pr e j ud gme n t Such subdi visi on wi l l be m ore effect ive and pract ical i f i t re- of t he geol ogi c r ecord by nom enclature, a tenuous l ink at bes t lect f s nat ur al geol ogi c gr oupi ngs or cycl es t hat r et ai n t hei r uni t y consideri ng the care taken in defi ni ti on, i s null i fi ed by t he back- when depicted on m aps, for exam pl e. There w il l be less need or gr ound of t he Precam bri an nom encl at ure provi ded to t he Inter n a- t empt at i on t o f or ce ( i ncorect l y) r ocks and event s i nt o ( unnat ur al ) t i onal Commi s s i on on St ra t i gr aphy ( I CS) a nd I UGS and r epr oduced bel ow ( i t ems 1-4) . The s cheme i s st r i ct l y chr onomet ri c. equal-uni t di vi si ons, whi ch cross nat ural groupings. It wi ll be si m pler t o assign rock uni ts on the basis of fi el d relat ionshi ps and correl ati on Nom enclat ure is an essent ial component of any subdi vi si on, but al one when pr eci se i sotopi c ages or ot her means are not avail able i t has proved t o be an elusi ve com ponent of the Subcom m i ssion's i m m ediat ely. Al though geologi c events are di achronous and any deli berat ions. Convenience in expressi on favors t he use of sim pl e, chosen boundari es or groupings are unli kely t o fit all regions equall y. si ngle nam es, which shoul d be unique in order t o avoi d confusion and alt hough some unit s wil l span boundari es and m ust be descri bed wi t h any ot her classi fi cati on. Nom encl at ure shoul d t ranscri be i nto as so, recent advances i n preci se geochronol ogy, sequence st rati gra- other languages wi th as l it tl e transl ati on as possi bl e, and t he term s p hy. a n d s o o n, a r e i n f a c t r e i nf o r c i ng t h e e xi s t e n c e of ma j o r c yc l e s t hat t ranscri be m ost equal ly i nt o al l languages are t hose t hat have and pa ral l el s i n geol ogi c evol ut i on over wi de ar eas. classi cal Greek r oot s. The logic of l inki ng a chronom et ri c scheme June 1992 125 a nd a numer ical l y bas ed nomencl a t ur e was r eadi l y appar ent t o t he Subcommi ss i on, but a pr oposed s i mpl e and easi l y under s t ood Gr eek- t hi ng l i ke 5 0 0, 1 , 00 0, 1 , 5 00, 2, 00 0, a n d 2, 5 00 Ma, or p e r ha p s j us t bas ed numeri c sc heme ( Pl umb and Gee, 1 986, 1987) was not r e- s hor thand nomencl at ur e f or r oundi ng ages and ( or ) gr oupi ng r ock c ei ved f avor abl y by t he geol ogi cal communi t y at l a r ge. The Subcom- mi ss i on gr at ef ul l y acknowl edges Dr. Hof mann' s wel come s uggest i on uni t s wi t hi n 1 00- M a i nt er v al s; anyone i s wel come t o us e s uch r ound- i ng i n appr opr ia t e ci r cums t ances as an al t er nat i ve t o t he us e of pr eci s e i s ot opi c ages al r eady encour aged by t he I UGS s cheme. The Subcommi s si on t hanks Dr. Hof mann f or hi s se ver al car e- ( per s onal c ommuni cat i on, 1987) of t he nomencl at ure t ha t s ubse- quent l y has been adopt ed f or t he e ras, but t he nomenc l at ur e f or per i ods has pr oved mor e el us i ve. The Subcommi s si on does adopt t he si mpl e pr inci pl es t hat no- me ncl at ur e for t i me uni t s s houl d be si mpl y a s et of conveni ent l a bel s, s o as t o ai d unambi guous n d a pr eci s e s ci e nt i f i c communi cat i on, and t hat no f undament al s i gni f i cance s houl d be at t a ched t o any t e rms. Sever al al t e n at r i ve t ypes of nomencl a t ur e wer e cons i der ed car ef ul l y a n d r e j e c t e d f o r v a r i o us r e a s o ns b y t h e Su bc o mmi s s i on. As i n mos t del i be ra t i ons of t hi s nat ur e, t he Gr eek names der ived f rom geol ogi c pr oc es ses, whi c h have been r ecommended and appr oved, re pr es ent ed t he mos t ac ce pt a bl e compr omi s e. I n expl ai ni ng t he backgr ound of t he Pr ecambr ia n nomenc l at ur e t o I CS and I UGS, t he Subcommi ss i on has s t r es sed t hat ( 1) rock uni t s s hal l be ass i gned t o t i me uni t s onl y on t he bas i s of age, or der of s uper pos i t i on, nd a ( or ) s t r at i gr aphi c cor r el a- t i on; ( 2) t he nomencl a t ure r ef l ect s but does not def i ne geol ogi c hi s t or y; ( 3) t he pr ocess es i mpl i ed by t he names ar e not di agnos t i c of or uni que t o t he t i me uni t s t o whi ch t he y appl y; and ( 4) t he pr oces ses s hal l not be cons i der ed i n as s i gni ng r ocks t o t he t i me s ca l e. The der ivat i on of t he r ecommended names ve y r pur pos el y pl ays no pa t, r nor i s even ment i oned, i n t he f or mal pr oposal f or t he s ubdi vi s i on and nomencl at ur e of t he Pr ecambr ian ( Pl umb, 1991 ). The der i vat i ons ar e desc ibed r onl y i n t he suppor ti ng bac kgr ound i nf or ma t i on t o I CS and I UGS and i n t he mor e det a i l ed publ i cat i on i n pr ogres s. W e ant i ci pat e t hat t he de ri va t i ons soon wi l l be l ost i n t he r ou t i n e o f ev e yda r y u s a g e, j us t a s i n t he pr e c e de n t p r ov i d e d by t h e Phaner ozoi c. Ther e t he gl obal s pr ead of geol ogi cal knowl edge has bl urr e d t he or i gi nal s i gni f i cance a nd de ivat r i ons of t he cl as s i c t er mi - nol ogy, as i s t ypi f i ed by t he curr ent s el ect i on a nd def i ni t i on of many of t he new boundar y st r at ot ypes fr om r egi ons t hat ar e f ar r emoved f r om t he cl as si c t ype s ect i ons. 1, 500 and 2, 500 M a. Dr. Hof mann' s geon r ecommendat i on i s but a f ul l y cons i der ed and l ea n ed r cont ibut r i ons t o t he pr obl ems of Pr e- cambr ian t i me sca l es and r emai ns gr at ef ul f or hi s par ti cul a rl y el e gant sol ut i on t o t he nom e ncl at ur e of t he eras, a nomencl at ur e now so obvi ous i n ret r os pect i n i t s pa ra l l el wi t h Phaner ozoi c equi val ent s. However, a deci s i on and a r ec ommendat i on ha ve been made now by I CS and I UGS a f t er exha us t i ve pr ocess es and cons i der at i ons of al l opi ni ons avai l abl e t o SPS. The t es t now r est s wi t h t he appl i cat i on and accept ance, or ot her wi s e, of t he J UGS s cheme by t he geol ogi cal communi t y a t l a rge; i ndi ca t i ons ar e t hat t hi s wi l l be s o. R eferences James, H. L. , 1973, Repor t on Subcommissi on meet i ng i n Adel aide, Aust ra- l ia, 1973: Geologi cal News let ter of t he I nt er n ati onal Uni on of Geol ogi cal Sci ences, v. 1973, no. 4, p. 282-285. Pl umb, K. A., 1991, New Pr ecambr ian t i me scal e: Epi sodes, v. 14, p. 139-140. Pl umb, K. A., and Gee, R. D. , 1986, Recent proposal s for subdi visi on of the Precambri an: Epis odes, v. 9, p. 243. 1987, Nomencl at ure for the Prot erozoi c: Recent act i ons by t he Sub- commissi on on Pr ecambri an St rat i graphy: Pr ecambr ian Research, v. 36, p. 185-187. Pl umb, K. A. , and Jam es, H. L. , 1986, Subdi vi sion of Precambr ian ti me: Recommendat i ons and suggest i ons by t he Subcommi ssi on on Pr ecambr i- an St rati gr aphy: Precambr ian Resear ch, v. 32, p. 65-92. 0 Kennet h A. Pl umb has been a Vot - i n g Me mb e r o f t h e Su bc o mmi s s i on o n Pr ecambri an St r at i gr aphy s i nce 1977 and i t s Chai r man si nce 1984. He has Dr. Hofma nn pl aces consi de ra bl e wei ght on consi s t enc y of t r eat ment i n a t i me sca l e, but Dr. Hof ma n n' s own r ecommendat i on i s wor k e d wi t h t he Bur e au o f Mi ne r al i nconsi s t ent. Dr. Hof mann adopt s t he s ame boundar i es, nome ncl a- t ur e, and uneven s i ze of hi s eons a nd er as as t he sc heme r ecom- mended by SPS and I UGS has and, t her ef or e, by def aul t , adopt s t he s el ect i on of boundar ies by r ef er ence t o t he geol ogi c r ecor d. Onl y f or Res our ces, Geol ogy, and Geophysi cs hi s ge ons ( uni t s t hat do not appea r cur r ent l y a nywher e i n any code of s t r at i gra phi c nomencl at ur e) ar e hi s equal - uni t pr inci pl es appl i ed. To st rat i gra phy and t ect oni c hi st or y be cons i s t ent , Dr. Hof mann' s er as s houl d be r epl aced wi t h s ome ot he r uni t and nomencl at ur e and s houl d have bounda ies r a t s ome- Epi s ode s, Vol . 15, no. 2 i n Can b er r a, Au s t r al i a , f o r o v e r 3 0 year s, whe re hi s res ear ch has c en- t e r e d o n ie f l d s t u d i e s o f t h e r e gi o nal o f t h e Au s t r al i a n Pr e c ambr i a n, par - t i c ul ar l y t h e Pr o t e r o z oi c o f n or t he r n -- Wl r } k " i f Aust ral i a.
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