New Precambrian time scale: Reply

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.