l^OTES
ON
THE
SECTION
AT
ONE
THEE
POINT,
NEAR
HOBART.
Plates iii. to ix.By
FiiiTzNoETLiNG,
M.A., Ph.D.(Read
July 14, 1913.)].
Introductoky Remarks.
More
than tJiirty years ago Mr. R.M.
Johnston, in apaper (1) read before this Society, pointed out the great
geological interest of asection exposed on the western side
of Brown's River-road, near
One
Tree Point.On
thewhole
Mr.
Johnston's observations hold goodup
to thepresent day, though
some
detailshave
to be altered. Mr.Johnston has
shown
thatwe
have
here a volcanic rock ofthe Basalt-aronp overlying sedimentHry rocks, which con-tain an
abundance
of plantremains.The
most
importantof these Mr. Johnston sent to the late
Baron
von Mueller,who
was
able to identifythem
with species well-knownfrom
the auriferous drifts of Victoria.In
itself thiswould 1e a
most
importantfact, but the flora, according tothe Baron's determination, is of such an extraordinary nature, thatit wouldbe almost unique, if his determinations
could be accepted as correct.
Many
years later Messrs.McLeod
and White examined
the Basalt,
and
came
to the conclusion that it is a basaltin which Fayalite, the red variety of Olivine, has replaced
the Augite
and
Olivine (2).I have frequently studied this section, remarkable in
more
than one way,and
I havecome
to the conclusionthat it allows for an intei-pretation quite different
from
that given
by
Mr. Johnston.The
main
pointon which
Idiffer
from
him
is of tectonical nature. Mr. Johnstonthinks that the southern pori}ion of the section has been
thrown
up, but a careful examination has convincedme
that it is not the southern portion thathas moved, but the
northern one; further, there is not an up-throw of the
(1) Notes showing that the Estuary of the Derwent was occupied by a fresh water lake during the Tertiary Period. These Papers and
Proeeedinos, 1881,p. 74-.
(2) Notes on a Favalite Basalt from One Tree Poini..—These Papers and
96 SECTION AT ONE THEE POINT,
U.S. TAS.
southern, but a
down-throw
of thenorthena portion.The
ends of the leaf beds are turned
upwards
near the fault,and
this admits of only one explanation, viz., thatthi«part
moved
downwards
in relation to the southern one.If this view be admitted, the interpretation of the section
tak«s a (^lifferenl aspect at once,
when
we
replace tliedown-thrown
portion to its original level. I further noticed that theleaf beds ofthe northern portion are considerably thicker than those of the southern one. Thereforewe
must
assume that, if the portions on either side of thefaultoriginally formed one continuouslayer, a considerablepart
of the leaf beds
must have
disappeared in the southern(remaining) portion. This observation holds good, whether
the southern portion is
thrown
up
asMr.
R.M.
Johnstonthinks, or
whether
the northern portion has slid down, asI opine.
The
origin ofthecurioiisBrecciaseems tome
stillsome-what
doubtful. Mr. Johnston thinks it represents old landslipswhich
fell in the "lake."There
are, however,many
objections to such a view,which
I will set forthlater on.
This section is perhaps the best illustration of the
more
modern
geological history of Tasmania,and
thesequence of strata, as well as its tectonical features, afford
a wonderful
mass
ofinformation regarding the latestphasesof the geological evolution of Tasmania. 1 think it will, therefore,
be
advisable to describe first the section as itnow
stands, then to' discuss its different membei-^, and,lastly, to deducethose conclusions
which seem
to afford anexplanation of thefacts observed.
2.
The
Leaf
Bed
Series.Though
not ecxposed atOne
Tree Point, it is prettycertain that the lowest
member
of thewhole
series is a yellowish sandstone, but so far the contact of Sandstoneand
Breccia has not been observed (3). Itmay
be that thelatter restsimmediately on the Sandstone, orthatsome
beds of different,
more
argillaceous nature, intervenebe-tween
the two.On
the whole this is of little importance as far as the questions here discussed arc concerned, ifwe
always bear in
mind
that the Breccia does not fo-rm th<>base of the series.
(3) A little further towards Hobart a sandstone of yellowish colour appears from underneath the Breccia. There is no doubt that this
sandstone belongs to tlie LeafBed series, of wlurli it most probably
BY FRITZ NOBTLING, M.A. PH.D. 97
^1913.
Mr. R.
M.
Johnston has already noticed that the leafbed series asa. whole, wherever they occur, are not found
higher than about 40 feet a,bovethe present sea level,
and
from
this psculiai^ deposition he concludes that, exceptingfor the south-easterly dip, the leaf bed series is practically
in the
same
position as itwas
when
it was deposited.He
turtJier assumes that the strata in question are of
lacus-trine origin,
and
in hisfirstpaper he draws a vivid pictureof the "lake" filling
up
the Dei^vent estuary.Assuming
that Mr. Johnston's hypothesis be correct, it requires
—
(a) Thatthevalley of the
Derwent
existed almost in its presentshape jDievious to the deposition ofthe leaf bedseries.
(I)) Thatthe stratadeposited in this lake gradually
thinouttowardsthe hillranges on either side.
The
first proposition assumes that the deep canyon ofthe Detrvvent
was
completed to about the present day levelin Pre-Eocene times (4). In other words, that the greatest
portion of the
Derwent
valley is geologically of very oldage (5). This is a very fascinating theory, but it mainly
depends
on the age of the leaf bed series. There is every reason to believe that the Tertiary strata of Australia aremuch
younger than Eocene; in fact, it ismore
thanpro-bable that they are not older than Miocene. Natui^ally
this
would
greatly reduce the age of the valley inwhich
the lake
was whose
waters deposited, the leaf beds- Thisis aquestion
which
cannot be decidedfor the present,and
w'e have to leave itan open
one, but Ido
not think thatthe
Derwent
valley dates back as far as theUpper
Cre-taceous period.As
regards the second proposition,we
should expect tliat the leaf bed series gi'adually gets
thinner towards the shore line of the old lake,
which
naturally
must
besomewhere
along theMount
Nelsonridge, for instance, on the western side.
Though
I ob-served the leaf bed series on the lower part of thenew
road to
Mount
Nelson, I have net been able to ascertainwhether
thevcomply
with this requirement of the lakehypothesis or not. If anything can be said from the
ratherunsatisfactoi*}' outcrops itis this, that the thickness
of the leaf beds is not reduced towards the hillside.
On
the otherhand,Mr.
R.M.
Johnston'sown
sections,(4) For the sake of argument I accept the view that the LeafBeds are of Eocene age, though according to more modern researches this
view is no longer tenable.
(5) Accepting the present view ofthe Eocene age of the Le-afBeds
98 SECTION AT ONE TREE POINT,
K.S. TAS.
particularly his Fig. 2, the section opposite
Sandy
Bay
Point, offer amost
serious objection to hisown
hypothesis;leastways they are capable of quite a different
interpreta-tion.
As
shown
bybim
in Figure 2, the leaf beds abutting against the Palaeozoicrocks are bent upwards.Now,
suchaneffect will never be produced during sedimentation; only
a subsequent
downward movement
of the strata canpro-duce such an eft'ect. Mr. R.
M.
Johnstons
Figure 2plainly suggests the existence of a trough fault; that is to
say, that the leaf beds were deposited at a
much
higherlevel than they are now,
and
that subsequently thewhole
mass
moved
downwards
alongtwo
great faults, one ofwhich
is running approximately parallel to theMount
Nelson range,
and
that during this downwai'dmovement
it
was
broken, the different pieces acquiring the southerntilt.
Of
course, ifit can be proved that the leaf bedseriesarepreserved in a trough fault, the "lake" hypothesis,
not-withstanding itsseductiveness,has to be abandoned. Before
we
decidewe
will havetomake
further investigations,and
I refrainfrom
expressingmy
opinion oneway
or other;forthe purposes of this paper I acceptMr. Johnston's lake
hypothesis, because the
main
results will not be affectedby
it. Should, however, future investigations prove that the leafbed
series restnow
inside a trough fault,many
of theseemingly incongruous features
would
easily explainthemselves.
Mr. B.
M.
Johnston has already noticed that the leafbed series have a very unifoi'm dip towards south,
and
are traversedby
a series of faults,which
run apparently nearlyparallel in north-west, south-eastern direction. These
faults have produced a feature
known
as stepfaults; thafcis to say, while one- end of the
mass
of rockremained
stationary, the other
moved
downwards.
In the leafbed
series apparently the southernend
of themass
between
two
faultsmoved
downwards,
while the northernend
re-mained
stationary. Imay
point out that this is a feature frequently obsein^ed in strata preserved in trough faults;for instance, in the coal basins of Giridih, in Bene^al.
3.
Description
ofthe Section
atOne
Tree
Point,The
sequence of the strata as seenin the above sectioiiisas follows (see Plate iv.):
—
(a)Northern
part.BY FRITZ NOETLING, M.A., PH.D. 99 1913.
(4) Leaf
beds-(b) Altered, 4 to 6 feet. (a)
Normal,
about 90 feet.(3) Breccia, about 12 feet.
(2) Leaf beds, normal, about 40 feet.
(1) Breccia, about 80 feet.
(b) Southern part.
iv.
Humus,
about4 to 5 feet.iii. Basalt.
(b) Normal,
(a) Vesicular.
ii. Leaf beds.
(a) Altered, about 4 to 5 fe^t. (b)
Normal,
about 40 to 42 feet.(c) Arenaceous, about 18 feet.
i. Breccia, about 130 feet.
The
sequence of strata as given above shows thatthe two parts of the section north
and
south of the faultare
somewhat
different.The
differences are slight only,and
may
not bematerial, yet theyhave
to be noted.The
most
important is the appearance of a Brecciabed in the lower portion of the leaf beds;
where
itreachesthe surface it has a thickness cf about 18 feet, but it very
rapidly becomes thinner in the direction of the dip, and
has probably died out completely before reaching- the level
of the road. This conclusively proves that though the deposition of the leaf beds
was
continuous, itwas
locallyinterrupted
by
layel's of Breccia.In
other words, the agencies which produced the Breccia continued tosome
extent during the deposition cf the leaf beds,
though
onthe whole it
would
appear that the Breccia is older thanthe leaf beds.
The
leaf beds are overlaidby
a fairly thick layer ofvesicular Basalt,
which
is, however, of small horizontal extension,and
rests unquestionably on altered leaf beds.The
southera portion issomewhat
obscured: the alteredleaf beds foi-m a conspicuous yellow band, which extends
from underneath the Basalt
up
to the fault; above thisare about five feet of leaf beds,
which
appear as if theyhad
beenworked
up,and
seemingly dip in northerndirec-tion; these are followed
by
a bed ofhumus
appearing asa conspicuous dark band.
The
remarkable feature is that thehumus
does notap-100 SECTION AT
ONE
TREE POINT,Pw.S. TAS. plies to the
worked up
leaf beds. I shallhave
to dealwiththisfeature presently.
The
southern portion appearsto be
more
regular.Under
a thick layer of Basalt which, rapidly thins out in northern direction follows a layer ofvesicular Basalt, which rests on altered leaf beds, of
con-spicuous yellow colour, of about 3 to 4 feet in thickness;
below these are the
normal
leaf bedswhich
g-et ratherarenaceous towards the base, resting
on
Breccia of gi'eat
thickness-These leaf beds are again observed at sea level, abo<ut
40 to 50 feet below the
main
section, overlaidby
massiveBasalt,
which
is dividedby
a vesicularband
(6).4.
Description of the Strata Observed.
A. The Breccia. (PI. v.
and
vi.)Mr. R.
M.
Johnston has given amost
accuratede-scription of this very rock,
whose
peculiar featureshe
well noticed. It is "a motley assortment of coarse
and
huge
angular blocks principally of the fossiliferousmud-stone." Thisfeature is exceedingly well
shown
in Plate vi.Though
withoutdoubt
the blocks ofmudstone form
far the majority, there occur
many
boulders of Diabasefrequently of large dimensions (See Plate v.). These
boulders are always well rounded,
and
thus sharplycon-trast with the angular blocks of mudstone.
The
bouldersare
imbedded
inan
argillaceous,somewhat
gritty matrix.At
the northern portion amudstone
boulder of fairlylarge size is deeply pressed into the underlying leaf beds. (See Plate vi.).
The
dark lines of stratification seen inthe white leaf beds closely follow the contour of tJie
boulder,
and
this proves conclusively that itmust have
pressed into the leaf beds while these were still in a
plastic state.
I
have
been greatly puzzled as to the origin of thisBreccia. It unquestionably closely resembles a
moraine
produced
by
the action of glaciers,and
I think thatunder
other circumstances
nobody would
hesitate to consider it of glacial origin.On
the other hand,no
scratched bouldershave
so far been found,and
I cannot quiteimagine
how
a glacier couldhave
passed over bodswhich were
unquestionably in a softand
pliable state (7)(6) To judge from Mr. Johnston's figures th^i Breccia could be observed at sea-level in 1881. During
my
visits the Breccia was notvisible, being covered under a layer of debris.
BY FEITZ NOETLING, M.A., TH.D. 101
1913.
without disturbing
them
further than pressing a boulderinto the top layer. Mr. R.
M.
Johnston thinks that theBrecciarepresents the debrisof landslips that fell into the
"lake." This is a very fascinating theory,
and
looks veryconvincing at first, but on closer inspection there are
many
objections to it.To
begin with, there are certainrea/sons against the lake hypothesis, but I will let that pass (8).
The
main
objection to the landslip theory isthe composition of the Breccia;
how
is it possible that ina debris produced
by
a landslip wellrounded
boulders aremixed
with angular blocks? In all the landslips I haveseen inthe Himala^^asthe debris consisted of shar]o angular
fragments,
which
were, moreover, neverimbedded
in amatrix as seen at
One
Tree Point. Further, if thelandslip fell into the water one should assume that the
action of the waves started at once the process of sifting.
Instead of preserving its present appearance,
where
blocksof all sizes are irregularly
mixed
up, the heavier blockswould
have eventually settled towards the bottom, leaving the lighter ones near the top. In fact, I cannot imaginehow
an entirely unstratifiedmass
remained
as such in thewater for
any
length of time.And,
again, if themass
ofdebris fell from above,
how
is it that only one solitaryblock has been observed to be pressed into the underlying
strata,
which
were, asitmust
be keptin mind, in a pliablestate
when
the Breccia was deposited? If such amass
ofdebris, as
shown
inPL
vi., fell on the top of a softmud
one should imagine that
numbers
of thehard
bouldersbecame imbedded
in it.But though some
large boulderswere within an inch or so
from
the top layer of tlie leafbeds, they were not pressed into it, as is well seen in Plate
vi.
The
whole appearance of the leaf beds and Brecciaproves that the deposition of the latter on the top of the
former
must
have been a quiet, rather than a violent,pro-cess. Further, the Breccia layer in themiddle of the leaf
beds in the northern portion
becomes
thicker towardsnorth; that is to say, towards a direction
where
therewere no cliffs
from
which
the debris could have fcillen,though
Iwould
not lay toomuch
stress on this.Taking
everything into consideration,this "landslip" hypothesis
af-fords, therefore, such a lot of difficulties, that it is
no
longer tenable.For
the present Iam
unable to replace itby
any
other theol'3^and
the origin of the Breccia is still aproblem. If
we
could conclusively prove that it were a102 SECTION AT ONE TREE POINT,
R.S. TAS.
moraine,
many
of the present difficultieswould
disappear.One
ofthe- chiefarguments
against the moraine hypothesis is the present position of the leaf bed series, but were theview that the leaf bed series rest in a trough fault, that
is to say, are
now
in a lower position than they wereoriginally, correct, one of the chief objections against the
glacial origin of the Breccia
would
disappear.B. The
Leaf
Beds.The
leaf beds represent a series of finely laminatedclay of whitish colour,
which
towards its base on thesouthern section
becomes
somewhat
arenaceous.When
exposed to the air the leaf beds soon crumble away,
and
this unfortunately renders the presei'\'ation of the fossil
remains
most
difficult (9).Mr. R.
M.
Johnston has given a largenumber
offigures of plant remains collected
by
him,and
describedby
Ettingshausenand
von Mueller.The
association of thisflora is rather a curious one; next to such genera like
Betuhij
which
occurs in coldand
Arctic climates, orFagiis,
which
is essentially a genus preferring a cooler climate, or Quercnsand
Salix, also genera existinor intemperate cooler zones,
we
have
in Clnnaynonum,Sapo-tacites, or Cassia, genera of essentially a tropical nature. It is very difficult to accept such conflicting evidence as final,
and
considering the very unsatisfactory state ofpreservation, I cannot help thinking that
somewhere
amistake has been
made
(10.)If it could be
shown
that the tropical generahave
been erroneously determined as such,
and
that only generaindicating a. cool climate occur, the theory of the glacial
origin of the Breccia
would
gain considerable support.Ill that case the leaf
bed
serieshad
to be included in the(9) At the same time it is pretty certain that a good many of the leaveswere fairly macerated when they bi^came deposited in the silt.
(10) During the discussion that followed the reading of this paper Mr, Rodway remarkedthat at thattime wlien Ettingsliausen determined
these leaf-remains, phyto-paheontology was in rather a backward state, a remark with which I heartily agree. When I remember the
unsatis-factory state of our knowledge of fossil i)lant remains, even as late as the end of the seventies or the beginning of the eighties, and when I
consider the great progress made sincf". it is more tlian probable that Ettingshausen's determinations are more or less wrong. Mr. Rodway
statedfossil leaves were usually giventhe names of tliose recent genera with which tliey liad ageneral resemblance,a practice which is entire-ly wrong, because, withoutthe knowledge ofthe fruitsorblossoms,which are farmore important than the leaves, the determination must always remain doubtful. I think that every jjala^ontologist will agree with
BY FRITZ NOETLING, M.A., PH.D. l03
1913.
Pleistocene,
and
the Basaltwould
be cf Post-Pleistoceneage. This view
would
much
better agree withobserva-tions
made
along the north coast of Tasmania. Ihave
repeatedly expressed the opinion that the view of the
Eocene
age of the fossiliferous beds nearWynyard
is erroneous.In
a,subsequent paper I willshow
that thesebeds are
most
probably contemporaneous with thestan-nifei-ous dnfts. Considering that Mr. R.
M.
Johnstonidentified one cf the leaves found in the
Wynyard
serieswith one of the
most
common
ones ofOne
Tree Point, viz.,Sapofacifes oiigonevrif^, thesynchronismof the leafbedsin
the
Derwent
valleywiththeWynyard
beds can betaken asan
established fact. Granting this, the leaf beds are ofPleistocene age,
and
another objection against the glacialorigin ofthe Breccia
would
disappear.On
tlie other hand, if it could be proved that thegenera indicating a cooler climate were wrongly
deter-mined,
and
the flora werereally one indicating a wai-merclimate, the glacial origin of the Breccia
would
not bequite disposed of, because it
would
be quite possible thatinstead of one glaciation only there were several
alternat-ing with
warmer
periods, as inEurope
and
Northern
America. This is a view that
must
not be entirelyover-IcK>keci.
For
the present this questionmust
be held inabeyance till a revision of the determination of the flora
has been made, but on the whole I
am
inclined to thinkthat thefinal verdictwill be toconsider the leafbedseries
as of Pleistocene but net of
Eocene
age.C. Thf^ BasaJt.
(PI. vii., viii.,
and
ix.).The
Basalt overlying the sedimentaiy rocks showssome
peculiar features. In the first instance itmust
have
had
a rather low t-emperaturewhen
erupted. Thisis conclusively proved
by
the small alteration ormeta-morj^hism the leaf beds sustained.
Nowhere
has anyevi-dence of fritting been observed; all the alterations
pro-duced ai"e a change of colour
from
white into yellow,and
even this is not always maintained.
The
low temperatureof the Basalt is further proved
by
the small alteration ofthe included fragments ofrocks through which the
magma
broke. This frequency cf inclusions is another peculiar feature;
some
of these are of large size, but unfortunat-elythey
have
so fai' not been examined. Mr. Johnston104 SECTION AT ONE TREE POINT,
R.S. TAS.
"surrounding stratified rocks. "
Now,
thoughI carefully
searched for fragments of the Palseozodc mudstone, I failed
to find them.
On
the other hand, fragments of schistoserocks,
which do
not occuranywhere
near, aremost
numer-ous. This seems to indicate that the
magma
only brokethrough beds belonging to the
Prc-Cambrian
schists(11)-Probably the explosions preceding the eruption completely
shattered the overlying Palaeozoic strata,
and
when
eventu-ally the eruption of the
magma
took place it only filledthe cauldron-like cavity produced
by
the explosions, en-closingand
enveloping only rocks from greater depths.The
examination of these inclusions is anotherproblem
that awaits solution,
and
probablymuch
information as t#the coimposition of the strata in greater depth
may
be
gleaned
from
it.Another
feature of the Basaltis its absence ofcoluni-nar structure (PI. vii.) ; instead of the customarv
columns
the Basalt is massive,
and
rather inclined to part inhori-zontallayers- This peculiarity
makes
it appear asif therehad
been several eruptions producing different flows.Of
these I
have
not foundany
evidence; the Basalt presentsthe appearance of one prciduced
by
a single eruption.Sometimes
it shews a vesicular appearance, particularly in contact with the underlying strata.But
such a layer ofvesicular Basalt right in the middle of the massive one is
also seen at the southern
end
of the section. I attributeit to a particularly strong percentage of water in this
par-ticular parti of the
magma,
but itmust
not be taken a^s proof of several eruptions (12).Even
a casualexamina-tion proves that
enormous
masseshave
been removed,and
that
what
we
seenow
forms only a small portion of theoriginal mass.
The
Basalt eruption atOne
Tree Pointwas
therefore not a fissure eruption, but is produced inharmony
Mnth other occurrencesby
a single eruption of acone-like mass, of
which
now
a small portion along itswestern edge is preserved. (PI. viii.).
Messrs.
McLeod
and
White
made
a chemicaland
microscopical examination of this Basalt,
and
have provedthe complete absence of magnesia.
They
have furthershown
that the small red grains represent the red varietyof Olivine, Fayalite ; Augite, according to the authors, is
(11) This would indicate that in tho Derwent Valley the Permian series most probably rests directly on Pre-Cambrian Schists.
(12) In tlie great Basalt stream flowing from the plateau of the Dscholan down the valley of the Yaniiuk numerous layers of vesicular Basalt can be observed in the massive ])ortions. These layers, which
BY FRITZ NOETLING, M.A., PH.D. 105
1913.
also absent. Accordingly the rock is considered a Basalt
in which Fayalite has replaced Augite
and
Olivine.Whether
under
these conditions it is correct to term thisrock a "basalt"' appears
somewhat
doubtful to me,par-ticularly
when
we
consider that it greatly differs in itsstructure
from
the true Olivine Basalt. However, itwould
be confusing to introduce anew
name,and
untilthe terminology has been revised, I prefer to use the
name
Basalt.Mr. R.
M.
Johnston statesthat he obtainedsome
bonesand
teeth probably belonging to a small marsupial relatedto Hi/pslprymnus in the cooling joints of an older sheet of
Basalt. I
have shown
above thatw©
have
no
reason tosuppose that tliere was
more
than one eruption,and
theprobability that these bones are of
much
later age, and werewashed
into a cooling joint, has to be taken into serious consideration.As
at thesame
time these bonesare very poorly preserved,
and
do not allowfordetermina-tion, except a vague resemblance toa family ofmarsupials,
they
might
as well be left out altogether.But
if the possibility that the bonesbecame
subse-quently
washed
into the cooling joints could be finallydis-proved,
and
Mr. Johnston's viewbecome
an establishedfact, the presence of the
modern
genusHypsiprymnus
would
speak for a very recent age of the Basalt,and
severely shake the opinion of its
Eocene
a^e.5.
Tectonical Features.
(PI. iv., viii., and ix.),
A
careful examination of the strata shows that theleaf beds of the northern portion are slightly bent
up-w^ards
where
they abut against the fault- This iscon-clusive proof that there is no overthrow, as Mr. R.
M.
Johnston assumes; butthat the northern portion has slid
down
along the fault. Therefore, ifwe
wish toknow
theposition of the strata before their dislocation
we
must
liftthe northern portion for about 80 feet,
and
place it ba-ckin its onginal position.
I have attempted such a reconstruction on Plate ix.,
but
we
see thatif thesunken
portion is liftedand
replaced inits original position a fresh difficultypresents itself. Ithas been
shown
that the total thickness of the leaf beds ismuch
smallerinthe southernthan inthe northernportion. Therefore, if the leaf beds of the northern poi-tion arecon-106 SECTION AT ONE TREK POINT,
R.S. TAS.
siderable pox'tion of the latter
must
have been destroyed,and
arenow
replacedby
Basalt(13).This is quite in
harmony
with the facts of volcanicactivity. Before the eruption of the
magma
took place a considerable portion of the overlying strata isblown
offby
explosions. It isvei^ probablej that the peculiar massof leaf bods above
mentioned
represents a portion of thedestroyed leaf beds
which
fell back.As
the isolated patch of ve«iciilar Basalt observed inthe northern portion
moves
to a higher level than it isnow, the
boundary
linebetween
Basaltand
leaf bedsmust
haverisen veryrapidlytowards north.This indicates the form.er existence of a cauldron-like
hole,
which
was
subsequently filledwith Basalt. In other words,the Basaltformed
originally a cone, a great portion ofwhich was
subsequently destroyed.We
cannow
trace the history of the eventswhich
eventually resulted in the present features with the greatestacciu-acy, butforthe
moment we
will refrainfrom
expressing a view as to thegeological time
when
they tookplace- In order to
make
the sequence of events clearer,we
will notwork backwards
in descending order, butwork
upwards from
the earliest eventwe
are able totrace.On
the supposition that Mr. R.
M.
Johnston's lake theory iscorrect,
and
thattbe leaf beds are practically in thesame
position
now
as they werewhen
deposited,we
have
thefollowing sequence ofevents:
—
1.
The
formationby
erosion of theDerwent
estuary tonearly its present depth. This
was
folkiwedby
theforma-tion of shore deposits, beginning with 2. Sandstone, followed
by
the deposit of(15) I maybe permitted topointout stillanotuer difficulty; suppos-ing that tlie"amount of the downthrow is more than 8o feet, in other words tliat tlie northern part of the section does not reprr->?ent the
direct'continuation of the southern one. Tliis would naturally mean
that the portions of the section on either side of tlie fault are not contemix>raneous, as lie assumed, but tlmt the southern jjortlon is
older than the northern one. In an undisturbed section we would have, then, the following
sequencer-Northern portion.
Southern portion.
In tills case the mass of strata removed above those that are now to beseen wouldbe far greater than here assumed; aU the strata, (4),
BY FRITZ NOETLING, M.A., PH.D. 107
1913.
3. Breccia(aperiodof intense denudation),
and
termi-nat-ed
by
the deposition .of4.
Well
stratifiedleaf beds. (TlieagenciesproducingtheBreccia
must have
continued during the formation of theleaf beds, because layers of Breccia are intercalated in the
leaf beds.)
.5. Pause in the deposition of sedimentary rocks,
dur-ing
which
the leaf bed series (including all the beds from2 to 4) were laid dry. (It is veiy probable that during this
process the leaf bed series acquire the remarkable regular
dip towards south.)
6. Partial destruction ofthe leaf beds
by
volcanic ex-plosions, followed immediatelyby
the erujotion of Basalticmagma
of probably very low temperature.7. Dislocation of the leaf bed series
and
the overlyingBasalt
by
faults striking in an approximate north-westerndirection,
and
having a northern dip, goinghand
inhand
with the destruction of the Basalt cone.
8. Period of extensive denudation
and
erosion of thebedofthe
Derwent
below thepresent sealevel. (Sealevellower than at tliepresent day.)
9. Depositionofarenaceous bedson Basaltat
Droughty
Point.
10. Rise of sea to its present level. (Partial filling
up
of theDerwent
valley with sea water.)11.
Formation
of hillwash
consisting of volcanicdebris, ashes, etc., representing the present
day
soil.12.
Formation
of shell depositsby
the aborigines.13. Present
day
deposits.All these events can be traced with the greatest
acciu'acy,
and
theymust have
followed each other in theabove order. There can be not theslightest doubt tliat the
period during
which
the Breccia was deposited in thelowerparts of the
Derwent
valleymust
represent a periodof
most
active denudation in other parts of Tasmania.Great masses of angulardebris of sedimentary rocks
mixed
with well
rounded
blocks of Diabase were quicklymoved
and
redeposited at convenient places. This period ofgreat activity
was
followedby
one of comparative rest,during which a fine silt
was
depositedwhich
preservedtheimpression of delicate leaves floating about in the water.
Occasional relapses of the fonner energetic denudation
108 SECTION AT ONE TREE POINT,
R.S. TA3. inthe deposition of Breccia beds bet-weenthe well stratified leafbeds.
The
climatic conditions of this periodmust have been
differentfrom
those prevailing at the present day, be-causethe listof plantsgivenby
Mr. E,.M. Johnston would
indicate asomewhat
milder climate than that of the pre-sent day.However,
as I pointed out above, thedeter-minations ofthe species are not
beyond
doubt.One
fact,however, appears to be certain, the annual rainfall
must
have
beenmuch
heavierthan
it isnow
during the timethe Breccia
was
dej)osited.It is impossible to say anything as to the length of
the period that lapsed after the leaf series
had
been tiltedup
aiid the outburst ofvolcanic action- In all probabilityit
was
not very long, however.Itfurther appears thatthedestruction
and
denudationof the newly-formed Basalt cone
commenced
no
soonerthan it
had
been built up. This destruction^must againhave taken place during a time of
most
energeticdenuda-tion.
The
RiverDerwent
cut its channel right throughthe leaf series, probably deep into the underlying
Permian
beds. Simultaneously the
whole
serieswas
brokenby
faults; the latter tectonicmovements
do not appear tohave
been very energetic,and
they probably ceased verysoon, while the denudation of the Basalt cone continued.
Until the exact extent of the former cone is
known,
it isimpossible to
form
an idea as to thequantity of matterre-moved,
but itmust have
been of great magnitude, becausewhat
is seen to-day represents only a very small portioRof the original bulk. (PI. viii.). It is, further, probable that the process of active denudation
came
to a standstillwhen
the level of the sea discontinued to recede,and
amovement
in theopposite directionsetin.By
thismovement
the valley of the Dei-went
became
filledup
to its presentlevel; in fact, it almost appears as if it
had
been higherstill-
The
data for this hypothesis are, however, ver}''insufficient,
and
I do not wish to saymore
on this point.6.
The Age
ofthe
Leap
Bed
Seriesand
the
Basalt.Mr.
R.M.
Johnston,when
speaking of the leafbed
series in his G-eology of Tasmania, refersitto the Tertiary periodgenerally,but on page 261 hestatesthatin oneofthe
leaf specimens of the Turritella group, near
Wynyard,
herecognised thewell-known
form
Sapotacites oliyonenrisEtting,whichoccursintheleaf beds oftheDerwent, notably at
One
infra-BY FPvITZ NOETLING, M.A., PH.D. 109
1913.
basaltic leaf beds belong to the Palseogene, and aslie
con-siders the fossiliferous beds of Table
Cape
to be ofEocene
age,we
mu&t
infer that he attributes thesame
age to theleaf
bed
series of the Deinvent valley.Under
thesecir-cumstancesthe erosion of the old
Derwent
valley inwhich
the leaf
bed
serieswas
depositedmust
have taken placein Pre-Eocene times; in other words, it
would
date backas far as the Cretaceous period.
More
recent researches, particularly of Victoriangeologists,
have
conclusively proved that Tate's views as tothe
Eocene
age of the Australian TertiaryBeds
areno
longer tenable. All the Tertiaries of Australia are
much
younger; in fact, it appears very doubtful
whether
theEocene
isrepresented at all in Australia. It ismore
thanprobable that the Table
Cape
beds are ofmuch
youngeragethan hitherto assumed,
and
consequentlythe leaf bedsof the
Derwent
aremuch
younger
than Eocene. Ifmy
views are correct, they cannot be older than Pleistocene,
and
we
need notassume
a Pre-Eocene erosion of theDerwent
valley if, as I believe, the leafbed
series were atthe time of their deposition in a
much
higher level than they are now,and
onlycame
to their present positionby
a
downward movement
which
immediately preceded theeruption of the Basalt.
The
Post-Pleistocene period represents, therefore, thetime during
which
the leafbed
series were raisedfrom
thewater,
and
it is probable that eruption of the Basalt tookplace during the
same
period,though
itisby no
naeansim-probable that the latter is
somewhat
younger-The
period following the eruption of the Basalt
was
a time ofextensive denudation
which
destroyed anenormous
portionof the Basalt ccne
and
a large portion of the leaf beds.During
thesame
period theDerwent
valleywas
scoopedout.
As
the sealevelwas
theii at its lowest, or nearly its lowest, this stagemust
represent the timewhen
Tasmania
was
connected with themainland
of Australia.Towards
the
end
of thePleistocene, long after the glaciershad
dis-appeared, a gradual rise of the sea level
commenced,
butprevious tofinal separation from the
mainland
the presentflora
and
faunahad
settled in Tasmania.The
remains ofthe former fauna were a few isolated specimens of gigantic
mai-supials
which
found ahaven
of refuge inNorthern
Tasmania,
where
they died out before tlieyhad
time tospread.
The
last to follow were the aborigines,and
veiy soon after their aiTival the rising seahad
completely110 SECTION AT ONE TREE POINT,
R.S. TAS.
The
above results have beensummarised
in theap-pended
table, which, however, requires a few words of ex-planation.Thcugh
the actual sequence of the strata isbeyond
doubt, the age attributed to the different beds isopen
to dispute. It all dependson
the age of the leafbed
series.As
far as this is concerned, one fact is cer-tain, that it does not belong to theEocene
epoch,and
we
need not attribute a Pre-Eocene age to a pre-leafbed Derweiit valley.
Even
ifmy
view as to the Pleistocenewere not correct, the leaf beds are not likely to be oldor
than Miocene.
Plates.
iii. General viewof
One
TreePoint.iv. fSectionat
One
Tree Point.V. Diabase boulderin breccia.
vi. Ijrecciaandleaf-beds— boulderpressed into leaf-bed^
vii. Basalt overlyingthe leaf-l^eds.
viii. Section at
One
TreePoint.BY FRITZ NOETLING, MA., PH.D. Zfa
c o
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< 2 D
p.
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p. R. S. Tas., 1913. PLATE III.p. &. p. R. S. Tas., 1913. PLATE IV.
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