Natural Science Forum / Biology / Paleontology / March 2007

One important old question of mine below copied (some writing errors
corrected):

What is the exact age (with +- error limits) of the Bulawayan
Stromatolite UCLA-Bul-7=P.P.R.G.253 ?

Hannu

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From: h.poropu...@luukku.com (Hannu K.J. Poropudas)
Newsgroups: sci.geo.geology,sci.astro,sci.physics
Subject: Age of the Huntsman Limesone (Bulawayan) Stromatolite (UCLA-
Bul-7 = P.P.R.G. 253) ?
Date: 2 Nov 2004 04:57:31 -0800
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Hi,

Q0. Could the age of the Bulawayan stromatolite UCLA-Bul-7 be
   3080 Ma + - 60 Ma ?

(Pannella G given name is UCLA-Bul-7, Mode = 10-20-40,
Highest Count = 41, No certain year marks found, Interpretation:
40-41 mean solar days per synodic month at this time)
(= P.P.R.G. 253, in Schopf's (editor) Earth Earliest Biosphere,
Its Origin and Evolution, 1983)

Some background:

Huntsman quarries (1973) was near 19 degrees 43 seconds South,
28 degrees 48 seconds East) where the stromatolite UCLA-Bul-7 =
P.P.R.G. 253 was found from tailings of pile or some of the dumps
(three possible places due sample's founder Dr. Kvenvolden did not
remember anymore which of those three possible (near each others)
places it exactly was).

Geology near the Huntsman quarries:

The succession in descending order is:

Greensone
(dolerite sill)
Greenstone
Upper limestone
Banded ironstone
Lower limestone (with stromatlites)
Quartz sericite schist

Field relations, Huntsman quarries (my remark:
map (geology of near Huntsman quarry) on the
figure is from the year 1973):

1. Dolerite (Intrusive)
2. Greenstone
3. Upper limestone and banded ironstone
4. Quartz-sericite schist
5. Lower limestone and banded ironstone
6. Quartz-sericite schist

3.- 6. = Zwankendaba Member,
2.- 6. = Lower Formation,
2.- 6. = Bulawayan Group

Some text parts from the reference (Bond G., Wilson J. F.,
Winnall N. J., 1973):

"The Huntsman limestone was included without question in
the lower part of the 'Zwankendaba Group' of the 'Lower
Series of the Volcanic Greenstone System', which later
become the 'Bulawayan System' (now Group).
Lenticular limestone is associated with more persistent
Banded Ironstones, and outcrops at intervals for several
miles along the strike.
Limestone has been quarred only at Huntsman, and fossils
are not known from the other limestone occurences."

"Because the outcrop of the Bulawayan Group is continuous
throughout the area, and the Heany, Essexvale and
Bulawayo granites are intrusive into those rocks we
conclude that the age of the Huntsman limestone and its
fossils must exceed 2900 m.y., and is probably greater
than 3000 m.y."

Reference (Prendergast M. D., 2004) gives:

Age of the Bulawayan Supergroup is 2600 Ma - 3000 Ma.

On pages 432-433:

"In this straticgraphic type-section in the Belingwe greenstone
belt (Fig 2), the Lower Bulawayan is represented by the
Koodvale Formation of folded coarse, clastic, and felsic
volcanogenic sedimentary rocks."

On page 439:

"The chronological development of the Bulawayan Supergroup
is at present constrained by very few precize isotopic age data.
The interpretations of four ages (2880 Ma, 2839 Ma, 2698 Ma,
2645 Ma, (some text removed)) are uncertain and beyond the scope
of this paper to discuss."

Q1. Does the 'Lower limestone and banded ironstone' (ref. 1.)
belong to the 'Koodvale Formation (ref. 2.) and, if so then
what is the age of the Koodvale Formation (with + - error limits)?

Q2. Could the 'Lower limestone and banded ironstone' (ref. 1.) be
something else than lower greenstone mentioned in (ref. 2) ?

Q3. In reference (Jahn B-M., Condie K. C., 1976)
got 'preferred' age 3080 Ma +- 60 Ma, could this be
something else than greenstone's age (about 2750 Ma),
for example the age of Bulawayan stromatolite
UCLA-Bul-7 = P.P.R.G. 253 ?

References:

1. Bond G., Wilson J. F., Winnall N. J., 1973
Age of the Huntsman Limestone (Bulawayan) Stromatolites.
Nature, vol. 244, August 3, 1973, pages 275-276.

2. Prendergast M. D., 2004.
The Bulawayan Supergroup: a late Archean passive margin-related
large igneous province in the Zimbabwe craton.
Journal of the Geological Society, London, Vol. 161, 2004,
pp. 431 - 445. Printed in Great Britain.

3. Jahn B-M., Condie K. C., 1976.
On the Age of Rhodesian Greenstone Belts
Contrib. Mineral. Petrol. 57, 317-330 (1976)
(Rhodesia name become later Zimbabwe)

Best Regards,

Hannu Poropudas
Vesaisentie 9E
90900 Kiiminki
Finland