K-Bentonites of North America and Gondwana
Stig Bergström
(Department of Geological Sciences, Ohio State University, Columbus, Ohio, USA)
11 May 1999
There
are 60 ash beds documented in the Ordovician of North America. Of these,
there are 2 big ones (the Millbrig & the Diecke). In Scandinavia,
there are 2 corresponding bentonites. North
America's Millbrig Bentonite (the thicker of the two) corresponds to
Scandinavia's “Big Bentonite” (now called the Kinnekulle Bed). The
Millbrig Bentonite thickens toward the southeast in North America. The
Kinnekulle Bed thickens toward the east in Scandinavia. Ordovician
paleogeography puts the original source volcano somewhere between Laurentia and
Baltica.
Recent
Ian Dalziel plate tectonic reconstructions show Laurentia adjacent to South
America during the Middle Ordovician. This puts the Baltica-Laurentia
connection in doubt, if correct. If correct, one should be able to
find corresponding bentonites in the Middle Ordovician of Argentina and
Bolivia, or elsewhere in South America.
The
Laurentia-South America connection during the Middle and Upper Ordovician -
there is a plate sliver between the two continents - the Precordillera
Terrane - it is now in Argentina. Bergström visited South America in
1994 to look for bentonites. Before this trip, no bentonites had been
recorded from the Paleozoic of South America. The Precordillera is
supposed to have come from the Ouchita Embayment of North America as a
microplate. It later became attached to South America. The
Precordillera is now east of the Andes. It is a separate microplate
sutured onto South America. The Famatina Range occurs to the
north-northeast of the Precordilleran Terrane. The Precordillera extends
from Mendoza in the south to San Juan to even farther north. Lots of wine
growing occurs in the Mendoza region. Ponon Tehue is a small Middle
Ordovician outcrop well south of Mendoza. The terrane is about 1000 km or
so in total length. Field work is best done in May (ideal time) or
October (roads are bad after summer rains, though). Since one needs to
drive up river beds, the summer is not a good time - rivers are flooded &
there will be lots of insects. Field work is different due to varying
road conditions.
Precordilleran
geology - to the west is the High Andes (mostly Tertiary rocks). To the
east of the High Andes is a subduction zone, and to the east of that are deep
water oceanic rocks (shales/mudstones/mafic & ultramafic
intrustions). To the east of that are shallow water carbonates and
clastics.
Looked
for bentonites in the deepwater shales at first - thought that the preservation
potential was higher in calmer, deeper water. There are some olistoliths
with Cambrian limestones and trilobites. Middle Ordovician shales are
slightly metamorphosed and are folded, but they have graptolites in them for
dating. Some basalts in these sections also. No bentonites found.
To
the east, rocks are less tectonized and less metamorphosed - the black
shales/mudstones there are fairly well preserved. No K-bentonites were
found there either. Then, went further east to the shallow water carbonate
sections.
Ordovician:
-
Los Azules/Invernada Formation - the basal part of this is carbonates; the rest
is mudstones-shales-siltstones with graptolites
-
San Juan Limestone
-
Zonda Formation - all carbonates, ~1500 meters thick.
Cambrian:
-
La Laja Formation
The
Cambrian carbonate section has North American-type shelly fossils (the same
trilobites as seen in the Great Basin and the Appalachian Basin). The San
Juan Limestone is very similar to the Knox of Laurentia, but the San Juan is a
limestone - it isn’t dolomitized like the Knox.
Conodonts
occur throughout this sequence. Rocks dipping to the west are common -
they are nappes thrust to the east during Andean orogenic movement.
Paleomagnetic
information has historically been difficult to get for the Early Paleozoic - it
all gives Permian latitudes. Lately, some Early Cambrian paleomagnetic
information has become available, which indicates a tropical latitude
(~10˚ S) - this matches up well with the Laurentian origin hypothesis for
the Precordillera Terrane.
The
first successful locality was found in May 1994 - found a yellowish, 50 cm
thick bentonite bed in a Middle Ordovician shale-mudstone-carbonate section
(San Juan Limestone + overlying fine-grained siliciclastics). The
bentonites occur in the lowest part of the Middle Ordovician
(Whiterockian). Went to other sections in the shallow water facies belt
and found other bentonites. Twenty different bentonites - previously
described by Argentinian geologists as clay beds. Found bentonites at
many subsequent localities in the ~same stratigraphic position. Watch out
for mountain lions!
To
the east of the Precordillera are Mesozoic/Cenozoic plains with volcanoes
sticking up out of them. A Precambrian-cored mountain range also occurs
to the east of the Precordillera Terrane, marking the boundary with the
Gondwana Terrane.
So,
found many localities with Ordovician volcanic ashes. All are in the
lower part of the Middle Ordovician, in the Gualcamayo Fm., in the Cerro Viejo
Fm., and in the uppermost San Juan Limestone. All of these are much older
than what we were looking for.
1:3
is the estimated compaction ratio for these bentonites. A 2 meter bed
today would be 6 meters thick originally.
Went
north to look for upper Middle Ordovician bentonites. The northern part
of the Precordillera Terrane is more difficult to get to. Did make it to
several localities and did find some bentonites - present in greater abundance
than expected. Found 180 individual bentonite beds at 1 stream cut - best
in the world. Few people ever had been there before. But, these
were lower Middle Ordovician again - wanted to find upper Middle Ordovician ash
beds. Went to the Famatina Range, which has a Gondwanan-type of fossil
fauna. The Famatina Range was a possible candidate for a source area for
North American & Precordilleran bentonites. Found no bentonites in
the Famatina Range, and the volcanics present there are the wrong chemistry to
have been the bentonite source.
Geologic
map coverage is not good - it is uneven. Just reconnaissance mapping is
available.
Despite
considerable efforts, couldn’t find upper Middle Ordovician bentonites in the
Precordillera. The lower Middle Ordovician of North America has few
bentonite beds. The lower Middle Ordovician of Baltica has quite a few
bentonites, actually, as in the Precordillera.
South
American & Laurentia are different worlds during the Ordovician.
Lithologically, South American and Laurentian rocks are not very similar.
Precordilleran docking with South America - this is a matter of some dispute;
some say it occurred in the Ordovician, and some say late Silurian or even
Devonian time.
So,
the Ian Dalziel plate tectonic reconstruction is unlikely. The
Laurentia-Baltica connection is still more likely. Several publications
are out so far on this - others are still pending. These bentonites are
chemically like subduction zone volcanoes - probably originally to the
northeast of the present Precordilleran Terrane.
Advantage
of bentonites - can be well dated radiometrically & biostratigraphically.
References:
Bergstrom
et al. (1996) - XIII Congreso Geologico Argentino y III Congreso de
Exploracion di Hidrocarburos, Actas 5: 481-490.
Cingolani
et al. (1997) - Revista de la Associacion Geologica Argentina 52(1):
47-55.
Krekeler
et al. (1995) - Ordovician Odyssey, 7th International Symposium on the
Ordovician: 355-356.
Huff
et al. (1995) - Ordovician Odyssey, 7th International Symposium on the
Ordovician: 343-348
Huff
et al. (1997) - Episodes 20(1): 29-on.
Huff
et al. (1998) - Ordovician K-bentonites... pp. 107-126 in The
Proto-Andean Margin of Gondwana.
Bergstrom
et al. (19xx) - Actas X Congreso Latinoamericano de Geologia y VI Congreso
Nacional de Geologia Economica 2: 439-444.
Two
Oliver Lehnert papers...
