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.
- 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.
- 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.
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...