UPPER DEVONIAN (FRASNIAN-FAMENNIAN) CONODONTS FROM THE HIGH-LATITUDE MADRE DE DÍOS BASIN, NORTHERN BOLIVIA

Seventy-five conodont elements recovered from organic-rich Upper Devonian northern Bolivian source rocks allow biostratigraphical refinement of the Frasnian-Famennian boundary. Eighteen horizons in a 19 m-thick interval (1564.2-1583.53 m depth) of the Mobil/Occidental Boliviana Pando X-1 slimhole core yielded well-preserved conodonts from bedding surfaces of organic-rich shales within interbedded sandstones and shales of the Tomachi Formation in the eastern Madre de Díos Basin. These organic-rich marine shelf shales (~16 wt% total organic carbon) are thermally immature based on a conodont color alteration index of 1 and a low palynomorph thermal alteration index of 1-2.

Diagnostic conodonts from the 1579 m to 1583 m interval include Polygnathus foliatus Bryant sensu Ziegler et al. 2000, Polygnathus sp., and Ozarkodina semialternans?. These are similar to upper Frasnian faunas reported from North America and Germany. The higher 1564 m to 1566 m interval yielded Branmehla bohlenana and Cryptotaxis sp., a fauna similar to that described from the grey to black shales of the Jandiatuba Formation in the Upper Amazon Basin of northwestern Brazil, and indicative of the middle-upper Famennian. The Frasnian-Famennian boundary occurs within the 1569.1 - 1579.3 m interval of the formation.

This is the first report of conodonts from Bolivia, and constrains biostratigraphic correlations based on palynomorphs, as well as a magnetic susceptibility curve that was also measured from the core. The conodonts commonly occur with scattered actinopterygian (ray-finned) fish scales and the green algal chlorophyte Tasmanites. The cosmopolitan nature of the conodonts and palynomorphs (acritarchs, spores) recovered from the core strongly suggests warm surface-water temperatures, open ocean connections between western Gondwana and Euramerica, and/or a lower latitudinal position for western Gondwana in the Late Devonian.