OCEANOGRAPHIC CONTROLS ON GRAPTOLITE SPECIES DISTRIBUTION IN THE TACONIC AND CENTRAL ANDEAN FORELAND BASIN SUCCESSIONS

The reliable use of fossils for biogeographic interpretation requires that environmental correlates of geography itself must be among of the dominant the factors that effect species distribution.  Although Ordovician graptolite faunas can be grouped broadly into latitudinally differentiated provinces, the difference between the isograptid biofacies (outer-shelf to open-oceanic biotope) and the didymograptid biofacies (inner to mid shelf biotope) is comparable to that among paleogeographic provinces.  Oceanographic differences associated with particular basins add a third set of major controls. Thus, the Late Ordovician Taconic Foreland Basin (despite its wide geographic extent and well-developed, basinal black shale facies with abundant graptolites) contains a restricted and partly endemic fauna that lacks many of the widely distributed species present in Pacific Province oceanic facies such as those of the Exploits Group in central Newfoundland or the Sebree Trough succession in the Laurentian midcontinent. The graptolite biogeography of the Andean margin of South America exhibits an even more striking and complex contrast.  The Precordilleran Terrain most likely occupied  peri-Gondwanan position outboard of the Famatinian Magmatic Arc during the Middle Ordovician.  The Precordilleran peripheral foreland basin in this interval yields Pacific Province isograptid biofacies faunas.  In contrast, strata from the back arc side of the Famatinian Magmatic Arc and the Central Andean Foreland Basin (CAFB; located in NW Argentina, Bolivia, and Peru) contain diverse didymograptid faunas of Atlantic Province type.  Recent discoveries indicate that these faunas also include oceanic isograptids (e.g., Isograptus victoriae, Arienigraptus sp. and Parisograptus caduceus).  This association indicates that the observed faunal differentiation is not of the on-shore to off-shore type.  Rather, we suggest in analogy with the Taconic case, that the CAFB was isolated from the oceanic system outboard of the Famatinian Arc.  This isolation allowed the CAFB to retain a strong Atlantic Province despite its oceanic character, relatively low paleolatitude, and close geographic proximity to the open oceanic regime that prevailed over the Precordilleran Terrain.