The proto-andean margin of Gondwana and accreted terranes: contrasting biogeographic signatures based on Late Cambrian-Early Ordovician trilobites

VACCARI, N.E.; WAISFELD, B.G.

Toledo, España

Conferencia; Fourth International Trilobite Conference Trilo 08; 2008

Institito Geológico Minero de España

According to the records discussed herein, Furongian-early Tremadocian trilobites from the proto-Andean margin of Gondwana are broadly composed of numerous taxa accounting for a wide geographic distribution.  However, the presence of biogeographically sensitive taxa, such as Onychpyge, Amzaskiella, Golasaphus, and Australoharpes is significant. The fauna as a whole exhibits close links with coeval trilobite faunas reported from the Oaxaca block, in agreement with its position adjacent to the margin of Gondwana suggested by previous authors (cf. Fortey and Cocks, 2003). Trilobite records throughout the Cambrian successions of the Precordillera point to a strong faunal identity with Laurentia, and also to the absence of faunal exchange with coetaneous areas of East Gondwana. This influence persisted until the Tremadocian. Accordingly, the Laurentian Precordillera terrane is as allochthonous to the margin of Gondwana as the Gondwanan Oaxaca terrane is to the margin of Laurentia. Aceñolaza et al (2002) proposed a parautochthonous model for the Precordillera, recently upheld also by Finney (2007). According to this model, Cuyania is located adjacent to the Gondwanan margin during the Cambrian, close to Antartica. In their reconstruction Australia, Antartica, and Cuyania are in a similar latitudinal position, with carbonate platforms developed in the three areas, without separation by oceanic barriers. Cambrian faunas from Antartica and Australia conform a closely related area within the well-known Redlichiid Realm. Surprisingly, no faunal elements are shared despite this geographic proximity. If the position of Cuyania  relative to Antartica and Australia was that supposed in the parautochthonous model, a fluid faunal exchange  among these blocks should be expected in both directions. Accordingly, Upper Cambrian-Lower Ordovician trilobite evidence does not support the parautochthonous model for the Precordillera proposed by Aceñolaza et al. (2002), and Finney (2007).