Deciphering the movement of the Argentine Precordillera from tropical to higher latitudes, Late Cambrian - Late Ordovician, through conodont δ18O paleothermometry.

ALBANESI G.L.; BARNES C.R; TROTTER, J. A.; WILLIAMS, I. S.

Harrisonburg

Simposio; 12 International Symposium on the Ordovician System; 2015

SOS, ICS, IUGS

The Cambrian-Middle Ordovician succession in the Precordillera of north-central Argentina is in several respects unique in South America. It is largely composed of richly fossiliferous carbonates whose lithology and diverse shelly and conodont faunas differ conspicuously from those of coeval poorly fossiliferous clastic successions in adjacent parts of Gondwana. Because of these differences, the Precordillera has during recent decades been regarded as an allochthonous tectonic unit of the exotic Cuyania Terrane whose geographic origin has been controversial. Hence, in some models, the Precordillera has been interpreted to be originally derived from a low-latitude segment of Gondwana and later in the Ordovician moved southward toward higher latitudes during periods of major faulting. In another, and perhaps more common model, the Precordillera is interpreted to have moved during the Late Cambrian-Early Ordovician from the southern margin of Laurentia across the Iapetus Ocean to finally dock at higher latitudes with Gondwana as suggested by the end of carbonate deposition and the presence of Late Ordovician glaciogenic sediments. However, many details in the Precordilleran drift history remain unclear.The Late Cambrian-Early Ordovician shelly faunas in the Precordillera are of general Laurentian type but stratigraphically slightly younger faunas have a paleogeographically mixed character where during the Dapingian-Darriwilian there is a gradually increasing number of colder-water taxa typical of the Avalonian and Baltic Provinces. This faunal change is exhibited in several fossil groups. Obviously, changes in ocean temperature are likely to reflect latitudinal changes of a moving microcontinent such as the Precordillera. In order to test if such temperature changes could be used for tracing the drift of the Precordillera, we used the oxygen composition (δ18Ophos) from well-preserved conodonts from Precordilleran and Laurentian successions. The objective was to test the possibility of a general temperature trend toward colder ocean water during the late Furongian (Late Cambrian)-early Sandbian (Late Ordovician) time that could reflect a latitudinal drift of the Precordillera. We used biostratigraphically well-dated samples from the Precordillera and two Laurentian control sites in Texas and Alberta. The δ18O composition in conodonts was measured in situ at high spatial resolution (30µ spots) using the sensitive high resolution ion microprobe (SHRIMP II) at the Australian National University. The conodont δ18O values range from 16.9 in the Tremadocian to 19.5 in the Sandbian for the Precordillera, whereas this degree of change is not seen in the values obtained from the conodonts of Texas and Alberta representing Laurentia.The resulting data show that there is a progressive change toward colder sea water temperatures in the Precordilleran samples after the early Darriwilian. This change can be correlated with the period of increased influx of colder-water faunal elements. Unfortunately, the virtual absence of conodont-bearing carbonates in the Upper Ordovician of the Precordillera makes it impossible to continue these studies into the Late Ordovician. However, we conclude that the new isotope data support the model of a drift of the Precordillera from tropical to higher latitudes across the Iapetus but further detailed studies in the Precordilleran and North American successions are needed to clarify unequivocally whether or not the Precordillera originated from southern Laurentia (Ouachita Embayment) as advocated by many authors.