A LARGE SEAWATER 87SR/86SR DROP (~ 0.0008) IN THE MIDDLE TO LATE ORDOVICIAN (LATE WHITEROCKIAN): EVIDENCE FROM CENTRAL NEVADA

The most striking feature of the Ordovician ⁸⁷Sr/⁸⁶Sr curve documented by Shields et al. (2003) is the large discontinuity between strata deposited during the Lower-Middle Ordovician and those of Upper Ordovician age. The ⁸⁷Sr/⁸⁶Sr drop of ~ 0.0008 is comparable in magnitude to the well-known Late Cenozoic rise but appears to have occurred over a shorter time interval (~5-6 myr) centered at about 460.5 myr in the middle to late Whiterockian (end Darriwilian) Stage. However, the precise timing and duration of the Sr drop are not well constrained in the Shields et al. (2003) composite due to biostratigraphic uncertainty and limited resolution in the critical interval. Here we present new ⁸⁷Sr/⁸⁶Sr data from a detailed measured section in the Monitor-Antelope Range region of central Nevada. This section contains biostratigraphically useful conodonts that are integrated as part of the Ordovician global Composite Standard Section in the graphic correlation scheme of Sweet and others.

A sample from the lower part of the Antelope Valley Limestone records a ⁸⁷Sr/⁸⁶Sr of 0.7089, while the overlying Copenhagen Formation yields a value of 0.7081 in its upper part. The entire measured section spans ~ 500 meters of section, and therefore our resolution and correlation with important sequence boundaries in the section is limited at this preliminary stage of our investigation. The abrupt transition between the pure, shallow-water limestones of the Antelope Valley and the clastic-dominated Copenhagen-Eureka succession marks a major influx of weathered material from cratonal source areas in western Laurentia. However, this regional input of radiogenic Sr was overwhelmed by events in other regions, perhaps the most important of which was the rapid erosion of juvenile volcanics (low ⁸⁷Sr/⁸⁶Sr) associated with the beginning stages of the Taconic orogeny in eastern Laurentia. The abrupt decrease in ⁸⁷Sr/⁸⁶Sr could also reflect a global increase in rates of sea-floor hydrothermal weathering or the net impact of a highstand of sea-level that flooded cratonal interiors (e.g., Tippecanoe transgression) in other parts of the world. For example, the Antelope Valley-Copenhagen transition correlates with the McClish (Tulip Creek)-Bromide clastic-carbonate transition in Oklahoma (P.serra/P.anserinus zones) that signals a reflooding of cratonal interiors.