Biostratigraphy

The Cordilleran region of western North America contains an abundance of fossiliferous marine sedimentary and interbedded volcaniclastic rocks of Late Triassic to Early Jurassic age that are ideal for calibrating biochronologic and geochronologic time scales. Previous work has established zonal schemes for many stages within this interval that are calibrated to variable degrees of certainty with dates from interbedded zircon-bearing ash beds (Smith et al., 1988; Tozer, 1994; Jakobs et al., 1994; Jakobs, 1997; Silberling et al., 1997; Guex, 1998; Pálfy et al., 2000; Taylor et al., 2001; Taylor and Guex, 2002; Guex et al., 2004; Longridge et al., 2006; 2008; Orchard et al., 2007; McRoberts et al., 2008; Orchard, 2010; Mundil et al., 2019). This is illustrated here on Figure 3 for two Stages of the Early Jurassic.

Today there is a continued effort to refine and improve this time scale by intercalating these biostratigraphic relationships with geochemical data (e.g. stratigraphic trends in carbon isotope stratigraphy). This powerful tool not only shows trends that appear globally correlatable, but can also be used to better understand changing environmental conditions which led to the apparent fluxes in the carbon cycle; leading to mass extinction(s). This is the foundation behind work at P.E.R.L., with continued efforts in several locations covering parts of Nevada, California, Oregon, British Columbia (Canada), Alberta (Canada), and Alaska. Pictures here represent ammonites collected from various stratigraphic sections from some of these sites.