Graptolite distributions in Ordovician shelf, slope, and basinal facies in the Great Basin indicate that graptolites were scarce in open oceanic waters oceanward of the Cordilleran shelf margin and that they thrived in waters above the margin. This pattern is consistent with that of most zooplankton in modern oceans. It follows from these observations that the depositional setting of typical graptolitic shale was the area of the sea floor under continental-margin upwelling zones where graptolites flourished and within the oxygen-minimum zone where their rhabdosomes were preserved. With changes over time in relative sea level, deep oceanic circulation, and wind-driven surface circulation, the upwelling and oxygen-minimum zones may have thickened or thinned, migrated landward or oceanward, and expanded laterally, contracted, or even disappeared. The observed graptolite occurrences suggest that the primary graptolite biotope—that is, the habitat of diverse and abundant faunas—was a relatively narrow belt of upwelling waters along, and extending somewhat open oceanward from, the continental margin. Provinces were maintained only to the extent that species could disperse along continental margins. Distribution of typical graptolitic strata may be used to interpret development of continental margins, because such distribution incorporates a signal of sea-level rise or fall, oceanographic changes (especially upwelling), and tectonic events that led to creation and deterioration of upwelling conditions in which graptolites flourished.
- Geological Society of America