Geology Department - Colorado College

Paul Myrow - Research Programs

Cambrian and Ordovician Strata of Rocky Mountains: Montana to Texas

Rocky Mountains Project Description:

Extinctions through the Cambrian-Ordovician boundary interval set the stage for the Ordovician radiation, one of the most important diversification events in the history of marine invertebrates. Stratigraphic data from carbonate platform and off-platform strata in the Great Basin and elsewhere have been used to propose process-response models that invoke "eustatic events" as a forcing mechanism for the extinctions. However, sedimentological interpretations that form the basis of these models are (at best) inconclusive. They rely on ambiguous lithologic information and/or lack the precision of correlation necessary to provide a rigorous test of the linkage between extinctions and paleoceanographic events. Also lacking are detailed data from nearshore and shoreline environments, settings where facies are highly responsive to relative sea level change.

For these reasons, an integrated biostratigraphic, chemostratigraphic, and sequence stratigraphic analysis of Upper Cambrian and Lower Ordovician rocks in Colorado (previous NSF grant to Myrow) was conducted. Despite the abundance of stratigraphic gaps in many sections, and the sparsely fossiliferous character of these inner shelf facies, this multidimensional study produced some of the most complete sections yet documented for the Cambrian-Ordovician "inner detrital belt". Additionally, trilobite faunas and lithofacies documented in Colorado more tightly constrain the position and timing of development of the Transcontinental Arch, which separated the eastern and western (modern coordinates) shelves of the continent at that time.

The proposed research will extend this study into coeval rocks in Wyoming/Montana and the southwestern United States, which offer sections from slightly more distal inner shelf settings on the west and east sides of the Transcontinental Arch, respectively. Continued use of multiple correlation tools, and utilization of the varied data in Graphic Correlation, will result in unprecedented precision of correlation of measured sections with one another, as well as with standard Cambrian-Ordovician successions in Utah and Oklahoma. This precise, integrated stratigraphic framework will be used to monitor sea level behavior (recorded in the vertical succession of lithofacies and sequence boundaries), paleoceanographic events (reflected in the isotope stratigraphy), and bioevents (represented by extinction horizons and intervals of adaptive radiation). This will allow us to test null hypotheses that horizons of extinction are marked by isotopic anomalies and/or lithofacies shifts, suggesting that paleoceanographic events forced the faunal changes. Comparison of relative sea level curves from opposite sides of the arch will test the validity of "eustatic events" proposed in previous studies of Cambrian-Ordovician strata on various continents. The precise chronostratigraphic framework developed in this study will also provide necessary temporal constraints for future chemostratigraphic, paleomagnetic, and paleobiologic studies of this important statigraphic interval.

*Note: Myrow et al. (2003; GSA Bulletin) argues against the presence of a Transcontinental Arch in the south-central Rockies until well into the Lower Ordovician.

Additional Details of the Rocky Mountains Project

Myrow et al. 2003

Photo: Upper Cambrian deposits of the Sawatch Formation near Horseshoe Mountain, outside of Fairplay, Colorado.

Photo: Glauconite-rich, large-scale, subaqueous, tidal dunes in the Middle Sawatch Formation in Manitou Springs, Colorado (see Myrow, 1996). The Pikes Peak Granite is in the foreground. The dunes have spacings of hundreds of meters and heights of over 3 m. This cross-bedded unit consists of a compound cross-bed set over 6 m thick on the right hand side of the picture.

Refereed Publications/Guidebooks:

Myrow, P., 1998, Transgressive Stratigraphy and Depositional Framework of Cambrian Tidal Dune Deposits, Peerless Formation, Central Colorado, U.S.A. in Alexander, C., Davis, R., and Henry, J., eds., Tidalites: Processes and Products, SEPM Special Publication No. 61, p. 143-154.

Myrow, P.M., 1995, Thalassinoides and the enigma of early Paleozoic open-framework burrow systems: Palaios, v. 10, p. 58-74.
Myrow, P.M., Ethington, R.L., and Miller, J.F., 1995, Cambro–Ordovician proximal shelf deposits of Colorado: Short Papers for the Seventh International Symposium on the Ordovician System, Ordovician Odyssey, p 375-379.

Myrow, P.M. and Grotzinger, J.P., 2000, Chemostratigraphic proxy records: forward modeling the effects of unconformities, variable sediment accumulation rates, and sampling-interval bias, in Grotzinger, J. and James, N., eds., Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World: SEPM Special Publication 67, p. 43-55.

Myrow, P., Taylor, J., Miller, J., Ethington, R., Ripperdan, R., and Allen, J., 2003, Fallen arches: Dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain region: Geological Society of America Bulletin, v. 115, p. 695-713.
Myrow, P., Taylor, J., Miller, J., Ethington, R., Ripperdan, R., and Brachle, C., 1999, Stratigraphy, Sedimentology, and Paleontology of the Cambrian-Ordovician of Colorado, in Lageson, D.R., Lester, A., and Trudgill, B., eds., Colorado and Adjacent Areas: Geological Society of America Field Guide, Boulder, CO, p. 157-176.
Myrow, P., Taylor, J., Miller, J., Ethington, R., Ripperdan, R., Brachle, C., and Owen M., 1999, Stratigraphic synthesis of the Cambrian–Ordovician rocks of Colorado: Acta Universitis Carolinae – Geologica, v. 43, p. 9-12.

Rigby, J.K. and Myrow, P.M., 1999, Lower Ordovician Sponges from the Manitou Formation in Central Colorado: Brigham Young University Press, v. 44, p. 135-153.