Upper Devonian strata of the western U.S.

I have been working on late Famennian (uppermost Devonian) to Mississippian strata since 2005 with a number of collaborators (D. Fike, Washington University; Sam Bowring and Jahan Ramezani, MIT; David Johnston, Harvard; Erik Sperling, Stanford; George Gehrels, U. of Arizona; and several Polish academics) as well as Colorado College students (Jessica Creveling, Justin Strauss, Karri Sicard, Anna Phelps, Annie Hanson, Devon Cole, Anne Hakim). Our first paper (Myrow et al., 2011), written in collaboration with Charles Sandberg (USGS) and Robert Ripperdan (St. Louis University) included detailed measured sections and carbon isotopic data from rocks of this age in the western United States and central Europe. Latest Famennian strata record the earliest signals of a major glaciation at the end of the Devonian. Our initial work in north-central Utah identified several major positive isotopic excursions, the first of which is an abrupt ~3‰ positive excursion referred to as ALFIE (A Late Famennian Isotopic Excursion). A subsequent paper (Myrow et al., 2013) showed that a record of this event in Colorado shows a shift to lighter δ13C values, in some cases by as much as 5‰. Oxygen isotopic values are extremely variable between measured stratigraphic sections, in cases invariant across the contact, and in other cases covarying with the δ13C values. The high variability is demonstrated to have resulted from diagenesis linked to a regional unconformity and associated relative sea-level fall. A second isotopic shift in younger strata coincides with a paleokarst breccia, and we link this to the globally significant latest Famennian Hangenberg Event, which includes a eustatic lowstand and subsequent transgression. In this case, both carbon and oxygen isotopes in strata below this regional unconformity surface show the variable nature of diagenetic alteration of carbonate units during lowstand conditions. This has broad implications for the evaluation of δ13C data in the rock record, particularly the assumption that extensive correlatable isotopic anomalies reflect global marine signatures.

In collaboration with Polish and U.S. colleagues, we dated volcanic ash beds above and below the Hangenberg Shale in the Holy Cross Mountains of Poland. This unit is linked to the terminal Devonian global shift from greenhouse to icehouse climate conditions, a global transgression, and widespread black shale deposition. Our precise UPb zircon ages constrain the duration of the event, one of the most significant palaeobiological events of the Palaeozoic Era, to ~50100 ka (Myrow et al., 2014). This is comparable to those of Quaternary glaciations, and is consistent with both a glacioeustatic origin for the eustatic fluctuations and changes in ocean chemistry that led to this major reorganization of the biosphere.

We extended our stratigraphic range in subsequent work on Upper Devonian to Lower Mississippian strata in Utah and Montana. Integration of sedimentology, physical stratigraphy, chemostratigraphy, and biostratigraphy allowed us to recognize important disconformities and regional stratigraphic patterns, and thus reconstruct depositional, tectonic, and eustatic history of the region (Cole et al., 20150. We document a significant shift in δ13C in Lower Mississippian strata in Utah and Montana, which represents one of the largest positive δ13Ccarb isotope excursions of the Phanerozoic, linked to drawdown of atmospheric CO2 and glaciation in the Early Mississippian. Detrital zircon spectra for strata in Utah and Colorado link zircon ages to possible source terranes in western Laurentia. We also focused on an unusual shallow water faunal assemblage of spinicaudatans (clam shrimp or conchostracans), which are extant, bivalved, chitinous, benthic crustaceans that live in fresh to brackish water, in ephemeral ponds, estuaries, and other shoreline settings (Myrow et al., 2015). The remarkably large areal extent (~1600 km north to south, and ~1000 km east to west) of this very thin interval is one of the most unusual paleoecological events of the latest Devonian in Laurentia. We posit that it was an artifact of exceptional depositional processes including passive transgression (i.e., little or no wave or tidal ravinement) along with rapid opportunistic takeover of habitats during transgression. The radiation, possibly associated with temporary hypoxia, was aided by the spinicaudatan reproductive strategy of numerous offspring, high growth rate, and rapid reproduction.


Cole, D., Myrow, P.M., Fike, D.A., Johnston, D.T., Hakim, A., and Gehrels, G.E., 2015, Uppermost Devonian (Famennian) to Lower Mississippian events of the western U.S.: Stratigraphy, sedimentology, chemostratigraphy, and detrital zircon geochronology: Palaeogeography, Palaeoclimatology, and Palaeoecology, v. 427, p. 1-19.

Myrow, P.M., Cole, D., Johnston, D.T., Fike, D.A., Sperling, E.A., and *Hakim, A., 2015, Passive transgression: Remarkable preservation and spatial distribution of uppermost Devonian (Famennian) nearshore marine facies and fauna of western Laurentia: Palaios, v. 30, p. 490-502.

Myrow, P.M., Ramezani, J.,Hanson, A.E., Bowring, S., Racki, G., and Rakociński M., 2014, High-precision U–Pb age and duration of the latest Devonian (Famennian) Hangenberg event, and its implications: Terra Nova, v. 26, p. 222-229.

Myrow, P.M., Hanson, A., Phelps, A.S., Creveling, J.R., *Strauss, J.V., Fike, D.A., and Ripperdan, R.L., 2013, Latest Devonian (Famennian) global events in western Laurentia: Variations in the carbon isotopic record linked to diagenetic alteration below regionally extensive unconformities: Palaeogeography, Palaeoclimatology, and Palaeoecology, v. 386, p. 194-209.

Myrow, P.M., Strauss, J.V., Creveling, J.R., Sicard, K.R., Ripperdan, R., Sandberg, C.A., and Hartenfels, S., 2011, A carbon isotopic and sedimentological record of the latest Devonian (Famennian) from the Western U.S. and Germany: Palaeogeography, Palaeoclimatology, and Palaeoecology, v. 306, p. 147-159.

Comments are closed.