Impact Markers in the Stratigraphic Record

Part of the series Impact Studies pp 315-332

Search for an Extraterrestrial Component in the Late Devonian Alamo Impact Breccia (Nevada): Results of Iridium Measurements

  • Christian KoeberlAffiliated withInstitute of Geochemistry, University of Vienna
  • , Heinz HuberAffiliated withInstitute of Geochemistry, University of Vienna
  • , Matthew MorganAffiliated withColorado School of Mines
  • , John E. WarmeAffiliated withColorado School of Mines

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The Alamo Breccia is a Late Devonian sedimentary layer as much as 135 m in thickness that is widespread over southern Nevada, USA. Direct evidence for an impact origin of the Alamo Breccia includes shocked quartz grains within the Breccia matrix and broken fragments of distinctive carbonate lapilli beds within the heterolithic Breccia clast population. The Breccia was studied as a potential impactite because it was recognized as an anomalous stratigraphic unit compared to the contrasting and predictable carbonate platform strata of the Devonian platform. To date, only one possible crater locality has been identified within the complex geology of Nevada.

A search was made for the presence of a meteoritic component in 23 samples: 16 from the Breccia matrix, 6 from the lapilli, and 1 background sample from below the Breccia. The samples were analyzed by multiparameter γ-γ coincidence spectrometry after neutron activation. The results vary between 19 and 51 ppt iridium, and indicate only minor possible Ir enrichment in the matrix and lapilli clasts compared to the background sample. The lapilli samples averaged slightly higher Ir values than the matrix. Because these are carbonate samples, and highly diluted by the massive Breccia, as little as 0.04 ppb matrix Ir may indicate a small meteoritic component. The lapilli population should have higher concentrations of rock from the target zone than does the mixed proximal and distal Breccia matrix, but they appear to contain insignificantly higher Ir values than the matrix. This leaves a variety of possibilities: either the Breccia does not contain much of an extraterrestrial component, or the Alamo impact event was caused by an Ir-poor projectile — either an achondrite, or maybe even an cometary nucleus.