CARBONATE ACCRETIONARY LAPILLI IN THE LATE DEVONIAN ALAMO BRECCIA: IMPACT GENESIS AND IMPLICATIONS FOR TIMING OF EJECTA EMPLACEMENT

The Alamo Breccia, a Late Devonian carbonate megabreccia in southeastern Nevada, is the result of an impact that occurred on carbonate rocks in a marine environment. Post-impact orogenic deformation has apparently obliterated the original crater morphology, however signatures of the impact event are still preserved. Carbonate accretionary lapilli (Alamo lapilli) occur in conjunction with shocked quartz and minor iridium enrichment and provide evidence for the impact origin of the Alamo Breccia. Concentrations of Alamo lapilli formed lapillistone that occurs as discontinuous, reworked clasts within the Breccia. The Alamo lapilli resemble accretionary lapilli described from other impact sites and from silicate and carbonatite volcanoes. The structure, texture, and composition of the Alamo lapilli are similar from all localities, generally consisting of a nucleus surrounded by a mantle of silt- and sand-sized particles, and a very fine-grained outer crust. They are composed predominantly of carbonate mineral fragments, with minor shocked quartz grains, opaque minerals, and fossil fragments.

The original depositional stratigraphy is preserved in a few undeformed fragments of lapillistone as poorly size-sorted layers with varying proportions of matrix. Rare lapillistone fragments exhibit normal grading of lapilli size. They may form spherical masses or broken and contorted fragments with serrated edges indicating their degree of hardening before they were damaged by post impact processes. Preservation of the lapilli and lapillistone occurred by impact calcination of the pulverized target rocks. Lapilli formed through agglomeration of dust particles within or near the impact plume, were partially cemented in flight by hydration, and precipitated as one or more beds over resurge debris flows and/or nearly contemporaneous tsunamites. Cementation of the lapilli beds continued, where some survived the reworking, settling, and dewatering of the enclosing Breccia. Fragments of lithified lapillistone masses have been found up to 25 m below the top of the Breccia, indicating the thickness of Breccia reworked by post-impact processes.