TWO-STAGE TSUNAMI RESURGE DEPOSITION IN THE WAKE OF THE ALAMO IMPACT, LINCOLN COUNTY, NEVADA

The terminal graded beds of the Alamo Breccia preserve the waning stages of tsunami deposition in the wake of the Alamo meteor impact. Determining the flow orientation of the tsunami deposit not only can identify seafloor topography caused by the Alamo impact, but also track the elusive processes of tsunami deposition for offshore settings. This study spans the offshore Ring realm and nearshore Runup realm where the carbonate platform was detached by the impact and is thought to have formed annular basins away from the impact crater. The trend and plunge of over 1,000 imbricates and inclined clasts from the uppermost 2 m of the tsunami breccia were measured at over 30 localities and corrected for tilt of bedding. These trends identify the axis of flow, but cannot discriminate direction.

At the onshore Runup realm localities, clasts align roughly E-W (256.5°, sd=22.0°). The Ring realm localities, by contrast, exhibit two separate modes of flow axis, either N-S (341.9°, sd=18.0°) or E-W (260.9°, sd=19.8°). Mixing of the two modes at one locality has not been observed, which suggests that the clasts record paleoflows that are distinct in their process and timing.

We propose a two-stage depositional model for the resurge of water and sediment in the waning tsunami. In this model, the E-W axis represents a high energy, westward (offshore) sheet-like flow that deposited clasts early in the tsunami resurge, whereas N-S oriented clasts represent later stage, channelized flow. E-W orientations in the Runup realm may represent either shoreward uprush or early resurge. Later-stage resurge likely recycled some earlier deposited E-W oriented tsunami breccias. Preservation of the earlier E-W directed clasts is greatest on seafloor that rose topographically higher, or was spared from excavation and reworking by later-stage channelized flow. The N-S orientation of the proposed late-stage tsunami breccia may follow a shore-parallel basin hypothesized for the Ring realm.