Sedimentary Stratigraphy of the Alamo Impact Site
Sedimentary Rocks in Devonian
The Alamo Impact Event is contained within the Late Devonian Guilmette Formation throughout southeastern Nevada. The oldest group of pre-impact strata within the Guilmette Formation is named the "Yellow Slope Former" or YSF because it weathers to form slopes with a distinctive yellow color. The YSF often contains stromatolites near its base and the majority of the rock is laminite, a sedimentary rock composed of very fine layers often associated with a tidal flat environment.
The next oldest group of pre-impact strata is named the "Ledge Forming Interval" or LFI because these rocks weather to form ledges. The majority of the LFI is limestone, representing the carbonate platform environment that existed prior to the Alamo Impact Event. The LFI is also known for its high abundance of marine fauna, such as stromatoporoid sponges and brachiopods.
Overlying the LFI are the fragmented rocks associated with the Alamo Impact Event named the Alamo Breccia. The Alamo Breccia has been separated into 4 different facies, D through A, based on the way it was transported after the impact. The majority of the fragmented rock contained within the breccia is made out of the LFI limestones that existed prior to impact. A smaller portion of the rock fragments represents older strata that existed below the LFI and were carved into by the impact.
At the end of the Alamo Impact Event the marine environment begins to calm back down and return to normal marine deposition. This shift back to normal marine deposition marks the beginning of the Upper Member of the Guilmette Formation. Like the LFI, the majority of these rocks are limestones representing the carbonate platform environment post-impact. The Upper Member contains a diverse suite of marine life, and in some areas, even includes fossilized reefs. In many of the Alamo Impact outcrops, the Upper Member is topped with thick deposits of quartz sandstone. The sandstone was likely sourced from nearshore environments to the east and transported/deposited onto the carbonate platform as sea level lowered across the carbonate shelf.