I would not be too surprised if there were more deposits in the US southwest. While the specific sequence of events for this one may be unique, there was tremendous volcanic activity there during the "Mid-Tertiary Ignimbrite Flareup". This event, from about 25-40 million years ago, produced enough pyroclastic deposits that (if evenly spread) would bury the entire continental US 55 meters deep. I believe the largest known supervolcanic eruption ever was part of this period.