STRATIGRAPHIC UNITS IN THE BASIN FILL OF LAS VEGAS VALLEY, NEVADA, WITH EMPHASIS ON ALLOSTRATIGRAPHIC UNITS

Donovan (1996) developed a set of allostratigraphic units for the upper 500 meters of basin fill of Las Vegas Valley, using the most formal data available. The same investigation also established a set of hydrostratigrahic units, to document the differences, spatially, between the two kinds of units. By intention, the two kinds of units were, spatially identical at 36°10'8.28"N, 115°11'28.38"W.

In a nearby area, a temporally, spatially, fossiliferous, and lithologically restricted unit (Las Vegas Formation - LVF) had been named in 1965 and is becoming better known, due to the establishment of a geologically based national monument in the northern part of the bolson in December, 2014. Although the definition of the LVF conformed to the current practices of the time, the 1983 revision of the stratigraphic code would have recommended a more precise descriptor such as “Las Vegas Fossiliferous Silt” or “Las Vegas Bone Beds”. Additionally, a local mapping unit known as “QTs” was widely used on the four Las Vegas quadrangle maps and the more recent (2005) Las Vegas 100,000 geologic map; following the example of the earlier maps. The 100,000 map, and subsequent mapping, in the 21^st century, have provided additional absolute dates and a possible new technique to better characterize the lithological variations. The bulk of the “QTs” deposits can be shown to be completely surrounded, and underlain by, Quaternary sediments. And, in its first use in 1977 was originally symbolized as “QT(?)s”.

The relationship between the various stratigraphic units will be presented, and some of the newer 21^st Century data.