Ajo Mountains -- eastern portion of the Monument.
This range is composed almost entirely of Miocene volcanics. The
approximately 5,000 square kilometer field also contains subordinate
sedimentary units, mostly conglomerates and sandstones. Three major
episodes of volcanism occurred: 1) the basal Childs Latite, 2) rhyolite
and rhyodacite flows with rhyolitic tuff, and 3) basalt and andesite
flows and flow breccia. The volcanics rest unconformably on late
Cretaceous and early Tertiary granite and gneiss. The youngest rocks were
not affected much by Tertiary orogenesis, so they remain essentially
horizontal.
Approaching the Ajo Mountains on the scenic drive, the stratigraphic
relationships of the flows are visible.
A natural arch in one of the Ajo Mountains basalt/andesite flows.
The lighter colored nature of the lower flows is visible here.
Basalt/andesite flows and some tuff is also present.
Some early, light colored flows are visible in the lower right of this
picture. Very dark basaltic material is seen at the top of the outcrop.
Ash deposits can also be seen.
A wall of lava along the trail in the Ajo Mountains. Note the think ash
deposit along the base of the wall.
This peak clearly shows the relationship between lava flows and ash
deposits in the Ajo Mountains.
An isolated occurrence of some Tertiary volcanic material in the Puerto
Blanco Mountains in the western portion of the Monument. This material is
most likely equivalent to the Childs Latite to the east. Although mapped
as volcanic, this material may also be hypabyssal in nature.
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