The Cenozoic rocks of the northeastern Cady Mountains can be divided into two separate sequences--an unnamed volcanic succession in the eastern Cady Mountains, and the volcanic and sedimentary strata of the Hector Formation in the northern Cady Mountains. In the eastern Cady Mountains, approximately 3,280 ft (1,100 m) of volcanic flows, flow breccias, domes, and laharic breccias overlie a basement complex of Mesozoic quartz diorite and quartz monzonite. These volcanic rocks have a continuous compositional variation from basaltic andesite to rhyodacite, although dacites and rhyodacites predominate. The age of this sequence is probably Oligocene and/or Miocene.

In the northern Cady Mountains, approximately 2,395 ft (730 m) of strata assigned to the Hector Formation (Oligocene? to Miocene) lie unconformably on a basement complex of Mesozoic quartz monzonite and metamorphic rocks. The upper part of the Hector Formation also unconformably overlaps the volcanic rocks of the eastern Cady Mountains. The Hector Formation consists of alluvial sandstone and conglomerate plus lacustrine and paludal sandstone, siltstone, mudstone, and limestone. Laharic breccias, ash-flow and air-fall tuffs, and andesitic to basaltic lava flows are interbedded with the sedimentary rocks.

Geochemical data indicate that the volcanic rocks from both areas are part of a calc-alkalic suite. The model proposed by various authors for the Cenozoic tectonic evolution of the western United States suggests that there should have been a transition from andesitic-type volcanism to basaltic-type volcanism, concurrent with the termination of subduction and the initiation of regional faulting. No such volcanic transition, however, can be identified within the northeastern Cady Mountains, nor has one been clearly identified in other late Oligocene to Miocene volcanic sequences within the Mojave Desert region. If this type of transition did take place within the Mojave Desert, it must have occurred after 18.6 ± 0.2 m.y.B.P. This is the date obtained from the youngest dated volcani rock that is unquestionably part of the calc-alkalic suite observed in the northeastern Cady Mountains.

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