Early Miocene basaltic volcanism across the Basin and Range was volumetrically minor in comparison to the widespread felsic-intermediate volcanism. Eruption of relatively small volumes of basaltic rocks generally preceded the larger volume felsic-intermediate eruptions. Across the southern part of the Transition Zone, alkaline basalts were interbedded with tuffs and fluvial- lacustrine sediments, together comprising the Chalk Canyon Formation. To the north of the Cave Creek quadrangle, a thick Chalk Canyon Formation section forms the steep slopes beneath a series of resistant, Middle Miocene Hickey Formation basaltic flows capping New River Mesa and Skull Mesa. In the Basin and Range to the south of these high-standing mesas, several small, tilted, fault-block ranges contain the same rock units as the homoclinally dipping basaltic and sedimentary sequence underlying New River Mesa (see figure 4b).