Age and chemistry of tertiary volcanic rocks in north-central Arizona and relation of the rocks to the Colorado Plateaus

During late Miocene (14.8 m.y.) to early Pliocene (10.1 m.y.) time, local latite and widespread basaltic flows accumulated with associated continental sedimentary deposits in north-central Arizona. Some of these rocks were displaced and tilted by normal faults, and new drainage, now occupied in part by basalt flows of late Pliocene age (average 5 m.y., maximum of 6 m.y.), was established at the southern margin of the Colorado Plateaus. The time of faulting and uplift of the Colorado Plateaus in this region is thus bracketed between about 10.1 and 6 m.y. ago.

Five analyzed basalts fall within the tholeiitic basalt field of the alkali-silica diagram and twenty are in the alkalic basalt field; in general, they are similar to other alkali-olivine basalts from the western United States.