Description:
The geology of the Mormon Flat Dam Quadrangle is dominated by a thick succession of
Miocene volcanic rocks, and by the consequences of two important structural events: 1)
formation of the Superstition Cauldron at approximately 18.6 Ma (McIntosh, personal
communication, Ferguson and others, in preparation), and 2) regional extension which was
active for a short period of time directly before, during, and after eruption of the Apache Leap
Tuff. The Tertiary stratigraphy consists of three broadly defined sequences: 1) pre-cauldron
lavas, 2) syn-cauldron units - chiefly Apache Leap Tuff, and 3) post-cauldron lavas, intrusions
and volcaniclastic sedimentary rocks.
Regional extensional tectonism was active over a broad area south of the Salt River
Canyon during formation of the pre-cauldron lava field (between approximately 20.5 Ma and
18.6 Ma), and then for a short time after cauldron formation, along a narrower zone along the
Salt River. Areas lying to the north of Salt River remained relatively stable and unextended
throughout the history of the volcanic field.
Report and geology of the Mormon Flat Dam Quadrangle, Maricopa County, Arizona, three sheets,
scale 1:24,000.