Wednesday, August 16, 2017

Remembering M. Lee Allison: one year later

Lee Allison examining a diversion ditch, Flagstaff, AZ. 

A year ago today Lee Allison, Director of the Arizona Geological Survey and State Geologist, passed away 3 days after suffering a tragic accident at his home. We cannot adequately express the scope of this tremendous loss to the AZGS and staff members. Lee was unique and irreplaceable, a dynamo who was primarily responsible for the survival and growth of the AZGS during the 10+ years when he was Director. For Lee, challenges were opportunities to do more, and obstacles never stopped him from going forward. The AZGS became involved in areas of research and outreach that we never would have imagined before his arrival, and the profile of the AZGS was raised within Arizona, nationally, and internationally.

Fortunately for us who have had to carry on without him, the legacy of Lee’s dynamic tenure as Director was incredibly influential as the state legislature considered re-establishing a state appropriation for the AZGS this past spring. It was much easier to explain the critical value the AZGS provides to the State of Arizona because of the work we completed under Lee’s leadership, addressing many different aspects of geology and its impact on society and commerce. Lee truly viewed geology as underpinning modern society, and he never passed up an opportunity to make that argument in public forums. (Photo by A. Youberg, 8/25/2010)

Phil Pearthree
AZGS Director & State Geologist

Monday, August 14, 2017

Eldred Dewey Wilson & the Proterozoic ‘Mazatzal Revolution’

Study area map of Wilson (1937)
In 1937, geologist Eldred Dewey Wilson coined the phrase ‘Mazatzal Revolution’ to describe mountain building along the western edge of the North American craton. While the Mazatzal Revolution occurred in the Proterozoic - more than 1.6 billion years ago - it continues to influence Arizona geology and mineral exploration to this day. Wilson’s 1937 Ph.D. research is now available online for the first time.

In about 1920, twenty-two-year old Eldred Dewey Wilson joined a handful of geologists - N.H. Darton, Carl Lausen and Olaf P. Jenkins, among them – wrestling with the complex geology of the rugged mountains of southern and central Arizona. Wilson was an assistant geologist at the Arizona Bureau of Mines and working on his M.S. thesis, ‘The Mazatzal Quartzite, a new pre-Cambrian formation of central Arizona’ at the University of Arizona. In 1924 Wilson was promoted to geologist at the Bureau, where he remained, with a short leave of absence to begin his doctoral research in 1931-1932 at Harvard University, until his death in 1965.

Wilson set out in 1930 to address, ‘the chief features of pre-Cambrian regional structure within part of central Arizona’, for his Ph.D. dissertation – ‘‘The Pre-Cambrian Mazatzal Revolution in Central Arizona’. His field area included the Mazatzal Mountains, Pine Creek, eastern Tonto Basin or northern Sierra Ancha, Del Rio, and the southern Black Hills areas, all of which contained extensive outcrops of Proterozoic-age rocks. Wilson concluded from his observations of the field relationships of rocks and structures that the ‘principal features of regional structure originated from a great pre-Cambrian crustal disturbance’, which he called the ‘Mazatzal Revolution’.

Wilson’s ‘Mazatzal Revolution’ was an early contribution to deconstructing the processes responsible for the geology of central Arizona. He noted, ‘The subparallel folds, thrust faults, and imbricate, steeply dipping reverse faults clearly resulted from intense north­west-southeastward regional compression. The transverse faults are believed to have been formed, also during the compression, by shearing normal to the trend of the folds.’

Wilson hypothesized, too, that, ‘structural weaknesses inherited from the Mazatzal Revolution may have influenced the localization of many of Arizona's prevailingly north­eastward-trending veins and the pattern of the Tertiary Basin and Range faulting.’ The orogenic Mazatzal Revolution continues to impact Arizona geology today. 

E.D. Wilson ca. 1960s.

Reynolds & Others (2013) on Eldred Dewey Wilson’s contribution to Arizona geology. Wilson published a number of important papers on Arizona geology. According to Reynolds and others (2013), Eldred D. Wilson provided the first geologic map and cogent discussion of the geology and mineral resources of southern Yuma County: “Wilson mapped this hitherto unknown area of southwestern Arizona from 1929-1932. In the process, he discovered a new set of mountains that had been overlooked by previous geologists and explorers. He named this range the Butler Mountains after G. M. Butler, former Director of the Bureau and Dean of the College of Mining and Engineering (Wilson, 1931). Wilson was the first person to describe and map the geology of a large number of mountain ranges in southwestern Arizona. The data from Wilson's 1933 geologic map were incorporated into the 1969 state geologic map.” 

See James T. Forrester and Richard E. Moore’s ‘Memorial to Eldred Dewey Wilson 1898-1967’ for more about the life and times of Dr. Wilson.

Note: AZGS thanks an anonymous patron who arranged at his/her own expense with Harvard University to scan Wilson’s dissertation and secure copyright permission from Dewey Wilson to re-release Dr. Wilson’s work as CR-17-C. 


Forrester, J.T. and Moore, R.E., 1965 Memorial to Eldred Dewey Wilson 1898-1967. Geological Society of America Bulletin, V. 76, p. 187-191. 

Reynolds, S., Spencer, J.E., Richard, S.M., Pearthree, P.A. 2013, The Geological Exploration of Arizona: The Role of State and Federal Surveys and the Geologic Map of Arizona, Arizona Geology Magazine, Winter 2013.

Wilson, E.D., 1922, The Mazatzal Quartzite, a new pre-Cambrian formation of central Arizona. Univ. of Arizona M.S. thesis, 40 p.

Wilson, E.D., 1937, The Pre-Cambrian Mazatzal Revolution in Central Arizona. Ph.D. Dissertation, Harvard University, Cambridge, MA, 335 p.

Thursday, August 10, 2017

Important San Pedro River Subflow Adjudication Ruling!

By Order dated July 13, 2017, Maricopa County Superior Court Judge Mark H. Brain (presiding judge in the Gila River adjudication) affirmed the most recent effort by the Arizona Dept. of Water Resources (ADWR) to delineate the lateral extent of certain hydrogeologic areas known as subflow zones in the San Pedro River watershed in southeastern Arizona. A well located within a subflow zone is presumed to be pumping subflow, which is water that is hydraulically connected to the surface flow of a perennial or intermittent stream.

This ruling stems from technical reports and oral testimony presented before and during a four-day long hearing conducted in Phoenix in August-September 2015. It is the latest chapter in a multi-decadal effort to differentiate between surface-water and groundwater in Arizona. The San Pedro River, the Babocomari River and Aravaipa Creek were test cases for this issue; if courts bless a methodology there, it likely will be applied to other perennial and intermittent streams within much of Arizona where surface water rights are being adjudicated.

Adjudicating Subflow – More complicated than anticipated. Since 1988, the Gila River adjudication court has been involved in developing a test to determine whether a well is pumping subflow based on its location and certain hydrogeologic factors. In 2000, the Arizona Supreme Court held that a well located within the saturated floodplain Holocene alluvium (FHA) of a perennial or intermittent stream (known as a subflow zone) is presumed to be pumping subflow rather than groundwater. Since then, water users, lawyers, geologists, hydrologists, and judges have been wrestling with how to delineate the subflow zone for the San Pedro River. In general, the combatants have broken into 2 camps: the surface-water users, who argued for a wide subflow zone to maximize the number of wells that would be subject to the adjudication, and groundwater users, who argued for a narrow subflow zone so that fewer wells would be impacted. ADWR has occupied a difficult neutral position as technical advisor to the adjudication court – imagine barrages of slings and arrows from both sides. Delineating the lateral extent of FHA proved to be much more difficult than was originally envisioned by the Arizona Supreme Court, and has involved numerous reports from ADWR and several evidentiary hearings with multiple parties.

A Role for Geology and Geomorphology. The AZGS has provided substantial technical assistance in this process since 2007, when ADWR contracted with us to map the lateral extent of Holocene river alluvium and other geologic units along the river corridors. In 2009, we released geologic strip maps that accurately depict the geologic framework of the entire San Pedro River and its major perennial tributaries, Aravaipa Creek and Babocomari River (AZGS DM-RM-01). 


Surface contacts between Holocene river alluvium and bounding geologic units were the primary data used by ADWR staff to define the lateral extent of FHA in maps released in 2009. After critical comments from technical experts and legal teams, this approach was rejected by the courts in 2012; the reasoning was that surface contacts underestimated the lateral extent of FHA in the subsurface (see schematic diagram below).

In response to that ruling, ADWR contracted with us again in 2013. This time, we visited and documented stratigraphic exposures that shed light on the lateral extent of FHA in the subsurface (AZGS OFR-15-02). We also developed an overview of the geology and geomorphology of San Pedro Valley, focused on the nature and timing of river and tributary erosion and deposition that has shaped the river valley (AZGS Special Paper 10). These geologic investigations provided a conceptual framework for understanding river behavior in the late Pleistocene and Holocene (the past 50,000 years or so), and provided important insights into how FHA is likely distributed in the subsurface of the valley.

The Result. There is unavoidable uncertainty when attempting to estimate the extent of FHA in the subsurface. With our assistance, however, ADWR staff developed a new, more realistic delineation of the extent of FHA that has passed the latest test of thorough judicial review. It is uncertain whether any of the parties will seek review by the Arizona Supreme Court of the Gila River adjudication court’s ruling.

Phil Pearthree