28 Geologic Mapping in the West (Posters)

This poster session highlights geologic mapping in the western Cordillera and Rocky Mountains. We encourage submissions that explore bedrock and surficial geology, geophysics, hydrology, hazards, and more. Both professional and student submissions are welcome.

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Proterozoic to Cenozoic Geologic Evolution of the Santa Maria River Region, West-central Arizona: Insights from 1:24,000 Scale Bedrock and Surficial Geologic Mapping

Paper number: 28-2
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM

Authors

Abstract

Combined bedrock and surficial 1:24,000 scale geologic mapping by the Arizona Geological Survey has documented the Proterozoic and Cenozoic evolution of the Santa Maria River region in western-central Arizona. The region is underlain by Paleo-Mesoproterozoic metamorphic and plutonic rocks, intruded by sparse Cretaceous dikes, blanketed with Paleo-Neogene sedimentary and volcanic rocks, and dissected by the Santa Maria River. Proterozoic metamorphic rocks include sillimanite and andalusite-cordierite schists, migmatitic gneisses, and amphibolites with a subvertical northeast trending schistosity to gneissosity. Intersection lineations and cm-scale folds indicate this foliation is isoclinally folded and transposed around an axis plunging shallowly to the northeast. Metamorphic rocks are intruded by the variably foliated ca. 1.68 Ga Granite of Thorn Peak, which crosscuts the foliation of country rocks and envelops them in sub-kilometer scale xenoliths, indicating the intrusion is post-kinematic relative to the main foliation. The Granite of Thorn Peak also hosts ubiquitous microgranular enclaves and schlieren that define a magmatic foliation parallel to the foliation of wall rocks and localized subvertical northeast striking meter-scale protomylonitic to mylonitic shear zones with predominantly dextral strike-slip kinematic indicators. The western margin of the Granite of Thorn Peak is crosscut by a 1415 ± 15 Ma megacrystic granite. The lack of Paleo-Mesozoic rocks suggests their removal possibly during Cretaceous-Paleogene contractional orogeny and/or subsequent Paleogene-Neogene extension. Paleogene to earliest Neogene alluvial conglomerates unconformably overly Proterozoic basement rocks and record a south-southwest directed drainage system. The onset of Neogene volcanism at ca. 21.8 Ma marks an angular unconformity with underlying Paleogene sediments. Explosive and effusive volcanism initiated with fractionated intermediate magmas and transitioned to dacite trachyte resulting in a thick (~450 m) package of interbedded lava flows, pyroclastic rocks, and volcaniclastic rocks which comprise much of the Tres Alamos Wilderness area. Faults that cut this volcanic sedimentary package strike ESE to SE. A transition from deposition to incision occurred during the late Neogene. River incision is spatially variable due to diverse bedrock lithologies that express varying resistance to erosion and recent drainage captures that occurred within the Santa Maria River system.  Deep seated landslides are prevalent in localities where river incision exposes layers of bentonite clay within Neogene sedimentary and volcanic rocks, and these features are a potential concern for infrastructure in the region.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410519

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