Revisiting the Arizona Red: Lower to Middle Triassic Strata of Arizona

Описание: DESCRIPTION: On 4 March 2025, Chad Kwiatkowski, Research Scientist at the Arizona Geological Survey, addressed members of the Arizona Geological Society (AGS) regarding his research on the Lower to Middle Moenkopi Formation of Arizona. Chad’s presentation is 52 minutes long followed by 9 minutes of Q&A. Thirty people attended in person and another 20 people watched via live-stream. AGS thanks Hexagon Mining for supporting our 2025 speaker series.


ABSTRACT: The brick-red Moenkopi Formation, exposed in the valley of the Little Colorado River in northern Arizona, was deposited along western equatorial Pangea during the Early to Middle Triassic Period. The Moenkopi Formation has been valued by humans for millennia as a building stone and lithic canvas to record intricate carvings. The pueblo walls at Wupatki National Monument are built using Moenkopi Formation blocks, with an assortment of petroglyphs etched into the calcite-cemented sandstone of the lower massive sandstone member.
The lower massive sandstone member was quarried in Flagstaff beginning in the 1880s, quickly becoming Flagstaff’s second-largest industry. Referred to as Arizona Red, it was used to rebuild Flagstaff after fires destroyed the original town. It was also transported by train to be used in buildings as far northwest as San Francisco and as far east as Chicago. The sandstone’s calcite cement proved to be an ideal canvas for stoneworkers to carve intricate patterns and inscriptions. Unfortunately, the calcite cement was easily weathered when incorporated into buildings in more humid climates.
Moenkopi sediment was deposited in fluvial, deltaic, tidal flat, sabkha, and shallow marine environments, with an increasing marine influence to the northwest. The depositional system conveyed sediment from the Central Pangean Mountains (collisional zone between Laurentia and Gondwana) through a fluvial network and across a coastal plain to the Sonoma sea. Due to the gentle slope of the coastal plain, even minor changes in eustatic or local sea level led to widespread marine transgressions. In the fluvial and tidal flat environments of north-central Arizona, pre-dinosaur reptiles walked the floodplains while fish and large amphibians navigated the stream and tidal channels. A diverse reptilian footprint assemblage, exemplified by Chirotherium (hand animal) is recorded in the Moenkopi Formation, in places preserving delicate details such as scale impressions, claw impressions, and tail-drag marks.
The Moenkopi Formation thickens to the west-northwest, from ~50 m in north-central Arizona to ~600 m near Las Vegas; this is compatible with foreland downflexure related to emplacement of the Golconda allochthon in central Nevada. However, the upper part of the Moenkopi Formation has a relatively uniform thickness throughout the region, suggesting that Golconda subsidence had waned by Middle Triassic time. Moenkopi stratigraphic thickness contours are truncated at the Mogollon Rim, indicating that the Moenkopi basin continued some distance to the southwest. Additionally, volcanic detritus is present in the Moenkopi Formation, increasing in abundance to the south, toward the early Cordilleran arc, which is delineated by a series of Permian to Middle Triassic plutons exposed in the Mojave Desert and northwestern Sonora.
In west-central Arizona, southeastern California, and northwestern Sonora, calc-silicate metasedimentary rocks of the lower Buckskin Formation correlate with the Moenkopi Formation. The detrital zircon signature of the lower Buckskin Formation is dominated by Permo-Triassic grains likely sourced from the nearby Cordilleran arc. At Sycamore Canyon along thee Mogollon Rim southwest of Flagstaff, a detrital zircon sample is also dominated by Permo-Triassic grains. Quantitative comparisons of the lower Buckskin Formation and Moenkopi Formation at Sycamore Canyon display a high degree of similarity and probably represent a fluvial linkage from the Cordilleran arc to the main Moenkopi depocenter.
In summary, the Moenkopi and lower Buckskin Formations represent a complex tectonic basin along western Pangea. Emplacement of the Golconda allochthon during the Sonoma orogeny flexed the crust into a foreland basin in which the lower Moenkopi Formation accumulated. Dynamic subsidence above the subducting Panthalassan lithospheric slab was ongoing during the entire timeframe of Moenkopi deposition and was the dominant source of subsidence after foreland subsidence associated with the Sonoma orogeny waned. The Cordilleran arc contributed sediment to the arc-proximal portions of the Moenkopi basin via fluvial networks that flowed generally northward toward the main Moenkopi deposystem, eventually overwhelmed by detritus along the main, northwest flowing fluvial system.