The engimatic Late Cretaceous mcDermott Formation
David A. Gonzales


The Late Cretaceous McDermott Formation is a distinctive maroon to purple unit that is exposed on the northwestern margin of the San Juan Basin. Previous workers have argued that this unit is composed of volcaniclastic deposits that were derived from a volcanic complex in the vicinity of the La Plata Mountains. There is no evidence for any volcanic features or events associated with the intrusive complex in the La Plata Mountains, and definitive field and petrologic evidence for volcanic deposits within the McDermott Formation has not been documented. The igneous material in the McDermott Formation is dominated by pebble- to boulder-sized fragments of diorite and monzonite that are similar to intrusive rocks exposed in the La Plata Mountains laccolithic complex. The deposits in the McDermott Formation mostly fine upward, and were deposited as debris flows and hyperconcentrated flows along with minor dilute stream flows.

An alternative hypothesis for the origin of this controversial unit is that dome collapse of the La Plata Mountains intrusive complex was created by magmatic inflation and roof-flank detachment creating gravity slides that were remobilized by streams flowing to the south and southeast. Reduction in lithostatic pressure may have also allowed for sudden release of pressure on the underlying magma body which could have produced minor volcanic-flank eruptions. This model is consistent with the dominant type of igneous material in the unit, and the fluvial-dominated systems that are preserved in these rocks.


  1. Gonzales, David A., 2010, The engimatic Late Cretaceous mcDermott Formation, in: Geology of the Four Corners Country, Fassett, James E.; Zeigler, Kate E.; Lueth, Virgil W., New Mexico Geological Society, Guidebook 61st Field Conference, pp. 157-162.

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