Age and Provenance of the Eocene Crandall Conglomerate: Implications for the Emplacement of the Heart Mountain Slide

Abstract Prior to the emplacement of the Heart Mountain slide of northwestern Wyoming, the world’s largest terrestrial landslide, an extensive fluvial system traversed the area later affected by sliding. Some of what remains of this fluvial system is called the Crandall Conglomerate, which is presently found in the upper and lower plate of the detachment. The Crandall Conglomerate occurs at 15 localities in the vicinity of the Clarks Fork of the Yellowstone River. These exposures can be as much as 105 m thick and are composed of clast-supported conglomerates that are predominantly derived from local Paleozoic carbonate rocks. Although the exact age of the Crandall Conglomerate was previously unknown, it was generally thought to have been deposited prior to Heart Mountain faulting. In this study, we sampled the matrix of the conglomerate for detrital zircons in order to assess their age and provenance at six different locations (North and South Dead Indian Creek, Beem Gulch, Lodgepole Creek, Dead Indian Hill, and Squaw Creek). We analyzed 544 matrix zircons of the Crandall Conglomerate and found that despite there being no clasts of Eocene volcanic material of the overlying Absaroka Volcanic Supergroup, a significant proportion of the matrix zircons (18%) are Eocene in age. Because slide motion dismembered the Crandall Conglomerate fluvial system, the youngest zircons found in the conglomerate constrain the earliest possible time for slide emplacement. Using an iterative Monte Carlo technique we have determined that the best representation of the youngest zircon age is 49.57 Ma with a + 0.51 Ma and −0.64 Ma 2σ error. Using our spectrum of detrital zircon ages, we also have been able to determine the likely provenance of sediment sources that supplied the Crandall Conglomerate during its formation. The six sampling localities (four in the upper plate and two in the lower plate) contain zircons of Eocene (18%) and Archean (18%) age. Although Eocene and Archean zircons were expected based on their proximity to the Beartooth Uplift and Absaroka volcanic vents, samples also contain surprisingly abundant proportions of Mesozoic (7%), Paleozoic (5%), and Proterozoic (52%) zircons. We therefore interpret the Crandall Conglomerate as part of a larger system of rivers that drained the Sevier highlands and not a local drainage with spatially limited sediment sources. The diverse detrital zircon spectra of the Crandall Conglomerate also indicate that the development of a significant drainage system in the Heart Mountain slide area may have been more extensive than previously envisioned. This active erosion along the toe of the Absararoka-Beartooth highlands also could have aided in the gravitational instability that eventually led to the Heart Mountain collapse.