Abstract

A 7-km wide circular structural feature on Black Mesa, Navajo Nation (Figure 1) has been proposed as representing an impact structure that formed in conjunction with nearby melt breccias. Some outcrops of mostly siliciclastic Mesaverde Group rocks on Black Mesa display various post-depositional thermometamorphic textures like melt brecciation and fusion of sand grains. Apatite fission track (AFT) analysis was applied to melt breccias and nearby unmelted sandstones to determine the age of melting at selected outcrops. For breccia sample BM-1: apatite is sparse, 1 of 9 AFT ages as young as 0 Ma (95%CI). Breccia sample BM-2B: apatite is sparse, 7 of 12 AFT ages as young as 0 Ma. Breccia sample BM-4B: 78 of 78 AFT ages as young as 0 Ma, pooled age <1 Ma. For unmelted sample BM-2A: 15 of 74 AFT ages as young as 0 Ma. Unmelted sample BM-3: 11 of 54 AFT ages as young as 0 Ma. Unmelted sample BM-4A: 76 of 87 AFT ages as young as 0 Ma. Sparse apatite likely indicates loss by thermal decomposition. A measured AFT pooled age <1 Ma for breccia sample BM-4B supports very recent melting, probably from an underground coal fire. Our evidence therefore supports an interpretation as clinker and contradicts the impact hypothesis at these outcrops.