Gallery of Geology: Permian Angular Uncomformity, Robledo Mountains, Dona Ana County, New Mexico

Abstract

During the late Paleozoic (Pennsylvanian–Early Permian), the collision of Gondwana and Laurussia (Euramerica) created the Pangean supercontinent . That collision caused extensive tectonism along a then nearly equatorial zone that extended from eastern Europe to western North America . In the United States, from Illinois to Idaho, the Ancestral Rocky Mountains (ARM) formed as a lengthy belt of basement-cored uplifts (e .g ., Kluth and Coney 1981; McBride and Nelson 1998; Dickinson and Lawton 2003) . In New Mexico, these uplifts were mostly north-south oriented islands surrounded by shallow seas during the Pennsylvanian that were ultimately worn down and buried by alluvial sediments during the Early Permian (Woodward et al . 1999) . Deciphering many aspects of ARM tectonism in New Mexico has been difficult because few late Paleozoic structures are preserved intact (e .g ., Woodward et al ., 1999) . Indeed, most of these structures were reactivated by tectonism during the Late Cretaceous–Eocene Laramide orogeny or during the late Cenozoic tectonism associated with Basin and Range extension, notably of the Rio Grande rift . Perhaps the best documented ARM structures are those described by Bachman and Hayes (1958), Pray (1961) and others in the Sacramento Mountains of Otero County, where folded Pennsylvanian strata are truncated by nearly flat lying Permian red beds . Here, we document a small ARM structure in the southern Robledo Mountains of Doña Ana County . This structure is within the confines of the Prehistoric Trackways National Monument (PTNM) . The PTNM is approximately 2,137 hectares (5,280 acres) of land protected by an act of the U . S Congress primarily because of its world-class Early Permian trace-fossil assemblages (Lucas et al . 2011) . The trace fossils come from a lithostratigraphic unit referred to as the Robledo Mountains Formation of the Hueco Group (termed Abo Tongue or Abo Member of the Hueco Formation in older stratigraphic terminology, Seager et al ., 2008) . In the PTNM, the Robledo Mountains Formation is composed of approximately two-thirds drab-colored marine limestone and shale interbedded with one-third red terrestrial mudstone, siltstone and sandstone . The formation has an average thickness of 120 m . The siliciclastic red beds yield the trace fossils for which the PTNM was created . Voigt et al . (2013) recently argued that these sediments formed on a coastal floodplain during alternating wet and dry conditions . They based this conclusion on detailed sedimentological and ichnological analysis that indicates most of the trace fossils found in the Robledo Mountains Formation red beds were preserved on mud-draped surfaces within distal crevasse-splay siltstone to fine-grained sandstone . The physical and biological structures recorded in their study support the reconstruction of a freshwater ecosystem dominated