between sedimentary layers as magma before cooling into rock. The sill contained a moderate abundance of quartz and plagioclase feldspar phenocrysts. To the west a second intrusive body, an igneous dike, with larger crystals and a different composition crosscut this sill indicating changing compositions of magma through time.
between sedimentary layers as magma before cooling into rock. The sill contained a moderate abundance of quartz and plagioclase feldspar phenocrysts. To the west a second intrusive body, an igneous dike, with larger crystals and a different composition crosscut this sill indicating changing compositions of magma through time.
intrabasinal conglomerate bedrock is seen in the shadows, to the left of the Juniper tree, and contains pebbles recycled from within the same sedimentary basin that it was deposited in. The rock in the foreground is an interbasinal conglomerate containing chert pebbles from more distal source areas. This was the lowest occurrence of the interbasinal conglomerate in the area and likely represents a stratigraphic contact between the middle and upper Triassic related to landscape changes with “missing time” in the unconformity between the layers.
intrabasinal conglomerate bedrock is seen in the shadows, to the left of the Juniper tree, and contains pebbles recycled from within the same sedimentary basin that it was deposited in. The rock in the foreground is an interbasinal conglomerate containing chert pebbles from more distal source areas. This was the lowest occurrence of the interbasinal conglomerate in the area and likely represents a stratigraphic contact between the middle and upper Triassic related to landscape changes with “missing time” in the unconformity between the layers.
area. As I map the landscape I observe and analyze different components of rocks and refer to literature that discusses the texture, composition, and extent of different formations/members to match my observations with established groupings of sedimentary rocks. This area has complex faulting, folding, and igneous intrusions that complicate the interpretation of the stratigraphic sequence.
area. As I map the landscape I observe and analyze different components of rocks and refer to literature that discusses the texture, composition, and extent of different formations/members to match my observations with established groupings of sedimentary rocks. This area has complex faulting, folding, and igneous intrusions that complicate the interpretation of the stratigraphic sequence.
By Jon Krupnick, Field Geologist
NMBGMR
White Oaks North and Ancho 7.5x15-Minute quadrangle
February 20, 2026
In the middle of February, I spent a few days mapping in the White Oaks North and Ancho 7.5x15-Minute quadrangle (1:24,000-scale). The weather was perfect with little wind, partial cloud coverage, and a mild 40-to-65-degree range in night to daytime temperatures. I spent my time familiarizing myself with, and mapping, Permian, Triassic, and Cretaceous stratigraphy that was intruded by middle-Cenozoic igneous rocks. Exposure of the rocks was poor and much of my time was spent making interpretations along ridgelines and arroyo bottoms, or from “float mapping” where you rely on observations of detached rocks, that are no longer in their original location, to interpret the concealed geology. In one instance I found myself in the Mancos Shale with the only indication of my stratigraphic position being a mound of gray fossiliferous shale debris that were excavated by a burrowing animal whose den was deep enough to access un-
weathered material below soils and colluvial cover.