New Mexico Geological Society Annual Spring Meeting — Abstracts

In Bull Canyon in east-central New Mexico (S1/2, T9N, R26E) a lacustrine interval within the eolian Exeter Sandstone contains numerous fossils of actinopterygian fishes. This lacustrine interval is up to 1.5-m-thick and consists of three units: (1) A basal clastic unit of cross-bedded sandstone, planar-bedded silty sandstone and varved shale with some sandstone lenses (70± cm), (2) a middle carbonate unit of flaggy and varved limestone overlain by varved calcareous shale (50± cm) and (3) an upper vug-limestone unit of varved limestone with vugs often filled with secondary calcite (16± cm). In the SW1/4, NE1/4, SE1/4, sec. 29, T9N, R26E fossils of actinopterygian fishes abound in the basal varved limestone of the middle carbonate unit. Two fish species ("Pholidoehorus" americanus and "Leptolepis" schoewei) are very common as articulated, entire fish that are dorsa-ventrally flexed on bedding planes. These fish species are the same taxa that are present in the Todilto Formation of the Colorado Plateau, the "Ralston Creek Formation" of south-central Colorado and the Sundance Formation of Wyoming, Montana and South Dakota. This indicates that the fish-bearing limestone in Bull Canyon is of middle Jurassic (Callovian) age. It also suggests that the lacustrine interval in Bull Canyon either represents part of the Todilto lake baain (and should be assigned to the Todilto Formation) or represents a separate lake contemporaneous with the Todilto lake. In addition, correlation with the Todilto Formation indicates that at Bull Canyon the Exeter Sandstone below the lacustrine interval is homotaxial with the Entrada Sandstone of the Colorado Rlateau, and the Exeter above the lacustrine interval is homotaxial with the Cow Springs/Bluff Sandstones of the Colorado Plateau.