Trace fossils in Upper Cretaceous argillaceous marine facies of the U.S. Western interior

Argillaceous strata of the Greenhorn and Niobrara depositional cycles contain an extensive, though spottily developed, record of tracemaking organisms. These beds are assignable to five shaly facies, four of which represent the gradation from nearshore sandy shales to far offshore calcareous shales, the fifth representing anoxic or nearly anoxic bottom conditions. In strata studied by us the best potential for trace fossil preservation was in deposits characterized by pronounced textural variation, such as shales with sandstone interbeds, shales with thin lenses of siltstone or sandstone, or sandy and silty shales (= mudstones). In pure clayey shales and calcareous shales, trace fossils are preserved best in concretionary structures. Our study suggests that except for shales rich in organics, trace-making organisms were common and widespread in argillaceous muds of the Western Interior. Initial fluidity, high degree of compaction, and textural homogeneity are principal reasons for the poor record of trace makers in pure, clayey shales.