Trace fossils of Fort Hays Limestone Member of Niobrara Chalk (Upper Cretaceous), west-central Kansas

Trace fossils are abundant and diverse, though poorly preserved, in the Fort Hays Limestone Member of the Niobrara Chalk. In west-central Kansas this trace-fossil fauna, which has not been studied previously, consists of at least 20 taxonomic and ethologic variants distributed among approximately 13 genera or morphologic forms. Named genera include Asterosama, Chondrites, Laevicyclus, Planolites, Teichichnus, Thalassinoides, Trichichnus (Frey, n. gen.), Zoophycos, and possibly Arthrophycus. Forms which cannot be assigned readily to distinctive trace-fossil genera include cylindrical shafts, mechanically filled burrows, and various calcite-, pyrite-, and limonite-filled tubular burrows. About 35 per cent of these taxa or forms represent feeding burrows or combined feedingdwelling burrows, and 65 per cent represent dwelling structures. Tracks, trails, and other surficial traces were not observed. The distribution and abundance of several of these trace fossils are correlative with distinct lithotypes, and the composition of individual assemblages changes gradually stratigraphically upward. Yet none of the trace fossils were observed above a bed of chalk in the lower few feet of the overlying Smoky Hill Chalk Member of the Niobrara. Evidence indicates that 1) except for brief influxes of terrigenous detritus, the carbonate muds of the Fort Hays accumulated slowly, 2) the substrate was soft and yielding to considerable depth, and remained so until late in diagenesis, 3) both the sediments and the overlying water were well aerated, 4) currents capable of substrate scour and shell fragment transport were common, especially during early episodes of Fort Hays deposition, 5) sediments accumulated initially in relatively shallow water, which deepened gradually with successive intervals of time, and 6) the overall depositional environment was more favorable to fossil organisms than is suggested by the diversity of preserved tests and shells. The dearth of trace fossils in the overlying Smoky Hill Member reflects the inception of a different depositional regimen, including increased deposition of terrigenous detritus and more poorly oxygenated sediments.