Cyclic sedimentation in the Fort Hays Limestone Member, Niobrara Formation (Upper Cretaceous) in northeastern New Mexico and southeastern Colorado
Allan P. Laferriere
Rhythmically interbedded isochronous limestones and shales of the Fort Hays Limestone Member were deposited during the transgressive phase of the Niobrara marine cycle in Late Cretaceous time. Limestone beds were formed from coccolith-rich pelagic ooze that was deposited during periods of minimal clastic-detrital influx. Dilution of carbonates during periods of increased runoff from siliciclastic source areas resulted in deposition of shaly beds. Non-carbonate material has been further concentrated in shaly horizons by pressure solution during burial diagenesis. Recent investigations of the Fort Hays suggest that shale-limestone interbedding was generated by orbitally-forced climatic variations. This hypothesis is supported by the occurrence of these cyclic deposits over much of Colorado, New Mexico and Kansas. Detailed regional correlation of individual cycles, however, indicates that climatically-induced bedding patterns were overprinted by effects of Upper Cretaceous tectonic activity along the Sevier orogenic belt and by erosional events associated with tectonoeustatic sea level oscillations. Correlation of the Fort Flays from Pueblo, Colorado into northeastern New Mexico documents progressive disappearance of climatically-induced depositional cyclicity with increasing proximity to terrigenous detrital source areas.
- Laferriere, Allan P., 1987, Cyclic sedimentation in the Fort Hays Limestone Member, Niobrara Formation (Upper Cretaceous) in northeastern New Mexico and southeastern Colorado, in: Northeastern New Mexico, Lucas, S. G.; Hunt, A. P., New Mexico Geological Society, Guidebook, 38th Field Conference, pp. 249-254.