Pelagic/hemipelagic rhythmites of the Greenhorn Limestone (Upper Cretaceous) of northeastern New Mexico and southeastern Colorado
Donald E. Hattin
Across much of the central Great Plains and Southern Rocky Mountains, the Late Cenomanianearly Middle Turonian part of the marine Cretaceous is represented by pelagic/hemipelagic sedimentary rocks classified as Greenhorn Limestone. Part of this formation, the Bridge Creek Member, is characterized by limestone/shale couplets that have been demonstrated to be time parallel. Bedding rhythmicity is believed to reflect climatic variations that were forced by Earth's orbital perturbations. The Bridge Creek contains readily identifiable limestone and bentonite marker beds that have been traced previously across Kansas and eastern Colorado and are shown in this report to extend also throughout the field-conference area.
Bridge Creek strata equivalent to the Jetmore Member of central Kansas display remarkable stratigraphic uniformity, and include some of the most widely traceable cratonic limestone beds reported from anywhere in the world. In contrast, Bridge Creek strata equivalent to the Hartland Member of central Kansas manifest considerable stratigraphic variation in number, thickness and spacing of pelagic limestone beds. Such variations may have resulted from such factors as scour, eastward pinchout of individual shaly beds, regional variations in effects of climatic signals on pelagic productivity or wedging out of beds against highs. Locally, the entire Bridge Creek section is greatly condensed. General correspondence of condensed sections with structural highs suggests depositional control by tectonic features.
Pelagic and hemipelagic Bridge Creek sediments were deposited in relatively deep water, and accumulated under conditions of generally poor benthic circulation. Isolated beds of skeletal grainstone are attributed to storms. Stratigraphic intervals characterized everywhere by abundance of skeletal debris are attributed to general shallowing related to eustacy. Skeletal grainstones occur essentially throughout condensed Bridge Creek sections, owing apparently to shallower water conditions over the postulated sea-floor highs.
- Hattin, Donald E., 1987, Pelagic/hemipelagic rhythmites of the Greenhorn Limestone (Upper Cretaceous) of 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. 237-247.