GREENHORN LIMESTONE (CRETACEOUS, U.S.WESTERN INTERIOR) : CASE STUDY IN LAYER-CAKE STRATIGRAPHY

Pelagic/hemipelagic rhythmites occur in many upper Cretaceous carbonate-rich formations, and include some of the world's finest examples of layer-cake stratigraphy. Typically, such rhythmites manifest regularity of lithologic alternation, uniformity of individual bed thickness and, commonly, wide lateral extent. Within such sequences individual beds are intuitively or demonstrably time parallel, and are interpreted by most students as climatically controlled results of Earth's orbital variations. Amongst these rhythmites, those in the Greenhorn Limestone (or Formation) are a crown jewel of layer-cake stratigraphy that is unique with respect to uniformity of bed succession and extraordinarily broad geographic distribution of individual and groups of beds. Four members; Hartland, Jetmore, and Pfeifer of central Kansas; and equivalent Bridge Creek Member farther to the west, evince rhythmicity which is nowhere better developed than in the Jetmore and equivalent part of the Bridge Creek. That interval comprises 13 subequally spaced decimeter-scale limestone marker beds which have been designated JT-1 thhough JT-13, and are separated by usually thicker beds of chalky or marly shale. Associated seams of bentonite, and statigraphic distribution of key macroinvertebrate species are evidence that each of the rhythmic couplets is time parallel. Jetmore and equivalent Bridge Creek strata are divisible into four areally persistent submembers, which constitute a bundle of stacked lithochronozones. Individual Jetmore marker beds and groups of marker beds are traceable for hundreds of kilometers, north to south and east to west. Bed JT-1 extends more than 1000km, and may be the world's most extensive time-parallel carbonate marker bed. In several Kansas, Colorado and New Mexico sections, each on a present-day structural "high," thickness of Greenhorn strata is greatly reduced, but at most such places the marker sequence is complete or nearly so. Such anomalies are evidence of uplift--Laramide-related events which affected deposition even in deeper water settings. Aside from benefits to regional studies of geochemistry and geobiology, precise chronologic correlation of Greenhorn rhythmic couplets thus affords unexcelled opportunity for assessment of syndepositional tectonism.