Geochronologic history of deposition of the Upper Cretaceous Pictured Cliffs Sandstone, Fruitland Formation, and Kirtland Formation, San Juan Basin, New Mexico and Colorado

For about 25 m.y., the Western Interior Seaway bisected the North American continent, withdrawing and disappearing from most of the area during Late Cretaceous time. In the San Juan Basin area of New Mexico and Colorado, the final regression of the western shoreline of the seaway was accompanied by deposition of the shoreface Pictured Cliffs Sandstone. Precise 40Ar/39Ar ages for sanidine crystals from eight altered volcanic ash beds precisely dated this regression across the San Juan Basin as beginning 76.28 Ma when the shoreline was in the southwest part of the basin and ending 73.55 Ma when the shoreline left the northeast part of the basin. (These ages are corrected from those reported in Fassett, 2000, per Kuiper et al., 2008 ? these authors assigned an age of 66.95 Ma to the K-Pg boundary vs. an age of 66.5 Ma that had been assigned earlier.) The Pictured Cliffs regression across the basin covered a distance of 145 km and took 2.73 m.y. The stratigraphic rise of the Pictured Cliffs across the basin was 384 m and thus the thickness of rock concurrently deposited was also 384 m. These precise geochronologic data make it possible to calculate the sedimentation and regression rates during the retreat of the Pictured Cliffs shoreline across the San Juan Basin.The overall average rate of sedimentation during that time, based on rock thickness, was thus about 140 m/m.y. and the average rate of regression of the shoreline was about 53 km/m.y. The decompacted 380-m rock thickness is 702 m yielding a corrected sedimentation rate of 257 m/m.y. These rates are averages, but because there are eight dated ash beds in this stratigraphic section, the rock thicknesses and time durations between these dated ashes can each be used to determine the variable rates of sediment accumulation and shoreline regression during the 2.73 m.y. it took for the Pictured Cliffs to regress across the basin. These data also make it possible to estimate the rate of tectonic subsidence and the creation of accommodation space during the PC regression; that is 114 m/m.y.