The Cretaceous-Tertiary boundary in the San Juan basin, New Mexico and Colorado, has been variously placed at the base, at the top, and somewhere within the Ojo Alamo Sandstone. New evidence bearing on the location of this boundary includes palynologic analyses, vertebrate paleontology, paleomagnetism, and trace-element analyses.

The Ojo Alamo Sandstone is a series of fluvial, high-energy, braided stream deposits consisting of from one to several conglomeratic sandstone sheets separated by overbank or floodplain shales. The conglomerates contain pebbles and cobbles of quartzite, jasper, and andesite on the west side of the San Juan basin. The pebbles and cobbles diminish in size eastward and are rare to nonexistent on the east side of the basin. There is a major unconformity at the base of the Ojo Alamo.

The Cretaceous-Tertiary boundary, as determined earlier from palynologic criteria, is below the base of the Ojo Alamo Sandstone in the uppermost part of the underlying Kirtland Shale on Mesa Portales. New evidence from the Star Lake area, New Mexico, tends to confirm that boundary location. Other new data based on vertebrate remains, also from the Star Lake area, indicate that the boundary is within the Ojo Alamo Sandstone. Recently published paleomagnetic data and trace element studies now underway may also help in solving the Cretaceous-Tertiary boundary problem.

End_of_Article - Last_Page 559------------