Observations on Marine Lower Jurassic Stratigraphy of North America, with Special Reference to United States

Recent work on the stratigraphy of Lower Jurassic sedimentary rocks in Nevada, Oregon, and California is reviewed, and certain ammonite identifications amended. The zonal succession in the western United States is compared with that in Canada and Mexico, and the invertebrate fauna compared with the European. It is concluded that the American fauna shows strong European affinities, with Mediterranean elements present. Indigenous forms are more common among the pelecypods than the ammonites, though still subsidiary. The stratigraphic review reveals several widespread horizons in North America, in the Sinemurian and Toarcian, and a major stratigraphic gap in the Pliensbachian. The bearing of these facts on the writer’s hypothesis of eustatic control of sedimentation is discussed and, in conclusion, a few comments are offered on the extent of the Lower Jurassic sea in the United States.