Gilbert—Bedding rhythms and geochronology

Gilbert interpreted rhythmic spacing in limestone units in the Upper Cretaceous of Colorado as a geological response to a planetary cause—the 21,000 yr precession of the equinoxes. In this light, he judged the Upper Cretaceous marine sequence of Colorado to have been deposited over a span of about 21 m.y.—a figure that seems remarkably close to that yielded by modern radiometric geochronology, 24 to 35 m.y. A reexamination of three of Gilbert’s four short rhythmic sequences, using the available radiometric data of Obradovich and Cobban, in conjunction with a model of subsidence and sedimentation, yields bedding rhythms in the 18,000 to 22,000-yr range, and seems to confirm Gilbert’s hypothesis. Most rhythmic Cretaceous sequences in other parts of the world also yield bedding rhythms close to the precessional period, according to the Obradovich-Cobban time scale. The alternative Van Hinte time scale, however, yields a wider scatter of values, and suggests that only some of the rhythms are related to the precession, others seeming to be closer to the 41,000-yr period of obliquity. The equinoctial precession can affect geology only when acting in conjunction with the eccentricity of the Earth’s orbit, which waxes and wanes irregularly with a mean period of 93,000 yr. One should therefore expect precessionally caused rhythms to occur in sets averaging 4.5. That limestone-shale bedding rhythms occur in sets was shown long ago by Schwarzacher for upper Paleozoic and Mesozoic sequences. His observation that the mean number of rhythms per set lies between 5 and 6 suggests that the orbital parameters may have changed. At least two of Gilbert’s four Cretaceous sequences of Colorado are bundled in this fashion. Thus, Gilbert’s suggestion that bedding rhythms provide a basis for geochronology takes on new interest—not to compete with radiometry in the rough calibration of Earth history, but as a refinement. It may also provide a means of tracing the evolution of the Earth’s orbital behavior.