A FIELD GEOLOGIST’S VIEW OF THE GEOLOGIC TIME SCALE

A subtitle for this presentation could be: ‘Why regional stratigraphy, structure and biostratigraphy need a detailed, reliable geologic time scale (GTS).’ The GTS is essentially a calibrated ruler of the range of paleontologic taxa through time. It is calibrated by appearance and disappearance (range) of taxa, geochronologic age dates, chemical variations (chemostratigraphy), astronomic cycles, and changes in magnetic polarity. The modern GTS that is being established by the International Commission on Stratigraphy provides time markers that are reliable at shorter and shorter time intervals; for example, in the Permian to a few million years. When linked via detailed biostratigraphic data at the regional scale the GTS forms the basis for an even more precise temporal framework. Thus, the GTS and the related regional framework allows us to time events even in the Paleozoic at intervals that reveal new scenarios for the regional geologic histories.

One example comes from the upper Paleozoic of the Great Basin. Because of the detailed biostratigraphic data allowed by conodonts, fusulinids, ammonoids and brachiopods, the classic picture of the upper Paleozoic continental margin has changed from one of being a passive continental margin following the Antler orogeny to one of being an active/periodically active margin. The same can be suggested for the lower Paleozoic continental margin, which raises the question, what did happen to the supercontinent Rodinia in the late Proterozoic and earliest Paleozoic that lead to what is now the western continental margin of North America? Detailed biostratigraphy allows us to calibrate stratigraphic and structural events against an equally detailed time scale, that then gives us the framework to reconsider long-held theories about the geologic development of the western U.S.