Refinement of the Late Paleozoic Time Scale: The Potential of Astronomical Tuning
An astronomical time scale has been established for the Neogene, and ‘floating astronomical time scales’ are being developed over extended intervals of geologic time as far back as the Late Triassic (Hinnov and Ogg, 2007). Improvements in stratigraphic data acquisition, time series analysis, geochronological techniques, and knowledge of Earth’s paleo-orbital parameters have created opportunities to extend the astronomical tuning approach to pre-Mesozoic stratigraphic successions. In this presentation, we review the occurrence of cyclic stratigraphic successions of Devonian through Early Triassic age that may contain periodic signals at orbital time scales. Such successions are widespread in time and space, suggesting that generation of an astronomically-tuned time scale for the Late Paleozoic is a realizable goal. In contrast to the Jurassic and later, however, cyclicity in older successions is found predominantly in epicratonic marine settings of comparatively shallow water depths. As a consequence, time-series analysis of Upper Paleozoic successions presents a unique set of challenges, in particular with regard to recognition of and compensation for sedimentary hiatuses (Meyers and Sageman, 2004). Another issue is uncertainties in the Earth’s orbital frequencies prior to ~50 Ma as a consequence of changes in orbital resonance among the planets and chaotic influences within the solar system. However, many stratigraphic successions of Late Paleozoic age yield significant signals at or close to present-day orbital frequencies, suggesting that changes in the Earth’s orbital characteristics over the last ~400 Myr have not been of sufficient magnitude to prevent use of the astronomical tuning procedure. Modeling studies suggest that the 405-kyr long-eccentricity period has been relatively constant over long intervals owing to the stabilizing gravitational influences of Jupiter and Venus on the Earth’s orbit (Laskar et al., 2004), making it the preferred frequency for tuning of long cyclic stratigraphic successions. The long-eccentricity orbital signal has now been identified in various Upper Paleozoic successions (Goldhammer et al., 1994; Yang and Kominz, 1999; Heckel, 2004). In order to ground orbitally-based time scales in an absolute time frame, cyclostratigraphic studies must be coordinated with high-precision U-Pb and Re-Os dating (Bowring et al., 1998; Mundil et al., 2004; Kaufmann et al., 2004; Selby and Creaser, 2005; Ramezani et al., 2007).
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009