TOWARDS AN ASTRONOMICALLY CALIBRATED TIMESCALE FOR THE JURASSIC

The useful marine magnetic anomaly record, on which the globally correlatable portion of the geologic time scale is based, ends in the Middle Jurassic. The lack of intact earlier Jurassic oceanic crust precludes a useful older oceanic record. Continental crust will therefore provide the basis for a timescale for most of the Jurassic and older time periods. Requirements for timescale development are as follows: 1, continuous sedimentation for very long periods (~107 years); 2, a recoverable and well characterized paleomagnetic polarity record; 3, independent time control, preferably both radiometric and astronomical; 4, correlatable biostratigraphy; and 5, meaningful isotopic record. A precedent for this is the continental rifts of Eastern North America. Their advantages include: 1, very high accumulation rates; 2, well-behaved and well-characterized magnetic behavior; 3, obvious and well-behaved Milankovitch cyclicity; 4, dated lava flows and other datable materials; and 5, a developing carbon isotopic record. The Triassic age part of this sequence provides the basis for the Newark basin astronomically calibrated geomagnetic polarity time scale for the Late Triassic, now successfully correlated at the substage level with marine sections. About 2.5 million years of Early Jurassic is present in the Hartford rift basin of CT and MA, most of it certainly of Hettangian Age and of nearly entirely normal polarity, with the presence of multiple zones of reverse polarity indicated the probable base of Sinemurian Age strata. For most of the younger parts of the Jurassic, sections examined thus far are entirely marine and too short to be very useful in time scale construction, although there have been several attempts at combining the scattered sections. Prospects for long, high accumulation rate sections are largely continental and include several basins in China, Greenland, Poland, Russia, and Australia. We can envision progressing down from long overlapping sections with well-characterized polarity timescales and Milankovitch cyclcostratigraphies linking the post-Early Jurassic marine magnetic timescale, which itself is being astronomically calibrated, to that of the astronomically tuned Late Triassic Newark basin GPTS.