This grant will date sedimentary and igneous rocks from within the Newark Supergroup in an attempt to both refine the Late Triassic and Early Jurassic timescale and to test the well-grounded hypothesis that the cycles observed in the supergroup are controlled by Milankovitch orbital forcing. This grant will provide a very important test of Uranium-Lead dating in carbonates, clarify the timing of terrestrial vs. marine extinctions at the Triassic/Jurassic boundary, and provide much more precise data on the actual dates and progression of the boundary.
The Newark Supergroup records the Triassic-Jurassic extinction and has been intensively studied in terms of bio-, cyclo-, and magentostratigraphy. Although the cyclostatigraphic record is often cited as recording orbital forcing, absolute age constraints on this high-resolution record are lacking. The only horizon that has been dated in this sequence is a series of basaltic flows and intrusions that were dated at 201 +/- 1 Ma. New work at MIT suggests it will be possible to date zircons from granophyric layers in basalts to a precision of 0.2 Ma or better. These basalts bracket units containing sedimentary carbonates that appear to have great potential for U-Pb dating. Comparison of the ages will test the potential for U-Pb dating of sedimentary materials for yielding accurate ages.
Testing the potential of U-Pb dating of carbonates is a key area for understanding the distribution of time in thick sequences of rocks that do not contain volcanic ashes. Geochronological data are needed to address fundamental questions about Earth processes, many of which involve questions about rates or correlations. Although magnetostratigraphy and cyclostratigraphy can be used to subdivide time in the rock record, assumptions about the detailed distribution of time are impossible without precise calibration. Although the precision of U-Pb ages of carbonates appears to be limited by geologic complexity (e.g., slight initial heterogeneity in the Pb isotopes; mobility of U and daughters during early diagenesis) the accuracy may well be equal to the stated uncertainty in the age. If so, the possibility of dating carbonates to 1% (2-sigma) uncertainty will provide a much-needed geochronometer for sequences throughout the Phanerozoic for which no air-fall ashes can be identified.
Graduate students at SUNY Stony Brook, MIT and Columbia University will be involved in this research. Due to the proximity of our institutions to each other as well as to some of the best studied parts of the Newark sequence we anticipate a very dynamic collaboration that will provide students with vast opportunities to use facilities at all three institutions and to enjoy a rich field experience.
