Paper No. 256-8
Presentation Time: 3:35 PM
ASTROCHRONOLOGY AND DEPOSITONAL RATES IN UPPER DEVONIAN SEDIMENTARY BASINS – TEPHRAS, CYCLES, MAGNETIC SUSCEPTIBILITY, AND WHAT UNDERGRADUATE RESEARCH CAN ACCOMPLISH – AN INSPIRATIONAL TRIBUTE TO ROBERT K. SCHWARTZ
Small scale depositional cycles are well developed in the Upper Devonian fine-grained offshore clastic strata of western New York, manifested as distinct changes in color and texture. In the Hanover Shale between the Lower and Upper Kellwasser beds magnetic susceptibility changes, measuring mineral differences in the stratigraphic interval as a proxy for sea-level change, were analyzed with frequency analysis which resolved into eight, 21, and 40 cycles, that in a 800,000 year framework assigned for the interval are proposed to be the short eccentricity, obliquity, and precession orbital perturbations. Similar scale cycles are found in the underlying Angola Shale, but absolute age constraints have not been established. In the Exshaw Shale of western Alberta magnetic susceptibility values between dated tephra horizons, a duration of 1.0 Ma, tentatively indicate 10 and 27 cycles in the 5 m interval – proposed to represent the eccentricity and obliquity perturbations. Higher level (shorter duration) cycles were not resolvable at the sampling scale in this more condensed stratigraphic interval.