Duration of volcanism in the Newark basin based on the Milankovitch cyles in lacustrine strata interbedded with and surrounding the basalt-flow units. Counting the number of Van Houten cycles in the Feltville and Towaco formations provides a minimum estimate, to which must be added the duration of the lava-flow units themselves. However, the basalt flows appear to have been erupted geologically instantaneously. This is shown as follows: We first scale the depth rank curves (so that each Van Houten cycle has the same thickness) and join them into a composite section (2nd column from left). We then filter the composite depth rank curve to generate the symmetrical curve in the 2nd column from the right. We can them compare the filtered depth rank with the "precession index," which describes the variations in sunlight reaching the Earth based on the variations in the Earth's orbit (Milankovitch cycles). The best match between the filtered curve and the precession index results when the basalts represent no appreciable geologic time. Therefore, the duration of the volcanic period in the Newark basin (base of Orange Mountain Basalt to top of Hook Mountain Basalt) is 580,000 years, that is, there are ~29 Van Houten cycles (duration of ~20,000 each) in the Feltville and Towaco formations. The diagram also shows that the Triassic-Jurassic boundary (green line) is ~40,000 years older than the Orange Mountain Basalt. Click on the image to see a larger version. Modified from Olsen et al. (1996).