Magnetic polarity stratigraphy: Stochastic properties of data, sampling problems, and the evaluation of interpretations

Abstract

A statistical model has been derived which can be used either to test the interpretation of a given magnetic stratigraphy or to make time estimates from the paleomagnetic data. The model is based on the presently accepted statistical properties of the earth's magnetic field and the stochastic nature of the paleomagnetic sampling process. Given knowledge of the number of paleomagnetic sites, the distribution of the sites, an estimate of the geologic time span sampled, and the mean length of the magnetic polarity zones, the expected success of the sampling program can be calculated and compared with its actual performance. In a number of test cases the theoretical results have agreed with realized results. The mean time span for polarity intervals in the late Neogene has been established from existing data at 120,000 years. Once the mean time span for polarity intervals has been established, estimates for stratigraphic time span can be made by means of the statistical model. Of three models studied, a time uniform sampling program is the most efficient for establishing a local magnetic polarity stratigraphy; the least efficient strategy is an exponential distribution of samples.

Magnetostratigraphy and depositional history of the Miocene Wushan basin on the NE Tibetan plateau, China: Implications for middle Miocene tectonics of the West Qinling fault zone

“…A statistical assessment of the number of reversals expected (42) given our sampling spacing for the periods 6-14.5 Ma, however, is greater than the actual number of reversals (35) that we detected (Johnson and McGee, 1983). This suggests that we have not recovered the complete magnetostratigraphy.…”

Pulsed Miocene range growth in northeastern Tibet: Insights from Xunhua Basin magnetostratigraphy and provenance

“…This result suggests that we have recovered more than 95% of the true number of magnetochrons. Additionally, a statistical assessment (Johnson and Mcgee, 1983) of the number of reversals expected with our site density (63) is commensurate with the number of reversals we detected (65). Questionable reversals defi ned by only a single specimen are shown with a half-bar (Fig.…”

Chronology of the Eocene continental deposits of Africa: Magnetostratigraphy and biostratigraphy of the El Kohol and Glib Zegdou Formations, Algeria

“…Using the statistical method of Johnson and McGee (1983), the amount of time, Δt, represented by a sampled section can be approximated using the formula Δt (m.y.) = SтN, where S = -ln (1 -2p)/2 (т = mean time span of polarity intervals; N = the number of paleomagnetic sites; and p = R/ [N -l], where R is the number of reversals encountered).…”