Time Not Our Time: Physical Controls on the Preservation and Measurement of Geologic Time

摘要

Sadler's (1981) analysis of how measured sedimentation rate decreases with timescale of measurement quantified the vanishingly small fractional time preservation-completeness-of the stratigraphic record. Generalized numerical models have shown that the Sadler effect can be recovered, through the action of erosional clipping and time removal (the "stratigraphic filter"), from even fairly simple topographic sequences. However, several lines of evidence suggest that most of the missing time has not been eroded out but rather represents periods of inactivity or stasis. Low temporal completeness could also imply that the stratigraphic record is dominated by rare, extreme events, but paleotransport estimates suggest that this is not generally the case: The stratigraphic record is strangely ordinary. It appears that the organization of the topography into a hierarchy of forms also organizes the deposition into concentrated events that tend to preserve relatively ordinary conditions, albeit for very short intervals. Our understanding of time preservation would benefit from insight about how inactivity is recorded in strata; better ways to constrain localized, short-term rates of deposition; and a new focus on integrated time-space dynamics of deposition and preservation.