The contemporary elevation of an ancient strandline provides a measure of eustasy at a single time in the geologic past. If the elevation of a paleoshoreline has not changed since deposition (or if it has changed by a known amount), then the measured (or corrected) evaluation equals the sea level at that time. The ideal location for measuring an ancient sea level is a paleoshoreline deposited high on the shoulder of a stable craton where postdepositional elevation changes through tectonic movement, lithospheric compensation to loads, and sediment compaction are minimized. The contemporary elevation of a Late Cenomanian (≈93 Ma) shoreline was determined at five localities along the tectonically stable, eastern margin of the Cretaceous Weastern Interior Seaway, North America.

This shoreline, represented by Marine-to-nonmarine facies transitions in strata of the Greenhorn sequence (UZA-2 cycle of Haq et al. (1987)), was identified from outcrop and borehold data. Biostratigraphic zonations contrained the geologic age at each locality. Sequence stratigraphic correlations, based on identifying discrete progradational units and the surfaces that separate them, were used to refine age correlations to better than 100 kyr between localities. A single Cenomanian shoreline was correlated within a single progradational unit, and its elevation was determined at five localities.

This paleostrandline occurs 265--286 m above present-day sea level, at an average elevation of 276 m. Isostatic and flexural correction were applied to remove the effects of postdepositional vertical movement, including sediment compaction by loading uplift due to erosion, and glacial loading and rebound. Errors inherent in each measurement and each correction were estimated. Corrections and their cumulative error estimates yield a Late Cenomanian elevation of 269¿87 m above present sea level. The corrected elevation approximates sea level at 93 Ma and provides a measure of Late Cenomanian eustasy prior to the Early Turonian highstand. Establishing the absolute value for eustasy at a single point in geologic time provides a frame of reference for calibrating relative sea level curves, as well as constraining the magnitudes of tectonic subsidence sediment flux, and other variables that controlled water depth and relative sea level. ¿American Geophysical Union 1991