Abstract

Two laterally traceable discontinuities (Montana, United States) that vary in character and origin provide a means of subdividing fully nonmarine strata into genetic packages that are likely related to changes in relative sea level. Both discontinuities are situated >100 km inland of contemporaneous shorelines, and both are characterized by features that render them distinguishable from other surfaces in the alluvial record (e.g., pervasive oxidation, persistent and unusually thick lags, extraformational clasts). The lower discontinuity probably formed due to a fall in eustatic sea level and appears analogous to a marine sequence boundary. The upper discontinuity probably formed due to a tectonically induced increase in the rate of base-level rise and lacks characteristic

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