Paleobotanical Estimates of Laramide Relief in the Western Interior of North America

The Paleocene topography of the Western Interior of North America is characterized by the Laramide orogeny, which produced a deformation belt extending from Montana to Mexico, as much as 1,500 km inboard of the nearest convergent margin. Isotopic proxies, fossil evidence, and climate-model results, indicate that Laramide topography had a profound influence on Paleocene climate dynamics and paleoecological gradients especially vegetation patterns after the K-T boundary. To assess Laramide topography during the Paleocene, we compare mean annual temperature (MAT) estimates derived from fossil-dicot leaves in Laramide intermontane and foreland basins. First, we show that modern meridional-MAT profiles from ~350 North American sea-level weather stations are well-fit to latitude by MAT = b₀cos(b₁l) + b₂z where b₀ = 29.3 °C and b₁ = 1.60 °C/°N (R² = 0.91). Subtracting this function from all station data (~2,000 records) gives latitude independent MAT results that demonstrate variations in temperature that are due to elevation. Using this method, elevations in the Western Cordillera can be detected above ~1.5 km. We apply this meridional temperature function to North American sea-level Paleocene floras to produce a series of meridional-MAT profiles at ~3 myr increments. These profiles are depressed, on average, ~50% relative to the modern gradient. Reducing the Paleocene floras by the meridional profiles gives a latitude independent temperature curve, which appears to mirror that of the deep marine record—a proxy for high latitude sea surface temperatures. Reducing all floral data available by these profiles demonstrates that MAT records in Laramide intermontane basins are decoupled from those in the foreland. Extreme examples demonstrate as much as 2 km of local relief. These results show that local Laramide relief during the Paleocene was significant and that terrestrial Paleocene temperature curves derived from proxies in some Laramide basins are considerably affected by paleoelevation.