Palaeobotanical investigation of early tertiary palaeoelevations in northeastern Nevada: initial results

Abstract

Physical models of surface uplift rates can be evaluated using palaeobotanically determined estimates of surface heights through time. Some classes of physical model imply that surface uplift rates are too great to be accounted for by known rates of crustal thickening. If these rates could be confirmed important information on lower lithospheric and upper mantle processes could be derived. Moreover, the temporal development of surface heights in mountain belts and plateaux has been suggested to have important effects on global climates. Mean Annual Temperatures (MAT) can be estimated from a fossil flora using Wolfe's recently developed method of multivariate (correspondence) analysis of the physiognomy of angiosperm leaves, Climate Leaf Analysis Multivariate Program (CLAMP). This technique is here applied to fossil floras in the Basin and Range province, western USA, a region which may have undergone a rapid height increase before extension began in Oligocene to Miocene time. Palaeoelevations, determined by comparing contemporaneous MAT data from floras deposited at sea level and from those in the continental interior, are estimated for two late Eocene floras in northeastern Nevada. Sea level data are derived from floras deposited in the Puget Group, Washington. One important source of uncertainty in estimating MAT using CLAMP, sample size, is evaluated here using the diverse Republic flora from Washington. These initial results suggest that in the late Eocene the elevation of northeastern Nevada was similar to that of today. However, the uncertainties inherent in using a small number of floras remain significant and to minimise these uncertainties additional floras of late Eocene, and also of Miocene age, from northeastern Nevada are being examined.