PALEOBOTANICAL INVESTIGATION OF EARLY TERTIARY PALAEOELEVATIONS IN NORTHEASTERN NEVADA - INITIAL RESULTS

Physical models of surface uplift rates can be evaluated using palaeob otanically determined estimates of surface heights through time. Some classes of physical model imply that surface uplift rates are too grea t to be accounted for by known rates of crustal thickening. If these r ates could be confirmed important information on lower lithospheric an d upper mantle processes could be derived. Moreover, the temporal deve lopment of surface heights in mountain belts and plateaux has been sug gested to have important effects on global climates. Mean Annual Tempe ratures (MAT) can be estimated from a fossil flora using Wolfe's recen tly developed method of multivariate (correspondence) analysis of the physiognomy of angiosperm leaves, Climate Leaf Analysis Multivariate P rogram (CLAMP). This technique is here applied to fossil floras in the Basin and Range province, western USA, a region which may have underg one a rapid height increase before extension began in Oligocene to Mio cene time. Palaeoelevations, determined by comparing contemporaneous M AT data from floras deposited at sea level and from those in the conti nental interior, are estimated for two late Eocene floras in northeast ern Nevada. Sea level data are derived from floras deposited in the Pu get Group, Washington. One important source of uncertainty in estimati ng MAT using CLAMP, sample size, is evaluated here using the diverse R epublic flora from Washington. These initial results suggest that in t he late Eocene the elevation of northeastern Nevada was similar to tha t of today. However, the uncertainties inherent in using a small numbe r of floras remain significant and to minimise these uncertainties. ad ditional floras of late Eocene, and also of Miocene age, from northeas tern Nevada are being examined.