LATE TRIASSIC CLIMATE RECONSTRUCTION USING PALEOSOL MINERAL ASSEMBLAGES

Paleosol mineral assemblages (PMAs) can be used to reconstruct climate evolution as soil mineral stability is particularly dependent on mean annual precipitation and precipitation frequency, the resulting drainage, and temperature. PMAs can be characterized by the abundance and type of minerals that are present using quantitative X-ray diffraction techniques. Both parent-mineral reactivity and the formation of authigenic weathering products provide useful information about environmental conditions. The use of parent-mineral reactivity as an indicator of climate is well illustrated by the variable weathering intensities experienced by feldspar types under the same environmental conditions. Authigenic phases that form under a range of climate conditions include Al oxide/hyroxides, Fe oxide/hydroxides, phyllosilicates (clays), and carbonates.

This study characterizes the PMAs for a succession of late Triassic paleosols from the Chinle Formation in Petrified Forest National Park. A shallow burial history for this cratonic depositional basin minimizes the likelihood of diagenetic modification. The most dramatic stratigraphic change in PMA occurs at 215 Ma where a climate shift marked by increasing aridity has been recognized using bulk paleosol element transfer functions. The distinctive changes in mineralogy above and below the climate shift include: 1.) higher total feldspar abundance in the upper Chinle (5% vs. 2 %) 2.) lower plagioclase abundance in the lower Chinle, 3.) abundant calcite in upper Chinle paleosols (10% average) compared to almost none in the lower Chinle , 4.) higher clay mineral abundance in the lower Chinle (69 vs. 61%) and, 5.) abundant kaolinite in the lower Chinle (8 %) compared to almost none up section.

In addition to the mineralogy changes at the climate shift, distinct PMAs occur throughout the stratigraphic succession particularly at the base of the section (K-feldspar, kaolinite, hematite, ferrous smectite) and the top of the section (calcite, palygorskite, plagioclase). The succession of PMAs throughout the Chinle represent a transition of declining weathering intensity controlled by increasing aridity that characterizes western Pangea towards the end of the Triassic.