MEAN ANNUAL PRECIPITATION ESTIMATES FOR THE MORRISON FORMATION

Geochemical analysis of paleosol B-horizons developed in the Upper Jurassic Morrison Formation of the western United States provides some of the first quantitative estimates of paleoclimate for this prolific fossil-bearing unit. Samples were collected from localities near Ghost Ranch, NM, and Shell, WY. Chemical index of alteration minus potassium (CIA-K) values determined from elemental analysis of these matrix samples range from 80 to 97, with an average of 86. These CIA-K values correspond to mean annual precipitation (MAP) estimates between 1100 and 1500 mm/yr, with an average value of 1200 ±150 mm/yr.

Previous studies of paleosol morphology in the Morrison Formation have inferred a semi-arid paleoclimate characterized by seasonal rainfall with drier conditions in the western and southern portions of the depositional area and a stratigraphic trend toward more humid conditions over time. The quantitative data presented here largely confirm these earlier qualitative interpretations. The average MAP estimates for the Ghost Ranch (1200 ±100 mm/yr 1σ) and Shell (1300 ±200 mm/yr 1σ) localities are statistically indistinguishable, but are not inconsistent with a spatial trend toward wetter paleoenvironmental conditions moving northward within the Morrison Formation. No temporal trends in MAP are discernible at either locality. Comparison of these results with the average MAP estimate previously calculated for the contemporary Upper Jurassic Lourinhã formation in Portugal (1100 ±200 mm/yr 1σ) indicates that the Morrison Formation was characterized by roughly equivalent levels of paleoprecipitation, contrary to comparisons of climatically sensitive sedimentary indicators and paleosol macromorphology that have been interpreted to reflect drier average conditions for Morrison paleoenvironments.