A combination of field, petrographic, and isotopic work has identified 2 horizons in Precambrian carbonate strata (800 m.yr. Beck Spring Dolomite, CA. and 1200 m.yr. Mescal Limestone, AZ) which were silicified during Precambrian erosion events. Microfossils have been discovered in chert samples from these horizons by R Horodyski and represent the first fossil record of organisms that inhabited the Precambrian land surface. Carbon isotope measurements in the associated carbonate reveal that the land surface was extensively occupied by a terrestrial photosynthesizing community. The microfossils in the chert are therefore probably not isolated examples but are elements of this extensive community. An attempt will be made to establish the first paleontologic record for the Precambrian terrestrial environment. Field, isotopic, and petrographic work will establish the paleoenvironmental conditions of the paleokarst horizons and will establish, in detail, which silica phases formed during Precambrian weathering as opposed to those which formed during early diagenesis of the original marine carbonates. Cherts with strong textural and geochemical characteristics indicative of subaerial formation will be examined in thick petrographic sections for the presence of microfossils. The biogenicity of microstructures will be evaluated, and an effort will be made to establish biologic affinities for any microfossils found. Particular attention will be given to comparing microfossils associated with the exposure surfaces with those in the underlying marine cherts to establish that the microfossils found associated with the exposure surfaces truly represent microorganisms that were living in a terrestrial setting.
