A RECONNAISSANCE STUDY OF VARIATIONS IN ELEMENTAL CONCENTRATIONS DURING WEATHERING AND TAFONI DEVELOPMENT IN 19TH CENTURY ARKOSIC SANDSTONE BLOCKS IN THE SPRAY ZONE AT SALT POINT STATE PARK, NORTHERN CALIFORNIA COAST, U.S.A

The Cretaceous arkosic sandstone sea cliffs at Salt Point State Park are famous with tourists and photographers for their deep, picturesque tafoni (pitted weathering in uniform rock). Tafoni has also developed in blocks of the native rock which were quarried nearby and abandoned in the surf and spray zone when the quarry closed in 1869. We have determined the elemental compositions of the weathering rinds of dated surfaces and the unweathered interiors of blocks, some of which developed tafoni, and contiguous blocks which did not. As, Al, Au, Ba, Be, Br, Ca, Cl, Co, Cr, Cs, Hf, K, La, Mg, Mn, Mo, Na, Ni, P, Pb, Rb, S, Sb, Sc, Si, Sr, Ta, Ti, Th, U, V, Y , Zr and eight of the lanthanides are found with about the same concentrations in the least weathered cores and in the weathered rinds of all but one of the boulders sampled. The only elements which showed clear changes in most blocks during weathering were copper and iron. The copper decreased about 20% from least weathered to most weathered in most of the samples. Ferrous iron was oxidized and decreased while ferric iron increased in roughly equal proportions during weathering, which is consistent with the color changes from medium gray to light orange. We have examined the cores in thin section, scanning electron microscope, and electron microprobe, and we do not find new minerals precipitating. The preliminary results show that there is oxidation, but little leaching or neomineralization during the development of significant tafoni over a period of more than a century. This is not consistent with the popular theories that tafoni are caused by the growth of crystals or development of concentrated solutions which pry grains off the surface of the blocks. Our thickest block and deepest core was anomalous. Many of the elements vary from the core to the rind, and the heavy lanthanides, Tb, Yb, and Lu, were preferentially leached.