CLAY MINERAL WEATHERING SEQUENCE IN A TROPICAL TERRACE PROGRESSION, COSTA RICA

Soils developed on Quaternary fluvial and marine fill terraces on the Pacific coast of Costa Rica display a pronounced progression of clay mineral assemblage with age. Clay minerals from the active floodplain are predominately smectite with lesser 7Å halloysite and traces of 10Å halloysite. Quaternary terraces at < 80m above MSL exhibit similar clay mineral assemblages. Terraces from 80 to 210m above MSL consist of 7Å halloysite with only traces of 10Å halloysite and smectite. A 7m soil profile taken at 140m above MSL is dominated by halloysite with traces of smectite at depths of 5.5 to 7m. The data from this profile and the terrace progression document a transition from smectite to halloysite with increasing age. Coincident with these soils is a transition from dark yellowish brown (10YR 3/4) to lateritic dark red (10R 3/6) soil. Current research consists of examining variation in bulk chemistry and bulk mineralogy of the terrace soils.

Fluvial and marine terraces on the Pacific coast range in age from a lower limit of mid to late-Holocene (Pazzaglia et al., 1998) to an upper limit of 352 ka (Marshall et al., 2003). Given an uplift rate of 1m/ka (Fisher et al., 1998), terrace ages are as follows: Qt Z (approx. 210 masl) equals 220 ka; Qt Y (120-140 masl) equals125 ka; Qt X (50-80 masl) equals 75 ka; and Qt W (5 to10 masl) equals Holocene. Our estimates assume that the fill terraces formed during periods of relative high sea level associated with interglacial periods. Applying a parabolic diffusion limited model to weathering rind thicknesses studied by Sak et al. (2004) produces results consistent with our age estimates. Using the time constraints we will extrapolate rates of mineral formation and chemical evolution within the tropical terrace progression.

Fisher DM, Gardner TW, Marshall JS, Sak PB, Protti M. 1998. Geology 26, 467-70.

Marshall JS, Idleman BD, Gardner TW, Fisher DM. 2003. Geology 31, 419-22.

Pazzaglia F, Gardner T, Merritts D. 1998. In: Rivers Over Rock: Fluvial Processes in Bedrock Channels, v 106 (ed. E. Wohl and K. Tinkler), pp. 207-36, AGU.

Sak PB, Fisher DM, Gardner TW, Murphy K, Brantley SL. 2004. Geochimica et Cosmochimica Acta (in press).