INSOLUBLE RESIDUES FROM CONODONT SAMPLES AND THEIR UTILITY IN SEQUENCE STRATIGRAPHY

Conodont specialists commonly discard light fractions after heavy-liquid separations of acid residues of carbonate samples. However, specially treated light fractions can help one to understand the sequence stratigraphy of strata. Cambrian-Ordovician carbonates in Utah and Texas show variations in quartz sand content that are related to sea-level changes. Sea-level drops exposed siliciclastic source areas and caused quartz sand to prograde into carbonate environments. Sea-level rises covered source areas and cut off the supply of quartz sand. Highstand carbonates in Utah and Texas often have less than 2 per mil quartz sand content, but lowstand carbonates may have more than 100 per mil. Vertical variations in quartz sand concentration are another tool to help one to delineate sequences and sequence boundaries, and these variations are useful in correlation. Obtaining insoluble residue data takes only one or two additional processing steps. Sample mass is measured before acetic or formic acid treatment, then the acid residue is washed through 16- and 200-mesh sieves. Mass of rock (including chert) remaining on the coarse sieve is measured and subtracted from the sample mass to obtain dissolved rock mass. Insoluble material caught on the fine sieve is dried and separated with tetrabromoethane; other heavy liquids should work equally well as long as all samples are treated consistently. Light fractions are cleaned with acetone, dried, and treated with HCl to remove all carbonate. The light fraction is washed through the 200-mesh sieve, dried, and glauconite is removed with a magnetic separator. The mass of the resulting insoluble material (mostly fine quartz sand) is measured. The masses of dissolved sample and of HCl-treated light fraction are entered into a spread sheet; the former number is divided by the latter, and the dividend is multiplied by 1000 to obtain per mil concentration. Finally, the per mil concentration of HCl-insoluble light fraction is graphed versus stratigraphic thickness. Comparisons among Utah sections show similar variations in concentration of insoluble residue at the same biostratigraphic levels. Comparisons between Utah and Texas sections show variations in concentration that are of quite different magnitude but occur at the same biostratigraphic levels. The insoluble residue graph essentially tracks sea-level changes.