Calcium-carbonate content of some California Mesozoic and Tertiary sediments

Abstract

The calcium-carbonate content of approximately 8500 outcrop and well samples of Mesozoic and Tertiary sediments in California was determined in order to ascertain the extent to which the quantity of calcium carbonate in the sediments might be related to the conditions under which the sediments were deposited. In general, the different formations have an average carbonate content of approximately 5 per cent. The Miocene sediments, with an average of about 6 per cent, are the richest; the Eocene sediments, with an average of about 2 per cent, are the poorest. At least 25 per cent of the samples analyzed contain less than 1 per cent calcium carbonate, and nearly 50 per cent contain 3 per cent or less. Only 10 per cent of the sequences of sediments that were examined have an average carbonate content of more than 10 per cent, and very few have more than 20 per cent. The quantity of calcium carbonate in outcrop samples commonly is less than in well samples of corresponding age, owing to the effect of leaching. Much of the carbonate content of some of the Tertiary sediments apparently is due to the presence of Foraminifera. The areal distribution of calcium carbonate among sediments of similar stratigraphic age seems to show little relationship to the environmental conditions of deposition. The carbonate content of the Pliocene sediments in the Los Angeles basin, however, in general tends to increase away from probable position of the shore line at the time the sediments were deposited. Dolomite apparently is fairly common in the Jurassic and the Cretaceous sediments, not so common in the Eocene, Oligocene, and Miocene sediments, and rare in the Pliocene sediments.