Differential Deposition of Clays: Pericontinental Fractionation

Clays can be excellent paleo-climatic indicators when they have time to form in soils and the soils are developed enough to represent the prevailing climate. The clays must be eroded from these soils and transported to a basin with little mixing from concurrently eroding sedimentary rocks or from soils from a different drainage basin that represents a different type of climate.

In addition to this stipulation, clays must be deposited en masse in a basin and not segregated prior to deposition. However, many studies indicate that segregation does occur, either because of differential flocculation (Parham, etc) or because of different settling velocities due to different shapes of clays (Gibbs).

The prevailing idea about the abundance of smectite in Cretaceous sediment in the North Atlantic was that the abundance of smectite indicated that there was a monsoonal climate. Smectite formation occurred in low-lying areas, presumably in vertisols, along the eastern seaboard of North America.

Actual paleosols (ancient soils) of Aptian-Albian age are preserved around Baltimore, Md. Here, paleoinceptisols are interbedded with crevasse splay deposits of the Potomac Group. Paleo-ultisols occur in areas farther from stream channels. These have clay-enriched E horizons. The clay mineral composition of these paleosols is dominantly kaolinite, with detrital illite that was probably shed from Paleozoic shales of the Appalachians just to the west. No paleovertisols were observed in this outcrop.
Aptian-Albian sediment from just offshore the New Jersey coast, in the Baltimore Canyon Trough, comprises deltaic sand and interbedded mud. The dominant clay is illite, here shown undergoing dissolution. There is an almost equal amount of kaolnite and no or only traces of smectite. Roll mouse over the image to see illite and kaolinite.
Turbidites interbedded with cyclic pelagic black shale and calcareous (nannofossil) ooze of Aptian-Albian age just offshore the Baltimore Canyon Trough. The black shales contain dominantly smectite (identified by x-ray diffraction). Turbidite beds are richer in kaolinite and illite than pelagic layers.

These data indicate that pericontinental fractionation, or the differential deposition of clays at the edge of a continent, has sorted clay and led to a skewed "paleoclimate" signal in sediment of the North Atlantic. Kaolinite and illite are trapped on the shelf, while smectite escapes to the deep sea.

Kaolinite may flocculate first as clays reach the salty sea because of its low charge and low cation exchange capacity (CEC), or it may settle first because of its equant shape. Illite has a higher CEC and a platy morphology. It is wafted farther out onto the shelf. Smectite has the highest CEC and a flaky morphology that resists deposition. It tends to accumulate in the pelagic environment, the deep sea. Turbidites bring sediment from the shelf to the deep sea, enriching pelagic sediment in "shelf" components, such as illite and kaolinite. Interpretation of paleoclimate from clay composition must be done with caution. The best way to do this is to use clays in place, in situ, such as in paleosols.

References

Chamley, H., 1979. North Atlantic clay sedimentation and paleoenvironment since the late Jurassic. In Talwani, M., Hay, W., and Ryan, W. B. (Eds.), Deep Drilling Results in the Atlantic Ocean: Continental Margins and Paleoenvironment. Am. Geophys. Union, Maurice Ewing Ser. 3:342-361.

Gibbs, R. J., 1977. Clay mineral segregation in the marine environment. Jour. Sed. Petr. 47(1):237-243.

Holmes, M. A., 1987. Clay mineralogy of the Lower Cretaceous deep-sea fan, deep sea drilling project site 603, lower conti-nental rise of North Carolina. Init. Repts., DSDP, 92:1079-1089 (U.S. Govt. Printing Office, Washington).

Lonnie, T. P., 1982. Mineralogic and chemical comparison of marine, nonmarine, and transitional clay beds on south shore of Long Island, New York. Jour. Sed. Petr. 52(2):529-536.

Parham, W. E., 1966. Lateral variations of clay mineral assemblages in modern and ancient sediments. Proc. Intn'l. Clay Conf. 1:135-145.

Porrenga, D. H., 1966. Clay minerals in recent sediments of the Niger delta. Clays Clay Minerals, 14th natl. conf., Pergam-mon, Oxford, New York:221-233.

Singer, A., 1984. The paleoclimatic interpretation of clay minerals in sediments - a review. Earth-Sci. Rev., 21:251-293.