46

action, is weak, on the east shore of the Baltic and the north shore of the Black Sea.

-The Ordovician rocks at Marathon contain a large amount of cherty material. This is particularly true of the Maravillas formation in which a large proportion of the strata consist of bedded chert. Lower down in the section considerable amounts of bedded chert are found in the Fort Peña formation, and locally there are also some chert beds in the Marathon limestone. It is noteworthy that other systems of the Paleozoic at Marathon also include layers of chert and siliceous shale, which suggest that conditions favorable for the accumulation of siliceous deposits were remarkably persistent in the area.

Part of the chert in the Ordovician of the Marathon region is of secondary origin. The original calcareous shells of fossils in some of the limestones of the Maravillas chert have been silicified, and some of the thicker limestone ledges have been replaced along the bedding planes by seams and irregular knots of siliceous material. Some of the thinner limestones can be traced along the outcrop into chert beds. It is probable that these features of obvious secondary origin are a minor part of the whole mass of siliceous deposits. Their formation must have been materially aided by the large amount of silica available in the primary chert deposits.

That a considerable part of the chert is of primary origin is indicated by much evidence. The intraformational conglomerates in the Maravillas formation contain pebbles of both chert and limestone. The cherts are identical with those in place in the formation- a fact which indicates that they assumed their present character at least shortly after deposition. Moreover, thin limestone beds in the chert and thin chert beds in the limestone are sharply set off from the enclosing strata, without gradation between them. The stratification of the cherts is quite different from that of the limestones. Most of them are more thinly bedded, some are nodular, and a few of the thicker beds are aggregates of large pillowlike masses. These features have not been seen in the associated limestone deposits.

Some of the chert layers are laminated and even cross-bedded. These parts are seen under the microscope to contain seams of fine embedded sand grains and various fossil fragments. Some of the cherts also contain siliceous and calcareous spicules.

The source of the material for the cherts lay to the southeast. Those in the Maravillas formation thicken markedly in that direction at the expense of the limestone beds, even within the 40-mile breadth of the Marathon Basin. To the northwest the equivalent of the Maravillas chert is a limestone, the Montoya formation. However, the Montoya contains a great deal more chert than the limestone beds that underlie and overlie it.

The silica that formed the cherts was derived from a land area that was able to supply a large quantity of material through a long period of geologic time. This may have been due to some peculiarity of the rocks of which the land was composed, or it may have resulted from volcanic activity, although direct evidence for such activity in Ordovician time is lacking. That the land from which the siliceous materials was derived was not far distant is suggested both by the rapid increase in volume of cherty material to the southeast and by changes in character in the associated sediments in a similar direction. The thickening of the chert beds to the southeast suggests that whatever process carried the silica into the sea was incapable of transporting it for any great distance.

It is possible that the cherts were derived from colloidal silica that was flocculated directly on the sea bottom. Most of the material thus flocculated was deposited in regular strata, but the nodular parts (fig. 13, B) may have been original gel masses of silica. Organisms with siliceous skeletons, such as sponges, must have found encouragement in this environment, as their spicules are found in the rock, but it is unlikely that they played more than an incidental part in the formation of the chert. The fine clastic materials present in parts of the chert represent an occasional deposition of sediments by mechanical processes but do not show that the chert as a whole had this origin.

-Boulders as much as 5 feet across are embedded in the upper part of the Woods Hollow shale and the basal part of the Maravillas chert. The origin, mode of transportation, and deposition of these masses present many puzzling features.

The boulders of the Maravillas formation are associated with coarse, poorly sorted cobbles in its basal conglomerate. The intercalation of thin layers of chert and limestone in the conglomerate and the water-rounded character of many of the fragments indicate that the rock is a marine rather than a terrestrial deposit and that it is not a breccia of tectonic origin. As the boulders and cobbles include rocks from nearly every older formation in the region down to the Cambrian, they must have been derived from an uplift several thousand feet in height. As the rocks are like those indigenous to the region, the uplift was probably within the Llanoria geosyncline, although "

Twenhofel, W. H., Treatise on sedimentation, 2d ed., pp. 262-263, 1932. Trask, P. D., Origin and environment of source sediments of petroleum, pp. 149-150, 1932.

Twenhofel, W. H., op. cit., pp. 541-542.

For a description and interpretation of the very similar cherts in the Bigfork formation see Hendricks, T. A., Knechtel, M. M., and Bridge, Josiah, Geology of Black Knob Ridge, Okla.: Am. Assoc. Petroleum Geologists Bull., vol. 21, pp. 6-7, 1937.