26
Woods Hollow Tank;MARATHON LIMESTONE
GENERAL FEATURES
The term "Marathon limestone", as here used, is a restriction of the term "†Marathon series" of Baker and Bowman to the limestones and associated rocks that crop out within the town of Marathon. These strata, which are a well-marked unit of Deepkill (Beekmantown) age, are exposed in the streets and vacant lots of the town. The basal beds, resting on the Dagger Flat sandstone, crop out a mile southwest of the railway station, on the north side of the old road to Alpine, and the highest layers, dipping beneath the Alsate shale, occupy a northeastward-trending belt of outcrop that crosses the Boquillas road 1¼ miles south of the town.
Other good exposures are found on the south side of the road to the Roberts ranch, 3 miles west-southwest of old Fort Peña Colorada. Part of the formation is also beautifully revealed in the bed of Alsate Creek, on the opposite side of the road (fig. 12). There are extensive exposures of the formation in the northeast end of the Dagger Flat anticlinorium, northeast of Woods Hollow Tank.
In aerial photographs the outcrops of Marathon limestone are lighter-colored than, those of adjacent formations but are streaked with faint light and dark bands that mark the outcrops of individual beds (pl. 17).
The Marathon limestone generally ranges between 500 and 1,000 feet in thickness but thins to only 350 feet in the southernmost exposures (pl. 2). The most conspicuous parts of the formation are beds of flaggy limestone that weather to an ashen-gray or bluish color, some of which contain graptolites. Partings of shale separate most of the limestone layers (pl. 3, A), and there are a few thick members of greenish clay shale. The argillaceous parts of the formation probably make up one-third or one-half its total thickness. Between the limestones are a few layers of sandstone and many beds of intraformational conglomerate. Near the middle of the formation is the Monument Spring dolomite member, which reaches 90 feet in thickness in the Marathon anticlinorium, where it has a wide and persistent development, but it thins and disappears southeastward in the Dagger Flat anticlinorium. The member is a massive mottled dolomitic limestone that contains fossils like those of the El Paso limestone in the region to the northwest.
LOCAL FEATURES
Marathon anticlinorium-The Marathon limestone has a wide exposure between the town of Marathon and the Roberts ranch along the crest of the Marathon anticlinorium, where it crops out in nearly level plains or in rolling hills covered with ashen-gray outcrops of its limestone flags. The formation is intricately folded and crumpled, and in places the weaker beds are cut out by squeezing or faulting, so that measurements of thickness are not altogether trustworthy, and thicknesses of individual members are quite different in closely adjacent sections. The competent Monument Spring dolomite member near the middle of the formation is broken and shattered, so that its outcrop is characteristically a chain of disconnected boulders. As a result of deformation the layer has been repeated in a most bewildering manner, and only careful field mapping does away with the impression that there are many beds of this sort in the section instead of one. The limestone flags of the formation in many places are traversed by veins of granular calcite and are cut at oblique angles to the bedding by cleavage planes that are strongly slickensided and coated with small calcite crystals. The interbedded shales are somewhat indurated but are not otherwise altered.
The Monument Spring dolomite member, near the middle of the formation, is named for its exposure half a mile west of Monument Spring, 12 miles southwest of Marathon. Its maximum thickness in the area near Fort Peña Colorada and Alsate Creek is 94 feet, but it thins to 25 feet 8 miles to the southwest, near Monument Spring. It consists of dense mottled dolomitic limestone, breaking with conchoidal fracture, and is constructed of small nodular masses of blue-gray dolomite, closely packed in a yellowish dolomitic matrix. In places it contains small angular calcareous fragments of similar appearance to the matrix. It weathers to light-colored, rounded boulders or disconnected ledges, which appear white from a distance (pl. 3, B). In places the rock is strongly silicified; the yellowish matrix is changed to brown chert, and the bluish nodules are relatively unaltered, so that the rock takes on a ribbed or spongy appearance. In other places the yellowish matrix has yielded more readily to decay, and the nodular portions remain as hard lumps. The exposures then resemble the nodular marls of the Comanche Peak and Georgetown formations in the Cretaceous. Near the top and base of the member the beds are less mottled and thick-bedded and are intercalated with thin layers of compact dark-gray or brown limestone. Fossils are common in parts of the dolomite, "
For a discussion and interpretation of limestones of this type see Twenhofel, W. H., Treatise on sedimentation, 2d ed., pp. 334-335, 1932.