22
Many of the Paleozoic strata contain fine fragmental material derived from crystalline rocks. Conglomerate beds in various parts of the Ordovician section, but particularly in the Maravillas chert, contain fragments of vein quartz. The Tesnus and Haymond formations (Pennsylvanian) contain beds of arkose, which increase in number and thickness toward the south. Their source perhaps lies in highlands in that direction. The arkose contains grains of microcline and other minerals, probably derived from the break up of granite, as well as small chips of slate and phyllite.
In the boulder-bed member of the Haymond formation near Haymond station there are fragments of ancient rocks of larger size. Most of these are well-rounded cobbles, some of which reach a foot in diameter. They consist of granite, aplite, pegmatite, vein quartz, rhyolite, quartz conglomerate, and possibly of schist. Several thin sections of the rocks have been examined. Some of the specimens consist of fine-grained granite and some of porphyritic granite. Others consist of rhyolites of various types that contain large phenocrysts of quartz and plagioclase, whose edges are rounded by resorption. The groundmass of the rhyolites is a finely crystalline aggregate, probably a devitrified glass. In some specimens it shows a well-developed sinuous flow structure. In the exposures of the boulder-bed member north of Haymond station cobbles of the igneous rocks are few, a fact which suggests that they came from a southern and probably distant source.
Depth of the pre-Cambrian floor in the Marathon Basin-There is no clear evidence to show at what depth the pre-Cambrian floor lies beneath the Marathon Basin. The oldest exposed parts of the Paleozoic section are of Upper Cambrian age. The character of the structural features of these and the overlying Ordovician rocks suggests that they are underlain by a considerable body of strata that is probably of incompetent character. Unfortunately, paleogeographic evidence furnishes no good clues to the age of these oldest members of the Paleozoic. They may be Middle or Lower Cambrian. The writer considers it possible that the thrust sheets in the Marathon Basin originated in wedges of crystalline rocks that lie at an unknown depth beneath (pl. 20, B, C, and D).
CAMBRIAN SYSTEM
DAGGER FLAT SANDSTONE
GENERAL FEATURES
The Dagger Flat sandstone was named by the writer in 1931 for exposures in Dagger Flat, 13 miles south of Marathon. The sandstones were first described by Baker and Bowman in their section on Threemile Hill, 3 miles northeast of Maravillas Gap, and were designated, without age assignment, as member 2 of their † Marathon series.
The formation is the oldest rock found in place in the Marathon region, and its base is nowhere exposed. Its strata are in all places so intricately contorted that the exposed thickness is not exactly known, but a probable maximum of 300 feet is found on the south side of Dagger Flat. It is exposed in long, narrow belts in the center of the anticlines in both the Marathon and Dagger Flat anticlinoria, with the most extensive exposures in the south, on the Buttrill ranch and near Threemile Hill.
The Dagger Flat sandstone consists of thick ledges of saccharoidal buff sandstone interbedded with shale. These pass toward the top into shales and thin flaggy sandstones, with some calcareous beds that contain a few Upper Cambrian fossils. The formation is overlain without apparent break by thin flaggy graptolite-bearing limestones of the Marathon formation (Lower Ordovician).
LOCAL FEATURES
Dagger Flat area--The Dagger Flat sandstone is well displayed for a distance of 4 miles northeast from the Buttrill ranch (pl. 23) in a series of narrow belts on the crests of anticlines. The southeasternmost of these belts is the broadest and has a width of 1,500 feet. The lowest beds are conspicuous ledges, each 3 or 5 feet thick, of white to buff sugary, moderately coarse- grained sandstone, weathering pale brown (sec. 12, pl. 2). In places these beds pass into a fine conglomerate of rounded pebbles of vein quartz, some of which show a notable secondary regrowth of quartz crystals. The massive sandstones are overlain by flaggy and thinly laminated brown and greenish micaceous sandstones weathering to angular blocks and flags, with much interbedded shale, particularly toward the top. In the upper part there are also several layers of laminated brown calcareous sandstone. Their bedding surfaces are strewn with fragmental fossils, including the cephalons of various trilobites, such as Agnostus and many shells of Lingula and Obolus. There are also several layers of conglomerate composed of small black chert, gray limestone, and clear quartz pebbles in a matrix of brown sandy limestone. The formation is "
King, P. B., Pre-Carboniferous stratigraphy of the Marathon uplift: Am. Assoc. Petroleum Geologists Bull., vol. 15, pp. 1064-1065, 1931.
Baker, C. L. and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, p. 83, 1917.
A dagger (†) preceding a geologic name indicates that the name has been abandoned or rejected for use in classification in publications of the U. S. Geological Survey. Quotation marks, formerly used to indicate abandoned or rejected names, are now used only in the ordinary sense.
The Buttrill ranch referred to in this report is the unnamed house east of the Marathon road in the northeast corner of the Santiago Peak quadrangle. Dagger Flat is in the southwest corner of the Marathon quadrangle. These two places should not be confused with the Buttrill ranch and Dagger Flat farther south, shown on the map of the Bone Spring quadrangle.