We studied two short, high-gradient river systems draining the Dolomite Alps in northeastern Italy in order to determine which grain types survive transport and to what extent sand grain types reflect source rocks. Grains of all the labile rock types in the source areas survived to lower reaches of the rivers. In one drainage (Boite-Piave), they reached the Adriatic coast. Carbonate grains (largely dolomite) in the Gadera-Rienza Rivers decreased abruptly, largely by dilution, from >50% to trace amounts in 100 km of travel. Percentage of carbonate grains in the lower reaches of these rivers was generally less than one-half the areal percentage of limestone and dolostone exposure in the source areas. However, in the Boite-Piave Rivers (200 km long), enrichment of carbonate grains in beach sand at the expense of polycrystalline quartz and volcanic rock fragments results in dolostone sand at the beach reflecting 78% of its outcrop abundance and limestone (calcite) sand reflecting 68% of its outcrop abundance. Polycrystalline quartz and mafic volcanic rock fragments are less abundant in the beach because of dilution by longshore drift or the breakdown of these grains by wave abrasion. The relative resistance of carbonate textural grain types to abrasion is micrite > spar > mixed micrite/spar. The results indicate that detritus from dominantly silicic and intermediate volcanic rocks can survive fluvial transport and at least moderate wave abrasion. Metamorphic rock fragments (mostly phyllite) in the Gadera-Rienza Rivers survived transport to the confluence with the Isarco River at Bressanone. In the Boite-Piave river system, metamorphic rock fragments survived fluvial transport to the beach plus some beach abrasion. They did so because the relatively rapid transport down the high-gradient, low-sinuosity streams did not permit extensive chemical weathering. Grains of calcite (micrite and spar), dolomite, and volcanic rock fragments increased in roundness by abrasion in the surf after undergoing only a few kilometers of transport along the coast.

We studied the composition and roundness of medium sand from 18 small beaches of Elba Island. Six are pocket beaches less than 100 m long; the longest is 1.3 km long. The drainage basins of streams that supply the beaches are all less than 25 km 2 ; most are less than 5 km 2 . Beach sands range widely in composition owing to diverse source terrane. For the various drainage basins, comparison of the outcrop areas of the different types of bedrock with the compositions of beach sand grains yields the following conclusions: (1) the relative area of granodiorite outcrop is accurately represented by the amount of quartz + feldspar + quartzofeldspathic rock fragments in all beaches, although plagioclase in beach sand is significantly reduced relative to K-feldspar; (2) the relative areas of outcrop of ophiolitic and limestone bedrock are accurately represented by beach sand in pocket beaches, but are only moderately represented (ophiolitic rocks) or poorly represented (limestone) in other beaches; (3) the relative area of outcrop of metamorphic rocks is poorly represented in beach sand (metamorphic rock fragments + polycrystalline quartz) except in one anomalous beach supplied in part by mine tailings; (4) the relative area of bedded chert outcrop is poorly represented in beach sand because bedded chert does not break down into sand-size grains; and (5) shale bedrock is not represented or is only marginally represented in beach sand. For Elba beaches in general, the order of increasing roundness of grains, and thus increasing rate of abrasion, is: quartz < plagioclase < K-feldspar and igneous rock fragments (quartzo-feldspathic) < serpentine < metamorphic rock fragments < carbonate rock fragments (CRFs). There are no significant differences in roundness with beach length for quartz, K-feldspar, or CRFs. There are also no significant differences in roundness for the same three grain types from beaches of different size drainage basins, which indicates there is no perceptible rounding of grains by streams. First-cycle monocrystalline quartz grains of medium sand are unrounded, although coarser grains show minor blunting of edges. The roundness of quartz, K-feldspar, and CRFs are all greater on the eastern, more protected part of the island. This reflects a significant proportion of recycled quartz and K-feldspar in the eastern beaches, but CRFs may undergo more rounding in beaches of low to moderate wave activity than in high-energy beaches.

Abstract Paleozoic rocks that were deposited along the southeastern margin of North America during Paleozoic time and that make up the Ouachita orogen extend from Arkansas across Oklahoma and Texas and have been traced almost to Mexico. Rocks at the southwestern end of the Ouachita orogen are exposed only in the Marathon and Solitario uplifts in west Texas, but the western edge of the sequence in the subsurface of Texas is fairly well known from well and seismic data (e.g., Flawn and others, 1961; Nicholas and Rozendal, 1975). The Marathon uplift is a broad domal uplift of early Tertiary age and is more than 125 km in diameter (King, 1937). Erosion of Cretaceous and younger strata from the crest of the uplift produced the topographic Marathon Basin in which are exposed, in an area approximately 50 by 75 km, deformed Paleozoic rocks that have a composite stratigraphic thickness of 5000 m. Permian strata in the Glass Mountains unconformably overlie older rocks alongthe northwestern edge of the Marathon uplift (Fig. 1). The Solitario uplift, 65 km to the southwest, provides exposures of Paleozoic rock approximately 8 by 15 km at the crest of a buried intrusion.

Abstract Roadcuts provide the only exposures of public access in the Marathon Uplift. However, roadcuts on U.S. 385 south of the town of Marathon and on U.S. 90 east of Marathon display spectacular features. Mileages to the first five stops in this guide are given from the junction of U.S. 385 south and U.S. 90 at the west edge of Marathon; mileages to the six remaining stops are from the junction of U.S. 385 north and U.S. 90 (Fig. 1).

ABSTRACT Upper Cretaceous sandstones of northern Mexico have similar framework composition for a distance of 400 km south of the Rio Grande but differ markedly in reservoir quality in the north versus the south chiefly because the basins in which the sandstones were deposited experienced different post-depositional histories. The sandstones are composed largely of detritus from volcanic and intrusive igneous rocks and were deposited in paralic and fluvial environments. Sandstones in the north were never buried more than 1500 m and were subjected to slow, gentle, basinward downwarping, followed by gentle uplift and denudation. They underwent a complex diagenetic history of cementation by chlorite, quartz, calcite, ferroan carbonate and kaolinite; the development of modest secondary porosity; and they form hydrocarbon reservoir rocks of moderate quality ( ϕ = 2 to 18%, k = 0.2 to 20 md). Sandstones to the south were buried rapidly by 1000 to 4000 m of younger strata and immediately thereafter underwent strong compressional folding and local thrustfaulting during the Laramide orogeny. These sandstones lost from 20 to 35% porosity by compaction and the remainder of the porosity by cementation with calcite and minor chlorite and quartz. They are tight, did not develop secondary porosity, and have no shows of hydrocarbons. During slow subsidence of sandstone-shale sequences in the north, the normal maturation of shale and associated organic matter occurred. Shale water was probably expelled in stages, organic matter evolved to produce some hydrocarbons, and acid formation water was generated. Diagenesis of the associated sandstones followed a typical sequence found in many sedimentary basins and included the development of secondary porosity by the dissolution of framework grains and cement. To the south, Laramide compressive forces caused strong compaction and more rapid than normal de-watering of the Upper Cretaceous shales. Most water present in the shales was expelled prior to development of conditions favorable for hydrocarbon maturation. Thus, the rapid and severe compaction precluded the opportunity for the development of typical acidic formation waters that might have contributed to dissolution of some labile constituents. Close to the Sierra Madre front, rapid and early expulsion of water produced a strong fracture cleavage in shale and siltstone.

The Caballos Novaculite is composed predominantly (90 percent) of bedded chert; the widespread distribution of two novaculite (milky white chert) lithosome marker units permits the formation to be divided into five members. From the base up, the members are listed with maximum thicknesses and rock type as follows: lower chert, 15 feet, light brown and off-white chert with shale partings; lower novaculite, 150 feet; lower chert and shale, 200 feet, chert of many different colors (chiefly green) and shale partings, and several beds of calcarenite; upper novaculite, 400 feet; and upper chert and shale, 400 feet, chert of many different colors and shale partings. Minor amounts of red and green shale, pebble conglomerate, sandstone and calcarenite are in the chert and shale units. The Caballos Novaculite in the Marathon Basin is a lens-shaped unit from 100 to 700 feet thick, based on 26 measured sections. The Caballos is conformable with the underlying Maravillas Formation of Late Ordovician age and with the overlying Tesnus Formation of Mississippian to Early Pennsylvanian age; and, therefore, probably includes rocks of Silurian, Devonian, and possibly Mississippian age. The novaculite members may be time- stratigraphic units in the center of the Marathon Basin. Chert formed by the diagenetic alteration of opaline skeletal particles, chiefly sponge spicules and Radiolaria. Quartz in the form of subsequent grains of microquartz (<35 μ ) and lesser megaquartz is the only silica phase present. Color varieties of chert differ in fabric of microquartz and content of pigmenting impurities; green chert contains abundant illite; tan, brown, and black chert contain iron and manganese oxides and organic matter; red chert contains hematite; blue chert contains apatite. Mottled chert beds formed by submarine slumping. Novaculite (snow-white chert) owes its lack of color to the absence of detrital impurities and chemical pigments, and its milkiness to the dispersion of light by minute water inclusions (Folk, 1965) and reflecting faces of microgranular quartz crystals. Novaculite has a distinct microscopic fabric; subspherical specks up to 200 μ in diameter composed of microquartz grains up to 10 μ long are scattered in a matrix of slightly larger uneven-grained microquartz grains, 5 to 25 μ long; the specks are probably relict Radiolaria. Skeletal ghosts of spicules visible in ordinary light range from 0 to 80 percent of novaculite beds, but average 10 percent. Radiolaria are more abundant than spicules in colored chert beds. The Caballos was deposited in a deep-marine trough adjacent to peneplaned land masses; rate of accumulation (assuming 50 percent compaction) was from 0.1 to 0.5 mm/1000 years. Terrigenous clay was absent during accumulation of proto-novaculite, but was supplied occasionally during accumulation of colored chert and shale beds. Green and red (originally brown?) colors of clay shale reflect original differences in bottom conditions during deposition. Quartz silt in chert includes wind-blown and storm-deposited grains; sandstone, calcarenite, and conglomerate beds were deposited by rare turbidity currents. Depositional fabrics of sediments were modified by lithification processes, burrowing infauna, escaping gases, and submarine slumping. Chertification occurred largely by solution of opal and precipitation of crystalline silica (cristobalite or low-quartz) from pore solutions relatively soon after deposition.