Angular Clasts Research Articles

Remnant Mountains in Early Ordovician Seas of the El Paso Region, Texas and New Mexico

The Thunderbird mass of Precambrian rhyolite porphyry in the Franklin Mountains of westernmost Texas is a low remnant mountain that protruded above the floor of the Early Ordovician seas. This extrusive rhyolite, dated at 950 m.y., is surrounded and overlapped by limestone of the Early Ordovician El Paso Group. Sparite-cemented masses of rhyolite talus border the mountain, grading into dolomitized biointrasparrudite containing many large angular clasts of rhyolite. This rhyolite mountain and other Precambrian-rock islands in the region provided detritus to the Early Ordovician seas of southern New Mexico, west Texas, and northern Mexico. Preservation of such highlands for much of Early Ordovician time appears to be extraordinary. The El Paso limestone of the Franklin Mountains, however, reflects in entirety an environment ranging from supratidal to shallow-water marine, as evidenced by the occurrence of such features as polygonally cracked micrites, algal stromatolites, digitate algae, and archaeoscyphid mounds. Protection of the remnant mountains from high-energy waves can be attributed to the regional, virtually intertidal, mudflatlike environment; the occurrence of archaeoscyphid biohermal barriers; periods of submergence below wave base; or a combination of these factors. The sedimentary features imply that a vast region of remarkably uniform shallow-water to supratidal environmentals existed during Early Ordovician time.

The Glacial Origin of Carboniferous Conglomerates West of Barraba, New South Wales

A Carboniferous terrestrial conglomerate near Barraba, in northern New South Wales, is considered to be a tillite, and a detailed examination shows that lithologic and fabric criteria authenticate its glacial origin. The tillite, of probable Middle Carboniferous age, occurs near the top of the Spion Kop Conglomerate, a lenticular terrestrial conglomerate formation reaching a maximum thickness of 1800 feet. The tillite is nonsorted and nonstratified, and is associated with stratified disrupted-framework conglomerates believed to be fluvioglacials. When compared with other rocks in the sequence, it is unique in that it contains no volcanic detritus, and is composed of mostly angular clasts of sedimentary, metasedimentary, and acidic igneous intrusive origin, some of which reach 4 m in length. Some smaller clasts are striated and even faceted. The lower boundary of the tillite is gradational with the underlying fluvioglacials, which include a compacted silty layer and varve-like layers containing dropped pebbles. Fabric data suggest that the tillite was mostly an ablation moraine, and that the glacier flowed toward the east. Fine-grained layers (probably varves) overlie the tillite, and it is considered that the onset of proglacial lacustrine sedimentary conditions protected the moraine from reworking by glacial meltwaters. The known paleogeography and restricted occurrences of glacial deposits in the Carboniferous rocks in this region of New South Wales suggest that the glaciation was alpine rather than ice-cap glaciation.

Chronology of Deformation of Paleozoic and Tertiary Succession Near Railroad Valley, Nevada: ABSTRACT

Principal structures occurring in the Paleozoic sedimentary section of the Horse Range are thrust faults (some placing older over younger rocks, but most placing younger over older), north-south-trending, asymmetrical, eastward-overturned folds, and high-angle faults. Pre-Oligocene deformation of these Paleozoic rocks is indicated by Oligocene volcanic rocks lying with angular unconformity on overturned Ordovician strata. In the dissected pediments west of the Horse Range, a 10,000-foot-thick sedimentary and volcanic succession of Oligocene, Miocene, and Pliocene rocks crops out (Moores, 1965). The Miocene-Pliocene part of the section lies disconformably on Oligocene volcanic rocks and consists of an assemblage of terrestrial strata, including indurated ash beds, lacustrine limestone, and immature detrital deposits which contain angular Paleozoic carbonate and Oligocene volcanic clasts derived from adjacent ranges. The general parallelism of the Paleozoic and Tertiary successions given as evidence for contemporaneous deformation (Moores, 1965) is expressed only in that they are in contact with one another for a distance of approximately 12 miles. There the parallelism ends. Folds and thrust faults in the Paleozoic rocks trend north-south, whereas folds within the Tertiary succession trend east-west and plunge steeply toward the west. These relations, together with the angular unconformity between the Oligocene volcanic rocks and Paleozoic rocks, temporally separate tectonic deformation of the Paleozoic and Tertiary successions. Principal deformation of the Paleozoic succession at least preceded the Oligocene volcanic rocks, and may be as old as Pennsylvanian (Ptacek, 1963). Deformation of the Tertiary succession occurred, at least in part, after the Pliocene rocks were deposited, and probably during the Oligocene and Miocene, as is suggested by several unconformities reported in the Tertiary succession (Moores, 1965). Some occurrences of Paleozoic rocks are in positions that suggest emplacement by gravity sliding during or after deposition of Tertiary rocks. These occurrences can be explained easily as gravity-slide blocks emplaced from adjacent ranges (e.g. Horse and Grant) as the ranges were uplifted along high-angle faults, but in no way imply that the principal deformation of the Paleozoic strata was contemporaneous with deformation of the Tertiary succession. End_of_Article - Last_Page 650------------