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Ahlitanah (talk) 18 March 2015 (UTC)

The Vinini Formation is a marine, relatively deep-water, sedimentary deposit of Ordovician to Early Silurian age in Nevada, U.S.A. It is notable for its highly varied, mainly siliceous composition, its mineral deposits, and controversies surrounding both its depositional environment and structural history. The formation was named in 1942 by Merriam and Anderson,^[1] for an occurrence along Vinini Creek in the Roberts Mountains of central Nevada and that name is now used extensively in the State.

Stratigraphic relations

Although the name Vinini is used extensively, the Palmetto Formation in southern Nevada ^[2] is of the same age and lithic composition. Originally the Vinini was considered to be wholly of Ordovician age, but the unit designated as Vinini on many published maps includes, at the top, a massive chert unit, the Cherry Spring chert, now known to be of Early Silurian age.^[3] The formation is considered to have been deposited in relatively deep water, outboard of the contemporaneous carbonate bank to the east, because of its generally dark gray color and the near absence of a shelly fauna.^[4] The identity of strata underlying the Vinini over most of its range is uncertain due to the prevalence of faults, but the equivalent Palmetto Formation of southern Nevada is known to be underlain depositionally by Upper Cambrian limestone deposits.^[5] The Vinini is overlain depositionally by Middle Silurian sandy strata. To the west, the Vinini grades laterally into the Valmy Formation, a somewhat similar unit, with the same age range.^[6] Together, the Vinini and Valmi formations constitute a principal part of lower Paleozoic deep-water units in Nevada known collectively as the "western assemblage."^[4] To the east, separated by faults, is a bank of mainly shallow-water shelf carbonate rocks ranging in age from Cambrian to Devonian--the "eastern assemblage".^[4,7] Throughout its extent, the Vinini can be, and has been, divided into two ^[7] or three ^[6][8] subunits. Regardless of how it is subdivided, the lower part of the formation, wholly of Ordovician age, is extremely heterogeneous and coarsely granular, and the upper part, of Ordovician and Silurian ages, is more uniform and fine-grained reflecting very different conditions of deposition compared with the upper part. The Vinini has remarkably similar lithic and temporal correlatives, with other names, in Idaho,^[9]Arkansas,^[10] Oklahoma,^[10] Texas, ^[11][12] and Mexico. ^[13]

Lithic composition

Lower part

Regionally, the lower, heterogeneous, part of the Vinini is composed principally of the following components: heterogeneous beds of sandstone composed of quartz grains, fragments of calcareous and phosphatic organisms, the alga Nuia, dolomite rhombs, sponge spicules, and chert grains; less common are beds of conglomerate, feldspathic siltstone, shale, bedded chert, and greenstone.^[8] The proportions of rock types vary stratigraphically and from place to place indicating a laterally varied and temporally shifting environment. Almost all the sandstone components appear to be debris shed from the eastern assemblage. However, some of it is composed almost entirely of quartz grains, and this presents a problem: correlative shallow-water carbonate deposits in the eastern assemblage are free from sandy deposits, and channels through the carbonate bank are unknown,^[7] making a source of the quartz grains in that direction unlikely. So-called "rip-up clasts" are common in some areas.^[8] These are large, angular fragments of sandy limestone that, apparently, were "ripped up" by strong currents, from existing strata in a submarine environment. The angularity of these clasts and their similarity to underlying, undisturbed beds indicate very little transport.

Upper part

Regionally, the upper part of the Vinini is composed largely of shale rich in graptolites, commonly overlain by a black, bedded chert unit, and topped by the conspicuously white Cherry Spring chert. Dark, very fine–grained limestone beds composed largely of the alga Nuia are locally present interbedded with the shale, as are quartz sandstone beds. The quartz sandstone beds, unlike those in the lower part of the Vinini, are approximately correlative with the Eureka Quartzite in the eastern assemblage to the east of the Vinini and are thought to be genetically related to the Eureka.^[7] The chert units are notable for their very thick beds. Except for bedding thickness, the black chert unit is like other Paleozoic bedded cherts in appearance, and is brittle, breaking into cubes. The Cherry Spring chert is thick bedded and is white or nearly so, and unlike ordinary dark chert, it is tough and breaks concoidally.^[6]

Mineral Deposits

Sulfide mineral deposits are an integral part of the Vinini, probably formed during deposition of the formation, and are not related to known igneous rocks. Iron sulfide (pyrite) and lead sulfide (galena) are present sporadically in the Vinini and, where oxidized, form colorful gossans.^[6,14] Some of these deposits are in the lower part of the Vinini, but most appear to occur within the Cherry Spring chert.^[6,14]

Origin

The Vinini was originally thought to be indigenous to the continental slope in Nevada, the area in which it is found,^[4] but, beginning in the nineteen seventies, with the development of plate tectonics, the prevailing theory held that the Vinini is an oceanic deposit originating in the open ocean or a back-arc oceanic basin. More recently, efforts were made to restore the depositional location of the Vinini to the continental slope in Nevada.^[6][7][17] Unquestionably, the formation was deposited in deep water relative to that of the eastern assemblage as indicated by its lithic composition as cited above, but its composition also precludes an oceanic origin.^[17] The two-fold stratigraphic nature of the Vinini points to a deepening of the sea during deposition. The chert at the top of the upper part was deposited when the sea extended across the continent^[18] and, presumably, was at maximum depth in Nevada.

Structure

The Vinini was strongly deformed by folding, and by both low-angle and high-angle faulting, most of uncertain age. In the 1970s and 80s, the folds and low-angle faults were ascribed to plate convergence by which the western assemblage was thrust or "obducted" from an ocean basin onto the eastern assemblage at the margin of the continent.^[15,16] The underlying thrust fault was the Roberts Mountains thrust, and the western assemblage constituted the Roberts Mountains allochthon. Blocky outcrops of carbonate rocks surrounded by exposures of the siliceous western assemblage were considered to be eastern assemblage rocks exposed in "windows" of the Roberts Mountains allochthon. Recently^[17] the existence of the Roberts Mountains allochthon has been challenged, and the blocky outcrops of the eastern assemblage were proposed as displaced blocks of that assemblage, disloged by the Alamo impact^[19] of Late Devonian age.

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