Roberts Mountains Thrust Relationships in a Critical Area, Northern Sulphur Spring Range, Nevada

Roberts Mountains thrust relationships in a critical area, northern Sulphur Spring Range, Nevada J. G. JOHNSON | Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506 ROBERT VISCONTI* ABSTRACT (oceanic) Ordovician rocks on top of eastern two units were recognized. The lower Vinini assemblage (shelf) Devonian carbonate rocks in consists of quartzite, arenaceous limestone, and At the orogenic front of the Antler orog- the Roberts Creek Mountain quadrangle (Fig. "fine laminated sandy and brownish-gray and eny, the Roberts Mountains allochthon lies 1). Roberts and others (1958) developed the greenish-brown silty sediments" (Merriam and on Lower Mississippian Dale Canyon Forma- concept that the Roberts Mountains thrust was Anderson, 1942, p. 1694). The upper Vinini tion and consists of Middle and Upper Devo- the principal structure of the Antler orogeny and consists of mainly black, bedded chert with thin nian Woodruff Formation, slivers of Silurian accepted the earlier postulate of Kay (1952) that argillaceous partings and black organic shale. to Lower Devonian Roberts Mountains For- thrusting had occurred as a middle Paleozoic Merriam and Anderson (1942) dated the Vinini mation, and Ordovician Vinini Formation, in event. Smith and Ketner (1968), mapping in the as Early to Middle Ordovician based on grapto- structural succession. Devonian Pilot Shale, Pinyon Range (Pine Valley and adjacent quad- lites. Murphy and others (1984) showed that the mapped unconformably below Dale Canyon rangles to the north), interpreted the Lower Mis- upper Vinini is Lower Ordovician, and that the flysch, was mapped previously as allochtho- sissippian Webb Formation to postdate thrust- lower Vinini is Middle to Upper Ordovician, nous Woodruff Formation. The Vinini recog- ing, but the Webb was not shown by mapping and that the two units are separated by a thrust nized here was also mapped previously as to be continuous across the thrust. Smith and fault. Woodruff. The Dale Canyon was mapped Ketner's (1977, 1978) structural analysis and In the northern Sulphur Spring Range, the previously as Chainman Shale. published maps postulated a complex history of Vinini Formation consists mainly of black, Recognition of Pilot Shale here necessi- events for rocks related to the Roberts Moun- bedded chert which weathers brown and gray. tates the elimination of a previously postu- tains allochthon (RMA). Later, a significantly Most beds are 5 to 15 cm thick, separated by lated thrust plate in which Woodruff was different interpretation of the structure and its thin argillaceous partings. Fine laminations are shown thrust upon Devonian carbonate age was published by Johnson and Pendergast visible on weathered surfaces. In thin section, rocks; it also makes more consistent the struc- (1981). the chert is very dark due to carbonaceous mate- tural sequence within the Roberts Mountains The present report summarizes results and rial; radiolarian molds are common. Black or- allochthon along its frontal margin. conclusions based on remapping an area that ganic shale composes a minor part of the Vinini Assignment of fine-grained siliciclastic tur- crosses the boundary between the Pine Valley and is poorly exposed. bidites in the flysch trough of the Antler fore- (Smith and Ketner, 1968, 1978) and Mineral Bedded chert was mapped as part of the land to the Dale Canyon Formation elimi- Hill (Carlisle and Nelson, 1990) 15-minute Devonian Woodruff Formation by Smith and nates anomalous use of Chainman Shale as a quadrangles (Fig. 1). This area proves critical to Ketner (1978), but it resembles the Vinini stratigraphic term and allows its useful reten- the solution of problems left unsolved by pre- Formation found in the Willow Creek area, tion for delta-slope deposits distal to the Di- vious mapping, and our results differ in some three miles north, where Smith and Ketner amond Peak molasse. fundamental aspects. (1975) collected Ordovician graptolites. South The so-called Dry Creek fault, previously of Pony Creek, bedded chert of the Vinini struc- interpreted to cross the mapped area, cannot ALLOCHTHONOUS ROCKS: turally overlies Woodruff dated by conodonts as be recognized. Some outcrops of Devonian VININI FORMATION Late Devonian (Smith and Ketner, 1975). carbonate rocks, previously mapped as in In the northern Sulphur Spring Range, the structural windows below Woodruff Forma- The Ordovician Vinini Formation belongs to Vinini Formation, along with the Woodruff tion, are actually Quaternary landslide the western assemblage of Roberts and others Formation and fault slices of the Roberts Moun- blocks. (1958), deposited in an oceanic (eugeosynclinal) tains Formation, forms the Roberts Mountains lithotope. Deposition in a deep-water, oceanic allochthon (RMA). The Vinini lies structurally INTRODUCTION environment is based on the association of above the Devonian Woodruff within the RMA bedded chert, shale, graptolite faunas, and trace (Figs. 2, 3). The Roberts Mountains thrust (Merrian and fossils. Chamberlain (in Stanley and others, Anderson, 1942) was first characterized as a re- 1977) reported a Nereites trace-fossil assem- ALLOCHTHONOUS ROCKS: gional structure that placed western assemblage blage in the Vinini of the Roberts Mountains. ROBERTS MOUNTAINS FORMATION The Vinini Formation was named by Mer- •Present address: 4904 S. Broadway, no. 227, St. riam and Anderson (1942) for exposures along The Roberts Mountains Formation (Mer- Louis, Missouri 63111. Vinini Creek in the eastern Roberts Mountains; riam, 1940) was assigned by Smith and Ketner Geological Society of America Bulletin, v. 104, p. 1208-1220, 7 figs., September 1992. 1208 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/9/1208/3381627/i0016-7606-104-9-1208.pdf by guest on 28 September 2021 ROBERTS MOUNTAINS THRUST RELATIONSHIPS, NEVADA 1209 (1975) to the transitional assemblage of Roberts 117° 116° 115° and others (1958), but we regard it as compris- ing rocks deposited in an outer shelf basin (Johnson and others, 1991). In the northern Sulphur Spring Range, along Pony Creek, the Roberts Mountains Formation is present in a thrust slice between the Vinini and Woodruff Formations, as it is in the Willow Creek area to the north (Smith and Ketner, 1978; Whitaker, 1985). Smith and Ketner (1975, 1978) assigned an age of earliest Devonian based on graptolites of the Monograptus uniformis Zone. Along Pony Creek, the Roberts Mountains Formation consists of dark gray shaley and platy carbonaceous, laminated, silty dolomite that weathers to a yellowish brown. Thin sections reveal that the rock consists of quartz silt (10% to 20%) set in a matrix of finely crystalline dolomite with abundant carbonaceous and argillaceous materials. The unit is coarser grained than the Woodruff and is distinguished from rocks of similar lithology in the Woodruff For- mation by the presence of graptolites, identified by W.B.N. Berry (1982, written commun.) as Monograptus birchensis Berry and Murphy and Linograptusl sp. The age range of M. birchensis is latest Silurian to early Early Devonian (Berry and Murphy, 1975, see their Fig. 15). AUTOCHTHONOUS ROCKS: WOODRUFF FORMATION Lithology The Devonian Woodruff Formation was named by Smith and Ketner (1968) for exposures along Woodruff Creek in the Pinyon Range. In the northern Sulphur Spring Range, the Woodruff consists of silty dolomite, dolomitic siltstone, chert, and siliceous mudstone. Although Smith and Ketner (1968) assigned the Figure 1. Index map of east-central Nevada and its counties. The frontal margin of the Woodruff to the siliceous western assemblage, Roberts Mountains thrust is shown as a dashed barbed line. Its position, but not its continuity we assign it to the transitional assemblage be- to the south, is questioned. The two lS-minute topographic quadrangles that encompass the cause of the presence of carbonate rocks. mapped area are Pine Valley (PV) and Mineral Hill (MH); nearby locations referred to are Laminated quartz-silty dolomite (Fig. 4A) is Cortez quadrangle (CZ) and Roberts Creek Mountain quadrangle (RCM). The mapped area is the dominant lithology of the Woodruff in the shown in Figure 2. northern Sulphur Spring Range. These rocks are dark gray to black and weather grayish orange. Thin sections show quartz silt (5% to 15%) set in a finely crystalline dolomite matrix, with abun- quartz silt composes as much as 50% of the rock, makes up a small but significant part of the dant carbonaceous material and minor pyrite and it contains less carbonaceous material. Mot- Woodruff and has yielded good conodont fau- and argillaceous materials. Radiolarian molds tling and small burrows are evidence of nas (see Appendix). The rock is dark gray with are rare. Laminae are defined by concentrations bioturbation. light gray laminations. Thin sections reveal of carbonaceous material and coarser dolomite Black chert that weathers to brown and gray abundant calcispheres in a matrix of carbona- rhombs. The dolomite, replacement in origin, is is interbedded with silty dolomite and siliceous ceous and argillaceous lime mud. Laminae are cloudy and dirty, suggesting original deposition mudstone. Thin sections of the chert show that it defined by dolomite rhombs and quartz silt. as an argillaceous lime mud, similar to some of differs from chert of the Vinini Formation in Dark gray to black, carbonaceous, siliceous the Pilot Shale. that it is generally coarser and contains less car- mudstone makes up a minor part of the Wood- Dolomitic

Load more