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Antler Orogeny and Foreland Basin: a Model: Discussion and Reply

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Antler Orogeny and Foreland Basin: a Model: Discussion and Reply Antler orogeny and foreland basin: A model: Discussion and reply Discussion J. G. JOHNSON ) „ , r, . _ „„,,, ANNE PENDERGAST I ®ePartmenl °J Geology, Oregon Slate University, Corvallis, Oregon 97331 We deny that rocks of the Antler allochthon have been southern part of the Pine Valley quadrangle, south of Pony Creek, redistributed by probable Mesozoic thrust nappes along the the sequence is: Nevada Group carbonates, Pilot Shale, "Chainman Roberts Mountains thrust front between the latitudes of Eureka Shale," Roberts Mountains allochthon (Johnson and Pendergast, and Elko, as was represented by Speed and Sleep (1982, Fig. 1). 1981, p. 653). The important paper on the Antler orogeny by Speed and Throughout the rest of the Pinyon Range, mapped by Smith Sleep (1982) proposed a model which should be revised in the light and Ketner (1975, PI. 1; 1978) or discussed by them as regards of new interpretations of central Nevada geology made by Johnson timing of deformation (1977), two facts stand out consistently. (1) and Pendergast (1981) and published while the Speed and Sleep Wherever the base of the allochthon has been mapped, it overlies manuscript was in press. The most significant of these is that Lower Lower Mississippian (Kinderhookian) rocks. (2) Wherever the Mississippian rocks are in both the autochthon and the allochthon stratigraphic sequence is exposed, the lowest Mississippian beds along the thrust front. Specifically, we refer to allochthonous overlie Devonian autochthonous rocks. Ordovician-Devonian rocks in the belt from Devils Gate to the Because Smith and Ketner (1968) mapped Kinderhookian Roberts Mountains, from the west flank of the Sulphur Spring Webb Formation as overlying both autochthon and allochthon, Range, and including rocks as young as Kinderhookian at several they (apparently) assumed that any structurally higher allochthon localities in the Pinyon Range. This information can easily be had been emplaced by post-Antler movements. All of the supposed incorporated in Speed and Sleep's model, but it requires the rejec- Jurassic-Early Cretaceous thrusting that involves the Woodruff, tion of their contention (Fig. 1 and p. 816-817) that rocks in the Lee, and Willow plates (Smith and Ketner, 1977, Fig. 2; Stewart, Roberts Mountains allochthon between the latitudes of Eureka and 1980, Fig. 40) results from the misconception that first overlap Elko are in probable Mesozoic thrust nappes. determines last movement. Because these thrust plates were already Speed and Sleep (1982) cite Winterer's (1968) work in the in place in the Mississippian, the supposed Mesozoic thrust nappes Roberts Mountains, but that refers to the redistribution of carbon- of the Pinyon Range are without basis. ate blocks by landsliding (not only to the east, but also to the west, Our evaluation of the deformational history of the Pinyon into the area classified by Speed and Sleep as unaffected by Meso- Range implies nothing about events to the north. In northeastern zoic nappes). A decade of mapping since 1968 has not sustained Nevada and in central Idaho, there is compelling evidence of Winterer's hypothesis that landsliding led to wholesale reshuffling Mesozoic thrusting (Riva, 1970; Oversby, 1972; Dover, 1980). In of the stratigraphic deck (Murphy and others, 1978); instead, a geographically intermediate areas such as the north end of the definite structural sequence has been worked out (Murphy and oth- Adobe Range (Ketner, 1970) and at Swales Mountain (Evans and ers, 1982). Ketner, 1971), it has long been known that the Roberts Mountains In the northern Mineral Hill quadrangle and in the adjacent allochthon lies on Mississippian rocks, a situation we now know can have resulted from movement on the Roberts Mountains thrust (Johnson and Pendergast, 1981). We conclude that each area is in The article discussed appeared in the Bulletin, v. 93, p. 815-828. need of its own evaluation. Geological Society of America Bulletin, v. 94, p. 684-686, May 1983. 684 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/685/3444820/i0016-7606-94-5-685.pdf by guest on 27 September 2021 DISCUSSION AND REPLY 685 Reply R. C. SPEED Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201 N. H. SLEEP Department of Geophysics, Stanford University, Stanford, California 94305 Conflicting interpretations of the structure and age of Roberts rocks are there exposed in an anticlinal crest, that Smith emplacement of the Roberts Mountains allochthon in Nevada have and Ketner's (1978) southwestern contact dips shallowly in the recently appeared in the Bulletin. Johnson and Pendergast (1981) opposite direction from that shown and is not a thrust, and that the claim that the allochthon reached its final position in early Osagean discordance at the northeastern contact is somehow compatible (late Early Mississippian) time and that between Eureka and Elko, with a depositional contact. Here and elsewhere in the Piñón the toe of the allochthon retains its Mississippian tectonostrati- Range, Johnson and Pendergast have converted Smith and graphic position. We argued (Speed and Sleep, 1982) for an Early Ketner's faults to depositional contacts and even generated a fault Mississippian emplacement and that between Eureka and Elko, the without supporting evidence or documentation of field relation- toe of the allochthon and its substrata probably underwent Meso- ships. On the basis of present understanding, a post-Paleozoic zoic thrust imbrication. The late J. Fred Smith, Jr. (Ketner and emplacement of Roberts rocks above late Paleozoic strata at Smith, 1982) may have held a view similar to ours. In contrast, Woodruff Creek (Smith and Ketner, 1977) seems the best K. B. Ketner (Ketner and Smith, 1982) proposed that the Roberts explanation. Mountains allochthon may have been fully translated within the 2. Johnson and Pendergast's claim of an early Osagean Mesozoic. emplacement of the Roberts Mountains allochthon is unsubstan- The particular area of concern is the Piñón Range, which con- tiated. Dating of the foreland basin strata is meager, and the dating tains the easternmost outcrops of Ordovician and Devonian rocks of beds by correlations is hazardous in flysch and, moreover, affiliated with the Roberts Mountains allochthon (hereafter, requires clear understanding of post-depositional structures. Fur- Roberts rocks). Published field work in the range is entirely that of thermore, the assumption of an isochronous boundary between Smith and Ketner (1977, 1978, and earlier publications). Below, we arbitrary lithic divisions in flysch (Chainman and Diamond Peak address Johnson and Pendergast's interpretations and then, Formations) is dubious. Nonetheless, accepting the Johnson and Ketner's. Pendergast (1981) hypothesis that certain Mississippian beds (Dia- Johnson and Pendergast (1981) propose that in the Piñón mond Peak Formation) are depositional on Roberts rocks and Range and south to Devils Gate, the Roberts Mountains allochthon using dating control in their Figure 3, we read the following con- together with local Kinderhookian cover was emplaced in early straints on ages of emplacement of Roberts rocks: Swales Moun- Osagean time above foreland basin sediments (Chainman Forma- tain,: Lee Canyon, and Devils Gate: Kinderhookian or younger; tion) and was then covered by younger Mississippian foreland basin Emigrant Spring: Kinderhookian to Meramecian; Willow Creek: sediments. They state that such tectonostratigraphic relations exist Late Devonian or Mississippian. They have no constraint against a in the Piñón Range today, undisturbed by Mesozoic tectonism. Kinderhookian emplacement or against widely different times of Their interpretations are appealing by their simplicity; they are, emplacement for the various slices of Roberts rocks. The conclud- however, derived from arbitrary selection of data and are hard to ing dictum of Johnson and Pendergast's commentary is that each believe, as indicated by the following three points: area is in need of its own evaluation; this should apply to strati- 1. Smith and Ketner (1978) show relationships of Roberts graphic as well as structural matters. rocks in the Piñón Range that are difficult to explain by thrusting 3. The Piñón Range lies in a north-south belt of deformation only in Osagean time. For example, at Woodruff Creek, a unit of that is demonstrably Mesozoic at places and post-Paleozoic at oth- Roberts rocks (including the Webb Fm.) is bounded to the south- ers. The belt is identified by folds and, locally, by thrusts that affect west and the northeast by Mississippian strata that young away in all Paleozoic rocks (as young as Late Permian) and by remnants of both directions and are succeeded within 1 to 3 km by beds of Late syntectonic sediments mapped as Newark Canyon Formation that Pennsylvanian and Permian age. The Roberts rocks are shown by are known to be Cretaceous at several sites. Just north of the Piñón Smith and Ketner (1978) as thrust over the Mississippian to the Range in the Adobe Range, Ketner and Smith (1974) dated major southwest and to be overthrust by the Mississippian on the north- deformation as Jurassic or Cretaceous. To the south, Cretaceous east. The existence of a fault at the northeastern boundary is and possibly older thrusting and folding occurred near Eureka especially convincing because of the large discordance between the (Nolan and others, 1974). Post-Paleozoic deformation is demon- contact and bedding in the immediately adjacent Mississippian strable or inferrable everywhere in between, including the Piñón beds. These map relationships do not admit the Johnson and Pen- Range (Larson and Riva, 1963; Winterer, 1969; Smith and Ketner, dergast interpretation. They would presumably argue that the 1977, 1978; Murphy and others, 1978, who show a recumbent fold Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/685/3444820/i0016-7606-94-5-685.pdf by guest on 27 September 2021 686 SPEED AND SLEEP of the Roberts Mountains allochthon and autochthon; and Hose Hose, R. K., Armstrong, A. K., Harris, A. G., and Mamet, B. L., 1982, and others, 1982).
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