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Stratigraphic and Structural Investigations in the Northern Heritage Range, Ellsworth Mountains: Evidence for the Ross Orogeny? MARGARET N

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Stratigraphic and Structural Investigations in the Northern Heritage Range, Ellsworth Mountains: Evidence for the Ross Orogeny? MARGARET N Chirino-Gálvez, L.A. 1993. Cenozoic crabs from Chile. (Unpublished Formation (Paleocene) of Brazil. Journal of Paleontology. Masters thesis, Kent State University, Kent, Ohio.) Feldmann, R.M., D. Tshudy, and M.R.A. Thomson. 1993. Late Creta- Feldmann, R.M., and I.W. Keyes. 1992. Systematic and strati- ceous and Paleocene Decapod Crustaceans from James Ross graphic review with catalogue and locality index of the Meso- Basin, Antarctic Peninsula. Paleontological Society Memoir 28, zoic and Cenozoic decapod Crustacea of New Zealand. New 1-41. [Supplement to Journal of Paleontology, 67(1).] Zealand Geological Survey Record. Lower Hut, New Zealand: Weaver, C. 1927. The Roca Formation in Argentina. American Journal Department of Scientific and Industrial Research, Geology and of Science, 15(5), 417-434. Geophysics. Zinsmeister, W.J., and R.M. Feldmann. 1984. Cenozoic high latitude Feldmann, R.M, and R. Martins Neto. In press. Costacopluma nordes- heterochroneity of Southern Hemisphere marine faunas. Science, tina n. sp. (Decapodal Retroplumidae) from the Maria Farinha 224(4646), 281-283. Stratigraphic and structural investigations in the northern Heritage Range, Ellsworth Mountains: Evidence for the Ross Orogeny? MARGARET N. REEs, Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154 ERNEST M. DUEBENDORFER and DONALD J. THORSTENSON, Department of Geology, Northern Arizona University, Flagstaff Arizona 86011 ur 1993-1994 field season in the northern Heritage Range e O was the initiation of a 4-year project to collect strati- oessel graphic, structural, geochronological, and geochemical data to evaluate currently conflicting models for the origin (Cam- I, brian) of the Ellsworth-Whitmore Mountains terrane and the timing and kinematics of its accretion to East Antarctica. The (\J ç Ellsworth Mountains, which are part of a California-sized ter- CD rane, are located between the Transantarctic Mountains of Li East Antarctica and the collage of amalgamated terranes of West Antarctica (figure 1). The approximately 13,000 meters (m) of Cambrian-through-Permian strata exposed in the Her- C)f\ •: itage Range appears to have paleogeographic and paleobio- -7930,. geographic affinities to the paleo-Pacific margin of Gond- wanaland (Webers and Sporli 1983, pp. 261-264; Craddock et al. 1986; Webers, Craddock, and Splettstoesser 1992, pp. 1-9), but the depositional and tectonic history of the terrane is equivocal. 1V Our field party consisted of Margaret N. Rees (stratigra- pher), Ernest M. Duebendorfer (structural geologist), Donald J. Thorstenson (graduate student), and Lucylle J. Smith CA (mountaineer). On 22 November 1993, an LC-130 aircraft with a VXE-6 crew put us into the field on the Balish Glacier (7902406"S 84 03222"W) near the fuel cache left by a 10 km 1992-1993 U.S. party and near four 55-gallon drums of motor- gas that were air dropped during a reconnaissance flight on 12 November 1993. Over the next 30 days, we used four snow- mobiles and four Nansen sledges to transport gear and estab- lish five temporary camps (figure 1). We met briefly with Mike Curtis (structural geologist) and Brian Hull (mountaineer) of 0 the British Antarctic Survey. We were pulled out of the field ., on 24 December, leaving only the nearly buried drums from the 1992-1993 expedition. The northern Heritage Range is dominated by lower Figure 1. Index map of field area in the northern Heritage Range, greenschist facies metasedimentary and metavolcanic rocks Ellsworth Mountains, Antarctica. Triangles are camp sites. Small filled circles are peaks. Dashed lines are snowmobile routes. Solid square of the Cambrian Heritage Group and the Late Cambrian to is put-in site. ANTARCTIC JOURNAL - REVIEW 1994 2 Devonian Crashsite Group (figure 2; Craddock et al. 1986; AL PolarstarFm. Webers et al. 1992, pp. 9-21; Sporli 1992, pp. 21-36) that record three phases of folding, the earliest of which had not Whiteout Conglomerate been reported previously. Evidence for this deformational event includes - g Mount Wyatt Earp Fm. • a west-northwest-striking, moderately north-dipping cleavage and mesoscopic folds in the Middle Cambrian Mount Uptak Fm. Springer Peak Formation that are cut by the dominant • north-northwest-striking cleavage; o £ • the presence of rotated xenoliths of phyllite interpreted as - - Howard Nunataks Fm. • U fragments of the Springer Peak Formation within a Devon- IL ian stock (369±18 million years ago: rubidium/ strontium Minaret Fm. whole-rock isochron date, Vennum et al. 1992, pp. 295-324) that carries a later north-northwest-striking Frazier Ridge-Springer Peak-Liberty Hill Formation cleavage; and C 41 • the presence of strongly cleaved phyllite clasts within basal N - 0. Conglomerate Ridge Fm. conglomerate beds of the Crashsite Group. .D Goldstrand et al. (1994) reported that the Crashsite Group E • Drake Icefall Fm. disconformably overlies the middle Late Cambrian Minaret a 2 Formation and attributed the unconformity to the Ross t Hyde Glacier Fm. Orogeny. The present data, however, can constrain the tim- I p. ing of the early phase of deformation only to postdeposition I. of the late Middle Cambrian Springer Peak Formation and I Union Glacier Fm. predeposition of the Crashsite Group. The basal Crashsite is very poorly dated: the beds are certainly older than Devonian and may be as old as late Late Cambrian (Shergold and Figure 2. Generalized stratigraphic section for the Ellsworth Moun- Webers 1992, pp. 125-168). Given present data, it is permissi- tains (after Webers, Craddock, and Splettstoesser 1992, PP. 1-9; ble to attribute the deformation reported here to the Ross Webers et at. 1992, pp. 9-21). Orogeny. The second deformational event, which produced the neither original sedimentary textures nor fabrics. Therefore, dominant structural trend in the northern Heritage Range, is the stratigraphic position of this unit is uncertain. The "mid- defined by north-northwest-trending folds and an associated dle unit" is also strongly sheared and highly folded. It is com- subvertical axial planar cleavage. This deformation has been posed predominantly of thin-bedded black limestone and attributed to the Triassic Ellsworth Orogeny (Craddock 1983, calcareous shale, and our preliminary faunal data suggest a pp. 449-455). We have recognized two previously unreported middle Cambrian age (Robison personal communication). east-vergent thrusts that are associated with this period of The previously reported Middle Cambrian fauna from this deformation: the Hurst Peak thrust and the Conglomerate unit (Jago and Webers 1992) came from one of the "pod-like, Ridge shear zone. The latter fault dips moderately northwest light-gray to black limestone lenses" (Webers et al. 1992, p. and is marked by a 10- to 30-meter-wide cataclasic zone that 15). We interpret the contact of these pod-like units with the records west-side-up, dextral movement. Large domains of thin-bedded limestone and shale succession as faults. There- east-dipping axial planar cleavage suggest opposite vergence fore, their stratigraphic position is uncertain, although at least for structures east of the Balish Glacier. Second-phase folds one of them is Middle Cambrian in age (Jago and Webers plunge variably to the northwest or southeast and may reflect 1992, pp. 101-124). The uppermost exposures of the "Drake post-D2 regional warping about approximately east-trending Icefall Formation" is an argillite with sandstone succession fold axes as suggested by Sporli and Craddock (1992, pp. that is indistinguishable from parts of the Spring Peak Forma- 375-392). tion. This upper unit is faulted against the "middle unit of the Our structural data and the recognition of an early Paleo- Drake Icefall Formation" on one side, and along its other zoic phase of folding indicate that previously described margin, it is juxtaposed against the Conglomerate Ridge For- stratigraphy in the Heritage Range (figure 2) needs to be mation by the Conglomerate Ridge tectonite zone. The age refined. For example, the Drake Icefall area had been inter- and stratigraphic position of this unit is unconstrained. Previ- preted as a simple west-dipping homocline with a con- ously recorded west-dipping bedding attitudes within the formable stratigraphic succession (Craddock et al. 1986; Conglomerate Ridge Formation are attitudes of metamorphic Webers et al. 1992, pp. 9-21). Where we observed the Drake cleavage; bedding attitudes dip gently east. Thus, in the Drake Icefall Formation, its base was highly sheared and the contact Icefall area, the stratigraphic relationships are still uncertain. with the Union Glacier Formation appeared to be faulted. The Our structural and stratigraphic fieldwork revealed sev- contact of the "lower" and "middle units" also appeared to be eral features that are evidence for an early Paleozoic deforma- faulted. The "lower unit" has yielded no fossils and carries tional event, possibly resulting from the Ross Orogeny. Ongo- ANTARCTIC JOURNAL - REVIEW 1994 3 ing paleontologic, microstructural kinematic analysis, and Spettstoesser (Eds.), Geology and paleontology of the Ellsworth argon-40/argon-39 and uranium! lead geochronologic inves- Mountains, West Antarctica (Memoir 170). Boulder, Colorado: tigations will constrain more closely the timing and kinemat- Geological Society of America. ics of this deformational event. If deformation resulted from Sporli, K.B. 1992. Stratigraphy of the Crashsite Group, Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Craddock, and J.F. the Ross Orogeny as
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