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Early Paleozoic Continental-Rise Deposition Off East Antarctica: the Patuxent Formation of the Pensacola Mountains A.J

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Early Paleozoic Continental-Rise Deposition Off East Antarctica: the Patuxent Formation of the Pensacola Mountains A.J Early Paleozoic continental-rise deposition off East Antarctica: The Patuxent Formation of the Pensacola Mountains A.J. ROWELL, W.R. VAN SCHMUs, L.W. MCKENNA, III, and K.R. EvANs, Department of Geology, University of Kansas, Lawrence, Kansas 66045-2124 rustal extension and deposition of deep-water turbidites 1990, but see Storey et al. 1992). A Sm-Nd mantle separation Calong the margin of East Antarctica were not confined to age of 700-800 million years from a pillow basalt in the Cocks a period of Neoproterozoic rifting. Our new uranium/lead Formation is also consistent with a Neoproterozoic rifting (U/Pb) isotopic dates indicate that at least part of the turbite event in the Skelton Glacier area (Rowell et al. 1993). succession of the Patuxent Formation in the Pensacola Nevertheless, our new data demonstrate that not all of Mountains (figure 1) is Early Paleozoic. These results are con- this belt of turbidites is Neoproterozoic. In the Pensacola sistent with our discovery of marine fossils in limestone ohs- Mountains, the basis for such a claim, interpretation of rubid- tolith blocks in the formation. Much of the Patuxent Forma- ium-stronium (Rb-Sr) isotopic analysis of the Gorecki Felsite tion may represent formerly unknown deep-water deposits Member of the Patuxent Formation (e.g., Eastin 1970, pp. that accumulated oceanward of, and contemporaneously 67-114; cited by Schmidt, Williams, and Nelson 1978 and with, the Early Paleozoic shelf succession exposed to the east Storey et al. 1992) is invalid. At its type locality in the Schmidt in the Transantarctic Mountains. Hills (figure 1), the Gorecki Felsite Member is underlain and The existence of a large supercontinent and its breakup in overlain by fine-grained turbiditic sandstones and argillites. It the Neoproterozoic is a central feature of present-day paleo- geographic models of late Precambrian time (Moores 1991; I / I Dalziel 1991, 1992). Breakup is thought to have led to the isola- / I / J tion of Laurentia and the formation of the proto-Pacific as western North America drifted away from a conjoined Aus- tralian-East Antarctic plate. Comparative studies of Precam- brian geology across the putative rifted margins generally sup- port such a geological history (Ross 1992; Stump 1992; Young Gorecki 1992). As Borg and DePaolo (1994) have observed, however, r / section the match of Precambrian isotopic age provinces is greatly I/ Lu improved if the position of inferred allochthonous segments of ;J4 the antarctic margin is restored by right-lateral motion to \\ remove effects of early Ross transpressive deformation (Row- Lu ell, Rees, and Evans 1992; Goodge, Walker, and Hansen 1993). O;; / II ALI The time of the breakup, however, remains a difficulty. III, Imprecise isotopic dates suggest initial separation of Antarc- I1Teeny tica-Australia from western North America between 700 and Rock .- U, 800 million years ago (Devlin and Bond 1988; Borg, DePaolo, II * Lu and Smith 1990). The evidence in Antarctica, however, is geo- graphically limited. Deformed turbidites, labeled with a variety Qr of formational names, crop out along much of the length of the Put-in A, Transantarctic Mountains (figure 1) from southern Victoria i; Land to near the Weddell These formations are commonly \\\\ dw assumed to be Neoproterozoic and to have developed in a con- tinental rise setting subsequent to rifting. The age of the tur- bidites is not well controlled but, at Cotton Plateau, near the E X 4L Mt•S Nimrod Glacier, a pillow basalt interbedded with turbidites of the Goldie Formation yielded a samarium-neodymium (Sm- WS Nd) mineral isochron age of 762±24 million years (Borg et al. PC Pens:aMts tWhile this manuscript was in press, we learned that our colleagues in the British Antarctic Survey have independently arrived at somewhat similar conclusions and have obtained almost identical uranium-lead zircon ages 10 for the Gambacorta Formation and the Gorecki Felsite Member. Their results will be published as follows: Figure 1. Map of Neptune Range in the Pensacola Mountains showing Millar, IL., and B.C. Storey. 1995. Early Palaeozoic rather than Neo- outcrops, principal localities mentioned in the text, and camp sites proterozoic volcanism and rifting within the Transantarctic Moun- indicated by stars. (WS denotes Weddell Sea. TAM denotes tains. Journal of the Geological Society, London, 152. Transantarctic Mountains.) ANTARCTIC JOURNAL - REVIEW 1994 42 consists predominantly of oligomictic megabreccias that cian age. On the eastern side of the Roderick Valley (figure 1), include large blocks of felsitic tuffs, tens of meters in maxi- probably separated from outcrops in the Williams and mum size, together with locally abundant limestone blocks Schmidt Hills by a fault that extends down the Roderick Valley that typically are less than 1 meter in diameter. The (Schmidt et al. 1978), strongly deformed argillites referred to megabreccias are poorly sorted and matrix supported. We the Patuxent Formation crop out beneath the late Middle interpret them as a volcanic apron deposit formed by subma- Cambrian Nelson Limestone. We have elsewhere suggested rine gravity-driven mass movement that probably was associ- that this eastern outcrop belt of the Patuxent may, in part, be ated with a caldera event. A diversity of carbonate rock types of early Cambrian age and that it may have been deformed by is present representing marine depositional settings from an additional phase of early Ross deformation (Rowell et al. peritidal to below storm-wave base. A few of the limestone 1992); in any case, it is clearly pre-late Middle Cambrian. We blocks are fossiliferous, and their sparse fauna includes recognize, however, that the oldest beds in this formation hyolithids and sponge spicules (figure 2). The Gorecki Felsite may possibly be Neoproterozoic and have formed along the Member is clearly Phanerozoic, not Neoproterozoic! continental rise in response to rifting, although currently we Felsic igneous activity is widespread in this part of the have no data to document this possibility. Pensacola Mountains (Schmidt et al. 1978). Quartz por- In summary, available information clearly demonstrates phyritic rhyodacite forms a thick intracaldera phase (Hawkes that a substantial part of the Patuxent Formation is Lower Member, Gambacorta Formation) of the Iroquois caldera, Paleozoic and does not represent Neoproterozoic extension. which crops out on the eastern ridges of Mount Hawkes The significance of the probable extension, recognized by (Schmidt et al. 1978) (figure 1). In addition, thin sills of felsitic Storey et al. (1992), has yet to be fully assessed by geochemi- quartz porphyry are intruded into Patuxent metasediments at cal examination and further geochronological study of collec- Teeny Rock in the Williams Hills. Both of these igneous rocks tions that we have made. have field petrographic characters that are closely similar to Our field party consisted of all the authors. We air- each other and to the felsite blocks in the Gorecki Felsite dropped four 209-liter barrels of mogas fuel and twelve 209- Member. Initial U/Pb isotopic ages for zircons from these liter barrels of JP-8 aircraft fuel on our reconnaissance flight units (Gorecki Felsite Member, Teeny Rock felsite, Hawkes of 17 November 1993 and were subsequently put in the field Member) indicate Cambro/Ordovician crystallization ages of by LC-130 aircraft on 18 November. Our put-in site in the 490 to 500 million years, although further analysis may allow Roderick Valley (83040S 58007W; see figure 1) served as our more precise definition of their ages. main depot from which we established camp sites with snow- Because of deformation of the Patuxent Formation and mobiles hauling sledges (figure 1). We were pulled out from lack of distinctive marker beds, it is virtually impossible to our main depot on 23 December. obtain a reliable estimate of its thickness. Schmidt et al. We thank the LC-130 crews of the U.S. Navy VXE-6 (1978) estimated that it was greater than 10 kilometers. Fold- squadron for assistance in putting us in the field and for col- ing and faulting likewise preclude placing relative ages on the lecting us at the end of the season in marginal flying condi- Gorecki Felsite Member and the thick pillow basalts that crop tions. We are indebted to our colleagues with Antarctic Sup- out in the Williams Hills. It is apparent, however, that not all port Associates and the National Science Foundation for their the Patuxent Formation is of Late Cambrian or Early Ordovi- ready help and logistical support, and to the Twin-Otter crew while supporting our work for several days in the Rambo and Postel nunatak areas. We particularly want to thank Alan Fetter for help in obtaining U/Pb isotopic ages. This work is supported by National Sci- ence Foundation grant OPP 91-17444 to the University of Kansas. References Borg, S.G., and D.J. DePaolo. 1994. Laurentia, Australia, and Antarctica as a Late Proterozoic supercontinent: Con- straints from isotopic mapping. Geology, 22(4), 307-3 10. Borg, S.G., D.J. DePaolo, and B.M. Smith. 1990. Isotopic structure and tectonics of the central Transantarctic Mountains. Journal of Geophysical Research, 95(B5), 6647-6667. Dalziel, I.W.D. 1991. Pacific margin of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent. Geology, 19(6),598-601. Dalziel, I.W.D. 1992. Antarctica; a tale of two superconti- Figure 2. Sponge spicules from limestone clast in Gorecki Felsite Member at its nents. Annual Review Earth and Planetary Sciences, 20, type locality. Field of view approximately 45 millimeters. 501-526. ANTARCTIC JOURNAL - REVIEW 1994 43 Devlin, WJ., and G.C. Bond. 1988. The initiation of the early Paleozoic Evidence from the Skelton Glacier area, Transantarctic Moun- Cordilleran Miogeocline: Evidence from uppermost Proterozoic- tains.
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