(oral p.)
John W. Goodge1, Paul Myrow2, Ian S. Williams3, David Phillips4 & C. Mark Fanning5
'Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812 USA;
<jgoodge@d.umn.edU>;
2Department of Geology, Colorado College, Colorado Springs, Colorado 80903 USA;
<pmyrow@ColoradoCollege.edu>;
3Research School of Earth Sciences, Australian National Univervisty, Canberra, Act 0200 Australia;
<lan.Williams@anu.edu.aU>;
4School of Earth Sciences, University of Melbourne, Melbourne, Victoria 30 I 0 Australia;
<dphillip@unimelb.edu.au>;
5Research School of Earth Sciences, Australian National Univ.ersity, Canberra, Act 0200 Australia;
<Mark.Fanning@anu.edu.au>.
Siliciclastic rocks in the Transantarctic Mountains record the tectonic transformation from a Neopro-terozoic rift-margin setting to an active early Paleozoic orogenic setting along the paleo-Pacific margin of East Antarctica. Detrital mineral ages from sandstones constrain their depositional age and sedimentary provenance, as well as denudation rates during the Ross Orogeny. In the central Trans-antarctic Mountains, quartz arenites of the late Neoproterozoic Beardmore Group contain Archean and Proterozoic zircons, reflecting distal input from the adjacent cratonic shield, Mesoproterozoic
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-igneous provinces, and Grenville-age parts of East Gondwana. Similarly, Lower Cambrian sandstones of the autochthonous Byrd Group (Shackleton Limestone) record a dominantly cratonic shield source.
Detrital zircons from the Koettlitz Group in southern Victoria Land show a similar age signature and constrain its depositional age to be "5;670 Ma. Significant populations (up to 22 %) of -1.4 Ga zircons in the Neoproterozoic and Lower Cambrian sandstones suggest a unique source of Mesoproterozoic igneous material in the East Antarctic craton; comparison with the -1.4 Ga trans-Laurentian igneous province suggests paleogeographic linkage between East Antarctica and Laurentia prior to -1.0 Ga. In strong contrast, detrital zircons from upper Byrd Group sandstones ("5:520 Ma; Early Cambrian or younger) are dominated by young components derived from proximal igneous and metamorphic rocks of the Ross Orogen. Sandstones from the Pensacola Mountains are dominated by Grenville and Pan-African zircon ages, suggesting a source in the western Dronning Maud Land equivalent of the East African Orogen.
Integration of stratigraphic relationships and detrital age patterns leads to a tectonic model involving Neoproterozoic rifting and development of a passive-margin platform, followed by rapid transition in the late Early Cambrian (Botomian) to an active continental-margin arc and forearc setting. Large volumes of molassic sediment were shed to forearc marginal basins in Middle Cambrian and Ordovi-cian time, primarily by erosion of volcanic rocks in the early Ross magmatic arc. U-Pb and 40 Ar/39 Ar mineral ages from detrital zircons and muscovites in the molasse deposits allow us to estimate cooling and denudation rates in the source terrain. Based on stratigraphic evidence for a common provenance, the detrital zircon and muscovite ages indicate cooling rates of 10-30°C/m.y. in the source area.
Evidence of rapid cooling and unroofing suggest that crustal thickening associated with both magmatic intrusion and structural shortening was balanced by erosional exhumation. The calculated cooling rates, along with a geotherm determined from crystalline basement of the orogen, indicate denudation rates of 0.3-1.4 mm/a, which are comparable to those in recent convergent or collision orogens. Profound syntectonic denudation, followed by Devonian peneplanation, removed the entire volcanic carapace and exposed the plutonic roots of the arc. Rapid erosion and unroofing in the axial Ross Orogen is consistent with a sharp carbonate-to-elastic stratigraphic transition observed in the upper Byrd Group, reflecting a sudden outpouring of alluvial-fan and fluvial-marine elastic detritus, and with denudation rates inferred from mineral cooling ages in the adjacent metamorphic basement.
The forearc deposits were themselves intruded by late-orogenic plutons as the locus of magmatism shifted offshore during oceanward trench retreat. Deposition of individual molasse units continued until -490-485 Ma (earliest Ordovician), based on ages from cross-cutting igneous bodies and neoblastic metamorphic phases. The entire episode of interrelated tectonic, denudational, sedimentary, deformational, and magmatic events is restricted to a period of 7-25 m.y. in the late Early Cambrian to earliest Ordovician, and it reflects a short time lag between tectonism and sedimentary response documented in other continental-margin arc systems.
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-Depositional environment of the Byrd Group, Byrd Glacier area:
A Cambrian record of sedimentation, tectonism, and magmatism on the Paleo-Pacific continental margin of Gondwana
(oral p.) Brian Gootee & Edmund Stump
Department of Geological Sciences, Arizona State University, Tempe, AZ, 85287-1404, USA;
<brian.gootee@asu.edu>, <ed.stump@asu.edu>.
The geology of the Byrd Group immediately south of Byrd Glacier records a major sequence of geologic events, beginning with the development of a carbonate platform (Shackleton Limestone) during the early Atdabanian (approximately 525 Ma), followed by a transitional interval of silici-clastic deposition and volcanism (Starshot Formation) during the late Botomian (approximately 512 Ma), and ending with a coarse cover of siliciclastic molasse deposition (Douglas Conglomerate), no younger than plutonism at 492 ±2 Ma. Thus, the Byrd Group was deposited during a span less than 33 m.y., representing passivie shelf-margin sedimentation into active uplift and erosion related to the Ross Orogeny.
The development of the carbonate platform is represented by the newly subdivided Early Cambrian Shackleton Limestone. The lower half of the Shackleton Limestone is a succession of high-energy.
shelf margin, limestone tempestites (Cross-bedded member), followed by low- to moderate-energy, semi-restricted to restricted shelf deposits (Cherty member), overlain by low-energy, tidal flat deposits (Butterscotch member). This nearly 1,000 m-thick shoaling upwards succession is newly interpreted to represent a first- to second-order progradation and offlap of carbonate shelf sediments, possibly in response to a eustatic lowering of sea-level during the Early Cambrian. The upper half of the Shackleton Limestone (Upper member) is a sequence, approximately 1500 m thick, of mainly shallow-water shelf deposits, interpreted to record sedimentation in response to the Cambrian Sauk transgression. However, the lower part of the Upper member records a significant emergence of the carbonate shelf, possibly the platform, resulting in an interval of karst development, most likely caused by local tectonism and uplift of the carbonate shelf. The uppermost Upper member of the Shackleton Limestone contains two closely-spaced lithosomes, a volcanic ash bed followed by a horizon of argillite. The argillite represents the first major pulse of elastic sedimentation into the Shackleton carbonate basin. The volcanic ash provides the first isotopic age of this event (512 ±5 Ma), and upper bounding depositional age of the Shackleton Limestone. These new geochronological data constrain the span of deposition of the Shackleton Limestone to approximately 13 m.y ..
Basalt flows and pillows of the Starshot Formation accompany the carbonate to siliciclastic transition, coeval with limestone deposition. Sandstones of the Starshot Formation coarsen upwards into the Douglas Conglomerate. Within the Douglas Conglomerate, second-generation clasts indicate that the basin was feeding on itself during continued tectonism, which produced a thick molasse in response to regional uplift of carbonate and siliciclastic sources. The clasts of the Douglas Conglomerate reveal that the primary sources of elastic detritus were derived from the Shackleton Limestone, Starshot basalt, and possibly much of the Starshot sandstone. Additionally, discoveries at previously unvisited nunataks extend the elastic basin further to the east in this area.
Refinement of the sequence of sedimentation, tectonism, and m~gmatism that deformed the Byrd Group has improved our understanding of the regional geologic history and has raised several questions regarding the evolution of the central Transantarctic Mountains and the Ross Orogen, such as the geometry and location of the Early Cambrian paleo-Pacific continental margin, the direction of siliciclastic sources, the location of the magmatic arc (thought to be the locus of tectonic uplift and molasse progradation), and the relationship with older terrain to the north of Byrd Glacier separated by the Byrd Glacier Discontinuity (Stump et al. this volume).
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