Barron, J., Larsen, B., et al., 1991 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 119 45. AMERY GROUP RED BEDS IN PRYDZ BAY, ANTARCTICA1 Barbara H. Keating2 and Hideo Sakai3 ABSTRACT A thick sequence of continental red beds was drilled at ODP Site 740, on the continental margin of Antarctica. The red beds lack dateable fossils. Paleomagnetic studies were undertaken to establish the age of formation for these rocks. Many of the rocks sampled were found to be thermally remagnetized and yielded paleomagnetic inclinations similar to those of intrusive units within outcrops of red beds in Antarctica. Based upon the similarities in lithology, sequence of rock types, thickness, magnetic properties, and thermal histories, a correlation is made with the Amery Group of Per- mian age. Red beds of this group are exposed in the Prince Charles Mountains, inland of Prydz Bay. INTRODUCTION the northern Prince Charles Mountains (Crohn, 1959; Mond, 1972; Bennett and Taylor, 1972). These units were mapped by During Leg 119 of the Ocean Drilling Program (ODP), sev- McKelvey and Stephenson (1990). At least 2000 m of upper Per- eral holes were drilled on the continental margin of Antarctica. mian Amery Group conglomerates and sandstones is overlain At Hole 740A, the hole nearest the shoreline (Fig. 1), continen- by coal measures, dipping to the south-southeast at about 10° tal red beds were recovered. Sedimentologic studies indicate that to 20°. Drilling at Site 741, which is roughly 25 km from Site these red beds represent fluvial deposits. No fossils were found 740, cored 104 m of black shales, sandstones, and coal placed in the red bed unit, apart from undatable algal spores. The age roughly 1500 m higher in the geologic section than the red beds was designated "unknown" (Barron, Larsen, et al., 1989), and of Hole 740A (Barron, Larsen, et al., 1989). The stratigraphic subsequent fossil studies have not produced age constraints for sequence and thickness found by ODP drilling in Prydz Bay are this unit (Truswell, this volume). Thus, paleomagnetic studies thus similar to those reported at Beaver Lake by McKelvey and were undertaken to estimate the age of the sequence. Correla- Stephenson (1990), but because the sediments at Site 741 are of tions are made with nearby outcrops of the Amery Group on post-rift Early Cretaceous age according to Truswell (this vol- the basis of lithology and stratigraphy and supported by paleo- ume), they are not equivalent stratigraphically to the lower Per- magnetic correlations (including the inclination and polarity, mian Amery sequence. the character of the secondary magnetization, as well as rock The Permian Amery Group is the only known in-situ Paleo- magnetic characteristics). An age of Permian is suggested for zoic sedimentary sequence in all of MacRobertson Land. The the red bed sequence cored at ODP Hole 740A. flora contained in these rocks are described by Kemp (1973) and White (1973). The correlation of these rocks with part of the BACKGROUND Beacon Supergroup of western Antarctica was suggested by Prydz Bay is located on the eastern Antarctic continental Mond (1972). Within the southern Prince Charles Mountains margin, seaward of the Lambert Graben and Prince Charles fine-grained red siltstone debris containing Glossopteris plant Mountains (Fig. 1). At Site 740A, 56 m of glacial and gla- fossils like those found in the red beds of the Amery Group at ciomarine overburden and 58 m of continental red beds were Beaver Lake and the Beacon Supergroup elsewhere (Ruker, 1963; penetrated. Seismic-reflection profiles, collected as part of pre- White, 1962) was recovered in a moraine. drilling site surveys of the seaward-dipping sedimentary section The Amery sediments at Beaver Lake have been intruded by at Prydz Bay, revealed the stratified sequence at Hole 740A two geochemically distinct intrusive units. The red beds are in- (Barron, Larsen, et al., 1989). Outcrops of rock formations up- truded by alnoite sills and strongly altered alkaline mafic dikes. dip from Hole 740A include a lithologically correctable red bed The chilled margins and calcite veins and coatings on joint sur- unit on shore in the vicinity of Prydz Bay (McKelvey and faces in the sills extend into the surrounding strata. McKelvey Stephenson, 1990). and Stephenson (1990) reported only one K-Ar age of 110 Ma The outcrops of the red beds described by McKelvey and (mid-Cretaceous). Red bed sequences, containing thick intrusive Stephenson (1990) occur in the Prince Charles Mountains and bodies that have baked adjoining sediments, have been recorded are drained by the Amery Ice Shelf system. The southern Prince throughout Gondwanaland. The red beds are loosely dated as Charles Mountains consist of numerous nunataks forming moun- Devonian through Triassic. It is suggested that the red bed se- tain fringes of the Lambert and Kaliningrad glaciers (Hofman, quence drilled at Hole 740A corresponds to the Amery Group 1982). A complex of Archean to upper Proterozoic metamor- in the Prince Charles Mountains. phic rocks, locally intruded by granites and unconformably The paleolatitude results derived from this study are dis- overlain by Permian sediments (Tingey, 1972), forms the moun- cussed in the framework of the Permian tectonic configuration tains (Grew, 1982; Ravich and Fedorov, 1982). Upper Permian of Gondwanaland. Low paleolatitudes were observed in the red clastic sediments are present in exposures around Beaver Lake in beds from Hole 740A. The paleoinclinations are consistent with those observed from intrusives found within red bed sequences throughout Antarctica. Paleomagnetic studies conducted on the 1 Barron, J., Larsen, B., et al., 1991. Proc. ODP, Sci. Results, 119: College Beacon Supergroup red beds (Turnbull, 1959; Bull and Irving, Station, TX (Ocean Drilling Program). 1960; Bull et al., 1962; Hamilton, 1965; Christoffel, 1981; 2 Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822, U.S.A. Funaki, 1983; Sakai and Funaki, 1988) evidence that thermal 3 Department of Earth Sciences, Toyama University, Gofuku 3190, Toyama alteration of the red beds is abundant and very similar in nature City 930, Japan. to that observed in ODP Hole 740A. 795 B. H. KEATING, H. SAKAI Lutzow-Holm Bay Enderby Land J Amery Ice Shelf 270° W. 90° E. 136°E 180 142°E \ 70° ( 60° 65° 80° Beardmore Glacier \ 65° 60° 1— 80c \ Lambert Glacier / 75° X Amery Ice Shelf / Kaliningard Glacier -~ v_^^^ ) Prydz V Bay Beacon Heights )C^\ Ferrar Glacier ..Windy Gully [Λ *π"*π"ayloa r Glacier C ^-70° 75 Site 740- Site741 ^N 80°^ McMurdo •> Sound \ 5 Mawson Glacier — 80° I c - 75° 150 7cθ 7 70° \ 180° K A ^ B Figure 1. Map of Antarctica showing the location of (A) Sites 740 and 741 and (B) other paleomagnetic studies of Beacon Group sedi- ments. 796 AMERY GROUP RED BEDS IN PRYDZ BAY RED BEDS FROM HOLE 740A Hole 740A. The lower unit is well stratified and corresponds to the lower part of the sequence in Hole 740A. Stagg (1985) ar- Turner (this volume) described the depositional environment gued that the only well-stratified sediments observed within the and petrography of the red sediments from Hole 740A. The red Prydz Bay area were the Permian red beds and suggested that sediments collected consist of 58 m of sandstone interbedded the lower unit consisted of red beds similar to those in the with siltstone and claystone. The siltstones and claystones repre- Prince Charles Mountains. sent fine-grained flood plain deposits. The numerous erosively based sequences observed are believed to represent sandy braided PALEOMAGNETIC STUDIES stream deposits. Discharge fluctuations in the braided streambeds Paleomagnetic studies were undertaken in order to place con- were not great because significant amounts of trapped fine- straints on the age of the red beds drilled at Hole 740A. The grained sediments occur throughout the sandstones. Turner studies included detailed stepwise thermal demagnetization. Nearly concluded that much of the deposition of sandstones took place 150 red bed samples were studied. under steady-state flow conditions. Turner observed that pedo- genic carbonate is absent other than in root structures and sug- Instrumentation and Laboratory Experiments gested that the climate may have been too wet for extensive cal- The samples were inscribed with orientation lines prior to ex- crete formation. traction from the core. The samples for this study consist of Modal analysis of sandstones and siltstones indicates they cubes cut from the lithified sediments using a rock saw and are arkosic wackes composed of quartz, mica, and feldspar with stored in plastic sample cubes. Generally, three samples were subordinate rock fragments, garnet, and opaque minerals set in taken from each core section. Shipboard pilot studies were con- a clay matrix that makes up 17%-66% of the rock. Chlorite and ducted using both alternating field demagnetization and ther- sericite, some of diagenetic origin, are common constituents of mal demagnetization; the thermal demagnetization studies sug- the matrix. Turner (this volume) found in some thin sections gested a complicated picture of multicomponent magnetization. that the matrix consists of a coarse mat of biotite, muscovite, Subsequent to the cruise, the samples were shipped to the Uni- sericite, and chlorite of medium-silt grade. Authigenic vermicu- versity of Hawaii. The plastic sample cubes were opened so that lar chlorite was also present in the matrix. In the more clay-rich the samples dried in air. The samples were labeled with white sandstones the red sands are impregnated with iron oxides (mainly ink so that they could be identified after thermal demagnetiza- hematite). The green sandstones are rich in chlorite. Some tion and were placed in a doubly shielded mu-metal room for 2 quartz grains show poorly developed secondary overgrowths de- weeks prior to measurement. They were measured within the fined by a thin dust and iron oxide.