Late Jurassic ammonite faunas from the Latady Formation, Orville Coast
Lassiter Coast copper deposit (Farrar et al. in press; age recalculated for the new decay constants of Dalrymple 1979). The west Behrendt batholith, occurring in scattered unnamed nunataks west of the Behrendt Mountains, has an exposed diameter of about 12 kilometers. The rock is a uniform medium- to coarse-grained high-quartz granodiorite. The K-Ar age is 104.5 million years (biotite) to 108.9 million years (hornblende). The ages reported here are similar to those reported for plutons from the Lassiter and Black Coasts (Farrar et al. in press; Mehnert, Rowley, and Schmidt 1975) and eastern Ellsworth Land (Halpern 1967). Clearly the plutons in the southern Antarctic Peninsula belong to a single restricted Late Cretaceous magmatic event, in contrast to other plutons in the northern Antarctic Peninsula, which range in age from Jurassic to Tertiary. This work was supported by grants from the National Science Foundation (DPP 78-24214) and the National Science and Engineering Research Council of Canada. We thank C. J. Adams and J. E. Gabites, Institute of Nuclear Science, Department of Scientific and Industrial Research, Lower Hutt, New Zealand, in whose laboratory the K-Ar analyses were performed, for their kind assistance. References Dalrymple, G. B. 1979. Critical tables for conversion of K-Ar ages from old to new constants. Geology, 7(11), 558-560. Farrar, E., McBride, S. L., and Rowley, P. D. In press. Ages and tectonic implications of Andean plutonism in the southern Antarctic Peninsula. In C. Craddock (Ed.), Third Symposium on Antarctic Geology and Geophysics, August 1977. Madison: University of Wisconsin Press. Halpern, M. 1967. Rubidium-strontium age measurements of plutonic igneous rocks in eastern Ellsworth Land and northern Antarctic Peninsula, Antarctica. Journal of Geophysical Research, 72(20), 5133-5142.
Late Jurassic ammonite faunas from the Latady Formation, Orville Coast M. R. A. THOMSON
British Antarctic Survey Natural Environment Research Council Madingley Road Cambridge CB3 OET, United Kingdom
Reconnaissance geologic mapping by a U.S. Geological Survey field party from 3 November 1977 to 2 February 1978 showed that the Jurassic Latady Formation of the Lassiter Coast (Williams, Schmidt, Plummer, and Brown 1972) and southern Black Coast (Rowley and Williams in press) 28
Laudon, T. S. 1972 Stratigraphy of eastern Ellsworth Land. In R. J. Adie (Ed.), Antarctic geology and geophysics. Oslo, Norway: Universitetsforlaget. Laudon, T. S., Lackey, L. L., Quilty, P. C., and Otway, P. M. 1969. Geology of eastern Ellsworth Land (Sheet 3, eastern Ellsworth Land). In V. C. Bushnell and C. Craddock (Eds.), Geologic maps of Antarctica (Antarctic map folio series, Folio 12, Plate 3). New York: American Geographical Society. Mehnert, H. H., Rowley, P. D., and Schmidt, D. L. 1975. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. U.S. Geological Survey Journal of Research, 3(2), 233-236. Rowley, P. D. 1978. Geologic studies in Orville Coast and eastern Ellsworth Land, Antarctic Peninsula. Antarctic Journal of the U.S., 13(4),7-9. Rowley, P. D. 1979. Orville Coast-eastern Ellsworth Land project, 1978-1979. Antarctic Journal of the U.S., 14(5), 21-22. Rowley, P. D., and Williams, P. L. In press. Geology of the northern Lassiter Coast and southern Black Coast, Antarctic Peninsula. In C. Craddock (Ed.), Third Symposium on Antarctic Geology and Geophysics, August 1977. Madison: University of Wisconsin Press. Rowley, P. D., Williams, P. L., and Schmidt, D. L. 1977. Geology of an Upper Cretaceous copper deposit in the Andean province, Lassiter Coast, Antarctic Peninsula. U.S. Geological Survey Professional Paper 984. Rowley, P. D., Williams, P. L., and Schmidt, D. L., Reynolds, R. L., Ford, A. B., Clark, A. H., Farrar, E., and McBride, S. L. 1975. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Economic Geology, 70(5), 982-992. Thomson, M. R. A., Laudon, T. S., and Boyles, J . M. 1978. Stratigraphical studies in Orville Coast and eastern Ellsworth Land. Antarctic Journal of the U.S., 13(4), 9-10. Williams, P. L., Schmidt, D. L., Plummer, C. C., and Brown, L. E. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula—Preliminary report. In R. J. Adie (Ed.), Antarctic Geology and Geophysics. Oslo, Norway: Universitetsforlaget.
can be traced southward into the Orville Coast area and the Behrendt Mountains (Rowley 1978; Thomson, Laudon, and Boyles 1978). Thus this formation occurs over approximately 40,000 square kilometers (figure) and perhaps considerably more if some areas of metasedimentary rocks in northeastern Palmer Land are proven eventually to be part of the same sequence (cf. Taylor, Thomson, and Willey 1979). Because the Latady Formation is strongly folded, is exposed in isolated nunataks and mountains, and fossils are rare or absent over substantial thicknesses of it, a clear stratigraphic picture may not emerge for a long time. However, ammonite faunas previously reported from the Latady Formation range from Middle to Late Jurassic in age (Quilty 1970; Rowley and Williams in press). In the Orville Coast area (figure), exposures in the Latady Formation demonstrate a north-to-south facies change from (1) lacustrine or lagoonal along the "southern" margin of a Mesozoic magmatic arc through (2) a high-energy nearshore or shallow-shelf environment with abundant ANTARCTIC JOURNAL
Sketch map of the Orville Coast region showing the principal mountains and nunataks mapped.
shale-pebble conglomerates to (3) open marine. Thus ammonites are found at only one place in the Sweeney Mountains, are uncommon through the Hauberg and Wilkins mountains, but are relatively abundant at Cape Zumberge. Rarely do more than one or two ammonite species occur at a single outcrop so that there are few assemblage faunas. This makes it difficult to judge the stratigraphic age relationships between the species present in the area. With the exception of a single fragment of an oppeliid from southern Wilkins Mountains and a new genus of uncertain affinities from northern Wilkins Mountains, all the ammonites obtained are perisphinctids. The most com mon are multispiral forms with bifurcate ribbing and depressed whorl cross sections, reminiscent of such genera as Subdichotomoceras Spath, Torquatisphinctes Spath, and possibly Pachysphinctes Dietrich. Precise identification of these genera is proving extremely difficult because of their fragmentary preservation and distortion of many specimens. Nevertheless, such forms suggest that much of the sequence may be Kimmeridgian or perhaps Early Tithonian in age. Local occurrences of Katroliceras Spath (?),Subplanites Spath (?) and Aspidoceras Zittel are also consistent with this broad age estimate. Although there are a few fragments that may refer to the Tithonian genera Virgatosphinctes Uhlig and Aulacosphinc1980 REVIEW
toides Spath, the Orville Coast faunas seem largely to predate those of eastern Alexander Island (Thomson 1979), and there is little in common between the ammonite faunas of the two areas. Kossmatia Uhlig, also known from western Palmer Land (Thomson 1975), occurs commonly at one location in the Wilkins Mountains and rare single specimens were collected at nearby outcrops. However, they seem to be closer to Himalayan and Indonesian species than their antarctic relatives. One fauna, from Cape Zumberge, stands out as distinct from the rest. Characterized by the genera Blanfordiceras Cossmann and Berriasella Uhlig, it is probably Late Tithonian in age and is comparable to a similar fauna from the Ablation Valley (70°49'S 68°25'W) area of Alexander Island (Thomson 1979). This is the youngest fauna reported so far from the Latady Formation. An interesting feature of these faunas is the complete lack of phylloceratids and lytoceratids, generally regarded as offshore or open-ocean types. This contrasts strongly with the Late Jurassic marine faunas in the Fossil Bluff Formation of Alexander Island (Thomson 1979) where such forms are common. The more sheltered paleogeographical position of the Latady Formation sea in a rear-arc position, as opposed to the fore-arc location of the Fossil Bluff Formation sea, may account for these differences. 29
The field work for this project was funded by National Science Foundation grant DPP 76-12557 to the U.S. Geological Survey. I am indebted to my colleagues P. D. Rowley, J. M. Boyles, P. Carrara, K. S. Kellogg, T. S. Laudon, and W. R. Vennum for collecting many of the specimens mentioned. References Quilty, P. G. 1970. Jurassic ammonites from eastern Ellsworth Land, Antarctica. Journal of Paleontology, 44(1), 110 - 116. Rowley, P. D. 1978. Geologic studies in Orville Coast and eastern Ellsworth Land, Antarctic Peninsula. Antarctic Journal of the U.S., 13(4),7-9. Rowley, P. D., and Williams, P. L. In press. Geology of the northern Lassiter Coast and southern Black Coast, Antarctic Peninsula. In C. Craddock (Ed.), Antarctic Geoscience. Madison: University of Wisconsin Press.
Sedimentology of the Polarstar Formation (Permian), Ellsworth Mountains
Taylor, B. J . , Thomson, M. R. A., and Willey, L. E. 1979. The geology of the Ablation Point-Keystone Cliffs area, Alexander Island. British Antarctic Survey Scientific Reports, 82, 37 pp. Thomson, M. R. A., 1975. Upper Jurassic Mollusca from Carse Point, Palmer Land. British Antarctic Survey Bulletin, 41 and 42, 31-42. Thomson, M. R. A. 1979. Upper Jurassic and Lower Cretaceous ammonite faunas from the Ablation Point area, Alexander Island. British Antarctic Survey Scientific Reports, 97, 65 PP. Thomson, M. R. A., Laudon, T. S., and Boyles, J . M. 1978. Stratigraphical studies in Orville Coast and eastern Ellsworth Land. Antarctic Journal of the U.S., 13(4), 9 - 10. Williams, P. L., Schmidt, D. L., Plummer, C. C., and Brown, L. E. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula— Preliminary report. In R. J . Adie, (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget.
dispersed pebbles occur within the lower 5 meters. A 2.5meter-thick, poorly sorted, fine-grained sandstone occurs 7.5 meters above the base. The lower argillite unit is exposed on the lower slopes of Mt. Ulmer and along a ridge extending eastward from Mt. Ulmer toward Mt. Wyatt Earp.
JAMES W. COLLINSON, CHARLES L. VAVRA, and JOHN M. ZAwIsKIE Institute of Polar Studies and Department of Geology and Mineralogy The Ohio State University Columbus, Ohio 43210
Sedimentologic studies of the Polarstar Formation were conducted by the authors from 11 to 29 December 1979. Most locations were reached by motor toboggan from a tent camp in the northern Sentinel Range (figure 1). Helicopter support from the Ellsworth Mountains base camp aided in setting up the tent camp and in visiting the less accessible sites. The Polarstar Formation consists mostly of argillite and sandstone. Craddock (1969) described the following stratigraphic sequence: (1) argillite at the base; (2) argillite and fine-grained sandstone in the middle; and (3) coal measures at the top. His estimate of the thickness, 1,700 meters, is greater than our more conservative estimate of 800 to 1,000 meters. An accurate determination of the thickness is not possible because of the lack of a complete sequence at any one locality, the intense deformation of these rocks, and the lack of distinct marker beds. Disharmonic folds and thrust faults in argillite units inflate the apparent thicknesses of sections. The base of the formation is well exposed on the north flank of Whiteout Nunatak, where a 50-meter-thick sequence of black argillite conformably overlies the Whiteout Conglomerate, an Upper Carboniferous-Lower Permian diamictite. The basal contact of the argillite is sharp, but 30
86000'W
86°30W
Mt. Lymr
^^Y
77O3O'
Olsen P e ak
rstor Mt WykEar pFpA
• Ulmer £ Whiteout
0 • TENT CAMP km MEASURED SECTIONS
5
Figure 1. Location of measured sections in northern Sentinel Range. ANTARCTIC JOURNAL
Late Jurassic ammonite faunas from the Latady Formation, Orville Coast M. R. A. THOMSON
British Antarctic Survey Natural Environment Research Council Madingley Road Cambridge CB3 OET, United Kingdom
Reconnaissance geologic mapping by a U.S. Geological Survey field party from 3 November 1977 to 2 February 1978 showed that the Jurassic Latady Formation of the Lassiter Coast (Williams, Schmidt, Plummer, and Brown 1972) and southern Black Coast (Rowley and Williams in press) 28
Laudon, T. S. 1972 Stratigraphy of eastern Ellsworth Land. In R. J. Adie (Ed.), Antarctic geology and geophysics. Oslo, Norway: Universitetsforlaget. Laudon, T. S., Lackey, L. L., Quilty, P. C., and Otway, P. M. 1969. Geology of eastern Ellsworth Land (Sheet 3, eastern Ellsworth Land). In V. C. Bushnell and C. Craddock (Eds.), Geologic maps of Antarctica (Antarctic map folio series, Folio 12, Plate 3). New York: American Geographical Society. Mehnert, H. H., Rowley, P. D., and Schmidt, D. L. 1975. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. U.S. Geological Survey Journal of Research, 3(2), 233-236. Rowley, P. D. 1978. Geologic studies in Orville Coast and eastern Ellsworth Land, Antarctic Peninsula. Antarctic Journal of the U.S., 13(4),7-9. Rowley, P. D. 1979. Orville Coast-eastern Ellsworth Land project, 1978-1979. Antarctic Journal of the U.S., 14(5), 21-22. Rowley, P. D., and Williams, P. L. In press. Geology of the northern Lassiter Coast and southern Black Coast, Antarctic Peninsula. In C. Craddock (Ed.), Third Symposium on Antarctic Geology and Geophysics, August 1977. Madison: University of Wisconsin Press. Rowley, P. D., Williams, P. L., and Schmidt, D. L. 1977. Geology of an Upper Cretaceous copper deposit in the Andean province, Lassiter Coast, Antarctic Peninsula. U.S. Geological Survey Professional Paper 984. Rowley, P. D., Williams, P. L., and Schmidt, D. L., Reynolds, R. L., Ford, A. B., Clark, A. H., Farrar, E., and McBride, S. L. 1975. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Economic Geology, 70(5), 982-992. Thomson, M. R. A., Laudon, T. S., and Boyles, J . M. 1978. Stratigraphical studies in Orville Coast and eastern Ellsworth Land. Antarctic Journal of the U.S., 13(4), 9-10. Williams, P. L., Schmidt, D. L., Plummer, C. C., and Brown, L. E. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula—Preliminary report. In R. J. Adie (Ed.), Antarctic Geology and Geophysics. Oslo, Norway: Universitetsforlaget.
can be traced southward into the Orville Coast area and the Behrendt Mountains (Rowley 1978; Thomson, Laudon, and Boyles 1978). Thus this formation occurs over approximately 40,000 square kilometers (figure) and perhaps considerably more if some areas of metasedimentary rocks in northeastern Palmer Land are proven eventually to be part of the same sequence (cf. Taylor, Thomson, and Willey 1979). Because the Latady Formation is strongly folded, is exposed in isolated nunataks and mountains, and fossils are rare or absent over substantial thicknesses of it, a clear stratigraphic picture may not emerge for a long time. However, ammonite faunas previously reported from the Latady Formation range from Middle to Late Jurassic in age (Quilty 1970; Rowley and Williams in press). In the Orville Coast area (figure), exposures in the Latady Formation demonstrate a north-to-south facies change from (1) lacustrine or lagoonal along the "southern" margin of a Mesozoic magmatic arc through (2) a high-energy nearshore or shallow-shelf environment with abundant ANTARCTIC JOURNAL
Sketch map of the Orville Coast region showing the principal mountains and nunataks mapped.
shale-pebble conglomerates to (3) open marine. Thus ammonites are found at only one place in the Sweeney Mountains, are uncommon through the Hauberg and Wilkins mountains, but are relatively abundant at Cape Zumberge. Rarely do more than one or two ammonite species occur at a single outcrop so that there are few assemblage faunas. This makes it difficult to judge the stratigraphic age relationships between the species present in the area. With the exception of a single fragment of an oppeliid from southern Wilkins Mountains and a new genus of uncertain affinities from northern Wilkins Mountains, all the ammonites obtained are perisphinctids. The most com mon are multispiral forms with bifurcate ribbing and depressed whorl cross sections, reminiscent of such genera as Subdichotomoceras Spath, Torquatisphinctes Spath, and possibly Pachysphinctes Dietrich. Precise identification of these genera is proving extremely difficult because of their fragmentary preservation and distortion of many specimens. Nevertheless, such forms suggest that much of the sequence may be Kimmeridgian or perhaps Early Tithonian in age. Local occurrences of Katroliceras Spath (?),Subplanites Spath (?) and Aspidoceras Zittel are also consistent with this broad age estimate. Although there are a few fragments that may refer to the Tithonian genera Virgatosphinctes Uhlig and Aulacosphinc1980 REVIEW
toides Spath, the Orville Coast faunas seem largely to predate those of eastern Alexander Island (Thomson 1979), and there is little in common between the ammonite faunas of the two areas. Kossmatia Uhlig, also known from western Palmer Land (Thomson 1975), occurs commonly at one location in the Wilkins Mountains and rare single specimens were collected at nearby outcrops. However, they seem to be closer to Himalayan and Indonesian species than their antarctic relatives. One fauna, from Cape Zumberge, stands out as distinct from the rest. Characterized by the genera Blanfordiceras Cossmann and Berriasella Uhlig, it is probably Late Tithonian in age and is comparable to a similar fauna from the Ablation Valley (70°49'S 68°25'W) area of Alexander Island (Thomson 1979). This is the youngest fauna reported so far from the Latady Formation. An interesting feature of these faunas is the complete lack of phylloceratids and lytoceratids, generally regarded as offshore or open-ocean types. This contrasts strongly with the Late Jurassic marine faunas in the Fossil Bluff Formation of Alexander Island (Thomson 1979) where such forms are common. The more sheltered paleogeographical position of the Latady Formation sea in a rear-arc position, as opposed to the fore-arc location of the Fossil Bluff Formation sea, may account for these differences. 29
The field work for this project was funded by National Science Foundation grant DPP 76-12557 to the U.S. Geological Survey. I am indebted to my colleagues P. D. Rowley, J. M. Boyles, P. Carrara, K. S. Kellogg, T. S. Laudon, and W. R. Vennum for collecting many of the specimens mentioned. References Quilty, P. G. 1970. Jurassic ammonites from eastern Ellsworth Land, Antarctica. Journal of Paleontology, 44(1), 110 - 116. Rowley, P. D. 1978. Geologic studies in Orville Coast and eastern Ellsworth Land, Antarctic Peninsula. Antarctic Journal of the U.S., 13(4),7-9. Rowley, P. D., and Williams, P. L. In press. Geology of the northern Lassiter Coast and southern Black Coast, Antarctic Peninsula. In C. Craddock (Ed.), Antarctic Geoscience. Madison: University of Wisconsin Press.
Sedimentology of the Polarstar Formation (Permian), Ellsworth Mountains
Taylor, B. J . , Thomson, M. R. A., and Willey, L. E. 1979. The geology of the Ablation Point-Keystone Cliffs area, Alexander Island. British Antarctic Survey Scientific Reports, 82, 37 pp. Thomson, M. R. A., 1975. Upper Jurassic Mollusca from Carse Point, Palmer Land. British Antarctic Survey Bulletin, 41 and 42, 31-42. Thomson, M. R. A. 1979. Upper Jurassic and Lower Cretaceous ammonite faunas from the Ablation Point area, Alexander Island. British Antarctic Survey Scientific Reports, 97, 65 PP. Thomson, M. R. A., Laudon, T. S., and Boyles, J . M. 1978. Stratigraphical studies in Orville Coast and eastern Ellsworth Land. Antarctic Journal of the U.S., 13(4), 9 - 10. Williams, P. L., Schmidt, D. L., Plummer, C. C., and Brown, L. E. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula— Preliminary report. In R. J . Adie, (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget.
dispersed pebbles occur within the lower 5 meters. A 2.5meter-thick, poorly sorted, fine-grained sandstone occurs 7.5 meters above the base. The lower argillite unit is exposed on the lower slopes of Mt. Ulmer and along a ridge extending eastward from Mt. Ulmer toward Mt. Wyatt Earp.
JAMES W. COLLINSON, CHARLES L. VAVRA, and JOHN M. ZAwIsKIE Institute of Polar Studies and Department of Geology and Mineralogy The Ohio State University Columbus, Ohio 43210
Sedimentologic studies of the Polarstar Formation were conducted by the authors from 11 to 29 December 1979. Most locations were reached by motor toboggan from a tent camp in the northern Sentinel Range (figure 1). Helicopter support from the Ellsworth Mountains base camp aided in setting up the tent camp and in visiting the less accessible sites. The Polarstar Formation consists mostly of argillite and sandstone. Craddock (1969) described the following stratigraphic sequence: (1) argillite at the base; (2) argillite and fine-grained sandstone in the middle; and (3) coal measures at the top. His estimate of the thickness, 1,700 meters, is greater than our more conservative estimate of 800 to 1,000 meters. An accurate determination of the thickness is not possible because of the lack of a complete sequence at any one locality, the intense deformation of these rocks, and the lack of distinct marker beds. Disharmonic folds and thrust faults in argillite units inflate the apparent thicknesses of sections. The base of the formation is well exposed on the north flank of Whiteout Nunatak, where a 50-meter-thick sequence of black argillite conformably overlies the Whiteout Conglomerate, an Upper Carboniferous-Lower Permian diamictite. The basal contact of the argillite is sharp, but 30
86000'W
86°30W
Mt. Lymr
^^Y
77O3O'
Olsen P e ak
rstor Mt WykEar pFpA
• Ulmer £ Whiteout
0 • TENT CAMP km MEASURED SECTIONS
5
Figure 1. Location of measured sections in northern Sentinel Range. ANTARCTIC JOURNAL
