Potassium-argon ages of Upper Cretaceous plutonic
ac -
•;;-
3
r7!;#!iW t -'
- . -
—
-Figure 2. Terrace 2 (4 meters) just north of the mouth of Cross Valley. Note rapid erosion of seacliff.
3. Southwest flank of the meseta (2) 4-meter terrace Figure 3. and (3) 18-meter terrace. Note flat surface of the top of the meseta.
sediments, the seacliffs were being eroded at an unusually high rate (Zinsmeister 1979). The regression of the seacliffs, especially along the northwest coast of the island, has led to the development of broad intertidal mudflats and sandspits unique to Antarctica (Zinsmeister 1976). In addition to the rapid regression of the seacliffs, slope wash and mass wasting of the sands and siltstones are rapidly modifying the topography around the meseta. The rapid rate of erosion on Seymour Island indicates that the island must be young and the result of recent tectonic activity and uplift. The existence of a series of marine terraces on the northward side of the meseta supports the hypothesis of recent and rapid uplift of Seymour Island.
References
Potassium-argon ages of Upper Cretaceous plutonic rocks of Orville Coast and eastern Ellsworth Land EDWARD FARRAR
Department of Geological Sciences Queen's University Kingston, Ontario, Canada PETER D. ROWLEY
U.S. Geological Survey Denver, Colorado 80225 The Orville Coast and parts of eastern Ellsworth Land, Antarctica, were first mapped geologically in reconnaissance from 3 November 1977 to 2 February 1978 (Rowley 1978). At that time 14 stocks and one batholith were discov26
Elliot, D. H., Rinaldi, C., Zinsmeister, W. J . , Trautman, T. A., Bryant, W. A., and del Valle, R. 1975. Geological investigations of Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 10(4), 182-186. Zinsmeister, W. J. 1976. Intertidal region and molluscan fauna of Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 11(4), 222-225. Zinsmeister, W. J . 1979. Coastal erosion on Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 14(4), 16-17.
ered and studied. Samples of five of the plutons, considered typical of all of them, were collected for potassium-argon (K-Ar) analysis (table). One of the sampled intrusive rocks, the Sky-Hi stock, may represent the upper barren part of a porphyry-type copper deposit (Rowley 1978, 1979). The geology of the Orville Coast and the part of eastern Ellsworth Land that was explored in 1977-78 is similar to that of the Lassiter and Black Coasts to the northeast (Williams, Schmidt, Plummer, and Brown 1972; Rowley and Williams in press) and to part of eastern Ellsworth Land previously mapped (Laudon 1972; Laudon, Lackey, Quilty, and Otway 1969). All plutons mapped during the 197778 season forcibly intruded either folded fine-grained sedimentary rocks of the Latady Formation (Williams et al. 1972), which is mostly Late Jurassic but perhaps partly Middle Jurassic age (Thomson, Laudon, and Boyles 1978), or folded calc-alkaline volcanic rocks, which are locally intertongued and generally contemporaneous with the Latady Formation. The Hagerty stock, at Hagerty Peak (75° 17'S 68° 1 1'W) in the southeastern Sweeney Mountains (figure), has an exposed diameter of 8 kilometers. The pluton appears to be zoned compositionally from fine- to medium-grained diorite ANTARCTIC JOURNAL
K-Ar ages of plutons in Orville Coast and eastern Ellsworth Land, Antarctica Location' (Lat 5, Long W)
Mineral analyzed
Hagerty stock
75018' 68 0 11'
hornblende
Witte stock
75028' 60 0 21'
hornblende
Sample
Unit
V148a
Ro441d
biotite
biotite
Age (million years ±
2ab)
112.6 ± 1.6 104.7 ± 1.5 116.0 ± 1.6 110.0 ± 1.6 112.3 ± 1.6 108.8 ± 1.6
C2
Smart stock
75016' 70 0 14'
biotite
103.4 ± 1.5 110.8 ± 1.5
Ro498j
Ski-Hi stock
75055 71 0 19'
hornblende
120.5 ± 1.7 123.1 ± 1.8
Ke193d
west Behrendt batholith
75014' 73 0 15'
hornblende biotite
108.9 ± 1.6 104.5 ± 1.5
'Locations of samples from 1:500,000 scale U.S. Geological Survey Antarctic Sketch Map of Ellsworth Land (East Part)-Palmer Land (South Part). b
= 0.581 X 10 10 a', = 4.962 X 10 = standard deviation
10 a 1 ,
40
K/K = 0.01167 atom %
and syenodiorite at the contacts through medium-grained granodiorite to medium- to coarse-grained quartz monzonite in the interior (compositions based on modal analyses). A biotite K-Ar age of rock from the interior is 116.0 million years, whereas ages of 104.7 and 112.6 million years were obtained on two splits of a cogenetic hornblende (table). The reason for the disagreement between the two hornblende ages is presently unclear and therefore, in view of the biotite age, the 112.6 million years age is preferred. The Witte stock, in the Witte Nunataks, has an exposed diameter of 1 kilometer. The stock consists of fine- to medium-grained diorite and granodiorite at the contact and medium-grained granodiorite in the interior. Concordant
70
72
68
66
74. $64
Horner Nunataks
NE
Sky6 NaIaks EASTERN Merrick MIS
(Y7 ELLSWORTH
7 6 LAND
J)
70
Janke og
N
Cape Zu,ge
0 20 40 60 I'll RONNE ICE SHELF KILOMETERS
Location map of Orville Coast and eastern Ellsworth Land, Antarctica.
1980 REVIEW
ages of 108.8 million years (biotite) and 110.0 and 112.3 million years (hornblende) were determined for a sample from the interior (table). The Smart stock, at Mount Smart (75°16'S 70°14'W) in the southwestern Sweeney Mountains, has an exposed diameter of 3 kilometers. The rock is medium- to coarsegrained diorite at the contact and is as silicic as coarsegrained quartz rnonzonite in the interior. Discordant ages from different splits of biotite from a sample of quartz monzonite are 103.4 and 110.8 million years (table). The explanation appears to be sample inhomogeneity within the split; we cannot choose which age is better, but the younger age is more consistent with the age of silicic plutons elsewhere in the southern Antarctic Peninsula (Farrar, McBride, and Rowley in press). The exposures of the Sky-Hi stock, in the southeastern Sky-Hi Nunataks, comprise one nunatak about 1 kilometer long; no contacts are exposed. The rock is medium-grained granodiorite that is largely hydrothermally altered (propylitic, argillic, phyllic, and potassic facies), pyritized, and intruded by mostly unaltered porphyry dikes of diorite; all rocks are cut by altered shear zones. The geologic setting is similar to that of the Lassiter Coast copper deposit (Rowley, Williams, and Schmidt 1977; Rowley, Williams, Schmidt, Reynolds, Ford, Clark, Farrar, and McBride 1975), nearly 200 kilometers to the northeast. Semiquantitative spectrographic analyses of the plutonic rock for trace elements indicate only low-grade anomalies of copper (Cu), zinc (Zn), and molybdenum (Mo); the exposed mineralized area consists largely of disseminated pyrite. Veins containing chalcopyrite, malachite, and molybdenite crystals and higher contents of Cu, Zn, and Mo, however, cut Jurassic volcanic country rocks exposed 1 to 2 kilometers to the west. Concordant ages of 120.5 and 123.1 million years (table) on different splits of hornblende from nearly fresh granodiorite are older than the 97 million years age of the 27
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
