Structural studies in the Scotia Arc and West Antarctica Marine ...
Structural studies in the Scotia Arc and West Antarctica IAN W. D. DALZIEL
Lamont-Doherty Geological Observatory of Columbia University Palisades, New York 10964
Scientists at Lamont-Doherty Geological Observatory continued their studies in the Scotia Arc region during 1979-80 by completing the mapping of the Sarmiento Ophiolite Complex in the southernmost Andes (Allen, Antarctic Journal of the U.S., this volume; Daiziel in press-a) and by expanding field studies in the Andean foothills of southern Chile. Richardson Allen was in the field during May and June 1979 (R/v Hero cruise 79-4), and Terry Wilson from December 1979 through April 1980. In addition, as
principal investigator, I took part during December 1979 and January 1980 in the Ellsworth Mountains expedition, led by Gerald Webers of Macalester College, Minnesota, in order to help tie this important mountain range into the "basement" geology of the Scotia Arc region (Dalziel 1980 and in press-b). This work was supported by National Science Foundation grants DI'!' 78-20629, EAR 79-06308, and INT 79-20213. References Allen, R. B. In press. R/V Hero cruise 794: The geology and setting of the Sarmiento Complex ophiolite, southern Chile. Antarctic Journal of the U.S., 15(4). Dalziel, I. W. D. 1980. Comment on: Mesozoic evolution of the Antarctic Penninsula and the southern Andes by Harrison, C. C. A., Barron, E. J., and Hays, W. W. Geology, 8, 260-261. Daiziel, I. W. D., In press. Back-arc spreading in the southern Andes: A review and critical reappraisal. Royal Society of London. (a) Dalziel, I. W. D. In press. Pre-Jurassic history of the Scotia Arc region. In C. Craddock (Ed.), Antarctic Geoscience. Madison: University of Wisconsin Press. (b)
Marine terraces of Seymour Island, Antarctic Peninsula
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WILLIAM J. ZINSMEISTER
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Institute of Polar Studies The Ohio State University Columbus, Ohio 43210
During the 1974-75 austral summer, a geological survey of Seymour Island was conducted by a joint party from the Institute of Polar Studies, The Ohio State University, and the Instituto Antarctico Argentino. The objective of the field season was to study the stratigraphy and invertebrate faunas of the Tertiary La Meseta and Cross Valley formations exposed around the prominent meseta at the north end of the island (Elliot, Rinaldi, Zinsmeister, Trautman, Bryant, and del Valle 1975). Four terraces are preserved on the relatively gentle slopes of the meseta in the vicinity of Cross Valley. In those areas around the meseta where the slopes are steep, slope wash and mass wasting have removed all traces of any terraces. The lowest terrace (1 to 2 meters) is preserved only at the mouth (figure 1) and along the banks of the intermittent stream in Cross Valley and at Larsen Cove. The second and most extensively preserved terrace (4 meters) forms a broad platform extending for several kilometers along the coast 1980 REVIEW
Figure 1. Lower two terraces at the mouth of Cross Valley (1) 1- to 2-meter terrace and (2) 4-meter terrace.
on the side of Cross Valley (figures 1, 2, and 3). The third terrace (18 meters) is relatively narrow and is preserved only for a short distance along the base of the southwest corner of the meseta (figure 3). The highest recognizable terrace (35 meters) occurs as discontinuous remanents along the southwest side of the meseta. The top of the meseta is a broad, relatively flat surface (200 meters) and may represent a fifth terrace. The presence of large numbers of erratics on the top of the meseta indicates that the surface may have been formed by glacial erosion, but at the present time it is not possible to determine if the meseta top is a marine or glacial feature. It was also observed during the course of the field season that, because of the unconsolidated nature of the Tertiary 25
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-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
Lamont-Doherty Geological Observatory of Columbia University Palisades, New York 10964
Scientists at Lamont-Doherty Geological Observatory continued their studies in the Scotia Arc region during 1979-80 by completing the mapping of the Sarmiento Ophiolite Complex in the southernmost Andes (Allen, Antarctic Journal of the U.S., this volume; Daiziel in press-a) and by expanding field studies in the Andean foothills of southern Chile. Richardson Allen was in the field during May and June 1979 (R/v Hero cruise 79-4), and Terry Wilson from December 1979 through April 1980. In addition, as
principal investigator, I took part during December 1979 and January 1980 in the Ellsworth Mountains expedition, led by Gerald Webers of Macalester College, Minnesota, in order to help tie this important mountain range into the "basement" geology of the Scotia Arc region (Dalziel 1980 and in press-b). This work was supported by National Science Foundation grants DI'!' 78-20629, EAR 79-06308, and INT 79-20213. References Allen, R. B. In press. R/V Hero cruise 794: The geology and setting of the Sarmiento Complex ophiolite, southern Chile. Antarctic Journal of the U.S., 15(4). Dalziel, I. W. D. 1980. Comment on: Mesozoic evolution of the Antarctic Penninsula and the southern Andes by Harrison, C. C. A., Barron, E. J., and Hays, W. W. Geology, 8, 260-261. Daiziel, I. W. D., In press. Back-arc spreading in the southern Andes: A review and critical reappraisal. Royal Society of London. (a) Dalziel, I. W. D. In press. Pre-Jurassic history of the Scotia Arc region. In C. Craddock (Ed.), Antarctic Geoscience. Madison: University of Wisconsin Press. (b)
Marine terraces of Seymour Island, Antarctic Peninsula
,; ,-0.0
t
2
WILLIAM J. ZINSMEISTER
- -=- I
Institute of Polar Studies The Ohio State University Columbus, Ohio 43210
During the 1974-75 austral summer, a geological survey of Seymour Island was conducted by a joint party from the Institute of Polar Studies, The Ohio State University, and the Instituto Antarctico Argentino. The objective of the field season was to study the stratigraphy and invertebrate faunas of the Tertiary La Meseta and Cross Valley formations exposed around the prominent meseta at the north end of the island (Elliot, Rinaldi, Zinsmeister, Trautman, Bryant, and del Valle 1975). Four terraces are preserved on the relatively gentle slopes of the meseta in the vicinity of Cross Valley. In those areas around the meseta where the slopes are steep, slope wash and mass wasting have removed all traces of any terraces. The lowest terrace (1 to 2 meters) is preserved only at the mouth (figure 1) and along the banks of the intermittent stream in Cross Valley and at Larsen Cove. The second and most extensively preserved terrace (4 meters) forms a broad platform extending for several kilometers along the coast 1980 REVIEW
Figure 1. Lower two terraces at the mouth of Cross Valley (1) 1- to 2-meter terrace and (2) 4-meter terrace.
on the side of Cross Valley (figures 1, 2, and 3). The third terrace (18 meters) is relatively narrow and is preserved only for a short distance along the base of the southwest corner of the meseta (figure 3). The highest recognizable terrace (35 meters) occurs as discontinuous remanents along the southwest side of the meseta. The top of the meseta is a broad, relatively flat surface (200 meters) and may represent a fifth terrace. The presence of large numbers of erratics on the top of the meseta indicates that the surface may have been formed by glacial erosion, but at the present time it is not possible to determine if the meseta top is a marine or glacial feature. It was also observed during the course of the field season that, because of the unconsolidated nature of the Tertiary 25
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
