Station no

2

3

4

0.931.030.74

0.58 0.95 0.87

1750.85 >140

0.490.710.43

2.70

2.40

>4.00

1.63

rateof erosionper year

0.9

0.8

1.33

0.54

Total19751976 1977 1978Erosion

0.0

0.0

0.0

0.0

50.00.481.82 >1.70>4.001.33

60.00.70 0.901.102.700.9

base of the cliffs. The net result of little wave action andrelatively steep beach profile is that abrasion of the baseof the sea cliffs is greatly reduced.

The rapid rate of erosion of Seymour Island has aninteresting implication for the tectonic history of theJames Ross Island area. Because of the rapid erosion ofits coastline, Seymour Island must be considered an ex-

tremely young tectonic feature that has been upliftedonly recently.

ReferenceElliot, D. H., C. Rinaldi, W. J. Zinsmeister, T. A. Trautman, W.

A. Bryant, and R. del Valle. 1975. Geological investigationson Seymour Island, Antarctic Peninsula. Antarctic Journal ofthe United States, 10(4): 182-86.

CapeWiman

Study Cross.

Area)Valley

BodmanPoint

\\ Seymour

\Island

All measurements in meters.Average rate of erosion per year at all stations. 0.97m

Figure 3. Study area of coastal erosion on Seymour Island, with annual erosion data for each station.

Scotia Arc Tectonics Project,1978-79

I. W. D. DALZIEL, R. B. ALLEN, D. L. ELTHON,R. D. FORSYTHE, E. P. NELSON, T. J . WILSON, and

M. A. WINSLOW

Lamont-Doherty Geological Observatory of Columbia UniversityPalisades, New York 10964

From May 1978 to May 1979, geologists from La-mont-Doherty Geological Observatory continued theirfieldwork on selected regional problems within the Pa-tagonian and Fuegian cordilleras of the southern Andes.

Working on the Atlantic side of the cordillera, TerryWilson, accompanied by a student from the Universityof Chile, Santiago, initiated a structural program of in-vestigation within the Cretaceous-to-Tertiary foreland

fold and thrust belt in the Ultima Esperanza region. Inaddition, final reports were being completed on field-work conducted by Margaret Winslow during the pastfour years in the foreland terranes of Peninsula Bruns-wick and northern Tierra del Fuego.

Within the principal cordillera, field studies werecompleted by two working groups and another fieldproject was under way. On the iIv Hero cruise 78-2, EricNelson, accompanied by Ian Dalziel and Ian Ridley (La-mont-Doherty Geological Observatory), A. Geoff Milnes(the Swiss Federal Institute), and Constantino Mpodozis(University of Chile), Luis Oviedo, (University of Con-cepcion), and Ricardo Guzman (Chilean Institute ofGeological Investigations) completed the detailed struc-tural field investigation of Cordillera Darwin startedwith the i'/v Hero cruise 77-4 (Nelson et al., 1977; Nel-son, Dalziel, and Milnes, 1980; Nelson, Daiziel, and Rid-ley, 1979).

Richardson Allen, working in the canals to the west ofPuerto Natales on the structural evolution of the Cre-taceous ophiolitic terranes, completed two months offieldwork and subsequently continuing this study with

17

i/v Hero cruise 79-4. Preliminary results of this work arebeing prepared (Forsythe and Allen, 1979).

Also with cruise 79-4, Don Elthon continued petrol-ogic and geochemical sampling of the ophiolitic com-plexes to further our understanding of the igneous andmetamorphic evolution of the rocks of the so-called fos-sil marginal basin.

Within the remote outer Pacific perimeter of the ca-nals, Randall Forsythe expected to use cruise 79-5 toinvestigate the north and south continuations of the pre-Upper Jurassic basement assemblages that were studiedin previous years within the Madre de Dios Archipelago(Forsythe and Mpodozis, 1979a; Forsythe and Mpo-dozis, 1979b; Forsythe, 1978).

Laboratory investigations and analysis of field dataassociated with all of the above field projects, as well aswith previous field seasons in both Antarctica and theAndes, are continuing at Lamont-Doherty GeologicalObservatory.

This work has been supported by National ScienceFoundation grant DPP 74-21415.

ReferencesForsythe, R. D. 1978. Geologic reconnaissance of the Pre-Late

Jurassic Basement: Patagonian Andes. Antarctic Journal of theUnited States, 13(4): 10-12.

Forsythe, R. D., and R. B. Allen. In press. The Basement ofPeninsula Staines, Region XII, Province of Ultima Esper-anza, Chile. II Congreso Geologico Chileno, August 6-11,1979.

Forsythe, R. D., and C. Mpodozis. 1979a. Proto-Pacific crust insouthern Chile. Geological Society of America, CordilleranSection Meeting (April 1979), San Jose, California.

Forsythe, R. D., and A. C. Mpodozis. 1979b. El ArchipielagoMadre de Dios, Patagonia Occidental, Magallanes: Rasgosgenerales de la estratigrafia y estructura del "Basamento"Pre-Jurasico Superior. In Revista de la Geologia de Chile, no.7, pp. 13-29.

Nelson, E. P., I. W. D. Dalziel, and A. G. Milnes. 1980. Defor-mation in Cordillera Darwin, southernmost Chilean Andes:A metamorphic core complex? Geological Society of Amer-ica, Cordilleran Section Meeting, San Jose, California.

Nelson, E., 1. W. D. Dalziel, and W. I. Ridley. 1979. Geologicalstudies in Cordillera Darwin, Tierra del Fuego, southernChile: idv Hero cruises 77-4 and 78-2. Antarctic Journal of theUnited States (this issue).

Nelson. E., R. D. Forsythe, F. Hervé, M. Suárez, E. Valenzuela,and T. J . Wilson. 1977. Observaciones estructurales en laCordillera Darwin, Provincias Antarctica y de Tierra delFuego: Crucero 77-4 del RJV Hero. Notas Cien4fIcas, Corn-municaciones, 21:32-35. (Santiago, Chile.)

Rubidium-strontiumgeochronology of plutonic

igneous rocks from Hobbs andWaigreen coasts, Marie Byrd

Land

MARTIN HALPERN

Geosciences ProgramThe University of Texas at Dallas

Richardson, Texas 75080

F. ALTON WADE*

Antarctic Research CenterThe Museum, Texas Tech University

Lubbock, Texas 79409

Rubidium-strontium (Rb-Sr) radiometric age analyseshave been carried out on biotite concentrates and totalrock specimens of plutonic igneous rocks collected by

* Deceased

members of the Texas Tech University Antarctic Re-search Center. A primary objective of this research cen-ter, while under the direction of the late F. Alton Wade,was the preparation of geologic maps of Marie ByrdLand at a scale of 1:250,000. Radiometric dating of theubiquitous igneous rocks was to assist in establishing achronology of geologic events within Marie Byrd Landfor comparison with other circum-Pacific areas of con-tinental lithosphere in Antarctica, South America, andNew Zealand.

Metamorphosed sedimentary and igneous rocks con-stitute the basement complex of Marie Byrd Land. Thebasement rocks are intruded by igneous rocks of pre-dominantly intermediate composition and these are ov-erlain by Cenozoic volcanic rocks. The plutonic intrusiverocks are generally hypidiomorphic-granular. At BilleyBluff, the plutonic rocks are obicular; at Holmes Bluff,they intrude metasedimentary and metaigneous rocks.

Calculated ages for Hobbs Coast and Walgreen Coastigneous rocks are listed in the accompanying table, andtheir sample locations are shown in the figure.

The whole-rock isochron age for plutonic rock fromBilley Bluff is the first whole-rock Rb-Sr isochron datereported from this sector of West Antarctica. Its initial"Sr/"Sr ratio of 0.7054 ± 0.0013 is similar to that re-ported for Cretaceous plutons from the Ford Ranges(Halpern, 1968) and from small islands off the CanisteoPeninsula near Thurston Island (Munizaga, 1972). Thebiotite dates are apparent minimum ages similar to Rb-Sr and potassium-argon (K-Ar) mineral dates for plu-

18