Colored Geologic Mapsof the Beardmore Glacier Area

PETER J . BARRETT and JoHN F. LINDSAY

Institute of Polar StudiesOhio State University

The idea of producing colored geologic maps of theBeardmore Glacier area developed after a very suc-cessful field program was carried out there in1966-1967 by an Institute of Polar Studies field partythat included the writers and Dr. David H. Elliot.The work of the party showed that the Beardmorearea has an unusually thick section of Beacon strata,that rock exposures are widespread, and that most ofthe area is accessible by motor toboggan. Discoveriesduring the season, such as grooved pavements in thePermian glacial beds, volcanic ashfalls and well-pre-served plant remains in the Triassic, and several fossillocalities within the Jurassic basãlts, all suggested thatthe area would be of continuing geologic interest.

In the summer of 1967, a manuscript was preparedfor a geologic map of the area between the Beard-more and Nimrod Glaciers at a scale of 1: 125,000. Itwas apparent from this preliminary work that theareas of exposure were relatively small and that somesmall exposures included as many as five formations.Clearly, patterning was not going to show the geologyadequately, but colored maps might.

On the advice of the U.S. Geological Survey(USGS), it was decided to publish at a scale of

250,000. By scribing, instead of using conventionaldrafting techniques, the geology could be portrayed inadequate detail, and the plates used in producing theUSGS 1:250,000 reconnaissance series could be usedfor the topography.

Following the 1967-1968 field season, information'was either available or could be obtained from aerialphotos for most of five 1: 250,000 reconnaissance topo-graphic sheets covering the Beardmore Glacier area.It was hoped that the USGS would undertake colorseparation and printing because of its long experiencein color map reproduction and so that if other geo-logic maps were prepared in the future, standardiza-tion of format and color would be easier to achieve.

In March 1969, the authors spent seven dayslearning about the techniques used in color-map pro-duction, especially scribing, at various branches of theUSGS in Washington, D.C. Some agreement was alsoreached on line width, colors to be used on the finalproduct, and the general layout of the map sheets.The main outstanding problem is whether the shadedrelief as used on the topographic sheets should beretained on the rock exposures as well as the snow. Itadds topographic detail on the areas of snow but itwill vary the color on areas of rock outcrop.

The first sheet of a new antarctic map series, theMount Rabot Quadrangle, has been scribed and sub-mitted for publication by the USGS, and work is wellunder way on the Buckley Island and Mount Eliza-beth Quadrangles. Publication of the Mount Rabotsheet is expected by June 1970, and the other foursheets will be completed following a further field sea-son in the Beardmore Glacier area by an Institute ofPolar Studies party led by Dr. Elliot.

Regional Earthquakes Recorded atByrd and South Pole Stations

JAMES F. LANDER

Coast and Geodetic SurveyEnvironmental Science Services Administration

In the period 1957 through 1968, 1,058 epicenterswere located south of 45 0 S. latitude, mostly in seg-ments of the mid-oceanic belts which, together, forma circum-antarctic belt. Of the total, 149 epicenterswere located in 1967 and 130 in 1968. The increasingdensity of data points clarifies the existence of zonesof low seismic activity and brings out a fine structurewithin the more active zones. The scisrnicity of thesemid-oceanic ridges, currently under study, plays animportant role in validating theories of global tec-tonics and continental drift.

Figs. 1-2 show centers located before and since1957, the beginning date of records from seismologicalstations on the Antarctic Continent. Among the manyinteresting observations is the trend of hypocentersfrom the southern end of the Scotia Arc across theSouth Atlantic Ocean. Such trends are associatedwith the system of the mid-oceanic ridges elsewhere,and it is expected that they define the location of aridge not yet mapped. No earthquake has been lo-cated south of 65.8° S. in this time period.

Three separate earthquake episodes occurred atDeception Island during 1967-1969 that were re-corded at South Pole and Byrd Stations. The firstshock large enough to be located occurred on De-cember 4, 1967 at 19 hr 00 min 22.6 sec at 63.0°S.60.5°W. with a magnitude of 4.7 on the Richterscale. This shock was also reported by the seismolog-ical stations at Argentine Islands and Scott Base, aswell as by stations in South America and Africa. Asecond event, about magnitude 4.5, occurred twohours later, at 20 hr 28 min 30.5 sec and was locatedat 63.2 0 S. 60.3 0 W. Four other events, all above mag-nitude 4, were reported by Argentine Islands, Byrd,and South Pole Stations with indicated times of originat 19 hr 32 min 44 sec, 19 hr 48 nun 46 sec, 19 hr 52min 57 sec, and 21 hr 28 min 42 sec.

212 ANTARCTIC JOURN.\L

Figure 1 (right). The seismicity of the antarctic regionas known prior to the IGY. The epicenters plottedwere selected from data obtained before 1950 byB. Gutenberg and C. F. Richter and published intheir Seismicity of the Earth and Associated Phe-nomena, and those since 1950 from the Interna

-tional Seismological Summary and the Bulletin duBureau Central International de Seismologie.

Figure 2 (right, below). The seismicity of the antarcticregion since the IGY. The X symbols are hypocen-ters located graphically before 1961 from the In-ternational Seismological Summary or listed in theBulletin du Bureau Central International de Seis-mologie. The * are computer-determined solutionsby the Coast and Geodetic Survey beginning with1961. Despite some apparent spread in the olderdata, this plot shows great improvement in qual-

ity of data and resolution of seismic zones.

The second episode, apparently a solitaryevent, occurred on September 17, 1968at 20 hr 47 min 26.4 sec at 63.0°S. 60.8°W.with a magnitude of 4.9. At this time, noother events could be identified from thereported data as originating from thissource, nor were there any reports ofvolcanic activity.

The third episode was marked by a largeearthquake on February 21, 1969 at 06 hr32 min 23.5 sec at 62.9°S. 60.2 0 W. withmagnitude 5.2. Antarctic stations have notreported seismic signals consistent withany other seismic events in this episode. Re-ports from the British and Argentine baseson Deception Island, which had been re-occupied this austral summer, indicatedthat the tremor began on February 14, in-creasing in intensity until eruption oc-curred on February 21 at 00 hr 43 mm.This earthquake was recorded by stationsin all parts of the world.

The earthquake occurred on a poorlydefined zone that extends down the em-bayed coast of Chile and across the south-ern limb of the Scotia Arc to the mid-Atlantic seismic zone. It may have beenlarge enough to have produced usableRayleigh waves at several antarctic stationsfrom which average structure may be in-ferred from the group velocity.

September—October 1969 213