Volcanic Rocks of the Ross Island Area

At least two periods of deformation are recogniza- ble in the area. Earlier isoclinal folds have been re- folded around tight folds with northwest-trending axes. Bouguer gravity anomalies range from —10 mgal along the coast to —150 mgal 70 km inland. The anomaly values reach —100 mgal at Lake Vida and just west of The Flatiron at Granite Harbor. A large gravity gradient of 6 mgal/km parallels the coast just inland from McMurdo Sound. The high gradient suggests a shallow source, but the source has not been found in the surface exposures. Volcanic Rocks of the Ross Island Area SAMUEL B. TREVES C NX. A - Department of Geology Photomicrograph of anorllwcla e trachyte fromiI Cape Royds. University of Nebraska and Institute of Polar Studies Ohio State University During the 1960-1961 and 1961-1962 field seasons, the volcanic rocks of the Capes Evans and Royds and McMurdo Station areas were studied and mapped (Treves, 1962). During the 1964-1965 season, a study of the entire Ross Island Volcanic Province was started; for that study, the author had proposed that sequential relationships, geologic history, mineralogy, petrography, chemistry, and petrology of the volcanic rocks be investigated. Rocks were studied, mapped, and collected for petrographic, chemical, and isotopic analyses near McMurdo Station, at Capes Evans, Royds, Bird, and Crozier, and at Minna Bluff, Black Island, White Island, Marble Point, Taylor Valley, Heald Island, Brown Peninsula, the Dailey Islands, of plagioclase basalt from Cape Royds. Tent Island, Inaccessible Island, the Dellbridge Photomicrograph Islands, Cape Barne, Turks Head, Tryggve Point, Field and petrographic data indicate that older and Big and Little Razorback Islands, Mount Discovery, younger basalts occur at Cape Royds. The relation- and Hut Point. During the 1967-1968 field season, ship of the trachyte at Mount Cis (which is really additional work was done on the volcanic rocks at only an ice-cored mound) to the anorthoclase Capes Royds, Evans, and Crozier, Black Island, trachyte is not clear. White Island, Brown Peninsula, Minna Bluff, Miers Chemical, isotopic, and petrographic studies of the Valley, and in the general area of the Royal Society rocks are continuing. Future investigations should be Range. concerned with the geology of the high peaks of Ross In general, field results obtained last season agree Island, the volcanic rocks of Mounts Discovery and with those of earlier years. Almost everywhere, an Morning, the subprovinces of the McMurdo volcanic older olivine basal t-basal t- trachyte series and a young- area, the relationship of the volcanic rocks to the er olivine basalt-basalt series occurs. The author crustal structure of the area (especially to the struc- believes that the antarctic kenyte (Treves, 1962) is ture indicated by recent gravity maps), and the re- a trachyte equivalent and that it should be called an lationship of volcanic vents to the gross structural anorthoclase trachyte, an opinion that is apparently features of the nearby Transantarctic Mountains. shared by Boudette and Ford (1966). A radiometric age determination of the anorthoclase trachyte at Cape Royds gave a value of 0.68 (±0.14) X 10 11 my. Unofficial name. 108 ANTARCTIC JOURNAL References Ford, A. B. and E. L. Boudette. 1968. On the staining of anorthoclase. American Mineralogist, 53: 332-334. Treves, Samuel B. 1962. The Geology of Cape Evans and MISTHOUND COAL MEASURES Cape Royds, Ross Island, Antarctica. In: American Geo- PERMIAN physical Union, Geophysical Monograph no. 7, p. 40-46. 65m UNCONFORMITY Sedimentological Aspects of the Darwin Tillite in the Darwin Mountains LAWRENCE A. FRAKES and JOHN E. MARZOLF Department of Geology University of California (Los Angeles) T. W. GEVERS University of the Witwatersrand 45 Johannesburg, South Africa Z and 40 U LLOYD N. EDWARDS and JOHN C. CROWELL 0 Subaqueous mass-movement Department of Geology 0/AS TEM University of California (Santa Barbara) 35 U The sedimentology of the Darwin Tillite was Ripple marks, sole marks U 30 studied during a visit to the Darwin Mountains of I- southern Victoria Land in December 1967. The I- 0 examination of exposures there is critical to our un- derstanding of the Late Paleozoic glaciation of Ant- 25_ Subaqueous moss -movement arctica because the rocks are the northernmost ones a known to crop out in the Ross Sea sector, with the Z r exception of thin diamictite of the dry-valley region pc^^ 20 (Pinet et al., 1967). The purpose of the work was to a determine the facies of the glacial rocks, as well as Stumped blocks the direction of glacial transport. /5 The stratigraphy of the Darwin Tillite and its relationships to adjoining rocks are shown in the figure. As noted by Haskell et al. (1965), the Darwin Tillite /0 unconformably overlies the Devonian(?) Hatherton Sandstone. The relationship is well exposed on Colos- seum Ridge near 79°45S. 156°30E., where gray 5 sandstone containing abundant slump structures over- Slumped sandstone lies quartz sandstone which weathers to a light brown color. As in other areas, angularity at the contact is 0— UNCONFORMITY not observable. The top of the Darwin Tillite is sharply truncated by an erosional unconformity on which rests the basal quartz sandstone and conglom- HATHERTON SANDSTONE erate of the Misthound Coal Measures. Fossils were (UPPER MEMBER) not observed in these units during the present study, EARLY CARBONIFEROUS-LATE DEVONIAN with the exception of trace fossils in the Upper Mem- ber of the Hatherton Sandstone. Haskell et al. (1965) reported Gangamopteris and other plant remains from the Misthound and assigned the unit a Permian July-August 1968 109.

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