Of DSDP leg 55 basalts. Initial Reports of the Deep Sea Drilling Project, Land: Revised chronology and evaluation of tectonic factors. In C. 55, 639-652. Craddock (Ed.), Antarctic geosciences. Madison: University of Wis- Kyle, P. R. 1976, Geology, mineralogy, and geochemistry of the Late consin Press. Cenozoic McMurdo Volcanic Group, Victoria Land, Antarctica. Unpub- LeMasurier, W. E., and Rex, D. C. In preparation. Hyaloclastites and lished doctoral dissertation, Victoria University of Wellington. glacial history in the Hobbs Coast sector of Marie Byrd Land. LeMasurier, W. E., and Rex, D. C. In press. Volcanic record of Cen- McIntosh, W. C. 1981. Rock magnetism and paleomagnetism of antarctic ozoic glacial history in Marie Byrd Land and western Ellsworth hyaloclastites. Unpublished masters thesis, University of Colorado.

Hyaloclastite at Minna Bluff, this sequence consists almost entirely of lava flows, typically 2 meters to 6 meters thick, whereas the lower portion consists McMurdo Sound region of interbedded lava flows and hyaloclastite units ranging in thickness from 3 to 12 meters. The lowermost hyaloclastite unit is exposed approximately 15 meters above the level of the WILLIAM C. McINTosH adjacent Ross Ice Shelf. It rests on a glacially polished and striated erosional surface developed on the top of an under- Department of Geology lying lava flow, suggesting an origin involving subglacial vol- University of Colorado canic eruption. Clastic dikes, 15 centimeters wide and com- Boulder, Colorado 80309 posed of hyaloclastite, are present in the lower portion of the sequence. These dikes are similar to those reported from the PHILIP R. KYLE Hallett volcanic province (Hamilton 1972) and are evidence for postdepositional remobilization of unconsolidated hyaloclas- Institute of Polar Studies tite. The Ohio State University Additional outcrops of layered volcanic rocks are exposed in Columbis, Ohio 43210 a 15-kilometer-long series of cliffs along the southwestern edge of Minna Bluff. Field observations were insufficient to deter- mine the amount or distribution of hyaloclastite deposits in Minna Bluff is a 35-kilometer-long, 5- to 10-kilometer-wide, this area, but one hyaloclastite unit, 3 meters thick, was sam- 1,000-meter-high peninsula surrounded on three sides by the pled at an outcrop approximately 550 meters above sea level Ross Ice Shelf and located 72 kilometers due south of McMurdo (locality 4, figure). Station (figure). It adjoins Mount Discovery and, like Hut Point Peninsula, is composed of numerous volcanic deposits erupted from an aimed series of vents. A geological reconnaissance visit was made to Minna Bluff during January 1981 to search for occurrences of hyaloclastite. The search was prompted by a report by Hamilton (1972) that hyaloclastites were present in the area. Hyaloclastites are frag- mented volcanic rocks, resembling tuffs, which form by sub- 78°30 S aqueous eruptions. Formation may occur subglacially, there- fore hyaloclastites can be used to determine old ice levels (see, for example, Kyle in press and LeMasurier 1972). Minna Bluff presently acts as a barrier to direct movement of the Ross Ice Shelf through McMurdo Sound (Kyle in press). The time of formation at Minna Bluff is unknown, but is critical to an understanding of movement and possible erosional 78°37.5S effects of the Ross Ice Shelf in McMurdo Sound. Volcanic rocks, including lava flows, agglomerates, welded spatter, and hyaloclastites, were observed and sampled at four localities (figure). A sequence of more than 30 subhorizontally layered volcanic units is exposed in cliffs along the southeast- ern tip of Minna Bluff (locality 1, figure). The upper portion of 166°E 1670E Generalized map of Minna Bluff showing sample localities and present address: Department of Geoscience, New Mexico Institute of observed rock types. f = lava flow, h = hyaloclastite, a = Mining and Technology, Socorro, NM 87801. agglomerate, and s = welded spatter.

1981 REVIEW 27 Preliminary petrographic examination of rock samples from References Minna Bluff suggests bimodal basanite to phonolite compo- Hamilton, W. 1972. The Hallett volcanic province. U.S. Geological Sur- sitions, similar to volcanic rocks found elsewhere in the Erebus vey Professional Paper 456-B. volcanic province (Kyle 1976). Kyle, P. R. 1976. Geology, mineralogy, and geochemistry of the Late Cenozoic McMurdo Volcanic Group, Victoria Land, Antarctica. Unpub- It is probable that the lava flows at Minna Bluff were lished doctoral dissertation, Victoria University, Wellington, New extruded subaerially and that hyaloclastites were erupted in Zealand. subglacial environments produced by thickening of the Ross Kyle, P. R. In press. Glacial history of the McMurdo Sound area as Ice Shelf. indicated by the distribution and nature of McMurdo Volcanic Group Rocks. In L. D. McGinnis (Ed.), The Dry Valley Drilling Proj- Many thanks to John Schutt and Gary MacKenzie for field ect. Antarctic Research Series, American Geophysical Union. assistance in this project and to VXE-6 for helicopter support. LeMasurier, W. E. 1972. Volcanic record of Cenozoic glacial history of This research was supported by National Science Foundation Marie Byrd Land. In R. J . Adie (Ed.), Antarctic geology and geophys- grants DPP 77-21590 and DPP 79-20316. ics. Oslo: Universitetsforlaget.

Geochemistry of The amphibole of these rocks is kaersutite, which occurs as phenocrysts, micro phenocrysts, and as reaction rims on oliv- some rocks from Dry Valley ine. Locally, it is replaced, partially or completely, by opaque Drilling Project borehole 1, minerals. Hut Point Peninsula, Ross Island Olivine occurs as microphenocrysts and phenocrysts and is magnesium-rich. Glass is present in almost every unit. It is commonly charged with opaque minerals and ranges from brown to tan to clear. SAMUEL B. TREvEs Opaque minerals and apatite occur in all units. Rhönite Department of Geology (Kyle and Price 1975) occurs in unit 33 and perhaps in some of University of Nebraska the older units. Lincoln, Nebraska 68588 Petrography. Mineralogy and texture, the parameters ordi- narily used to define rock types and to assign rock names, cannot be used exclusively to name these rocks because most Dry Valley Drilling Project borehole 1 was drilled to the units contain a great deal of glass. The nomenclature must, northeast of McMurdo Station at the foot of Twin Craters therefore, be based, at least in part, on chemical analyses of (70°5045"S 166°4011"E), an extinct volcano on Hut Point Pen- the rocks. insula. Drilling began 21 January 1973 and was terminated 29 Geochemistry. Forty chemical analyses were made of these January 1973. During that time, 196.54 meters of core were rocks in the laboratories of the Japanese Geological Survey recovered. under the direction of Dr. Kurasawa (table 2). These analyses Detailed relogging of the core is now in progress. Data in show clearly that the rocks are alkaline. If the nomenclature hand indicate that the core consists of 31 or 32 flow units, a used by Goldich and others (1975) is used to name these rocks, paleosol, 7 or 8 pyroclastic units, and 2 dikes. The flows are most are trachybasalts, four are basanitoids, and one is a phon- generally thin (table 1) and slightly oxidized. Pyroclastic units olite. Trachybasalts of Goldich and others are resolved into are slightly thicker than the flows and consist of tuff, lapilli nepheline hawaiites, mugeantes, and nepheline benmoreites tuff, and breccia. The basal pyroclastic unit is probably a if Coombs and Wilkinsons (1969) nomenclature is adopted. hyaloclastite. Drilling was terminated in the hyaloclastite after The flows do not occur in any well-defined differentiation penetrating 52.52 meters; hence, its true thickness is not series, except that the basanites occur near the bottom of the known. The paleosol is less than 1 meter thick and consists pile and the intermediate rocks near the top. primarily of rounded fragments of basalts. The dikes are grey and, hence, distinctive. Contacts with flow units are sharp In conclusion, the rocks of this hole resemble the surface and inclined. Streaky-flow banding is common. flows of the Ross Island province described by Goldich and Mineralogy. The flow units consist primarily of plagioclase, others (1975). The AFM (A = alkalies; F = iron oxide; clinopyroxene, amphibole, olivine, and glass. Plagioclase M = magnesium oxide) plots (figure) show the similarity occurs as microphenocrysts and microlites and ranges in com- clearly. Hence, these rocks are also differentiations of a bas- position from bytownite to oligoclase (table 1). The clinopy- anitoid magma that probably was generated initially by partial roxene occurs as microphenocrysts in the groundmass, and as melting of the mantle. xenocrysts. Brown to pink titan-augite is most common; aegi- The most striking feature of the flows of this hole is the rine-augite occurs in the more sodic units. Xenocrysts consist abundance of intermediate rock types represented. In contrast, of diopsidic-augite and are commonly rimmed with titan- the surface flows of this province are primarily basanites and augite. phonolites.

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