This raises the question of what the food of the antarctic The Fremouw Formation of the Cumulus Hills represents proto-lizard may have been. Gow (1975), in his monograph a small part of a large river system that probably drained the on Prolacerta from South Africa, suggested that this reptile fed Gondwanian orogenic belt (Elliot, 1975). Streams in the upon "the young of many small synapsids [mammal-like rep- Cumulus Hills area flowed toward the southwest from a tiles] procolophonids and labyrinthodonts [amphibians], source terrane that included volcanic, metamorphic, and as well as insects." Gow also suggests that Prolacerta—"a light, sedimentary rocks. Rocks in the lower and middle Fremouw swift creature"—may have been to some extent arboreal. Ap- are interpreted to represent broad, relatively straight, sand- plying these considerations to the presumed Prolacerta in Ant- bed variable discharge rivers separated by well-vegetated arctica, we can speculate on the environment of this part of flood plains. The upper Fremouw represents a system of Gondwanaland in early Triassic time. We know there were braided streams in which the flood-plain facies is poorly trees and other forms of vegetation. There must have been in- represented. This change was possibly related to renewed sects, for the antarctic proto-lizard is of such small size that uplift or increased volcanism in the source area or to climatic insects probably formed a large part of its diet. But we still change. have much to learn about the fauna and flora of the Fremouw The Fremouw Formation in the Cumulus Hills is approx- Formation. imately 700 meters thick (Kitching et al., 1972) and can be The study of Prolacerta from Antarctica was supported in divided into 3 informal members, although thicknesses of part by National Science Foundation grant DPP 75-23136. these members vary greatly from locality to locality. The lower member consists of approximately equal amounts of ledge-forming sandstone and slope-forming mudstone; the middle member is predominantly slope-forming siltstone and References mudstone with varying proportions of ledge-forming sandstone; and the upper member is mostly ledge-forming sandstone. The lower member was studied at Thrinaxodon Colbert, E. H. 1975. Further determinations of Antarctic Triassic Col and Collinson Ridge, the lower and middle members at tetrapods. Antarctic Journal of the Us., 20(5): 250-252. the head of LaPrade Valley and Shenk Peak, and a part of Gow, C.E. 1975. The morphology and relationships of Youngina the upper member near the top of Mt. Kenyon. capensis Broom and Prolacerta broomz Parrington. Palaeontologia Two major facies occur in the Fremouw Formation, chan- Africana, 18:89-131. nel and flood-plain. Channel-lag deposits of conglomerate commonly occur above the basal erosion surfaces of channels and above large scour surfaces within sandstone units. Most clasts are locally derived mudstone chips, pebbles, and cob- bles, but quartz pebbles and vertebrate bones occur sporadically. Characteristic features of channels are large scours with single trough-fills [or cosets] of festoon cross- beds, which probably represent high-flow scour and fill Stratigraphy and sedimentary and/or fill at lower flow by linguoid dunes and bars (figure 1). Beds are commonly distorted by soft-sediment deforma- petrology of the Fremouw tion. Tops of some major scours and truncation surfaces are Formation (Lower Triassic), overlain by thin, silty mudstone drapes that are commonly burrowed and mud-cracked. Cumulus Hills, Central The flood-plain facies comprise: (a) fining-upward se- Transantarctic Mountains quences of greenish-gray, fine-grained argillaceous sandstone, siltstone, and mudstone; (b) tabular sheets of low angle cross-bedded and horizontally bedded, medium- grained sandstone (figure 2); and (c) fining-upward se- JAMES W. COLLINSON, KENNETH 0. STANLEY, and quences of red, sandy siltstone and mudstone. Fine-grained CHARLES L. VAVRA sandstone commonly exhibits parting lineation and small- Institute of Polar Studies and scale cross-bedding. Root casts and burrows are most abun- Department of Geology and Mineralogy dant in the greenish-gray rocks; only the tops of redbeds con- The Ohio State University tain root casts. Tabular sheets of medium-grained sandstone, Columbus, Ohio 43210 intercalated in the flood-plain facies, are characterized by bottom surfaces that are not scoured. In some cases sandstone has filled mudcracked surfaces on the underlying mudstone (figure 3). These sandstones represent rapid progradation of Sedimentologic investigations in the Cumulus Hills, com- sand across the flood plain during overbank flow. Their inter- bined with vertebrate paleontologic studies (Cosgriff et al., nal structures suggest lobate-bar, sand-delta, and sheet-sand 1978), are providing new information about Early Triassic deposition where the stream expanded onto the flood plain. paleogeography in Antarctica. A seven-man camp was Streams that deposited Fremouw sandstone in the established on the McGregor Glacier (175°W.85°S.) on 21 Cumulus Hills were of low sinuosity and flowed toward the November 1977 near the site of the 1970 camp. The three southwest. This is in contrast with the northwesterly sedimentologists left the field with the midseason supply paleocurrent directions reported by Barrett (1970) for the Fre- flight on 11 December 1977. Localities in the Cumulus Hills mouw Formation in the Queen Alexandra Range. Of 508 were reached by motor toboggan via LaPrade Valley or the cross-bed readings in the Cumulus Hills, the mean direction Shackleton Glacier. was 223°, s=±62°. In the lower and middle members, 427

October 1978 21 Figure 1. Channel fades in the lower Fremouw Formation at Figure 3. Mudcracks on bottom surface of sheet sandstone In Shenk Peak showing cross-bedded sandstone and large flood-plaIn fades. scour surface overlain by a pebble lag and filled with festoon cross-bedded sandstone. the source were pyroclastic flows. Post-depositional modifica tions to Fremouw sandstones include formation of cements, destruction and replacement of grains, and dissolution of grains. Authigenic cements and minerals that replace detrital grains include zeolites, prehnite, and phyllosilicates. The important volcanic components in sandstones of the lower Fremouw in the Cumulus Hills are not characteristic of the formation in the Queen Alexandra Range, where Barrett (1969) reported subarkose. However, in both areas the mid- dle and upper members are similarly dominated by volcanic lithic fragments. Our interpretations of the sandstone petrology in the Cumulus Hills differ from those of LaPrade (1970), who described these rocks as feldspathic graywackes. This research was supported by National Science Founda- tion grant DPP 76-23431.

References

Figure 2. Flood-plain fades at Shenk Peak In middle Fremouw Barrett, P. J 1969. Formation showing fining-upward, . Stratigraphy and petrology of the mainly fluviatile greenish-gray siltstone to Permian and Triassic Beacon rocks. Beardmore Glacier area. mudstone sequences and tabular sheets of medium-grained Report Institute Polar Studies, Ohio State University, 34. sandstone. Barrett, P. J. 1970. Paleocurrent analysis of the mainly fluviatile Per- mian and Triassic Beacon Antarctica: Journal of Sedimentary readings indicate a mean direction of 219, s= ±45°. Fewer Petrology, 40(1): 395-411. readings in the upper Fremouw (78) indicate a mean direc- Cosgriff,J. W., W. R. Hammer, J. M. Zawiskie, and N. R. Kemp. tion of 246°, s = ±36°. Most of the dispersion occurs in sepa- 1978. New Triassic vertebrates from the Fremouw Formation of rate but closely spaced sandstone units. Dispersal directions the Queen Maud Mountains. Antarctic Journal of the US., 13(4). are similar in flood-plain and channel facies. The variation in 23-24. direction from large- and small-scale cross-beds and from Elliot, D. H. 1975. Gondwana basins of Antarctica. In: Gondwana parting lineation is insignificant. Geology (K, S. W. Campbell, ed.). A.N.U. Press, Canberra. pp. 493-536. Channel and coarser flood-plain deposits of the Fremouw Kitching, J Formation are dominantly medium- to fine-grained, well . W., J. W. Collinson, D. H. Elliot, and E. H. Colbert. 1972. Lystrosaurus sorted volcanic sandstone. Detrital rock fragments, quartz, zone (Triassic) fauna from Antarctica. Science, 175(402): 524-527. feldspar, and heavy mineral grains indicate a source terrane LaPrade, K. E. dominated by felsic volcanic rocks with subordinate 1970. Permian-Triassic Beacon group of the Shackleton Glacier area, Queen Maud Range, Transantarctic metamorphic and sedimentary rocks. Relic shard and Mountains, Antarctica. Geological Society America, Bulletin, 81(5): pumaceous structures indicate that some of the volcanics at 1403-1410.

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