Turnbull, I.M.; Allibone, A.H.; Forsyth, P.J.; Heron, D.W. 1994 Geology of the Bull Pass-St Johns Range area, southern Victoria Land, Antarctica, scale 1:50 000 (flat map). Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 14 52 p. + 1 map
Abstract: The Bull Pass - St Johns Range map covers an elongate area from the central Wright Valley, north through the upper Victoria Valley, to the Mackay Glacier. It includes the eastern Olympus and Insel ranges, the St Johns Range, and the eastern Clare Range. The intervening valleys have been extensively glaciated since the Miocene, with numerous alpine glaciers remaining. The Cotton, Miller, Frazier and Mackay glaciers still occupy major valleys in the north. Valley sides and floors are underlain by basement plutonic and metamorphic rocks of Precambrian to Ordovician age, largely mantled by glacial, fluvioglacial, and periglacial deposits. Small remnants of Devonian sediments are preserved above the basement rocks, and form a more extensive area on the Olympus Range. Basement rocks, and the sediment-basement contact, are intruded by extensive sills of Jurassic dolerite which form spectacular cliffs in many places. The oldest rocks in the map area are multiply deformed Koettlitz Group comprising Hobbs Formation schists, amphibolites and gneisses, and Salmon Marble gneisses, amphibolites, and marbles. Mineral assemblages indicate mid to upper amphibolite facies metamorphism. Intercalated hornblende-biotite, biotite and gabbroic to dioritic orthogneisses were deformed and metamorphosed with the Koettlitz Group. Younger plutons forming the bulk of the Granite Harbour Intrusives invade the Koettlitz Group and intercalated orthogneisses. Major plutons and dike swarms, from oldest to youngest, are Bonney (monzodiorite - granodiorite), Wheeler (quartz diorite to quartz monzodiorite), Valhalla, Suess and St Johns (all granodiorite to granite), biotite granodiorite and granite dikes and plugs, Vanda dike swarms, and Orestes (granite), Swinford (granite), and Harker (granite). The gabbroic to dioritic Packard Pluton contains orthogneiss rafts, and is cut by Vanda mafic dikes and forms rafts in Orestes Pluton, but its age is otherwise unconstrained. The Granite Harbour Intrusives were emplaced over a period from 501 to 477 Ma (Early Ordovician). The regionally extensive Kukri Erosion Surface truncates all basement lithologies, and dips gently northwest. Up to 77 m of relief is recorded in fossil channels, and a major high on the surface is preserved in the St Johns and Clare ranges. Lower Taylor Group, Beacon Supergroup sediments overlie the Kukri Erosion Surface. On Mt Jason, four formations of predominantly quartzose sandstone are recognised. These are absent in the Clare Range, where the two overlying formations rest on basement. Undifferentiated Taylor Group sediments occur on the St Johns Range, where a polymict conglomerate formation is also mapped. Two sills of dolerite (Ferrar Dolerite Group) occur throughout the map area, one of which has disrupted and rafted the Beacon Supergroup sediments. Dolerite dikes may be partly fault-controlled. Diamicton (Jason Glaciomarine Diamicton, Onyx Pond Diamicton, Heimdall Glaciomarine Diamicton, Prospect Formation) in the central Wright Valley indicates at least three periods of fiord development between about 9 and 3 Ma (Miocene and late Pliocene). Interglacial expansion of the inland ice sheet resulted in extensive till deposits throughout the map area (Peleus Till, Insel Drift, Bull Drift, and higher altitude deposits adjacent to the Mackay, Frazier and Cotton glaciers). Cool glacial Ross Sea ice advances produced Ross Sea Drift in the lower Victoria Valley and at lower elevations adjacent to the Mackay, Frazier and Cotton glaciers. Local alpine glacier expansions (also interglacial) resulted in Alpine Drift, and the Vida and Packard drifts in the upper Victoria Valley. Periglacial processes have modified extensive areas of till, producing rock glaciers and gelifluction lobes, and talus mantles much of the area. Meltwater has locally resulted in fan development and extensive alluvial plains in the Wright Valley, and isolated lake benches.
