ABSTRACT. Several species of fossils that indicate an age almost precisely at the Cambrian-. Ordovician boundary were collected from a limestone olistolith ...
Newly discovered youngest Cambrian or oldest Ordovician fossils from the Robertson Bay terrane (formerly Precambrian), northern Victoria Land, Antarctica Thomas 0. Wright Earth S c i e n c e s Division, National S c i e n c e Foundation, Washington, D.C. 2 0 5 5 0
Reuben J. Ross, Jr. D e p a r t m e n t of G e o l o g y , C o l o r a d o S c h o o l of Mines, Golden, C o l o r a d o 80401
John E. Repetski U.S. G e o l o g i c a l Survey, National M u s e u m of Natural History, Washington, D.C. 2 0 5 6 0
ABSTRACT Several species of fossils that indicate an age almost precisely at the CambrianOrdovician boundary were collected from a limestone olistolith found in turbidite sandstone and shale from Handler Ridge (lat 72°30.5'S, long 167°03'E), an area previously mapped as late Precambrian Robertson Bay Group. The rocks enclosing the olistolith are therefore of this age or younger, not Precambrian, and their presence in this position has important implications to the early Paleozoic history of the Transantarctic Mountains.
INTRODUCTION Here we report and document several invertebrate fossils found during the 19821983 German Antarctic Northern Victoria Land Expedition (GANOVEX) III. Except for one of the conodonts, none of these fossils has been previously reported from the Antarctic continent, but similar faunas are known from several places around the world; they are near the Cambrian-Ordovician transition in age. Antarctica plays a key role in reconstructions of Gondwana, with the Transantarctic Mountains (Fig. 1) generally following the Pacific margin of the old supercontinent. The Precambrian and older Paleozoic geology of the Transantarctic Mountains records compressive tectonic activity for most of this time (Craddock, 1982). In several places, thick piles of turbidites were deposited during the Precambrian and deformed during the late Precambrian Beardmore orogeny. Later, a fossiliferous Cambrian sequence was deposited and was subsequently deformed in the widespread Late Cambrian-Ordovician Ross orogeny. Tectonic compression may have continued in northern Victoria Land during the Silurian-Devonian Borchgrevink orogeny. No marine Ordovician rocks have been described from Antarctica, and the Ordovician was thought to be a time of uplift and orogeny. GEOLOGY, v. 12, p. 301-305, May 1984
The geology of northern Victoria Land (Fig. 1) has received considerable attention in the past several years (Wright, 1981; Laird et al., 1982; Bradshaw et al., 1982; Cooper et al., 1983; etc.). Prior to the discovery of the fossils described here, the Robertson Bay Group (a quartzose turbidite and hemipelagic shale unit) was thought to be late Precambrian (Wright, 1981) or possibly Cambrian (Field and Findlay, 1983), but fossil evidence was limited to acritarchs and trace fossils found at only a few widely spaced locations. The Bowers Supergroup contains a lower volcanic and volcanogenic sedimentary rock unit (Sledgers Group), a middle shallowmarine sequence of clastics and limestone (Mariner Group), and an upper, mainly fluvial, quartzose sandstone and conglomerate unit (Leap Year Group). The ages of the two lower units are largely Middle Cambrian and Middle Cambrian to Late Cambrian, respectively. The Leap Year Group has only sparse trace fossils and may be Late Cambrian or possibly Ordovician in age (Cooper et al., 1983). The Bowers Supergroup has not been reported east of the Leap Year fault, nor has the Robertson Bay Group been reported west of it except for possible correlatives in the Rennick Glacier area. The geologic relationships between the Bowers Supergroup rocks west of the Leap Year fault and the Robertson Bay Group east of it are not well known
(Tessensohn et al., 1981; Grindley and Oliver, 1983) and have led to the concept of "tectonic zones" or terranes. A more complete report of the findings of the 1982-1983 expedition is in preparation, and a discussion of the tectonic significance of this material will be included in the GANOVEX special volume of the Geologisches Jahrbuch of the German Federal Republic Geological Survey (BGR). HANDLER RIDGE LOCALITY Handler Ridge and the fossil site (Fig. 1) are east of the Leap Year fault and within the Robertson Bay terrane. The lithology in the vicinity of the fossil site consists of gray shale and dark, immature sandstone, some beds of which have high amounts of calcite cement, and several conspicuous 10-cm to 2-m-thick allochthonous blocks of light-gray limestone of shallow-shelf origin. The blocks of limestone are restricted to a 30-m-thick horizon in which severe bed disruption has occurred, most likely caused by soft-sediment slumping rather than tectonic processes. The presence of these carbonate blocks in the turbidite and hemipelagic shale sediments implies downslope movement from a carbonate shelf; however, no appropriate shelf of this age has been recognized in northern Victoria Land or elsewhere in Antarctica, and the transport distance is unconstrained. The blocks were of 301
several lithologies, and two contained recognizable fossils. The fossils reported here came from a small block measuring about 30 x 30 >< 15 cm. It was about 2 m from another, larger limestone block having visible pelmatozoan columnals. The limestone olistoliths are surrounded by a shale matrix and are fairly well rounded. They show little internal deformation. The small fossil-containing block is a calcite micrite-supported wackestone with sand-size particles consisting of lime-mud pellets, quartz sand grains, Nuia (a presumed alga of uncertain affinities; Toomey, 1970), and
Figure 1. Sketch map showing geology of northern Victoria Land and location of Transantarctic Mountains. V pattern indicates Tertiary and Holocene volcanic rocks; diagonal-line pattern represents Admiralty intrusive rocks; cross-hatch pattern represents Granite Harbor intrusive rocks.
minor chert and metamorphic lithic fragments. The micrite is about 70% of the rock. The trilobites and other fossils were clearly concentrated and deposited by bottom currents—they are mostly packed in several zones in the block, but some large trilobite fragments are randomly oriented. The fossils,
although disarticulated, are not significantly abraded. AGE OF THE COLLECTION Trilobites The collection includes at least four genera of trilobites (Fig. 2)—Harpides or a close rela-
Pacific Ocean
Cape Adare
Figure 2. Selected trilobites from Handler Ridge, Antarctica. A: Saukiid cranidium, x4, lacking glabellar lobe Ip and occipital ring, USNM 374066. B: Saukiid? pygidium, x4, latex replica, lacking axis, USNM 374067. C: Saukiid? pygidium, x4, latex replica of small specimen showing five axial rings and left margin, USNM 374068. D: Saukiid free cheek, *2, lacking eye and much of anterior genal platform and border, USNM 374069. E: Tsinania cf. T. nomas, pygidium, x2, USNM 374070. F: Tsinania sp., free cheek, x2, USNM 374071. G: Tsinania cf. T. nomas, pygidium, x2, USNM 374072. H: Tsinania cf. T. neiopina, pygidium, x2, USNM 374073.1: Pseudokainella sp., anterior of glabella and pregtabellar field, x3, USNM 374074. J: Pseudokainella sp., cranidium, x4; right palpebral rim missing; USNM 374075. K: Pseudokainella sp., free cheek, x8, USNM 374076. L: Harpidid, partial cephalon, >
