Depositional history of pre-Devonian strata and timing of Ross orogenic tectonism in the central Transantarctic Mountains, Antarctica Paul M. Myrow* Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA Michael C. Pope Department of Geology, Washington State University, Pullman, Washington 99164, USA John W. Goodge Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA Woodward Fischer Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA Alison R. Palmer Institute for Cambrian Studies, 445 North Cedarbrook Road, Boulder, Colorado 80304-0417, USA ABSTRACT small fraction of its previous extent. The into trilobite- and hyolithid-bearing calcar- basal unit of the Byrd GroupÐthe predom- eous siltstone of the Starshot Formation A combination of ®eld mapping, detailed inantly carbonate ramp deposits of the and alluvial-fan deposits of the Douglas sedimentology, carbon isotope chemostra- Shackleton LimestoneÐrest with presumed Conglomerate. Trilobite fauna from the tigraphy, and new paleontological ®nds unconformity on the restricted Goldie For- lowermost siltstone deposits of the Starshot provides a signi®cantly improved under- mation. Paleontological data and carbon Formation date the onset of this transition standing of the depositional and tectonic isotope stratigraphy indicate that the Low- as being late Botomian. history of uppermost Neoproterozoic and er Cambrian Shackleton Limestone ranges The abrupt transition from the Shackle- lower Paleozoic strata of the central Trans- from lower Atdabanian through upper ton Limestone to a large-scale, upward- antarctic Mountains. On the basis of these Botomian. coarsening siliciclastic succession records data, we suggest revision of the existing This study presents the ®rst description deepening of the outer platform and then stratigraphy, including introduction of new of a depositional contact between the deposition of an eastward-prograding mo- formations, as follows. The oldest rocks ap- Shackleton Limestone and overlying clas- lassic wedge. The various formations of the pear to record late Neoproterozoic deposi- tic units of the upper Byrd Group. This upper Byrd Group show general strati- tion across a narrow marine margin under- carbonate-to-clastic transition is of critical graphic and age equivalence, such that lain by Precambrian basement. Siliciclastic importance because it records a profound coarse-grained alluvial-fan deposits of the deposits of the Neoproterozoic Beardmore shift in the tectonic and depositional history Douglas Conglomerate are proximal equiv- GroupÐhere restricted to the Cobham of the region, namely from relatively pas- alents of the marginal-marine to shelf de- Formation and those rocks of the Goldie sive sedimentation to active uplift and ero- posits of the Starshot Formation. Paleocur- Formation that contain no detrital compo- sion associated with the Ross orogeny. The rents and facies distributions from these nents younger than ca. 600 MaÐoccupied uppermost Shackleton Limestone is capped units indicate consistent west (or southwest) an inboard zone to the west. They consist by a set of archaeocyathan bioherms with to east (or northeast) transport of sediment. of shallow-marine deposits of an uncertain up to 40 m of relief above the sea¯oor. A Although the exact structural geometry is tectonic setting, although it was likely a rift widespread phosphatic crust on the bio- unknown, development of imbricate thrust to passive margin. Most rocks previously herms records the onset of orogenesis and sheets in the west likely caused depression mapped as Goldie Formation are in fact drowning of the carbonate ramp. A newly of the inner margin and rapid drowning of Cambrian in age or younger, and we reas- de®ned transitional unit, the Holyoake For- the Shackleton Formation carbonate ramp. sign them to the Starshot Formation of the mation, rests above this surface. It consists This tectonic activity also caused uplift of Byrd Group; this change reduces the ex- of black shale followed by mixed nodular the inboard units and their underlying posed area of the Goldie Formation to a carbonate and shale that ®ll in between, basement, unroo®ng, and widespread de- and just barely above, the tallest of the position of a thick, coarse clastic wedge. *E-mail: [email protected]. bioherms. This formation grades upward Continued deformation in the Early Ordo- GSA Bulletin; September 2002; v. 114; no. 9; p. 1070±1088; 16 ®gures. For permission to copy, contact [email protected] 1070 q 2002 Geological Society of America PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA Figure 1. Geologic map of central Transantarctic Mountains with inset (upper left) of Antarctica showing location of ®eld area. CPÐ Cotton Plateau, CRÐCobham Range, MUÐMount Ubique, PAGÐPrincess Anne Glacier, and PPÐPanorama Point. Close-up of Holy- oake Range on right shows basic geologic units. Details of thrust slices of transitional units cannot be shown at this scale, but rectangular blocks indicate locations where upper Shackleton through Douglas transitions occur, each with well-developed bioherms that all dip vertically and face west. S1 (section 1), S2 (section 2), and HR (High Ridge) are described in text. vician (younger than 480 Ma) in turn af- because of a lack of biostratigraphic con- Formation. Fossil-bearing strata of the Byrd fected these synorogenic deposits, causing straints, detailed sedimentology, and physical Group are generally considered to have been folding and thrust repetition of all pre- stratigraphy of these units. The traditional deposited unconformably upon the Goldie Devonian units. view of stratigraphic relationships in this re- Formation. The basal formation of the Byrd gion, as reviewed by Stump (1995) and dis- Group, the Lower Cambrian Shackleton Lime- Keywords: Archaeocyatha, Cambrian, Neo- cussed in more detail herein, is that basement stone, consists of dominantly carbonate rocks proterozoic, reef, Ross orogeny, Transant- rocks of the Nimrod Group (Archean and Ear- with a range of facies including archaeocyathan- arctic Mountains. ly Proterozoic) are directly overlain by Neo- bearing bioherms. Clasts of the Shackleton proterozoic rocks of the Beardmore Group, al- Limestone occur within overlying synorogenic INTRODUCTION though this relationship has not been conglomerate- and sandstone-rich units of the observed. The latter consists of unfossiliferous Douglas and Starshot Formations, respective- Stratigraphic relationships of pre-Devonian metamorphosed siliciclastic and carbonate ly. The stratigraphic relationships and ages of formations in the central Transantarctic Moun- rocks of the Cobham Formation and overlying these younger units were poorly de®ned; pos- tains (Fig. 1) have been the subject of much less metamorphosed siliciclastic rocks (mostly sible ages range from latest Early Cambrian conjecture and debate (Fig. 2), in large part sandstone with minor shale) of the Goldie to Devonian (Fig. 2). Geological Society of America Bulletin, September 2002 1071 MYROW et al. Figure 2. Interpretations of stratigraphic relationships of pre-Devonian Formations in the central Transantarctic Mountains. Right-hand column is revised view of these relationships. Formations: StÐStarshot Formation, ShÐShackleton Limestone, DoÐDouglas Conglom- erate, DiÐDick Formation, GoÐGoldie Formation. Authors: AÐLaird (1963), BÐSkinner (1964), CÐLaird et al. (1971), DÐLaird (1981), EÐRowell et al. (1986), FÐRowell et al. (1988b), GÐRees et al. (1989), HÐRowell and Rees (1989), IÐRees and Rowell (1991), JÐGoodge et al. (1993) and Goodge and Dallmeyer (1996), KÐGoodge (1997). For reports simply describing ``Lower Cambrian'' strata, base of formation arbitrarily chosen as upper Tommotian. New geochronologic data recast the age Group, and these ``outboard'' Goldie strata are of the upper Byrd Group from the southern and stratigraphic relationships of these units. now assigned to that formation. A very nar- Holyoake Range. Sedimentological aspects of Detrital-zircon geochronology on sedimentary row exposure of enigmatic, generally more this transition from massive, archaeocyathan units of the region indicates that the bulk of metamorphosed, rocks in the vicinity of Cot- bioherms to overlying siliciclastic-dominated outcrops mapped as Goldie Formation are in ton Plateau and the eastern Cobham Range strata allow us to reconstruct the depositional fact younger than the Shackleton Limestone were also originally mapped as Goldie For- response to Ross orogenesis. In addition, a new (Goodge et al., 2000, 2002). Detrital zircons mation. These have older detrital-zircon sig- trilobite collection in strata above the bioherms from these sandstone deposits are as young as natures (1000 Ma or older) and probably rep- provides constraints on the ages of several for- ca. 520 Ma, making their depositional age resent bona ®de Neoproterozoic deposits mations and the timing of tectonic events. younger than the Atdabanian Stage of the (``inboard assemblage'' of Goodge et al., These new ®ndings, in conjunction with the Lower Cambrian (Landing et al., 1998; Bow- 2000, 2002). geochronological data already mentioned, have ring and Erwin, 1998; Fig. 3). These young This study provides new ®eld, fossil, and profound implications for the depositional and provenance ages come from strata in an are- isotopic data as the basis for additional strati- tectonic history of the Transantarctic Moun- ally