Late Cenozoic Stratigraphy of the Southern Terror Rift, Antactica: Implications for Tectonic and Climatic Evolution

Abstract

This thesis presents an integrated analysis of late Cenozoic (last 14 Ma) glacimarine stratigraphy within the Terror Rift in southern McMurdo Sound, Antarctica. The study area is located in the Windless Bight region of the McMurdo Ice Shelf (NW corner of the Ross Ice Shelf), which overlies a 600 to 1000 m-deep bathymetric moat surrounding Ross Island in the depocentral axis of the Victoria Land Basin (VLB). The VLB, one of a number of extensional sedimentary basins within the West Antarctic Rift System, comprises a 7-km-thick succession of syn- and post-rift glacimarine sediments that accumulated during the Oligocene and Miocene periods, respectively. Renewed rifting, known as the Terror Rift, in the centre of the VLB began between 17 to 14 Ma and has accommodated as much as one third of the entire Cenozoic basin-fill. The development of rift-related alkalic volcanoes associated with the Erebus Volcanic Province from c. 5 Ma has produced localised flexural basins/moats around Ross Island that have preserved a thick Pliocene-Pleistocene stratigraphic record. A new stratigraphic architecture is presented for southern Terror Rift based on: (1) the acquisition, processing and interpretation of 73 kilometres of over-ice shelf multi-channel seismic reflection data; (2) correlation of seismic stratigraphy with the integrated chrono-, litho- and cyclostratigraphy of the 1285 m-deep ANDRILL McMurdo Ice Shelf (MIS) Project drill core (AND-1B) using a synthetic seismic-well tie, and time-depth data from a vertical seismic profile (VSP). Five seismic units bounded by regionally-mappable unconformities, which thicken into the centre of the basin, are identified and their relationship to the existing seismic stratigraphy of western VLB (Fielding et al. 2007) is established. In addition, a further 17 mappable seismic surfaces bounding higher-frequency depositional units are identified. The seismic units are interpreted on the basis of characteristic features, seismic facies, and correlations with the lithostratigraphy of the core. 1). Rg (light green) surface is the oldest regionally-mappable reflector. It directly overlies a redeposited volcanic unit sampled by the AND-1B core and yielded an age of ~13.8 Ma, dated using the 40Ar/39Ar method. The unconformity is undulating, dislocated by normal faults, truncates underlying strata, and is characterised by stratal onlap above. The 500 m-thick interval of Late Miocene strata between surface Rg (1260 metres below sea floor (mbsf) in the AND-1B core) and surface Rh (760 mbsf) comprises seismic Unit M1. Within Unit M1 a prominent reflector Rg1 (1100 mbsf) separates a subjacent, c. 160 m-thick interval of diamictite-dominated glacial-interglacial sedimentary cycles deposited from a proximal, predominantly-grounded, cold polar-style ice sheet, from an overlying c. 250 m-thick interval diamictite-mudstone-dominated cycles representing a warmer, wet-based more dynamic ice sheet in the western Ross Sea. The considerable thickness of Unit M1, its association with normal faulting, and correlation with the Rg surface in western VLB implies that this unit is associated with renewed rifting and initiation of the Terror Rift. However the unit may have a polyphase origin as the erosion associated with the Rg1 unconformity and the overlying dry-based, polar glacial deposits also correspond to a major period of global cooling and inferred Antarctic ice sheet expansion at 13.8 Ma, in marine oxygen isotope records (the Mi-4 glaciation). 2). Rh (dark green) surface is also characterised by truncation of underlying strata and onlap by overlying strata, is also dislocated by normal faulting, and marks the base of a c. 150 m-thick unit of proximal sub-marine volcanic deposits. Within the study area, Rh (~770 mbsf) is correlated with the base of White Island basaltic deposits dated at c. 7 Ma. Unit M2 comprises in its lower interval the proximal volcanic material associated with the submarine apron of White Island volcano, which is overlain at the Rh1 seismic reflector (600 mbsf), of alternating cycles of ice proximal diamictite and open marine diatomite and terrigenous mudstone. Unit M2 represents continued rift-related subsidence and sedimentation based on its association with syn-depositional normal-faulting and volcanism. The core stratigraphy implies that a wet-based margin of a dynamic Antarctic ice sheet oscillated across the study area during the Late Miocene (13.8-7 Ma). 3). Ri (red) surface is correlated with the "b-clino" surface in western VLB and marks regional subsidence possibly associated with marine transgression. In the study area the surface is correlated with the onset of load-induced sea-floor subsidence associated with the emplacement of Mt. Bird (e.g. 4.6 Ma). It is generally characterised as a downlap surface and mildly truncates underlying strata. The lower c. 80 m of the c. 180 m-thick, overlying seismic Unit M3 is dominated by seismically-homogenous diatomite representing up to 300 ka of open-marine deposition in western Ross Sea. The upper part of Unit M3 corresponds to cycles of diatomite and diamictite interpreted as successive glacial-interglacial advances and retreats of the grounding-line. Noteworthy is the coincidence of a dynamic ice margin and periodically open Ross Sea with global warmth of the Early and middle Pliocene period. 4). Rj (turquoise) surface is scoured and undulating, and is marked by underlying stratal truncation. Overlying Unit M4 (c. 300-150 mbsf) comprises five distinct Milankovitch-scale, glacial-interglacial diatomite-diamictite cycles that can be mapped across the entire study area and are coincident with Late Pliocene cooling and expansion of ice sheets: both on the Northern hemisphere and in the Ross Embayment. The Rj surface has an age c. 3 Ma and is coincident with expansion of grounding-lines elsewhere on the Antarctic margin onto the continental shelf. This surface is also correlated with the beginning of a phase of significant cone building on Ross Island that promoted the local creation of accommodation space and preservation of strata. 5). Rk (pink) surface represents an unconformity dated at c. 2 Ma. It is expressed by pronounced truncation of underlying strata and onlap by strata above. Overlying Unit M5 (c. 145-0 mbsf) is characterised by diamictite dominated cycles and a return to a cold, polar, dry-based ice sheet with conditions similar to today.