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

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

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

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.</p

Heat Flow and Hydrologic Characteristics at the AND-1B borehole, ANDRILL McMurdo Ice Shelf Project, Antarctica

The Antarctic Drilling Program (ANDRILL) successfully drilled and cored a borehole, AND-1B, beneath the McMurdo Ice Shelf and into a flexural moat basin that surrounds Ross Island. Total drilling depth reached 1285 m below sea floor (mbsf) with 98 percent core recovery for the detailed study of glacier dynamics. With the goal of obtaining complementary information regarding heat fl ow and permeability, which is vital to understanding the nature of marine hydrogeologic systems, a succession of three temperature logs was recorded over a five day span to monitor the gradual thermal recovery toward equilibrium conditions. These data were extrapolated to true, undisturbed temperatures, and they define a linear geothermal gradient of 76.7 K/km from the seafloor to 647 mbsf. Bulk thermal conductivities of the sedimentary rocks were derived from empirical mixing models and density measurements performed on core, and an average value of 1.5 W/mK ± 10 percent was determined. The corresponding estimate of heat fl ow at this site is 115 mW/m2. This value is relatively high but is consistent with other elevated heat-fl ow data associated with the Erebus Volcanic Province. Information regarding the origin and frequency of pathways for subsurface fluid flow is gleaned from drillers’ records, complementary geophysical logs, and core descriptions. Only two prominent permeable zones are identified and these correspond to two markedly different features within the rift basin; one is a distinct lithostratigraphic subunit consisting of a thin lava fl ow and the other is a heavily fractured interval within a single thick subunit

Comprehensive downhole and core physical-property measurements at the AND-1B Drillsite, ANDRILL McMurdo Ice Shelf Project

As part of the ANDRILL McMurdo Ice Shelf (MIS) Project two comprehensive sets of geophysical data were collected on ice at the AND-1B drillsite. Whole-core physical properties were determined with high vertical resolution to a depth of 1285 mbsf. A multi-sensor-core-logger was used to determine bulk density, sonic velocity, magnetic susceptibility and electrical resistivity. After drilling, a set of downhole measurements was collected, which consisted of caliper, temperature, fluid conductivity, induction resistivity, magnetic susceptibility, natural gamma activity, acoustic televiewer, borehole deviation, and dipmeter. In addition, three vertical seismic profiles (VSP) were obtained. Physical properties were used for initial core characterization and on-site correlation with seismic modeling. Lithology and stratigraphic units are in good agreement with changes in the pattern of the physical properties. The resulting data are amenable to studies of cyclicity and climate, cementation and compaction history, heat flux and fluid flow, and structure and stress

Neogene tectonic and climatic evolution of the Western Ross Sea, Antarctica — Chronology of events from the AND-1B drill hole

Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87 m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600 m and 40Ar/39Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well‐dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre‐existing strata. The combined age model implies relatively rapid (1 m/2–5 ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin‐wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies: 1. The base of the cored succession (1275–1220 mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5 Ma by several reworked volcanic clasts; 2. A late-Miocene sub-polar orbitally controlled glacial–interglacial succession (1220–760 mbsf) bounded by two unconformities correlated with basin‐wide reflectors associated with early development of the terror rift; 3. A late Miocene volcanigenic succession (760–596 mbsf) terminating with a ~1 my hiatus at 596.35 mbsf which spans the Miocene–Pliocene boundary and is not recognised in regional seismic data; 4. An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (590–440 mbsf), separated from; 5. A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (440–150 mbsf) by a 750 ky unconformity interpreted to represent a major sequence boundary at other locations; 6. An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150–80 mbsf), and; 7. A late Pleistocene glacigene succession (80–0 mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment