Geology Reports⌕ Search

Geology topics

P. E. O’Brien

Publications and source records attributed to P. E. O’Brien.

8 recordsLinked to original sources
USGS2007

Mega debris flow deposits on the western Wilkes Land margin, East Antarctica

Multichannel seismic data collected off Western Wilkes Land (East Antarctica) reveal the occurrence of mega debris flow deposits on the lower slope and rise that were formed throughout the Miocene. Commonly, debris flow units are separated by thin deposits of well-stratified facies, interpreted as predominantly glaciomarine mixed contouritic and distal turbidite deposits. These units could act as weak layers and could have played a major role in the slope instability. High sedimentation rates, due to large amounts of sediment delivered from a temperate, wet-based ice sheet, constituted a key factor in the sediment failures. The main trigger mechanism would probably have been earthquakes enhanced by isostatic rebound following major ice sheet retreats.

Open-File Report↗
USGS2007

Tectonic elements of the continental margin of East Antarctica, 38-164ºE

The East Antarctic continental margin from 38–164ºE is divided into western and eastern provinces that developed during the separation of India from Australia–Antarctica (Early Cretaceous) and Australia from Antarctica (Late Cretaceous). In the overlap between these provinces the geology is complex and bears the imprint of both extension/spreading episodes, with an overprinting of volcanism. The main rift-bounding faults appear to approximately coincide with the outer edge of the continental shelf. Inboard of these faults, the sedimentary cover thins above shallowing basement towards the coast where crystalline basement generally crops out. The continental slope and the landward flanks of the ocean basins, are blanketed by up to 9–10 km of mainly post-rift sediments in margin-parallel basins, except in the Bruce Rise area. Beneath this blanket, extensive rift basins are identified off Enderby and Wilkes Land/Terre Adélie; however, their extent and detailed structures are difficult to determine.

Open-File Report↗
USGS2004

Prydz channel fan and the history of extreme ice advances in Prydz Bay

During the late Neogene, the Lambert Glacier-Amery Ice Shelf drainage system flowed across Prydz Bay in an ice stream that reached the shelf edge and built a trough mouth fan on the upper continental slope. The adjacent banks saw mostly subglacial till deposition beneath slower-moving ice. The fan consists mostly of debris flow deposits derived from the melting out of subglacial debris at the grounding line at the continental shelf edge. Thick debris flow intervals are separated by thin mudstone horizons deposited when the ice had retreated from the shelf edge. Age control at Ocean Drilling Program Site 1167 indicates that the bulk of the trough mouth fan was deposited prior to ~780 ka with as few as three debris flow intervals deposited since then. This stratigraphy indicates that extreme advances of the Lambert Glacier-Amery Ice Shelf system ceased during the mid-Pleistocene. Possible causes for this change are progressive over-deepening of the inner shelf, a reduction in maximum ice volumes in the interior of the East Antarctic Ice Sheet caused by temperature change, and a change in the interaction of Milankovich cycles and the response time of the ice sheet.

Prydz Bay↗
USGS2004

Synthetic seismograms linking ODP sites to seismic profiles, continental rise and shelf of Prydz Bay, Antarctica

Synthetic seismograms provide a crucial link between lithologic variations within a drill hole and reflectors on seismic profiles crossing the site. In essence, they provide a ground-truth for the interpretation of seismic data. Using a combination of core and logging data, we created synthetic seismograms for Ocean Drilling Program Sites 1165 and 1166, drilled during Leg 188, and Site 742, drilled during Leg 119, all in Prydz Bay, Antarctica. Results from Site 1165 suggest that coring penetrated a target reflector initially thought to represent the onset of drift sedimentation, but the lithologic change across the boundary does not show a change from predrift to drift sediments. The origin of a shallow reflector packet in the seismic line across Site 1166 and a line connecting Sites 1166 and 742 was resolved into its constituent sources, as this reflector occurs in a region of large-scale, narrowly spaced impedance changes. Furthermore, Site 1166 was situated in a fluvio-deltaic system with widely variable geology, and bed thickness changes were estimated between the site and both seismic lines.

Proceedings of the Ocean Drilling Program: Scienti↗
USGS2001

Leg 188 synthesis: Transitions in the glacial history of the Prydz Bay region, East Antarctica, from ODP drilling

Drilling during Leg 119 (1988) and Leg 188 (2000; Sites 1165–1167) of the Ocean Drilling Program (ODP) provides direct evidence for long- and short-term changes in Cenozoic paleoenvironments in the Prydz Bay region. Cores from across the continental margin reveal that in preglacial times the present shelf was an alluvial plain system with austral conifer woodland in the Late Cretaceous that changed to cooler Nothofagus rainforest scrub by the middle to late Eocene (Site 1166). Earliest recovered evidence of nearby mountain glaciation is seen in late Eocene–age grain textures in fluvial sands. In the late Eocene to early Oligocene, Prydz Bay permanently shifted from being a fluvio-deltaic complex to an exclusively marine continental shelf environment. This transition is marked by a marine flooding surface later covered by overcompacted glacial sediments that denote the first advance of the ice sheet onto the shelf. Cores do not exist for the early Oligocene to early Miocene, and seismic data are used to infer the transition from a shallow to normal depth prograding continental shelf with submarine canyons on the slope and channel/levees on the rise. Cores from the continental rise at Site 1165 show long-term (millions of years) early Miocene and younger decreases in sedimentation rates as well as short-term (Milankovitch periods) cyclicity between principally biogenic and terrigenous sediment supply—resulting from the cyclic presence of onshore glaciers and changes in ocean circulation. Middle Miocene transitions include rapid decreases in sedimentation rates, increased ice-rafted debris, shifts in clays and other minerals, and regional erosion of the slope and rise. These transitions may reflect enhanced glacial erosion and reduced glacial meltwater from progressively colder ice. At this time, seismic data show that depocenters began to shift from the outer continental rise to the base of the continental slope coincident with the initial stages of the glacial erosion and overdeepening of the continental shelf. During the late Miocene to early Pliocene there was a transition to greater subglacial activity on the shelf and more pronounced cyclic facies variations on the continental rise. At this time, severe glacial morphologies initiated on the shelf with the erosion of Prydz Channel and other troughs by fast-moving ice and the deposition of overcompacted glacial diamictons by slow-moving ice on adjacent banks. The Prydz trough-mouth fan also began to form with alternating deposition of debris flows (ice at shelf edge) and muddy units (reduced ice) (Site 1167). The fan also records a transition during the late Pleistocene for times younger than 780 k.y. when short-term glacial variations continued but ice reached the shelf edge only a few times. Both short-term and long-term transitions characterize the Cenozoic evolution of the Prydz Bay region from the Cretaceous nonglacial to late Neogene full-glacial paleoenvironments. These transitions are known only from ODP cores, and further insights will require additional drilling.

Prydz Bay region↗
USGS2000

Milestones in Antarctic Ice Sheet history: Preliminary results from Leg 188 drilling in Prydz Bay Antarctica

The Antarctic Ice Sheet is one of the great features of our planet. It plays a pivotal role in global atmospheric circulation and the sea-ice zone around it produces cold waters that control much of the ocean’s deep circulation. The Antarctic Ice Sheet is also the largest store of fresh water on earth and controls short-term sea level changes. The history of the Antarctic Ice Sheet has been pieced together from various sources. For the late Quaternary, ice cores contain a detailed record of accumulation, air temperature and atmospheric composition. For the Cenozoic, information has come from distal marine oxygen isotope records, records of detrital output from the continent and fragmentary outcrops in ice-free areas on the continent. These records have been augmented by drilling on the Antarctic continental margin to try and recover direct evidence of glacial ice and to investigatethe transition from pre-glacial to the full polar glacial conditions that we see today.

JOIDES Journal↗
USGS1998

Ice sheet history from Antarctic Continental Margin sediments: The ANTOSTRAT approach

The Antarctic Ice Sheet is today an important part of the global climate engine, and probably has been so for most of its long existence. However, the details of its history are poorly known, despite the measurement and use, over two decades, of low-latitude proxies of ice sheet volume. An additional way of determining ice sheet history is now available, based on understanding terrigenous sediment transport and deposition under a glacial regime. It requires direct sampling of the prograded wedge of glacial sediments deposited at the Antarctic continental margin (and of derived sediments on the continental rise) at a small number of key sites, and combines the resulting data using numerical models of ice sheet development. The new phase of sampling is embodied mainly in a suite of proposals to the Ocean Drilling Program, generated by separate regional proponent groups co-ordinated through ANTOSTRAT (the Antarctic Offshore Acoustic Stratigraphy initiative). The first set of margin sites has now been drilled as ODP Leg 178 to the Antarctic Peninsula margin, and a first, short season of inshore drilling at Cape Roberts, Ross Sea, has been completed. Leg 178 and Cape Roberts drilling results are described briefly here, together with an outline of key elements of the overall strategy for determining glacial history, and of the potential contributions of drilling other Antarctic margins investigated by ANTOSTRAT. ODP Leg 178 also recovered continuous ultra-high-resolution Holocene biogenic sections at two sites within a protected, glacially-overdeepened basin (Palmer Deep) on the inner continental shelf of the Antarctic Peninsula. These and similar sites from around the Antarctic margin are a valuable resource when linked with ice cores and equivalent sections at lower latitude sites for studies of decadal and millenial-scale climate variation.

Terra Antarctica↗