Brunswick, Australia
Brunswick, Australia

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Green P.F.,Geotrack International | Machado V.,Sonangol Pesquisa and Producao
Geological Society Special Publication | Year: 2017

A series of cooling events in the development of the Namibe margin of Angola is defined by apatite fission track analysis data from samples of outcropping Cretaceous sandstones and crystalline Precambrian basement. Regional exhumation in the Late Carboniferous-Early Permian and Jurassic preceded Early Cretaceous rifting. Further episodes of uplift and erosion affected the margin in the Early and Late Cretaceous before it was buried by up to 2 km of post-break-up section, which was subsequently removed during Cenozoic uplift and erosion beginning between 35 and 20 Ma. This timing is consistent with published analyses of river profiles suggesting that uplift of the margin began at c. 30 Ma. Estimates of between 1.5 and 2 km of section removed during Cenozoic exhumation are consistent with burial depths estimated from the diagenesis of former evaporite horizons now at outcrop. These results add further support to a growing body of evidence showing that the evolution of 'passive' margins is anything but passive. The key episodes of exhumation defined in this study are broadly synchronous with events identified in many areas of the West Africa margin from Namibia to Equatorial Guinea and are regarded as representing a continent-scale response to stresses related to tectonic plate movements. © 2017 The Author(s).


Stoker M.S.,British Geological Survey | Holford S.P.,University of Adelaide | Hillis R.R.,University of Adelaide | Green P.F.,Geotrack International | Duddy I.R.,Geotrack International
Geology | Year: 2010

The Cenozoic sedimentary basins on the Atlantic margin of northwest Britain contain a remarkable record of tectonically influenced post-breakup sedimentation. We have mapped the distribution and quantified the solid grain volume of four unconformity-bound successions in the region, the Eocene (~6-8 × 104km3), Oligocene (~2 × 104km3), Miocene-lower Pliocene (~4-5 × 104km3) and lower Pliocene-Holocene (~4-5 × 104km3), complementing previous work on the Paleocene succession. Of the total Cenozoic sediment volume on the Atlantic margin of northwest Britain, ~80% was deposited in Eocene and later time. The relative volumes of the Cenozoic succession do not support previous claims that the Paleocene was the main period of Cenozoic uplift and erosion of sediment source areas. Rather, the Cenozoic sedimentary basins on the Atlantic margin of northwest Britain record the detritus of four major episodes of Cenozoic uplift of the British Isles (Paleocene, Eocene-Oligocene, Miocene, and Pliocene-Pleistocene). © 2010 Geological Society of America.


Holford S.P.,University of Adelaide | Green P.F.,Geotrack International | Hillis R.R.,University of Adelaide | Underhill J.R.,University of Edinburgh | And 2 more authors.
Journal of the Geological Society | Year: 2010

The post-Caledonian exhumation history of NW Scotland is a controversial issue, with some studies advocating largely continual emergence whereas others suggest dominantly early Palaeogene plume-driven exhumation. Apatite fission-track analysis (AFTA) data for samples of Precambrian basement and Permian-Cretaceous sediments from onshore and offshore reveal multiple phases of post-Caledonian cooling: Triassic (beginning 245-225 Ma), Cretaceous (140-130 Ma; 110-90 Ma) and Cenozoic (65-60 Ma; 40-25 Ma; 15-10 Ma), all of which are interpreted at least in part as recording exhumation. Basement and sedimentary cover rocks display similar thermal histories, emphasizing the regional nature of these episodes and implying that sedimentary outliers represent the remnants of previously more extensive sequences. Significant thicknesses of Jurassic rocks may once have covered NW Scotland. Palaeocene palaeothermal effects are most pronounced in the vicinity of igneous centres, probably reflecting combined effects of heating by elevated heat flow, deeper burial and hydrothermal activity. Most of the region underwent kilometre-scale Neogene exhumation. Contrary to the common assumption of monotonic cooling and denudation histories, integration of geological evidence with AFTA data defines an episodic thermal history involving repeated cycles of burial and exhumation. We suggest that onshore passive margins and continental interiors may also best be characterized by similar histories. © 2010 Geological Society of London.


Tassone D.R.,University of Adelaide | Holford S.P.,University of Adelaide | Duddy I.R.,Geotrack International | Green P.F.,Geotrack International | Hillis R.R.,Deep Exploration Technologies Cooperative Research Center
AAPG Bulletin | Year: 2014

In prospective basins affected by exhumation, uncertainty commonly exists regarding the maximum burial depths of source, reservoir, and seal horizons. One such basin is the Otway Basin, an important gas province in southeastern Australia, which has witnessed several exhumation events. Here, we present estimates of net exhumation magnitudes for 110 onshore and offshore petroleum wells based on the sonic transit time analyses of Lower Cretaceous fluvial shales. Our results show significant post-Albian net exhumation in the eastern onshore Otway Basin (>1500 m [̃4920 ft]) and a generally minor net exhumation (<200 m [̃655 ft]) elsewhere in the Otway Basin, consistent with estimates based on thermal history data. The distribution of net exhumation magnitudes in relation to mid-Cretaceous and Neogene compressional structures indicates that exhumation was dominantly controlled by short-wavelength basin inversion driven by plate-boundary forces. Deeper burial coupled with high geothermal gradients in the onshore eastern Otway Basin and along the northern basin margin during the early Cretaceous have rendered Lower Cretaceous source rocks mostly overmature, with any remaining hydrocarbons from the initial charge likely to be trapped in tightly compacted reservoirs and/or secondary (fracture-related) porosity. However, the embrittlement of these reservoirs during their deeper burial may present opportunities for the development of low-permeability plays through hydraulic fracturing where smectite clay minerals are illitized. Source rocks at near-maximum burial at present day are at temperatures suitable for gas generation, with key controls on prospectivity in these areas including the sealing potential of faulted traps and the relationship between charge and trap development. ©2014. The American Association of Petroleum Geologists. ©2014. The American Association of Petroleum Geologists. All rights reserved.


Japsen P.,Geological Survey of Denmark | Green P.F.,Geotrack International | Bonow J.M.,Geological Survey of Denmark | Bonow J.M.,Södertörn University College | And 2 more authors.
Global and Planetary Change | Year: 2014

In southern East Greenland (68-70°N), voluminous flood basalts erupted onto a largely horizontal lava plain near sea level at the Paleocene-Eocene transition when sea-floor spreading started in the NE Atlantic. Based on synthesis of geological observations, stratigraphic landform analysis and apatite fission-track analysis data in 90 rock samples, we show how three regional phases of uplift and exhumation subsequently shaped the present-day margin and controlled the discontinuous history of the Greenland ice sheet. A late Eocene phase of uplift led to formation of a regional erosion surface near sea level (the Upper Planation Surface, UPS). Uplift of the UPS in the late Miocene led to formation of the Lower Planation Surface (LPS) by incision below the uplifted UPS, and a Pliocene phase led to incision of valleys and fjords below the uplifted LPS, leaving mountain peaks reaching 3.7. km above sea level. Local uplift affected the Kangerlussuaq area (~. 68°N) during early Eocene emplacement of the Kangerlussuaq Intrusion and during late Oligocene block movements, that may be related to the detachment of the Jan Mayen microcontinent from Greenland, while middle Miocene thermal activity, coeval with lava eruptions, heated rocks along a prominent fault within the early Cretaceous to Paleocene Kangerlussuaq Basin. The three regional uplift phases are synchronous with phases in West Greenland, overlap in time with similar events in North America and Europe and also correlate with changes in plate motion. The much higher elevation of East Greenland compared to West Greenland suggests support in the east from the Iceland plume. These observations indicate a connection between mantle convection, changes in plate motion and vertical movements along passive continental margins. © 2014 Elsevier B.V.


Japsen P.,Geological Survey of Denmark | Bonow J.M.,Geological Survey of Denmark | Green P.F.,Geotrack International | Cobbold P.R.,French National Center for Scientific Research | And 4 more authors.
Bulletin of the Geological Society of America | Year: 2012

It is a common assumption that elevated passive continental margins have remained high since rifting and breakup. Here, we show that the Atlantic margin of NE Brazil has under gone a more complex history. Our synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) After Early Cretaceous breakup, the margin under went burial beneath a thick sedimentary cover; (2) uplift episodes in the Campanian and Eocene led to almost complete removal of these deposits; (3) the resulting large-scale, low-relief erosion surface (peneplain) was deeply weathered and finally reburied at the Oligocene-Miocene transition; and (4) Miocene uplift and erosion produced a new, lowerlevel peneplain by incision of the uplifted and re-exposed Paleogene peneplain. Previous studies have identified aspects of this interpretation, but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed Early Cretaceous rifting and Eocene-Oligocene peneplanation. We suggest that a late sedimentary cover protected Paleogene weathering profiles until the present day. The uplift phases in Brazil are synchronous with uplift phases in Africa and the Andes. The Andean phases coincided with rapid convergence on the western margin of South America, and the Campanian uplift coincided with a decline in spreading rate at the Mid-Atlantic Ridge. Consequently, we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause, which is lateral resistance to plate motion. © 2012 Geological Society of America.


Green P.F.,Geotrack International | Duddy I.R.,Geotrack International | Japsen P.,Geological Survey of Denmark | Bonow J.M.,Mid Sweden University | Malan J.A.,New Age African Global Energy Ltd London UK
Basin Research | Year: 2015

Despite many years of study, the processes involved in the development of the continental margin of southern Africa and the distinctive topography of the hinterland remain poorly understood. Previous thermochronological studies carried out within a monotonic cooling framework have failed to take into account constraints provided by Mesozoic sedimentary basins along the southern margin. We report apatite fission track analysis and vitrinite reflectance data in outcrop samples from the Late Jurassic to Early Cretaceous sedimentary fill of the Oudtshoorn, Gamtoos and Algoa Basins (Uitenhage Group), as well as isolated sedimentary remnants further west, plus underlying Paleozoic rocks (Cape Supergroup) and Permian-Triassic sandstones from the Karoo Supergroup around the Great Escarpment. Results define a series of major regional cooling episodes. Latest Triassic to Early Jurassic cooling which began between 205 and 180 Ma is seen dominantly in basement flanks to the Algoa and Gamtoos Basins. This episode may have affected a wider region but in most places any effects have been overprinted by later events. The effects of Early Cretaceous (beginning between 145 and 130 Ma) and Early to mid-Cretaceous (120-100 Ma) cooling are both delimited by major structures, while Late Cretaceous (85-75 Ma) cooling appears to have affected the whole region. These cooling events are all interpreted as dominantly reflecting exhumation. Higher Late Cretaceous paleotemperatures in samples from the core of the Swartberg Range, coupled with evidence for localised Cenozoic cooling, are interpreted as representing Cenozoic differential exhumation of the mountain range. Late Cretaceous paleotemperatures between 60°C and 90°C in outcropping Uitenhage Group sediments from the Oudtshoorn, Gamtoos and Algoa Basins require burial by between 1.2 and 2.2 km prior to Late Cretaceous exhumation. Because these sediments lie in depositional contact with underlying Paleozoic rocks in many places, relatively uniform Late Cretaceous paleotemperatures across most of the region, in samples of both basin fill and underlying basement, suggest the whole region may have been buried prior to Late Cretaceous exhumation. Cenozoic cooling (beginning between 30 and 20 Ma) is focussed mainly in mountainous regions and is interpreted as representing denudation which produced the modern-day relief. Features such as the Great Escarpment are not related to continental break up, as is often supposed, but are much younger (post-30 Ma). This history of post-breakup burial and subsequent episodic exhumation is very different from conventional ideas of passive margin evolution, and requires a radical re-think of models for development of continental margins. © 2015 The Authors.


Chalmers J.A.,Geological Survey of Denmark | Green P.,Geotrack International | Japsen P.,Geological Survey of Denmark | Rasmussen E.S.,Geological Survey of Denmark
Journal of Geodynamics | Year: 2010

Nielsen et al. (2009a) suggested that the high mountains of southern Scandinavia are the result of protracted exhumation since the Silurian. The evidence cited by Nielsen et al. (2009a) in support of this hypothesis is, however, very selective and other published evidence shows that their hypothesis is untenable. We suggest that an objective review of available evidence shows that the mountains of Norway are the result of uplift resulting from Cenozoic tectonism amplified by the isostatic response to resulting erosion, similar to many other continental margins around the world. © 2010 Elsevier Ltd.


Japsen P.,Geological Survey of Denmark | Chalmers J.A.,Geological Survey of Denmark | Green P.F.,Geotrack International | Bonow J.M.,Geological Survey of Denmark
Global and Planetary Change | Year: 2012

Many studies of elevated, passive continental margins (EPCMs) assume that their characteristic, large-scale morphology with high-level plateaux and deeply incised valleys has persisted since rifting and crustal separation, and that the absence of post-rift sediments is evidence of non-deposition. The high mountains in West Greenland, however, expose evidence of km-scale, post-rift subsidence, and recent studies showed that typical EPCM morphology with elevated plateaux formed c. 50. Myr after breakup through a process of uplift and dissection of a regional, post-rift erosion surface. Since the West Greenland margin shares all the morphological characteristics of EPCMs, the results from West Greenland lead us to question the common assumption that EPCMs have remained high since the onset of continental separation. We present published evidence of post-rift burial followed by uplift and exhumation from a number of EPCMs and their adjacent basins to support the notion that EPCMs are not permanent highs and that their morphology is unrelated to rifting and continental breakup. Geodynamic models that explain EPCMs as permanent highs since the time of rifting require either no lithospheric mantle extension below extending crust or effective elastic thicknesses > 100. km. Such models are, however, not consistent with the subsidence history inferred from actual rifts and their margins. Geodynamic models using low elastic thicknesses and a much more uniform distribution of strain within the lithosphere are more consistent with observations of early post-rift behaviour, but some additional process is needed to uplift the margins later. We suggest that EPCMs represent anticlinal, lithospheric folds formed under compression where an abrupt change in crustal or lithospheric thickness occurs between cratons and rift basins. We propose that EPCMs are expressions of episodes of post-rift burial followed by compression-induced uplift and exhumation; one episode of uplift results in erosion of the region to produce a low-relief surface near the level of the adjacent, opening ocean, and a second (or more) episode(s) raises the plateau to its present elevation, after which the plateau is dissected by fluvial and possibly glacial erosion. © 2011 Elsevier B.V.


Green P.F.,Geotrack International | Duddy I.R.,Geotrack International
Petroleum Geology Conference Proceedings | Year: 2010

In many areas of the Arctic, sedimentary sequences have been exhumed from significantly greater depths during the Cenozoic, with 2 km of section or more removed in some areas. Implications for exploration include enhanced maturity levels, possible loss of reservoired hydrocarbons as a result of seal breach, and phase changes due to pressure reduction. While the importance of Cenozoic exhumation to hydrocarbon prospectivity in individual basins is widely recognized, less well recognized is the regional synchroneity in the main phases of Cenozoic exhumation over wide areas of the Arctic and North Atlantic. Thermal history reconstruction studies in the Barents Sea and the Alaskan North Slope, based on application of apatite fission track analysis and vitrinite reflectance, reveal three main episodes of exhumation, in Paleocene, Eocene-Oligocene and Miocene times, and correlative exhumation episodes have been identified in a number of published studies in these and other areas. Previous attempts to explain these episodes of exhumation have been focussed on local mechanisms. However, our results reveal a pattern of regionally synchronous exhumation over a wide region, not only of the Arctic but also in many areas around the European North Atlantic margin, suggesting that events in each area are a regional response to events at plate boundaries, perhaps coupled to imbalances of crustal forces at continental boundaries. To date, no convincing mechanism has been put forward for producing such regional exhumation episodes, despite the fact that in many areas they exert critical control on regional hydrocarbon prospectivity. We suggest that serious attention should be directed to investigating the underlying mechanisms. © Petroleum Geology Conferences Ltd. Published by the Geological Society, London.

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