Johan M. Bonow

Episodic burial and exhumation in NE Brazil after opening of the South Atlantic

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 undergone 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, lower-level 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.

Episodic uplift and exhumation along North Atlantic passive margins: implications for hydrocarbon prospectivity

Abstract We present observations that demonstrate that the elevated passive margins around the North Atlantic were formed by episodic, post-rift uplift movements that are manifest in the high-lying peneplains that characterize the coastal mountains, in the unconformities in the adjacent sedimentary basins and in accelerated subsidence in the basin centres. Results from West Greenland show that subsidence of the rifted margin took place for c . 25 Myr after rifting and breakup in the Paleocene, as predicted by classical rift theory, but that this development was reversed by a series of uplift movements (starting at c . 35, 10 and 5 Ma) that remain unexplained. East Greenland and Scandinavia seem to have had a similar evolution of post-rift subsidence followed by uplift starting at c . 35 Ma. There was no notable fall in sea-level at this time, so the subsiding basins must have been inverted by tectonic forces. We speculate that the forces causing this phase were related to the plate boundary reorganization in the North Atlantic around Chron 13 time. One feature that these areas have in common is that uplift took place along the edges of cratons where the thickness of the crust and lithosphere changes substantially over a short distance. It may be that the lateral contrasts in the properties of the stretched and unstretched lithosphere make the margins of the cratons unstable long after rifting. These vertical movements have profound influence on hydrocarbon systems, not only in frontier areas such as West and East Greenland, where Mesozoic basins are deeply truncated and exposed onshore, but also for the understanding of near-shore hydrocarbon deposits in mature areas such as the North Sea Basin, where low-angular unconformities may represent episodes of deposition and removal of significant sedimentary sections.

Palaeosurfaces and major valleys in the area of the Kjølen Mountains, southern Norway - consequences of uplift and climatic change

Palaeosurfaces and major valleys in the area of Kjolen Mountains, southern Norway : consequences of uplift and climatic change

Saprolite Remnants as Indicators of Pre‐Glacial Landform Genesis in Southeast Sweden

Abstract Twenty‐six sites with remnants of gravelly saprolites (grus) have been located in southeast Sweden. Joint block hills (castle kopjes) and steep rock walls with weathered joints as well as rounded boulders are documented to have an origin in deep weathering and subsequent stripping of saprolites. The saprolite remnants and landforms result from the fragmentation of the re‐exposed sub‐Cambrian peneplain along fracture systems. Only shallow saprolites occur on the elevated intact parts of the sub‐Cambrian peneplain, while saprolites up to 20 m thick are encountered in areas where the sub‐Cambrian peneplain is fractured and dissected. Neogene uplift with reactivation of the weathering system is thought to be the main cause of saprolite formation. Deep weathering is thus judged to have been the major agent of landform formation in the study area, while glacial and glaciofluvial erosion has contributed mainly by stripping saprolites, detaching corestones, and plucking joint blocks along weathered joints.

Post-breakup burial and exhumation of the southern margin of Africa

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. Previo ...

The South Swedish Dome : a key structure for identification of peneplains and conclusions on Phanerozoic tectonics of an ancient shield

Abstract The relationships between different denudation surfaces/peneplains formed across crystalline basement rocks give valuable information to the tectonic development of ancient shields. The denudation surfaces can be identified by the aid of their landforms, tilt and remnant weathering mantles in relation to cover rocks. Three types of denudation surfaces are identified across south Sweden (1) a tilted flat plain, (2) a tilted hilly surface with relative relief below 150 m and (3) stepped horizontal plains with residual hills. All three types of denudation surfaces are peneplains, denudation surfaces graded to specific base levels. The re-exposed parts of the inclined flat sub-Cambrian peneplain (SCP) extend as a landscape feature from below cover rocks in the north and east and reaches up on the highest summits of the South Swedish Uplands. The SCP (the exact unconformity) is encountered again below Cambrian covers outside the west coast. Thus south Sweden is a geological dome, the South Swedish Dome (SSD), in relation to the Cambrian cover. The southern and western low flanks of the exposed part of the dome are instead characterized by a hilly peneplain, the inclined sub-Cretaceous denudation surface, with remnants of thick, kaolinitic, clayey saprolites. This sub-Cretaceous peneplain is cut off at a distinct level in the south and west by the almost horizontal South Småland Peneplain, a never covered, epigene, peneplain. The uplift history of the SSD aids to the understanding on the development of late Tertiary drainage systems of the Baltic Basin by the Eridano River.

Morphostructural patterns and landform generations in a glaciated passive margin: the Kobberminebugt-Qaqortoq region of South Greenland

On the west coast of South Greenland, the passive continental margin displays a left-lateral step which is not apparent on the east coast and coincides with a topographic change that marks the transition between Archaean basement rocks to the north and the Ketilidian orogenic belt to the south. Two issues are considered: (1) the origin of the regional topographic asymmetry between the mountains of southeast and southwest Greenland; (2) the origin of the Kobberminebugt-Qaqortoq lowlands (differential early Cenozoic tectonic movements and/or subsequent differential erosion in the uplifted landmass). By identifying and mapping stepped landforms, morphostructural patterns and weathering features, we discriminate between control by differential erosion and tectonic movements. We suggest that the stepped patterns sof erosion surfaces represent a pre-glacial landform evolution related to the uplift of two antiforms along the west and east coast of southern Greenland. In addition, late fault reactivation may have favoured the development of a low coastal surface in less uplifted blocks. A relative chronology of landform development in the study area is given. Although implying a strong involvement of differential erosion, our interpretation of the geomorphology is compatible with a late reactivation of an oblique transfer zone formed during the early Cenozoic rifting.

Burial and Exhumation History of the Labrador-Newfoundland Margin and Implications for Hydrocarbon Exploration

The stratigraphic record along the continental margin of Labrador and Newfoundland provides ample evidence for vertical movements both prior to and after break-up that could have importance for hydrocarbon exploration in the region. Publically available vitrinite reflectance datasets from a well in the Jeanne d’Arc Basin indicate that the Cenozoic and deeper sections has been hotter in the past, presumably due to deeper burial prior to Late Cenozoic cooling. Furthermore, one of the datasets shows evidence of a major degree of deeper burial on the baseTertiary unconformity, but additional data are required to investigate the reality of the base-Tertiary episode of burial and exhumation. Over much of the Labrador shelf, Miocene deposits are absent, and we show evidence based on vitrinite reflectance and sonic data that indicate that Miocene deposits of significant thickness may have been present prior to uplift and exhumation. Onshore Labrador, the presence of a Cretaceous outlier on Precambrian basement adds to the evidence of one or more events of exhumation that has removed pre-Cretaceous sediments on a regional scale, similar to the offshore Labrador where Cretaceous rocks rest on Precambrian basement over most of the shelf. We also present results from a pilot study comprising apatite fission-track analysis (AFTA) data that reveals a Phanerozoic history involving a series of burial and exhumation episodes. The pilot study is a forerunner for a study of the onshore and offshore domain with three components: (1) A thermochronological study based on samples from outcrops and from onshore and offshore boreholes, (2) A stratigraphic landform analysis of the onshore study area based on mapping of erosion surfaces that will provide evidence of the vertical motion onshore and a relative denudation chronology, and (3) An integrated interpretation of the geological, geomorphological and thermochronological data to provide a coherent model of the timing and magnitude of the vertical movements along the margin both prior to and after break-up. Failure to account for the effects of uplift and erosion, such as greater depths of burial prior to exhumation, may lead to serious underestimation of the petroleum resource maturity and to erroneous estimates of the timing of hydrocarbon generation, and not least to changes in migration routes and in the source-to-sink system of sediment input into offshore basins.

Episodic Burial and Exhumation along Atlantic Margins - Implications for Hydrocarbon Prospectivity

It is a common assumption that elevated passive continental margins (EPCMs) have remained high since rifting and breakup. Absence of post-rift sediments in the highlands is often taken as evidence of non-deposition, and the presence of high-level plateaux along many EPCMs is unexplained in many studies. However, the post-rift stratigraphic record in the adjacent sedimentary basins often contains significant unconformities and in some cases, post-rift, marine sediment crop out at high elevation. We present evidence of post-rift vertical movements along Atlantic margins and discuss how these movements affect the hydrocarbon prospectivity of these margins. We maintain that folds, reverse faults, reactivated normal faults and strike-slip faults that are typical of EPCMs, are a result of post-rift compression that leads to the formation of EPCMs, and that the necessary forces build up during changes in plate motion. One episode of uplift results in erosion of the region to produce a low-relief surface and a second (or more) episode(s) raises the plateau to its present elevation, after which fluvial and possibly glacial erosion dissects the plateau. Failure to realize that EPCMs are post-rift features have far-reaching implications for understanding basin development, hydrocarbon systems and the properties of the lithosphere itself.

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