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- Volume 5, Issue 1, 1993
Basin Research - Volume 5, Issue 1, 1993
Volume 5, Issue 1, 1993
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Volcanism and sedimentation in part of a Late Archaean rift: the Hartbeesfontein basin, Transvaal, South Africa
More LessAbstractThe Hartbeesfontein basin is one basin within the Late Archaean rift system of South Africa. This rift system has been recently compared to the Basin and Range province in western North America and may therefore be an ensialic extensional back‐arc basin. Structurally, the Hartbeesfontein basin is a half‐graben structure bounded to the south‐east by a major, normal, listric fault and to the north‐east and south‐west by strike‐slip (transfer?) fault zones. It is infilled by over 2000 m of diamictites, shales, lavas and chemical sediments. Initial basin formation appears to be accompanied by phreatomagmatic volcanic activity caused by the interaction between basic tholeiitic magmas rising along fractures and groundwater. Volcaniclastic debris from these eruptions was incorporated into laharic debris flows and deposited on basin marginal alluvial fans. At the same time a deep, permanent lake formed within the basin in which silts and muds accumulated. Major fissure eruptions of basic, tholeiitic lavas followed, their eruptive centres being apparently located along the strike‐slip (transfer?) fault /ones. Initially, these fissure eruptions had high rates of magma discharge accompanied by intense fire fountaining that resulted in the rapid accumulation of aa type flows. Later lava discharge rates decreased and more quiescent pahoehoe type flows were erupted. Localized centres of acid volcanism within the basic lava pile were located along the south‐western strike‐slip fault zone. These acid volcanic rocks are interpreted as co‐ignimbrite lag breccias and pyroclastic flow deposits and tuffs produced by the repeated formation and collapse of Plinian eruption columns. Towards the top of the basic lava pile, two breaks in volcanism permitted the formation of dolomitic playa lakes. Sedimentation in these lakes was terminated by further basic lava flows. At the top of the basin fill sequence is a thick, bedded chert interpreted as a magadiitic, alkaline playa lake fed by silica‐rich hot springs located along the south‐eastern edge of the basin. Quartzites and conglomerates deposited by braided rivers unconformably overlie the basin‐fill sequence and probably represent a through flowing river system signifying termination of the Hartbeesfontein basin as a separate basin. The Hartbeesfontein basin and its fill demonstrate that a close relationship exists between fissure volcanism, sedimentation and basin evolution and that the strike‐slip, transfer faults acted as the loci of volcanic activity.
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Differential response of a Devonian‐Carboniferous platform‐deeper basin system to sea‐level change and tectonics, N. Chilean Andes
Authors HEINRICH Bahlburg and CHRISTOPH BreitkreuzAbstractDeposition of a 2700‐m‐thick clastic platform succession in a N‐S striking basin in northern Chile began in the Early Devonian during a global sea‐level rise. A transition to terrestrial facies took place at the Early‐Late Carboniferous boundary when the Gondwana glaciation began and global sea‐level dropped. On the platform, interbedded cross‐bedded or bioturbated sandstones, offshore tidal dunes and sand waves, and mudstones and tempestites suggest switching intertidal and shallow or deep subtidal environments. However, evidence for subaerial erosion indicates a significant regression during the Early Devonian.
In an adjacent and deeper N‐S striking sub‐basin to the W, up to 3600 m of turbidites were deposited from the Late Devonian to the Late Carboniferous by mainly southerly palaeocurrents. Turbidites accumulated in coarse‐grained proximal sand lobes in the N, and in fine‐grained lobe fringe and basin plain environments in the S, with alternating upward‐thinning and upward‐thickening cycles typical of tectonically controlled aggradational turbidite systems.
The sedimentological data indicate that the deeper basin depositional system evolved to a large extent independently from the platform system. Sediment in the deeper basin is less mature and more poorly sorted than that on the platform, suggesting that detritus bypassed the platform and was shed directly from the source areas into the western basin. The only depositional link between the platform and deeper basin systems seems to be longshore platform currents which may have funnelled minor quantities of mature sand into the deeper basin via bypass canyons. Although platform and deeper basin evolved in a common extensional tectonic setting, the platform reflects eustatic changes of sea‐level whereas deposition in the deeper basin records syndepositional tectonics.
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Structural evolution of African basins: stratigraphic synthesis
More LessAbstractThe structural and stratigraphic character of African interior sedimentary basins is highly variable, indicating contrasting basin‐forming mechanisms and subsequent subsidence histories. A stratigraphic database has been compiled for African interior depositional basins for the purpose of better understanding basin thermal and structural development. Data are recorded in the form of stratal age, lithology, thickness and elevation of top with respect to present sea level. The data are obtained from published structure contour maps, well sections, and outcrop geology and elevation. There are various degrees of data coverage of the basins, proportional to the amount of water and oil drilling activity. Consequently, there is excellent coverage of North African basins such as the Algerian basin and the Sirte basin, while there is little known about the subsurface of the Congo basin. The stratigraphic data are used to reconstruct the depositional history of the basins, while backstripping leads to the quantification of the thermo‐tectonic component of basin subsidence. The nature of basement subsidence can provide constraints on lithospheric flexural rigidity. In addition, the depositional and thermo‐tectonic history of each basin bears upon the mechanisms of basin formation and subsidence. Virtually all types of basins are represented in interior Africa, including thrust‐loaded basins (Algerian), passive‐margin rift basins (Algerian, Sirte), modern active rift basins (East African), ancient rift basins (Benue, Abu Gabra), basins caused by uplift of their margins (Congo, Chad, Illumeden) and even basins that may be related to thermal subsidence of hot‐spot domes (Algerian, Sirte).
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Pseudo‐toplap in seismic models of the Schlern‐Raibl contact (Sella platform, northern Italy)
Authors JAN Stafleu and WOLFGANG SchlagerAbstractIn the Sella platform of the Dolomite Alps, the horizontal beds of the Raibl Formation directly overlie the Upper Schlern Dolomite (Cassian Dolomite of some authors). The Schlern Dolomite comprises steep clinoforms, together with a few tens of metres of horizontal topset beds in the platform interior.
Two‐dimensional seismic modelling techniques were used to determine the seismic‐response of this interesting situation. To perform this, two different lithological models were constructed based on outcrops of the Sella south face. The first model is that of a rapidly prograding platform with slow aggradation in the platform interior. The second model shows clinoforms toplapping against the topset beds of the platform interior.
P‐wave velocities and bulk densities were assigned to the lithostratigraphical units in accordance with values from a similar study, involving the same formations. The vertical‐incidence method was used to construct perfectly migrated time sections and depth sections of reflectivity. These were convolved with zero‐phase Ricker wavelets of different peak frequencies to produce the final synthetic seismic sections.
Using conventional, low frequencies (e.g. 25 Hz), the seismic response of the two models is almost identical. The topset beds and the Schlern‐Raibl contact appear as one event. In a real seismic survey, both sections would be interpreted as toplap of Schlern clinoforms against the Raibl Formation. At higher frequencies (75 Hz), however, differences are revealed. The angle of progradation in the progradation & aggradation model becomes visible, as opposed to the horizontal surface in the progradation & toplap model. Topset beds are resolved separate from the Raibl Formation, but still appear to form a single dipping pseudo‐toplap surface.
Another modelling technique, simulating unmigrated sections, shows few differences between the two models even at high frequencies. In addition, the clinoforms are disturbed by the refraction of rays. This study demonstrates that, even under ideal acquisition and processing conditions, the seismic tool can introduce a pseudo‐toplap. This implies that toplap in a seismic section is not necessarily toplap in outcrop.
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BOOK REVIEWS
Book reviewed in this article:
AAPG Atlas of Oil and Gas Fields–Stratigraphic Traps III.:N. H. Foster & E. A. Beaumont (Eds) AAPG, Tulsa, 445 pp., 1992.
Sedimentology & Sequence Stratigraphy of Reefs and Carbonate Platforms:Wolfgang Schlager AAPG Continuing Education Course Note Series No. 34, 71 pp., 1992.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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Volume 2 (1989)
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Volume 1 (1988)