Sediment supply and climate change: implications for basin stratigraphy

The rate of sediment supply from erosional catchment to depositional basin depends primarily upon climate, relief, catchment slope and lithology. It varies in both time and space. Spatial changes in erosion rates due to variations in lithology are illustrated by contrasting rates of drainage divide migration away from faults of known ages. Time variations in relative sediment supply are extremely complex and vary widely according to the direction and magnitude of climate change. In many parts of the Great Basin and south-western USA, glacial maximum climates were characterized by higher effective moisture and the altitudinal downward spread of woods and forests. Sparse data from alluvial fans indicate reduced sediment supply, despite the increased runoff evident from higher lake levels. The situation in Mediterranean areas is less clear, with rival climatic scenarios for vegetation ecotypes predicting contrasting runoff. In order to test these latter we run Cumulative Seasonal Erosion Potential [CSEP] experiments for present-day and a variety of full-glacial Mediterranean candidate climates. The results indicate the likelihood of enhanced sediment supply and runoff compared to the present day during full-glacial times for a cool wet winter climate and a reduction in sediment supply and runoff for a full-glacial cool dry winter climate. We then explore the consequences of such phase differences in sediment supply, and sea and lake levels for the stratigraphy of sedimentary basins. Highstands and lowstands of sea or lake may be accompanied by greater or lesser sediment and water supply, as determined by the regional climate and the direction of climatic change. Thus marine lowstands are not necessarily periods of great transfer of coarse clastic sediments to shelves and deep water basinal environments. Unsteady sediment supply has greatest implications for alluvial systems, in particular the effect that changing relative supplies of water and sediment have upon river and fan channel incision.     

Orthogonal to oblique rifting: effect of rift basin orientation in the evolution of the North basin, Malawi Rift, East Africa

The East African Rift system has long been considered the best modern example of the initial stages of continental rifting. The Malawi Rift is characteristic of the western branch of the East African Rift system, composed of half-grabens of opposing asymmetry along its length. There are striking similarities between basins within the Malawi Rift, and others along the western branch. Each exhibits similar bathymetry, border-fault length, rift zone width and fault segment length. The North Basin of the Malawi Rift differs from others in the rift only in its orientation: trending NW–SE as opposed to N–S. Although there is general agreement as to the geometry of the Malawi Rift; debate as to the amount of strike–slip vs. dip–slip deformation and the influence of underlying Pan-African foliation remains. This study presents new data from a closely spaced shallow [2 s two-way travel time (TWT)] seismic reflection data set integrated with basin-scale deeper (6 s TWT) seismic reflection data that document the structural evolution of the border and intra-basin faults. These data reveal that the different trend of the North Basin, most likely to have been influenced by the underlying Pan-African foliation, has played an extremely important role in the structural style of basin evolution. The border-fault and intra-basin structures nucleated during extension that was initially orthogonal (ENE). During this time (>8.6 to ∼0.5–0.4 Ma) intra-basin faults synthetic to the west-dipping border-fault nucleated, whereas strain was localised on the segmented border-fault early on. A later rotation of extension orientation (to NW) led to these established faults orienting oblique to rifting. This generated an overall dextral strike–slip setting that led to the development of transfer faults adjacent to the border-fault, and the generation of flower structures and folds over the greater displacement intra-basin faults.     

Regional stratigraphy and subsidence of Orphan Basin near the time of breakup and implications for rifting processes

The stratigraphic, subsidence and structural history of Orphan Basin, offshore the island of Newfoundland, Canada, is described from well data and tied to a regional seismic grid. This large (400 by 400 km) rifted basin is part of the non‐volcanic rifted margin in the northwest Atlantic Ocean, which had a long and complex rift history spanning Middle Jurassic to Aptian time. The basin is underlain by variably thinned continental crust, locally <10‐km thick. Our work highlights the complex structure, with major upper crustal faults terminating in the mid‐crust, while lower crustal reflectivity suggests ductile flow, perhaps accommodating depth‐dependent extension. We describe three major stratigraphic horizons connected to breakup and the early post‐rift. An Aptian–Albian unconformity appears to mark the end of crustal rifting in the basin, and a second, more subdued Santonian unconformity was also noted atop basement highs and along the proximal margins of the basin. Only minor thermal subsidence occurred between development of these two horizons. The main phase of post‐rift subsidence was delayed until post‐Santonian time, with rapid subsidence culminating in the development of a major flooding surface in base Tertiary time. Conventional models of rifting events predict significant basin thermal subsidence immediately following continental lithospheric breakup. In the Orphan Basin, however, this subsidence was delayed for about 25–30 Myr and requires more thinning of the mantle lithosphere than the crust. Models of the subsidence history suggest that extreme thinning of the lithospheric mantle continued well into the post‐rift period. This is consistent with edge‐driven, small‐scale convective flow in the mantle, which may thin the lithosphere from below. A hot spot may also have been present below the region in Aptian–Albian time.     

Cooling of the Sverdrup Basin during Tertiary basin inversion: implications for hydrocarbon exploration

ABSTRACT The regional thermal history of the north‐eastern Sverdrup Basin, Canadian Arctic Archipelago, has been assessed using apatite fission‐track thermochronology and vitrinite reflectance data. Fission‐track data for 27 samples from six wells through the Mesozoic section on Axel Heiberg and Ellesmere Islands reveal significant Palaeocene cooling associated with basin inversion during the Eurekan Orogeny. Fission‐track data for 29 outcrop samples, ranging in stratigraphic age from Cambrian to Tertiary, also reveal significant Palaeocene cooling. Vitrinite reflectance data from carbonaceous shales and coal seams in well and outcrop samples are consistent with these conclusions. The degree of Palaeocene cooling observed is greatest for well and outcrop samples in the cores of anticlines or the hanging walls of thrust faults, such as the Fosheim anticline, and faults, such as the Lake Hazen fault system, and the East Cape and Vesle Fiord thrust faults. Palaeocene cooling is largely attributed to the denudation of structures during the Eurekan Orogeny. At one locality on north‐western Ellesmere Island, which is on the northern flank of the Sverdrup Basin, the underlying Franklinian basement rocks yield Early Cretaceous fission track ages with relatively long mean track lengths. This indicates that this part of the basin was uplifted at this time and that subsequent sedimentation and subsidence in the Cretaceous and early Tertiary were modest. This locality thus appears to be on the rift shoulder, which developed along the flank of the Amerasia Basin in the Lower Cretaceous. At a locality on western Axel Heiberg Island, which is downflank from the rift shoulder, the Upper Jurassic Awingak sandstone has a Late Cretaceous fission track age. This is best explained by heating above the total annealing temperature for fission‐tracks in apatite by extensive Lower Cretaceous intrusions and subsequent heat dissipation and cooling in the Late Cretaceous followed by further substantial cooling due to Tertiary denudation. These results indicate that maximum burial temperatures occurred in the presently exposed Mesozoic section prior to basin inversion during the Eurekan Orogeny. It can therefore be inferred that peak hydrocarbon generation and primary migration predated the formation of structural traps during the Tertiary at shallow depths within the northern Sverdrup Basin.     

The ecologo-geomorphological consequences of conservation of agricultural lands within the Lake Baikal watershed basin

The results derived from analyzing the long-term dynamics of the runoff of sediment loads of the main rivers in the agricultural areas of the Lake Baikal watershed basin are discussed. The analysis revealed a descending trend of the erosion processes caused by an abrupt change of agricultural activity in the region, i.e. a reduction in the area of arable and pasture lands in the late 20th century. It is shown that a decrease in basin erosion intensity promotes stabilization of desertification processes and, in general, an improvement of the ecological situation in the Baikal region.     

Universality and variability in basin outlet spacing: implications for the two-dimensional form of drainage basins

It has been observed that the distance between the outlets of transverse basins in orogens is typically half of the distance between the main divide and the range front irrespective of mountain range size or erosional controls. Although it has been suggested that this relationship is the inherent expression of Hack's law, and/or possibly a function of range widening, there are cases of notable deviations from the typical half‐width average spacing. Moreover, it has not been demonstrated that this general relationship is also true for basins in morphologically similar nonorogenic settings, or for those that do not extend to the main drainage divide. These issues are explored by investigating the relationship between basin outlet spacing and the 2‐dimensional geometric properties of drainage basins (basin length, main valley length and basin area) in order to assess whether the basin outlet spacing‐range width ratio is a universal characteristic of fluvial systems. We examined basins spanning two orders of magnitude in area along the southern flank of the Himalayas and the coastal zone of southeast Africa. We found that the spacing between basin outlets (L_os) for major transverse basins that drain the main divide (range‐scale basins) is approximately half of the basin length (L_b) for all basins, irrespective of size, in southeast Africa. In the Himalayas, while this ratio was observed for eastern Himalayan basins (a region where the maximum elevations coincided with the main drainage divide), it was only observed in basins shorter than ～30 km in the western and central Himalayas. Our analysis indicates that basin outlet spacing is consistent with Hack's law, apparently because the increase in basin width (represented by outlet spacing) with basin area occurs at a rate similar to the increase in main stream length (L_v) with basin area. It is suggested that most river systems tend towards an approximately diamond‐shaped packing arrangement, and this applies both to the nonorogenic setting of southeast Africa as well as most orogenic settings. However, in the western Himalayas shortening associated with localised rock uplift appears to have occurred at length scales smaller than most the basins examined. As a result rivers in basins longer than ～30 km have been unable to erode in a direction normal to the range front at a sufficiently high rate to sustain this form and have been forced into an alternative, and possibly unstable, packing arrangement.     

Patterns of basin fill in Triassic turbidites of the Nanpanjiang basin: implications for regional tectonics and impacts on carbonate‐platform evolution

The Nanpanjiang Basin occurs in a key position for resolving controversies of basin tectonics and patterns of plate assembly at the junction between south China and Southeast Asian plates. Paleocurrent measurements indicate that siliciclastic turbidites in the basin were sourced by the Precambrian Jiangnan uplift to the northeast, the Precambrian Yunkai uplift to the southeast and the Triassic Songma suture to the south. Detrital zircon geochronology reveals Archean (2500 Ma), Paleoproterozoic (1800–1900 Ma), Neoproterozoic (900–1000 Ma) and Paleozoic (420–460 Ma) ages consistent with derivation from the Jiangnan and Yunkai uplifts. A large Permian‐Triassic peak of 250 Ma is present in the southern basin and attenuates northward suggesting derivation from an arc developed along the Songma suture. Sandstone QFL compositions average 65/12/23% and plot in the recycled orogen field except for a few samples in the southern basin that fall in the dissected arc field. The compositions are consistent with derivation from Precambrian basement that includes orogenic complexes. In the southern basin, Middle Triassic turbidites contain greater lithics and feldspars and Lower Triassic turbidites have volcaniclastic composition consistent with derivation from a southerly arc. Our preferred interpretation is evolution from remnant basin to a large peripheral foreland with southward subduction and convergence with Indochina along the Songma suture. The previously proposed Dian‐Qiong zone is not a suture as its map location places it within carbonate platforms bounded by identical stratigraphy. The Nan‐Uttaradit zone is too distant to have provided voluminous siliciclastic flux to the basin. The Nanpanjiang Basin provides an example of the evolution of an exceptionally large foreland with far‐field rejuvenation of Precambrian uplifts and carbonate platforms that were significantly influenced by siliciclastic flux. The timing and pattern of turbidite basin fill impacted platform evolution by enabling margin progradation in areas proximal to siliciclastic sources, whereas platforms distant from sources were driven to aggradation and extreme relief with large‐scale gravitational sector collapse.     

An integrated model of clastic injectites and basin floor lobe complexes: implications for stratigraphic trap plays

Injectites sourced from base‐of‐slope and basin‐floor parent sandbodies are rarely reported in comparison to submarine slope channel systems. This study utilizes the well‐constrained palaeogeographic and stratigraphic context of three outcrop examples exposed in the Karoo Basin, South Africa, to examine the relationship between abrupt stratigraphic pinchouts in basin‐floor lobe complexes, and the presence, controls, and character of injectite architecture. Injectites in this palaeogeographic setting occur where there is: (i) sealing mudstone both above and below the parent sand to create initial overpressure; (ii) an abrupt pinchout of a basin‐floor lobe complex through steep confinement to promote compaction drive; (iii) clean, proximal sand beds aiding fluidization; and (iv) a sharp contact between parent sand and host lithology generating a source point for hydraulic fracture and resultant injection of sand. In all outcrop cases, dykes are orientated perpendicular to palaeoslope, and the injected sand propagated laterally beneath the parent sand, paralleling the base to extend beyond its pinchout. Understanding the mechanisms that determine and drive injection is important in improving the prediction of the location and character of clastic injectites in the subsurface. Here, we highlight the close association of basin‐floor stratigraphic traps and sub‐seismic clastic injectites, and present a model to explain the presence and morphology of injectites in these locations.     

The contribution of peat compaction to total basin subsidence: implications for the provision of accommodation space in organic‐rich deltas

Provision of accommodation space for aggradation in Holocene deltaic basins is usually ascribed to eustatic sea‐level rise and/or land subsidence due to isostasy, tectonics or sediment compaction. Whereas many Holocene deltas contain peat, the relative contribution of peat compaction to total subsidence has not yet been quantified from field data covering an entire delta. Subsidence due to peat compaction potentially influences temporal and spatial sedimentation patterns, and therefore alluvial architecture. Quantification of the amount and rate of peat compaction was done based on (1) estimates of the initial dry bulk density of peat, derived from a relation between dry bulk density and organic‐matter content of uncompacted peat samples and (2) radiocarbon‐dated basal peat used to reconstruct initial levels of peat formation of currently subsided peat samples. In the Rhine‐Meuse delta, peat compaction has contributed considerably to total basin subsidence. Depending on the thickness of the compressible sequence, weight of the overburden and organic‐matter content of peat, subsidence of up to approximately 3 m in a 10‐m thick Holocene sequence has been calculated. Calculated local subsidence rates of peat levels are up to 0.6 mm year^?1, averaged over millennia, which are twice the estimated Holocene‐averaged basin subsidence rates of 0.1–0.3 mm year^?1 in the study area. Higher rates of subsidence due to compaction, on the order of a few mm year^?1, occur over decades to centuries, following a substantial increase in effective stress caused by sediment loading. Without such an increase in effective stress, peat layers may accumulate for thousands of years with little compaction. Thus, the contribution of peat compaction to total delta subsidence is variable in time. Locally, up to 40% of total Holocene accommodation space has been provided by peat compaction. Implications of the large amount of accommodation space created by peat compaction in deltaic basins are: (1) increased sediment trap efficiency in deltas, which decelerates delta progradation and enhances the formation of relatively thick clastic sequences and (2) enhanced local formation of thick natural levees by renewing existing accommodation space.     

New Late Jurassic palaeomagnetic data from the northern Sichuan basin: implications for the deformation of the Yangtze craton

Upper Jurassic red sandstones and red siltstones were collected from 67 layers at 12 localities in the Penglaizhen formation. This formation is in the north of Bazhong county (31.8°N, 106.7°E) in the Sichuan basin, which is located in the northern part of the Yangtze craton. Thermal demagnetization isolated a high-temperature magnetic component with a maximum unblocking temperature of about 690 °C from 45 layers. The primary nature of the magnetization acquisition is ascertained through the presence of magnetostratigraphic sequences with normal and reversed polarities, as well as positive fold and reversal tests at the 95 per cent confidence level. The tilt-corrected mean direction of 36 layers is D = 20.0°, I = 28.8° with α ₉₅ = 5.8°. A Late Jurassic palaeomagentic pole at 64.7°N, 236.0°E with A ₉₅ = 7.0° is calculated from the palaeomagnetic directions of 11 localities. This pole position agrees with the two other Late Jurassic poles from the northern part of the Yangtze craton. A characteristic Late Jurassic pole is calculated from the three poles (68.6°N, 236.0°E with A ₉₅ = 8.0°) for the northern part of the Yangtze craton. This pole position is significantly different from that for the southern part of the Yangtze craton. This suggests that the southern part of the Yangtze craton was subjected to southward extrusion by 1700 ± 1000  km with respect to the northern part. Intracraton deformation occurred within the Yangtze craton.     

The timing of salt structure growth in the Southern Permian Basin (Central Europe) and implications for basin dynamics

In this paper, a literature‐based compilation of the timing and history of salt tectonics in the Southern Permian Basin (Central Europe) is presented. The tectono‐stratigraphic evolution of the Southern Permian Basin is influenced by salt movement and the structural development of various types of salt structures. The compilation presented here was used to characterize the following syndepositional growth stages of the salt structures: (a) “phase of initiation”; (b) phase of fastest growth (“main activity”); and (c) phase of burial’. We have also mapped the spatial pattern of potential mechanisms that triggered the initiation of salt structures over the area studied and summarized them for distinct regions (sub‐basins, platforms, etc.). The data base compiled and the set of maps produced from it provide a detailed overview of the spatial and temporal distribution of salt tectonic activity enabling the correlation of tectonic phases between specific regions of the entire Southern Permian Basin. Accordingly, salt movements were initiated in deeply subsided graben structures and fault zones during the Early and Middle Triassic. In these areas, salt structures reached their phase of main activity already during the Late Triassic or the Jurassic and were mostly buried during the Early Cretaceous. Salt structures in less subsided sub‐basins and platform regions of the Southern Permian Basin mostly started to grow during the Late Triassic. The subsequent phase of main activity of these salt structures took place from the Late Cretaceous to the Cenozoic. The analysis of the trigger mechanisms revealed that most salt structures were initiated by large‐offset normal faults in the sub‐salt basement in the large graben structures and minor normal faulting associated with thin‐skinned extension in the less subsided basin parts.     

The relationship between basin and margin thermal evolution assessed by fission track thermochronology: an application to offshore southern Norway

Abstract We use a quantitative model of apatite fission track (AFT) annealing to constrain the thermal evolution of a sedimentary basin and its margin. Apatites deposited in a basin contain several types of information. Provided temperatures remained below ?70°C, they retain much of their provenance thermal signatures and mainly record the thermal evolution of their source area. Above 70°C, the fission tracks in apatite rapidly fade, reflecting the thermal evolution of the basin. Therefore, downhole AFT dates in a well section can in principle be used to assess both the provenance detail (from shallow/cool samples) and the subsequent thermal history in the basin (from the deeper samples). We apply this concept to the south Norwegian offshore and onshore using AFT and ZFT (zircon fission track) data; the latter constrain maximum palaeotemperatures and provide additional provenance information. AFT and ZFT data from three offshore wells in the northern North Sea are shown to contain a record of palaeogeographical and tectonic evolution, closely associated with the Norwegian basement. ZFT data from Middle Triassic sediments suggest a Permian volcanic source. Modelling of AFT data from Jurassic sediments presently residing at temperatures below 70°C indicate rapid cooling during the Late Triassic to Early Jurassic, similar to onshore AFT data. During the Cretaceous minor sediment supply was derived from the Norwegian basement, as evidenced by ZFT ages that do not correlate to the onshore, suggesting that parts of southern Norway were covered with sediments at this time. At the end of the Palaeogene and during the Neogene, the south Norwegian basement again became a major source of elastics. AFT and ZFT data indicate that all wells are presently at maximum temperatures. No significant (> 500 m) erosion events are indicated in the three wells since the Jurassic. AFT data have not yet effectively equilibrated to present-day temperatures as nonzero fission track ages are maintained in sediments currently at temperatures of > 120°C. This implies that the present-day thermal regime has only recently been installed. Probable causes include rapid subsidence and an increase in the geothermal gradient during the last 5 Myr.     

The Stanley unconformity in Lake Huron basin: evidence for a climate-driven closed lowstand about 7900 <Superscript>14</Superscript>C BP,with similar implications for the Chippewa lowstand in Lake Michigan basin

J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 ¹⁴C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.3¹⁴C ka, determined from the inferred ¹⁴C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.