5 results found
Lodhia B, Roberts G, Fraser A, et al., 2019, Observation and simulation of solid sedimentary flux: examples from northwest Africa, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 20, Pages: 4613-4634, ISSN: 1525-2027
The sedimentary archive preserved at passive margins provides important clues about the evolution of continental topography. For example, histories of African uplift, erosion, and deposition of clastic sedimentary rock provide information about mantle convection. Furthermore, relating histories of uplift and erosion from regions where sediment is generated to measurements of efflux is important for understanding basin evolution and the distribution of natural resources. We focus on constraining Mesozoic to Recent solid sedimentary flux to northwest Africa's passive margin, which today is fed by rivers draining dynamically supported topography. Histories of sedimentary flux are calculated by mapping stratigraphy using seismic reflection and well data courtesy of Tullow Oil Plc and TGS. Stratigraphic ages, conversion from two‐way time to depth and compaction, are parameterized using biostratigraphic and check‐shot records from exploration, International Ocean Discovery Program and Deep Sea Drilling Project wells. Results indicate that Late Cretaceous to Oligocene (∼100–23 Ma) sedimentary flux decreased gradually. A slight increase in Neogene sedimentary flux is observed, which is concomitant with a change from carbonate to clastic sedimentation. Pliocene to Recent (∼5–0 Ma) flux increased by an order of magnitude. This history of sedimentary flux and facies change is similar to histories observed at other African deltas. To constrain sources of sedimentary flux, 14,700 longitudinal river profiles were inverted to calculate a history of continental uplift. These results were used to parameterize a simple “source‐to‐sink” model of fluvial erosion and sedimentary efflux. Results suggest that increased clastic flux to Africa's deltas from ∼30 Ma was driven by denudation induced by dynamic support.
Lodhia BH, 2019, Dynamic subsidence, continental uplift and sedimentary deposition to West Africa’s passive margin
The relationship between offshore subsidence, continental uplift and sedimentary deposition to passive margins is poorly understood. A history of dynamic subsidence, continental uplift and sedimentary deposition to West Africa’s passive margin was synthesised to produce a framework for predicting patterns of sedimentary flux to the Mauritanian basin. Stratigraphy was mapped using seismic/well data and Cretaceous–Recent sedimentary flux was calculated. Clastic sedimentary flux to the centre of the basin increased significantly during Neogene times and accelerated across the basin at ~ 5 Ma. The origins of the Mauritanian basin were explored by backstripping wells from the study area to calculate subsidence histories. Seismic, well, gravity, magmatic and tomographic information was used to show that < 0:8 km of rapid Neogene– Recent subsidence occurred due to the initiation of shallow mantle convection. Neogene–Recent mantle draw-down caused the creation of accommodation within the Mauritanian basin that was subsequently filled by clastic sediments delivered by rivers draining West Africa’s passive margin. The causes of increased Neogene–Recent sedimentary flux were explored by inverting 14700 African longitudinal river profiles using a calibrated stream–power model to calculate a history of continental uplift and to predict the pattern of sedimentary flux to the Mauritanian basin. Results are consistent with independent stratigraphic observations which indicate that from ~ 30 Ma–Recent, sedimentary flux to Africa’s major deltas increased and induced a change from dominantly carbonate to clastic deposition. This pattern of sedimentary flux is consistent with calculations of offshore sedimentary volumes made using seismic and well data. I suggest Neogene clastic flux to the Mauritanian margin was generated by denudation of the uplifting Fouta-Djallon topographic swell and Mauritinides fold-belt, which appears to
Roberts GG, White N, Lodhia BH, 2019, The generation and scaling of longitudinal river profiles, Journal of Geophysical Research: Earth Surface, Vol: 124, Pages: 137-153, ISSN: 2169-9003
The apparent success of inverse modeling of continent‐wide drainage inventories is perplexing. An ability to obtain reasonable fits between observed and calculated longitudinal river profiles implies that drainage networks behave simply and predictably at length scales of O(102–103) km and timescales of O(100–102) Ma. This behavior suggests that rivers respond in a predictable way to large‐scale tectonic forcing. On the other hand, it is acknowledged that stream power laws are empirical approximations since fluvial processes are complex, non‐linear, and probably susceptible to disparate temporal and spatial shocks. To bridge the gap between these different perceptions of landscape evolution, we present and interpret a suite of power spectra for African river profiles that traverse different climatic zones, lithologic boundaries, and biotic distributions. At wavelengths ≳ 102 km, power spectra have slopes of −2, consistent with red noise, demonstrating that profiles are self‐similar at these length scales. At wavelengths ≲ 102 km, there is a cross‐over transition to slopes of −1, consistent with pink noise, for which power scales according to the inverse of wavenumber. Onset of this transition suggests that spatially correlated noise, perhaps generated by instabilities in water flow and by lithologic heterogeneities, becomes more prevalent at wavelengths shorter than ∼100 km. At longer wavelengths, this noise gradually diminishes and self‐similar scaling emerges. Our analysis is consistent with the concept that complexities of river profile development are characterized by an adaptation of the Langevin equation, by which simple advective models of erosion are driven by a combination of external forcing and noise.
Roberts GG, Lodhia B, Fraser A, et al., 2018, Continental margin subsidence from shallow mantle convection: example from West Africa, Earth and Planetary Science Letters, Vol: 481, Pages: 350-361, ISSN: 0012-821X
Spatial and temporal evolution of the uppermost convecting mantle plays an important role in determining histories of magmatism, uplift, subsidence, erosion and deposition of sedimentary rock. Tomographic studies and mantle flow models suggest that changes in lithospheric thickness can focus convection and destabilize plates. Geologic observations that constrain the processes responsible for onset and evolution of shallow mantle convection are sparse. We integrate seismic, well, gravity, magmatic and tomographic information to determine the history of Neogene-Recent (<23 Ma) upper mantle convection from the Cape Verde swell to West Africa. Residual ocean-age depths of +2 km and oceanic heat flow anomalies of +16 ± 4 mW m−2 are centered on Cape Verde. Residual depths decrease eastward to zero at the fringe of the Mauritania basin. Backstripped wells and mapped seismic data show that 0.4–0.8 km of water-loaded subsidence occurred in a ∼500 × 500 km region centered on the Mauritania basin during the last 23 Ma. Conversion of shear wave velocities into temperature and simple isostatic calculations indicate that asthenospheric temperatures determine bathymetry from Cape Verde to West Africa. Calculated average excess temperatures beneath Cape Verde are View the MathML source °C providing ∼103 m of support. Beneath the Mauritania basin average excess temperatures are View the MathML source °C drawing down the lithosphere by ∼102 to 103 m. Up- and downwelling mantle has generated a bathymetric gradient of ∼1/300 at a wavelength of ∼103 km during the last ∼23 Ma. Our results suggest that asthenospheric flow away from upwelling mantle can generate downwelling beneath continental margins.
Kovin ON, Blinov SM, Belkin PA, et al., 2014, Results of Integrated Investigation of Collapse Sinkhole in Sarkayevo Village, Вестник Пермского университета. Геология, Vol: 1, Pages: 35-43, ISSN: 1994-3601
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