Autoacceleration of clinoform progradation in foreland basins: theory and experiments     

J. Leva López  W. Kim  R. J. Steel 《Basin Research》2014,26(4):489-504

Understanding the relationship between sedimentation and tectonics is critical to the analysis of stratigraphic evolution in foreland basins. Previous models of foreland basins have explained stratal development, but were done generally under the assumption that steady allogenic forcing produces a steady stratigraphic response. They did not consider autogenic shoreline behaviour during the development of the subsidence pattern characteristic of foreland basins. We present a mathematical model and flume experiments that explore how subsidence and sediment‐supply rates control the shoreline trajectory and the stratal patterns that fill foreland basins. Through these models, we found differing autogenic responses in the rate and direction of shoreline migration, and these generated three distinct styles of stratal architecture, despite the constant external forcing (i.e. constant sediment discharge and basin substrate tilting). The first response was ‘autoretreat’, where shoreline migration switched from initial progradation to retrogradation. The second response was progradation followed by constant aggradation of the shoreline. The third response was maintained progradation with a markedly accelerating rate. We termed this latter newly observed autogenic behaviour ‘shoreline autoacceleration’. These three modes of shoreline behaviour and their accompanying stratal architecture provide a basic framework for the relationship between sedimentation and tectonic activity in foreland basins under the simplified conditions presented here.  相似文献   

A Mariner/MESSENGER global catalog of mercurian craters     

Robert R. Herrick  Lorna L. Curran  Adrian T. Baer 《Icarus》2011,215(1):452-454

Using MESSENGER and Mariner 10 flyby images, we have compiled a global catalog of impact structures with diameters D > 10 km. The distribution of impact structures shows a factor of 10 range in areal crater density. Most regions of low crater density are located within large impact basins, consistent with the idea that these were low-lying areas that have been filled by subsequent volcanism over an extended period.  相似文献   

The Pterosaur Collection at the Natural History Museum,London, UK: an Overview and List of Specimens,with Description of Recent Curatorial Developments     

Lorna STEEL 《《地质学报》英文版》2012,86(6):1340-1355

The pterosaur collection at the Natural History Museum is of great historical importance. Most of the material is figured or cited, and several specimens are types or casts of types. Pterosaurs from UK localities are well represented, but foreign material is also present as original specimens and as high quality casts. The entire collection has been recorded in a database and curatorial improvements have been made, which has improved physical and virtual access to the collection. Although many of the specimens were collected as long ago as the early 1800s, they are still useful today. A full list of Natural History Museum pterosaur specimens is published, for the first time since 1888.  相似文献   

Influence of point-source sediment-supply on modern shelf-slope morphology: implications for interpretation of ancient shelf margins     

Cornel Olariu  Ronald J. Steel 《Basin Research》2009,21(5):484-501

Present sea-floor bathymetry indicates that the continental-shelf and shelf-break morphology have some unique and predictable characteristics in areas with and without high sediment supply. Using a global bathymetry dataset in open shelf areas in front of rivers that discharge over 25 × 10⁶ tons of sediment per year, five distinct accretionary types of shelf-break are distinguished based on along-shelf gradient variability and inferred shelf-break trajectory. Morphological characteristics of river-mouth shelves (compared with adjacent areas lateral to the immediate fairway of the river) are: (1) an overall lower gradient and greater width, and (2) a relatively high slope gradient/shelf gradient ratio. The exceptions are shelves with active shelf-edge deltas; these are narrower, steeper and have an attenuated shelf break in front of rivers. These observations are at seismic scale and have direct implications for the recognition and positioning of principal cross-shelf, supply fairways on ancient shelves or shelf margins, and therefore the potential by-pass routes for deepwater sands. Higher slope/shelf gradient ratios in areas of actively accreting margins, where the shelf-break is more prominent and easier to recognize on seismic data compared with adjacent areas, predict areas with high sediment supply. Along-strike morphological changes on supply-dominated shelves suggest that identification of the sediment-feed route and depocenter relative to the shelf break during a relative sea level cycle are critical for understanding/predicting the 3-D architecture of the shelf-slope-basin floor clinoform.  相似文献   

River-dominated and tide-influenced shelf-edge delta systems: Coarse-grained deltas straddling the Early–Middle Jurassic shelf–slope break and transforming downslope,Lajas–Los Molles formations,Neuquén Basin,Argentina     

Flavio N. Almeida Junior  Ronald J. Steel  Cornel Olariu  Yuqian Gan  Paulo S. Gomes paim 《Sedimentology》2020,67(6):2883-2916

The three-dimensional facies and architecture variability of shelf-edge deltaic units cropping out at the transition between the Lower–Middle Jurassic Lajas and Los Molles formations of southern Neuquén Basin, Argentina, is presented here, as well as their stratigraphic relationship to uppermost deep-water slope channel systems. Deep-water, slope mudstones with thin turbidite beds merge upward with prodelta mudstones and thin sandstones, which are truncated by delta-front to mouth-bar sandstones. The latter sandstones are then downcut by large-scale, trough cross-stratified coarse-grained sandstones and conglomerates of distributary channel systems and along-strike, amalgamated with cross-bedded sandy units showing evidence of tidal reworking. Proximal–distal facies and architecture variability within a shelf-edge deltaic succession demonstrates that distributary channel-complexes become wider and deeper basinward, forming channelized river-dominated distributary fairways separated by tidally reworked inter-distributary sand belts at the shelf edge. Evidence from depositional-dip oriented outcrops shows a lack of collapsed and slumped strata at the shelf edge, and that the coarse shelf-edge distributary channel fills continue far down the deepwater slope, and conglomerates transform to become high-density turbidites to mainly thick-bedded, sand-matrix-supported debrites. The interplay between flood tides and river currents is interpreted to have primarily modulated the focusing of river drainages, and consequently coarse-grained sediment transport, along preferential routes on the outer-shelf to shelf-edge and down onto the slope. This contribution documents a unique example of coarse-grained (mostly conglomeratic) shelf-edge delta systems, tying bed-scale facies and architecture data to a seismic-scale shelf-margin morphology, thus providing outcrop analogue data for the characterization of shelf-edge delta systems in the subsurface.  相似文献   

Astrometry of near-earth objects using small telescopes     

D. I. Steel  B. G. Marsden 《Earth, Moon, and Planets》1996,74(2):85-92

Currently, when a near-Earth object (NEO: any asteroid or comet approaching the terrestrial orbit about the Sun) is discovered, there is a distinct chance that it will be lost again due to the lack of suitable follow-up astrometry. The burgeoning international Spaceguard program — which has an aim of discovering and determining the orbits of all possible Earth-impactors so that calamitous collisions can be avoided-will make this problem more acute, as the discovery rate increases. When NEOs reach visual magnitudes brighter than 21, they may be observed with suitable CCD-equipped telescopes with apertures of one metre or less; in fact, even 0.25-m aperture instruments are now being employed to great effect in this regard. This means that in the present situation, countries or individuals having access to quite modest-sized and modest-priced equipment can make a real, valuable contribution to this international effort. When Spaceguard begins in earnest it will be vital that astronomers spread around the world, using small telescopes, take over the tracking of NEOs discovered by the large, expensive, search instruments. The Spaceguard project therefore represents an excellent opportunity for international collaboration, with scientists in developing nations being valuable contributors in a space program with real significance for all humankind. Presented at the UN/ESA Workshop on Basic Space Science: From Small Telescope to Space Mission, Colombo, Sri Lanka, 11–14 January 1996.

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The ABC of ACM: asteroids, Buffon and comets     

D.I. Steel 《Planetary and Space Science》1997,45(12):1501-1503

Most of the participants in the ACM 96 conference would have made use of facilities in a building named for Georges-Louis Leclerc, the Compte de Buffon (1707–1788). Buffon made many major contributions to the natural sciences, and may be considered to be one of the founders of planetary science. He proposed a theory for the origin of the planets which involved a massive comet having an oblique impact upon the Sun, the ejected material condensing so as to form a regular system of planets. Amongst his mathematical contributions is what is known as Buffon's Needle, whereby experimental evaluations of π may be made by randomly dropping a needle onto a set of parallel lines of separation greater than the needle length, and accumulating the fraction of times that the needle cuts one of the lines. Near-Earth asteroid (NEA) trails imaged onto a CCD chip provide a two-dimensional analogue of this, and where the pixel size is very large (this having some advantages for NEA searching) an analysis based on Buffon's Needle provides probabilities of the NEA trail lying within one, two or three pixels, such probabilities affecting the chances of detection. It is therefore appropriate that Buffon and his contributions to studies of comets and asteroids be remembered in these conference proceedings.  相似文献   

Rethinking the longitudinal stream temperature paradigm: region‐wide comparison of thermal infrared imagery reveals unexpected complexity of river temperatures          下载免费PDF全文

Aimee H. Fullerton  Christian E. Torgersen  Joshua J. Lawler  Russell N. Faux  E. Ashley Steel  Timothy J. Beechie  Joseph L. Ebersole  Scott G. Leibowitz 《水文研究》2015,29(22):4719-4737

Prevailing theory suggests that stream temperature warms asymptotically in a downstream direction, beginning at the temperature of the source in the headwaters and levelling off downstream as it converges to match meteorological conditions. However, there have been few empirical examples of longitudinal patterns of temperature in large rivers due to a paucity of data. We constructed longitudinal thermal profiles (temperature vs distance) for 53 rivers in the Pacific Northwest (USA) using an extensive data set of remotely sensed summertime river temperatures and classified each profile into one of five patterns of downstream warming: asymptotic (increasing then flattening), linear (increasing steadily), uniform (not changing), parabolic (increasing then decreasing), or complex (not fitting other classes). We evaluated (1) how frequently profiles warmed asymptotically downstream as expected, and (2) whether relationships between river temperature and common hydroclimatic variables differed by profile class. We found considerable diversity in profile shape, with 47% of rivers warming asymptotically and 53% having alternative profile shapes. Water temperature did not warm substantially over the course of the river for coastal parabolic and uniform profiles, and for some linear and complex profiles. Profile classes showed no clear geographical trends. The degree of correlation between river temperature and hydroclimatic variables differed among profile classes, but there was overlap among classes. Water temperature in rivers with asymptotic or parabolic profiles was positively correlated with August air temperature, tributary temperature and velocity, and negatively correlated with elevation, August precipitation, gradient and distance upstream. Conversely, associations were less apparent in rivers with linear, uniform or complex profiles. Factors contributing to the unique shape of parabolic profiles differed for coastal and inland rivers, where downstream cooling was influenced locally by climate or cool water inputs, respectively. Potential drivers of shape for complex profiles were specific to each river. These thermal patterns indicate diverse thermal habitats that may promote resilience of aquatic biota to climate change. Without this spatial context, climate change models may incorrectly estimate loss of thermally suitable habitat. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Inferring autogenically induced depositional discontinuities from observations on experimental deltaic shoreline trajectories          下载免费PDF全文

Daniel Mikeš  Johan H. ten Veen  George Postma  Ronald Steel 《地学学报》2015,27(6):442-448

Palaeo shoreline is a commonly used proxy for palaeo sea level, but only if deposition is continuous and constant will shoreline trajectory T(l) completely capture sea‐level time‐series E(t). Artificial deltas were generated in the Eurotank flume facility under stepwise tectonic subsidence, periodic sea‐level fluctuation and two periodic water‐discharge scenarios, one in‐phase and the other out‐of‐phase with sea level. Independent input variables tectonic subsidence Y, sea level E and water discharge Q (controlling sediment supply S) were varied and dependent output variable shoreline trajectory T was monitored. These experiments confirm that deposition is discontinuous even for continuous sediment supply, and this hinders the inference of sea‐level curve from shoreline trajectory. These results justify the here‐developed methodology for converting shoreline trajectory from the space domain to the time domain, thereby improving the accuracy of the inferred sea‐level curve.  相似文献   

Forensic geology at the International School Science Fair 2013     

Duncan Pirrie  Laurance Donnelly  Gavyn K. Rollinson  Alan R. Butcher  Lorna A. Dawson  Heather A. Pirrie 《Geology Today》2013,29(6):222-228

In July 2013 the International School Science Fair (ISSF) was hosted by Camborne Science and International Academy, Cornwall, UK. This meeting brings young talented scientists together from around the world to participate in workshops and activities highlighting current scientific developments. As part of ISSF 2013, a workshop on forensic geology was delivered to some of the international participants. This included the preparation of a map to show the mineralogical composition of the soils of the participating schools. The soil mineralogy was determined using automated mineral analysis based on scanning electron microscopy. In addition there were workshops on the recovery of geological trace evidence in a forensic context and the theory and practice of carrying out a geophysical search for hidden items. Data generated as part of this workshop are available to download from the International Union of Geological Sciences, Initiative on Forensic Geology website ( http://www.forensicgeologyinternational.com ).  相似文献