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.     

Geoarchaeological investigations of San Mateo and Las Flores Creeks,California: Implications for coastal settlement models

The late Quaternary stratigraphy and geochronology of San Mateo and Las Flores Creeks indicates that both streams, which drain into the Pacific Ocean along the southern California coast, had a synchronous landscape history. Both San Mateo and Las Flores Creeks cut deep valleys during the late Pleistocene in response to the worldwide drop in sea level. A long period of aggradation followed as sea level rose to its present position during the late Pleistocene and Holocene. Around 500 years ago, the channels of both streams downcut into their respective floodplains creating a prominent terrace (T-2). This was followed by renewed deposition and later channel incision which created a second terrace (T-1). These channel changes are probably the result of a complex response of the fluvial system to major flooding during the late Holocene. The geologic history of these drainages provides a framework that can be used to assess the buried archaeological record along the coast and evaluate regional prehistoric settlement patterns. Based on this study, the post-4000 B.P. valley floor archaeological record along the southern California coast will be more complete and accessible, resulting in a better understanding of the spatial distribution of sites. On the other hand, the pre-4000 B.P. valley floor record lies deeply buried and inaccessible to researchers, which results in a distortion of settlement patterns before 4000 B.P. Finally, examination of post-4000 B.P. sites in the alluvium of San Mateo and Las Flores creeks yielded a pattern of large sites that were occupied extensively for multiple seasons and had considerable evidence of marine exploitation. These findings are contrary to models of cultural development along this portion of the southern California coast. © 1999 John Wiley & Sons, Inc.     

Sand transport and deposition within arrays of non-erodible cylindrical elements

The study concerns sand deposition within a regular array of vertical cylinders placed in the path of a sand-laden wind. Twelve wind tunnel experiments using three preselected shear velocities (28·78, 32·86 and 45·1 cm s⁻¹), with associated rates of sand feed (0·3, 2·0 and 3·8 g cm⁻¹ s⁻¹), and four roughness element concentrations (λ = 0·046, 0·092, 0·184 and 0·369) were carried out to evaluate the factors that affect sand deposition and sand flux in the presence of immobile rough elements. The measurements showed that as the concentration of non-erodible elements increased, the percentage reduction in the initial sand flux increased and a particularly sharp reduction occurred when λ ≥ 0·18. The pattern of reduction was found to be q_red = q_eq (d/H) [Δy/(Δy−d)](0·68 −3·5λ) when λ ≤ 0·18, and q_red = q_eq(d/H)[Δy/(Δy−d)](0·025) when λ > 0·18, where q_eq is the equilibrium rate of sand transport arriving at the best bed, d is the diameter of the cylinder, H is the height of the cylinder, and Δy is the width of unit area associated with a cylinder. The experimenal results also showed that the sand flux downstream of the array started to increase immediately upon the commencement of burial of the array's cylinders. Thus the sand deposition and sand flux along an array consisting of regularly distributed, non-erodible elements were shown to be neither uniform nor steady. Copyright © 1999 John Wiley & Sons, Ltd.     

Trace element soil contamination from smelters in the Illawarra region,New South Wales,Australia

Boundaries of densely populated areas can approach with time to old subsurface repositories of radioactive waste due to growth of settlement territories. P     

The baseflow and storm flow hydrology of a precambrian shield headwater peatland

A hydrological investigation was conducted in a small headwater peatland located in the Experimental Lakes Area, north-western Ontario, Canada, to determine the subsurface and surface flow paths within the peatland, and between the peatland and an adjacent forested upland during baseflow and storm flow conditions. Distinct zones of groundwater recharge and discharge were observed within the peatland. These zones are similar to those found in much larger flow systems even though the peatland was only influenced by local groundwater flow. Groundwater emerging in seeps and flowing beneath the peatland sustained the surface wetness of the peatland and maintained a constant baseflow. The response of the peatland stream to summer rain events was controlled by peatland water table position when the basin was dry and antecedent moisture storage on the uplands when the basin was wet. The magnitude and timing of peak runoff during wet conditions were controlled by the degree of hydrological connectivity between the surrounding upland terrain and the peatland. © 1998 John Wiley & Sons, Ltd.     

Recycled insular phosphates and coated grains: Case study from Little Cayman,British West Indies

Phosphatic limestones on the west end of Little Cayman, at an elevation of 3 to 4 m above sea level and ca 320 to 550 m inland of the coast, lie on top of a phytokarst surface that defines the upper boundary of the Pedro Castle Formation (Pliocene). These phosphatic limestones are formed of phosphatic lithoclasts, detrital phosphate grains, coated grains (glaebules), composite coated grains, biofragments and phosphate rafts that are held in a matrix that is formed of micrite, calcite cement, and non-crystalline masses formed of P, Al, Si and Fe. The phosphate in these limestones is primarily hydroxylapatite, whereas the overlying soils, found in some areas, are formed of hydroxylapatite, crandallite and minor amounts of boehemite, kaolinite and quartz. Textures in the lithoclasts and detrital phosphate grains indicate that they were derived from older insular phosphates that that were largely removed by mining in 1890 to 1895. The coated grains (glaebules) typically have nuclei formed of a detrital phosphate grains that are encased by non-crystalline cortical laminae that are composed primarily of Al with their variable red colour reflecting the variable Fe content. The phosphatic limestones developed in a low-lying coastal area where the guano produced by a large seabird colony that was close to or mixed with terra rossa and marine carbonates that were washed onshore during storms/hurricanes. The Al, Fe, Si and rare earth elements found in the phosphatic limestones came from the terra rossa. Critically, this study documents the complex depositional regimes and diagenetic processes that can exist at the interface of marine carbonates, coastal phosphates and terrestrial soils as sea-level fluctuations control phases of sediment accumulation that were periodically interrupted by periods of non-deposition.     

Real-time hybrid simulation for structural control performance assessment

Real-time hybrid simulation is an attractive method to evaluate the response of structures under earthquake loads.The method is a variation of the pseudodynamic testing technique in which the experiment is executed in real time,thus allowing investigation of structural systems with rate-dependent components.Real-time hybrid simulation is challenging because it requires performance of all calculations,application of displacements,and acquisition of measured forces,within a very small increment of time.Furthe...     

Macro- and Micro-Purge Soil-Gas Sampling Methods for the Collection of Contaminant Vapors

Purging influence on soil‐gas concentrations for volatile organic compounds (VOCs), as affected by sampling tube inner diameter and sampling depth (i.e., system volume) for temporary probes in fine‐grained soils, was evaluated at three different field sites. A macro‐purge sampling system consisted of a standard, hollow, 3.2‐cm outer diameter (OD) drive probe with a retractable sampling point attached to an appropriate length of 0.48‐cm inner diameter (ID) Teflon^® tubing. The macro‐purge sampling system had a purge system volume of 24.5 mL at a 1‐m depth. In contrast, the micro‐purge sampling systems were slightly different between the field sites and consisted of a 1.27‐cm OD drive rod with a 0.10‐cm ID stainless steel tube or a 3.2‐cm OD drive rod with a 0.0254‐cm inner diameter stainless steel tubing resulting in purge system volumes of 1.2 and 7.05 mL at 1‐m depths, respectively. At each site and location within the site, with a few exceptions, the same contaminants were identified in the same relative order of abundances indicating the sampling of the same general soil atmosphere. However, marked differences in VOC concentrations were identified between the sampling systems, with micro‐purge samples having up to 27 times greater concentrations than their corresponding macro‐purge samples. The higher concentrations are the result of a minimal disturbance of the ambient soil atmosphere during purging. The minimal soil‐gas atmospheric disturbance of the micro‐purge sampling system allowed for the collection of a sample that is more representative of the soil atmosphere surrounding the sampling point. That is, a sample that does not contain an atmosphere that has migrated from distance through the geologic material or from the surface in response to the vacuum induced during purging soil‐gas concentrations. It is thus recommended that when soil‐gas sampling is conducted using temporary probes in fine‐grained soils, the sampling system use the smallest practical ID soil‐gas tubing and minimize purge volume to obtain the soil‐gas sample with minimal risk of leakage so that proper decisions, based on more representative soil‐gas concentrations, about the site can be made.     

Exploring the nature of non-Fickian transport in laboratory experiments

Observations of non-Fickian transport in sandbox experiments [Levy M, Berkowitz B. Measurement and analysis of non-Fickian dispersion in heterogeneous porous media. J Contam Hydrol 2003;64:203–26] were analyzed previously using a power law tail ψ(t) ∼ t^−1−β with 0 < β < 2 for the spectrum of transition times comprising a tracer plume migration. For each sandbox medium a choice of β resulted in an excellent fit to the breakthrough curve (BTC) data, and the value of β decreased slowly with increasing flow velocity. Here, the data are reanalyzed with the full spectrum of ψ(t) gleaned from analytical calculations [Cortis A, Chen Y, Scher H, Berkowitz B. Quantitative characterization of pore-scale disorder effects on transport in “homogeneous” granular media. Phys Rev E 2004;10(70):041108. doi: 10.1103/PhysRevE.70.041108], numerical simulations [Bijeljic B, Blunt MJ. Pore-scale modeling and continuous time random walk analysis of dispersion in porous media. Water Resour Res 2006;42:W01202. doi: 10.1029/2005WR004578] and permeability fields [Di Donato G, Obi E-O, Blunt MJ. Anomalous transport in heterogeneous media demonstrated by streamline-based simulation. Geophys Res Lett 2003;30:1608–12s. doi: 10.1029/2003GL017196]. We represent the main features of the full spectrum of transition times with a truncated power law (TPL), ψ(t) ∼ (t₁ + t)^−1−βexp(−t/t₂), where t₁ and t₂ are the limits of the power law spectrum. An excellent fit to the entire BTC data set, including the changes in flow velocity, for each sandbox medium is obtained with a single set of values of t₁, β, t₂. The influence of the cutoff time t₂ is apparent even in the regime t < t₂. Significantly, we demonstrate that the previous apparent velocity dependence of β is a result of choosing a pure power law tail for ψ(t). The key is the change in the log–log slope of the TPL form of ψ(t) with a shifting observational time window caused by the change in the mean velocity. Hence, the use of the full spectrum of ψ(t) is not only necessary for the transition to Fickian behavior, but also to account for the dynamics of these laboratory observations of non-Fickian transport.