Fossil flat-slab subduction beneath the Illinois basin, USA

The Illinois basin is one of several well-studied intracratonic sedimentary basins within the North American craton whose formational mechanisms and subcrustal structure are not well understood. We study the S-velocity structure of the upper mantle beneath the Illinois basin and its surrounding area through seismic tomography. We utilize continental scale waveform data of seismic S and surface waves, enhanced by regional earthquakes located near the Illinois basin. Our 3D tomographic model, IL05, confirms the existence of a slow S-velocity structure in the uppermost mantle beneath the Illinois basin region. This anomalously slow region exists from the base of the crust to depths of  90 km, and is slower than the North American cratonic average by about 200 m/s. This anomalous uppermost mantle beneath the Illinois basin is underlain by a faster lithosphere, typical of the surrounding craton, to depths of  200 km. Excluding the formation of the Reelfoot Rift, this area of North American has been stable for over 1.0 Gy. Thus, we do not expect thermal anomalies from before that time to persist into present day S-velocity anomalies and we consider a delamination origin as an explanation of Illinois basin subsidence unlikely. We cannot rule out that the slow mid-lithosphere beneath the Illinois basin is caused by an uppermost mantle enriched by a deep, but weak plume. We attribute the slow mid-lithosphere to the presence of either oceanic, hydrous crust, or, a relatively cool mantle wedge with preserved hydrous minerals in the Illinois basin's uppermost mantle, related to a fossilized flat subduction zone.     

Effect of acid mine drainage on a karst basin: a case study on the high-As coal mining area in Guizhou province,China

Acid mine drainage (AMD) is a common pollution in mining areas due to the oxidation of pyrite and associated sulfide minerals at mines, tailings and mine dumps. Elevated metals (Fe, Mn, Al) and metalloids (As, Hg) in AMD would deteriorate the local aquatic environment and influence the water supply. A carbonate basin with deposits of high-arsenic coal in Xingren County, southwestern China, was chosen to study the behavior of As and other chemical constituents along a river receiving AMD. Heavy metals (Fe, Mn) and major ions such as (Ca²⁺, Mg²⁺, Cl⁻, SO₄ ²⁻) in surface water, and As in sediment and surface water were analyzed. It was found that high concentrations of SO₄ ²⁻ (1,324–7,560 mg/L) and Fe (369–1,472 mg/L) in surface water were mainly controlled by the interactions between water and rocks such as the oxidation of pyrite in the local coal seams, precipitation and adsorption of iron minerals. Although ubiquitous carbonate minerals in the bedrock and the riverbeds, low pH (<3) water was maintained until 2 km downstream from the AMD source due to the Fe(hydro)oxide minerals coating on the surface of carbonate minerals to restrain the neutralization of acidic water. Moreover, the formation of Fe(hydro)oxide precipitations absorbed As was dominated the attenuation of As from water to sediment. Whereas, the dilution also played an important role in decrease of As in river water.     

Leaching of coal-mine tips (Nord-Pas-de-Calais coal basin,France) and sulphate transfer to the chalk aquifer: example of acid mine drainage in a buffered environment

In northern France, the weathering and oxidation of pyrite-containing coal mine spoils can lead to sulphate enrichment of the underlying chalk aquifer. Two sites have been studied: in a free water-table zone and in a confined-aquifer zone. Solid-fraction analyses have shown a release of carbon and sulphur from the waste dump surfaces. The groundwater isotopic analysis (notably ³⁴S) showed the existence of two sulphate sources (mine spoils and gypsum contained in Cenozoic formations). This study also saw evidence of bacterial sulphate reduction in the confined zone as a result of the release of carbon from mine spoils. The effects of acid mine drainage were the presence of secondary minerals as small jarosite dots (cm) present at depth, gypsum or ferrihydrite present on the bottom of temporary ponds, and an increase in sulphate content of groundwaters sampled downstream of the waste dumps. This acid mine drainage is buffered during its infiltration into the chalk aquifer. This is evidenced by the increase in magnesium, calcium, bicarbonate or strontium, resulting of carbonate digestion downstream of the sites. No significant leaching of metals, even those associated with the sulphide fraction, was seen at the two studied sites.     

A strontium isotopic study of formation waters from the Illinois basin, U.S.A.

The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated ⁸⁷Sr/⁸⁶Sr rations is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian water show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Regional migration of the waters and associated petroleum within the Silurian-Devonian strata, proposed in other studies, is consistent with the Sr isotopic data. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic recor of their evolution.     

废弃矿井充水问题及其研究和治理方法——以美国匹兹堡煤田为例

以美国匹兹堡煤田为例,介绍了废弃矿井地下水回充、相邻矿井水连通和自流排泄出地表,以致造成河水污染,生产矿井涌水量增加,甚至导致生产矿井水害事故等问题。强调应基于GIS和GPS等技术对这类矿区进行水文地质填图,设立长期水位水质观测站,并有针对性的设置矿井水处理厂。针对我国废弃矿井充水问题提出了建议。     

神府东胜矿区煤炭开采对水资源的影响机制——以窟野河流域为例

研究神府东胜矿区煤炭开采对水资源的影响机制,对于保护当地水资源及脆弱的生态环境具有重要意义。以窟野河为例,分析了煤炭开采对该流域水资源量与水质的影响机制,并计算出吨煤开采的基流损失量约为2.038 m³(1997—2005年)。研究结果表明,煤炭开采时形成的裂隙将萨拉乌苏组含水层中的水导入矿坑中,导致潜水由水平径流、排泄为主转化为以垂向渗漏为主,引发地下水位大幅下降,河流基流量减少以及泉流量衰减甚至干枯;煤炭开采使含水层中的水进入到矿坑,在物理作用和化学作用下形成硬度更大、矿化度更高的矿坑水,矿坑水排入河流后对河流造成污染。     

A geochemical study of the peat and soft-brown coal from the Zhenan basin,Western Yunnan,China

Little is known about the geochemistry of peat and soft-brown coal The analytical chemical dataof forty eight samples have been obtained for the peat and soft-brow n coal in the 7henan basin, western Yunnan. njection of continental detritus into theswamp is favourable for the degradation of plant remains and thefor-oration of humic acid. The oxide compositions of the ash of the peat and soft一)rown coal and their distribution-typeshave indicated that the continental detritusome from two kinds of parent rocks.The Al₂O₃ and SiO₂have a positive correlation with humic acid(Hmz)，showing that the organic matter is ad-vantageous to the formation of aluminosilic;ate mineral(mainly kaolinite, authigenic organic clay minerals). The TiO₂ enrichment is mainly related to mineral materials. The Ge content in the peat and soft-brown coalranges from 0. 2- 2.6×10-6, and it is mainly bound to those minerals with Al₂O₃and organic matters. The GaContent is from 2. 3- 19.1×10-6，and it is associated with minerals that are MgO一Baring aluminosilicate minerals.The uranium (0.3- 4.9×10-6) is mainly bound in the Ca-and Mg-bearing minerals. They are not enriched andnot related to organic matter.     

Holocene valley-floor deposition and incision in a small drainage basin in western Colorado, USA

The valley floor of a 33.9 km² watershed in western Colorado experienced gradual sedimentation from before ∼ 6765 to ∼ 500 cal yr BP followed by deep incision, renewed aggradation, and secondary incision. In contrast, at least four terraces and widespread cut-and-fill architecture in the valley floor downstream indicate multiple episodes of incision and deposition occurred during the same time interval. The upper valley fill history is atypical compared to other drainages in the Colorado Plateau.One possible reason for these differences is that a bedrock canyon between the upper and lower valley prevented headward erosion from reaching the upper valley fill. Another possibility is that widespread, sand-rich, clay-poor lithologies in the upper drainage limited surface runoff and generally favored alluviation, whereas more clay-rich lithologies in the lower drainage resulted in increased surface runoff and more frequent incision. Twenty-two dates from valley fill charcoal indicate an approximate forest fire recurrence interval of several hundred years, similar to that from other studies in juniper-piñon woodlands. Results show that closely spaced vertical sampling of alluvium in headwater valleys where linkages between hillslope processes and fluvial activity are relatively direct can provide insight about the role of fires in alluvial chronologies of semi-arid watersheds.     

煤矿采空塌陷的危险性预测评估与防治措施——以华亭矿区大柳矿井为例

煤矿开采后引发地面塌陷的危险性预测评估是煤矿地质灾害危险性评估的核心内容。以华亭矿区大柳矿井地质灾害危险性评估为例,介绍了煤矿采空引发地面塌陷危险性预测评估的基本内容及评估深度,并提出了较全面的矿区采空塌陷的防治措施,可以为矿井开采后防治地面塌陷提供指导。     

岩浆侵入煤层中锑的赋存特征——以淮北卧龙湖矿为例

为探讨岩浆蚀变作用对煤层中锑赋存特征的影响,系统采集安徽淮北煤田卧龙湖煤矿岩浆侵入煤层侵入岩和全煤层样品共12个,利用原子荧光光谱法（AFS）测定样品中Sb含量,并对煤质参数进行分析。结果表明：卧龙湖煤矿岩浆侵入区煤表现为超低挥发分,中等灰分,特低硫的特点,煤中的硫主要以有机硫和黄铁矿硫存在。受岩浆热液影响,煤中灰分增加,挥发分减少;岩浆蚀变煤层中锑明显富集,算术平均值达到10.48 mg/kg,且侵入岩上方煤中Sb的平均含量明显增高,煤岩接触带位置Sb的含量达到最高值（13.93 mg/kg）;岩浆蚀变煤中的锑主要以无机结合态形式存在（相关系数r为0.74）,有机硫与煤中Sb呈显著负相关（相关系数r为-0.60）。岩浆侵入作用导致卧龙湖煤矿煤的煤质特征及煤层中锑的赋存方式受到不同程度的影响,研究结果可为特殊地质作用下煤中锑的环境地球化学特征提供参考。     

Fracture and stress orientation from borehole image logs: A case study from Cambay basin,India

The Deccan trap basalt, laid down by multiple lava flows during upper Cretaceous to Paleocene times forms the basement of current study in Cambay basin. As such, there is great interest and value in fracture detection and evaluation of fractured basement reservoirs in the Cambay basin. The procedure for identification and evaluation of natural as well as induced fractures in basaltic basement of the Cambay basin is presented in this work. In this study formation micro-imager (FMI) and extended range micro-imager (XRMI) log data for fracture identification is used. The Deccan trap basaltic basement of the study area, comprising five wells in the Tarapur-Cambay block, has potential for holding commercial hydrocarbon due to the presence of fractures and weathered basement. Both image logs (FMI, XRMI) identify three types of fracture including open (conductive), partially open and closed (resistive) fractures, of which open and partially open fractures are important for hydrocarbon accumulation. Fracture dip ranges from 10° to 80°. Image logs have also identified washout, breakout and drilling-induced fracture zones. The strike direction of the open natural fractures for four wells varies from N60°E to N30°E whereas the strike direction of most natural fracture in the fifth well is oriented towards N20°W. The orientations of drilling-induced fractures and breakouts may be interpreted for the in-situ stress direction over the logged interval. Drilling-induced tensile fractures, identified over the depth interval of 1969–1972 m, and borehole breakouts over the interval of 1953–1955 m in one well, suggest an orientation of maximum in-situ horizontal compressive stress (S_H) lies in the north-south direction. The azimuths of open natural fractures in the same well vary from north-south to N30°E. It is expected that the direction of fluid flow will be controlled by open natural fractures and therefore would be in a direction parallel to the S_H direction, which is orthogonal to the minimum horizontal stress (S_h) direction. The orientations observed are consistent with the present day S_H direction in the study area of Cambay basin.     

气候与土地利用变化对流域水资源的影响——以美国北卡罗莱纳州Trent流域为例

分布式水文模型PRMS可为气候与土地利用变化对流域水资源影响的研究提供技术和理论支撑.对Trent流域产流过程采用PRMS模型进行模拟检验,结果表明,Nash模型确定性系数达到0.8以上.水文响应单元(HRU)划分尺度减小,可以有效地提高PRMS模拟精度达7%左右,划分尺度缩小到71个HRU时,模拟精度不再提高.流域蒸...     

Growth-strata geometry in fault-propagation folds: a case study from the Gafsa basin, southern Tunisian Atlas

The structural and sedimentological study of fault-propagation folds in Southern Tunisia highlights a special geometry of the growth strata (strata deposited simultaneously with the formation or growth of a fold). This distinct geometry is visible in the uppermost growth-strata beds and consists of one flank with unconformity as opposed to the other flank with perfect conformity. This geometry can be explained by the mechanism of fault-propagation folding, with asymmetrical flank dips and hinge migration kinematics. This kinematics was originally predicted by the fault-propagation fold model, which facilitates the study of this special geometry in a narrow domain of sedimentation-to-shortening ratios. A plot projection provides a generalisation of the results of all types of fault-propagation folds by revealing the expected geometry of the growth strata. This study constitutes one of the most complete examples of kinematic model validation on a field scale.     

大陆盆地的聚敛-闭合过程研究:以塔里木盆地为例

印度与欧亚大陆第三纪以来碰撞汇聚,造成亚洲大陆内部强烈缩短变形。塔里木盆地如何发生相应的变形调节和应变分解,成为中亚板内构造的重要问题。塔里木陆块新生代以来被板内造山带及走滑断裂系环绕,盆地内部以刚性为特征,未发生强烈构造变形。区域大断裂与塔里木盆地的冲断、走滑构造边界共同作用,形成盆地边缘复杂的构造系。其新生代构造变形主要集中于盆地的构造边界上,4条构造边界显示差异性的运动特征和构造交切关系。盆地边缘构造带叠加并向盆内扩展,造成盆地总体上水平缩短,并发生应变分解。盆地内部发生沉积-构造分异,发育前陆盆地、前缘隆起、复合前陆盆地、拉分盆地等单元。其中,盆地西北缘及西南缘发生陆内俯冲,形成前陆盆地及前陆冲断带,对盆内构造演化有重要影响。区域构造研究表明,塔里木盆地新生代主要发生了4期区域构造变形,第三纪以来还发生顺时针旋转。大陆盆地构造边界上的运动组合、盆内不均匀阻挡和滑脱拆离,造成其变形扩展方式的差异,并影响盆内单元构造演化。因此,塔里木盆地是认识大陆盆地聚敛与闭合过程的天然实验室。     

酸性矿坑废水对流域酸化的影响——以贵州兴仁县典型废弃煤矿区小流域为例

人们在开采使用矿产资源的同时,堆弃大量含有硫化物的废弃矿石和废渣于周围环境中。矿山环境中因硫化矿物氧化,导致采矿产生大量的酸性矿坑排水。这种水体具有低pH值,高电导率,高硫酸根和高重金属含量的特征。酸性矿坑排水对下游水生生物及植物等具有很强的毒性,大量排放引起的环境问题受到广泛关注。为了了解酸性矿山排水对流域水体和土壤的影响,本文选择位于贵州省西南部兴仁的一个典型废弃煤矿区进行研究,通过测定矿坑排水、水库水、河水的pH值和EC,以及土壤的pH值,分析矿坑排水、地表水以及土壤pH值的空间变化情况,在此基础上对矿坑排水对流域酸化的影响进行了综合评价。调查结果表明,酸性矿坑排水和受其影响的水库水体的电导率很高,且pH值均小于3。研究区域地表水（水库水、河水）本底水化学类型为Ca2+-HCO3-型,其pH值在7左右,反映了流域内有碳酸盐岩广泛分布的自然环境特征。当受到酸性矿坑排水影响后,水化学类型转变为Ca2+-SO42-型,pH值则低于4.0。通常,酸性矿坑排水在流动过程中与河床的碳酸盐岩发生中和反应,促使水体的pH升高。野外考察发现,研究区河道中碳酸盐岩中空易碎,其CaCO3成分因长期与酸性矿山排水发生反应而被耗尽。同时,在氧化条件下,酸性矿坑排水中的铁在流动过程中生成大量的氢氧化物覆盖了沿程的河床。这种覆盖作用抑制了酸性矿山排水进一步与碳酸盐岩发生中和反应。因此,在研究区分布有广泛的碳酸盐岩情况下,受酸性矿坑排水影响的河水到下游5 km处仍保持较低的pH值。研究区的主要农作物是水稻,其灌溉水源主要是水库水。为了了解酸性矿坑排水对土壤的影响,对水库下游流域土壤pH值的空间分布进行普查,统计其出现的频率。结果表明,以受酸性矿坑排水影响的水库水作为灌溉水源的土壤,其表土的pH值较低,平均值在5.0左右。反之,土壤表土的pH值平均值在6.5左右。此外,通过对受到酸性矿坑排水影响显著的土壤进行剖面调查,发现从地表到深度90 cm的土壤的pH值均小于4.0。结合受酸性矿坑排水影响的河水pH值普遍偏低的情况可以推测流域酸化与酸性矿坑排水有密切关系。      

大陆盆地的聚敛-闭合过程研究:以塔里木盆地为例

印度与欧亚大陆第三纪以来碰撞汇聚,造成亚洲大陆内部强烈缩短变形。塔里木盆地如何发生相应的变形调节和应变分解,成为中亚板内构造的重要问题。塔里木陆块新生代以来被板内造山带及走滑断裂系环绕,盆地内部以刚性为特征,未发生强烈构造变形。区域大断裂与塔里木盆地的冲断、走滑构造边界共同作用,形成盆地边缘复杂的构造系。其新生代构造变形主要集中于盆地的构造边界上,4条构造边界显示差异性的运动特征和构造交切关系。盆地边缘构造带叠加并向盆内扩展,造成盆地总体上水平缩短,并发生应变分解。盆地内部发生沉积-构造分异,发育前陆盆地、前缘隆起、复合前陆盆地、拉分盆地等单元。其中,盆地西北缘及西南缘发生陆内俯冲,形成前陆盆地及前陆冲断带,对盆内构造演化有重要影响。区域构造研究表明,塔里木盆地新生代主要发生了4期区域构造变形,第三纪以来还发生顺时针旋转。大陆盆地构造边界上的运动组合、盆内不均匀阻挡和滑脱拆离,造成其变形扩展方式的差异,并影响盆内单元构造演化。因此,塔里木盆地是认识大陆盆地聚敛与闭合过程的天然实验室。     

A topo-geochemical sequence study of groundwater in Asa drainage basin, Kwara State, Nigeria

 A strong geochemical gradient was observed in the thick overburden aquifer of the Asa drainage basin. Different types of groundwater occur at different (downslope) locations and groundwater table depths. The following sequence was noticed with increasing distance downslope or with increasing groundwater table depth: 1. Ca–Mg–HCO₃ water at about 390-m groundwater table elevations or upslope locations. 2. Ca–Mg–HCO₃–Cl water at middle-slope locations or groundwater table elevations of about 350 m above sea level; 3. Ca–Mg–SO₄–Cl water at downslope locations or groundwater table elevations of about 300 m above sea level. In this basin, changes in the type of water are expected at about every 40–50 m depth from the surface. Statistical analysis via the determination of the correlation coefficient (r) and regression analysis shows that about 80–99% of the variation in groundwater chemistry is accounted for by the topography, using the model presented in this paper. The rate of change in the sequence will depend on the permeability of the aquifer, which determines the rate of groundwater flow and the residence time, and the nature of recharge. Received: 4 February 1997 · Accepted: 22 July 1997