Bedrock, soils, and hillslope hydrology in the Central Texas Hill Country, USA: implications on environmental management in a carbonate-rock terrain

The Hill Country of Central Texas, USA, is undergoing rapid socioeconomic development, but environmental management of this region is hampered by misconceptions about local bedrock, soils, terrain, and hydrologic processes. The Hill Country is underlain mostly by Glen Rose Limestone (Lower Cretaceous) and exhibits a stepped terrain, which has been incorrectly attributed to alternating hard and soft bedrock strata. Other characteristics mistakenly attributed to this landscape include thin soils with scant water-retention capabilities, and rapid runoff as the dominant hydrologic process. This report presents new findings: unweathered bedrock is well indurated, but interbeds exhibit variable weathering rates. Recessive slopes (“risers”) on this stepped terrain result from rapid deterioration of strata having generally heterogeneous depositional fabrics (bioturbation and irregular clay partings) in contrast to ledge-forming strata having homogeneous fabrics. A stony regolith is thus formed beneath risers, providing porous and permeable ground that retards runoff and promotes the formation of moderately deep to deep (two-tiered) regolith/soil zones. These surficial materials on local steep slopes compose important natural environmental buffers; they support diverse biota and enhanced geochemical cycling of nutrients; they also exhibit significant water retention and enhanced erosion abatement. Proper land management demands recognition of these attributes in the siting, design, and construction of facilities.     

Determination of geomechanical properties and collapse potential of a caliche by in situ and laboratory tests

Working in complex grounds have always been difficult for engineering geologists because of the heterogenous nature of such geo-materials, which results in different behaviors under stresses. The caliches outcropping in the Adana basin and its close vicinity are selected as the study material because of their highly complex nature and also understanding their mechanical behaviour and collapse potential is too difficult, although their aerial extent is large in the region. For this reason, investigation of the collapse potential and mechanical behaviour of the caliches by applying new approaches besides the conventional in situ and laboratory tests is the purpose of the study. The study includes five main stages such as measurements of caliche profiles, in situ tests (plate loading), sampling, shooting photographs for photoanalyses and laboratory studies. Four different levels such as hard pan; silty, sandy layer; gravelly, blocky layer and clayey level are described for the caliches employed. The hard pan level is a weak rock with an average uniaxial compressive strength of 11.89 MPa while the others have typical soil characteristics. A series of plate loading tests are applied on the blocky, gravelly level of the caliche to determine the modulus of elasticity. The modulus of elasticity and the allowable bearing capacity are determined between 28.6–65.3 and 1.5–2.0 MPa, respectively. To determine the grain size distribution curve, in addition to sieve analyses, a photoanalysis technique is also applied and a combination procedure between the results from both sieve analyses and photoanalyses is introduced and the grain size curves for the blocky, gravelly level of the caliche are obtained. According to the results of collapse potential index tests performed on the samples collected from 20 locations of the study area, the soft pan level of the caliche has slight to moderate degree of collapse indices. In the final stage, various simple and statistically meaningful empirical equations are proposed for the indirect determination of the collapse index by employing simple paramateres.     

塔里木盆地玉北5井下奥陶统蓬莱坝组沉积期的暴露标志及其意义

塔里木盆地玉北地区玉北5井下奥陶统蓬莱坝组,属于局限台地潮坪沉积,主要为一套灰、浅灰色泥晶白云岩、灰色藻纹层(藻席)、灰质云岩与细粉晶云岩。首次在岩芯中发现多个层位发育暴露标志,包括钙结壳、渗流豆石、碴状层、溶洞。溶洞主要发育在第10回次下部,碴状层、钙结壳、渗流豆石发育在第10回次中上部至第5回次。蓬莱坝组这些暴露标志的确认,说明玉北地区早奥陶世特马豆克期有一定规模的海平面下降事件,可能是储层形成的重要控制因素。     

Ground penetrating radar imaging of cap rock, caliche and carbonate strata

Field experiments show ground penetrating radar (GPR) can be used to image shallow carbonate stratigraphy effectively in a variety of settings. In south Florida, the position and structure of cap rock cover on limestone can be an important control on surface water flow and vegetation, but larger scale outcrops (tens of meters) of cap rock are sparse. GPR mapping through south Florida prairie, cypress swamp and hardwood hammock resolves variations in thickness and structure of cap rock to 3 m and holds the potential to test theories for cap rock–vegetation relationships. In other settings, carbonate strata are mapped to test models for the formation of local structural anomalies. A test of GPR imaging capabilities on an arid caliche (calcrete) horizon in southeastern Nevada shows depth penetration to 2 m with resolution of the base of caliche. GPR profiling also succeeds in resolving more deeply buried (5 m) limestone discontinuity surfaces that record subaerial exposure in south Florida.     

Elemental composition of calcites in late Quaternary pedogenic calcretes from Gujarat, western India

Pedogenic calcretes commonly exhibit clotted micrite, circum-granular calcite (grain coats) and microspar/spar veins. The three calcite-types with different dimensions were analyzed for their magnesium content to determine the relationship between crystal elongation and magnesium incorporation. The results show a very low MgO content for grain coats and microspars and high values for clotted micrite indicating that the ideal kinetic model does not hold true and several variables govern the end composition of calcites. The magnesium concentrations of meteoric calcites are genetically linked to the evolutionary history of the soil and climate. Grain coats, which are elongated calcites, contain the least amount of Mg and is related to the initial stages of pedogenesis wherein the limiting factor is the Mg/Ca ratio of the parent fluid. Lower magnesium contents arise due to smaller quantities of Mg being released during incipient weathering. Micrite morphology and composition is controlled by the greater availability of Mg ions through weathering, higher pCO₂ in soil due to increased time-dependent soil respiration, which causes a rise in calcite precipitation rates and clay authigenesis. This in turn exerts a physical control on morphology by occluding pore space and providing numerous nuclei for calcite precipitation. The wide variability in spar cements is inherently controlled by inhomogeneties in parent fluid compositions with lower-than-micrite values on account of slower precipitation rates.     

The genesis of late Quaternary caliche nodules in Mission Bay, Texas: stable isotopic compositions and palaeoenvironmental interpretation

Caliche is a fairly widespread pedogenic carbonate that commonly has been used to reconstruct palaeoclimatological conditions. Stable isotopic analyses of three types of caliche nodules from Mission Bay, Texas, provide insights into the values and limitations in palaeoenvironmental interpretations. Soft incipient nodules (type I) and partially lithified chalky nodules (type II), composed of low Mg‐calcite, are in situ pedogenic products in the late Quaternary soils; they represent young to intermediate caliche nodules with no obvious diagenesis and, with constraints, can be used to investigate palaeoenvironmental conditions. The well‐lithified hard nodules (type III) dispersed on the beach and shallow bay exhibit complex shapes, fabrics, mineralogy and geochemical compositions. They are mature nodules that have undergone substantial diagenesis and, therefore, are believed to have lost their initial environmental signatures. The incipient nodules in the presently active grey soil and the older subjacent brown soil display significantly different δ¹³C values, ?8·4 and ?4·4‰, respectively, which indicates a change in palaeovegetation from C₃/C₄ mixed to C₃‐dominated flora. The δ¹³C values probably reflect a marked climate shift from warm and dry to cool and wet conditions in the middle Holocene. However, in the same grey soil, there is a sub‐set of incipient caliche nodules with δ¹³C values around 0·1‰, which is probably due to the input of localized carbon sources in the soil (e.g. shell fragments). The occurrence of essentially identical nodules appearing from the same modern soil horizon with significantly different δ¹³C values questions the universal reliability of this type of data for palaeoenvironmental interpretation. This study demonstrates that, whereas the stable isotopic compositions of caliche nodules can be used for palaeoenvironmental reconstruction, diagenesis and the influence of localized carbonate sources in the soils could lead to erroneous interpretations.     

砾石钙膜显微纹层定年方法及黄河黑山峡高阶地形成年代厘定

砾石钙膜是一种在干旱—半干旱环境中形成，反映环境和气候变化的沉积物，保留了比较完整的环境和气候信息，可用来分析古气候和测定形成年代．本通过砾石钙膜的沉积原理讨论，认为钙膜中的显微明暗纹层变化反映了气候变化过程，其明暗度变化曲线可与其它测年方法(深海岩芯氧同位素曲线、黄土剖面粒度曲线、古地磁变化、构造事件等)作对比，得到其形成年代的信息．而砾石钙膜形成年代可代表所在地貌面的年龄．本以此方法对黄河黑山峡高阶地的形成时代进行了厘定．