David R. Soller

The use of mineralogic techniques as relative age indicators for weathering profiles on the Atlantic Coastal Plain, USA

Abstract Textural, geochemical, and mineralogic study of soils and weathering profiles has led to the practice of applying varioys weathering parameters as relative age indicators. In our studies examined the entire thickness of weathered sediment (i.e., the weathering profile) for evidence of weathering-induced changes in both sand- and clay-sized mineralogy, and used two techniques for relative age determinations. These techniques were developed as tools to support geologic mapping. One of our techniques for determining relative ages is based on the depth of weathering as recorded by progressive loss of denrital sand-sized minerals upward in the weathering profile. This is our preferred tool, especially in areas where weathering profiles have been truncated. We have found a gradual trend of increasing loss of labile sand-sized minerals (e.g., hornblendes, feldspars) and increasing depth of weathering with increasing age of the deposit. Of significance to many research programs, this technique does not require expensive instruments such as an X-ray diffractometer. Our other technique depends on accumulation of stable, secondary clay-sized minerals in the upper part of the weathering profile. In our study area on the Atlantic Coastal Plain of the United States, the stable assemblage consists of vermiculite, kaolinite, gibbsite, and iron oxides and hydroxides. This technique can be effective for relative age determinations where profiles have not been truncated, and can provide useful information on depositional and erosional history. However, in areas of widespread erosion and profile truncation, such as the Carolinas, the utility of this technique for relative age determinations is limited. There, soils were partially or completely removed in many localities in relatively recent times.

A model for the assessment of aquifer contamination potential based on regional geologic framework

The texture and three-dimensional framework of geologic materials should be considered in assessments of groundwaters vulnerability to contamination because geology controls the movement of contaminants and groundwater and influences groundwater quality. Contaminants are introduced into, transmitted through, and stored by geologic materials. We present a model that identifies aquifers and ranks sequences of geologic materials by their relative potential for transmitting water and contaminants from land surface. With this basis, the model can be used to assess the potential for contamination of aquifers by surface activities such as landfitling of wastes or application of agricultural chemicals. A regional map of aquifer contamination potential can be generated from the model; it retains the geologic map information intact and available for reinterpretation or other uses.The model was developed using broad, regional map information and is intended to be a general tool for assessing the regional vulnerability of aquifers to contamination. It is not intended for local, site-specific use, but for prioritizing local areas where contamination potential and/or land-use history warrant more detailed assessment or monitoring. Because it provides a regional view of contamination potential, regional patterns or trends of map units should be evaluated, rather than using the map information literally to assess local areas. Methods of applying this model and contamination potential map to groundwater protection and management are currently being studied; research includes an attempt to statistically validate the model with water-quality data, and to identify natural groupings of the ranked contamination potential map units.

Thickness and character of Quaternary Sediments in the glaciated United States east of the Rocky Mountains

Publisher Summary A l:l, 000,000-scale map of Quaternary deposits has been compiled for the glaciated area of the United States east of the Rocky Mountains, which includes parts of southern Ontario, areas beneath the Great Lakes, and parts of the submerged eastern seaboard. The chapter discusses that the map has three components that, together, provide the first regional three-dimensional view of these deposits. These components are the surface distribution of Quaternary sediments, the total thickness of Quatemary sediments and the distribution of significant buffed Quaternary units. For many areas, this is the first map of Quatemary sediment thickness published at any scale. The map also shows the glacial limits for the Late Wisconsinan and for the maximum extent of ice prior to Late Wisconsinan time; this information was derived from many of the source maps cited in Soller. Those ice limits have been included in the digital map set of the current INQUA project. The map was designed to complement more traditional surficial geological maps that generally emphasise stratigraphical or geomorphological map units associated with the chronology of geological events such as ice advances and retreats. Other maps have provided a rigorous stratigraphy supported by extensive sediment texture information. In support of such maps, this map shows the thickness and character of these sediments without regard to age of the deposit. Justification for this approach is found in the emerging need for three dimensional, textural-based mapping of surficial deposits, particularly for this region where approximately 40% of the U.S. population resides.

Mapping time-dependent changes in soil slip-debris flow probability

Risk of personal injury or property damage from rainfall-triggered debris-flow events can be expressed in terms of expected losses if the probability that a potentially hazardous event will occur can be combined with economic data about the value of property and infrastructure that could be lost from such an event. A realistic forecast of expected losses could provide an economic basis for community choices about whether, when, and how to impose regulations for mitigation (such as grading or land use constraints) and assign responsibilities for warning and hazard response. Experience in some landslide-prone communities where grading codes require preconstruction site studies has demonstrated that sufficiently detailed geotechnical studies can provide site-specific deterministic assessments of risks that can be reduced through mitigation (see, for example, Slosson and Krohn, 1982). However, because detailed site studies are relatively costly, geotechnical studies are not commonly made at sites where no significant financial commitment has already been made. Cost also commonly prohibits application of those methods to the kinds of regional assessments that are needed to support responsible decisions for community action. To be timely, regional risk assessments should take advantage of earth-science data (especially topographic, geologic, and soils maps) that already exist or can be acquired rapidly by reconnaissance techniques. To the extent that the elements of the map data are analogous to variables in established geotechnical models, their relations are expected to parallel those in the models; yet the use of existing regional map data and rainfall records from scattered gaging localities introduces greater uncertainty about spatial and temporal variations in earth materials and rainfall than is normally applied in geotechnical analyses of stability. Probability offers a means to characterize that uncertainty quantitatively. A map sequence that displays changes in the spatial distribution of probabilities provides a way to identify the areas of greatest hazard potential and a visual way to evaluate their relations to topographic or cultural features that may be in harms way. It has been a fairly common practice for regional data to be synthesized into qualitative susceptibility maps delineating, for example, map units of high, moderate, and low potential for hazard. The qualitative maps can be used by skilled professional planners to guide decisions about land use regulation. However, they cannot be applied directly to a quantitative economic assessment of risk, and maps made by different individuals and agencies for different areas may not be comparable even in a qualitative sense. Probability maps, which can be prepared by

The U.S. National Geologic Map Database Project: Overview & Progress

The National Geologic Map Database (NGMDB) project continues to fulfill its mandate. Some of its accomplishments are specific and tangible, and others are more general in nature — for example, the NGMDB contributes to advancements in digital mapping techniques and database design by agencies in the United States and internationally. However, without extensive collaboration from enthusiastic and highly skilled members of the state geological surveys and the Geological Survey of Canada, these accomplishments would not have been possible. Highlights of the past year include: 1) the Geoscience Map Catalog now contains bibliographic records for more than 61,000 map products published by more than 270 organizations including the U.S. Geological Survey(USGS), 43 state geological surveys, universities, and scientific societies and organizations, 2) the Geologic Map Image Library has evolved from a concept to a prototype Web site that serves high-resolution images of nearly 1,000 geologic maps, 3) the project contributed significantly to evolution of the North American standard data model, science language, and data-interchange format, and to the cartographic standard for the U.S. Through discussions with ESRI, this data model may form the basis for their Geology Data Model for Arc Geodatabase. Internationally, NGMDB staff participated in “DIMAS”, the map standards committee of the Commission for the Geological Map of the World, 4) the seventh annual Digital Mapping Techniques workshop was a success, bringing together 90 technical experts from 36 agencies, and 5) the third phase of the project — the design and implementation of an online, vector-map database — was reoriented mid-year, and began to focus on data input tools and standardized science language.

CACOXENITE IN MIOCENE SEDIMENTS OF THE MARYLAND COASTAL PLAIN

Cacoxenite having the composition (Al4.0Fe22.5O7.1(OH)14.3(PO4)17(H2O)23.7)·50.3H2O was identified in a bed of mature quartz sand in the Miocene Calvert Formation near Popes Creek, Maryland. This is the first reported occurrence of this mineral in Atlantic Coastal Plain sediments north of Florida. The cacoxenite occurs as silt-size to sand-size grains, both as irregularly shaped aggregates and as radiating arrays of delicate acicular crystals. The presence of discrete cores and overgrowths in some grains indicates at least two generations of crystal growth. Electron microprobe analyses reveal excess Si and Al (relative to the ideal composition), which is believed to reflect ultra-fine clay particles within the cacoxenite grains. Admixed clays probably served as a substrate for the formation of ferric oxyhydroxides, which were subsequently converted to cacoxenite through the addition of dissolved phosphorus.