Geochemical Characteristics of the Conterminous United States: % CaO

This raster depicts the percentage of lithological calcium oxide (CaO) content in surface or near surface geology. We derived these rasters by calculating the average percent CaO content for each map unit in combined surficial-bedrock geologic maps. We used state geologic maps (Preliminary Integrated Geologic Map Databases for the United States, Open File Reports 2004-1355, 2005-1305, 2005-1323, 2005-1324, 2005-1325, 2005-1351, and 2006-1272), which depict surficial geology instead of bedrock when the surficial layers are sufficiently deep. For the state maps that do not incorporate surficial geology (i.e., midwestern states), we overlaid surficial geologic map units with thicknesses greater than 100 feet (from Soller and Reheis [2004]) to produce combined surficial-bedrock geologic maps that were similar to other states. We characterized geology based on the 201 different lithologies that the Geologic Map Database lists as occurring in the conterminous United States. Because some of these lithologies are known to have chemical attributes that vary widely, we created an additional 78 lithologic classes based on the common modifiers used in the geologic unit descriptions to better parse chemical variability within the lithologies (e.g., calcareous and noncalcareous sandstone). Modifiers were assigned base on descriptions of geologic formations obtained through either the Lexicon of Geologic Names of the United States or literature searches. Fifteen lithologic classes were not characterized because the class was not a specific rock type (e.g., mélange, water, and landslide). These classes were characterized as no data. We translated each state’s combined surficial-bedrock geologic maps into characteristics following the methods in Olson and Hawkins (2012) by assigning an estimate of each map unit’s percent CaO content to every occurrence of that map unit in the combined surficial-bedrock geologic map. This estimate was calculated as the average of literature or database values of the respective property for each lithological class contained within the map unit weighted by the prevalence of each lithological class within the map unit. The accompanying Excel workbook (Lith-MajorOxides.xls) contains a summary of all of the average geochemical characteristics for each lithology (“Lith Summary” tab) and tabs for each individual lithology that include the source of each record (e.g., originating from the Earth Chem Database or the specific literature reference), as well as the calculations used to determine the measure of central tendency (mean or median depending on the data). The final national raster was created by merging each of the individual state rasters. Users should be cognizant that some differences will exist in chemical and physical characterizations across state lines that are caused by unreconciled differences in lithologic descriptions or mapping scales used among the underlying state source maps.本栅格数据展示了地表及近地表地质中，岩性层面的氧化钙（CaO）含量百分比。本数据集的栅格通过整合表层-基岩地质图中各制图单元的平均氧化钙含量百分比生成。本次研究采用了美国州级地质图（《美国初步整合地质图数据库》，公开文件报告2004-1355、2005-1305、2005-1323、2005-1324、2005-1325、2005-1351及2006-1272），当表层沉积物厚度足够大时，此类图件将展示表层地质而非基岩地质。对于未纳入表层地质的州级图件（如美国中西部各州），我们叠加了厚度超过100英尺的表层地质制图单元（数据源自Soller与Reheis[2004]），以生成与其他州格式一致的整合表层-基岩地质图。本研究基于美国本土地质图数据库收录的201种不同岩性对地质特征进行表征。鉴于部分岩性的化学属性差异显著，我们依据地质单元描述中常用的修饰词额外划分了78个岩性类别，以更好地解析岩性内部的化学变异（如钙质砂岩与非钙质砂岩）。修饰词的划分依据源自《美国地质名称词典》或文献检索得到的地质地层描述。其中15个岩性类别因不属于特定岩石类型（如混杂岩、水体及滑坡堆积）未进行表征，将其标记为无数据。我们遵循Olson与Hawkins(2012)提出的方法，将各州的整合表层-基岩地质图转换为特征参数：为整合图中每个制图单元的所有出现位置赋予其氧化钙含量百分比的估算值。该估算值通过以下方式计算：以制图单元内各岩性类别的占比为权重，对单元内各岩性类别的对应属性的文献或数据库值取平均值。随附的Excel工作簿（Lith-MajorOxides.xls）包含了各岩性的平均地球化学特征汇总表（"Lith Summary"工作表），以及各单岩性的详细工作表：其中收录了每条记录的来源（如源自Earth Chem数据库或特定文献），以及用于计算集中趋势指标（均值或中位数，依数据情况而定）的计算过程。最终的全国范围栅格数据通过合并各州单独的栅格文件生成。使用者需注意：由于各州原始源地图在岩性描述或制图比例尺上存在未统一的差异，跨州边界的化学与物理特征表征可能存在一定差异。