Calcic pedocomplexes are associated with regional unconformities in the Great Plains and western United States that have approximate ages of 30 Ma, 20 Ma, and 4 Ma. In southwestern Montana, the calcic pedocomplexes are readily identifiable on the surface, and a pedocomplex typically contains several partial soil profiles. In the most complete scenario, an individual profile may contain an argillic or argillic/calcareous (Bt or Btk) horizon, a K horizon, and a C horizon. Often, however, the Bt(k) horizon is truncated or can be entirely absent from an individual profile. The K horizon contains an upper laminated zone that is underlain by an indurated carbonate sheet. Carbonate nodules and chalky micritic matrix materials underlie the sheet carbonate. The calcic paleosols display carbonate morphology ranging from stage IV to stage VI. The calcic pedocomplexes also possess distinct physical properties that aid in subsurface identification. The combined density and velocity differences between paleosols and nonpedogenic strata result in bright reflections on seismic sections and distinct well-log signatures. Although the calcic pedocomplexes and regional unconformity associations were first described within Tertiary strata of southwestern Montana, the same associations exist in numerous localities in the Great Plains and in other parts of the western United States. The extensive occurrence of the calcic paleosols and regional unconformity associations throughout this large area underscores their utility as a regional correlation tool. Moreover, the delineation of regional unconformities that are large-scale sequence boundaries by pedocomplexes has broad implications for continental sequence stratigraphy. En las Great Plains y oeste de Estados Unidos, los complejos edáficos cálcicos están asociados con las discontinuidades regionales cuyas edades aproximadas son: 30 Ma, 20 Ma, y 4 Ma. En el suroeste de Montana, estos edafocomplejos cálcicos se observan fácilmente en afloramientos de superficie y contienen varios perfiles edáficos incompletos. En los casos en los que estos complejos edáficos están más completos, un perfil individual puede contener un horizonte argílico (Bt) o argílico/calcáreo (Bt[k]), un horizonte K, y un horizonte C. Sin embargo, a menudo el horizonte Bt(k) está truncado o puede estar ausente totalmente en un perfil determinado. El horizonte K tiene una zona superior laminada que se sitúa por encima de una capa carbonática endurecida. Por debajo de la capa dura se reconocen nódulos carbonáticos y material micrítico pulverulento. Los paleosuelos carbonáticos tienen estadios morfológicos que varían entre IV y VI. Los complejos edáficos cálcicos también presentan propiedades físicas que facilitan su identificación en el subsuelo. Las combinación de las variaciones de densidad y velocidad en paleosuelos y estratos sin paleosuelos da lugar a reflexiones importantes en los perfiles sísmicos y a rasgos distintivos en sondeos. Si bien la asociación entre complejos pedocálcicos y las discontinuidades regionales se describió por primera vez en estratos Terciarios del suroeste de Montana, estas mismas asociaciones se reconocen también en muchas otras zonas de las Great Plains y en otras partes del oeste de Estados Unidos Norteamericanos. La frecuente presencia de estas asociaciones en una zona tan amplia indica su utilidad como herramienta de correlación regional. Además, la delimitación de las discontinuidades regionales que constituyen límites de secuencias de gran escala, y que están marcados por estos complejos edáficos, tiene implicaciones importantes para aplicar en la estratigrafia secuencial de cuencas continentales.

Abstract The Western Mountain Ranges region (Fig. 3, Table 2, Heath, this volume) owing to its areal extent (more than 1.5 million km 2 ) and large range in altitude (5,000 m) and latitude (35° to 62° N), includes perhaps the greatest diversity of geologic conditions and processes of all the regions of North America. Consequently, ground-water conditions also are highly varied. The region includes 15 subregions. Of the subregions, 12 are mountainous subregions and 3 are intermontane and lowland subregions. Included with the mountainous subregions are intermountain basins in Montana and Colorado and individual coastal lowlands along the entire Pacific Coast. The generalized boundaries and subregions of the Western Mountain Ranges region, as described in this chapter, are shown in Figure 1. Although the region boundaries shown in Figure 1 are based mainly on topographic breaks in slope, at places they represent compromises between lithologic and physiographic boundaries. The northern boundaries also are compromises between major topographic features and the areas of permafrost (subregions 4 through 7). Most of the mountain ranges trend generally northsouth or northwest–southeast. Notable exceptions are ranges in Alaska and the Uinta Mountains in Utah, which trend nearly east–west.