Evolution of a Humid Tropical Landscape in Northcentral Costa Rica as Deduced from Geomorphic and Pedogenic Evidence

The University of Kansas has long historical connections with Central America and the many Central Americans who have earned graduate degrees at KU. This work is part of the Central American Theses and Dissertations collection in KU ScholarWorks and is being made freely available with permission of the author through the efforts of Professor Emeritus Charles Stansifer of the History department and the staff of the Scholarly Communications program at the University of Kansas Libraries’ Center for Digital Scholarship.Progressive landscape changes in humid tropical provinces of southern San Carlos Canton in northcentral Costa Rica can be attributed to tectonic, volcanic, mass-wasting, and fluvial events. Spatial distribution of principal landform types and variations in degree of soil development of associated modern and buried soils are used to determine major episodes of landscape evolution- The two lines of evidence complement each other in that pedogenic evidence permits an assessment of temporal relationships between spatially disjunct landforms and of the magnitude of age differences between landforms, information unattainable from geomorphic evidence alone. Small hills of laharic origin, alluvial plains, and paired terraces are present in the Atlantic Lowland Province of southern San Carlos. The Piedmont Province contains tilted fault-block ridges, alluvial/laharic fans, alluvial plains, cinder cones, and volcanic ash mantles. Spatial segregation of differing landform types prevents assessment of relative landform ages for the region as a whole Duration of soil formation has exerted the greatest influence on pedogenesis? hence, degree of soil development provides a qualitative measure of soil age. Differences in soil development are revealed by silt/clay ratios, soil texture, free iron-oxide content, soil color, and illuvialclay content, in order of decreasing usefulness. Soils at an early, at an intermediate, and at an advanced stage of soil development are recognizable; ranking of individual soil groups within general age categories is possible on the basis of silt/clay ratios and soil texture. Absolute soil ages are inferred from silt/clay ratios in subsoil horizons. Knowledge of relative landform ages is greatly enhanced by pedogenic evidence. Pliocene tectonism, accompanied by explosive volcanic activity and laharic deposition, led to formation of four cinder cones In the eastcentral part of the Piedmont Province and small hills of laharic origin in the Atlantic Lowland Province of southern San Carlos. Further tectonism during the early Pleistocene created three tilted faultblock ridges in the western and central parts of the Piedmont Province. Consequent increase in fluvial aggradation initiated formation of alluvial plains in the Piedmont and Atlantic Lowland Provinces and, aided by intermittent laharic deposition, caused the buildup of alluvial/laharlc fans against the backslopes of tilted fault-block ridges. Upper portions of most alluvial/lahaxic landforms are late Pleistocene, those of alluvial plains in the Atlantic Lowland Province Recent in age. Explosive volcanic activity during the middle Pleistocene and late Holocene is responsible for pyroclastic surface deposits in the northeastern and southeastern parts of the Piedmont Province, respectively. The more recent volcanic event possibly took place at the time when increased fluvial degradation led to formation of paired terraces in the Atlantic lowland Province of the region