| dc.description.abstract | 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. | |
