Bob Blank
ABSTRACT
| Conceptually,
duripans are subsurface soil horizons cemented by silica. In
semiarid climates, calcite often augments or is dominant to silica cement.
There is a considerable lack of understanding in the genesis of duripans,
particularly: (1) the role of volcanic tephra in the silicification
process; (2) mineral neogenesis; (3) the morphogenic pathways of
concretion development; (4) the relatedness of duripans to overlying
horizons. To more completely understand duripan genesis, a range of
duripan-containing pedons throughout southern Idaho were field and
laboratory studied. While the collected samples differed in
horizonation and duripan expression they basically fit into two categories:
(1) marginal (weakly silicified) duripans formed in mixed loess-limestone on
alluvial fans - the Arco-Salmon sites and (2) duripans with maximal
silcification formed in loess over basalt - the Owyhee Plateau sites. The evidence suggests that volcanic tephra, while present in significant amounts, does not greatly influence the duripan silicification process. Rather, plant recycling of silica and the emplacment of loess within the duripan matrix appears to control the rate and timing of silicification. Dissolution of plant phytoliths and siliceous material around plant roots contributes soluble silica which then precipitates in the form of opal-A to fill pores and pseudomorphically replace calcite. Loess is altered via a closed system process to form a microgranular calcite-silica matrix. Opal, a major alteration product in this process, occurs as either striations or web-like arrangements. Sepiolite and opal-A are the dominant neogenetic substances in the duripans studied. Opal-A occurs as spherical units (175 nm to 550 nm in diameter) in a close-packed array. Individual spheres exhibit sperical bubble-like substructures heretofore unrecognized. Most often, sepiolite and opal are intimately associated. Sepiolite occurs as spear-like acicular crystals that radiate outward from an opal-A surface. Sepiolite and opal-A are covered by amorphous coatings which suggest that they are, in part, synthesized via gel reorganizaton. A multistage process is offered to explain the nature of and origin of concretions. The process begins immediately above the duripan - in loess - with the envelopment, by calcite, of small sand-sized soil granules. Upon complete envelopment and emplacement of the granules in the duripan matrix, aluminosilicate (loessial) material rapidly alters to a microgranular calcite-silica matrix composed dominantly of amorphous material, opal-A, calcite and quartz. Later, in an unexplained process, concentric laminae composed of siliceous material form at the interface of the central altered loess material and the outer calicite zone. Evidence presented suggests divergent pathways in duripan genesis between the Arco-Salmon and Owyhee Plateau pedons. The Arco-Salmon duripans appear related to their overlying horizons in a single-cycle genetic event. However, the Owyhee Plateau duripans appear unrelated to their overlying horizons and have experienced one or several episodes of overlying soil truncation and subsequent reburial. |