Origin and age of Fe-Mn-P concretions and nodules in an Oregon wetland

by Stewart, Scott R.

Wetland soils with complex stratigraphy and redoximorphic features occur on the

margin of the Willamette Valley, Oregon. In the Jackson-Frazier wetland, vertisols

show three distinct stratigraphic units that rest uncomformably upon each other and

correspond to the Bashaw series Bss, 2Bt and 3BC horizons. Holocene alluvium

deposited as distal alluvial fan sediment overlies partially eroded valley floor lake

deposits (Willamette formation) and is mineralogically distinct from the lower two

strata. Fe-Mn concretions differ among horizons in distribution, morphology,

mineralogy and chemistry. Pressure oriented clays coat the exterior of the Bss and

2Bt concretions; low porosity, stable isotopic composition and subtle differences in

mineralogy appear to indicate an early onset of formation and a long-term protected

weathering environment within the concretions and 3BC nodules. Photomicroscopy

provides evidence that despite distinct, sharp boundaries the concretions/nodules

began forming in their current location after the transport and mixing of materials

with disparate provenance. The concretions/nodules are enriched in Fe and P,

relative to the matrix soil, by as much as an order of magnitude. Mn enrichment can

exceed two orders of magnitude. During the reduced phase of the annual redox

cycle, concentration values for dissolved and KCl/HCl-Fe?² are in the range of 40-60

and 5OO-6OOOmgFe?²kg?¹1soil, respectively, and are sufficient to support Bss

concretion formation within the known age of the associated geomorphic surface.

Comparing the age of the concretion-C¹? and the stratum in which the concretions

are formed, places the larger, older Bss concretions between 1500-3300y old. The

amount of Fe+2 available in the reduced-soil phase compared to the degree of

Fe[subscript DCB] enrichment within the concretions suggests 2300-3500y for this enrichment

to occur, with an average near 2800y. Some of the 3BC nodules may be near 38ky

of age while the 2Bt concretions must be between 3.3ky and l5ky. The Bss-2Bt

concretions and 3BC nodules appear to be currently forming in situ as a function of

climate, redox and local valley stratigraphy.