ORIGIN AND POTENTIAL PALEOENVIRONMENTAL SIGNIFICANCE OF CALICHE CRUSTS ON BASALT AND LIMESTONE SUBSTRATES IN AND NEAR THE SNAKE RIVER PLAIN, SOUTHEASTERN IDAHO

Analysis of carbonate crusts on basalts and limestones from the high desert of southeastern Idaho suggests that these coatings have mostly formed pedogenically as caliche and that they potentially provide a significant paleo-environmental record.

Crusts on basalt have a coralloidal micro-morphology and consist largely of microcrystalline low-Mg calcite. They contain angular silt-sized quartz grains compatible with deposition of aeolian dust, which is commonly believed to be the source of constituents of caliche formed on basalts in deserts. They have δ¹⁸O values from -14.7 to -10.7 ‰ vs. VPDB, compatible with deposition of CaCO₃ from the low-δ¹⁸O meteoric precipitation of the area, and they have δ¹³C values from -4.7 to -4.2 ‰ vs. VDPB, compatible with deposition from DIC influenced by the C4 vegetation expected given the area’s dry climate.

Crusts on limestone are likewise coralloidal and consist of localized layers of drusy calcite, microcrystalline calcite, and aragonite. The drusy calcite is more abundant near the base of the crusts and thus earlier. Microcrystalline zones also contain mineralized traces of roots and other plant matter, compatible with a pedogenic origin. Some crusts also contain relict limestone grains derived from the substrate. Crusts on limestone have δ¹⁸O from -12.3 to -8.0 ‰ vs. VPDB, compatible with precipitation from the area’s low-δ¹⁸O meteoric precipitation with variable contamination by material derived from the substrate. Values of δ¹³C from -3.8 to +2.4 ‰ vs. VPDB in these crusts largely support formation under C4 vegetation. However, the material with the greatest δ¹³C is an early (basal) calcite with low δ¹⁸O, suggesting precipitation from meteoric water below the zone of soil CO₂, from water in which DIC was dominated by limestone-derived bicarbonate. These thoughts combine to suggest that crusts on limestone have a non-trivial component derived from the limestone substrate, unlike the purely aeolian and pedogenic origin inferred for crusts on basalt. Alternation of drusy and thus seemingly phreatic calcite with microcrystalline and seemingly vadose pedogenic calcite suggests changes between wet and dry climate through time, and in general a change from wetter to drier, but the thin layers preclude radiometric dating.