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A Geologic Odyssey: Discussion of Model and Variations

The estimated distances from the simple steady-state, plug-flow model match field evidence well. The distances would increase with less evapotranspiration or greater fault permeability. Evapotranspiration should decrease with depth, so the cemented zones probably would not have a precisely linear upper boundary, as suggested by the model.

The amount of time estimated to fill the pores of a pebbly sand with 30 percent porosity by evapotranspiration alone is 206,000 years for a 1-cm gouge thickness. Because the flow rate is faster with greater head, the time works out to be approximately the same for proportionate distances of the cementation zone for a given gouge thickness and constant hydraulic conductivity. The thicker the gouge, the shorter the distance the cementation is likely to extend from the fault and the shorter the time the cementation process can continue (assuming head is constant and does not dissipate by some other perturbation) before porosity is plugged. If the water is understaurated with respect to calcite, if porosity is higher, or if evapotranspiration is less, it would take longer to plug the pores with calcite. Degassing of CO2 would enhance the rate of cementation. Conversely, if porosity were lower and evapotranspiration rates were higher, the amount of time to plug the zone would be less.

Some of the best exposures of fault-related cementation of the offset ancestral Rio Grande deposits are where two faults step down rather than one. The intermediate blocks have more pervasive development of calcified root mats and tubular-nodular cementation. Our model would explain these intermediate levels as being fed from the uplifted main block and passing the remaining water to the lower block.

Multiple movements along faults could conceivably result in stacked calcified root mats, various types of nodular-tubular zones, and/or soil horizons at depth.


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Calcium-carbonate horizons in hanging wall adjacent to fault (locality 6)
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We have found such stacks, but can not be sure that the zones are related to episodic movements on the fault.

These simple models do not allow water movement in the hanging wall parallel to the fault. Such water movements would tend to negate the effects of the gouge-dependent rate of flow and evapotranspiration. However, evapotranspiration of shallow water moving parallel to a fault along the top of the hanging wall might produce shoe-string-like bodies of similar carbonate cements. Mack et al. (2000) describe extensive sheet-like bodies of calcified root mats, cemented massive sandstones with upper tubular fringes, and cemented massive sandstone beds descending the lower parts of large hanging wall alluvial aprons toward large faults in south-central New Mexico.

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Definitions | Geologic setting
Field observations | Laboratory observations | Hypothesis
Develop a semi-quantitative model to test the hypothesis
Discussion and variations | Abstract | References