Date of Graduation

12-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Geosciences (PhD)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Covington, Matthew D.

Committee Member

Cothren, Jackson D.

Second Committee Member

Shaw, John B.

Third Committee Member

Marshall, Jill A.

Keywords

Caves; Erosion; Karst; Speleogenesis

Abstract

Existing models of speleogenesis neglect the shape of cross-sections, which can hold information related to climate, tectonics, and sediment supply in their widths. The first study of this dissertation simulates cross-sections of phreatic tubes, vadose canyons, and paragenetic galleries using a method developed for bedrock channels. Successful simulation of these cross-sections depends on erosion scaling with shear stress, in conflict with speleogenesis theory. Scaling of equilibrium width in paragenetic galleries was explored through analytical derivation and simulations, showing that width scales positively with discharge to the 1/2 power, and negatively with a weak power of sediment supply. Negative scaling of width to sediment supply is the opposite of scaling in surface bedrock channels.

Mechanisms of erosion were explored in Parks Ranch Cave, New Mexico, and Copperhead Cave, Arkansas by comparing simulated relationships between scallop ratios and incision angles varying with an exponent in the erosion model, and field data measured from 3D scans of meandering passages. Data indicate that the exponent is between 0.5 and 2.5, with a best fit of 0.5 for Copperhead, and 1 for Parks Ranch. These values arise due to a mixing of dissolution and abrasion. This study also developed a tool to estimate discharge in partially air-filled passages by minimizing the discrepancy between measured and calculated scallop size.

The last study extends the cross-section model into multiple cross-sections simulating a single conduit, with flow calculated using a stormwater management code. This model includes the ability for base level to change, and erosion weighted by the probability of a particular discharge. Single cross-section simulations with weighted erosion shows that equilibrium width in vadose canyons scales similarly when only the mean discharge is simulated, though the magnitude of widths is lower. Magnitude is controlled by an extremity parameter in the probability distribution, with distributions with less extreme events having larger widths. The multiple cross-section model simulates vadose canyon formation from a phreatic tube, and simulates vadose canyons propagating downstream, the opposite of knickpoint propagation in surface channels. The model also shows that keyhole passages are only successfully simulated when mean discharge lowers during conduit formation.

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