Studying the geometry and kinematics of gypsum-filled fractures in the Permian strata of Caprock Canyons State Park, Texas, USA

Date

2015-08

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Abstract

Characteristic sets of veins can be found in the Permian rocks within the Southern High Plains. This network of veins are cut by the South Prong of the Little Red River in Caprock Canyons State Park, creating excellent 3-D exposures for study. These exposures are valuable in studying vein propagation, geometries, thickness, density, orientation, and other kinematic and geometrical information about vein networks at Earth’s surface. This research re-typifies the vein typology based on internal fabrics and geometries of the vein and crystal internal growth morphology and kinematic direction of opening. There are three main vein types found in Caprock Canyons: Type I- Syntaxial veins, which have mineral growth from a single plane, growing toward the rock wall interface. Type II- Antitaxial veins, which have mineral growth from two planes, growing inward. The growth planes meet somewhere near the center of the vein, displaying a medial scar. Type III- Accretion Veins, which have multiple episodes of cracking, creating veins that are a composition of many veining episodes. Structures in Caprock Canyons were studied in order to test the following hypotheses: Hypothesis 1: Vein thickness varies with the host rock lithology. Hypothesis 2: Vein thickness varies within individual sedimentary layer packages, thickening upward in stratigraphic section. Hypothesis 3: Veins mineralize in fractures and planes of weakness, therefore the fractures are prekinematic or synkinematic with vein formation. Hypothesis 4: Vein density varies with the host rock lithology. Hypothesis 5: Vein density increases upward in stratigraphic section, and decreases downward toward the base of veining layers. The macroscopic data collected included vein thickness, orientation, and density data. Vein thickness data was gathered at various station locations, and varying veining units, then plotted using box-and-whisker plotting methods. Orientation data, including vein orientation, fracture orientation, and vein fiber orientations were plotted using equal-area stereonets. Vein density data was collected every meter along each station location, and plotted using the box-and-whisker potting method. These plots were compared at each station, to determine vein density distributions throughout the study area. Using two different density data collection methods, the window and scanline methods, data was plotted to compare collection methods. The field method and computer method of data collection was then plotted, comparing collection methods. Hypothesis 1and Hypothesis 4 were confirmed comparing the vein thicknesses and vein density respectively in the Quartermaster Formation to the Cloud Chief Formation. Hypothesis 2 and Hypothesis 5 was disproven, using thickness and density data respectively in individual veining units. Hypothesis 3 was confirmed using vein and fracture orientations, showing that veins and fractures have similar orientations.

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Keywords

Geology, Gypsum, Caprock Canyons, Caprock, Anhydrite, Veins

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