Fracture Fillings at Weaver Pit, Walla Walla Valley, WA

Fractures in the basalt bedrock exposed at Weaver Pit, a quarry located along the base of the Horse Heaven Hills near Walla Walla, contain a variety of fillings.

Yellow Cemented Fills. Yellow colored silica (and some CaCO3) fills deep vertical fractures that cut the entire high wall, a distance greater than 6 meters.

Green Opal. Opal fills vertical joints in the basalt.

White Calcium Carbonate Stringers. CaCO3 fills thin horizontal fractures formed in the exposed and weathered surface of the uppermost basalt flow at the quarry. Calcium carbonate also fills a somewhat wider vertical fracture located just left of the hoe. A half meter of Holocene (modern) loess overlies the Miocene basalt.

Sideways Opal. Green opal (amorphous silica precipitated from hot fluids) occurs along subhorizontal fractures, around pillows with palogonite, and along contacts between flows in the basalt.

Brown Weathering. A thin fracture with no filling forms a pathway for surface water infiltration and weathering. Though the fracture itself is barely visible, its weathered zone measures more than 10cm wide.

Sheeted Clastic Dikes. Vertically-sheeted silt-sand dikes, composed of sediment sourced in the overlying Touchet Beds, exploit vertical tension fractures near the hinge of a gentle fold in the basalt. They pinch downward and penetrate several meters into the basalt bedrock.

Orange Gouge & Clastic Dike. Some fractures exhibit a small amount of offset (shear), thus qualify as faults. Broken chunks of rock and rock powder were formed by movement along the fault plane. This gouge has weathered orange (oxidized). A thin, gray silt-filled clastic dike, seen at right and originating in overlying Touchet Beds, penetrates and post-dates the gouge zone. The gouge-filled shear zone provided a low-resistance pathway for the both the clastic dike and infiltrating rain water.

Mixed Media. The vertical fracture contains some broken basalt (gouge), Touchet Bed sediment, pedogenic CaCO3 cement, and a plug of loess (silt diamict). Note how the CaCO3 overprints the rubbly, subhorizontal surface of the basalt on either side of the fracture. The cement (caliche) formed by soil processes and is part of a weak dryland soil profile developed into the weathered bedrock surface. Also note the thin stringers of carbonate that extend horizontally away from the fracture and the chips of caliche contained in the plug of tan sediment that just slumps into the top of the fracture. A lot is going on in this one little fracture.

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