GEOL 110 Sedimentary Layering Geologic Structures (faults, joints, folds), Unconformities, Karst Layering = horizontal, continuous banding produced during sediment deposition Sedimentary Layering Geologic Structures I) Geologic Structures = fractures or bends in rock layers Produced by plate tectonic (mountain-building) forces that deform previously unbroken and unbent rock layers. Geologists study geologic structures for clues about mountain-building during Earth's past Types of Geologic Structures - Faults A. Faults - fractures along which rocks have moved past each other; earthquake occurs when rocks move along a fault footwall - underlying surface of inclined fault (where your foot would stand) hanging wall - overlying surface of inclined fault (where your lantern would hang) Types of Geologic Structures-Normal, Reverse Faults Normal fault = hanging wall moves down; commonly caused by stretching (extensional) forces. Reverse fault = hanging wall moves up; commonly caused by squeezing (compressional) forces. 1
Types of Geologic Structures - Joints B. Joints - fractures or cracks along which rocks have not moved; can be produced by squeezing or stretching forces. Also, they can form by cooling of igneous rock (columnar joints - 6-sided columns) or by pressure release due to uplift. C. Folds - bends in rock layers; often due to squeezing (compressional) forces Anticline - arch-like fold (looks like letter "n") Syncline - trough-like fold (looks like letter "y") columnar joints in basalt Monocline - fold with only one side; often produced by fault in igneous or metamorphic rocks below, causing overlying sedimentary rocks to drape over fault ("drape fold"). Dome - upwarping of crust at central point, resembles inverted bowls; Rock layers dip (tilt) away from center of dome; in map view-geologic contacts are concentric circles with oldest rock in middle; cross-sectional view is anticline; e.g., Ozark Dome in SE Missouri; large-scale features (10s-100s of km across) Basin - downwarping of crust at central point, resembles normally oriented bowls; Rock layers dip (tilt) towards center of basin; in map view geologic contacts are concentric circles with youngest rock in middle; cross-sectional view is syncline e.g., Illinois Basin; large-scale features Unconformities II) Unconformity = surface between rock units that represents break in geologic record with rock above unconformity considerably younger than rock below. Usually caused by long periods of erosion; sedimentary rock below unconformity can have same or different orientation than overlying rock 2
Formation of an angular unconformity involves folding, uplift and erosion of rocks below unconformity Unconformities Caves and Karst Topography III) Cave = natural cavity in rock large enough for someone to enter (travel far enough to reach total darkness) Niah Cave, Malaysia Cave - Formation caves form most often in carbonate bedrock (limestone, dolomite) when slightly acidic, flowing groundwater moves along joints and dissolves bedrock; Slightly acidic rain moves downward through soil and becomes more acidic from decomposing organics that produce CO 2 gas, which dissolves in water to produce carbonic acid (H 2 CO 3 ), which dissolves calcite Caves and Karst Topography Sinkhole - bowl-like depressions at Earth s surface produced when underground cave collapses or from downward dissolution at Earth s surface Karst topography - area with abundant caves, sinkholes and related features incipient sinkhole Sinkhole formed over natural cave Karst topography Speleothems Chemically precipitated rock (usually calcite formed from dissolved minerals in water) found in caves Can form from dripping water: stalactites - thin icicles that grow from ceiling, stalagmites - wide cones that grow from ground Or from flowing water: flowstone Water with abundant dissolved calcite loses CO 2 gas (opposite of calcite dissolution reaction) or evaporates, both cause calcite precipitation Formation of speleothems 3
Joints enlarged by dissolution (2 sets at 90 ) 4
Geologic Structure Geologic Feature 5
Geologic Feature Geologic Feature Speleothems Speleothems of Onondaga Cave (Missouri trip) Sinkholes Sinkholes 6
Sinkholes 7