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Today’s Lecture: Chapter 7: Sedimentary Rocks Sedimentary structures: • Inferring depositional processes from sedimentary rocks •Sea-level changes & the facies concept Sedimentary structures: Features observed within a single bed. Within sedimentary beds, distinctive structures can usually be seen. These include systematic variations in grain size and sorting, internal bedding features, etc. that are diagnostic of particular depositional processes. Sedimentary Structures Graded beds: Show a gradual change in particle size as you move from the bottom of a bed to the top. Bed 2 Bed 1 Fig. 7.26a Stephen Marshak Sedimentary Structures Graded beds: Show a gradual change in particle size as you move from the bottom of a bed to the top. Bed 2 Bed 1 Bottoms of beds: Coarser Sedimentary Structures Tops of beds: Finer Graded beds: Show a gradual change in particle size as you move from the bottom of a bed to the top. Bed 2 Bed 1 Graded Bedding Finer The bed to the right shows a change from large grains at the bottom, to small at the top. This is called “normal” grading. Coarser Graded Bedding Lower Velocity As transport velocity declines, coarser particles settle out first (see video on turbidity currents). Higher Velocity Graded Bedding Lower Velocity Thus, graded beds tell us how flow velocity changed during deposition! Higher Velocity Sedimentary structures: Cross-Bedding Cross-bedding is internal bedding that is tilted at an angle to the primary bedding. Cross beds are formed by a scour and fill transport process involving either wind or water (see ripple movie). First we need to distinguish between primary bedding vs. internal layering. Primary bedding vs. internal layering Contacts between sedimentary beds Primary Bed Primary bedding vs. internal layering Internal, inclined layers Contacts between sedimentary beds Primary bedding vs. internal layering Internal inclined layers Contacts between sedimentary beds More cross-bedding Bed contacts” Fig. 7.25abc W. W. Norton More cross-bedding Cross beds Bed contacts” Cross-bedding Transport direction Tilt-direction of cross beds indicates the direction of transport (e.g., wind direction or direction of water flow). Large-scale cross-beds like these What a geologist sees. Which way did the wind blow? What a geologist sees. Paleowinds Sedimentary Structures: Ripple Marks Ripple marks form when moving wind or water causes sedimentary grains to “hop” along the bottom. Fig. 7.27a Stephen Marshak Ripple marks can be either symmetrical (formed by waves sloshing back and forth), or symmetrical (formed by water or wind flowing in one direction). Transport and Deposition in Running Water Ripple formation movie Sedimentary Structures: Ripple Marks Look closely at the ripples on this surface. Are they symmetrical, or asymmetrical? Which way did the water flow? Sedimentary Structures: Ripple Marks Look closely at the ripples on this surface. Are they symmetrical, or asymmetrical? Which way did the water flow? Paleocurrent direction Sedimentary Structures: Ripple Marks Look at these! Study these ripple carefully. Are they symmetrical or asymmetrical? What do they suggest about paleocurrent direction? Sedimentary structures: Ripple Marks Oscillation ripples (back and forth) Interpretation: Paleoshoreline Sedimentary Structures: Mud Cracks Ancient mud cracks (cross-sectional view) Fig. 7.27d Stephen Marhsak Sedimentary Structures: Mud Cracks Mud cracks form when mudcovered shorelines or lake bottoms, dry up. This produces an irregularlycracked surface. Margin of a dry lake with mud cracks. Note ripple-marked sand dunes at top of picture. Sedimentary Structures Note ripplemarked sand dunes at top of picture. Sedimentary structures: Mud Cracks Say you find mud cracks in an ancient sedimentary rock. What does that suggest about the environment where the rock formed? Fig. 7.27c Stephen Marhsak What a geologist sees. In class exercise: Describe what you see in this outcrop and interpret the geologic history and conditions of deposition. Contains “clasts” of older rocks. Igneous clast Metamorphic clast Wavy basal contact Fine layering Ripple crossbedding Wavy basal contact Lower contacts cut into units underneath. Erosional! Bed shows size grading. Finer top Single bed Coarser base What a geologist sees: Uplift of a deep-seated igneous pluton (granitic), with subsequent erosion by running water which transported igneous and metamorphic clasts to a river which then carried them some distance from the source area, to a site of deposition (stream channel). Transport by running is inferred by the rounding of the clasts, size grading, and sorting. Erosional contacts at the bases of beds indicate initial turbulent transport, followed by declining flow velocity (flood event?). Sedimentary environments Concept of Sedimentary “Facies” “Facies” are representations of sedimentary environments defined by the overall association of features preserved in rocks.