Today’s Lecture:  Sedimentary structures: Inferring depositional processes from sedimentary rocks Sea-level changes & the facies concept Chapter 7: Sedimentary Rocks

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 1 Bed 2

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. Bottoms of beds: Coarser Bed 1 Bed 2

Sedimentary Structures Graded beds: Show a gradual change in particle size as you move from the bottom of a bed to the top. Tops of beds: Finer Bed 1 Bed 2

Graded Bedding 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 Finer

Graded Bedding As transport velocity declines, coarser particles settle out first (see video on turbidity currents). Higher Velocity Lower Velocity

Graded Bedding Thus, graded beds tell us how flow velocity changed during deposition! Higher Velocity Lower 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.

Contacts between sedimentary beds Primary bedding vs. internal layering Primary Bed

Contacts between sedimentary beds Internal, inclined layers Primary bedding vs. internal layering

Contacts between sedimentary beds Internal inclined layers Primary bedding vs. internal layering

More cross-bedding Bed contacts”

Fig. 7.25abc W. W. Norton

More cross-bedding Bed contacts” Cross beds

Cross-bedding Tilt-direction of cross beds indicates the direction of transport (e.g., wind direction or direction of water flow). Transport direction

Large-scale cross-beds like these are formed by sand dune migration

Which way did the wind blow? What a geologist sees.

Paleowinds What a geologist sees.

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).

Ripple formation movie Transport and Deposition in Running Water

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 Study these ripple carefully. Are they symmetrical or asymmetrical? What do they suggest about paleocurrent direction? Look at these!

Sedimentary structures: Ripple Marks Oscillation ripples (back and forth) Interpretation: Paleoshoreline

Sedimentary Structures: Mud Cracks

Fig. 7.27d Stephen Marhsak Ancient mud cracks (cross-sectional view)

Sedimentary Structures: Mud Cracks Mud cracks form when mud- covered shorelines or lake bottoms, dry up. This produces an irregularly- cracked surface. Margin of a dry lake with mud cracks. Note ripple-marked sand dunes at top of picture.

Sedimentary Structures Note ripple- marked sand dunes at top of picture.

Sedimentary structures: Mud Cracks

Fig. 7.27c Stephen Marhsak Say you find mud cracks in an ancient sedimentary rock. What does that suggest about the environment where the rock formed?

What a geologist sees.

Describe what you see in this outcrop and interpret the geologic history and conditions of deposition. In class exercise:

Contains “clasts” of older rocks. Igneous clast Metamorphic clast

Lower contacts cut into units underneath. Erosional! Fine layering Wavy basal contact Ripple cross- bedding

Bed shows size grading. Coarser base Finer top Single bed

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?). What a geologist sees:

u “Facies” are representations of sedimentary environments defined by the overall association of features preserved in rocks. Sedimentary environments Concept of Sedimentary “Facies”