In last weeks exercise you were introduced to a variety of sedimentary rocks and their properties. Properties such as grain size, sorting, and rounding are important for reasons beyond simple rock classification and description. In this exercise we will explore ways in which these and other properties (including sedimentary structures) are used to interpret the environment in which these rocks were deposited. Correctly determining the environments of ancient deposits is a key goal in studying sedimentary rocks in part because certain valuable resources (such as oil, gas, and coal) are formed only in limited environments.

1. Grain size: This rock property is a measure of the energy of the environment. In general, coarser grain size indicates a higher energy environment. Thus muds accumulate in quiet settings (such as lakes), but sands and gravels accumulate on higher energy environments (such as beaches). Changes in grain size with energy can be observed along most continental margins, (which is where most sediments end up and where the thickest sedimentary deposits are located). If you could walk from the coastal beach to the sea you would see a change in grain size from gravel and coarse sands to finer sands to eventually muds.
2. Grain sorting: This rock property is controlled in part by rates of sedimentation. Sands deposited by storms tend to be poorly sorted because there was so little time to sort the grains. Sands deposited on beaches tend to be better sorted because the sediments are reworked by waves continually and therefore become well sorted.
3. Grain rounding: This property provides information about how far and for how long clasts were transported. Most particles start out angular when they are liberated from their original source (you can see this if you beat on a rock with a hammer). However, they become rounded through milling as they are transported great distances (like across continents). Of course roundness also has to do with the grain composition (i.e. harder grains are more difficult to round) and with the transport process.
4. Sedimentary rock color: Color can be influenced by grain composition, by cement, and by % of organic carbon or iron content. In general for mudrocks, reds and purples reflect continental environments whereas green, gray, and black colors reflect marine settings (where iron is reduced).

SEDIMENTARY STRUCTURES: Read these descriptions then visit the PHOTO GALLERY to view examples 

Asymmetrical ripple marks (current ripples)

• Formed by currents of water or wind over sand
• These ripples have a sloping (stoss) side and a steep (lee) side
• The steep side faces downstream or downwind

Symmetrical ripples (wave - oscillation ripples)

• These are made by waves in shallow water
• They have sharp crests and tend slope downward on both sides of the crest
• The troughs are rounded

Cross-bedding

• Found in deltas, streams, sand dunes, beaches and alluvial fans
• Cross-bedding is basically asymmetrical ripples stacked on top of one another (in cross-sectional view)

Plane Bedding

• Parallel layers of sediment

Graded bedding

• This is bedding that consists of a coarse-grained layer on the bottom and sequentially finer grains deposited on top.
• It forms when sediments of all sizes are mixed together in water
• As the water flow slows the coarse material is deposited first making the coarse layer
• The finest grains settle out more slowly making the finest layer at the top of the bed.

Inverse graded bedding

• This type of bedding consists of fine-grained sediments on the bottom of the bed covered by coarser-grained sediments

Mud cracks

• These are typically formed on mud flats
• When the mud dries out it shrinks and cracks
• The are preserved when sediments cover the cracks
• Mud cracks form polygon shapes usually with 5-sides
• In cross-section, the cracks taper down an are usually filled with different sediment

Cut & Fill

• These features form when water or wind cuts into preexisting sediment
• The carved out channel is the filled in with a different type of sediment
• An example is a stream cutting through a silt layer
• The stream deposits coarse-grained material into the channel until it is filled

Soft sediment deformation

• this happens when heavier & coarser sediments are deposited onto finer less dense sediments
• usually, it is a layer of sand deposited onto mud
• the sand then compresses the mud down and makes the contact between the layers uneven

Bioturbation

• the layers of sediment become disrupted by plants or organisms that borrow into the layer
• this process can disrupts the layering formed when the sediment was deposited

Alluvial fan

• lots of coarse particles
• usually sandstone and conglomerate
• poor sorting
• deposited by high energy floods or mudflows

Fluvial (stream channel)

• sandstone and conglomerate
• ripple marks and cross-beds
• graded bedding

Fluvial (floodplain)

• usually plane bedding
• fine-grained sand, silt, and clay

Paludal (swamps)

• dominantly fine-grained
• coal common

Lacustrine (lakes)

• beaches along edge; coarse-grained, well-sorted sandstone
• sediments in lake are finer grained
• can find evaporites such as gypsum and halite

Glacial

• extremely poorly sorted, usually coarse material in moraines
• better sorting in outwash deposits
• large cobbles may be striated

Aeolian (desert)

• sand usually fine-grained
• very large scale cross-bedding
• well sorted
• usually well rounded and frosted
• rocks may be polished and faceted (windblown)

Beach

• sizes range from cobbles to fine sand
• well sorted and well-rounded
• may be plane bedded or cross-bedded dunes

Estuary and Tidal Flats

Shallow marine (less than 200 meters)

• reef structures
• limestone, dolomite, and gray shale common
• sediments rich in glauconite and phosphate

Deep marine (greater than 200 meters)

• graded gray sandstone
• interbedded with gray shale
• bedded chert