Sedimentary Rocks

Terminology Sedimentary Rock - a rock formed through the deposition of sediments from weathering or biologic activity. Diagenesis- Refers to all the chemical, physical, and biological changes that occur after sediment is deposited and before and after lithification Lithification - The process by which unconsolidated sediments are transformed into sedimentary rocks.

Lithification: Two Processes control Lithification: Compaction and Cementation Compaction- As sediment accumulates the weight of the overlying material compresses the lower levels. This leads to a reduction in pore space (Imagine a garbage compressor) The size and shape of the grains that make up the sediment will directly determine the amount of compaction that occurs. Sands and other course-grained sediment are less compressible than fine grained sediments such as clay.

Cementation Cementation- Refers to the glue (cement) that holds the sediments together) For the most part, cementing materials are the by product of chemical weathering (They are the dissolved components of other rocks). Important Cementing agents: Calcite (CaCO3) Silica (SiO4) Iron Oxide (Fe2O3, FeO) These materials precipitate out of solution and become deposited within the pore spaces between the sediment grains

Types of Sedimentary Rocks Detrital- Accumulated material that is transported as solid particles that originated from chemical or mechanical weathering. Chemical- Formed as previously dissolved minerals precipitate out of solution.

Identification of Sedimentary Rocks Detrital sedimentary rocks are identified by both their composition and their grain size Chemical Sedimentary rocks are classified by their chemical composition

Sedimentary Environments Sedimentary rocks form in many different depositional environments ie. beaches, oceans, mountains Each area depositing a similar material is considered a sedimentary facies. As we move from the Continental Environment to the Marine Environment there will be a gradual transition between the facies due to the matter of transport and other factors affecting deposition.

Grain Size vs. Sedimentary Environment Detrital grain size gives some indication of the energy conditions during transport and deposition High-energy processes such as swift-flowing streams and waves are needed to transport large grains such as gravel Conglomerate (composed mostly of gravel) must have been deposited in areas of high energy Sand transport also requires vigorous currents Silt and clay are transported by weak currents and and therefore only accumulate under low-energy conditions such as in lakes and lagoons

Examples of Sed. Facies

Continental Environments Dominated by erosion and deposition associated with streams Channel Deposits Alluvial Fans Flood Plains In colder areas glacial movement takes the place of rivers and streams Morrains In more arid areas wind is the more dominant factor Dunes Playa Lakes

Transitional Environments (the shore line) Beaches Tidal flats Lagoons Spits, bars and barrier islands

Marine Divided according to depth Shallow Marine- reaches to depths of about 200 meters (~700ft) Extends from the shore to the outer edge of the continental shelf In some areas can reach up to 1500km, however, average is about 80 km Types of sediment deposited depend on several factors Distance from shore Elevation of adjacent land Water depth Water temperature Climate

Deep Marine – Seaward of the continental shelf and reaches depth greater than 200 m Deposition in these environments are composed of tiny particles that drift in the ocean current as well as skeletal remains of microscopic animals such as diatoms.

Sorting and Rounding Sorting and rounding are two textural features of detrital sedimentary rocks that aid in determining depositional processes Sorting refers to the variation in size of particles making up sediment or sedimentary rocks It results from processes that selectively transport and deposit sediments of particular sizes

Sorting If the size range is not very great, the sediment or rock is well sorted If they have a wide range of sizes, they are poorly sorted For example Wind has a limited ability to transport sediment so dune sand tends to be well sorted Glaciers can carry any sized particles, because of their transport power, so glacier deposits are poorly sorted

Rounding Rounding is the degree to which detrital particles have their sharp corners and edges warn away by abrasion Gravel in transport is rounded very quickly as the particles collide with one another Sand becomes rounded with considerably more transport

Rounding and Sorting A deposit of well rounded and well sorted gravel Angular, poorly sorted gravel

Sedimentary Structures Sedimentary structures are visible features that formed at the time of deposition or shortly thereafter Represent manifestations of the physical and biological processes that operated in depositional environments Structures seen in present-day environments or produced in experiments help provide information about depositional environments of rocks with similar structures

Bedding Sedimentary rocks generally have bedding or stratification Individual layers less than 1 cm thick are laminations common in mudrocks Beds are thicker than 1 cm common in rocks with coarser grains

Graded Bedding Some beds show an upward gradual decrease in grain size, known as graded bedding Graded bedding is common in turbidity current deposits form when sediment-water mixtures flow along the seafloor As they slow, the largest particles settle out then smaller ones

Cross-Bedding Cross-bedding forms when layers come to rest at an angle to the surface upon which they accumulate as on the downwind side of a sand dune Cross-beds result from transport by either water or wind The beds are inclined or dip downward in the direction of the prevailing current They indicate ancient current directions, or paleocurrents useful for relative dating of deformed sedimentary rocks

Ripple Marks Small-scale alternating ridges and troughs known as ripple marks are common on bedding planes, especially in sandstone Current ripple marks form in response to water or wind currents flowing in one direction have asymmetric profiles allowing geologists to determine paleocurrent directions Wave-formed ripple marks result from the to-and-fro motion of waves tend to be symmetrical Useful for relative dating of deformed sedimentary rocks

Current Ripple Marks Ripples with an asymmetrical shape In the close-up of one ripple, the internal structure shows small-scale cross-bedding The photo shows current ripples that formed in a small stream channel with flow from right to left

Wave-Formed Ripples As the waves wash back and forth, symmetrical ripples form The photo shows wave-formed ripple marks in shallow seawater

Cross-Bedding Tabular cross-bedding forms by deposition on sand waves Typically produce where streams enter large bodies of water (AKA Deltas) Tabular cross-bedding in the Upper Cretaceous

Animation of crossbedding

Cross-Bedding Trough cross-bedding formed by migrating dunes Trough cross-beds in the Pliocene Six Mile Creek Formation, Montana

Mud Cracks When clay-rich sediments dry, they shrink and crack into polygonal patterns bounded by fractures called mud cracks Mud cracks require wetting and drying to form, lakeshore river flood plain or where mud is exposed at low tide along a seashore

Ancient Mud Cracks Mud cracks in ancient rocks in Glacier National Park, Montana Mud cracks typically fill in with sediment when they are preserved as seen here

Biogenic Sedimentary Structures Biogenic sedimentary structures include tracks burrows trails Also called trace fossils Extensive burrowing by organisms is called bioturbation may alter sediments so thoroughly that other structures are disrupted or destroyed

Sedimentary Structures Sedimentary rocks form as layer upon layer of sediment accumulates in various environments. These layers are known as Strata or Beds. Each Stratum or bed of a sedimentary rock is unique Texture and composition of the bed reflects the different conditions under which each layer was deposited.

Between the Lines Separating each strata are bedding planes. Bedding Planes are flat surfaces along which the rock tends to break. Caused by changes in grain size. May also occur after pauses in deposition. In general most deposition occurs due to sediments settling out of water therefore most bedding is horizontal. In wind driven deposition, layers are not always horizontal. Sand dunes are a prime example. In these cases the bedding is described as Cross Bedding

Bedding Planes