EAS 222 Midterm 2

Midterm 2 Study Guide:

-The exam will include material from lectures 5 through 8 and 11.

Lecture 5

-You’ve already memorized the size Wentworth-Udden size classifications. Let’s cement that and make sure we know the grain-size cutoffs for the main siliciclastic rocks types.

• Conglomerate 2mm-4096mm

• Siltstone 1/256mm-1/16mm

• Claystone 1/256mm-smaller

• Sandstone 1/16mm-2mm

-What are the four major components of sandstone?

• Framework, matrix, cement, pores

-Know how to use a QFL diagram.

-What are the main rock-forming minerals?

• Clay minerals, quartz, feldspars, rock fragments

-What are accessory minerals? How are they important?

• Accessory minerals are minerals that are present in small amounts, and generally not taken into consideration when estimating the mineralogy of a rock. They are useful for provenance studies (where did the rock come from, and how did it get to it’s location).

-What is an ‘arenite’?, a ‘wacke’?, an ‘arkose’? 

• Arenite has less than 15% matrix present.

• Wacke has more than 15% matrix present.

• Arkose has more than 25% feldspar present.

-What is an olistolith’?, an ‘olistostrome’?

• An olistolith is a very very large clast, and an olistostrome is a rock make up of olistoliths. Like conglomerate but huge.

-What is the difference between fabric-selective and fabric-destructive porosity?

• Fabric selective pore development is controlled by grains.

• Fabric destructive porosity is when the destruction of fabric impacts the porosity of the rock.

-What three main processes affect sandstone porosity?

• Cementation - Destroys pore space.

• Dissolution - Creates pore space.

• Compaction - Decreases pore space.

Lecture 6

-What are the four main carbonate minerals we talked about in class and what are their main differences?

• Calcite

• Halite

• Dolomite

• Aragonite

-Where are each of the main carbonate minerals likely to form on the earth’s surface?

-What are the main components of carbonate rocks?

• Biological material (shells and other bone fragments).

• Calcium carbonate

-What are the five types of carbonate grains?

• Clasts, coated grains, peloids, aggregate grains, skeletal grains.

-What happens when seawater is progressively evaporated? What is precipitated out and in what order?

• Calcium carbonate, gypsum, halite, salts

-What affects the solubility of CO₂?

• Temperature and pressure.

-What is the Carbonate Compensation Depth?

• Depth below which the rate of calcium carbonate lags behind the rate of dissolution such that no carbonate is preserved.

• Determined by pressure, temperature, and seawater composition.

-Know the carbonate classification scheme of Dunham (1962) as modified by Embry and Klovan (1971).

-Know the names for the various types of coated / accretionary grains.

• Ooids

• Oolites

• Pisoids

• Pisolites

• Oncoids

• Oncolites

• Rhodoid (rhodolith)

• Peloids

-How have carbonate sediments changed from the Palaeozoic until the present?

-What are the most common skeletal grains in carbonate derived from?

• Crinoids

-Where does most dolomite form?

-Where at the earth’s surface / near surface might dolomite form at present?

• In sabkha settings

What are the basic porosity types and what are their characteristics?

• Intercrystal - The pores occur between crystals.

• Moldic - Pores left by removal of grains.

• Fenestral - Aligned ellipsoidal pores, characteristic of supratidal settings.

• Shelter - Formed under convex upwards shells, kind of like a carbonate umbrella.

• Fracture - Porosity associated with fractures is generally very low.

• Channel - More irregular.

• Vug - Single irregular cavity.

• Burrow - Grains are replaced by infauna rather than truncated.

• Boring - Truncation of grains and cements at bore walls.

Lecture 7

-What happens when seawater is progressively evaporated? What is precipitated out and in what order?

-Where and how do evaporites form?

-Where do siliceous rocks and minerals form? 

-What are the two main sources of the silica and what causes silica precipitation?

• The two main sources are biogenic and volcanic sources.

• Silica precipitation is caused by basins being supersaturated in silica, and precipitation onto clay minerals.

-What are the principal types of iron-bearing minerals?

• Sulfides and oxides.

-What is glauconite? Where and how does it form?

• Glauconite is a ferris-iron silicate mineral that forms on the sea floor. It forms by alteration of the fecal pellets of bottom dwelling organisms, modification of particles of lithic and biotitic clay by sea water, and direct precipitation from sea water.

-What is siderite? Where and how does it form?

• Siderite is an iron carbonate that forms under shallow burial in marginal marine and non-marine successions. It forms under anoxic conditions due to need for reduced dissolved ferrous elements to react with the carbonate iron.

-What is BIF? Where and when did they form?

• Banded iron formations are layered formations created in bodies of water when there were high amounts of iron and oxygen present in the water. These came together to form iron oxides which were deposited in layers leading to the creation of banded iron formations. They formed in the precambrian.

-What types of grains are sedimentary phosphorites composed of? 

• Ooids, oolites, pisoids, peloids, skeletal fragments, lenses, nodules, and amorphous apatite mud and cement.

-Where are phosphorites deposited? What mechanism is commonly invoked to explain their occurrence?

-When is the first (oldest) time interval of major coal accumulation in the rock record?  What events caused this interval to preserve so much more coal than later time intervals?

• The carboniferous.

• The earth was largely dominated by plants, which allowed large amounts of plant material to be buried and preserved as coal.

-What is required for the precursors of coal to be deposited and preserved?

• Large amounts of plant material to be buried at depth very quickly.

Lecture 8

-What is the difference between an alluvial fan and a fluvial fan?

• Alluvial fans are characterized by sedimentary gravity flows, while fluvial fans are characterized by sedimentary fluid flows. Fluvial fans are also much bigger than alluvial fans in area and radius, however alluvial fans have a higher slope.

-Be able to draw a proximal-distal cross-sectional profile of an alluvial fan and a fluvial fan.

-What are the main processes involved in the development of an alluvial fan?

• Alluvial fans occur where a channel encounters a significant decrease in slope or loss of confinement, typically where a mountain stream exits onto a plain or valley.

-What is a fan delta?

• Where an alluvial fan leads into a standing body of water. The fan delta is the space between the channel and the body of water.

-What are the main river types discussed in class? Draw a vertical profile for each and label the expected bedforms and physical/biogenic structures you would expect in each.

• Meandering river

• Straight river

• Braided river

  • Overbank deposits layered with channel deposits. On top former bars are covered in vegetation.

-What is a crevasse splay? What are their characteristics.

• When a river or stream breaks its levee and deposits sediment on a floodplain in a pattern similar to an alluvial fan.

• They commonly occur on the outside banks of meandering rivers.

• They often display graded bedding.

-What mechanism is responsible for most sediment movement in deserts?

• Flash floods.

-What is the grain-size ‘ceiling’ for sediment transported via traction on land?

• Medium grained sand

-What is a ventifact?

• A ventifact is a rock that has been weathered by wind driven sand or ice particles.

-What are the main dune types discussed in class? Under what conditions do each of these form?

• Barchan Dunes

  • Not a lot of sand, consistent wind direction.

• Transverse Dunes

  • Form where there is a large supply of sand.

• Linear Dunes.

  • Form when there are two prominent wind directions, low sand supply.

• Star Dunes.

  • Form under multiple wind directions, good sand supply.

-Where do lakes form?

• Lakes form where there is a supply of water to a topographic low.

-What is a hydrologically closed lake? A hydrologically open one?

• Closed system, no exchange of material or energy with other systems.

• Open system, exchange or material or energy with other systems is present.

-What are the main difference in tropical and temperate lakes?

• Temperature.

• Turnover in temperate lakes not present in tropical lakes.

• Tropical lakes will have more vegetation which can impact rates of erosion.

• Lakes have overall lower energy levels, and higher sedimentation rates.

• Tidal forces are minimal even in the smallest lakes.

• Water is the primary mixing force in temperate lakes, while air is the primary mixing force in tropical lakes.

-What are the layers in a stratified lake?

-What are the differences between an overfilled, balanced fill and underfilled lake?

• Underfilled lake means the rate of evaporation is greater than the rate of water supply.

• Overfilled lake means the rate of evaporation is smaller than the rate of water supply.

• Balanced fill lake means the rate of evaporation and water supply are relatively equal.

-What is the main difference in the lake margin sediments of temperate versus arid lake margins.

-What would an idealized vertical section through an arid, ephemeral lake look like?

Lecture 11

-What are the five principal carbonate depositional settings discussed in class? What are their basic characteristics?

-We talked about three main carbonate associations in class.  What are they are what are their main characteristics?

• Foramol associations, bryozoa, benthic formainifera, red corraline aglae, and mollusca, inhabiting seas where the temperature often falls below 15 degrees celsius.

• Chlorazoan associations, warm water, includes hermatypic corals, calcareous green algae, some bryozoans are present, barnacles are absent.

• Chloralgal associations, dominated by green algae, typical of saline environments.

-What are the main reef types and their basic characteristics?

• Barrier reef

  • Extends outwards towards the sea, deep inner lagoonal belt, linear.

• Lagoonal reef

• Fringing reef

  • Extends outwards in both a seaward and landward direction, no inner lagoonal belt, backreef zone commonly exposed during low tide.

• Atolls

  • Ring shaped reefs that created protected lagoons, usually in the middle of the sea.

• Platform reef

  • Occur in open ocean and on continental shelf, grow in all directions, commonly consist of primarily framestone, commonly occur in the lagoon behind a barrier reef.

• Pinnacle reefs

  • Bioconstructed isolate carbonate buildup, consist primarily of framestone.

• Mounds/reef mounds

  • Carbonate buildups that lack large skeletal frame builders, formed by the in situ production and trapping of skeletal and non-skeletal grains, commonly early cemented.

• Patch reef

• Bioherm/biostrome

  • Lens shaped carbonate buildup enclosed by sediments of a different lithology.

  • Biostromes are similar, but are laterally extensive tabular sheets.

-What are the difference between a fringing reef and a barrier reef? Between a bioherm and a patch reef? Etc…?

-What are ooids and where are they most likely to form?

• Ooids are small coated grains, usually made of calcium carbonate. Ooids usually form in the upper shore face.

-How do intertidal / supratidal successions differ between arid and humid depositional settings?

• Supratidal : Occur above mean high tide and are submerged only during storms in humid settings. In arid climates it is a sabhka, and evaporitic setting. Arid regions may have nodular or bedded anhydrite.

-In an arid setting, what type of sediment and characteristics would you expect to see in the shallow subtidal and lower intertidal.  What would you expect to see in the upper intertidal? How about in the supratidal?