Name: Reg. lab day: Tu W Th

Geology 1023

Environments & Facies (Lab #3, Winter 2010) Introduction This lab will review some general aspects of sedimentary environments and introduce you to the concept of "facies". Most sedimentary rocks have been deposited in bodies of water (marine environments) near the continental margin (coast). It is important to have an understanding how deposition of sediment varies in both time and space within marine environments. Continental Margin Profile Marine continental margins, where most of the sedimentary rocks are deposited, can be divided into three major regions: continental shelf, continental slope, and continental rise. The deep sea beyond the continental rise is the very flat, sediment-covered, abyssal plain where there may be volcanic island arcs and deep-sea trenches (Fig. 1).

Marine sediments The type of sediment being deposited at a given place on the seafloor depends on many factors that include distance from shore, kinds of source rock, seafloor topography, climate, etc. Coarser sediments tend to be deposited close to shore and finer sediments farther away. Most clastic material deposited in marine environments is originally derived from an adjacent landmass transported by rivers to the coast, then reworked along the shoreline and spread offshore across the continental shelf and into the deep ocean. Such clastic material is commonly called terrigenous sediment. For obvious reasons, only small quantities of the finest sediment make it to the deep ocean. Limestone is usually formed in clear shallow water in warm climates where

Answers

Environments & Facies — Winter 2010 Page 2 of 11 the carbonate grains, which are formed slowly, are not overwhelmed by terrigenous material. In the deep, open ocean, far from shore, typically only very fine clay, volcanic ash, meteoric dust, and the skeletons of microscopic organisms settle to the seafloor. A kind of sediment-laden density current called a “turbidity current” can transport sediment onto the continental rise and locally onto the portion of the abyssal plain nearest to continental margin. Turbidity currents are channelled down submarine canyons that cut into the continental slope (Fig. 2). Deposits from turbidity currents are called turbidites. Lithologically, most turbidites are greywacke. Turbidites are characterized by poor sorting, graded bedding and current marks on their bases (flutes and grooves). Turbidite deposits commonly show many repeated cycles of graded greywacke and finer shale (Fig. 2).

Environments & Facies — Winter 2010 Page 3 of 11 1. Label the features indicated on the shaded relief map of portion of the continental shelf off

the coast of Nova Scotia (Fig. 3).

2. What do you think would happen to the sediments on the Scotia Shelf (and their equivalents

in Europe and Africa) when North America and Europe/Africa converge again?

In the lab, there are some closed jars containing sediment and water. Look at one, then shake it vigorously. Now, put it down and watch what happens. 3. What sedimentary process are you simulating here? 4. Describe what you see after allowing the sediment to settle for 5 minutes or so.

5. What is the name of this sedimentary structure?

canyon

shelf

rise slope

plain

Build a new mountain chain

Turbidity current

Coarser grains settle on bottom finer ones on top Graded bedding

Environments & Facies — Winter 2010 Page 4 of 11 Facies

Some rock formations change lithology horizontally. We call this a “facies change”. The term "facies" refers to all of the physical, chemical and biological characteristics of a sediment or sedimentary rock that was deposited in an area. It is usually representative of a particular environment of deposition. Different environments coexisting side-by-side will produce different facies adjacent to each other. The lateral transition from one to the other is the facies change. We can see facies changes in large outcrops, or interpret them from stratigraphic columns and extrapolate them to cross-sections and illustrate them on maps. The following list gives lithologies and sedimentary structures that indicate various environments. In all the following discussions the term proximal means close (or closer) to the source or sediment, distal means far (or farther) from the source of sediment. Lithology Environment Arkose Alluvial. The coarser the grains the more proximal. Breccia Scree slope and perhaps the most proximal portion of an alluvial

fan. Conglomerate Distal portion of alluvial fan and proximal portion of river (braided

river). Greywacke Most commonly deep sea fan. Also distal portions of a delta if in

deep water. Limestone Shelf environment away from significant terrigenous input. Shale Indicates sediment input into still and usually deep water e.g.,

alluvial floodplain (subaerial), deeper portions of a lake, marine lagoon, or deepest marine environment (abyssal plain).

Siltstone Alluvial plain proximal to the channel or distal portion of beach. Sandstone Indicative of relatively fast-moving water or air such as in a river

channel, or in a beach or dune. The better sorted the sand the more proximal to the source.

Structure Environment Asymmetrical ripples Indicate current action in river channel or on beach. Size of ripples

generally indicative of water speed and depth (large ripples indicate strong currents in deep water, smaller ripples indicate slower currents).

Cross bedding Indicates deposition from a flowing medium (air or water). If crossbeds are very large (more than 1-2 m) may indicate aeolian environment (dunes), otherwise current action in river channel, delta or beach environment.

Graded bedding Indicative of changing flow conditions; river flood plains (proximal to the channel) or deep sea fans (turbidites).

Symmetrical ripples Indicate wave action, i.e., beach (on a lake if obviously terrestrial — red colour, rain prints, cracks, plant fossils — otherwise assume marine).

6. Using the above criteria, determine the environments of deposition for the samples A–K (at

Environments & Facies — Winter 2010 Page 5 of 11

the back of the lab) and indicate them on the diagram of clastic environments below (Fig. 4).

Environments & Facies — Winter 2010 Page 6 of 11 7. The map (rectangle) below you will see a series of letters that correspond to a facies as

represented by the samples in the lab (X indicates erosion/non-deposition).

a) Draw boundaries between the different facies indicated on the map. b) Label the depositional environments represented.

c) Indicate on the map the direction towards the sourceland for the sediment.

Isopach maps Isopach maps show how rock units change in thickness across any given area using lines drawn thorough points of equal thickness of the unit. The lines of equal thickness are called isopach lines. On a map, the edge of the outcrop marks the maximum lateral extent of the sedimentary rocks. In so far as the outcrop is the result of erosion, the original extent of the unit would have been greater. If little erosion has occurred the outcrop may represent the edge of the basin of deposition and would indicate the original extent of the unit being mapped. Isopach maps are extensively used in constructing a paleogeographic map. It is important to note that isopach maps are difficult to construct for even moderately deformed rocks. Deformation, even slight deformation, can significantly change the thickness of stratified units. However, in flat-lying rocks, such as are found on cratons, isopach maps are an essential

Environments & Facies — Winter 2010 Page 7 of 11 tool in reconstructing paleoenvironments and predicting the size, shape, and position of potential oil or groundwater reservoirs. 8. Figure 6 below shows the distribution and thickness of Cretaceous rocks in parts of British

Columbia, Alberta and Saskatchewan.

a) Draw boundaries between the different facies shown.

Environments & Facies — Winter 2010 Page 8 of 11

b) Using the spot thicknesses shown on the map, draw isopach lines for 250, 500, 750, 1000 and 1250 m.

c) What is the spatial relationship between the isopach lines and the facies boundaries?

d) Suggest why conglomerate is present in the west-central part of the area but not to the north or south.

e) What is the depositional environment of the conglomerate? f) Note the "limit of outcrop" for these Cretaceous rocks. Give 3 reasons why Cretaceous

rocks are not found further east.

1.

2.

3.

Transgressions and regressions Knowing the position of the ancient shoreline is one of the fundamental necessities for correctly interpreting the geologic history of a transgressive sequence of sedimentary rocks. However, the position of any coastline continuously changes as continents move and as global climate changes. Therefore, the sea has advanced across (transgressed) and receded from (regressed) the continental cratons many times during geologic history. These advances and retreats are due to fluctuation in either sea level and/or land level. Tectonic activity alters the relative positions of crustal blocks vertically and horizontally and has both local and global effects. For example, the break up of the supercontinent Pangaea significantly increased the area of shallow seas. But individual segments of continental masses may be uplifted when others are depressed. Likewise, tectonic activity will change the size, configuration and average depth of the ocean basins leading to global changes in sea level. In addition, major climatic changes (e.g., glaciation) will change the amount of water in the ocean basins, which will also change global sea level. Worldwide changes in sea level are called "eustatic" changes. The advance of the sea onto a continent, a transgression, produces a phenomenon known as

Sub-parallel

High mountains = coarser and thicker sed.

Alluvial fan

Erosion Non-deposition Covered up by younger rocks.

Environments & Facies — Winter 2010 Page 9 of 11 onlap. Onlap occurs when a series of sedimentary facies moves landward. Offshore facies will move landward and bury older onshore facies below. In a cross section, evidence of a transgression can be found by looking at the vertical sequence of rocks. In a local stratigraphic column, coarser rock units will be on the bottom and finer-grained ones at the top. In contrast, a regression will be indicated by the reverse — coarser-grained rocks at the top. Walther's Rule Because sea levels are constantly changing it follows that the lateral facies variation will be found in the vertical succession of rock units if there are no significant breaks in the rock record. This generalisation is sometimes called Walther's Rule. Thus, to determine what facies are to be encountered laterally from an outcrop, one only has to examine the vertical sequence of rocks at that locality. 9. Figure 7 (on the next page) shows three facies maps for the same area but three different

times (T1, T2 and T3).

a) Complete the maps by drawing facies boundaries (as before) and by filling in lithological patterns as indicated at the bottom of the page.

b) Mark the shoreline on each map in red.

c) Has the shoreline moved toward or away from the continent?

towards

Environments & Facies — Winter 2010 Page 10 of 11

Environments & Facies — Winter 2010 Page 11 of 11 Figure 8 below is the cross section for the three maps in Figure 7.

d) Transfer the facies data from each map (T1, T2, and T3) to the appropriate time line in the box. Draw slanting zig-zag lines between the facies. This demonstrates the facies change with time.

e) Does your completed cross-section show a transgression or regression?

f) Describe how Walther's Rule is illustrated for position " "as shown on this diagram

transgression

The vertical variation in one location is

the same as the horizontal variation at

any one time.