you will need additional pictures to do this assignment please let me know once you need them I can send them also please complete my other assignment order number 95790
Microsoft Word - Assignment 5.docx 8 ERSC 2P16 Clastic Sedimentology Fall 2021 Assignment 5: Identification of primary sedimentary structures and their use in determining “which way is up”. Introduction Sedimentary structures found in sedimentary rocks can be divided into three broad groups: 1. Primary structures: those that form by physical processes during or very shortly after deposition and well before consolidation and cementation. 2. Biogenic structures: those that result from the activities of organisms living in the environment of deposition (i.e., trace fossils). 3. Secondary structures: those which form in response to their physical environment well after initial deposition (e.g., pore water pressures that invoke soft sediment deformation, the chemistry of the pore waters, load pressure, temperature). In this assignment you will examine sedimentary structures that fall into the first class of structures, above, that are described in the course material for Chapters 5 and 6. An aim of the assignment is to give you an opportunity to gain some experience in identifying primary sedimentary structures that are described in the course notes. This is a fundamental skill for any geologist who plans to work on sedimentary strata for purposes such as hydrocarbon exploration or as a geoscientists whose interest is in the interpretation of the history of the Earth. In addition, this assignment will introduce you to techniques that are often used in structural geology to establish whether or not rocks have been overturned by tectonic processes over the course of their history, a task that often relies on primary sedimentary structures. Please watch the video that is provided in the Assignment 5 folder in Sakai that will aid you with this assignment. Part A: Name that primary sedimentary structure. Identifying sedimentary structure requires experience, much of which comes from field work or core analysis over the course of a career. However, long before you have much “real-world” experience you can compare what you see in sedimentary rocks to photos or sketches to help develop some skill at identifying the structures. In nature it’s often more difficult to “see” the structures than it is in the best specimens or photos that are selected for teaching purposes. The real world is always more complex than the classroom! 2 Part A of this assignment involves identifying primary sedimentary structures in photographs based on what has been information that is covered in Chapters 5 and 6 of the ERSC 2P16 course notes, particularly as they are illustrated in the PowerPoint presentations that accompany the notes. You may also search the internet for pictures of the named structures for comparison to the photos provided. This is fair game for any practicing geologist who wants to become proficient at recognizing primary sedimentary structures. Of course, as always when using the internet as a source of “good” information, use your best judgement to use only “reliable” web sites. Beware of sites that are just a random person’s thoughts about the subject of bed forms or sites that are based on “alternate facts” that are actually demonstrably false information. Within the Assignment 5 folder in Sakai there are a set of “Part A Photos” that include 10 digital photographs, labeled Photo 1 to Photo 10, and each photo displays a different primary sedimentary structure or structures. On page 8 of this assignment is the “Part A Answer Sheet” that must be completed and handed in. The right hand column of the answer sheet provides brief descriptions or names of structures shown in each of the 10 photos. On the left hand column you must write down the number corresponding to the photograph that shows the structure(s) listed in the corresponding line of the right hand column. Part B: Primary structures as way up indicators. My own interest in bed forms and cross-bedding in sedimentary rocks has always been directed at what they can tell us about the processes (blowing wind, flowing water, gravity flows, wet/dry cycles, the presence of fossils and their traces, etc., etc.) that were active in the depositional environment when the sediment that makes up the rocks that display the structures was deposited. Recognition and interpretation of the evidence of formative processes is the first step towards identifying the environment in which those processes were active; depositional environments are defined by the processes that act within them and those processes, in turn, determine what will be preserved in the geological record. Another very practical value of sedimentary structures is that many of them are useful in determining the “way up” at the time that the sediment displaying them was deposited. The question of “which way is up” may seem rather pointless unless you’re buried by a snow avalanche, a situation where knowing the upwards direction could save your life. However, in geology, ancient rocks may have gone through a variety of styles of deformation, some of which cause them to be overturned so that today they are upside down compared to their position at the 3 time of deposition. When working in structurally deformed terranes, where layered sedimentary rocks have been overturned, an ability to recognize indicators of direction to original top is often necessary to interpret the structural deformation (e.g., are the rocks just tilted or have they been over-turned during regional deformation?) and also to determine the age relationships of the rocks. The determination of the relative age of rocks commonly relies on Steno’s Law of Superposition which states that in a sequence of layered rock units the layers become younger as you move “upward” in the sequence. This principle can only be applied provided that the true “up” direction at the time of deposition is known. So, in structurally complex terrains a geologist may not be sure of the up direction at the time that the rocks were laid down unless they can find definitive evidence of that direction within the rocks themselves. Some bed forms and types of cross-bedding provide the answer to the question of “what direction was up”. In Part B of this assignment you will determine the “way up” based on photographs of rocks that display some primary sedimentary structures that are specifically associated with bed forms. Some criteria for recognizing “way up” based on sedimentary structures and stratification. Note that the video that will accompany this assignment will include more examples of determining “way up” using the criteria outlined here. The video will be available a few days prior to the start date for this assignment. Internal cross-strata Tangential internal cross-strata are concave in the direction towards top (tiny black arrow in the figure on the left, below). Note that the view shown is that seen on a vertical plane that is parallel to the flow direction of the depositing current (flow was left to right in this case), and the red arrows indicate the “up” direction that is evident from the cross-strata. Such internal cross-strata may be found in planar and wedge-shaped cross-stratification and similar cross-strata may be visible within flow parallel vertical sections of trough-cross-stratified deposits Note also that the tangential contact of internal cross-strata is at the underlying bounding surface 4 which is an erosional surface on the underlying cross-strata set (not shown). So, the tangential contact always appears at the bottom of the cross-strata, not on the top. In addition, the tops of the internal cross-strata have abrupt angular contacts, rather than tangential contacts, with the overlying erosional bounding surface. If the internal cross-strata are sigmoidal in form look closely at the top and bottom for evidence of abrupt truncation by erosion but this is often difficult to discern. Erosional truncation is always a good indicator of the direction to top (more on that below). Trough cross-stratification In the diagram below the left-hand block diagram shows top indicators in the case where top is up and the right-hand block shows the same but it is upside down for comparison. Erosional troughs (thickest lines in the diagram below) and internal cross- strata (thin lines in the diagram below), as seen in the view that is in the vertical plane aligned perpendicular to the flow direction, are strongly concave upward (“festoon” shaped). Along the view that is parallel to the flow direction the erosional surfaces and internal cross-strata are moderately concave upward. The most striking indicators of the top direction are seen in the view that is perpendicular to flow where the abrupt erosional truncations of the tops of well- defined troughs and internal cross-strata by the next overlying erosional trough are commonly easy to discern in the field. In the flow-parallel view the tops of internal cross-strata are also abruptly truncated by overlying erosional surfaces. Erosional scour, in general, is a good indicator of top direction at the time of deposition. The erosional surface in the hummocky cross-stratified (HCS) sandstone bed shown in the figure at the top of the following page provides a basis for determining the direction to top; HCS displays internal erosional surfaces are visible due to their truncation of underlying lamina. 5 In general, if you see an erosional surface you should look for truncations in internal strata above or below that surface. If the surface truncates strata beneath it then the rocks are NOT overturned (left-hand image, above). If the surface truncates overlying internal strata then the rocks are overturned (right hand image, above). Basically, erosion takes place at the sediment surface and that erosional surface becomes buried by subsequent deposition and is overlain by the younger deposits. Erosion of the surface affects only the underlying, older deposits so only layers that are beneath that surface are truncated by a given erosional surface. The video that accompanies this assignment includes an animation that shows schematically how the layering in the photo (above left) develops. So when you recognize an erosional surface that abruptly truncates layering in underlying deposits you know that the rocks have not been overturned. If an erosional surface