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This assignment will introduce you to one of the most fundamental tools an Engineering Geologist uses to develop a site characterization - geologic cross-sections. Geologic cross-sections provide a representation of the subsurface structure and stratigraphy based on surface mapping data, subsurface drill hole data, geophysical data, and other clues as to what lies beneath the ground's surface. The cross-section itself is a vertical panel that illustrates the subsurface layers and structure to a given depth along a straight line shown on a map, usually marked as A-A', B-B', etc. (typical geologic representation) or simply as A - A, etc., together with tick marks indicating the direction of view (typical engineering representation). Exercise Tasks In this exercise, you will develop a cross section in support of a mythical highway project using subsurface data from drill hole logs and surficial data from a geologic map of the area (sorry, no geophysical data with this one). You will draw the cross-section to accurately show the vertical orientation and extent of the geologic formations shown in the drill logs and geologic map. Additionally, you will need to calculate the regional strike and dip of the subsurface units from drill hole elevation data (three-point problem). and develop "corrected" drill hole logs for those that require projection to the cross-section line. The "corrected" logs are to be drawn with corresponding unit elevations annotated on the log sheet. This exercise will require you to download the PDF file that contains these instructions, a geologic map, drill hole log sheet, and blank cross-section plotting sheet from the Assignment folder in the Files section of Canvas. Your completed assignment submittal should include the following: • Geologic map showing your graphic work to determine the regional strike and dip of the Miocene sandstone (Mss) unit from drill holes DH-3, DH-4, and DH-5. • Geologic map showing your graphic work to determine the regional strike and dip of the Cretaceous intrusives (Kgr) unit from drill holes DH-1, DH-2, and DH-3. *Strike & dip of the Kor unit is the instruction guide example. • Drill hole logs sheet with your calculated adjustments to the geologic unit intervals drawn and annotated for DH-1 (projected) and DH-4* (projected). "DH-4 will be used as the example in the instruction guide.
Geologic cross-section completed with: Topographic profile of the ground surface drawn in above the 1200-foot elevation reference line Drill hole log "sticks" annotated with unit contact elevations drawn in at the appropriate locations and elevations along the section. Geologic unit contacts drawn to connect the unit contact elevations marked on the drill hole log "sticks” using appropriate line characteristics (solid, dashed, dotted, etc.) to represent the certainty of the contact location. Appropriate contact arrangement between the juxtaposed Qal and Qt units on the cross section (think superposition). Geologic units colored and labeled to match the geologic log color and label annotations (as close as is reasonable). Completed calculation and written / fill-in summary answer sheet that includes your calculations and solutions for the regional strike and dip of the Mss and Kgr units, and the apparent and true dip calculations and unit elevation adjustments for the DH-1 and DH-4 projected drill hole logs.
Assignment Components The following sections will step you through each of the tasks to complete the assignment. Before you start, you should print out the assignment package located in the Assignment No. 3 folder in the files section on Canvas. In addition to this assignment description and instruction sheet, the package contains a geologic map, a sheet with simplified drill hole logs, a cross section plot sheet for section A-A', and a summary answer sheet for you to fill in the various answers and calculations completed through this exercise. Two instruction guides to help you solve for regional strike and dip, and adjust the elevations in projected drill hole logs are also located in the Assignment No. 3 folder for your use.
Geologic Map Locate the geologic map in the assignment package. The map is a scaled topographic map that is overlain with color representations of the geologic units exposed at the ground surface. In this case two units are shown: Quaternary Alluvium of the Pilgrim formation (Qal), and Pleistocene Terrace Deposits of the Bee Conglomerate (Qt). The color and symbol legend for these units is shown at the upper left corner of the map. The line on the map that separates the Qal from the Qt is the surface contact between the two units. The map (horizontal) distance scale is shown in the lower right margin of the map. All topographic contour and site elevations shown on the map are given in feet above mean sea level (ams). The drill hole site locations are marked on the map by the double circles and annotated with the name (DH#) and the elevation of the ground surface at the drill hole site. The topographic contours on the map represent lines of equal elevation, and are drawn at 5-foot intervals. Elevations of locations between the contours can be interpolated. An example of this is shown at drill hole site DH2. DH2 lies between the 1225-foot and 1230-foot contours. Given that DH2 appears closer to the 1230-foot contour, you can estimate the site elevation as about 1228 feet amsl. Five drill hole sites, DH1 through DH5 are shown on the map. Three of the sites, DH2, DH3, and DH5 are located along the section line A - A'. The remaining two sites, DH1 and DH4 are located away from the section line A - A'. The distances that DH1 and DH4 lie from A-A' are 400 feet and 300 feet respectively along lines perpendicular to A- A'. To include the geologic unit contact elevations from DH1 and DH4 on section A-A', DH1 and DH4 must be projected along the perpendicular lines to the section line. Section A - A' is 1,500 feet long and is oriented North 30 degrees East (N30E) or 030. The map is oriented with north at the top of the sheet as shown by the compass rose. To determine the orientation of any line on the map, use a protractor to measure the angle between the line and the right or left margin (North-South direction). The section line lies at a 30-degree angle to the right (East) of the margins (North), thus the orientation is N30E or 030. The 030 value is the direction or Azimuth given in a 360- degree clockwise representation. In the 360-degree convention, simply use the 360 degrees of a circle starting at 0 degrees (North), then proceed clockwise around the circle to 90 degrees (East), 180 degrees (South), 270 degrees (West) and back to 0 (360) degrees (North).
Drill Hole Logs Locate the drill hole logs sheet in the assignment package. The drill hole logs are simplified representations of the geologic unit arrangement measured in each drill hole. The drill holes are all 150 feet deep from the ground surface. The five logs that are labeled "site" represent actual drill hole measurements. The two logs labeled "projected)" are blank logs for you to fill in with the adjusted unit dimensions from the corresponding "site" logs. Each of the geologic units intercepted in the drill hole are represented by the corresponding colors and symbols shown in the legend blocks at the lower left side of the sheet. The upper and lower boundaries of each unit are annotated with the depth of the contact below the ground surface (bgs), and the corresponding elevation (amsl). It is usually good practice to work with the elevation values rather than the depth below ground surface as the elevations share a common baseline (mean sea level) and can be directly compared between locations. Two of the units shown in the drill hole logs, Mss (Miocene sandstone - Jaymee Sirewich Sandstone) and Kgr (Cretaceous intrusives - Matt Cain Granodiorite) do not appear on the geologic map as they do not outcrop (appear on the surface) within the mapped area Cross section plot sheet - Section A-A' Locate the cross-section plot sheet in the assignments package. If possible, print this sheet using 8.5 x 14 (legal size) paper. The plot sheet gives you the basic scale and framework to develop the geologic cross section. The scale for the sheet is shown in the lower left corner of the sheet. Two scales are shown; horizontal and vertical. Engineering sections are typically developed using a horizontal to vertical ratio of 1:1 - no vertical exaggeration. This section will follow the 1:1 scale ratio. The arrangement of A and A' indicate which direction you are viewing the section. In this case, you will be viewing the section to the north west (see map). A common annotation for engineering is to place tick marks on the ends of the section in map view. The tick marks point in the direction that you are viewing the section. Compare the A-A' orientation with the tick mark directions on the map. The elevation scales shown on each end of the section are for you to mark the contact elevations at each drill hole site or projection. The locations of the drill hole sites and
projections on the section are noted by the labels and arrows along the top of the section. The scales have tick marks that indicate five-foot increments. When drawing in the unit contacts, you can round the measurement at each drill hole stick to the nearest five-foot interval Assignment Tasks Determine the regional strike and dip of the Mss and Kar units This task will follow the procedure for determining strike and dip shown in the Three-Point solution guide located in the Assignments folder of the Canvas Files section. The example given in the guide is the solution for the strike and dip of the Kor unit. Follow the Three-Point Solution guide's procedure to use the elevation data and map distance measurements from the map scale to determine the regional strike and dip of the Miocene sandstone (Mss) unit and the Cretaceous Intrusives (gr) unit. The dip you determine for each unit is the true dip. Show your graphic work on the map and enter your answers on the summary answer sheet. Determine the contact elevation adjustments for the projected DH-1 and DH-4 Logs This task will introduce you to the use of apparent dip to adjust unit elevations in drill hole logs for projection on to a cross section. Follow the procedure in the Apparent Dip Solution guide in the Assignment folder to determine the unit apparent dip angle between the drill hole sites DH-1 and DH-4 and their respective projection locations on the cross section. From the apparent dip angles and the horizontal distances between the drill hole site and their respective projection locations, adjust the unit contact elevations and draw them in on the drill hole log sheet. Annotate the projected log with the calculated unit contact elevation and depth below ground surface. Fill in the appropriate answers on the summary answer sheet. Complete the Geologic Cross Section for Section A-A' Use the cross-section plot sheet to draw the geologic cross section for A-A'. The sheet is formatted to fit on an 8.5 x 14-inch sheet of paper to retain the 1:1 horizontal to vertical scale proportion.
Complete the following steps to develop the cross-section: • Draw the topographic profile along the section from A to A'. The 1200-foot elevation line corresponds to the lowest elevation along the profile. Using the geologic map and an engineer's scale, measure the locations along A -A where the topographic contours intersect the section line and mark each with the corresponding elevation from the contour. Transfer these elevations to the cross-section plot sheet (you'll need to scale up the map distance intervals as the lines aren't the same length) and mark them along the top of the profile using the vertical scales on the ends of the cross-section plot sheet to set the points at the appropriate scaled elevation. Draw a line connecting the elevation points, completing the topographic profile. • Draw vertical drill hole log "sticks" at each of the five drill hole locations noted on the cross-section plot sheet. Each "stick" should be scaled to 150 long and have the upper end coincide with topographic profile at that location. Using the drill hole log sheet, mark the depth or elevation of each unit contact in each log onto the sticks". Don't forget to use the projected logs for DH-1 and DH-4. • Draw the unit contacts on the cross-section sheet by connecting the corresponding markings on the sticks. The most honest way to do this is with a straightedge between each stick as we do not have any information about folding or faulting between the drill hole locations, and we assumed the units are planar. You will notice that the Kgr contact doesn't continue all of the way across the section as it doesn't appear in logs for DH-4 and DH-5. The contact is there, but you just don't know where it is. To represent this, you can project the contact along a straight line that continues from the known contact until it goes off the section margin. To indicate that you are estimating the contact location to the left of DH-3. you draw the contact with a dotted line with question marks alongside it. this is called a "queried" contact. • Draw the unit contact between the Qal and the Qt units. You should notice from the geologic map that the Qal and Qt share a contact at the surface, and each directly overlies the Mss. To represent this on the section, you need to estimate a vertical contact between the two. To do this, locate the contact on the map where it intersects the section A - A' and transfer its location onto the topographic profile in the cross-section sheet. Draw a short line segment from the contact on the topographic profile to the top of the underlying Mss. But which way? Without any other information as to its whereabouts, we are left to geologic principles to estimate its orientation. I'll give you a hint: it is very rare that a contact like this between two unconsolidated units will be vertical - this would imply the older unit was a cliff that withstood river deposition next to it without erosion...not likely. So, the line is probably diagonal. I'll leave it to you to figure that out, but consider the relative ages of the units and the geologic principle of superposition. Finally, should the contact be "queried"?
• Once the unit contacts are all drawn in, complete the cross section by coloring the units to match (as close as is reasonable) the drill hole log and geologic map colors. The colors are not arbitrary, they follow the USGS guideline for unit colors based on type of unit, age, and so forth. I have included a copy of the USGS guideline in the supplemental materials folder for your reference. Annotate each unit in the cross section with the unit abbreviation. • The cross section should now be complete. Take a look at it and see if the contacts and measurements are consistent with the map and drill hole logs. The section line is not parallel to the true regional dip direction, so the dip in the section will be less than the one you calculated. Given that the direction the section line is oriented is N30E (030), what are the apparent dip angles for the Mss and Kgr units that you should see in the section? (show your work and answers on the summary answer sheet). • Finally - Geologic unit names are often assigned by the Geologists who discover or characterize a new unit. Because the units in this exercise are fictitious, I named them. Can you tell me the one place where I got the names? This isn't worth any points, but I'll give you a shout out.
Highway Exploration Project Geologic Map with Section A-A' Qui Quaternary alluvium - Pilgrim Formation Qt Pleistocene terrace deposits - The Bee Conglomerate Qt A Qt 1240 1240 1235 1235 400 ft DH2 1228 kg 1143 1230 DHI 1238 1230 Kgr = 1170 1225 1220 DH3 +217 Kr 1097 Mss=1196 1230 1225 1215 Qal 1220 DH4 300 ft 1213 Mss = 1168 Section A-A 1,500 ft long Bearing 030 1210 1215 1205 Qal 1210 A 1205 DHS 1200 1200 Mss = 1147 Contour and Site Elevations in Feet AMSL Top of unit elevations given in red (kgr) and green (Mss) at corresponding hole 0 200 100 feet
Drill Hole Logs - Highway Exploration Project The drill holes are noted as DH-1(site) through DH-5 (site). DH-1 (proj) and DH-4 (proj) are projections of DH-1 (site) and DH-4 site on to the section line A-A' as shown on the map. Your task is to calculate the adjusted contact elevations for the projected logs and draw in the contacts, annotate the depth below ground surface and elevation of each contact, and label the unit type. The attached instructions will demonstrate how to calculate the projected contact adjustments DH-1 (site) DH-1 (proj DH-2 (site) DH-3 (sitel DH4 (sitel DH-4 (proj) DH-5 (site) 1238 ft amsi 1232 ft amal 1228 ft amsi 1217 ft amst 1213 ft amst 1211 ftams! 1200 ft amsi 0 Ot 43 bgs Ot (1233.77 8.0bgs (122007 Qal 20.1' bgs (1196.97 Qal Qal Mss Mss 45.0 bgs (1168.00 50 52.8 bgs (1147.1) 675' bgs (1170.59 Mss Feet (bgs) 85.0bgs (1143.0) Mas 100 Kor Mss Kor 120.0 bgs (1097.07 kor 150 Oal Quaternary alluvium - Pilgrim Formation Qt Pleistocene terrace deposits - The Bee Conglomerate MSS Miocene sandstone - Jaymee Sirewich Sandstone Kor Cretaceous intrusives - Marr Cain Granodiorite Surface Elevations given in feet above mean sea level (ams) Contacts given in feet below ground surface (bg) Elevation above mean sea level in parentheses) and
Cross section plot sheet - Section A-A' A A + 1 12 200 1.200 150 LI 1.100 al 1.100 Sofert 100 0 50 100 font Scales vertical & horizontal (11)
Assignment No. 3 - Geologic Cross-Section - Summary Answer Sheet Regional Strike and Dip of Mss unit: (show graphic solution on the geologic map) Elevation used to plot the strike line: Strike direction: Dip direction: Length (map scale) of line segment from DH-3 to the intersection with the strike line (S1-S2): Elevation change of the top of the Mss unit from DH-3 to the strike line: Dip angle (8) for the top of the Mss unit: Regional Strike and Dip of Kor unit (show graphic solution on the geologic map) Elevation used to plot the strike line: Strike direction: Dip direction: Length (map scale) of line segment from DH-1 to the intersection with the strike line (S1-S2): Elevation change of the top of the Kgr unit from DH-1 to the strike line: Dip angle (6) for the top of the Kgr unit: Adjustment of Projected Log Elevation for DH-1 (show graphic work on map and log sheet) Direction (azimuth) of 400-foot projection line for DH-1: Angle (B) between the strike direction and the apparent dip direction: Apparent dip (a) of the top of the Mss unit along the projection line: Apparent dip (a) of the top of the Kgr unit along the projection line: Elevation change in the Mss contact from the DH-1 site to the DH-1 projection: Elevation change in the Kgr contact from the DH-1 site to the DH-1 projection: Annotate projected log DH-1 with the adjusted depths and elevations for Mss and Rgr contacts.
Cross-Section Reality Check: Regional apparent dip (a) of the top of the Mss unit in the direction of A-A': Regional apparent dip (a) of the top of the Kgr unit in the direction of A-A': Calculations: Show your calculations for true and apparent dips, and elevation adjustments in the space below. All graphic solutions should be drawn on the geologic map.