GEOL 3000 Geologic Maps Name Lab 7 Geologic Maps of Deformed Lithostratigraphic Units Masonville Quadrangle Colorado Objective With this lab exercise we will be making geologic cross sections of an area with significant topographic relief that is underlain by lithostratographic units which are tilted folded faulted and locally intruded by igneous rocks Procedure Working individually or in pairs you will construct two cross sections corresponding to lines A A and B B on the Masonville quadrangle geologic map USGS Map GQ 832 If you are working individually you need only construct one cross section In constructing the cross sections here are some guidelines Base draw each cross section on 5div inch graph paper will need to tape two sheets together Scale show no vertical exaggeration 1 2000 or one small square 400 Topographic profile In constructing the topographic profile again with no vertical exaggeration it is usually enough to just note where the bold contours every 200 cross your profile line Take note of the approx elevations of the high and low points however Dip of contacts there is not always a strike and dip of bedding symbol right adjacent to your profile or in some cases there are several in the immediate area This requires that you project a symbol that occurs nearby and along strike If several measurements qualify take the average dip Apparent Dip If the strike of your units are at less than 70 from the cross section profile line it is not necessary to determine the apparent dip For most of the true dip angles here it will reduce them by only a degree or two If the strike to profile angle is less than 70 then use the nomograph on the next page Quaternary Cover On this complete geologic map Quaternary sediments are mapped Ignore these thin horizontal units on your cross sections If a Quaternary unit is covering a rock unit contact along your profile estimate where it might project beneath the cover Linework Show map unit contacts as normal solid lines show faults as bold dashed lines and show unconformities as bold solid lines Note that you should portray geologic contacts and fault in the subsurface similar to how they appear at the surface For example if a contact is very straight and planar at the surface show it that way in the cross section if the contact is very irregular at the surface and not the result of topographic expression show it as irregular in the section Fault orientations Although the orientation of fault planes might be measured and shown with a strike and dip symbol on the map more often faults are weak zones that are poorly exposed But because faults are planar we can infer their general dip by how they interact with the topography A low angle fault can wiggle across a highly dissected terrane whereas a near vertical fault will show as nearly straight line regardless of topography Intrusions Although you should show the large masses of Precambrian igneous rocks don t bother to show the narrow dikes blue and red lines in the cross sections Subsurface Cartooning of igneous intrusions It is impossible for your profile line cross to every interesting feature on your map While the geology along the surface of your cross section should be accurately portray the geology along the profile the subsurface part of your cross section gives you the ability to speculate as to what might exist at depth While you want to make reasonable speculations based on how things project into the subsurface you are free to illustrate some features that may be part of a unit but don t show on the surface along your profile An example of this would be deeper units that are in the map area but don t cross your profile line an inclusion of country rock in an igneous body or an igneous intrusion into another rock type In this map note that several types of Precambrian intrusive igneous rocks intrude the Precambrian metasedimentary rocks However few of these intrusions actually cross the profile lines You are free to cartoon in these intrusive relationships at depth In doing so try to replicate the contact geometry that you see on surface e g if the intrusive contacts on the map are boxy show them as boxy in the cross section Another aspect of cartooning is to show how bedding internal to a map unit is oriented especially if that bedding is deformed and your unit is thick e g units mks and qms This bedding can be portrayed with thin discontinuous lines that show possible fold patterns