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Alessandro Grippo, Ph.D.

A key to lab 7 (Earth History), Age Relations and Unconformity

answers to Lab 7 questions Last Updated  •  November 1, 2016    

Figure 7.1, page 63
All of these questions are based on the Principle of Cross-Cutting Relationships


  • 1a - Basalt is older
  • 1b - Granite is an intrusive igneous rocks, it cannot form at the surface. In this case, granite intrudes the basalt.
    The opposite idea, basalt covering an eroded granite, would be theoretically possible, but definitely not in this case, because of the unlikely erosive features the granite would have.


  • 2a - G, N, W, Y.
  • 2b - A granite was crossed by dike N, which was then cut by dike W, which is eventually cut by dike Y.


  • 3a - The only thing you can say is that the shale is older than 50 m.y.

Figure 7.2, page 63
All of these questions are based on the Principle of Inclusions


  • 1a - The granite is older.
  • 1b - Inclusions are older than the rock in which they are found


  • 2a - The sandstone is older.
  • 2b - Inclusions are older than the rock in which they are found


  • 3a - The rhyolite is older.
  • 3b - Inclusions are older than the rock in which they are found

Figure 7.3, page 64

  • 1a - Fault B is older than fault A
  • 1b - Fault B is cut by fault A. Because of the principle of cross-cutting relationships (what cuts is younger than what has been cut), A is younger than B
  • 2a - To the right of fault A
  • 2b - By matching the layers that have been displaced by the fault (correlation), it is possible to notice that the block to the right of fault A has been moved upward, thus exposing older rocks (you know they are older because of the principle of superposition)

Figure 7.4, page 64

  • 1a - Toward the left.
  • 1b - There are two explanations: because of the presence of shale inclusions in the sandstone, the shale on the left is older than sandstone on the right. Another reason is that, since the sandstone presents visible coarser grains at its base and finer grains at its top. This characteristic feature is clearly visible in the picture and is a sedimentary structure defined as "graded bedding". even if you could not see graded bedding in the image, the rock is defined as having graded bedding in the figure caption.
  • 2a - Turbidity currents are events that repeat over time scales of centuries to millennia. A turbidity current develops at the shelf's edge and moves quickly through a submarine canyon, cuts into the slope of a continental margin, finally reaching the continental rise where it settles. In the time interval between turbidite deposition, pelagic clay accumulates at a very low rate on the rise (and in the abyssal plain). The speed of the incoming turbiditic flow is often enough to erode the partially lithified pelagic clay, ripping "clay chips" from the bottom and carrying them in the current's flow over a short distance. They would thus become part of the flow and settle in its lower part (because a clay chip, even if made of clay, is usually the size of gravel).
  • 2b - Tectonic setting is most likely orogenic
  • 2c - Continental Rise (Deep Marine)

Figure 7.5, page 65

  • 1 - Marine, probably deep. The presence of chert indicates a deep marine environment, while clastic limestone might also imply a shallower environment. Keep in mind that silica can be easily dissolved and mobilized, hence forming chert nodules at other locations. Also, clastic limestone could have been produced in shallow waters and then transported to deeper locations.
  • 2 - skipped

Figure 7.6, page 65


Figure 7.7, page 66

  • 1 - Cratonic setting (the sand is very mature: rich in quartz, rounded and sorted). The depositional environment is probably a coastline or a shelf affected by wave motion. (You were only required to indicate a tectonic setting)
  • 2a - The overgrowths formed after deposition
  • 2b - You can tell because this is a mature quartz sandstone, as you can see from the grains' roundness. If these grains were angular, all the other parameters (sorting and composition) would not make sense. Also, you would not be able to see a rounded "ghost" grain inside. Finally, if you cannot see this evidence, the caption of the figure defines this as a "mature quartz sandstone". As such, grains are rounded and sorted. As a consequence, the overgrowth of straight crystal faces must be posterior to deposition.

We skipped the whole section "The Method of Multiple Working Hypotheses"
All of the following questions relate to unconformities

Figure 7.11, pages 68 and 69

  • 1a - All of the Devonian is missing, maybe some Silurian and Mississippian too. As a consequence, a minimum of 57 my (measuring from the top Silurian to the base Mississippian, 416-359) and in any case less than 125 my (from base Silurian to top Mississippian, 443-318) is missing; in this second case, it would have to be LESS than 125 my because you do have some Silurian and some Mississippian rocks in the image, so not ALL of them is missing). These numbers are obtained using the Geologic Time Scale on the inside front cover of your lab manual, as indicated during the lab.
  • 1b - Non-deposition
  • 2a - No
  • 2b - The fossils indicate the same age. There are changes in lithology and faunas, but they simply indicate a change in the environment of deposition, and not a missing record.
  • 3a - If both beds are marine, and are separated by an unconformity caused by a stream channel, it means that sea level dropped at first, so that the sequence was eroded while emerged, then sea level rose once again covering the erosional surface
  • 3b - If both beds are non-marine fluvial, they would very likely represent floodplain sediments. A buried stream channel would indicate a meander shift or, in general, a change in position of the stream bed

Notice that in figure 7.11.1, all rocks are the same, and there is an unconformity. In figure 7.11.2 all rocks are different but there is no unconformity. Imagine yourself in the field, looking at rocks. You see limestones stacked on top of each other and you think that there is no unconformity. This simple exercise shows that as a matter of fact there is a major one instead (in this case, a disconformity). Moral: do not take the sedimentary record for granted!

Figure 7.12, page 69

  • 1 - Before (the unconformity cuts both the fault and the dike intrusion)
  • 2a - Angular Unconformity (beds meet at an angle at the unconformity)
  • 2b - Before (the unconformity and the overlying layers are not folded; the unconformity cuts the fold)
  • 3 - After (the unconformity and the sequences above and below it are all folded in the same way)
  • 4 - After (the unconformity is offset by the fault)

Figure 7.13, page 69

  • 1a - Angular Unconformity
  • 1b - The two sequences are at an angle. The upper sequence cuts the lower sequence
  • 2a - Orogenic
  • 2b - Deep marine: Continental Rise (graded bedding indicates turbidites)
  • 3a - Orogenic
  • 3b - Continental (red, immature, cross-bedded sandstone, typical of subaerial environments)
  • 4 - because mountain building, which occurs mostly at convergent boundaries, is the only possible cause for such a magnitude of tilting and deformation

Figure 7.14, page 70 (skipped)

  • 1a - Cratonic
  • 1b - Shallow marine
  • 2a - Angular Uncomformity
  • 2b - It is possible to see strata the angle between the two sequences above and below the unconformity

Figure 7.15, page 70

  • 1a - A non-conformity. An angular unconformity could also be inferred if you consider the lower sequence as sedimentary. Still, the lower sequence is identified and defined as a quartzite, which is a metamorphosed sandstone. In such a case, even if the original layers are still recognizable, the rock is classified as metamorphic. The unconformity is a non-conformity.
  • 2 - Cambrian, Ordovician, and Silurian are missing between the Proterozoic and the Devonian. That is at least 126 million years (Top Precambrian dated at 542 my and base Devonian dated at 416 my according to the inside cover Geological Time Scale). It cannot exceed 2.084 billion years (see reasoning used for Figure 7.11, question 1a)

Episodes of Precambrian History

Questions 1, 2, 3, 4 and 5, page 70; questions 6, 7 and 8, page 71. All questions based on Figure 7.16, page 71
(these question were skipped; you do not need to prepare them for your next exam, but it might be useful for you to review the solutions)

  • 1a - Not probable
  • 1b - Because of the immaturity of the few clastic sediments present
  • 2a - Not probable
  • 2b - Because of the abundance of conglomerate (in both Co and Br) and arkose (in Br), that would both be deposited at the foot of a mountain chain
  • 3a - Probable
  • 3b - There is no evidence of intrusion, and an unconformity separates the unit Br from granite Gg.
  • 4a - Probable
  • 4b - The granite Gr intrudes unit As and very likely also unit Br. As such, it is younger than both (principle of cross-cutting relationships). We have seen earlier that granite Gg (previous question) is older than unit Br, and as a consequence granite Gr will be younger than granite Gg.
  • 5a - Probable
  • 5b - If it happened before the deposition of unit Co, then said unit would still display horizontal layers.
  • 6a - Not Probable
  • 6b - Unit Asc is a metamorphic rock cut by the sequence of unit Br, and as such it is older than Br.
    Unit As is a sedimentary rock found below unit Br, and as such it is older than said unit (principle of superposition).
  • 7a - Probable
  • 7b - Unit Asc is a metamorphosed oceanic crust with adjacent deep marine rocks. These must have been part of a mountain chain that was later eroded and flattened, before sedimentation started again with unit Br. Unit Br also shows conglomerate at its base, denoting erosion of a mountain chain by a river.
  • 8a - Probable, at least in the left part of the image
  • 8b - Unit Br is directly above (along a non-conformity) the eroded roots of an ancient mountain belt. To the right, unit Br is above another sedimentary unit (As) which would also be deposited directly on granite Gg (here not visible because intruded by granite Gr, as discussed previously in question 4b)

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