Page > of 012 : -ZOOM 6. Which of the 3 dikes (see image below) is the youngest?
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Page > of 012 : -ZOOM 6. Which of the 3 dikes (see image below) is the youngest? Dike B 7. What principle of stratigraphy did you use to determine the answer for question 6? 8. You find potassium feldspars in a sandstone. Dating these feldspars will tell us when the sandstone formed 9. You find a fluvial-sandstone deposited on top of a deep-marine shale. The fossil assemblages indicate that there is a gap in time. What kind of unconformity is this? 10. What geologic process can explain this sudden change in lithologies?Explanation / Answer
6. The sequence of dike intrusion, in this case, can be found using the cross-cutting relation which basically states that if a feature cross cuts any other feature, then it is younger that the feature it cross cuts. Based on this principle we can see dike A cross cuts dike B which in turn cross cuts dike C, therefore we can write the sequence as dike A is the youngest followed by dike B an the oldest is the dike C.
7. For finding out the sequence of events for features like dikes we use the ' Principle of cross-cutting relations' which states that if two features cut each other its the younger feature that cuts the older feature,
8. Potassium feldspars have been used to date the formation of sandstones using the luminescence dating technique. This method dates the minerals based on when the mineral was last exposed to sunlight. Now during the formation of the sandstone its only till the mineral is not covered by another sufficient layer of sediment that it gets sunlight. After it is covered it receives no sunlight and hence when the mineral, is dated it gives the time of deposition.
9. We see that both the layers are deposited parallel to one another but there is a gap in the deposition of the beds as indicated by the fossil assemblage, this type of unconformity is known as disconformity. Disconformities are characterized by the absence of an erosional surface and are based on only the age gap given either by fossil assemblages or dating.
10. The deposition of a fluvial sandstone bed on top of a deep marine shale indicates that there has been a change in the depositional environment from the deep marine to the fluvial environment. This could be due either the regression of the sea in which the sea regresses back and the depth of the water decreases or it could also be due to the suuden upliftment of the region which would bring the earlier deep marine sediments very near to the surface for fluvial sediments to be deposited above it.
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