a) Determine the strike and dip of i) bedding in the sedimentary rocks and ii) t
ID: 286710 • Letter: A
Question
a) Determine the strike and dip of i) bedding in the sedimentary rocks and ii) the contact between the rhyolite and the sedimentary rocks.
b) Consider that contact between the rhyolite and the sedimentary rocks. What types of contact are permitted by the rock types and geometrical relationships? List all possibilities permitted by the map information.
c) identify two pieces of additional geologic information that could resolve uncertainty about the nature of rhyolite contact and briefly explain how that information applies to this example.
Jcg Jss North Ksh v v rhyolite Kis limestone Ksh shale Jss sandstone KIs Jcg A' Jss line of v profile Jeg conglomerate Ksh 200 m Jcg Topographic contours are in metersExplanation / Answer
Sedimentary rock units are initially flat lying sequences of deposits. Each layer reflects a change in some aspect of the depositional environment: water velocity, size of the clasts (sediment fragments, be they clay or boulders), degree of rounding of the clasts, etc. Once sedimentary rocks are subjected to tectonic forces, they may be folded, tipped on edge, chopped up by faults, or even overturned. The orientation of the rock beds is described using ‘strike’ and ‘dip’. Dip refers to the downward angle at which the tilted beds now lie. A dip of 90 degrees is vertical. The dip might be stated as “beds dipping 45 degrees to the north”. Strike is more complicated. This is defined as the direction the contacts between the beds point when looking along the contact, and is always a compass direction exactly 90 degrees to the direction of dip. If the beds are folded, this direction will change over some distance. So, it is important that you are using the correct direction when describing the strike. Beds dipping to the north could conceivably be said to strike either to the east or the west. So a convention is necessary, since no direction is given on a map when the dip is recorded. We use the ‘right hand rule’ – when your fingers point down along the dip, your thumb points in the direction of strike. You have to orient your hand so your fingers point down the steepest angle of the tilted beds, the direction a drop of water would run down the rock.
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