geology-Show an angular unconformity

GEY 100 Roadcut Cross-Section Assignments – Sedimentary Roadcut
Consider an eroded mountain range along the outer flanks of the deformation zone where
early flat-lying sediments have been tilted (perhaps even gently folded) and uplifted,
subjected to erosion before being buried by a fresh pulse of erosional debris from the
nearby rising mountain range. Now 400 my later, imagine a small roadcut outcrop
exposing the field evidence for this two stage sedimentation history as an “angular
1. Show an angular unconformity (the type seen by Hutton at Siccar’s Point for example)
with older lower set of sedimentary beds that have been steeply tilted, eroded flat and
buried by an upper set of beds that are horizontal and have been deposited on top of
the old eroded surface. This will give you the all-important “T” structure that is the
hallmark of this sedimentary structure.
2. Sedimentary bed contacts within each of the upper and lower stratigraphic sections
should all be parallel to each other so that each bed is of uniform thickness. Make sure
that the lower bed contact lines are truncated by the unconformity surface.
3. All rocks need patterns appropriate for the particular rock type and the sedimentary
rock type patterns include tiny dots for sandstone, dashes for shale, dot-dash for siltstone,
brick patterns for limestone, larger dots or small circles for conglomerate with individual
elongate pattern elements (such as dashes or bricks) parallel to the bed contacts.
4. All rock types need a pattern and a color shading. Rock types (and patterns) can
repeat but must have unique colors and each represented by its own box in the legend
with its own specific geologic age and descriptive text entry. For example, if two
different sandstone layers are used with the sandy dot pattern the layers need to be
differently colored according to their different geologic ages and then represented by two
boxes in the legend stack.
5. Upper stratigraphic units should be Late Devonian, lower stratigraphic units should be
Early Silurian for this early Devonian age unconformity where Early Silurian units were
tilted by collision to be buried by the younger units as erosional debris was shed off of
the uplifted Devonian age mountain range.
6. Make the color spectrum work for you by choosing colors for the upper and lower
sedimentary sequences from opposite ends of the color spectrum to highlight the two
differently positioned and differently aged sets of sedimentary beds that define the
angular unconformity.
7. Don’t forget to indicate the position of the unconformity surface within the stack of
legend boxes (it will separate the upper sediment beds from the lower set of sediment
beds), and its position within the cross section. Title, and scale bar as usual.

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