Draw cross sections along A-A’ and B-B’ traverses on Figure 2 (you can assume a

Question

Draw cross sections along A-A’ and B-B’ traverses on Figure 2 (you can assume a flat topography)

b) Considering contact relationships, what type of contact is present between Precambrian basement and younger sedimentary units? Be specific.

c) If Tertiary basalt flow is still horizontal, what would be the contact type between Tertiary basalts and Mesozoic sedimentary rocks?

d) Figure 3 shows a simplified geologic map of another locality 20 miles away (westward) from the project area. Rock units are labeled 1 (oldest) through 7 (youngest) and the CC’ line represents the faulted contact. If the contact C-C’ is a fault plane that you identified as ……………………, mark the orientation of principal stresses (1, 2, 3) on your cross-section and explain how this fault plane would affect rock units in a well log (repeated section or omission?)

Figure 2

Figure 3

Simplified Geologic Map of Golden, CO North Table Mesa Ms Production well #1 Clear Greek njection well pcis 4.5 km A' tion well #2 Ms Pf B' pcis South Table Mesa 1 km Explanation Tv Tertiary volcanics-basalt now Ms Mesozoic Sedimentary Rocks, undivided-interbedded sandstone and mudstone Pf Permian Fountain Formation sandstone pCis Precambiran Idaho Springs Formation metamoprhosed granite Injection Well Production

Explanation / Answer

b)

There are three basic types of contacts:
(1) depositional contacts, where a sediment layer is deposited over preexisting rock.
(2) fault contacts, where two units are juxtaposed by a fracture on which sliding has occurred.
(3) intrusive contacts, where one rock body cuts across another rock body.

Relatively continuous sedimentation in a region leads to the deposition of a sequence of roughly parallel sedimentary units in which the contacts between adjacent beds do not represent substantial gaps in time. Gaps in this context can be identified from gaps in the fossil succession.

The boundary between adjacent beds or units in such a sequence is called a conformable contact. It is seen that new formations and magmatic and metamorphic complexes are formed. However, it is unclear whether many of these really exist or are only a result of overinterpretation of these isotopic data. Meanwhile, the physical criteria for separating basement from cover rocks are the metamorphic grade, contact relationships, cartographic limits and style differences in deformation. However, many contact relationships between basement and low-grade cover successions have been neglected, and cartographic boundaries changed with each stratigraphic interpretation. Unconformities represent gaps in the rock record that can range in duration from thousands of years to billions of years.Pleistocene till buries Proterozoic and Archean gneisses. In the classic unconformity between Paleozoic sedimentary rocks and Precambrian gneisses is shown and many introductory geology books show this contact in the Grand Canyon.

c) The Cenozoic Era was divided into the Tertiary and Quaternary periods, separated at the boundary between the Pliocene and Pleistocene epochs (formerly set at 1.8 million years ago). The name Tertiary was introduced by Italian geologist Giovanni Arduino in 1760 as the second youngest division of Earth’s rocks. The oldest rocks were the primitive, or “primary,” igneous and metamorphic rocks (composed of schists, granites, and basalts) that formed the core of the high mountains in Europe. Arduino designated rocks composed predominantly of shales and limestones in northern Italy as elements of the fossiliferous “secondary,” or Mesozoic, group. He considered younger groups of fossiliferous sedimentary rocks, found chiefly at lower elevations, as “tertiary” rocks and the smaller pebbles and gravel that covered them as “quaternary” rocks. Although originally intended as a descriptive generalization of rock types, many of Arduino’s contemporaries and successors gave these categories a temporal connotation and equated them with rocks formed prior to, during, and after the Noachian deluge.

Basalt flows and associated tuff, breccia, and conglomerate of late-volcanic bimodal suite ((Phanerozoic, Cenozoic, Tertiary) Includes basalts of Hinsdale Fm in San Juan Mountains - Servilleta Fm in San Luis Valley and many other occurrences.Basaltic flows in Denver Fm near Golden (Phanerozoic , Cenozoic , Tertiary). Basaltic intrusive rocks related to basalt flows (Phanerozoic, Cenozoic ,Tertiary) In dikes and plugs.

Felsic and hornblendic gneisses, either separate or interlayered (Proterozoic , Paleoproterozoic). Includes metabasalt, metatuff, and interbedded metagraywacke; locally contains interlayered biotite gneiss. Derived principally from volcanic rocks.

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