48 Exercise 4 Depositional Environments 2. What does the red color suggest about

Question

48 Exercise 4 Depositional Environments 2. What does the red color suggest about the envi PROCEDURE ronment of deposition? Figures 4.7 through 4.12 show outcrop images of Mesozoic formations from central Utah along with sedimentary data collected at each locality. Careful- ly study the images and accompanying information. Compare outcrop data with information provided 3. Rocks of the North Hom Formation were most in table 4.1 to determine the depositional environ ment in which each of the formations was formed. Answer all of the accompanying questions. likely deposited in which of the sedimentary environments listed in table 4.1? Explain your PART A North Horn Formation, Cretaceous, Red Narrows, Utah (Figure 4.7) 1. What is the dominant sedimentary rock type shown in figure 4.7? 05 m Cretaceous North Horn Formation, Utah Rock Type: Conglomerate with interbeds of sandstone Composition: Boulders, cobbles, and pebbles comprised of sandstone and limestone. Matrix comprised of quartz, lithics, and mud Texture: Poorly sorted, large clasts subrounded, sand-size material angular to subrounded Color: Red to reddish brown Sedimentary Structures: Thick-bedded with primary dips of a few degrees to the east; gravel-filled channels Fossils: None Regional Trend: Formation is over 2,000 feet thick in centra Utah; it becomes thinner and finer-grained to the east 1 m FIGURE 4.7 Cretaceous North Horn Formation, Utah

Explanation / Answer

1. The dominant sedimentary rock type shown in Fig. 4.7 is conglomerate as the outcrops in the images A, B, C show that the texture of the rocks is poorly sorted and the clasts are mostly angular.

2. The red color suggests that the rocks were deposited under warm, humid climatic conditions in a terrestrial environment. Such an environment causes the formation of iron oxides as the iron-rich minerals get oxidized in an oxygen-rich environment.

3. The rocks of the North Horn Formation were most likely deposited in an alluvial which favored the transportation and sedimentation of a wide variety of sediments ranging from sand to cobble. The lack of fossils. The sediments are mostly angular which suggests that the clasts of different sizes did not move a long distance from the source. The red color in rocks indicates that the deposition took place in an oxygenated environment. All such pieces of evidence are indicative of the fact that the rocks of the North Horn Formation were deposited in a high energy, non-marine environment, i.e. an alluvial fan.

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