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Name ________________________________________________________________________ Activity 8 – Demise of the Ancestral Rockies GEOL 1040 Learning Goals: • Use your knowledge of sedimentary grain size and the significance of unconformities to reconstruct how long it took for erosion to erase the topography of Colorado’s Ancestral Rockies (25 points total) Background Information: In Activity 7 we deduced that the base of the Ancestral Rocky Mountains was marked by the boundary between a nonconformity and the deposition of boulder and cobble conglomerate deposited during the Pennsylvanian period. Those Pennsylvanian-aged (about 300 Ma) conglomerate formations go by different names on either side of the Ancestral Rockies. For the basin to the east side of the range they are called the Fountain Formation. For the western basin they are called the Maroon Formation. 1. As erosion wears a mountain range away, consider the trend through time you expect in the grain size of the sediments that are being deposited at the base of the range. a. List the trend through time you expect (from older to younger rock formations) for the deposition of the following sediment sizes: a combination of fine-grained sandstone+siltstone+shale, conglomerate, medium-grained sandstone, coarse-grained sandstone. (3) ° young mountains ------------------------ → later time, as the mountains erode fine-grained sandstone+siltstone+shale -> medium-grained mudstone -> coarse-grained sandstone -> conglomerate b. Which of the above sediment grain sizes do you expect to be deposited when the mountain range has been eroded to a nearly flat plain? Explain your reasoning. (3) I expect the fine-grained sandstone+siltstone+shale to be deposited when the mountain range was nearly flat because these would be the result of the larger rock formations breaking down over time and would be left after everything else eroded away.
2. Fig. 1 has descriptions of the following rock formations on the legend for the Horsetooth Reservoir geologic map (which lies in the basin east of the Ancestral Rockies): Fountain Formation, Lyons Formation, Lykins Formation. List both the age (i.e. Period of deposition) and the maximum sediment grain size present in each formation. (3) - Lykins Formation: Fine-grained sandstone, 260-250 Ma - Lyons Sandstone: medium-grained quartz sandstone, 250 Ma - Owl Canyon Formation: Ripple-laminated Sandstone, 280 Ma - Ingleside Formation: Fine-grained sandstone, 280 Ma - Fountain Formation: Reddish-brown to purplish-gray arkosic conglomerate, 290-340 Ma 3. Fig. 2 has descriptions of the following rock formations on the legend for the Sawmill Mountain geologic map (which lies in the basin west of the Ancestral Rockies): Maroon Formation, Schoolhouse Tongue of the Weber Formation, Moenkopi Formation. List both the age (i.e. Period of deposition) and the maximum sediment grain size present in each. (3) - Moenkopi Formation: Algal Dolomite, 240 Ma - Weber Formation: feldspathic or orthoquartzitic sandstone, 250 Ma - Maroon Formation: arkosic sandstone, 310 Ma 4. Considering your answers to questions 1b, 2, and 3, list when (list both the period and the time in Ma) the Ancestral Rockies were no longer a significant topographic feature (i.e. eroded nearly to a flat plain). Explain your reasoning. (3) - The Ancestral Rockies were no longer a significant topographical feature around 290- 340 Ma because the conglomerates would be the last to erode away, since the smaller sediments would have been removed and relocated. - 5. Tectonic activity caused uplift of the Ancestral Rockies beginning about 300 Ma. Considering your answer to Question 4, how long were the mountains part of the Colorado landscape (list a duration in millions of years)? (2) - These mountains seem to have been around for 100 Ma 6. Look at Fig. 3, which shows part of the Royal Gorge geologic map and its key. The pink rock (Xgn) is Precambrian gneiss. All the other colors represent younger sedimentary rocks that overlie the metamorphic gneiss. The blue (PPf) is the Fountain Formation, which was deposited at the base of the Ancestral Rockies. The crest of the Ancestral Rockies range will be marked by the nonconformity with the longest time gap. a. List the age (i.e., period) of deposition and formation abbreviation (e.g. PPf, etc.) of the youngest sedimentary rock formation that contacts Xgn in a nonconformity (bold lines are faults, and a fault conta ct isn’t an unconformity). (2)
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