TEACHING STUDENTS TO RECOGNIZE CONTINENTAL DEPOSITS AT THE I-70 SOUTH ROADCUT NEAR MORRISON, COLORADO

A complete section of the Morrison Fm in the I-70 south roadcut at exit 259, west of Denver, provides an excellent opportunity for students to practice identifying and interpreting continental sedimentary deposits. The section is located 2-5 km from the old quarries where Arthur Lakes excavated a variety of dinosaurs. These rocks thus provide a unique glimpse into the world of the dinosaurs, the real "Jurassic Park."

The section is reached by a walkway extending from the parking lot in the southeast corner of the interchange. The concrete pyramids on the handrail are used as reference points. The base of the Morrison Fm occurs at pyramid 11, at the base of the sandstone beds. The sandstones occur in lens-shaped bodies, and contain lag deposits with mud chips and dinosaur bone, as well as trough cross-bedding, ripple cross-lamination, and discrete, steep-sided bulges interpreted as dinosaur tracks in profile view. These rocks are interpreted as fluvial deposits. At pyramid 18 there is a change to red and gray mudstone and siltstone. Carbonate nodules and root traces are present; these rocks are interpreted as floodplain deposits with paleosols. Pyramid 27 marks a change to gray mudstone, claystone, and limestone. Bentonites can be seen near pyramids 31 and 46. Other workers have discovered flora, fauna, and geochemistry indicative of freshwater lakes; these rocks are interpreted as lake deposits with volcanic ash beds. Pyramid 48 begins an interval of gray mudstone and sandstone. The sandstone bodies are similar to those at the base of the Morrison Fm, and are also interpreted as fluvial deposits. One contains a "worm's-eye view" of sauropod tracks. The mudstone contains clay-coated joints and blocky peds, interpreted as paleosols. At pyramid 56, the mudstone turns a dark maroon color with mottling and blocky peds; these are interpreted as paleosols formed at the unconformity separating the Morrison Fm from the overlying Lytle Sandstone at pyramid 58. Sandstone bodies in the Lytle Fm are very similar to those in the Morrison Fm but their petrology is different. The Lytle is a clean quartz sandstone, whereas the Morrison sandstones contain clay, red oxidation spots, feldspars, and carbonate cement. Thus, hand-specimen identification skills are important in properly locating the contact and the position of the K-1 unconformity.