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<br />. <br /> <br />. <br /> <br />. <br /> <br />209(' <br />u L. <br /> <br />Jurassic-Cretaceous Boundary <br /> <br />The Jurassic-Cretaceous boundary cannot be precisely <br /> <br />located in most of the project area because of inter- <br /> <br />tonguing, lithologic gradation, and lack of fossils. <br /> <br />Intertonguing and lithologic gradation between the Jurassic <br /> <br />Brushy Basin Shale Member of the Morrison Formation and <br /> <br />the Cretaceous Burro canyon Formation has been observed <br /> <br />in many areas on the Colorado plateau. In the Navajo Indian <br /> <br />Reservation in Utah, Repenning, and Irwin (1954a) mapped <br /> <br />the conglomeratic sandstone of the Burro Canyon Formation <br /> <br />as lensing into Brushy Basin mudstone to the south. T11e <br /> <br />Burro Canyon is absent or cannot be recognized as a mappable <br /> <br />unit south of the San Juan River. Harshbarger, Repenning, <br /> <br />and lrwin (1957, p. 57) described the Jurassic-Cretaceous <br /> <br />relation on the Navajo Indian Reservation as follows: <br /> <br />"Where the Jurassic rocks are overlain by the Burro <br /> <br />Canyon Formation, the contact is extremely arbitrary and <br /> <br />no indication of a break in sedimentation can be found. <br /> <br />T11erefore, the upper boundary of the Brushy Basin Member <br /> <br />of the Morrison is questionable, and the time boundary <br /> <br />between the Cretaceous and Jurassic periods cannot be <br /> <br />precisely located." <br /> <br />tt <br />