The Sedimentology Of Chalk

Author : James E. De Long
Publisher :
ISBN 13 :
Total Pages : 91 pages
Book Rating : 4.3/5 (91 download)

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Download or read book Geology of the Chalk Mountain Area, Climax, Colorado written by James E. De Long and published by . This book was released on 1966 with total page 91 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Chalk l\lountain area is located on the west side of the Tenmile Range a short distance west of the Climax molybdenite deposit in parts of Summit, Lake, and Eagle Counties, Colorado. The area covers about 5 square miles with altitudes ranging from 10,600 to 12,014 feet. The Continental Divide follows the crest of Chalk Mountain. The geologic rock units consist of Pennsylvanian sedimentary rocks, Tertiary intrusives, and Quaternary glacial drift, alluvium, and talus. The sedimentary rocks comprise about the lower 2000 feet of the Ninturn Formation and are predominantly conglomerates and sandstones but include several thin, discontinuous limestone beds. A thin green shale bed on the east side of Chalk Mountain is tentatively correlated with the Jack 8 member, a marker bed. Seven varieties of Tertiary porphyry intrusives range in composition from rhyolite to quartz diorite. The relative ages of four of these are known. From oldest to youngest these are: Elk Mountain porphyry, Sacramento (1) porphyry, Lincoln porphyry, and Chalk Mountain porphyry. The position of a quartz diorite porphyry and two intensely altered porphyries in this sequence is no~ known. The largest intrusive body 1s a stock of Chalk Mountain porphyry. Most other bodies are small sills; a few are dikes. Glacial drift is the most widespread Quaternary deposit. The drift was derived from two sources, the Tenmile cirque east of Climax and a cirque at the head of the East Fork of the Arkansas River. The ice probably reached an elevation of about 11,400 feet on Chalk Mountain and encircled it except for the western side. Drift covers the area between the base of Chalk Mountain and Climax. Alluvium is restricted to local spots along the streams. Talus is present on a few steep slopes. Structures include folds, faults and a dome. The dominant structure of the region (not included in the mapped area) is the Mosquito fault, a north-trending normal fault with a large throw which separates Pennsylvanian sediments on the west from Precambrian igneous and metamorphic rocks on the east. Part of the north-trending Kokomo syncline and the east-west trending Robinson syncline occur in the area. An elongated dome was formed by the intrusion of the Chalk Mountain stock. Most structures were formed during the Laramide orogeny although the Robinson syncline may have formed in the Pennsylvanian. The top of Chalk Mountain and other flat surfaces at about 12,000 feet in the region represent an erosion surface, probably the Rocky Mountain peneplain of Miocene age. Numerous prospect pits, adits, and shafts dot the area but only the Grand Union tunnel is extensive. No records of production are known. Prospecting was originally for precious metals and more recently for molybdenum, but with no success. No conclusive evidence of the age of the Chalk Mountain porphyry relative to the Mosquito fault and the Climax deposit was found. The Chalk Mountain porphyry is similar to the Tertiary rhyolites which are closely related to the ore deposits at Climax, but certain textural and compositional differences distinguish it from them and suggest that the Chalk Mountain porphyry is not the same age. However, if the Chalk Muntain porphyry is older than the Mosquito fault, the level of the porphyry now observed is more than 8000 feet below its original position which makes correlation difficult. It is suggested that the Chalk Mountain porphyry may have been the source of some of the topaz in the Climax deposit and therefore younger than most of the molybdenite mineralization.