Memorial to Ogden L. Tweto 1912-1983

P. K. SIM S and T. A. STEVENU. S. Geological Survey, B ox 25046, Federal Center, Denver, Colorado 80225

Ogden Tweto, a distinguished Fellow of the Geological Society of America and a true gentleman, passed away quietly on November 23. 1983, at his home in Lake­wood, Colorado. He is survived by his wife. Marguerite (Nikki); a son, John; a daughter, Barbara; and four grandchildren. F or more than 45 years he was identi­fied with the U.S. Geological Survey, and for much of this time he was deeply involved in the geology of Colorado. So deeply was he steeped in all aspects of the history of his beloved state that he had become “Mr. C olorado” to colleagues and friends.

Tweto was born of Norwegian parentage in Aber­crombie. N orth D ako ta , a rural town in the Red River Valley. A move to Missoula, M ontana, in his early youth exposed him at an impressionable age to the

fascination of mountains and geology in the “Big Sky” country. He graduated fromMissoula C ounty High School in 1930, and earned B.A. and M.A. degrees at theUniversity of M ontana in 1934 and 1937, respectively. Tweto entered graduate school at the University o f Michigan in 1937, and by 1939 had satisfied the course requirement for the Ph.D . degree. Thesis preparation, however, was delayed by higher-priority work connected with critical minerals investigations during World War II, so it was not until1947 tha t all formal requirements were satisfied and the degree was granted. At Michigan, Tweto was influenced strongly by T. S. Lovering and F. S. Turneaure, who more than any others fixed the destiny o f his career. Lovering in troduced Tweto to the wonder of C olorado mountain geology; Turneaure implanted the spark of interest in complex ore deposits that grew to an intense but carefully focused flame.

Tweto began his association with the U.S. Geological Survey as a summer field assistant to W. C. Alden in 1937, studying glacial geology and geomorphology in western M ontana and northern Idaho. He began his Colorado work as field assistant to T. S. Lovering in the Boulder tungsten district in 1938. In 1940 he became a permanent employee of the Survey, assisting Lovering in mapping the Minturn 15-minute quad­rangle, Colorado, and studying the contained replacement ore deposits at Gilman. These studies were interrupted during World War II by the urgent need to increase the Nation’s supply of strategic minerals. Tweto moved to independent assignments, mainly short­term investigations of known or potential mineral deposits in Colorado and adjacent areas. During the Strategic Minerals Program, his work in the Boulder tungsten district contributed significantly to a 40-fold increase in annual production from that district.

Immediately after the war, Tweto expanded his studies in central C olorado to a regional investigation of the heavily mineralized area including Gilman, Leadville, and Climax, near the center of the Colorado mineral belt. In 1954 he began detailed geologic mapping in the western part of the Holy Cross quadrangle, a key area for understanding the complex geology o f this part of the mineral belt. Northeast-trending mylonite shear

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zones m apped here led to the recognition (with P. K. Sims) that the whole belt was localized along a zone of en echelon shears of Precam brian ancestry.

In 1961 Tweto was m ade Branch Chief, Southern Rocky M ountains and C olorado Plateau, in which capacity he supervised geologic research projects from Texas to U tah. He moved to W ashington, D .C., in 1965 to become Assistant Chief G eologist for Econom ic Geology, responsible for managing the Survey’s efforts on m inerals and m ineral fuels. In this position, he helped develop and then managed an accelerated program of research on precious metals for the D epartm ent o f the Interior, and he was largely instrum ental in setting up the W ilderness Program , as required by the W ilderness Act of 1964 (Public Law 88-577).

In 1968 Tweto left W ashington and came home to the Rockies, first com pleting some long-pending reports prepared jo intly with Lovering, and then beginning work on a new geologic m ap o f C olorado, which was published in black and white in 1976 and in co lor in 1979. P reparation of the geologic m ap was a true labor o f love, and it was approached as a prim ary research project, not ju s t a com pilation of existing data. He m eticulously compiled geologic m aps of the 1° x 2° quadrangles o f the state, including reconnaissance m apping of poorly known areas, and then combined and condensed these into a coherent, understandable geologic map for publication at 1:500,000. In terpreta­tions reflected in this new state m ap give insight into Tw eto’s rem arkable knowledge and imaginative grasp of the complex geology of this im portant area. A com m on judgm ent by knowledgeable geologists is that the Geologic M ap of C olorado is the finest such product ever assembled.

As an outgrow th of the C olorado map, Tweto em barked on a new round of studies o f the larger features o f the geology of the Southern Rocky M ountains; the results include a regional synthesis o f the tectonism o f west-central C olorado, a tectonic map and report on the Rio G rande rift system in C olorado, a geologic m ap and accom panying report on the Precam brian basem ent, and a basem ent configuration m ap of C olorado. These syntheses chronicle T w eto’s rem arkable ability to patiently assemble and synthesize regional geologic data into coherent and unifying concepts.

Tweto earned m any honors and aw ards during his distinguished career. The D epart­ment of the Interior presented him with its highest honor, the D istinguished Service A w ard, in 1970. In 1978 he was nam ed Scientist o f the Year by the Rocky M ountain Association of Geologists, and in 1983 he was the recipient of the first D istinguished Geological P ioneer Award of the D enver section of the Society of Economic Paleon­tologists and M ineralogists. The Rocky M ountain Association of Geologists volum e on Colorado Geology was dedicated to him, and he was granted honorary mem bership in both the Rocky M ountain Association of Geologists and the C olorado Scientific Society.

He was a dedicated and responsible m em ber o f several national and local scientific societies; he was councilor of the Society of Economic Geologists from 1969 to 1972 and later declined nom ination as society president. He was an active mem ber and leader in the C olorado Scientific Society and served as secretary from 1948 to 1951 and as president in 1952.

Tw eto’s bibliography consists of 79 papers, not including abstracts and reports now being processed for publication. The papers are mainly a mix of regional geologic investigations and studies of ore deposits, and they include a rem arkable spread of subject m atter. Each report is of exceptional quality and is highly readable, and many are im portan t contributions. Tw eto’s papers reflect his conservative and disciplined approach to scientific thinking; he believed that a body of good data should stand on its own and was reluctant to extrapolate interpretations beyond the limits of the data.

M E M O R I A L T O O G D E N L. T W E T O 3

Tw eto was a modest m an of great integrity, and he had a truly uncom m on am ount o f com m on sense. He was an unselfish public servant who m any times put organizational loyalty above personal goals. His wisdom was always available to those who sought his counsel, including, fo r example, au th o r Jam es M ichener, who acknowledges the help received on geologic and historical m atters for the epic novel Centennial. To those of us who knew him well, he was a generous friend who enriched our lives.

As a fitting and lasting m em orial, colleagues in the C olorado Scientific Society have established the Ogden Tweto M em orial Fund to provide support for graduate-student research in C olorado geology.

S E L E C T E D B IB L IO G R A P H Y O F O. L. TW ETO

1951 Form and structure o f sills near Pando, C olorado: Geological Society o f America Bulletin, v. 62, p. 507-531.

1953 (with Lovering, T. S.) Geology and ore deposits of the Boulder C ounty tungsten district, C olorado: U.S. Geological Survey Professional Paper 245, 199 p.

1960 Scheelite in the Precam brian gneisses o f C olorado: Economic Geology, v. 55, p. 1406-1428.

1963 (and Sims, P. K.) Precam brian ancestry o f the C olorado mineral belt: Geological Society of America Bulletin, v. 74, p. 991-1014.

1965 (with R ichm ond, G. M., and M alde, H. E.) G laciation of the C olorado Plateau and S outhern Rocky M ountains in C olorado, P t. 1 in Guidebook for Field Conference E, N orthern and M iddle Rocky M ountains— International Association Q uaternary Research, 7th Congress, United States o f America, 1965: Lincoln, Nebraska, N ebraska Academy of Science, p. 117-125.

1968 Leadville district, C olorado, in Ridge, J. D ., ed.. Ore deposits o f the United States, 1933-1967 (G raton-Sales Volume): New York, American Institute o f M ining M etallurgical Engineers, p. 681-705.

------ Geologic setting and interrelationships of m ineral deposits in the m ountain provinceo f C olorado and south-central W yoming, in Ridge, J. D., ed.. Ore deposits of the United States, 1933-1967 (G raton-Sales Volume): New York, American Institute o f M ining M etallurgical Engineers, p. 551-588.

1970 (and Bryant, Bruce, and Williams, F. E.) M ineral resources o f the Gore R ange- Eagles Nest Primitive Area and vicinity. Sum m it and Eagle Counties, Colorado:U.S. Geological Survey Bulletin 1319-C, 127 p.

1972 (and Case, J. E.) G ravity and magnetic features as related to geology in theLeadville 30-minute quadrangle, C olorado: U.S. Geological Survey Professional P aper 726-C, 31 p.

1974 Geologic m ap and sections o f the Holy Cross quadrangle. Eagle, Lake, Pitkin, and Sum m it Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Series M ap 1-830.

1975 Laram ide (Late C retaceous-early Tertiary) orogeny in the Southern Rocky M ountains: Geological Society o f America M em oir 144, p. 1-44.

1977 N om enclature o f Precam brian rocks in C olorado: U.S. Geological Survey Bulletin 1422-D, 22 p.

------ (and Lovering, T. S.) Geology of the M inturn 15-minute quadrangle. Eagle andSum m it Counties. C olorado: U.S. Geological Survey Professional P aper 956, 96 p.

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1978 (and M oench, R. H., and Reed. J. C., Jr.) Geologic m ap of the Leadville 1° x 2° quadrangle, northw estern Colorado: U.S. Geological Survey M iscellaneous Investigations Series M ap 1-999.

____ Tectonic m ap of the Rio G rande rift system in C olorado, in Hawley, J. W .. ed..G uidebook to the Rio G rande rift in New Mexico and C olorado: New Mexico Bureau of Mines and Mineral Resources Circular 163.

____ (with I.overing, T. S., and Lovering, T. G.) Ore deposits of the G ilm an district.Eagle County, C olorado: U.S. Geological Survey Professional Paper 1017, 90 p.

1979 Geologic map of C olorado: U.S. Geological Survey Special M ap. scale 1:500,000.____ The Rio G rande rift systems in C olorado, in Riecker, R. E., ed., Rio G rande rift:

Tectonics and magm atism : W ashington, D .C., Geophysical M onographs of the American Geophysical U nion, p. 33-56.

1980 Tectonic history of C olorado, in Kent, H. C., and Porter, K. W.. eds., Colorado geology: Denver, Rocky M ountain Association of Geologists, p. 5-9.

____ Precam brian geology of C olorado, in Kent. H. C., and Porter, K. W.. eds.,C olorado geology: Denver. Rocky M ountain Association o f Geologists, p. 37-46.

____ Sum m ary of Laram ide orogeny in C olorado, in Kent, H. C., and Porter, K. W.,eds.. Colorado geology: Denver, Rocky M ountain Association of Geologists, p. 129-134.

1983 Las Animas Form ation (upper Precam brian) in the subsurface of southeastern C olorado: U.S. Geological Survey Bulletin 1529-G.

____ Geologic section across C olorado: U.S. Geological Survey M iscellaneous Investi­gations Series M ap 1-1416.

Printed in U.S.A. 9/84