Locally Sponsored Research

Guam Hydrologic Survey

Geologic Maps


In 2004, WERI hydrogeologist Dr. John Jenson approached his now-retired former WERI colleague, Dr. Galt Siegrist, along with Dr. Mark Reagan of the University of Iowa, Department of Geology, about collaborating to update the Geologic Map of Guam (Tracey et al., 1964), which was by then more than 40 years old. Dr. Siegrist had long studied the field geology of Guam, specializing in the petrology of the limestone units that comprise the northern half of the island (Randall et al., 1984; Siegrist et al., 1984a; Siegrist, 1988; Siegrist et al., 1984b; Siegrist and Randall, 1992). Dr. Reagan has long specialized on the volcanic geology of southern Guam (Meijer et al., 1983; Reagan and Meijer, 1984) and the related rock units of the region.


Method and Products

Fieldwork began in the summer of 2005 and continued through spring and summer of 2006, during which time Dr. Siegrist and Dr. Reagan visited Guam to study sites of interest, and work with Dr. Jenson to assemble previous and new data. Reinterpretation and revision of the map benefited from additional radiometric dating of selected volcanic rocks and thin-section study of certain limestone specimens by Dr. Reagan and Dr. Siegrist, respectively, during the fall and winter of 2006. Dr. Siegrist coordinated the compilation and production of the new map with US Geological Survey cartographers during 2007. The new 1:50,000-scale map, based on the current USGS 15-minute topographic quadrangle, along with the nine 1:24,000 scale maps, based on the current USGS 7.5-minute quadrangles that cover the island of Guam, were published in early 2008 and are available below in downloadable pdf format.

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  • Meijer, A., M. Reagan, H. Ellis, M. Shafiqullah, J. Sutter, P. Damon, and S. Kling, 1983, Chronology of volcanic events in the Eastern Philippine Sea: AGU Monograph, v. 27.
  • Randall, R. H., H. G. Siegrist, Jr., and A. W. Siegrist, 1984, Community structure of reef building corals on a recently raised Holocene reef on Guam, Mariana Islands: Palaeontogr. Am., v. 54, p. 394-398.
  • Reagan, M. K., and A. Meijer, 1984, Geology and geochemistry of early arc-volcanic rocks from Guam: Geological Society of America Bulletin, v. 95, p. 701-713.
  • Siegrist, A. W., R. H. Randall, and H. G. Siegrist, Jr., 1984a, Functional morphology group variation within an emergent Holocene reef, Ylig Point, Guam: Palaeontogr. Am., v. 54, p. 390-393.
  • Siegrist, H. G., 1988, Miocene reef carbonates of the Mariana Islands: American Association of Petroleum Geologists Bulletin, v. 72, p. 248.
  • Siegrist, H. G., Jr., R. H. Randall, and A. W. Siegrist, 1984b, Petrography of the Merizo Limestone, an emergent Holocene reef, Ylig Point, Guam: Palaeontogr. Am., v. 54, p. 399-405.
  • Siegrist, H. G., and R. H. Randall, 1992, Carbonate geology of Guam: Proceedings of the Seventh International Coral Reef Symposium, p. 1195-1216.
  • Tracey, J. I., Jr., S. O. Schlanger, J. T. Stark, D. B. Doan, and H. G. May, 1964, General Geology of Guam, US Government Printing Office, Washington, D.C., U.S. Geological Survey Professional Paper.

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