Geology of Isla Desecheo, Puerto Rico, With Notes on the Great Southern Puerto Rico Fault Zone and Quaternary Stillstands of the Sea GEOLOGICAL SURVEY PROFESSIONAL PAPER 739 Prepared in cooperation with the Industrial Research Department, Puerto Rico Economic Development Administration GEOLOGY OF ISLA DESECHEO, PUERTO RICO Aerial view of Isla Desecheo from the north. Photograph by Pedro Sola Amadeo, Laboratory of Perinatal Physiology, U.S. National Institutes of Health, San Juan, Puerto Rico. Permission to publish the photograph granted by Dr. Ronald E. Myers, Chief, Laboratory of Perinatal Physiology. Geology of Isla Desecheo, Puerto Rico, With Notes on the Great Southern Puerto Rico Fault Zone and Quaternary Stillstands of the Sea By VICTOR M. SEIDERS, REGINALD P. BRIGGS, and LYNN GLOVER III GEOLOGICAL SURVEY PROFESSIONAL PAPER 739 Prepared in cooperation with the Industrial Research Department:, Puerto Rico Economic Development Administration The surjicial deposits and volcanic bedrock of an island in Mona Passage with comments on regional structure and higher sea levels UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 459-607 O - 72 - 2 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600060 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 45 cents (paper cover) Stock Number 2401-2106 CONTENTS Page Page Abstract................................................................................ 1 Stratigraphy Continued Introduction......................................................................... 1 Quaternary System...................... ........................... 11 Previous work...................................................................... 4 Marine-terrace deposits.........................- ........ 11 Geography............................................................................ 4 Deposits chiefly between 6- and 12-meter Stratigraphy..... ..... .. .. ........................... ....... 5 elevation.................................................... 11 Tertiary System............................................ .... 5 Deposits at about 2-meter elevation.... .. 13 Rio Culebrinas Formation(?).......................... 5 Colluvium.................................................--..--..... 13 Volcaniclastic rocks.................................... 5 Beach deposits.........-....---..-.--. ....................... 14 Calcareous mudstone and sandstone........ 7 Structural geology........ ... .. .-- 15 Calcarenite................................................... 7 The great southern Puerto Rico fault zone............ -... 17 Penecontemporaneous deformational Quaternary stillstands of the sea..................................... 19 structures......................................................... Upper Desecheo stillstand......................................... 19 Conditions of deposition..................................... Lower Desecheo stillstand......................................... 20 Alteration..--.... - --........................................ 10 Regional relations and age........................................ 20 Age.............................. ................................... 10 References cited............................................................ .. 21 ILLUSTRATIONS Page FRONTISPIECE. Aerial view of Isla Desecheo from the north. FIGURE 1. Geologic map and sections of Isla Desecheo............................................................. ................ 2 2. Bathymetric map of Mona Passage.--.... ..-.- . ..---.-- ........................ ..... ........... .... ..... 4 3. Photograph showing mudstone slabs oriented parallel to bedding in volcanic sandstone, Rio Culebrinas Formation (?)............................... ..... ............................................... ..-. 4-6. Photomicrographs: 4. Volcanic sandstone............................................................................................................... ...... 5. Volcanic sandstone............................................................................................... ...................... 6. Calcareous foraminiferal mudstone........................................................................................... Sketch showing eroded and upturned mudstone beds in lower part of volcanic sandstone bed. Photographs: 8. Recumbent slump fold and slide breccia.............. ....................................................... ...... 10 9. Zeolite mottling in zeolitized volcanic sandstone.............. .......... ................. 11 10. Marine-terrace deposits at Puerto de los Botes.................................................................... 12 11. Marine-terrace deposits at Puerto Canoas............ .. .............................. .... .......... 12 12. Marine-terrace deposits in irregular depression in volcanic bedrock just west of Puerto de los Botes......................................................................................................... ..- 12 13. Overturned syncline near cable house. ......-- ............. ..... .................... .. ...... 15 14. Puerto Manglillo fault..... ....................................................................................................... 16 15. Joints filled with calcite and zeolites....................................................................................... 17 16. Map of the Greater Antilles showing major fractures........................................................................... .. 18 17. Photograph of northwest coast from near helicopter pad showing lower Desecheo stillstand platform. 20 TABLE Page TABLE 1. Chemical analysis and molecular norm of cemented-colluvium sample from south cut of helicopter pad, Isla Desecheo.................................................................................................................................. .. .. 13 GEOLOGY OF ISLA DESECHEO, PUERTO RICO, WITH NOTES ON THE GREAT SOUTHERN PUERTO RICO FAULT ZONE AND QUATERNARY STILLSTANDS OF THE SEA By VICTOR M. SEIDERS, REGINALD P. BRIGGS, and LYNN GLOVER III ABSTRACT Survey in cooperation with the Puerto Rico Eco­ Isla Desecheo is in Mona Passage between Puerto Rico and nomic Development Administration. Although most Hispaniola. The bedrock, 700-1,100 meters of middle Eocene efforts have been concentrated on geologic mapping volcaniclastic rocks, consists chiefly of dacitic volcanic sand­ of the main island of Puerto Rico, the goal of this stone, fine-grained volcanic breccia, and mudstone. Deposition program is knowledge not only of the main island, in at least moderately deep water is indicated by the abun­ dance of graded beds and by the fact that interbedded mud- but also of the many smaller adjacent islands. stone contains exclusively planktonic faunas. Sparse rounded In this report, Seiders and Glover are chiefly grains and shallow-water fossil debris in the graded beds indi­ responsible for data and discussion on bedrock cate that the volcaniclastic debris came from volcanoes in geology, and Briggs, for surficial geology. The field- shallow seas. Steep submarine slopes are suggested by sub­ work was done during two periods: Seiders, Briggs, marine-slide deposits and penecontemporaneous deformation structures. The rocks are extensively zeolitized. and Glover were on Isla Desecheo April 15-19, 1966, The southern two-thirds of the island, south of an east- and Seiders and Glover returned to the island May west vertical fault, has west-trending nearly isoclinal folds 8-10, 1967, accompanied by Eduardo Aguilar-Cortes, which are overturned to the north. North of the fault, the geologist with the Puerto Rico Economic Develop­ main structural feature is a gentle anticline which plunges ment Administration. south. The contrasting structural features are believed to represent different tectonic levels that are separated by a For the first fieldwork period, transportation to low-angle thrust fault. The strongly deformed beds may be Isla Desecheo was by RV Shimada of the Puerto the remnant of a gravity-glide sheet that was eroded from Rico Nuclear Center; the return trip was made in the northern part of the island. a small craft of the National Institutes of Health. Isla Desecheo lies along the seaward projection of the great For the second period, transportation was by a southern Puerto Rico fault zone, which is, perhaps, the major fracture zone of Puerto Rico. Large-scale transcurrent and helicopter of the U.S. Navy Fleet Composite Squad­ vertical faults and associated gravity-glide structures seem ron 8 (VC-8), Commander M. E. Smith command­ closely related in age and geometry to similar features in ing. Arrangements for transportation to and from Hispaniola and Cuba. Isla Desecheo were made through the cooperation Surficial deposits include marine terrace deposits, colluvium of the staff of Dr. R. E. Myers, Chief, Laboratory (in part phosphate cemented), and beach deposits. There is of Perinatal Physiology, San Juan, and J. A. Mor- evidence of two stillstands of the sea, one at 12-13 meters and the other at about 2 meters higher than present sea level. rison, Acting Chief, Section on Primate Ecology, The higher level is probably of Pleistocene age and the lower, Punta Santiago, Puerto Rico, both of the National of Holocene age, perhaps about 3,300 years before the present. Institutes of Health. Food and water were supplied Tectonic