Geology of Saipan Mariana Islands Part 4. Submarine Topography and Shoal- Water Ecology GEOLOGICAL SURVEY PROFESSIONAL PAPER 280-K Geology of Saipan Mariana Islands Part 4. Submarine Topography and Shoal- Water Ecology By PRESTON E. CLOUD, Jr. GEOLOGICAL SURVEY PROFESSIONAL PAPER 280-K Description and interpretation of the submarine topography and of the sediments^ biotas^ and morphology of the reef complex adjacent to a geologically diverse tropical island UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. S EATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. GEOLOGICAL SURVEY PROFESSIONAL PAPER 280 Geology of Saipan, Mariana Islands Part 1. General Geology A. General Geology By PRESTON E. CLOUD, Jr., ROBERT GEORGE SCHMIDT, and HAROLD W. BURKE Part 2. Petrology and Soils B. Petrology of the Volcanic Rocks By ROBERT GEORGE SCHMIDT C. Petrography of the Limestones By J. HARLAN JOHNSON D. Soils By RALPH J. McCRACKEN Part 3. Paleontology E. Calcareous Algae By J. HARLAN JOHNSON F. Difcoaster and Some Related Microfossils By M. N. BRAMLETTE G. Eocene Radiolaria By WILLIAM RIEDEL H. Smaller Foraminifera By RUTH TODD I. Larger Foraminifera By W. STORRS COLE J. Echinoids By C. WYTHE COOKE Part 4. Submarine Topography and Shoal-Water Ecology K. Submarine Topography and Shoal-Water Ecology By PRESTON E. CLOUD, Jr. CONTENTS Page Page Abstract_________________________________________ 361 Shoal-water and shoreline ecology and sediments—Con. Introduction. ______________________________________ 362 Habitat descriptions—Con. Purpose and scope of the work_____________________ 362 Organic reefs and reef benches______________ 383 Field methods and acknowledgments-_______________ 362 Minor reef structures______________________ 384 Systematic identifications and other research aid____ 363 Biotope X. Larger reef patches and reef clus­ Interpretation of the submarine topography__________ 363 ters of lagoon and offshore Significance of topographic criteria.-..______________ 363 shelf_______________ 384 Inferred structural pattern_______________________ 364 Peripheral reef surfaces______________________ 384 Gravity slides and density currents _________________ 364 Biotope XI. Lagoon fringe of northern barrier Shoal-water and shoreline ecology and sediments_____ 365 reef_________________ 385 General setting, procedure, and limitations_______ 365 Biotope XII. Reef flat of barrier reef and con­ Substrate characteristics and sediment distribution__ 368 tiguous fringing reef______ — 385 Description. _______________________________ 368 Biotope XIII. Narrow fringing reef of east, Comparison of sediments with those of other areas.. 371 north, and south coasts. _____ 385 Approach to ecologic analysis.. ____________________ 373 Reef front_________________________ 386 Broad ecologic pattern____________________________ 374 Biotope XIV. Reef front of barrier and fringing Organic community______ _________________________ 375 reefs_______________ 386 Habitat descriptions______________________________ 376 Shallow sea beyond the barrier reef_______________ 386 Shoreline. _______________________________ 376 Biotope XV. Limesand bottom___________ 387 Aggrading beach____________________________ 376 Biotope XVI. Interspersed reef patches and Western beaches____________._______ 376 limesand- ________________ 387 Southern and eastern beaches________________ 377 Nature of the organic reef complex________________ 387 Biota of the beaches_________-.___________ 377 Basic features___________________________________- 387 Eroding coast._______________________________ 377 Symbiosis and comparable associations ______________ 389 Bluffs.________________________ 377 Boring and imbedding organisms _________ __ ______ 391 Rubble coast______________________ 377 Other distinctive adaptations_____________________ 393 Erosion surfaces.___________________________ 378 Biogeography___ — ______ —— __——————————— 395 Biota of the eroding coast___ — ___———____ 378 Origin of sediments_____________________________-_ 397 Barrier reef lagoon and contiguous fringing reef Sediments of direct organic origin.__ — ___—_____ 397 moats. _.________________________ 378 Detrital sediments produced or modified by organ­ Lagoon proper and marginal limesand shelves. ___ 379 isms________________________——— 398 Biotope I. Marginal limesand bottom with Mechanical breakup of rock weakened by organic seaweed.____.__________ 379 agencies and solution________________________ 400 Biotope II. Shallow area of gravel trains with Sediments resulting from mechanical factors alone __ 400 scattered living coral and al­ Orientation of sediment trains and growth features gae———————————————— 380 parallel to current flow_____-____________-____-_- 400 Biotope III. Staghorn Acropora zone of outer Erosion and precipitation of calcium carbonate in the lagoon shelf_____________ 381 intertidal zone______________________________ 401 Biotope IV. Interspersed reef patches and Chemical factors______________________________ 401 limesand of lagoon proper. 381 Organic effects____________-_____-----____---__- 403 Biotope V. Dredged areas) mainly of lagoon Physical suggestions of intertidal solution __________ 403 proper.____________________ 382 Precipitation. __________________________________ 404 The contiguous moats_____________________ 382 Variation in characteristics of the peripheral reef flat Biotope VI. Outer moat floor of interspersed and reef front._______________________________ 405 coral-algal bosses with gravel Reef flat proper._______________________________ 405 and sand__------______--_-_ 383 Groove and spur system.________________________ 406 Biotope VII. Bottom matted with living coral­ Algal ridge and surge channels__________________ 410 line algae_________________ 383 Gross morphology and evolution of the reef complex.___ 410 Habitats common to lagoon and moat___________ 383 Concentric zonation___________________________-___ 410 Biotope VIII. Clean limesand with little or no Evolution__ ____________________________________ 411 seaweed-___________________ 383 References cited___-_-______--_-___-_-----_--_--_-_- 412 Biotope IX. Mixed limesand and calcareous Appendix.—Tabulation of animals and plants found in gravel at reef fringe of very the shoal marine habitats of Saipan_________________ 416 shallow lagoon or moat___.__ 383 Index.__________________________________ 443 VI CONTENTS ILLUSTRATIONS [Plates 2 and 120-122 in pocket; plates 123-139 follow index] PLATE 2. Generalized geologic map and sections of Saipan, Mariana Islands. 120. Bathymetric and tectonic profiles adjacent to Saipan. 121. Bathymetric and tectonic chart, Saipan to Aguijan. 122. Shoal marine habitats of Saipan. 123. General views of western reef complex. 124. Reef patches, reef clusters, and transition of western barrier reef to fringing reef. 125. Western fringing reef ends, southern barrier reef complex, and examples of shoal marine erosion. 126. Close views of limesand bottom and staghorn Acropora zone. 127. Reef builders. 128. Some reef building and sediment forming algae. 129. Some distinctive reef-associated gastropods. 130. Work of sediment-producing fish. 131. Work of sediment-producing fish. 132. Fringing reefs with shallow moats and reef fronts with abundant grooves and spurs. 133. Erosional fringing benches with thin organic veneer and reef fronts with abundant grooves and spurs. 134. Surge channels and surfaces of fringing benches. 135. Fringing bench in volcanic conglomerate and groove and spur system on volcanic substratum. 136. Narrow fringing bench of limestone veneered with algae and vermetid gastropods, and reef front with tiered groove and spur system. 137. Narrow algal-veneered and pitted fringing surfaces and terraced ramp. 138. Terraced ramps and pedestals, blunt surge channels, and irregularly pitted low rocky shore with spray pools. 139. Reef veneering gastropods, terraced basins, and cove reef. Page FIGURE 36. Cumulative semilogarithmic screen analyses of representative shoal marine sediments of Saipan ____________ 366 37. Histograms of grain size of representative shoal marine sediments of Saipan——_—— ——— _________ ——— 367 38. Size-frequency distribution of 63 shoal sands from Saipan_.___________ — _____ —— __ — ___________——— 369 39. Sorting-frequency distribution of 64 shoal sediments from Saipan___ —— _________________________ —— — 369 40. Change in alkalinity components of sea water with changing pH______________________________________ 402 41. Reef front grooves and pits near station (?^________________________--_-----_-__--------_____----_____ 407 42. Reef front grooves, spurs, and pits between Susupe and Afetfia points.__________________________________ 408 43. Reef front grooves and spurs between Afetfia and Agingan points.._____-_-_--__-___---__-_____-__-_____ 409 TABLES Page TABLE 1. Shoal marine limesands of Saipan in order of increasing average coarseness____—___________ ——— ——— 371 2. Shoal marine limesands of Saipan in order of average sorting-_______--_____________---_-__-___----___-___ 371 3. Grain diameter and sorting characteristics of samples of shoal marine limesands from Saipan________ _________ 371 4. Summary of the identified marine organisms of Saipan____________——_________—__ —— _________ —— — 376 5. Numbers of mollusk and annelid species common to Saipan, Cocos-Keeling, and the Marshall Islands.________ 395 6. Classification and distinctive