Haggerty, J.A., Premoli Silva, I., Rack, F., and McNutt, M.K. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 144 45. GEOLOGIC HISTORY OF WODEJEBATO GUYOT 1 Jonathan M. Lincoln, 2 Paul Enos, 3 Gilbert F. Camoin, 4 Jim G. Ogg, 5 and Douglas D. Bergersen 6 ABSTRACT Wodejebato Guyot, in the northern Marshall Islands, underwent two distinct eruptive phases during construction of its volcanic edifice. The first began before or during mid-Cenomanian time and ended at the end of Cenomanian time. This phase of eruption produced thick deposits of volcaniclastic debris at Site 869, 83 km southeast of the guyot, and a vegetated volcanic island that was at least partially submerged before onset of the second eruptive phase. The second phase of volcanic activity was confined to early Campanian time. Once again, a volcanic island rose above sea level. During this emergent period, a tropical weathering profile, up to 24 m thick, developed on top of the basalts and breccias of the volcanic island. Carbonate sands, which were later stabilized by encrusting organisms such as red algae, were laid down in late Campanian time as subsidence of the underlying edifice carried the summit down to, and eventually below, the sea level. Following the initial transgression of the volcanic island, an algal-coral- rudist boundstone reef formed a perimeter ridge at Sites 874 and 877 that resulted in progressively quieter, more restricted depositional environments at Site 873 in the interior of the platform. The carbonate platform may have experienced exposure above sea level in mid-Maastrichtian time. At this time, any sediments at Sites 875 and 876, seaward of the initial perimeter ridge, eroded down to the underlying basalt. The basalt at these seaward sites was soon covered with a ridge, or spit, formed from sands and other debris eroded from the adjacent reef. As the sea level rose again and a new coral-algal-rudist reef reestablished itself on the initial, inner perimeter ridge, sands continued to accumulate into an essentially parallel outer perimeter ridge. Sand shoals also migrated across the lagoonal sediments on the interior of the platform, followed by colonization of these sands by algae andrudist colonies. A second relative sea-level fall in late Maastrichtian time may have eposed the surface of the platform to erosion and meteoric diagenesis. Evidence includes scouring of the terminal carbonate deposits, dissolution in the upper limestone subunits, and isotopic compositions characteristic of diagenesis in meteoric waters. Possible cooling of mid-Maastrichtian oceans and high nutrient levels encountered as the guyot approached the equator may have slowed carbonate production to the point that the platform could not keep up during the subsequent sea-level rise and drowned. INTRODUCTION Wodejebato Guyot (12.0°N, 164.9°E; formerly Sylvania Guyot) is located approximately 74 km northwest of Pikinni Atoll (formerly Bikini Atoll) in the Ralik Chain of the northern Marshall Islands (Fig. 1). Wodejebato Guyot and Pikinni Atoll share the same volcanic pedestal and are connected by a volcanic ridge at a water depth of approximately 1500 m. The Ralik Chain is the central of three north- west-trending, en echelon chains of seamounts, guyots, and atolls that make up the Marshall Islands. During Ocean Drilling Program (ODP) Leg 143, Site 869 was drilled into the apron southwest of the Wode- jebato-Pikinni guyot-atoll pair through nearly 800 m of a layered succession of sediments that include nearly 600 m of volcaniclastic debris shed from the nearby atoll-guyot pair (Sager, Winterer, Firth, et al., 1993). Five holes were drilled on Wodejebato Guyot during Leg 144 (Premoli Silva, Haggerty, Rack, et al., 1993): Site 873 is located within the center of the broad guyot, Sites 874 and 877 are located approximately 20 km north of Site 873 on the inner of two perimeter ridges that bound the northeast side of the guyot, and Sites 875 and 876 are located on an outer perimeter ridge, about 1.5 km east of Sites 874 and 877, respectively (Fig. 1). At all five sites drilled on Wodeje- bato Guyot during Leg 144, limestone typical of a shallow-water carbonate platform was recovered. The results of drilling are herein used to reconstruct the geologic history of Wodejebato Guyot from Haggerty, J.A., Premoli Silva, I., Rack, R, and McNutt, M.K. (Eds.), 1995. Proc. ODP, Sci. Results, 144: College Station, TX (Ocean Drilling Program). 2 Department of Earth and Environmental Studies, Montclair State University, Upper Montclair, NJ 07043, U.S.A. 3 Department of Geology, University of Kansas, 120 Lindley Hall, Lawrence, KS 66045, U.S.A. 4 Centre de Sédimentologie et Paléontologie, U.R.A. 1208 du C.N.R.S., Université de Provence, 3 place Victor Hugo, F-13331 Marseille Cedex 3, France. 5 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, U.S.A. 6 Department of Geology, University of Sydney, New South Wales 2006, Australia. the eruption of a volcanic pedestal in Cenomanian and in early Cam- panian times, through its subsidence and accumulation of shallow- water carbonate sediments in late Campanian to Maastrichtian time, to its final drowning in mid to late Maastrichtian time. PREVIOUS WORK IN THE NORTHERN MARSHALL ISLANDS Pikinni and Anewetak Atolls were drilled in 1947 and 1952, re- spectively, as part of Operation Crossroads (Emery et al., 1954). The deeper of the two holes drilled at Pikinni Atoll penetrated 780 m of coral-algal, reef-related sediments and reached strata of early Mio- cene and Oligocene(?) age; volcanic basement was not reached in either hole (Emery et al., 1954; Cole, 1954). A stratigraphic section equivalent to that drilled at Pikinni was penetrated at the top of two holes drilled on Anewetak Atoll, 300 kmwest of Pikinni Atoll, in 1952 (Cole, 1957; Schlanger, 1963). Basalt, directly overlain by reef, fore- reef, and outer-slope sediments of late Eocene age, was reached at depths of 1271 and 1388 m below Anewetak Atoll. This basalt has recently been dated at 75.9 ± 0.6 Ma (late Campanian) by 40 Ar/ 39 Ar methods (Lincoln et al., 1993). Seismic-refraction studies at Pikinni Atoll revealed a reflector with high surface relief and a velocity of about 4 km/s, thought to be the top of the volcanic edifice, at a depth of about 1300m(Raitt, 1954). Abasement depth of 1500-1600mwas estimated for Pikinni Atoll from aeromagnetic surveys (Alldredge et al., 1954). Based on similar depths to volcanic basement and similar subsidence histories inferred from biostratigraphic correlation (Cole, 1957), Pikinni Atoll was assumed to be contemporaneous with the edifice beneath Anewetak Atoll. Dredging on Wodejebato in 1950 recovered basalts and a tuff breccia with cracks filled with phosphatized, planktonic foraminifers of Eocene age (Hamilton andRex, 1959). Emery et al. (1954) inferred eruption in shallow water from the highly vesicular and scoriaceous nature of the rocks; the high proportion of hyaloclastic material in the dredge hauls indicated submarine eruptions. Similar rocks were re- 769