Silver, E. A., Rangin, C., von Breymann, M. T., et al., 1991 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 124 30. SUBSIDENCE AND SEDIMENTATION ANALYSIS OF MARGINAL BASINS: CELEBES SEA AND SULU SEA, LEG 124, SITES 767 AND 7681 Zehui Huang,2 Felix M. Gradstein,2 and Keith E. Louden3 ABSTRACT Based on the chronostratigraphy and porosity of the sediments recovered from ODP Sites 767 and 768 drilled in the Celebes Sea and Sulu Sea, respectively, we examine the decompacted sedimentation and basement subsidence rates using a quantitative method. The results of this study help us to recognize several important tectonic events in these two marginal basins which improve understanding of the regional tectonic development. Among the recognized events are: (1) a pronounced increase in decompacted sedimentation rate in the Celebes Sea at 23.4 Ma, indicating basin transformation from a larger, more open oceanic setting to a marginal basin; (2) an increased basement subsidence rate in both basins between 10.8 and 8.9 Ma associated with the initiation of the Sulu Trough and the Sulawesi Trough; (3) appearance of a decompacted sedimentation rate peak between 10.0 and 8.9 Ma in both basins, indicating the combined effect of tectonic uplift in the source area and a sea-level drop; (4) uplift of the basement since 2.0 Ma, first in the Celebes Sea and then in the Sulu Sea, probably occurring as the result of collision events. INTRODUCTION strike-slip fault system that cuts through Sulawesi, which is a Neogene volcanic arc. The Molucca Sea collision zone, which is The Sulu and Celebes Seas are two small marginal basins in nearing completion and features incipient thrusting on its west- the west Pacific region, a complicated geotectonic region ern boundary, lies to the southeast of the Celebes Sea (Hamilton, marked by dynamic arc-arc and arc-continent collision zones, 1977; Moore and Silver, 1982). The Sangihe Arc, stretching from subduction zones, and long strike-slip fault zones. Sites 767 northeastern Sulawesi to south-central Mindanao, has active and 770 in the Celebes Sea and Site 768 in the southeast volcanoes in its southern and central portions (Hamilton, 1977). sub-basin of the Sulu Sea penetrated the sediment cover and In the northeast section of the Celebes Sea, an incipient east- reached the basaltic basement. The age of basement is early ward-directed subduction zone appears to be developing (Moore Eocene in the Celebes Sea and early Miocene in the Sulu Sea. and Silver, 1982). The drilling results from Sites 767 and 768 provide a good Site 768 was drilled in the southeast basin of the Sulu Sea, opportunity to examine the subsidence history of small, which is separated from the Celebes Sea by the Sulu Archi- marginal basins in the west Pacific region in a quantitative pelago. Mascle and Biscarrat (1978) demonstrated that the fashion. The subsidence histories of small, marginal basins southwest-northeast-trending Sulu Trough that lies north of associated with convergent margins have not been nearly as the Sulu Archipelago is an inactive subduction zone. The well studied using quantitative techniques as have basins on Negros Trench along the southeast side of the basin is active, passive margins. In this study, we will use decompaction and but features very slow subduction (Mascle and Biscarrat, backstripping techniques to examine changes in decompacted 1978). The Sulu Archipelago is a Neogene volcanic arc. sedimentation rate through time and to analyze the subsidence On-board trace-element geochemical analysis of the basalts history on the basis of sediment thickness and chronostratig- recovered from Sites 767 and 768 indicated that the plagio- raphy from Sites 767 and 768. We expect not only to improve clase-olivine phyric basalts from the Celebes Sea are charac- the understanding on the evolution of these two basins, but teristic of normal mid-ocean ridge basalts (MORB), while the also to extract information on the regional geotectonism, such olivine basalts from the Sulu Sea appear to be transitional as the timing of tectonic events in the region. In addition, this between MORB and island-arc tholeiites (Rangin, Silver, von study provides examples of the evolution of small marginal Breymann et al., 1990). basins along an active margin. Figure 2 summarizes the lithology and chronostratigraphy at Sites 767 and 768. The sedimentation through time in these GEOTECTONIC SETTINGS, SEDIMENTATION, two basins can be briefly described as follows: AND CHRONOSTRATIGRAPHY Figure 1 shows the location of Sites 767 and 768 and outlines 1. The sediment column penetrated at Site 767 is 786.6 m the geotectonic settings in the two basins under study. Site 767 thick. Overlying the oceanic basement are middle Eocene was drilled in the Celebes Sea. Hamilton (1977) has shown that reddish brown clay stones, indicating an open oceanic envi- the Celebes Sea is nearly encircled by subduction zones. To the ronment. The sediments above the basement suggest that the south, the Sulawesi Trough is an inactive subduction zone, Celebes Sea originated in a position protected from terrige- terminating in the west at the projection of an active left-lateral nous sedimentation (Rangin, Silver, von Breymann et al., 1990). Until the early Miocene, the sediments are reddish brown to grayish brown clay stone. Miocene sediments are 'Silver, E. A., Rangin, C, von Breymann, M. T., et al., 1991. Proc. ODP, dominated by claystone, siltstone, and some quartz-rich sand- Sci. Results, 124: College Station, TX (Ocean Drilling Program). stone of turbiditic origin, interbedded with hemipelagic clay- department of Geology, Dalhousie University, Halifax, N. S. B3H 3J5, Canada. stone. From late Miocene to the Quaternary time, the Celebes department of Oceanography, Dalhousie University, Halifax, N. S. B3H Sea received volcanogenic clayey silt to silty clay and some 3J5, Canada. ash, with carbonate silt to sand. 399 Z. HUANG, F. M. GRADSTEIN, K. E. LOUDEN Acti ve a u b d ucti on z on e 10°N- Inactive subduction zone Strike-slip fault Active volcano Inactive volcano 10βS- 100"E Figure 1. Location of Leg 124 Sites 767, 768, and 770, and the geotectonic settings of the Celebes and Sulu Seas (revised from Hamilton, 1977; Silver and Moore, 1978; and Weissel, 1980). The subduction zones are indicated by the number in circles. 1. Manila Trench; 2. Palawen Trough; 3. Negros Trench; 4. Sulu Trough; 5. Philippine Trench; 6. Cotabato Trench; 7. Sulawesi Trough; 8. Sangihe Trench. 2. The sediment cover at Site 768 is 1046 m thick. Upper explains why the stratigraphic resolution is higher in the Sulu Miocene sediments are exclusively coarse to fine vitric tuff and Sea than in the Celebes Sea. Unfortunately, in both the Sulu lapillistone formed mostly by pyroclastic flow, with some inter- Sea and Celebes Sea, the microfossil stratigraphy is not well bedded dark-brown claystone beds representing hemipelagic resolved between 10 and 16 Ma (middle Miocene). A hiatus sedimentation immediately overlying the basaltic basement. Fol- could have existed between nannofossil Zones NN8 and NN5 lowing the active pyroclastic flow phase, there was a period of at Site 768 around 714.0 meters below sea floor (mbsf) in Core hemipelagic sedimentation, indicated by brown and grayish 124-768C-38R. At Site 767, there may be an unrecognized green claystone of late early Miocene to middle Miocene age. hiatus in this interval. The stratigraphic resolution is poor at During most of the middle to late Miocene, however, turbiditic both sites in this interval. The numeric age of the fossil datums sediments prevailed. The sediments are claystone, siltstone, and and paleomagnetic events is from Berggren et al. (1985). A thick quartz-rich sandstone with minor chalk of turbiditic origin, time span from 10.8 to 14.5 Ma is tentatively assigned to the interbedded with hemipelagic claystone and ash layers. In the hiatus found at Site 768. Pliocene, with less turbiditic mass flow, more hemipelagic clay- The age of the basement at Site 767 in the Celebes Sea is stones were deposited along with some volcanic ash. Pelagic determined to be 43.0 Ma, according to the occurrence of a carbonate deposition persisted throughout the Pleistocene along radiolarian assemblage resembling the Podocyrtis chalara with increased input of volcanic ash. The tuff and lapillistone Zone of late middle Eocene age in the sediments 4 m above the above basement indicate that the Sulu Sea formed in a back-arc basement. However, an older basement age of 55 Ma is also environment. possible, as the examination on deep-water agglutinated for- aminifers reveals an assemblage of early Eocene age (Kamin- The microfossil and paleomagnetic records of the sediment ski and Huang, this volume). The errors caused by different columns at Sites 767 and 768 (Rangin, Silver, von Breymann basement ages will be examined below. The numeric age of et al., 1990) serve as the basis for the chronostratigraphy of the basement in the Sulu Sea is 19 Ma, according to both the two basins (Fig. 2). At Site 768 more paleomagnetic events radiolarians (Stichocorys wolffii Zone) and paleomagnetic and nannofossil datums were detected than at Site 767, which polarity zones (5Cr to 5E). 400 MARGINAL BASIN SUBSIDENCE AND SEDIMENTATION Site 768 Site 767 0 - 0.73 0.98 Brown/red claystone Carbonate Hemipelagic [ΛΛΛ ] Green claystone Green silty clay/clayey silt Turbidite IBBSBBBI Silty claystone to claystone £500 fggffl Quartz sandstone/siltstone Q. Q |'!'i'!| Bioclastic carbonates Other Volcanogenic sediments 10.7 j Basalt 14.5 35.2 16.2 " Hiatus 43.0 •17.57 17.9 1000 - •19.0 Notes on the numeric age (Ma) in sediment columns (* Radiolarian zone): 0.73 Top of Matuyama chron 0.98 Bottom of Jaramillo anomaly 1 98 Base of NN19 2.20 BaseofNN18 2.40 BaseofNN17 3.40 BaseofNN16 3.70 BaseofNN15 5.60 BaseofNN12 8.20 BaseofNN11 8.90 BaseofNNIO 10.0 BaseofNN9 10.8 Near base of NN8 14.5 Near top of NN5? 16.2 BaseofNNS 17.57 Top of 5D anomaly 17.9 Bottom of 5D anomaly 23.4 Tase of L.