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The upper Aptian to Albian carbonate platform series at Site 879 consists of three main episodes of transgressive deepening followed by a general upward-shallowing succession. These depositional sequences typically have a transgressive to early-highstand depositional systems tract of quiet-water lagoon facies and a late-highstand to lowstand systems tract of shallow-lagoon to intertidal facies. The shallow-lagoon to intertidal facies display an internal cyclicity of ~3-m-thick upward-shoaling successions. If these short-term upward-shallowing cycles are related to Milankovitch climatic-oceanographic cooling cycles of the 100-k.y. period of eccentricity modulation of precession, then the 3-m-average thickness of these upward-shallowing cycles would then imply a 5-m.y. duration for the 164-m-thick carbonate platform. The basal biostratigraphic age, relative thicknesses, and a constant subsidence rate of 30 m/m.y. for these carbonate platform successions are consistent with assignments to late Aptian Sequence LZB-4.1 through early Albian Sequence LZA-1.2. However, benthic foraminifer assemblages from the upper 40 m of the platform suggest a longer age span of accumulation. The lowest depositional sequence (126-164 meters below seafloor [mbsf] ,39 m thick) onlaps a 23-m-thick clay-rich, weathering zone on volcanic breccia and contains a total of 12 cycles. The cycle set is developed in mixed transgressive-carbonate and prograding-clastic facies in the lower 12 m and algal-cyanobacterial-rich shallow-lagoon to emergent facies in the upper 27 m. The second depositional sequence (88-126 mbsf, 38 m thick) is a 24-m-thick, deep-lagoon, foraminifer-pellet packstone followed by a 12-m-thick interval with four main cycles of shallow-lagoon to intertidal facies. The third depositional sequence (4-88 mbsf, 84 m thick) begins with 49 m of quiet lagoon facies, including 22 m of an unrecovered, low-porosity interval interpreted as uncemented pelletal lime mud. The upper portion of this third depositional sequence begins with a thin, oncolite-rich interval. This shallow-lagoon facies is followed by a series of four main episodes of winnowed gastropod-rich facies interbedded with redeposited coral-rich floatstone. These reworked deposits may represent the Cretaceous equivalent of storm-debris accumulations that construct the island rings of modern atolls, and such deposits may have also played a major role in constructing the rim-lagoon geomorphology of Takuyo-Daisan Guyot. However, the origin of the 70-m-high southern rim cannot be deduced from the limited logging data and core recovery, although the drilling results did not provide any evidence for either a wave-resistant bioherm construction or a solution edge-effect during prolonged subaerial exposure. The highest sediments in Hole 879Aare a winnowed gastropod-rich peloid grainstone-coquina, which may be bioclastic sand shoals deposited during the initial transgressive stages of the final submergence of the guyot. The drowning of Takuyo-Daisan Guyot may have resulted from the combined effects of an accelerated transgressive rise of sea level during the early Albian and unfavorable environments in proximity to the paleoequator.

Takuyo-Daisan Guyot: Depositional History of the Carbonate Platform from Downhole Logs at Site 879 (Outer Rim)