Cold-seep Carbonate Hardgrounds as the Initial Substrata of Coral Reef Development in a Siliciclastic Paleoenvironment of Southwestern Taiwan

Abstract

Abrupt facies changes from the underlying terrigenous mudstone of deep-water facies upward into reefal limestones were observed on Pleistocene scleractinian reefs in southwestern Taiwan. To reveal the initial mechanisms of reef development, we examined the lithologies and vertical facies changes of 7 outcrops and 37 borehole cores from the Takangshan Reef and performed petrographic and isotopic studies. Various occurrences of dolomitic mudstone were observed from 6 outcrops and in 11 borehole cores, containing massive dolomitic mudstones, carbonate pipes, dolomitic cobbles, and dolomitic pebbles. The δ 13 C values of 27 samples ranged from-53.7 0 00 to-10.4 0 00 , indicating that the carbonate cements of these mudstones were all cold-seep carbonates in origin. The majority of the coldseep carbonates and a funnel-shaped structure packed with dolomitic cobbles were precipitated and formed within fine-grained siliciclastic mudstones. The wide occurrence of seep carbonates in the study area suggests hydrocarbon seepage having occurred extensively. The compact nature and associated large lucinid bivalves in massive cold-seep carbonates further indicate a pronounced, long-lasting seepage of methane occurring antecedently to the development of Takangshan Reef. A schematic model was proposed to illustrate the occurrence of various associations of lithologies and lithofacies. The erosional surfaces on siliciclastic mudstones and the funnel-shaped structure, as well as the exhuming of massive cold-seep carbonates may have occurred concurrently during a tectonically unstable time in SW Taiwan. The deposition of fossiliferous mudstone interfingered

Patterns and Sizes of Authigenic Carbonate Formation in the Pliocene Foreland in Southwestern Taiwan: Implications of an Ancient Methane Seep

“…In addition, ACCs from the Shenhu area at the northern slope of the South China Sea (to the west and near the Kaoping Slope) also have similar carbon and oxygen isotopic compositions (δ 13 C: -57.5 to -35.7‰; δ 18 O: +1.7 to +5.3‰; "the Jiulong methane reef", Han et al 2008;Ge and Jiang 2012). A summary of the spatial relationship, ages, and carbon/oxygen isotopic compositions of the asynchronous cold seep carbonates (Huang et al 2006;Wang et al 2006; this study) is given in Fig. 6.…”

“…4a and listed in Table 1 Table 1. Published data for the inland Pleistocene Takangshan cold seep carbonates in southwestern Taiwan (Wang et al 2006) and the ACCs in the Kaoping Slope offshore southwestern Taiwan (Huang et al 2006) are also plotted to compare with the carbon and oxygen isotopic compositions in this study. O values in Exposure B (range, -6.90 to -0.66‰).…”

“…The onshore Pleistocene Takanshan cold seep carbonates (Wang et al 2006) and the offshore Pleistocene-Present ACCs from the Kaoping Slope (Huang et al 2006) are also plotted in Fig. 4a.…”

“…There are many related geological phenomena, including active offshore mud diapirs (Chiu et al 2006), cold seep sites (Huang et al 2006), onshore mud volcanoes (Shih 1967;Yang et al 2004;Chiu et al 2006), and cold seep remnants from the Miocene (Chien et al 2012) to the Pleistocene (Wang et al 2006), dispersed within the Taiwan orogenic belt.…”

“…These ACCs were previously ignored or were misinterpreted as mysterious trace fossils (Yeh and Chang 1991). We also compared these paleoseep ACCs to two previously studied cold-seep-origin carbonates also located in southwestern Taiwan area but with younger ages (Huang et al 2006;Wang et al 2006), and attempted to reveal the evolutionary relationship of the formations and preservations of the cold seeps in southwestern Taiwan.…”