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

Several dolomitic authigenic carbonate concretions (ACCs) are preserved in the Pliocene foreland sequence in southwestern Taiwan. Carbon isotopic signatures and the morphology of these carbonates and associated chemosymbiotic bivalve fossils reveal their methane seep origin. There are three types of ACCs: (1) massive brecciated blocks (MBBs; delta C-13 = -49.57 similar to-38.20 parts per thousand; delta O-18 = 1.59 similar to 4.25 parts per thousand); (2) giant chimneys (GCs; delta C-13 = -43.14 similar to -17.73 parts per thousand; delta O-18 = -7.64 similar to 3.36 parts per thousand); and (3) slender pipe networks (SPNs; delta C-13 = -43.51 similar to 5.91 parts per thousand; delta O-18 = -6.90 similar to -3.57 parts per thousand). Different shapes, sizes, stratigraphic positions, and carbon isotopic compositions are due to different flux/intensities and flow pattern of discharging methane. MBBs were derived from diffusion of methane and formed in deeper positions, whereas GCs mark the locations of feeder faults and main outlets of methane emission. SPNs formed along fractures or bedding planes; they were (1) accessories of MBBs and GCs, or (2) products of minor releases of residual geofluids. We compared ACCs of the Chiahsien Paleoseep to two previously studied cold seep carbonates within the same foredeep in southwestern Taiwan, which represent similar occurrences and stable carbon isotopic compositions. However, the older ACCs in the Chiahsien Paleoseep have undergone longer diagenesis and weathering processes and have more complicated and lower 8180 signatures. These asynchronous cold seep carbonates can indicate hydrocarbon migrations and fault activities within the orogenic belt of Taiwan.