경상남도 사천시 일대의 진주층에서 산출되는 백악기 비해성 스트로마톨라이트
Cretaceous non-marine stromatolites around the Sacheon City, Gyeongsangnam-Do, Korea
백악기 스트로마톨라이트 사천시;
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The sedimentary sequence of the non-marine Cretaceous Jinju Formation around Sacheon City contains various types of stromatolites. They are distinguished by gross morphology as 4 types, which includes rod-shaped, curd-shaped, stratiform, and small columnar stromatolites. Three stromatolite types, except for the rod-shaped, are autochthonous growing upward in situ, while the rod-shaped stromatolites are found as solitary specimens in sandstone or mudstone beds. The rod-shaped stromatolites are most abundant, occurring as randomly isolated specimens or a tomb-like aggregation. They also form a thick bed (up to about 0.6 m in thickness), within which the rod-shaped stromatolites are tightly compacted showing no preferable direction. The gross morphology of the rod-shaped stromatolite is almost identical to broken plant twigs or stems. Specimens with branching patterns of twigs are sometimes observed, and plant tissues remain less-altered in the central part of the stromatolite, all of which strongly suggest that the rod-shaped stromatolite were formed by microbial activity growing on the ancient plant twigs. All the stromatolites found in the Jinju Formation are known to have formed in a shallow depositional environment, particularly in a marginal lake. Sporadic occurrence of the reworked stromatolite fragments within sandstone beds also implies that the ancient lakes are experienced sporadic inland storms in the course of development of the Jinju Formation. Stromatolites, including reworked stromatolite chips, show typical stromatolitic fine lamination, characterized by an alteration of thin, organic-rich layers and thick, sediment-rich layers. The organic-rich layers are mainly composed of tiny micrite with abundant microfossils, whereas sparite is a major mineral composition in the sediment-rich layers with a few microfossils remained. Filamentous microfossils are found in the stromatolite, particularly within the organic-rich layers; no coccoid fossils are observed. They are calcified tubes with micritic walls, erected, and attached each other, forming a thin organic-rich layer. Two filamentous groups are identified on the basis of diameter of the filaments, ecological role, and morphology like tapering or branching patterns of the filament; one is cyanobacterial filament and the other is green algal filament. The diameter of cyanobacterial filaments is smaller than that of green algal ones, ranging from 1 to 11 um, while the larger filaments with about 32 um in diameter are considered as green algae. The cyanobacterial fossils played a key role in the formation of stromatolite, while green algal filaments were auxiliary stromatolite-builder. Together with the filamentous microfossils found within the stromatolitic laminae, some of planktonic and benthic animals including bivalves, gastropodes, ostracodes who lived in the Cretaceous lake are also encountered in between the stromatolitic domes, or sometimes in the stromatolitic laminae. Such all fossils found in the Jinju Formation may yield a clue in understanding of biodiversity in Korea during the Cretaceous time.