Preliminary study on enigmatic synsedimentary cracks in the Lower Cretaceous lacustrine deposits, Jinju Formation, Korea

Unusual synsedimentary cracks with enigmatic origin were described from the Cretaceous lacustrine deposits of the Jinju Formation at Jahyeri in southern coast of Korea. These cracks occur within 2 m-thick beds composed of grey mudstones and dark grey shales (lake margin and sublacustrine deposits). These cracks are classified into three types according to their shapes, orientations, and formation sequence. Type A is calcite-filled subhorizontal sheet cracks with a few cm stratigraphic interval. The thickness ranges from a few mm to 2 cm, and tabular to lenticular. Type B is polygonal shrinkage cracks with 2 to 3 cm width. It occurs on the top surface of the crack-bearing deposits and extends downwards over 1 m deep. It is unusual that it is not filled with sandstone like normal desiccation cracks. It is filled with calcite, and central thin cores are filled with mudstone. Type C is isolated sublinear cracks with 0.5 cm more or less in width, and resembles incomplete desiccation cracks. It is also filled with calcite, and calcite-filled subconical holes are in places associated. On the basis of their transection features each other, the paragenetic sequence of these cracks is determined as A - B - C. All of these cracks are cut by sandstone-filled synsedimentary clastic dykes which are 1 to 20 cm wide and over 2 m deep. The paragenetic relationship between the unusual cracks and clastic dykes indicates that these cracks are interpreted to have been formed before deep burial. It is suggested that earthquake or gas expansion might be related with the formation of these calcite-filled cracks. The common development of soft sediment deformation structures with paleoseimic origin in the deposits below and above these crack-bearing deposits suggests that these cracks might be related with near-surface paleoseismic activity, whereas the shrinkage features and the filling of calcite suggest that they might be generated by gas expansion within organic-rich lacustrine sediments. The details of their occurrences and origins will be figured out by further study.