Desiccation cracks in vertic palaeosols of the Cretaceous Hasandong formation, Korea: genesis and palaeoenvironmental implications

The Hasandong Formation (Upper Cretaceous), Korea contains diagnostic vertic features such as pedogenic slicken-sides, pseudoanticlines, and large desiccation cracks. The Hasandong vertic palaeosols developed on floodplain deposits and display a diverse range of desiccation cracks, including calcite-filled, vertically stratified, and thin varieties. The calcite-filled cracks are 5-10 cm wide on the surface and up to 60 cm deep, and may have been formed by selective calcite precipitation in the cracks due to preferential vegetation. The vertically stratified cracks are those with crack-fillings having alternating sandstone and mudstone subparallel to crack walls. They are interpreted to have been formed by repeated crack opening and filling at crack margins due to seasonal drying and wetting. The thin cracks are less than 1 mm to a few mm wide and a few cm long, and are recognized by light colour mottling around them. Thin crack development may have resulted from intermittent incomplete cracking during sedimentation. The Hasandong vertic palaeosols have stratigraphic and lateral variations in vertic features, which are attributed to the changes in palaeoenvironments. Deep and large desiccation cracks with unbridged sediment-infilling formed in flood basins. The low topographic position of the distal floodbasin area resulted in weakly developed vertic palaeosols with calcite-filled desiccation cracks and some rhizocretions, and proximal location to a main channel may be responsible for rare development of deep and wide cracks and intercalation with calcrete-intraclast conglomerates. Differences in these vertic features along with interbedded calcic palaeosols may provide useful criteria for the palaeoenvironmental changes in and correlation of the floodplain deposits. (C) 1998 Elsevier Science B.V. All rights reserved.