NONMARINE SEDIMENTATION OF SHEET-LIKE SANDSTONE ON THE GREAT UNCONFORMITY IN THE EASTERN MARGIN OF THE NORTH CHINA BLOCK, GONDWANA: A CASE OF THE JANGSAN FORMATION IN THE NORTHEASTERN PART OF THE TAEBAEKSAN BASIN, KOREA

Basal Cambrian sandstone units deposited worldwide coinciding with global sea-level rise, exhibit various lithologies above the Precambrian basement, forming the Great Unconformity. While these sandstones were deposited from diverse depositional settings ranging from shallow marine to terrestrial environments, the nonmarine aspects of the basal units in the North China Platform have not been thoroughly investigated despite their lateral heterogeneity of facies.

In the northeastern part of the Taebaeksan Basin, Korea (eastern margin of the North China Block), sheet-like sandstone of the basal Jangsan Formation exhibits coarse (coarse sand to gavel dominated), immature, and poorly sorted textures, and dm-scale beds. These sandstones display predominantly massive (no apparent structure) and planar stratification, indicative of en masse deposition of the heavy sediment-laden flows and upper flow regime conditions. Based on these characteristics, a proximal sandy or gravel-bed shallow ephemeral sheet braided deposits are suggested. In the upper part of Jangsan Formation, thin bedded fine-grained sediments displaying normal grading, current ripple-cross lamination, massive, and planar or low-angle cross lamination are interbedded with sheet sandstone beds. These are interpreted as deposition from suspension and weak traction currents of floodplain environment. The Jangsan Formation is, in turn, conformably overlain by the Myobong Formation showing flaser to wavy bedded sandstones, wave rippled sandstones, and alternations of sand and mud of tidal flats.

The application of this fluvial interpretation to these sheet sandstones may be extended to other parts of the North China Platform, where precise interpretation was hindered by its non-channelized geometry, characteristic of fluvial deposits before the advent of the terrestrial land plant. Possible basal fluvial deposition of the sandstone units in other portions of Taebaeksan Basin and the North China Platform is suggested by the occurrence of coarse-grained deposits with similar characteristics described herein for the Jangsan Formation in these basal units.