TOKUOKA, TAKAO, SEIKI YAMAUCHI, YOSHIKAZU SAMPEI, and YUICHIRO MIYATA, Geological Survey of Japan, Tsukuba, Ibaraki, Japan

ABSTRACT: Mudlumps Formed at the Mouth of the Iinashi River, Shimane, SW Japan

Two eyebrow-shaped mud lump islands formed at the mouth of the Iinashi River in Lake Nakaumi, a coastal lagoonal lake facing the Japan Sea, in January 1991. These were located about 70 m in advance of the delta front, and were both 70 m long by 30 m wide and their maximum height was 1.7 m above sea level in May 1991. The mud lumps were attacked by wave action and submerged below sea level at the end of July 1991. One of them was again uplifted in March 1992 to 1 m above sea level at the same location but slightly larger than previously and was submerged at the end of August 1992. Aerophotographic, echo-sounding and drilling surveys, as well as on-land geologic survey of the mudlumps. have been done at the delta front area. Their mechanism can be interpreted as a rotational sliding phe omenon related to a rapidly growing delta-front sand body. The mudlump islands have often been witnessed from 1940s to 1960s. It seems likely that after channel construction along the river, such islands occurred in advance of the delta front during its progradation. The Iinashi River mudlumps are similar in their occurrences to those at the mouths of the Mississippi as reported by Shaw in USGS Professional Paper 85-B in 1913 in the early stages of Jetty construction and are different from diapiric intrusion which is generally accepted as the origin of mudlumps.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.