D. I. Lim á Y. A. Park á J. Y. Choi J. W. Cho á B. K. Khim Glauconite grains in continental shelf sediments around the Korean Peninsula and their depositional implications Received: 4 October 1999 / Revision accepted: 18 May 2000 Abstract Glauconite grains are an important compo- nent of the sur®cial sediments on the Korean continental shelf. Relatively high glauconite contents >20%) occur near Huksan Island in the Yellow Sea and on the outer shelf of the southern East Sea where they are associated with relict, sandy sediments. By contrast, glauconite grains are rare <1%) in clay-dominated sediments. The grains can be classi®ed into four major categories: 1) very abundant, green to dark green, spheroidal and lo- bate grains displaying bulbous and honeycomb exteriors with well-developed surface cracks; 2) dark green, discoidal grains with poorly de®ned cracks; 3) scarce, moderately dark green, accordion-shaped grains char- acterized by series of closely spaced, parallel surface ridges; 4) scarce, green to dark green, foraminifer- shaped grains. Microprobe analyses as well as optical microscopy and SEM observations show that most grains are composed of mineral mixtures e.g., quartz, mica, feldspar) rather than a single mineral species. The mineralogy, morphology, and textural properties sug- gest that the grains may have formed mainly by replacement of fecal pellets, and the alteration of mica and clays which have ®lled foraminifera tests. The high potassium contents, rosette-shaped clay structures, and bulbous shapes re¯ect an ``evolved'' mature) stage of glauconitization. Glauconite grains in Korean shelf sediments are presumably relict, and have been produced by the reworking of older glauconitic sedi- ments during the Holocene sea-level transgression. Introduction Glauconite is a common authigenic constituent of modern continental shelf sediments as well as of Pre- cambrian±Pleistocene sedimentary deposits Odin and Letolle 1980). The formation of glauconite grains glauconitization) occurs commonly in shelf environ- ments with low sedimentation rates, low terrigenous material supply, suitable substrates, and the requisite physicochemical conditions e.g., low turbulence, low temperatures, slightly reducing conditions, and an abundant supply of iron and potassium; Odin and Matter 1981; Hughes and Whitehead 1987). Thus, glauconite is not only a stratigraphic tool but also an indicator of depositional environment type. The inner continental shelf surrounding the Korean Peninsula is covered with muddy Holocene sediments while coarse-grained, sandy material is dominant on the outer shelf Choi 1990). The sands are comprised mainly of quartz and feldspar Choi and Park 1993). The presence of iron-stained quartz grains, and the associa- tion of stable heavy minerals indicate a relict origin Lee et al. 1988; Choi and Park 1993). Recently, glauconite grains have been discovered in the sandy sediments of the Yellow Sea KIGAM 1992; Lee et al. 1995). They were interpreted as being authigenic, and having formed under reducing conditions Lee et al. 1997). To date, however, little is known of the geographical distribution, geochemical composition, and detailed external and internal texture of the glauconite grains in the region. In this paper, we document the occurrence of glau- conite grains in continental shelf sediments in the vicinity of the Korean Peninsula, and attempt to inter- pret their origin and evolutionary stage on the basis of morphological, geochemical, and external as well as in- ternal textural characteristics. To further our under- standing of the depositional history of shelf sediments in Geo-Marine Letters 2000) 20: 80±86 Ó Springer-Verlag 2000 D. I. Lim &) á Y. A. Park Department of Oceanography, Seoul National University, Seoul 151-742, Korea e-mail: oceanlim@snu.ac.kr J. Y. Choi Department of Oceanography, Kunsan National University, Kunsan 573-701, Korea J. W. Cho Department of Earth Science, Chosun University, Kwangju 501-759, Korea B. K. Khim Polar Research Center, KORDI Ansan 4rt-600, Korea