. Collected reprints, Essa Institute for Oceanography. Oceanography SEDIMENTS OF THE MALACCA STRAIT, SOUTHEAST ASIA 123 eroded from the neighboring land areas. Large ash concentrations off the Sumatra coast in the vicinity of rivers draining tuff deposits on Sumatra lends support to this conclusion. The numerous ash layers in relatively short intervals of the core section, 25 laminae in a 10 cm in- terval, may be more indicative of heavy runoff during the monsoons than volcanic eruptions. Reworking by currents may also account for the ash distribution. Authigenic Constituents Gla
. Collected reprints, Essa Institute for Oceanography. Oceanography SEDIMENTS OF THE MALACCA STRAIT, SOUTHEAST ASIA 123 eroded from the neighboring land areas. Large ash concentrations off the Sumatra coast in the vicinity of rivers draining tuff deposits on Sumatra lends support to this conclusion. The numerous ash layers in relatively short intervals of the core section, 25 laminae in a 10 cm in- terval, may be more indicative of heavy runoff during the monsoons than volcanic eruptions. Reworking by currents may also account for the ash distribution. Authigenic Constituents Glauconite is found extensively throughout the strait occurring mainly as pellets of dark- green grains and to a lesser extent occupying the cavities of foraminiferal tests. Percentages of glauconite range from a trace to 80 percent of the light fraction with a mean of about 6 percent. The highest concentrations occur in the Central region along the Sumatra margin. Only traceable amounts are found in the coarser sediments be- tween Malacca and Port Swettenham. No ap- parent correlation is observed between the occur- rence of glauconite and grain size because of the way in which the pellets are deposited. Glau- conite pellets become part of the sediment as the foraminiferal tests surrounding the pellets are broken. Most of the broken tests are winnowed away leaving the pellets behind. Excluding the narrow portion of the strait in the vicinity of Port Swettenham, there appears to be a general inverse relationship between the presence of foraminifera and glauconite as a result of the removal of the broken tests. Pyrite occurs in the form of internal molds of foraminifera and radiolaria and is frequently found in the surface sediments. The possibility that reducing conditions may exist in the micro- environment within cavities of various tests and shells may be significant in explaining the origin of such authigenic minerals as pyrite and glauco- nite in an environment often oversatur
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