Aquifers and ferric spring deposits south of Miri, Sarawak: impact on facies characterisation of coastal sediments

Observations on artesian springs indicate the presence of at least three groundwater bodies that are chemically distinct. Artesian springs are observed from both high and low iron-content ground waters. Fe2+ rich groundwater is rapidly oxidized at surface, and orange deposits of Fe3+ minerals are precipitated. These are either parallel-layered, or form concentric ferric conglomerates in coastal caves. Once eroded, Fe-coated pebbles are scattered along the coast and embedded in recent sediments. Similar deposits are also seen in the Late Miocene Lambir and Miri formations. Field observations supportive the idea of a strong tidal influence on the dynamics of aquifers, and iron mobility in coastal areas. Re-deposition of the Fe in oxidized form could be in combination with other metallic cations, anions or even in chelated forms with organics. The reduction and movement of Fe in aquifers, and iron deposits (coated pebbles, filled burrows) could be linked to the facies signature of coastal Miocene rocks, as it does in present-day sediments. This process might be related to the development of source rocks in coastal offshore deposits.