(Table 1) Types of carbonate sediments and origin of carbonate constituents at DSDP Leg 57 Holes

Petrographic and mineralogical studies using optical and scanning electron microscopes and X-ray diffraction and X-ray microprobe analyzers on 137 samples of Pleistocene to Upper Cretaceous sediments at Sites 438 and 439 off northeast Honshu in northwest Pacific yielded the following major results:
1) Sites 438 and 439 are characterized by carbonate-free terrigenous and diatomaceous sediments throughout the Pleistocene to the Upper Cretaceous. However, carbonate concretions, thin limestone beds, and carbonate-cemented sandstones are sporadically intercalated. Moreover, terrigenous and diatomaceous sediments occasionally contain significant amounts of carbonates.
2) Carbonate minerals encountered include calcite, dolomite, ankerite, and siderite. Calcite and dolomite are either of authigenic or detrital origin. Biogenic calcite grains are not uncommon. Ankerite and siderite are of detrital origin.
3) Authigenic calcite ranges in chemical composition from high Mg calcite in the Pleistocene sediments to Fe calcite in the uppermost Oligocene sediments through FeMg calcite in the Miocene sediments. Composition is probably controlled by diagenetic alteration of initially precipitated high Mg calcite during burial.
4) Mg ions released from high Mg calcite during the transformation seem to cause dolomitization.
5) The transformation and dolomitization in the uppermost Oligocene sandstone were probably accelerated by the recharge of fresh or brackish water from the underlying subaerially formed conglomerate and/or from the nearby Oyashio ancient landmass.