Cenozoic sedimentation and clay mineralogy in the northern part of the Magellan Trough, Central Pacific Basin

Assemblages of clay-sized minerals in twelve deep-sea cores from the northern part of the Magellan Trough in the Central Pacific Basin clearly show that authigenic smectite together with clinoptilolite formed subsea in situ and predominated throughout Palaeogene time. Clastic clay minerals such as chlorite, illite and kaolinite increased at the expense of smectite in sediments deposited since the Pliocene. Most of the authigenic smectites are of the di-octahedral, iron-rich type. The most probable origin of the authigenic smectites is formation from precursors such as volcanic glass. As part of this process biogenic silica evidently contributed to the formation of smectite and zeolites as clinoptilolite. The presence of a tri-octahedral saponite and the TEM morphology of some smectites, on the other hand, suggests that not all of the smectite studied was formed subsea in situ but that some had an allogenic origin. Systematic vertical changes in the assemblage of clay-sized minerals suggest that the authigenesis of submarine smectite and clinoptilolite prevailed in conditions of reduced aridity and weak wind activity throughout the Palaeogene in the study area. However, allogenic clay minerals such as chlorite and illite increased since the Olduvai event under conditions of severe aridity and strong wind activity caused by global climatic change. The west-northwest movement of the Pacific plate must have partly played a role in deposition of the clastic clay-sized minerals from the Asian continent by westerlies in the study area. The widespread occurrence of Cenozoic hiatuses in the study area is attributed to the influence of Antarctic Bottom Water (AABW). The latter must have influenced non-deposition of the clay-sized minerals at times during the Cenozoic.