28. CLAY PETROLOGY OF MUDSTONES, LEG 18, DEEP SEA DRILLING PROJECT

Clay minerals in mudstones from Leg 18 drilling sites offshore of Oregon and in the Gulf of Alaska have potential value for interpreting diagenetic history, provenance, sediment dispersal mechanisms, and relative movements of lithospheric plates. Clay mineral suites from both regions are similar to most from the Cenozoic of the northeast Pacific. They contain detrital, terrigenous chlorite, mica, and vermiculite. Mixed-layer clays are also abundant; these can either be weathering products of igneous and metamorphic minerals and volcanic debris, or submarine alteration products of volcanic material of continental or oceanic origin. This question was asked about both regions: do crumpled mudstones at the continental margin represent trench fill which was deeply buried in the subduction zone and subsequently folded and uplifted against the continental slope? If so, the clay minerals may have undergone diagenetic reactions as a result of burial. In fact, there is no such evidence in the nature of mixed-layer clays progressively deeper in any hole examined to suggest diagenetic clay alteration of the type observed in Cenozoic shales of the Gulf of Mexico region. However, this does not rule out the possibility of the mudstones once being more deeply buried than they are now; it simply means that temperatures were not great enough to cause noticeable clay diagenesis. The Columbia River and the Rogue River have delivered in the Pleistocene, and are delivering today, different clay mineral assemblages to the Oregon margin. These differences are related to rock types, climates, and weathering reactions in the respective drainage basins. These clay mineral differences between provenances persist and may even be enhanced as fine-grained sediments are carried several miles away from river mouths. This raises the possibility of using clay minerals for provenance recognition. Because clays tend to remain suspended longer than sands and are more widely dispersed in the oceans accordingly, many factors of sediment transport and dispersal need further evaluation before clay minerals have the same potential for determining provenance as have heavy minerals. Clay-mineral stratigraphy at two sites (174 offshore of Oregon and 178 on Aleutian Abyssal Plain) on oceanic lithospheric plates can be interpreted to mean ever greater influence of terrestrial sedimentation upward through time. In other words, oceanic and continental plates moved closer together through time as the oceanic plate was swallowed in the subduction zone at the continental margin. Site 178 shows this particularly well, with Early Miocene clay suites being nearly pure mixed-layer clay of probable submarine volcanic origin, followed by ever increasing amounts of detrital, well crystallized, terrigenous mica and chlorite upward through the late Pleistocene. INTRODUCTION sediments at Sites 175 and 181 on the continental slope? Clays from adjacent abyssal plain, trench, and continental Is it possible that detailed study of clay minerals can shelf sites were examined for comparison. The sites fall into shed light on processes taking place in subduction zones? two somewhat analogous groups: (1) Sites 174, 175, and Specifically, is there clay-mineral diagenetic evidence in 176 offshore of Oregon; and (2) Sites 178, 180, and 181 in mudstones for deep burial and subsequent uplift of trench the Gulf of Alaska. In addition to the matter of burial