| dc.description.abstract | We analyzed clay minerals and their relative abundances of Miocene to Pleistocene shales as well as mineral compositions of sandstones sampled from northwestern Taiwan. Results of vertical changes on clay mineral species and abundances yield information on degree of burial diagenesis along with possible rock types in sediment source terrains. Samples were collected from
two localities, one is from the core materials of KY‐1 well, locating on the western coast of the Taoyuan Tableland, a series of 30 samples was collected from the Pleistocene Toukoshan Formation to the Miocene Taliao Formation, ranging from 181 to 2603 m in depth. The other is from three outcrop sections along the Dahan River, Tzukeng River and Sanshia River, respectively, in the NW
Foothills. We collected 35 samples from the Pliocene Chinshui Shale to the Miocene Mushan Formation with an estimated maximum burial depth ranging in between 2550 to 5200 m deep.
Results from X‐ray powder diffraction analysis on collected samples show that KY‐1 samples are rich in expandable clays, mainly smectite and minor mixed‐layer illite/smectite. There is no characteristic vertical change for the relative abundances of expanding clays and non‐expanding clays (e.g., illite) at the KY‐1 well. For the samples from the Foothills, the abundance of expanding
clays is high in the upper section and decreases its relative abundance when increasing burial depths. In addition, it appears that the depth range of 4000 to
4500 m reveals an ordering mixed‐layered illite/smectite. We therefore interpret that the above depth range is the depth where significant transformation of smectite to illite occurs. However, illites are generally high in
abundance throughout the composite sections of the Foothill. As illite is abundant for all studied samples and it coexists with large amount of expanding clays in rocks experiencing shallow burial, we examined the illite crystallinity in order to understand its origin. For burial depths shallower than 4000 m (i.e. for all formations at the KY‐1 well and Peiliao and younger formations in the foothills), illites show a crystallinity with a Kübler Index of 0.25 to 0.40 。2θ, indicating that most of the illites are sourced from low‐grade metamorphic
terrains. For burial depths deeper than 4000 m (i.e. for Shiti and older formations in the Foothills), some of illites show lower crystallinity with a Kübler Index of 0.40 to 0.44 。2θ. Through microscopic and SEM/EDS analysis in the samples collected from the Foothill regions, the chemical compositions changed among the mixed‐layered Illite/smectite, which were iron and magnesium ions decreased and silicon and aluminum ions increased with
increasing depth. Moreover, in the deeper burial depth, some of detrital illites were altered to mixed‐layered illite/smectite, which is probably the reason why
decreasing illite crystallinity decreases in the deeper depth in the Foothills.
These results indicate that the strata in the Foorhill region experienced more diagenetic effects than strata in the KY‐1 well.
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