Diagenetic Trends in the Silica Lithofacies of the Monterey Formation in the Southwestern Casmalia Hills: Santa Maria, CA

Ijeoma, Idu Opral C. and Behl, Richard J.
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A detailed diagenetic analysis of a well-exposed section through the Sisquoc and Monterey Formations does not reveal simple expected trends in ordering of opal-CT as expressed by d101-spacings in X-ray diffraction data. This suggests that in some locations factors other than the burial depth and temperature reached are important in the crystallographic ordering of siliceous minerals. In the southwest Casmalia Hills, a well exposed 1100 meter thick section that spans the Sisquoc and Monterey Formation was analyzed for composition by X-ray diffraction and X-ray fluorescence and the opal-CT phase rocks were characterized for their d101-spacing. No simple trends were found for the widely scattered data for the d101-spacings which ranged from 4.05-4.11 Å from a compositional range of 13 to >90 % silica (carbonate-, apatite- and organic-matter-free basis). The opal-A diatomaceous sediments are at the top of the section in the Sisquoc Formation with the uppermost opal-CT rocks in the lower Sisquoc. Quartz phase rocks are not concentrated in the lower section of the Monterey but exist interbedded with opal-CT rocks throughout the middle to lower section. The stratigraphically highest quartz phase rocks are primarily the most siliceous, cherty-porcelanites and cherts. This confirms that the most siliceous rocks undergo a silica diagenesis that is not simply related to greatest burial. Opal-CT phase rocks from siliceous mudstone to cherty porcelanites were analyzed in groups of narrow 10 % silica compositional steps. None of the groups show simple trends of decreased d101-spacing with burial as would be expected. These results may relate to the burial and uplift history of the area and related recrystallization or out-of-sequence diagenesis during uplift, or perhaps localized variations in pore-water chemistry or porosity and permeability. These unexpected findings suggest that simple and general diagenetic trends may not be applicable in all settings.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013