Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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A Test Of Diagenetic Ordering In Siliceous Lithofacies, Monterey Formation, Southwestern Casmalia Hills: Santa Maria Basin, CA


A detailed analysis of a well-exposed section through the Sisquoc and Monterey formations test the results of previous silica diagenesis studies by Isaacs (1981), Pisciotto (1981), and Murata and Nakata (1974). From these studies, a key understanding was that for each compositional range, there would be a regular progression of opal-A → opal-CT→ progressive ordering of opal-CT d101 spacing → quartz. This progression should apply to any stratigraphic succession for strata of a particular compositional range, and opal-CT d101 could be used as a geothermometer and indicator of maximum burial depth. Surprisingly, these classic studies analyzed only limited numbers of samples in single successions, so we decided to test if the same trends are evident in a very dense data set in one stratigraphic sequence, 230 outcrop and road-cut samples were collected from a 700-meter thick sections and analyzed for composition, silica phase, and d101-spacing using combined EDS/XRF and XRD. The field area has a down-section stratigraphic trend from opal-A to opal-CT to interbedded opal-CT and quartz-phase rocks. Samples ranging from siliceous mudstone to cherty porcelanite were plotted in narrow, 10 percent compositional brackets on a carbonate-, apatite-, organic matter-free basis. The data displayed a wide range of opal-C d101 spacing for all depths and compositional ranges, and do not show a simple trend of decreased d101-spacing with depth as would be predicted by previous, well-cited studies. A literature search to understand the discrepancy in the results found several publications that also found opal-CT d101- spacing decreasing only subtly with depth and with wide degree of scatter. It is little appreciated that Isaacs' well-known silica diagenesis diagram that shows the differences in d101-spacing with composition was not constructed from a stratigraphic succession, but from relationships at multiple locations with different burial depths and temperatures — working geologists have inferred that this trend is applicable to single stratigraphic sections. Our new results do not necessarily refute the conceptual model of increased ordering (decreasing d101) with burial depth and temperature, but strongly suggest that other spatially important variables besides burial depth/temperature and composition are critical. Consequently, use of opal-CT d101-spacing should be used as a geothermometer only with great caution.