Print this pageHigh-resolution geochemical and petrographic study of the opal-A to opal-CT transition in the silicious Miocene Monterey Formation of California will address the origin of ribbon bedding in chert. Whether the distinctive cyclic alternation is a primary or a diagenetic feature has long escaped resolution. At Mussel Rock, the development of ribbon bedding has been "caught in the act" by rapid tectonic uplift.
X-ray diffractometry of eleven samples, including a nodule, identified and characterized the opal-A to opal-CT transition. The diffractograms show a gradual transition from opal-A to opal-CT over a 33-meter section. No distinct opal-CT is found before the stratigraphically uppermost nodular layer. Smooth opal-A humps transition to serrated opal humps within 10 meters from the formation of opal-CT. This shape may reflect precipitation of inorganic opal-A' from dissolution of opal-A diatom frustules. Sediments within the same layer as the opal-CT nodule, and immediately below, contain this intermediate opal phase as well. Besides opal peaks, quartz and feldspar peaks are prominent.
Additional samples will be analyzed in detail for elemental composition, mineralogy, microfabric, total organic carbon, and porosity. Comparisons of ratios of mobile versus immobile sedimentary components will define the diagenetic modification of the original sediments. Changes in microfabric will be correlated with changes in porosity and mineralogy to learn the diagenetic mechanisms. A pilot study suggests that diagenetic enhancement of ribbon bedding occurred by both compositionally conservative and modifying processes within the same stratigraphic interval.
AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid