MALONE, MITCHELL, PAUL BAKER, and CHRIS LABER, Department of Geology, Duke University, Durham, NC, and STEPHEN BURNS, Geologisches Institut, Universitat Bern, Bern, Switzerland

ABSTRACT: Burial Diagenesis of Dolomite from the Miocene Monterey Formation

Isotopic and minor element analyses and a qualitative assessment of cation ordering in dolomites from the Miocene Monterey Formation at Shell Beach and at three sections from the Santa Barbara coastal area of California show that these dolomites have undergone variable degrees of stabilization during burial diagenesis. Shell Beach was sampled throughout the entire section. Dolomites from three sections (Naples Beach, Gaviota Beach, and Damsite Canyon) of the upper calcareous shale member of the Monterey Formation west of Santa Barbara were also sampled. Previous work in this basin by Isaacs and coworkers has shown that burial depths and temperatures were greater in the western sections than farther to the east.

At Shell Beach, there is a positive correlation between (isotope){18}O and the Sr content of the dolomites. We believe this is due to progressive loss of Sr during crystallization of dolomite in marine pore fluids. A previous study at Shell Beach has shown that dolomites must have formed in the upper few meters of the sediment column making the alternative explanation, that the dolomites precipitated at highly variable burial depth, less likely. Preliminary results on cation ordering show that dolomites with negative (isotope){18}O values negatively correlate with degree of order. Likewise, there is a good negative correlation between degree of order and the Sr contents of the dolomites.

Preliminary data from the Santa Barbara Coastal Area show that mean cation ordering and mean mole-percent MgCO[3] increase to the west, whereas mean Sr content decreases westward. Stable isotopic analyses for the Santa Barbara sections will also be presented.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.