Lithology of the Monterey Formation in the Western Santa Maria Valley Field, Santa Maria Basin, California

Abstract Three hundred and fifty feet of conventional core were recovered from the Union Leroy 51-18 well in Western Santa Maria Valley field, Santa Barbara County, California. The core contains unweathered Monterey Formation lithologies in both the opal-CT- and quartz-grade silica phases. The core spans the opal-CT to quartz silica-diagenetic transition. The silica-phase change occurs within a transition zone in which opal-CT and quartz-phase rocks are interlayered with one another. It is an observation that within the interlayered zone, rocks composed of nearly pure biogenic silica are preserved as opal-CT, while more detrital-rich siliceous shales and clay shales have had all their biogenic silica altered to the quartz silica phase. The stratigraphic succession of lithologies within the core shows a clear subdivision into Lower Monterey and Upper Monterey Members. The Lower Member is characterized by phosphatic, organic-rich shale interlayered with one- to two-foot thick dolomite beds. There are no layers of pure biogenic-siliceous rocks present within this lower shale and carbonate sequence. In the Upper Member however, beds of nearly-pure biogenic silica appear for the first time within the Formation. Extremely organic-rich shales also are present within the basal portion of the Upper Member. Incipient formation of glassy quartz chert is observed within the Upper Member. The glassy quartz chert is seen to form from diagenetic replacement of pre-existing pure-silica opal-CT beds.