AIELLO, IVANO W., Universitci di Firenze, Firenze, Italy and Univ. of California, Santa Cruz; DEBRA STAKES, Monterey Bay Aquarium Research Institute, Moss Landing CA; MIRIAM KASTNER, Scripps Institution of Oceanography, La Jolla, CA; and ROBERT E. GARRISON, Univ. of California, Santa Cruz and Moss Landing Marine Laboratories, Moss Landing, CA
Abstract: Carbonate Vent Structures in Fractured Miocene Biosiliceous Rocks at Santa Cruz, California
Prominent authigenic carbonate vent structures formed by fluid flow occur in the Santa Cruz Mudstone, an upper Miocene biosiliceous unit correlative with the Monterey Formation, at Santa Cruz, California. The structures occur in highly fractured opal-CT porcelanites near the top of the unit, just below an angular unconformity separating the Santa Cruz Mudstone from the overlying Mio-Pliocene Purisima Formation. The structures include bedding-parallel slabs, pipes ("chimneys"), mounds, and massive beds. Many of these contain circular feeder conduits, which in some cases remain open, in others are filled with host sediment or with authigenic carbonates. The structures are composed of low magnesium calcite whose oxygen and carbon isotopes indicate precipitation at low temperatures with the carbonate derived from anaerobic sulfate reduction of organic matter, an interpretation supported by the presence of finely crystalline pyrite. The shapes and orientations of the carbonate vent structures are closely aligned with two prevalent fracture sets in the porcelanites, one striking N30° E, the other N60° W. Linear prolongations of the bedding-parallel slabs trend predominantly in these directions, as do the plunge of inclined pipes and their conduits.
Formation of these structures occurred during late Miocene uplift and folding of the Santa Cruz Mudstone, probably related to movements on the nearby San Gregorio or Monterey Bay fault zones, when porcelanite beds became intensely fractured. These fractures funneled the expulsion of fluids and gases expelled during compression of organic-rich sediments and localized the sites of calcite precipitation.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California