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Authigenic carbonate structures in Miocene biosiliceous sediments are well exposed near a late Miocene angular unconformity in coastal cliffs at Santa Cruz, California and closely resemble carbonate structures formed at modern seep sites on the seafloor, including the adjacent Monterey Bay. The Miocene vent structures show varied morphologies, including pipes (“chimneys”) and bedding-parallel slabs., but, unlike many modern seep carbonates, they lack an associated vent macrofauna. They are composed of low magnesium calcite which cements and partly replaces the host sediment, indicating that the structures formed below the sediment-water interface and not above the seafloor. Carbon and oxygen isotopic compositions suggest carbonate precipitation occurred in a low temperature pore fluid environment fairly near the seafloor, within the zone of bacterial sulfate reduction.

The host rock, the Santa Cruz Mudstone, comprises interbedded siliceous mudstones and thin, brittle opal-CT porcelanite layers which show two dominant fracture sets, one striking N30°E, the other N60°W. Most of the carbonate vent structures occur within the porcelanite layers, and the orientations of many pipes and slabs parallel the strikes of the two fracture sets. This suggests that the fluids which precipitated the carbonates were channeled along fractures. This structural control and the proximity of the vent structures to an angular unconformity indicates that deformation was a major factor, creating fracture permeability and probably also causing tectonic compaction of sediments as well as expulsion of fluids. The main Miocene vent locality lies near three major fault zones (San Gregorio, Monterey Bay, and Ben Lomond), and we speculate that the deformation was related to tectonism on one or more of these faults.

Among the unresolved issues is the time and burial depths at which the carbonate vent structures formed. Some evidence (e.g. preservation of opal-A diatoms in the calcite structures) favors carbonate precipitation prior to the opal-A to opal-CT phase transformation, while other evidence (e.g. lack of compaction around the carbonate structures) suggests precipitation occurred after or contemporaneously with the silica phase change. These conflicting scenarios might be reconciled if the silica phase transformation occurred relatively early at shallow burial depths in an environment of advecting fluids with low silica concentrations.

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