Abstract

The Burns limestone, located in the central region of the La Honda Basin, California, represents reworked shallow-water calcareous sediments mixed with volcaniclastic sediments, deposited on the shelf edge and upper flanks of a submarine basaltic topographic high in an otherwise deep, siliciclastic basin. The bank-top environment, created by volcanic activity in a pull-apart basin, provided a shallow-water, sediment-starved setting where calcareous organisms could flourish, and produce large localized volumes of carbonate sediment. Volcanic activity in the area introduced chemically unstable ash and finely crystalline basalt to the bank setting. During reworking and deposition of the sediments along the bank-edge and bank flanks the two sediment-types were mixed to varying degrees. With increased burial the chemically unstable bioclasts began to undergo diagenetic alterations. Equally chemically unstable, the tuff and volcaniclastic sediments altered, forming authigenic clay minerals and zeolites. The presence of calcium carbonate ions in the pore waters appears to have influenced the sequence of zeolite formation, enabling the precipitation of sedimentary analcime at somewhat shallower burial depths than clinoptilolite and/or heulandite. In turn, the formation of diagenetic clay minerals and zeolites within the existing voids may have influenced the composition and rates of carbonate diagenesis by gradually decreasing the amount of pore waters circulating through the limestone and by providing a source of magnesium for the formation of dolomite in the deeper buried sections of the limestone lens.