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ABSTRACT: Depositional Environment and Diagenesis of a Miocene Tuffaceous Limestone in the La Honda Basin, California

Dallilah El-Sabbagh

Limestone bodies can form on localized topographic highs in deep, pull-apart, siliciclastic depositional basins, potentially becoming reservoir rocks. In a tuffaceous limestone in the La Honda basin the uncommon mixture of sediment types influenced the rates and formation sequences of diagenetic minerals.

The Burns Limestone, a subsurface oil-bearing tuffaceous limestone was deposited on the shelf-edge and upper flanks of a Miocene basaltic topographic high within the La Honda basin. The bank-top setting created by volcanic activity in a pull-apart basin created a shallow-water environment starved of clastic sediment, permitting calcareous organisms to flourish and providing localized large volumes of carbonate sediment.

Volcanic activity contributed chemically unstable finely crystalline basalt and ash. Local reworking and allodapic deposition along the top and flanks of the bank mixed the two sediment types. Deposits proximal to the bank are more carbonate-rich and massive, forming porous, oil-bearing horizons. The more distal tuffaceous-carbonate mixed deposits are thinly bedded and less porous, partially due to the presence of clays (smectite) and zeolites within the matrix.

Increased burial, compaction, and alteration of the tuff and basaltic fragments to clays and zeolites, reduced the limestone's permeability, decreasing the flow of circulating groundwater. Pore waters became more reducing, reflected by increasingly more ferroan calcite in the later stage, stratigraphically deeper cements, and the presence of dolomite at the base of the limestone. The dominant zeolite in the limestone is analcime, whereas clinoptilolite/heulandite is dominant in the underlying tuff. This slightly reversed order of zeolite formation may have been influenced by the presence of calcium carbonate ions in the pore waters.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990